I I jiiii i iliiiJ I 11 ill??: 11 iilil llitiiiJiWl (Hi ill I III' if lii .,.„.,,i 11 I id P I * ill i mm m\mm mm mmm liiii I . U. Ml Slibrarg tlf Qlaroltna BUU lok was presented by sri ck L. Wei Iman 0^45 S122 This book is due on the d below and is subject to an* as posted at the Circulatioi MAY 1 7 1967 WG %^ ^"uuV 9 1993 / lY '1 f>*^: (^ /> /f 5^ TEXT-BOOK OF BOTANY ^^ CHS ilontion MACMILLAN AND CO. PUBLISHERS TO THE UNIVERSITY OF TEXT-BOOK OF BOTANY MORPHOLOGICAL AND PHYSIOLOGICAL BY JULIUS SACHS PROFESSOR OF BOTANY IN THE UNIVERSITY OF WURZBURG TRA NSLA TED A ND A NNO TA TED BY ALFRED W. BENNETT, M.A., B.Sc, F.L.S. LECTURER ON BOTANY AT ST. THOMAS'S HOSPITAL ASSISTED BY W. T. THISELTON DYER, M.A., B.Sc, F.L.S. SOMETIME JUNIOR STUDENT OF CHRIST CHURCH, OXFORD AT THE CLARENDON PRESS M DCCC LXXV [All rights reserved] PREFACE. This Text-book of Botany is intended to introduce the student to the present state of our knowledge of botanical science. Its purpose is not only to describe the phenomena of plant-life which are already accurately known, but also to indicate those theories and problems in which botanical research is at present especially engaged; the arrangement of the material and the mode of treatment of the separate subjects are adapted to this purpose. Detailed discussions of questions of minor importance have been avoided, as these would only mar clearness of oudine in the design; critical remarks have been introduced occasionally where they seemed necessary, in order to determine facts, or to justify the views taken on matters of fundamental importance. The historical development of botanical views and theories does not seem to come within the scope of a Text-book of Botany, and would only interfere with the unity of design of the work. It would therefore be superfluous to quote scientific works which have only a historical interest. In the references which will be found in the work the chief object has been to introduce the student to those writings in which he will find a fuller discussion of those parts of the subject which have been only touched on briefly. In some cases the writings of others have been quoted because they represent views difl'erent from those of the author, and because it is desirable to place the student in a posi- tion to form a judgment for himself. Others again of the references are simply for the purpose of citing the authorities on which reliance is placed for state- ments that have not come within the range of the author's own observation. The reader of this work will at least learn the names and standing of those w^orkers who have in recent times contributed most essentially to the science of which it treats. By far the greater number of the illustrations are original, many of them the result of laborious investigation. Where they have been copied the name of the author from whom they are borrowed is in each case given in the descrip- tion; iUustrations from other sources are used only when the objects themselves have not been accessible, or when it seemed impossible to obtain better ones. The Table of Contents will give sufficient indication of the plan of the work; the Index should be consulted for references to other parts of the book where an explanation of technical terms will be found when their meaning does not appear in any particular passage. TRANSLATOR'S PREFACE. In introducing to the English public Sachs's ' Lehrbuch der Botanik ' in an English form, the translator believes that he is supplying a want which has long been felt by English botanical students. Our own Hterature has not at present produced any work at once so comprehensive in its scope and so minute and accurate in its details, — qualities which have recommended the German work to every one familiar with that language. In the notes the citations of authorities have been somewhat increased. It has also seemed desirable sometimes to depart from the author's rule of passing over authorities whose interest is now chiefly historical. References have been given to English and French translations of many of the papers and memoirs quoted, as these are at any rate often more accessible in this country than the originals. On several points additional matter has been introduced into the footnotes. With respect to these the translator has to acknowledge the kind help of numerous scientific friends, amongst whom he may more especially mention Professor W. C. Williamson, Mr. H. C. Sorby, and Professor W. R. M^Nab. In the selection of English expressions for German technical terms he has also in many instances had recourse to their advice. One case of great difficulty may be pointed to in ' Stoff- wechsel'; as an equivalent to this the term * Metastasis' has been used. It had already been employed in a more restricted although analogous way by Graham; speaking of the mutability of colloids due to internal molecular rearrangements, that distinguished chemist says, ' Their existence is a continued metastasis ' (Journ. Chem. Soc. 1862, p. 217). The fourth edition of the German work has been passing through the press concurrently with the printing of this edition. Where possible the new matter introduced into it, or the new views adopted by the author^ are referred to in the footnotes to the present edition. The new classification of Cellular Crypto- gams adopted by Sachs will be found at p. 847. A. W. B. London, February 1875. ERRATA. P. 16, 1. 10 from bottom, and description to Fig. 13 ; for antheridium read globule. P. 25, 1. /[from bottom; after Bordered Pits insert reference to footnote 2. P. 30, 1. 18, for exospores read exospore. P. (i\, first line of footnote ; for p. 254 read p. 252. P. 65, 1. 12 from bottom ; for Salms-Laubach read Solms-Laiibach. P. 65, last line] for Gingko biloba read Salisburia adiantifolia. P. 82, 1. 2 ; for colourless read coloured. P. 1 01, 1. 5 ; dele previous. P. loi, 1. 6 ; for cannot be read has not been. P. 106, 1. 19 ; for bundle read bundles. P. 1 12, 1. 3 ; for Asclepiadse read Asclepiadeae. P. 147, 1. 40 ; for root-bearers read rhizophores. P. i\8, footnote ] for Oaniopsis read Calliopsis. P. 241 ; omit "^rd footnote. P. 2i\, footnote ; for Rees read Reess. P. 279, 1. 6 from bottom ; for p. 287 read p. 291. P. 287, description to Fig. 207 ; for antheridia read globules. P. 288, description to Fig. 208 ; for antheridium read globule. P. 296, 1. 3 of footnote ; for Synopsia read Synopsis. P* I'^liff-^t line of description to Fig. 222 ; for its read a. P. 327, 1. gfrom bottom; for colourless cells elongated in a parenchymatous manner read elongated colourless parenchymatous cells. P. 340 ; the reference in footnote to Ray Societfs publication should follow that to Hofmeister's papers. P. 361, 1. \from bottom; for the single read a single. P. 454, 1. II of footnote; dele entire. P. 456, 1. d from bottom; dele the first archegonium. P. 585, 1. 14; /or Balanophorae read Balanophoreae. P. 647, 1. ^from bottom; for Sect. 8 r^^/^ Sect. 7. P. T TO, footnote; for Ailanthus malabarica read Ailantus excelsa. CONTENTS. BOOK I. GENERAL MORPHOLOGY. CHAPTER I. Morphology of the Cell. PAGE Sect. I. Preliminary Inquiry into the Nature of the Cell i 2. Difference in the Forms of Cells 5 3. Formation of Cells 7 4. The Cell- Wall 19 5. Protoplasm and Nucleus 37 6. The Chlorophyll-Bodies and similar Protoplasmic Structures ... 45 7. Crystalloids 49 8. Grains of Aleurone (Proteine-grains) 51 9. Starch Grains 56 10. The Cell-sap 62 11. Crystals in the Cells of Plants 64 CHAPTER II. Morphology of Tissues. Sect. 12. Definition ^^ 13. Formation of the Common Wall of Cells combined into a Tissue . • . 7° 14. Forms and Systems of Tissues 77 15. The Epidermal Tissue 79 16. The Fibro-vascular Bundles 9^ 17. The Fundamental Tissue ^°^ 18. Laticiferous and Vesicular Vessels; Sap-conducting Intercellular Spaces; Glands i°9 19. The Primary Meristem and the Apical Cell "7 CONTENTS. Sect 20. )) 21. » 22. » 23. 5) 24. V 25. ) 26. }} 27. »1 28. 29. CHAPTER III. Morphology of the External Conformation of Plants. PAGE Difference between Members and Organs. Metamorphosis . . .128 Leaves and Leaf-forming Axes 131 Hairs (Trichomes) 138 The Root 140 Various Origin of Equivalent Members 148 Different Capacity for Development of the Members of one Branch-system 155 The Relative Positions of Lateral Members on a Common Axis . . 166 Directions of Growth .182 Characteristic Forms of Leaves and Shoots 190 Alternation of Generations ' . 202 BOOK II. SPECIAL MORPHOLOGY AND OUTLINES OF CLASSIFICATION. Group L Thallophytes Class I. Algae „ 2. Fungi . Group n. Characeae Class 3. Characeae Group in. Museineae Class 4. Hepaticae „ 5. Mosses Group IV. "Vascular Cryptogams Class 6. Ferns . „ 7. Equisetaceae „ 8. Ophioglossaceae „ 9. Rhizocarpeae „ 10. Lycopodiaceae Group V. Phanerogams Class II. Gymnosperms Sub-group. Angiosperms . Class 12. Monocotyledons „ 13. Dicotyledons 207 208 238 278 278 292 296 311 335 340 362 378 383 400 421 433 466 541 556 CONTENTS. xi BOOK III. PHYSIOLOGY. CHAPTER I. Molecular Forces in the Plant. PAGE Sect. I. The Condition of Aggregation of Organised Structures .... 587 „ 2. Movement of Water in Plants 598 „ 3. Movements of Gases in Plants 614 CHAPTER II. Chemical Processes in the Plant. Sect. 4. The Elementary Constituents of the Food of Plants 618 „ 5. Assimilation and Metastasis 626 „ 6. The Respiration of Plants 644 CHAPTER III. General Conditions of Plant-life. Sect. 7. The Influence of Temperature on Vegetation 647 „ 8. Action of Light on Vegetation 659 „ 9. Electricity 687 „ 10. Action of Gravitation on the Processes of Vegetation 690 CHAPTER IV. The Mechanical Laws of Growth. Sect. II. Definition 692 „ 12. Various Causes of Growth 695 „ 13. General Properties of the Growing Parts of Plants 697 „ 14. Causes of the Condition of Tension in Plants 7^8 „ 15. Phenomena due to the Tension of Tissues in the Growing Parts of Plants . 715 „ 16. Modification of Growth caused by Pressure and Traction .... 729 „ 17. Cause of the Growth in Length under constant External Conditions . . 735 „ 18. Periodicity of Growth in Length caused by the alternation of Day and Night 743 Xil CONTENTS, •PAGE Sect. 19. Effect of Temperature on Growth 74^ „ 20. Action of Light on Growth. — Heliotropism 752 „ 21. Influence of Gravitation on Growth. — Geotropism 758 „ 22. Unequal Growth 7^5 „ 23. Torsion 77° „ 24. The Twining of GHmbing Plants 772 „ 25. The Twining of Tendrils 775 CHAPTER V. Periodic Movements of the Mature Parts of Plants and Movements dependent on Irritation. Sect. 26. Definition 782 „ 27. Review of the Phenomena connected with Periodically Motile and Irritable Parts of Plants 783 „ 28. Mobile and Immobile Condition of the Motile Parts of Plants . . . 788 „ 29. Mechanism of the Movements 792 CHAPTER VI. The Phenomena of Sexual Reproduction. Sect. 30. The Essential Element in the process of Sexual Reproduction . . . 801 „ 31. Influence of the origin of the Reproductive Cells on the product of Fertilisation 807 „ 32. Hybridisation 816 CHAPTER VII. The Origin of Species. Sect. 33. Origin of Varieties . 822 „ 34. Accumulation of new characters in the Reproduction of Varieties . . 826 „ 35. Causes of the Progressive Development of Varieties 830 „ 36. Relationship of the Morphological Nature of the Organ to its adaptation to the Conditions of Plant-life 835 „ 37. The Theory of Descent 842 Appendix . . 847 Index 849 BOOK I. GENERAL MORPHOLOGY, CHAPTER I. MORPHOLOGY OF THE CELL. Sect. i. Preliminary Inquiry into the Nature of the Cell. — The substance of plants is not homogeneous, but is composed of small structures, generally indistinguishable by the naked eye ; and each of these, at least for a time, is a whole complete in itself, being composed of solid, soft, and fluid layers, different in their chemical nature, and disposed concentrically from without inwards. These structures are termed Cells. For the most part, a group of them are in close contact and firmly united ; they then form a Cell-tissue. But every plant which completes its term of life has at least one period in which certain cells separate themselves at definite points from the union, and, after isolation, each begins for itself a separate course of life (spores, pollen-grains, ovum-cells, gemmae). The shape and size of the whole plant, the form, structure, and volume of the cells are subject to regular changes, and their nature cannot therefore be inferred from the knowledge of one single phase, but rather from the sum of changes which may be called the life-history of the cell. And as, moreover, each cell fulfils its own definite part in the economy of the plant, i. e. is specially intended for certain chemical or mechanical purposes, so also cells show a variety in form, which corresponds to the different functions. These differences, however, do not usually arise until the cells have passed through their earlier stages; the youngest cells of a plant are only slightly distinguishable from one another. The law of configuration that prevails in all cells is also more clearly evident in the young state ; the more the developing cells assume the special purposes for which they exist, the more difficult it becomes to recognise this law. The morpho- logical law of cells, thus briefly pointed out, we will now endeavour to expound more in detafl. By far the largest proportion of cells in the living succulent parts of plants, e. g. young roots, lea^ves, internodes, fruits, are seen to be made up of three ' B MORPHOLOGY OF THE CELL. concentrically-disposed layers : firsdy, an outer skin, firm and elastic, the Cell-mem- brane or Cell-wall, consisting of a substance peculiar-to itself, which we call Cellulose (Fig. I, B, C, h). Close up to the inner side of this entirely closed membrane is a second layer, also entirely closed, the substance of which is soft and inelastic, and always contains albuminous matter ; H. V. Mohl, who first dis- covered this substance, has given it the very distinctive appellation of Protoplasm ^. In the condition of cells now under consideration it forms a sac enclosed by the cell-wall, in which usually also other por- tions of protoplasm are pre- sent in the form of plates and threads (Fig. i, B, C,p). Ab- sent from some of the lowest organisms, but present in all the higher plants without ex- ception, there lies imbedded in the protoplasm a roundish body, the substance of which is very similar to that of the protoplasm, the Nucleus (Fig. i, A, C,k). The cavity enclosed by the protoplasm-sac is filled with a watery fluid, the Cell- sap (Fig. I, B, C, s). And .besides this, there are also very commonly found in the interior of the cell granular bodies, which however may be passed over for the moment. Thus, then, cells in the stage of development now described consist of a firm membrane, soft protoplasm (including the nucleus), and fluid cell-sap. At first, however, the sap is wanting; if the same cells be examined in a very early state of their development they are smaller (Fig. i. A), their cell- wall thinner, and the protoplasm forms a solid body in the middle of which lies the nucleus, at this time relatively very large (k). The cell-sap first appears when the whole cell is increasing quickly in volume (Fig. i, B); it presents itself originally in the form of drops (vacuoli) in the interior of the protoplasmic body (Fig. i, B, s); at a later period these usually coalesce, and form a single sap-cavity (Fig. i, C, s) which is enclosed by the now sac-like hollow substance of the protoplasm. Fig. I.— Parenchyma-cells from the central cortical layer of the root oi Fritillaria imperialis ; longitudinal sections ( x 550). A very young cells lying close above the apex of the root, still without cell-sap. B cells of the same description about 2 mm. above the apex of the root ; the cell-sap s forms separate drops in the protoplasm /, behind which lie walls of protoplasm ; C cells of the same description about 7—8 mm. above the apex of the root ; the two cells to the right below are seen in a front view ; the large cell to the left below is in section ; the cell to the right above is opened by the section ; the nucleus shows, under the influence of the penetrating water, a peculiar appearance of swelling (x, y). ^ H. V. Mohl, Ueber die Saftbewegungeii im Inneren der Zellen.— Bot. Zeitg. 1846, p. 7, PRELIMINARY liVQUIRV INTO THE NATURE OF THE CELL. 3 In their earliest state the cells of the waod and cork of trees show also conditions of development which correspond essentially to those represented in Fig. I. In these cells, however, a new condition foMows very soon after the appearance of the cell-sap; the protoplasm containing the nucleus disappears, leaving the cell-wall filled either with air or with water. Older wood and cork when completely formed thus consist of a mere framework of cell- walls. But now arises an important difference between the behaviour of those cells which enclose a protoplasmic body, and of those from which it has already dis- appeared. The former only can grow, develop new chemical combinations, and, under certain conditions, form new cells. The latter are never capable of further development ; if they are wood, they are of service to the plant only from their firm- ness, power of absorbing water, and from their peculiar form; if cork, they form protecting envelopes which surround the living succulent cellular tissue. Since then no further process of development can take place in the cells which no longer contain protoplasm, it may be concluded that the latter is the proximate cause of growth. We shall see in a future paragraph that the de- velopment of each cell begins with the formation of a protoplasmic body, and that i.^— > FIG. 2.— Sexual reproduction of Fucks vesiculosus ; A cellular filaments bearing antheridia ; B spermatozoids ; / Oogonium, Og with paraphyses / ; // the exterior membranes of the oogonium is split, the inner 2 protrudes, containing the ova; /// an escaped ovum, with spermatozoids swarming round it ; A' first division of the fertilised ovum ; IV 3. young Fucus resulting from the growth of the fertilised ovum (after Thuret, Ann. des Sci. Nat. 1854, vol. ii). (B X330; all the rest X160.) the cell-wall is also generated from it ; but the relation of the protoplasm to cell- formation is still more strikingly conspicuous in those cases in which it continues its life for some time as a naked sharply-defined solid body, and only at a later period clothes itself again with a fresh cell-wall, and again takes up cell-sap within itself. We have an excellent example of this in the reproduction of the Fucace^. On the fertile branches of these great marine Algae, of which we may take Fiiciis vesiculosus as an example, large cells are formed in peculiar receptacles, the Oogonia (Fig. 2, /, Og) ; the space enclosed by the cell- wall is densely filled with fine-grained protoplasm, which at first presents a homogeneous mass, but at last B 2 MORPHOLOGY OF THE CElt. falls into eight portions, and these, completely filling up the cell-cavity of the oogonium, press against one another, and become polygonal. The wall of the oogonium consists of two layers ; the outer one splits, and the inner one pro- trudes in the form of a sac, which distends by absorption of water ; in this enlarged sac the portions of protoplasm become globular (Fig. 2, U); then this also bursts, and the protoplasmic bodies, now completely spherical, escape. By the fertilising action of other smaller protoplasmic structures, the spermatozoids, these spherical bodies are excited to further development; out of the interior of the ball of protoplasm (the fertilised ovum) a colourless substance next makes its appear- ance, which hardens into a closed cell-wall. The newly-formed cell now grows in two different direc- tions in different modes, and produces after further transform- ations (Fig. 2, Fand IV) a young Fucus-plant. Still more clearly than in these cases does the inde- pendence of the protoplasmic body of a cell show itself in the formation of the swarm- spores (zoospores) of Algae and of several Fungi. Here in many cases, as in Stigeodonium insigne (Fig. 3 ; B, a), the protoplasm- sac of a cell filled with cell-sap contracts, lets the water of the cell - sap pass out, and forms a solid roundish lump, which, escaping through an opening in the cell-wall, and impelled by an internal force, swims about in the water (C). While it is passing out of the cell-wall, the proto- plasmic body shows, by its mo- tions and changes of form, that it is soft and extensible ; but, once freed, it assumes a definite specific form, con- ditioned by an internal force. At last, usually after some hours, the swarm-spore comes to rest; if killed by the proper means, the protoplasmic body contracts {E, F, p), and a fine cell- wall may now be recognised, which it did not possess at the time of its exit, and at the beginning of its swarming. When once at rest, it also changes its form, and increases in volume, w^hile fluid cell- sap collects in the interior. The cell formed in this way now grows in a manner dependent Fig. 3. — Stigeoclomunt tnsigne (after Nageli, Pflanzenphysiol. Untersuch- ungen, Heft i) ; A a. branch of the Alga consisting of one row of cells, with a lateral branch ; cl green-coloured protoplasmic structures (chlorophyll), imbedded in the colourless sac of protoplasm of each cell not shown in the drawing ; B the protoplasmic bodies of the cells contracting and protruding through openings in the cell- wall ; C swarm-spores still without cell-wall ; D one come to rest ; at E and F killed ; the protoplasm / contracts and allows the newly-formed cell-wall h to be recognised ; H a young plant grown from the swarmspore; G two cells of a filament in the act of dividing; the protoplasmic body of each cell {.v, y) has temporarily split into two equal parts, and contracted by addition of a re-agent. DIFFERENCE IN THE FORMS OF CELLS. C on the specific nature of the plant; — in our example it specially elongates itself (Fig. 3, D and //), — whereon new changes (in this case, e.g., cell-divisions) begin. These examples, and many more might be added, show us that the protoplasmic body forms the cell ; the cell, in the sense defined above, is evidently only a further form of development of it ; the formative forces proceed from it. It has hence become usual even to consider a protoplasmic body of this kind as a cell, and to designate it as a naked membraneless cell or Primordial Cell ; its relationship to a cell provided with membrane and cell- sap is somewhat like that of a larva to the per- fect insect, which is developed from the larva into the more perfectly matured form. The development of a swarm-spore, Uke that of a Fucus-ovum, shows, — as may also be proved in the case of every other cell, — that the substance out of which the cell-wall is formed was already contained in the protoplasm in some form or other which could not be recognised; and so the formation of the cel]-v\^all must be regarded as a separation of matter hitherto contained in the protoplasm. ' In the same manner the water of the cell-sap, although taken up from without, must nevertheless pass in through the protoplasm ; and, while it gathers inside as cell-sap, it takes up from it soluble substances ; and so far the formation of the cell-sap also appears as a separation of matter hitherto contained in the protoplasm. We shall see, further on, that the substance of the nucleus also, where it is present, was originally distributed in the protoplasm, and that the nucleus is formed by the collection of certain particles of protoplasm at the centre of the growing cell. Thus the cell provided (by development) with membrane, nucleus, and cell-sap appears as the result of a differentiation of particles of matter hitherto contained in the protoplasm. The essential point is this,— that this difi"erentiation always leads to the formation of concentrically dis- posed layers, the outermost of which, the cell-wall, is firm and elastic, the middle one, the protoplasm-sac, soft and inelastic. If the cell, as is usually the case, is at first without any sap-cavity, the protoplasm is the less firm and more watery in the middle, or a nucleus in this case is formed, which, at least in young cells, is always more watery than the surrounding protoplasm. When at last the cell-sap makes its appearance, the inner cavity of the cell is always filled with actual fluid, m which the nucleus often takes up a central position surrounded by protoplasm, or, more usually, it approaches, together with the protoplasm, the circumference of the sap-cavity, and becomes parietal. So long as that condition of cell-de- velopment in which the cell appears as a sap-cavity bounded by a membrane— certainly the one most commonly seen — had alone been observed, it was correct enough to define the cell as a vesicle ; it is obvious, however, that this view does not apply to many true cells, e. g. to young tissue- cells (as Fig. i, A), of the true nature of which we should get but a very ill-defined conception were we to regard them as vesicles. The term applies still less to the structure of swarm-spores and of the ova of Fuci. Sect. 2. Difference in the Forms of Cells.-In the conformations de- scribed in the previous paragraphs, the developnjent of the cells seldom remains stationary. Still further changes of form usually take place in the separate parts of the cell. The collective volume of the whole cell generally increases for a 6 MORPHOLOGY OF THE CELL. considerable time with corresponding increase of the cell- sap ; not unfrequently it mounts up to a hundred or even a thousandfold the volume of the cell at the time of its formation. During this increase, the contour — the collective form — of the whole cell, commonly undergoes a change ; if it was at first roundish or polyhedral, it may afterwards become elongated, filiform, bag-like, prism- shaped in length or tabular in breadth, many-armed, or branched. The cell-wall may increase very considerably in thickness, and this thickening is usually not uniform ; single spots remain thin, in others the thickened membrane becomes prominent without or within ; strap -shaped prominences, spines, knobs, &c. appear. In the substance of the cell-wall itself, differences also manifest themselves, which result in imparting to it greater firmness, elasticity; or hardness, or, on the other hand, greater softness or pliancy. The protoplasm may, in these processes, decrease more and more in quantity, until at last it forms an extremely thin membrane, which lies so close to the cell-wall that it does not become visible till contraction takes place ; after the completion of the growth of the cell-wall it may even entirefy disappear. But in many other cases the protoplasm increases with the increase in volume of the cell ; it forms a thick- walled sac, the substance of which is endowed with constant motion, while filiform or strap-shaped strings of protoplasm often pass through the sap-cavity of the cell In those cells which appear externally green, certain portions of the protoplasm become separated, and assume a green colouring ; these particles of chlorophyll may appear in the form of bands, stars, or irregular masses ; but they usually form numerous roundish granules, and the particles of chlorophyll always appear as parts of the collective protoplasmic substance of a cell. Sometimes, mixed with the green colouring matter which tinges these por- tions of the protoplasm, are pigments of other colours, red, blue, or yellow (as in Floridese, Oscillatorieae, and Diatomaceae) ; or the particles of chlorophyll assume, through changes in their colouring matter, other tints, mostly yellow or red. Colouring matters may also appear as dissolved in the cell-sap. The other chemical compounds which are formed in extremely large numbers in the cell, are mostly dissolved in the cell-sap ; but many of them assume definite forms ; thus arise granules of fat, drops of oil, and frequently true crystals or crystalline bodies. One of the commonest granular compounds present in almost all plants, with the exception of Fungi and some Algae and Lichens, is Starch, the grains of which often accumulate in the cell in numbers greatly exceeding all other substances. The most perfectly developed form of cells is found in certain families of Algse, the Conjugatae, Siphoneae, and Diatomaceae. Since in these cases one and the same cell unites in itself the organs for all vegetative functions, and at the same time a many-sidedness in the phenomena of life presents itself, the whole cell attains a high degree of differentiation ; the separate parts, — the cell-wall, the protoplasmic body, and its contents, — show a variety of structure which does not occur elsewhere concurrently in the different parts of one and the same cell. Hence it happens that the same cell has in these cases often to go through the most diverse metamorphoses, so that besides its manifold development as to size, it also undergoes a series of temporary changes of form. Hence these forms of Algae become of great importance for an accurate comprehension of the nature of the cell. (Book 4 1. Algae.) But above all, these cells are distinguished by this, — that, after they have attained the highest grade of development, they are in a condition to divide and to multiply, and at length, sooner or later, give up their FORMATION OF CELLS. ' 7 cell-wall, contract their protoplasmic body, together with all its serviceable contents (starch, oil, chlorophyll, &c.), expel the water of the cell-sap, and form a new cell. We may pass over the innumerable intermediate forms, and turn cur attention at once to the other extreme, namely those plants of which each usually consists of thousands or even millions of cells, as is the case with Vascular Cryptogams and Phanerogams • and in which at the same time the different parts of the plant undergo an entirely different morphological development, and are adapted to different functions for the support of the v.hole. Here then we find that certain cells never attain their full development, they remain . constantly in the condition of youth which is represented in Fig. I, A ; these however assist the whole by continually giving rise to new cells by division, which then, on their part, undergo a further development. Such cells, which serve exclusively for the purpose of producing new ones, are found at the extremities of all roots and branches, and abundantly at the base of leaves. The cells produced in these positions undergo a different development according to their situa- tion, and usually in such a way that whole aggregations of them into layers or strings follow the same course of development. Some grow quickly in all directions, their wall remains thin, the great bulk of their protoplasm becomes transformed into chlorophyll, they are rich in cell-sap, and serve, as we shall see hereafter, for as- similation, /. e. the production of new organic substance, which is formed out of the elements of the absorbed nutritive material ; in other parts of the same plants the cells extend greatly in length, their diameter remains small, they form no chlorophyll ; a certain number remain succulent and serve to conduct certain assimilated sub- stances ; other cells of the Same string thicken their walls rapidly in many ways, their septa become absorbed, numerous cells in the same row join into a long tube (vessel), from which the protoplasm and the cell-sap disappear ; they serve then as air-passages for the interior of the plant. In their neighbourhood are formed the wood-cells ; they are mostly fibre-like, extended in length, their wall greatly thickened, and its substance chemically changed (lignified) ; they form collectively a firm frame- work which supports the remaining tissues, lends firmness and elasticiLy to the whole, and is especially adapted for the rapid conduction of water through the tissues of the plant. In the tissue of tubers, bulbs, and seeds, most of the cells remain thin- walled ; they become filled in the interior with albuminous substances, starch, fat, inuline, &c., which afterwards, when new organs are being formed, serve as material for the construction of new cells. In the same manner a considerable series of other forms of tissue could be adduced, cork, the testa of seeds, the stone of stone-fruit, &c., which all alike attain the needful firmness and strength by a peculiar development of their cell-walls, in order to serve as protective envelopes for the other masses of cells which are still capable of further development ; their contents disappear as soon as the cell-wall has assumed these properties, and their purpose has been fulfilled. Each of the forms of cell hitherto spoken of, occurring in one and the same plant, thus serves especially or exclusively for one purpose only ; in correlation with this, either the cell-wall, the protoplasmic body, the chlorophyll, the cell-sap, or its granular deposits, is specially developed. Very commonly these specialised cells lose the power of reproduction and of multiplying by division ; when they have fulfilled their function, they disappear, or their woody frame-work, the cell-wall, alone remains. The whole plant, of which these cells form a part, continues to remain as such ; at definite places it possesses cells, which, at the proper time, again produce new masses of cells, and these again are adapted to fulfil for the time all these functions. Sect. 3. Formation of Cells ^ — The formation of a new cell always comnjences with the re-arrangement of a protoplasmic body around a new centre ; * H. von Mohl, Vermischte Schriften botanischen Inhalts, Tubingen 1845, pp. 6t., 84, 362.— Schleiden in Miillei's Archiv, 1838, p. 137.— Unger, Botan. Zeitung, 1844, p. 489 ; H. v. Mohl, 8 MORPHOLOGY OF THE CELL. the materi^J required is always afforded by protoplasm already present; the newly constituted protoplasmic body clothes itself, sooner or later, with a new cell-wall. This is the only process common to all reconstruction of cells. A description which goes more into detail requires a distinction to be at once drawn between different cases, or we shall be led into erroneous generalizations, since there is great variety in the mode in which new cells are formed. It appears to me convenient and natural to distinguish three principal types : — ( I ) The Renewal or Rejuvenescence of a Cell ; i. e. the- formation of one new cell from the whole of the protoplasm of a cell already in existence; (2) The Conju- gation or Coalescence of two (or more) protoplasmic bodies in the formation of a Cell ; (3) The Multiplication of a Cell by the formation of two or more protoplasmic bodies out of one. Each of these types shows a series of variations and transidons into the others. Great diversity arises, especially in the multiplicadon of cells. Two cases are here to be distinguished first of all, according as a part only of the protoplasm of the mother-cell is applied to the formation of the new cells (Free Cell-formation), or as the whole mass passes over into the daughter-cells (Division). The last, by far the most common case, again exhibits variations, according as the masses of protoplasm, w-hich become separated and then collect around new centres, expel water and contract, and become globular, or not, and according as the cell-wall is secreted during the division or only after the complete formadon of the new cell, and even after the appearance of cell-sap and nuclei. In the course of the vegetation of a plant, different forms of cell-formation are brought into play. On Cell-division depends the formation of the vegetative parts of the plant, the production of the Cell-tissue ; Free Cell-formation occurs in the production of the ascospores of Fungi and Lichens, and in the embryo-sac of Phanerogams ; Cell-formation by Conjugation is limited, in its typical form, to single groups of Algae and Fungi for the purpose of reproducdon ; the Renewal or Rejuvenescence of Cells is found in the formation of a single swarm-spore out of the whole contents of a vegetadve cell in many Algae ; and analogous phenomena occur in the sexual reproduction of Cryptogams. In what follows I purpose to give a summary of the different kinds of cell-forma- tion according to the principles already indicated. The brevity required in an introductory treatise will be my excuse if I omit the details necessary for a more accurate knowledge. A. Cell-formation by Renewal or Rejuvenescence of a Cell. — A good ex- ample is afforded in the formation of the swarm-spores of Stigeoclonium insigne (Fig. 3, Botan. Zeitung, 1844, p. 273. — Nageli, Zeitschrift fiir wiss. Botanik, I. 1844, p. 34, III, IV, 1846, p. 50. — A. Braun, Verjiingung in der Natur, Freiburg 1850, p. 129 et seq. — Hofmeister, Vergleichende Unsersuchungen iiber die Embryobildung der Kryptog. u. Conif., Leipzig 1851.— De Bary, Untersuch- ungen uber die Familie der Conjugaten, Leipzig 1858. — Nageli, Pflanzenphys, Untersuchungen, Heft I.— Pringsheim, Jahrb. fiir wiss. Botanik, I. 1858, pp. i, 284, 11. p, i.— Hofmeister, Lehre von der Pflanzenzelle, Leipzig 1867. [Schleiden's Contributions to Phylogenesis are in Taylor's gcient. Mem., vol. II. pp. 281-312, and Sydenham Society, 1847; Braun's Rejuvenescence was published by the Ray Society in Bot. and Phys. Mem. 1853; and Nageli on Vegetable Cells by the same Society in their Reports and Papers on Botany, 1845 and 1849.] FORMATION OF CELLS. page 4) ; the whole contents of a vegetative cell of a filament contracts, expels a portion of the water of the cell-sap ; the arrangement of the differentiated proto- plasmic body is changed, the bands of chlorophyll disappear ; its form alters as it escapes from its cell-wall ; from almost cylindrical, the protoplasmic body becomes ovoid, and shows a broad green and a narrower hyaline end ; after the swarming is completed, the latter becomes the base, the green end alone grows at the apex as soon as the new cell clothes itself with a cell-wall. The observa- tions of Pringsheim on Oedogonium also show that the direction of growth of the renewed cell is at right angles to the original direction of growth before the renewal ; for the hyaline, or rooting-end of the swarm- spore, which afterwards attaches itself, is formed on the side (Fig. 4, ^, E), not at the upper or lower end of the protoplasmic body. An essentially differ- ent arrangement in space of the entire protoplasmic body of the cell also takes place ; the transverse be- comes the longer diameter of the cell and of the plant arising from it. The material remains, as far as can be seen, the same, but its arrangement is different ; this is morphologically determinate, and every new formation of cells depends essentially on a fresh arrangement of protoplasm already in existence ; hence the rejuvenescence of a cell not only may but must be regarded morphologically as the formation of a new one. B. Cell-formation by Conjugation. — The proto- plasmic bodies of two or more cells coalesce to form one common protoplasmic body which surrounds itself with a cell-wall, and becomes endowed with the other properties of a cell. For the elucidation of this process, which presents many variations, we may observe the conjugation of one of our commonest filamentous Algae, Spirogjra longata (Figs. 5, 6). Each filament (Fig. 5) consists of a row of similar cylindrical cells, each of which contains a protoplasm - sac ; this encloses a re- latively large quantity of cell - sap, in the midst of which hangs a nucleus, enveloped in a small mass of protoplasm, and attached to the sac by threads of the same substance ; in the sac lies a band of chlorophyll, which is spirally coiled, and at definite places contains grains of starch. In this case the conjugation always takes place between the adjacent cells of two more or less parallel filaments. A preparation is made for it by the formation of lateral protuberances, as represented in Fig. 5, a ; these continue to grow until they meet {b). The protoplasm-sac of each cell concerned then contracts ; it detaches itself sharply from the surrounding cell- wall ; rounds itself into an . ellipsoidal form, and contracts still more by expulsion of the water of the cell-sap. This occurs simultaneously in the two conjugating cells. ' Next the cell-wall opens between the two protuberances (Fig. 6, a), and one of the two ellipsoidal protoplasmic bodies forces itself into the connecting channel thus formed ; it glides slowly through it into the other cell-cavity, and as soon as it touches the protoplasmic body con- tained in it, they coalesce (Fig. 6, a). After complete union (Fig. 6, b) the united body is again ellipsoidal, and scarQely larger than one of the two which compose it ; during the union a contraction has evidently taken place with expulsion of Fig. 4. — A, B escape of the swarm-spores of an Oedog^onium ; C one free in motion ; D the same after it has become fixed and has formed the attaching disc ; E escape of the whole protoplasm of a germ -plant of Oedogonium in the form of a swarm-spore (X350). (After Pringsheim, Jahrb. fijr wiss. Bot. I. pi. I.) lO MORPHOLOGY OF THE CELL. water. The coalescence gives the impression of a union of two drops of fluid ; but the protoplasm is never a fluid ; and, independently of other circumstances, there is a fact that shows that altogether peculiar forces are here active which are absent from all fluids; — the spiral band of chlorophyll of each of the two conjugating protoplasmic bodies is preserved in the contraction ; it only becomes closely drawn together ; during ^ 1., Fig. S- — Spiro^yra loiigata. Cells oi two filaments preparing for conjugation, they show the spirally coiled bands of chlorophyll in which, at different places, lie wreath-like arrangements of starch- grains ; small drops of oil are also distributed through them (cf. sect. 6) ; this is the behaviour of the chlorophyll after the action of strong sun- light ; the nuclei are also to be seen in the cells, each surrounded by protoplasm, threads of which touch the cell-wall in different places ; a and b are the protuberances preparing for conjugation (X550). Fig. 6. — Spirogyra lo7igata. A Cells in the act of conjugation ; at a the protoplasmic body of one cell is passing over into the other; at b this has already taken place ; the band of chlorophyll together with starch-grains is still partially recognisable. B the young zj'gospores surrounded by membrane ; the protoplasmic body contains numerous drops of oil {XS50). the coalescence the ends of the two bands of chlorophyll place themselves together in such a manner as to form one band. The conjugated protoplasmic body clothes itself with a cell-wall, and forms the body called a Zygospore, which germinates after a period of repose of some months, and developes a new filament of cells. With greater or smaller deviations from this plan, conjugation takes place in a group of Algge comprising a large number of species, the Conjugatae, among which the Diatoms must be included, and in some Fungi. In the latter more considerable deviations occur {e.g. Syzygites, Mucor stolon'ifer). In Spirogyra nitida it also happens (according to De Bary, Conjugaten, p. 6) that one cell conjugates with two others, and takes up their masses of protoplasm ; in these cases a Zygospore is the product of the contents of three cells. In the IVIyxomycetes the swarm- spores (Myxo-amoebae), which are endowed with a peculiar motion, coalesce gradually in great numbers, and finally form large, motile, membraneless protoplasmic bodies, the Plasmodia, which only at a subsequent period are transformed into numerous cells. In the cases hitherto considered, the uniting protoplasmic bodies are of equal size ; the process of fertilisation in many Cryptogams differs only in the fact that the two protoplasmic bodies which coalesce are of unequal size, and otherwise of different properties. In Book II we shall treat in detail of the reproduction of Cryptogams; here we need only state that the male, motile fertilising bodies (Sper- matozoids) of Cryptogams are naked protoplasmic bodies, which are considered to be primordial cells ; in the female organ of these plants is a cell which opens outwardly, and contains a protoplasmic body which is fertilised by the spermatozoids. In cases which have been accurately observed (Oedogonium, Vaucheria), these coalesce FORMATION OF CELLS. II with the former; and from this the reconstruction of a cell results. Here, as with the conjugation of the Gonjugatae and some Fungi, the cell which results in this manner from the coalescence is always a reproductive cell ; with it begins the formation of a new individual plant. In fertilisation one of the two bodies is evidently- very different from the other ; it may therefore be assumed that in conjugation also a difference exists, although at present undiscovered, between the coalescing cells. C. Free Cell-formation. — In the pro- toplasmic body of a cell new centres of formation arise, around each of which a portion of the protoplasm gathers, and forms a cell. Another portion of the proto- plasm rema'ms o'ver, and represents the still- persisting protoplasmic body of the mother- cell, which survives for a longer or shorter time. The new centres of formation may or may not be indicated by the previous appearance of nuclei. Generally, the num- ber of daughter-cells which arise in this manner is considerable ; as an instance may be mentioned the formation of spores in a small Ascomycete, a Peziza^ (Fig- 7)- The sac-like mother-cells of the spores {a) are at first densely filled with pro- toplasm, and contain only one small nucleus. This disappears ; /. e. its sub- stance becomes distributed through that of the protoplasm ; this latter becomes frothy, and roundish drops of sap make their appearance in it {b, c'). Preparation is made for the formation of spores by the condensation of the protoplasm in the upper part of the sac, while in the lower part it remains frothy {e, /). The forma- tion of spores does not in this case precede the appearance of the nuclei ; and the spores always remain devoid of a nucleus ; and this is the more instructive as in other Pezizae {e. g. P. confluens^ according to De Bary) nuclei are formed in the first place, around each of which a lump of protoplasm collects-, which then forms the spore. In this case eight spores are always formed in each sac within the upper dense proto- plasm ; /. e. around each of eight points a portion of the protoplasm collects in an ellipsoidal mass (^) ; each such collection consists at first of coarse-grained protoplasm surrounded by a clear space ; a portion of Fig. T.—Peziza convexula. A vertical section of the whole plant { X about 20) ; h hymenium, i. e. the layer in which the spore- forming sacs lie ; 5 the tissue of the Fungus enveloping the hymenium at its edge 4^ in a cup-like manner; at the base of the tissue 5 fine threads arise, which grow between the particles of earth. B a smaller portion of the hymenium (x 550) ; sh sub- hymenial layer of densely interwoven cell-filaments (hyphae); a—f spore-forming sacs ; among them thinner sacs, the. para- physes, in which lie red granules. ^ It appears in considerable quantity on the ground among Phascum along forest-paths in the neighbourhood of Bonn in the month of March. The cup is from |-i inch broad, brick-red, sessile, with slightly projecting rim. According to Rabenhorst, Deutschlands Kryptogamenflora, 1844, p. 368, it may be P. convexula. ^ In the embryo-sac of Phanerogams fresh nuclei are formed in the protoplasm, and around each of these one cell. (Cf. Book II. Conifers, Monocotyledons, Dicotyledons.) 12 MORPHOLOGY OF THE CELL. fine-grained protoplasm forms the ground, so to speak, in which the spores are imbedded. Afterwards each spore becomes more sharply defined, the clear space disappears (e), its substance becomes more fine-grained and clearer ; and in one of its foci is formed a vacuole, /. e. a transparent drop of fluid. Finally, each spore surrounds itself with a firm membrane, the vacuole disappears, and in the centre is formed a large drop of a strongly refractive oil, as well as numerous smaller drops of oil. D. Formation of Cells by Division of the Mother- Cell. — In the protoplasm of a cell new centres of formation arise ; around each of these a portion of the proto- plasm of the mother-cell gathers, in order to form a new cell ; in this manner the entire protopJasni of the mother- cell is completely used up ; its cell-wall alone remains, if it pos- sess one, which is not always the case. If the mother-cell has a nucleus, this is usually dissolved in the protoplasm ^ ; and as many new nuclei are produced- as daughter- cells originate ; or the nucleus of the mother-cell di'vides into t^vo nuclei, while the whole protoplasm sepa- rates into tavo portions (see Han- stein). ist Case. Cell - DiiHsion ^vith Contraction and Rounding-off of the Daughter-cells. a. A Cell-^vall is not secreted till daughter-cells, already isolated, haue becot?ie completely separate. An ex- ample is afforded by the formation of the oospores of Achlya (Fig. 8). At the end of a sac-shaped cell or of a branchlet of one, the protoplasm collects, the larger end itself swells up into a globular form [A, E), and, by the formation of a sep- tum (C), becomes an independent cell (the oogonium). Nucleus-like structures sometimes, but not usu- ally, form in the protoplasm (as in C). The whole protoplasmic body then breaks up into two, three, four, or more parts, which very quickly round themselves off into a perfectly spherical form; (in a large number of observations I have never seen an intermediate form between C and Z).) The parts thus formed {e, e in Z)) contract violently during their separation ; i. e., their protoplasm becomes denser by expulsion of Fig. 8. — Oogonia and antheridia of Achlya lignicola, growing on wood in water ; the course of development is indicated by the letters A — F. a the antheridium, b its sac penetrating into the oogonium (X550). ^ An exception occurs in the formation of spores of Anthoceros, where the nucleus of the mother- cell is not absorbed until four new nuclei are formed. ^ In Spirogyra, Mougeotia, and Craterospermum, the new nuclei only arise during the progress of the division of the protoplasm (De Bary, Die Familie der Conjugaten, Leipzig 1858). In the formation of the stomata of Hyacitithiis orie?italis, I was unable either before, during, or immediately after the division of the mother-cell, to perceive a nucleus; it did not appear in any of the deriva- tive cells until a considerable time after the division. FORMATION OF CELLS. ^3 water; and only after they have become fertihsed by the antheridium-tubes («, l? in D) do they surround themselves with a cell-wall. This form of cell-dfvision evidently bears, throughout its whole course, a close re- semblance to Free Cell-formation ; it is distinguished only by the circumstance, that here the whole protoplasm collects round several centres. If the whole protoplasmic body, in its contraction, were to form only one ball, which also happens, the case would be analogous to that of Renewal or Rejuvenescence. If the balls, during their separation, were to surround themselves with copiously secreted cellulose, the process would bear a strong similarity to the formation of pollen in many Dicotyledons [see below]. There occurs also in this same plant (Fig. 9) a varia- tion of this process of division, when it forms its swarm- spores ; here the protoplasm breaks up in the club-shaped swollen end of a sac into a large number of small portions (yl), which become completely rounded off only (egetationis of roots and stems, one sees at a glance hundreds of cells which are in process of division at the same time ; and yet it is seldom possible to see the condition in question. This however shows at the same time that the partition-wall always arises in these cases simultaneously over the whole surface ; if it grew from without inwards, this would actually be seen, since Fig. 13. — Formation of tlie anthericlium n{ Nitella flexilis (cf. Book II). ^ The firm connexion of the two daughter-cells before the formation of the partition-wall occurs also in a different manner, e.g. in Oedogonium (Hofmeister, I.e. pp. 84 and 162). The preliminary indication of the partition-wall by the appearance of a disc of granules in the boundary plane is not universal, as is shown in the formation of the pollen of Funkia and of the spores of Funaria. (Hofmeister, /. c. Fig. 20.) FORMATION OF CELLS. 17 FIG. 14.— Embryos in the embryo-sac oi Allium Cepa; the cells contain very large nuclei, each with two nucleoli. At / the spherical apical cell contains two nuclei {a) ; at // it has already divided (a has split up into a' and a"), and same manner the cell c (in /) has split up into c and c'. ithe all the steps of the development in this case come easily into view ; here and there half- formed partition-walls would be found. So is it also with the first cell-divisions of the embryos in the embryo-sac ; here the circumstances are peculiarly favourable ; but here also the next stage which comes into view after the for- //^.^^"^^^^^ ^n mation of two nuclei "(Fig. 14, 7) is usually the presence of a complete thin partition-wall (//}. I was also successful in crushing an embryo of Al- lium Cepa {III) in iodine-solu- tion in such a manner that it was evident that the younger derivative cells were not yet separated by a partition-wall, although sharply defined. b. While the diiusion of the protoplasmic body is taking place from Qvithout inwards, cell- membrane is formed- a ridge of cellulose intrudes into the dividing fold which arises in the protoplasmic body ^. A clear and well-studied example is afforded in the stouter forms of the genus Spi- rogyra. In order to observe the divisions here, it is neces- sary to place strongly vege- tating filaments after midnight in very dilute alcohol, that they may be examined later, the divisions taking place only by night. Fig. 15 shows a living cell of a filament of 5. longata by day \ B to E the conditions of division at night ; the protoplasm - sacs of the cells are contracted by the life-destroying reagent. B and C (Fig. 15) show, at q and q the folding-in of the protoplasm - sac, and the an- nular ridge of cellulose which is growing into it. While the folding - in advances further and further, the lamella of cel- lulose does the same; finally the channel closes, the annular lamella Fig. xz.—Spn-o,i:yra loiitrata {X550). ^ a cell in the living state; B, C cells laid in dilute alcohol during the division by night ; D, E central portion of cells in the act of division. becomes a disc, and now lies between two new completely closed sacs of protoplasm as a partition -wall. Sometimes the the ^ This case was the first of all processes of cell-foraiation that was accurately examined; H. von Mohl first described it in 1835 in Conferva glomerata. (Mohl : Vermischte Schrifteu hot. Tnhalts. Tubingen 1845.) l8 MORPHOLOGY OF THE CELL. folding-ill of the protoplasmic body makes great progress, pushing on even to the separation into two sacs, before the partition - wall of cellulose begins to form {q and q" in D and E) ; an abnormal condition which shows plainly that it is not the band of cellulose which infolds the sac, but that this latter becomes constricted by a process of growth of its own, which takes place independently of the formation of the partition-wall. The behaviour of the nucleus, and generally the arrangement of the portions of protoplasm during the division, here shows considerable deviation from other similar processes ; this one thing however must be clearly borne in mind, that the formation of two nuclei, and their position in the middle of the newly-formed cells, does not here precede the division, but proceeds along with it. Not till the begin- ning of the infolding which takes place in the circumference of the central nucleus, are two nuclei to be observed in the central mass of protoplasm ; these separate slowly from one another, each surrounded by protoplasm, while the folding-in proceeds, so that, by the time the division is completed, the nuclei have reached nearly the centres of their cells. In some cases modes of division occur which appear, at the first glance, to differ essentially from any hitherto described ; e. g. the production of the basidiospores of the Basidiomycetes (as Agaricus, Boletus). A closer study, however, shows that such processes follow more or less exactly one of the types described. Thus, for example, all possible connecting links are to be found between the usual mode of division and the peculiar process in the production of the spores of Agaricus and other Fungi. If the behaviour of the two daughter-cells, rather than the process of division itself, were made the prin- ciple of classification, many other cases would have to be considered. On this I will touch only very briefly. The daughter-cells resulting from the division may be of equal size or not ; in the first case they may be so similar to the mother-cell that they have only to grow in a direction at right angles to that of the division in order to be- come exactly like it (as in Spirogyra) ; but the daughter- cells, even when like one another, may nevertheless be from the first different from the mother-cell ; and this may happen in very different ways, and the difference may constantly increase. But, in other cases where the daughter-cells are from the commencement unlike, this difference usually increases later, especially in the formation of the spores of Fungi on the so-called basidia. A small portion of the end of a long cell becomes divided off; the septum splits into two lamellae, and the separated piece (the basidiospore) falls off; the part which remains attached to the plant shows scarcely any change, and can again and again repeat the same process. The portion of the mother-cell which remains behind, called the basidium, has evidently become a daughter-cell as truly as the detached spore ; but, while the spore is very unlike the mother-cell, the other daughter-cell, the basidium, remains very like it. Hence has arisen the pardonable, but very incorrect expression, that the basidium forms several spores in succession ; whereas properly the formation of each spore is a bipartition, the basidium being always as much a daughter-cell as is the spore (cf. Book II. Fungi). In the same manner the apical cell at the end of a growing stem is the sister-cell to the segment last formed ; but since the former is always renewing itself, it is more convenient to express oneself as if the apical cell always remained the same, and to treat the segments as its products. Behaviour of the Nucleus during the Di'vision. Where the cell-division is combined with contraction and rounding off of the newly-formed portions of protoplasm, as in the formation of spores and pollen-grains, it is the rule that the new nuclei become A?isible in the centres of the future daughter-cells, whether, as is usually the case, the nucleus of the mother-cell have previously disappeared, or whether it remain during the process, as in the formation of the spores of Anthoceros (pp. 14-16). From these cases, a clear observation of which was easy, the opinion has hitherto prevailed that in the bipartition of the tissue-cells of growing parts the nucleus of the mother-cell also be- comes absorbed in the protoplasm, and that in this latter two new nuclei arise in the centres of the forming derivative-cells. But the bipartition of the cells of Spirogyra THE CELL- WALL. 19 (p. 17) does not justify this conclusion, in so far as it is only during the folding-in of the protoplasm - sac that the two new nuclei slowly separate from one another; whe- ther they are formed afresh after the absorption of the mother-nucleus, or arise from its division, is still uncertain. According to the more recent researches of Hanstein', the bipartition of the parenchyma-cells of the pith of Dicotyledons {e. g. Sambucus, Helianthus, Lysimachia, Polygonum, Silene) really precedes the division of the mother- nucleus ; a mass of protoplasm, enclosing the latter, places itself in the centre of the mother-cell. Even before the cell-division, two nucleoli at least are to be detected in the nucleus, and soon afterwards a fine line divides the nucleus into two halves ; ' directly afterwards, or at the same time, the whole layer of plasma which surrounds it shows a free intersecting division surface, in which the new wall of cellulose then gradually arises.' The nuclei of the two sister-cells thus lie, immediately after their production, on the new division-wall : but they usually soon leave this situation ; very commonly they move in opposite directions along the wall, until they arrive at spots over against those at which they arose, and there they come to (temporary) rest on the older septa. Since these parenchyma-cells usually divide in regular succession, two newly-formed nuclei of different origin thus lie opposite to one another on each side of all the older septa. Whether these processes also take place in the primary parenchyma of the same plants, and whether possibly they occur in all plants the cells of which are united into tissues, Hanstein has not yet definitely stated. Sfxt. 4. The Cell-Wall ^ — The substance of the cell-wall is secreted from the protoplasm. In what form it is contained in the protoplasm immediately be- fore the secretion is not yet certainly known ; it always appears as a solution, and then becomes first organised on the surface into a thin membrane. The substance capable of forming cell-wall always consists of a combination of water, cellulose, and incombustible materials (ash-constituents), but may afterwards undergo further chemical changes. By the continual secretion of substance which forms cell-wall out of the proto- plasm, and the deposition of this between the molecules of the membrane already formed, this latter grows in such a manner that on one hand its surface, and on the other hand its thickness, increases. The mode of both processes of growth is dependent on the specific nature of the cell, and on the function which it has to fulfil in the life of the plant ; it therefore varies almost infinitely. Generally the growth in surface first preponderates, afterwards that in thickness. Neither the one nor the other is uniform over all points of a cell-wall ; hence each cell, during its growth, also changes its form ; moreover the growth of a cell-wall continues only so long as it is in immediate contact on its inner side with the protoplasm. The want of uniformity of the growth in surface at different points causes cells which are at first, for example, spherical, ovoid, or polyhedral, to become subse- quently cylindrical, conical, bag-shaped, tabular, bounded by waved surfaces, &c. The want of uniformity of the growth in thickness usually brings about sculpture of the surface, which is very characteristic. The thickened parts may project either ' Sitzungsbeiichte der niederrheinischen Gesellschaft in Bonn, Dec. 19, 1870, p. 230. 2 H. von Mohl. Vermischte Schriften hot. Inhalts. Tiibingen 1845 (numerous treatises).— Schacht, Lehrbuch der Anat. und Phys. der Gewachse, 1856.— Niigeli, Sitzungsbeiichte der Miinch. Akademie, 1864, May and July.— Hofmeister, Die Lehre von der Pflanzenzelle, Leipzig 1867. Also numerous treatises in the Botanische Zeitung. C 3 20 MORPHOLOGY OF THE CELL. outwardly or inwardly. The former occurs commonly in the free-lying surface of cell-wall, the latter in the partition-walls of adjoining cells. The thickenings which project outwardly may appear in the form of knots, humps, spines, or ridges ; but those which project on the inside are much more various. In this case peg-shaped protuberances occur but seldom ; much more common are annular ridges or spirally-curved bands ; these latter may be united in a reticulate manner, so that thin polygonal interstices remain ; or the thicknesses may spread, and the thin parts then appear in the thick wall as fissures or roundish pits. If the wall is very thick, the latter become channels, which pass entirely or partially through the wall. Not unfrequently the thin portion of the wall, which at first closes such a channel on the outside, becomes absorbed, and the cell- wall is then perforated. But as, when contiguous cells are united into a tissue, the partition-wall usually becomes thickened in the same manner on both sides, the pits and pit-channels of both sides meet, and the intermediate thin portion of membrane becomes absorbed ; a channel thus arises uniting two cell-spaces (Bordered Pits, perforated septum of vessels). During the increase of the surface and of the thickness of the wall -by depo- sition of new substance in both a tangential and radial direction between the molecules already formed, a finer internal structure usually becomes visible, which is termed Stratification and Striation. Both are the result of a different regularly alternating distribution of water and solid substance in the cell- wall ; at every visible point water is combined with cellulose, but in different proportions ; por- tions less and more watery, denser and less dense, alternate. Thus, in every cell- wall sufficiently thick, a system of concentric layers becomes visible, of which the outermost and innermost are always denser, while between them alternate more and less watery layers. The stratification is visible on the transverse and longi- tudinal sections of the cell-wall, the striation also on the surface being usually most evident there, but is in general less easily seen than the stratification ; it consists in the presence of alternately more and less dense layers of cell- wall, cutting its surface at an angle. Mostly two such systems of lamellae may be recognised mutually intersecting one another. There are thus altogether three kinds of stra- tification present in a cell-wall, one concentric with and two vertical or oblique to the surface, cutting one another or mutually intersecting, like the cleavage-plain of a crystal cleaving in three directions (Nageli); and as this cleavage takes place in different directions, at one time the stratification, at another the striation is more evident. Independently of this internal structure, chemical changes arise in the cell- wall which never affect the whole mass uniformly, but usually divide the thickened cell-wall into concentric layers which differ from one another chemically and phy- sically. These chemical differentiations, which are always combined with an alter- ation of physical properties, show a great variety, but can conveniently be reduced to three categories; — Conversion into Cuticle or Cork (Verkorkung), Lignification (Verholzung), and Conversion into Mucilage (Verschleimung). The first consists in the change of the outer layers of the cell-wall into a plastic very elastic substance which water cannot at all, or scarcely penetrate or cause to swell (as the outer cell -wall -layer of the epidermis and of pollen-grains and spores and cork). Lignification occasions an increase in the hardness of the cell-wall, a decrease of its THE CELL-WALL. ZI plasticity, and easy permeability to water without any considerable swelling. The conversion into mucilage at length causes the cell-wall to become capable of ab- sorbing great quantities of water, so as to increase its volume to a corresponding extent, and to assume a gelatinous consistence. In the dry state such cell-walls are hard, brittle, or flexible like horn (as the cell-walls of many Algse, the so-called intercellular substance of the endosperm of Ceratonia Siliqua, of linseed, and quince- mucilage). Several of these changes may occur simultaneously in a cell-wall, so that, for instance, the outer layers become woody and the inner mucilaginous {e.g. wood-cells of the root of Phaseolus). Besides these changes in the substance of the cell-wall, which are not unfre- quently correlated with peculiar colourings, changes in its chemico-physical behaviour may also be induced by the interposition between its molecules of considerable quantities of incombustible substances, especially hme and silica. If the deposition of these substances take place in sufficient quantities, they remain behind, after destruction of the organic groundwork of the cell-wall, in the form of what is termed an ash-skeleton. (a) The Surface-gro^vth causes not only an increase of the size of the cell, but also changes of form, in proportion as it is wanting in uniformity at different parts of the circumference ; hence cells of originally dissimilar form may become similar by un- equal growth ; but it is much more common for cells origi- nally alike in form to become entirely unlike. This is most usually the case with the Hiulti- cellular organs of the higher plants, leaves, stems, and roots; cells in their infancy can here often scarcely be distinguished from one another ; whereas in the completely developed or- gan the most various forms are contiguous (Fig. 1 6). It is only rarely, as in the growth of some spores and pollen-grains, that the surface-growth is so uniform that the original form is nearly retained even after considerable increase in volume {e. g. pollen of Cucurbita and Althaea). But even in these cases the uniformity is only temporary, for the pollen- grains subsequently emit their pollen-tubes or the spores germinate, in both cases by the local growth of their inner layer of cell-wall. This also shows at the same time that the surface-growth of a cell-wall may be very different at different times ; and this indeed is usually the case. From the infinite variety of the surface-growth of cell-walls, it is convenient, for the sake of arrangement, to reduce the different cases to classes, and to bestow names upon them'. Thus it is usual to distinguish between inter- Fig. i6. — From the transve section of a leaf of Camellia japo)iica; P paren- chyma-cells with grains of chlorophyll and drops of oil ; /•" a very thin tlbro- vascular bundle; v v z. large, branched, thick-walled cell, which intrudes its arms between the parenchyma-cells. ^ A good classification of the processes of growth is, of course, still more important for the study of the mechanics of growth; but little has, however, yet been done in this duection, and we can only give a brief abstract. 22 MORPHOLOGV OF THE CELL. calary and terminal growth of the cell-wall. Terminal growth takes place when the surface-growth attains a maximum at any one part of the circumference (by inter- position of new particles of cell-wall), while the intensity of this process decreases on all sides of this point, and at a definite distance attains a minimum, so that this portion of the cell- wall projects as a point, or appears as the rounded end of a prominence, or of a cylindrical sac {e. g. hairs, filamentous Algae). If several points of terminal growth occur in a cell which was originally round, it may become star-shaped ; if new points of growth are formed beneath the continuously growing end of sac, the sac-like cell branches (as in many filamentous Algae, hyphae of Fungi, Vaucheria, Bryopsis). Hofmeister^ distinguishes as a peculiar form of terminal growth the case in w^hich, instead of a point, a line is rapidly raised on the cell-wall ; this may occur as the terminal line or intersecting edge of two curved surfaces. Intercalary growth of the cell- wall occurs in a typical form in the case in which the deposition of new substance within a belt lying in the surface of a cell takes place in such a manner that this belt extends, and a fresh interposed piece of the cell-wall from time to time makes its appearance. To the last-named case may be referred the common phenomenon of the occurrence of growth in the whole of the side-wall of a cubical, tabular, or cylindrical cell, as, for example, in the cells of Spirogyra, and the parenchyma- cells of growing roots and stems of Phanerogams (cf. Fig. i). Oedogonium presents a peculiar case of intercalary surface- growth (Fig. 17). Inside below the septum is formed a project- ing annular cylindrical deposit of cellulose {A, qu) ; there the cell-wall splits, as if separated by a circular cut, into two pieces ; and these now, retreating from one another, remain united by a zone of cell-wall {B, nv') formed by extension of the cylinder ^u. After the interposition of this new cylindrical zone, cell-division follows ; and, sipce this is repeated many times, the appearance is presented which is figured at A, c (the so-called formation of a cap'). (b) The Gro^vth in Thickness of a Cell-^joall is usually strictly localised, so that the thicker parts appear mostly as very abrupt projections on the thinner parts of the cell-wall, either on the outside or the inside. The collective impres- sion made by the sculpture is especially dependent on whether the extension of surface is less on the thicker or on the thinner parts. If the thickening is especially strong on certain points, the structure takes the form outwardly (Fig. iq) or inwardly (Fig. 18, C, D) of projecting warts, pegs, or spines ; if the thickening occurs most strongly in linear or strap-shaped spots of the cell-wall, projecting cylinders, ridges, bands, or combs are formed on the inner or outer side. These ridge-like projections may form reticu- lated figures on the inner or outer side (Fig. 18, 5, Fig. 20, /), or rings, or spiral bands, a development especially frequent in those thickenings of certain tissue-cells which project from within. If the internally projecting rings or spiral bands are thick and firm, and the intermediate portions of cell-wall thin and easily destructible, these thickenings may become free even within the plant, and remain lying as isolated threads of cellulose in channels of -Intercalary surfat of Oedogoniiini. * Handbuch der physiol. Botanik, 1. p. 162. 2 For further details of these somewhat complicated processes see Pringsheim, Jahrbuch fur wissen. Bot. 1; Hofmeister, Handbuch der phys. Bot. I. p. 154, and Niigeli und Schwendener, Mikroskop, II. p. 549. THE CELL-WALL. 33 the tissue (annular vessels in the fibro-vascular bundle of Equiseta, Zta Mais, &c.); but the thickenings formed like spiral bands may often be drawn out to considerable length as isolated fibres (very striking examples of these so-called untwisting spiral vessels are found in the rachis of the inflorescence of Richius communis and in the leaves of Agapanthus). If the thickening of the cell-wall takes place over more extended portions of the surface, and if only smaller portions remain thin, these latter appear as pits of very various outline, either roundish or like fissures, or, when the thickening of the cell-wall is very considerable, as channels, which perforate them. These kinds of thickening most frequently project on the inner side of the cell-wall ; the channels therefore run from the Fir,. iS.— Cell -forms of Marchaiilia foly- 7norpha with tliickeninjTS projecting inwards; A an elater (Schleuderzelle) (one-half) from the sporangium, with two spiral bands ; A' a portion more strongly magnified ; B a parenchyma-cell from the centre of the thallus, with thickenings projecting inwards in a reticulate manner ; C a thin root-hair with thickenings projecting inwards, these are arranged on a spiral constriction of the cell-wall ; at Z> a thicker root-hair, with pro- jections thicker and branched, and spiral ar- rangement still clearer. L- rh'l i "^ifi^ Fig. 19.—^ a young pollen-cell of Funkia ovata; the knob-like thickenings projecting out- wardly are still small ; in the older pollen-cell C they are larger; they are arranged in lines united into a net-work.. Fig. 20.— Ripe pollen -grain of Cichormvi Intybus ; the almost spherical substance of the cell-wall is furnished with ridge-like thickenings united into a net-work ; each of these bears thickenings which project still more, in the form of spines arranged like a comb. Fig. 18 di's.— Piece of an annular vessel from the fibro-vascular bundle of the stem of Zea Mats (X550). A h the thin cell-wall of the vessel, on which the boundary lines of the adjoining cells are clearly seen, r r the annular thickenmgs of the wall of the vessel ; y the inner substance of one of the rings laid open ; i the denser layer which extends over the inner side of the ring projecting into the cavity of the cell. cavity of the cell outwards, and are there closed by a thin membrane^; when the cell loses its protoplasm and dies, the latter is in many cases destroyed, and the pit or the channel then becomes open (as, for instance, in Sphagnum and many wood-cells). The pits, especially in elongated cells, appear to be generally arranged in spiral rows, but in other cases are peculiarly grouped (Fig. 21, A). A remarkably striking form of this grouping is the Sieve-structure which occurs in the sieve-cells of the fibro-vascular bundles of vascular plants, generally in the septa, but also in the longitudinal walls. In ^ Sometimes strongly thickened cell-walls with branched pit-channels show a very complicated structure, e. g. in the hard testa of Bertholletia. (Cf. Millardet in Ann. des Sciences Nat., fifth series, vol. vi. part 5.) 24 MORPHOLOGY OF THE CELL. Fig. 21. — A, a parenchyma-cell of the cotyledon oiPhaseoliis nuiltiflortis isolated by maceration ; i i the parts of the cell-wall where it is bounded by intercellular spaces ; t, t the cell-wall furnished with numerous simple pits, but not greatly thickened ; the thinnest parts of the pits are drawn dark. B epidermis (c) and collenchyma (c/) of the leaf-stalk of a Begonia ; the epidermis-cells are uniformly thickened on the outer wall where they adjoin the collenchyma, but are thickened like the collenchyma at the angles where three cells meet ; these thickenings have great power of swelling; chl chlorophyll grains; / parenchyma-cell (X550). FIG.22.— A cell beneath the epi- dermis of the underground stem of Pteris aquilina, isolated by boiling in a solution of potassium chlorate in nitric acid ; it is more strongly thickened on the left side ; the unthickened places here ap- pear as branched channels (xsso). J '--r'l-i-^' Figs. 23, 24.— Young sieve-cells oiCjicurbita Pepo (X5S0) ; the preparation has been taken from pieces of the stem which by having lain for a long time in absolute alcohol have allowed thel production of extremely clear sections. The sieve-plates do not at present show anything of the subsequent more complicated structure, which may be examined in Nageli, /. c. ; the opening of the sieve-pores has not yet begun; they are as shown in Fig. 24, sp, still closed, their contents not yet combined. Fig. 23. Transverse section ; c c cambium ; / parenchyma ; si the septa of the sieve-cells, developing into sieve-plates. Fig. 24. Longi- tudinal section ; g the transverse view of the sieve-like septa ; si a sieve-plate on the side-wall ; x thinner parts of the longi- tudinal wall, seen at / in longitudinal section ; in them are subsequently formed a number of small sieve-pores, at present they are still homogeneous ; ps the contracted protoplasm-sac, lifted off at sp from the septum ; z parenchyma-cells between the sieve-cells. THE CELL-WALL. 25 the simplest case the thin places (pits) are densely crowded, only separated by thicker ridges, and polygonal (Fig. 23, 24, si) ; they very often appear as sharply circumscribed groups of numerous points ; the whole surface of such a group may then be thinner than the rest of the cell-wall. But in many cases the thin part of such a pit becomes absorbed, and the protoplasmic contents of adjoining cells enters into communication through these narrow channels. (Fig. 88.) Sometimes the structure of these sieve- plates {e.g. in Cucitrbita Pepo) becomes, when old, very peculiar and complicated from further thickening and swelling of the thickened portions \ Fig. 25. — Pinus syl-oestris ; radial longitudinal sec- tion through the wood of a rapidly growing branch ; cb cambial wood-cells ; a—e older wood-cells ; t ^ t" bordered pits of the wood-cells, increasing in age ; st large pits where cells of the medullary rays lie next the wood-cells (X550). Fig. 26.—/'u/iis sylvcstris; .1 transverse section of mature wood- cells (XSoo) ; ni central layer of the common wall ; i inner layer, cloth- ing the cavity ; z intermediate layer of the cell-wall ; t a mature pit cut through the middle ; t' the same, but at a thicker part of the section, the part of the cavity of the pit lying beneath is seen in perspective ; t" a pit cut through beneath its inner opening ; B transverse section through the cambium (x8oo); c cambium; h wood-cells still young ; between them two very young wood-cells with the formation of pits beginning/ t; C—/=" diagrams. One form of the internally projecting thicknesses which is of extremely common occurrence in wood-cells and vessels, viz. the formation of Bordered Pits, deserves a fuller exposition at this place. The formation of Bordered Pits arises thus : comparatively large spaces remain thin at the commencement of the thickening of the cell-wall (Fig. 25, t] Fig. 26, B^t); and ' Compare Niigeli, Ueber die Siebrohren von Cucurbita, in the Sitzungsbeiichte der k. bayerischen Akad. der Wissenschaften. Miinchen 187 1 ; and Hanstein, Die Milchsaftgefasse. Berlin 1864. 2 The development of these was first accurately recognised by Schacht, De maculis in plantarnm vasis, &c. Bonn i86o. 26 MORPHOLOGY OF THE CELL. after increased thickening, the thickening-mass which is always projecting inwardly, acquires a larger surface, and forms an arch over the thin part of the wall (Fig. 25, a~e\ Fig. 26, C-F). The outline of the thin parts of the wall in the wood of Pinus syl'vestris appears circular on a front view ; the rim of the thickening-mass which becomes arched over it grows also in a circular manner, gradually contracting the opening ; and thus the front view of such a pit appears in the form of two concentric circles, the larger of which represents the original dimensions of the thin parLs of the cell-wall (Fig. 25, cb, t), and the inner one the gradually widening circular rim of the thickening (Fig. 25, a-e; Fig. 26, C, D). Now since this process takes place on both sides of a partition-wall of two cells, a lenticular space is enclosed by the two overarchings, which is divided in the middle into two equal parts by the original thin lamella of the cell-wall (Fig. 26, F, nv) ; each half of this pit-cavity communicates with the cell-cavity by a circular opening. If the wood-cells lose their protoplasm, and become filled with air and water, this thin mem- FIG. ■2'!.— Dahlia variabilis ;\\2i\\ of a vessel with bordered pits from the succulent root-tuber ; A front view of a piece of the wall of a vessel from without ; B transverse section of the same (horizontal, at right angles to the paper) ; C longitudinal section (vertical, at right angles to the plane of the paper) ; q septum ; a the original thin thickening-ridges; b the expanded part of the thickening-masses, formed later and over-arching the pit ; c the fissure through which the cavity of the pit connnunicates with the cell-cavity. At a and ^ the corresponding front view is appended in order to make the transverse and longitudinal sections more clear (x 800). FIG. 28. — Dahliavariabilis, from the root-tuber ; P parenchymatously deve- loped wood-cells ; Fa piece of the wall of a vessel, where it adjoins a paren- chymatous wood-cell ; a b the thicken- ing-masses of the wall of the vessel cut through at right angles ; r, t fissure of the pit ; d simple pits in the parenchy- matous wood-cells (X800). brane is destroyed (as in Fig. 26, E)\ the pit-space forms a single cavity, which is en- closed between the over-arching thickening-masses of the partition-wall, and is united, right and left, with the adjoining cell-cavities by a circular opening (Fig. 26, A,D,E). In Pinus syl'vestris the pits are large and distant from one another, and the whole pro- cess may be easily traced step by step. The process appears somewhat different when pits lie very near to one another, as in Pitted Vessels, In this case the thickening first presents itself in the form of a net-work, which surrounds the thin parts of the cell- wall in the form of roundish polygonal meshes, as may be very easily recognised in young maize-roots, for instance. Fig. 27, ^, represents a portion of the side-wall of an already mature vessel ^ of the root-tuber of Dahlia. The ridges which originally ap- pear on the thin cell-wall are indicated by a and are left clear ; they enclose elliptical meshes pointed at both ends. As the thickening continues, each ridge retains its On the idea of a vessel, see chap. ii. THE CELL-WALL. 27 original breadth, where it is raised on the thin cell-wall ; but the free rim which grows further inwards, expands, and becomes arched over the thin parts of the cell-wall. But in this case the overarchings do not grow uniformly, but in such a manner that their rims form at least a fissure (c, in A and B). Here also, when two similar cells adjoin, the same process takes place on both sides of the partition-wall ; and here also lenticular spaces are formed by the overarchings ; these are at first bisected by the original thin lamella of the cell-wall, which afterwards disappears, and the two cell-cavities are placed in communication at each bordered pit ; the chan- nel or bordered pit which unites them is wide in the middle, and opens right and left into each cell by a narrow fissure (Fig, 27, B, C). If, on the other hand, a vessel of this kind adjoins a parenchyma-cell which remains always full of sap and closed, the thickening and overarching of the pit occurs only on the side of the vessel (Fig. 28, V); the thin parts of the cell- wall are retained 1, and the bordered pits remain closed ; from the cell-cavity of the vessel a narrow fissure {c) proceeds between the expanded thickening -masses [b) to a wider cavity, which is bounded on the sides by the narrow part of the thickening-masses {a), on the outside by the primary cell-wall. These processes can only be seen in sec- tions of extraordinary tenuity ; but these are easily obtained if larger pieces of the parts to be observed are allowed to lie for months in plenty of absolute alcohol, then taken out before the preparation is made, and the alcohol allowed to evaporate : in this manner pieces of some hardness and toughness are obtained, which may be cut extremely well and smoothly if the knife is very sharp. In the walls of vessels thickened like ladders or steps, which are developed with peculiar beauty in the higher Cryptogams, the bordered pits are fis- sure-like ; they are often as broad as the partition- wall of two adjoining cells, but very narrow in the direction of the longitudinal axis of the cell. In Fig. 29, A, is shown the lower half of a vessel of this kind with the fissure-like pits, between which the thickening-masses of the wall lie like rungs of a ladder ; the larger clear spaces are the angles of the contiguous cells. The formation of such a scalariform thickening begins by the growth, on the originally very thin wall which separates two vessels (C, /), of transverse ridges of thickening (y), which pass over, right and left, into that thickening which always lies on the angle of a cell- wall. C shows this horizontally, D in vertical section. When completely developed, the thin lamella (/) has disap- Flf,. 29. — Pteris aquilina, vessel from the un- derground stem thickened in a scalariform man- ner ; .'/ a half-vessel, isolated by Sthulze's mace- ration ; B — D obtained from pieces of the stem liardened in absolute alcohol; B after a very clean section, represented half as a diagram ; to the right, front view of the wall of the vessels from within ; c c vertical section of the same ; C front-view of the young wall of a vessel ; D its vertical section; E place where a vessel adjoins a succulent cell, in section vertical to the thickening-ridges of the vessel (X8oo). ^ These thin pieces of cell- wall which close up bordered pits may, by rapid surface- growth, form bag-like prominences, which grow through the pores of the pits into the vessels, spread them- selves out there, become separated by septa, and thus form a thin-walled tissue, which not unfre- quently fills up the whole of the cavity. These formations were long known under the name of 'Tiillen '; they are abundantly and easily seen, for instance, in old roots of Cucurbita, and in the wood of Rohinia pseiidacacia, &c. [These cells contained in the ducts are, according to Mohl and Reess, really hernioid protrusions from adjacent cells; see Journ. of Eot. 1872, pp. 321-323, t. 126; and Reess, Bot. Zeitg. 1868, pp. i-ii, 1. 1.] 28 MORPHOLOGY OF THE CELL. peared (c, c, in B), the thickening-ridges have become overarched, growing inwards, so that now only a narrow fissure (?> of cell-walls enclosed within one another are gradually formed, while from time to time the older masses of layers cease growing, and are pierced by the growing filament, which now forms new layers of cell-wall (cf. Niigeli und Schwendener : Das Mikroskop, II. p. 551). It need scarcely be mentioned that tehse appearances do not contradict the theory of the growth of the cell-wall by intussusception, but only represent, in general, par- ticular modifications of the life of the cell. (e) Differentiation of the Cell- ivall into Systems of Layers [Shells) > and r) remain unchanged by this maceration, except that they lose their colour ; and hence the shell c is shown to be composed of some more and some less watery layers (Fig. 30, C, c). The three shells also show a different behaviour on treatment with concentrated sul- phuric acid : a becomes a dark reddish brown, and does not swell, or only slightly ; b swells in the radial direction and becomes thicker ; while c swells in the radial, tangential, and lon- gitudinal directions (cf. Fig. 40, C, c, and Z), c) ; in transverse sections c breaks away from b, and curves spirally (C) ; in longitudinal sec- tions it is bent in a wavy manner (D). In true wood-cells, e. g. in Pinus syl-vestris (Fig. 26, A), three shells are likewise gene- rally to be distinguished : a central one (Fig. 26, A, w), next a thicker one (2), and an inner (/) ; the two first turn yellow on treatment with solution of iodine or iodine and sulphuric acid, the innermost blue with the latter reagent ; 2 and / are dissolved by concentrated sulphuric acid, and the central lamella m remains ; here also the possibility of isolating the cell depends on the circumstance that the central lamella m may be dissolved by boiling in nitric acid with potassimn chlorate ; and thus the iso- lated cells consist only of the two inner shells. In many wood-cells (the 'Libriform Fibres' of Sanio) the inner thickening-layers form a shell of cartilaginous and gelatinous consistence (as in the wood of many Papilionaceae). D 2 Fig. \o.—Pteris aquilina; structure of the brown-walled sclerenchyma in the stem (X55o)- ^^ a fresh thin transverse section ; B the longitudinal wall between two cells, fresh (a curved pit-channel at the lower end) ; C transverse sec- tion in concentrated sulphuric acid; Z) longitudinal section of the wall in sulphuric acid ; a tbe central lamella of the wall; b second shell; c third, inner shell of the cell-wall; p pore-channels ; / cavity of the cell. 36 MORPHOLOGY OF THE CELL. When the outermost layers of cells which are combined into tissues become gelatinous or mucilaginous, the boundary-line readily disappears ; and the appearance may then be presented as if the cells, enclosed by the inner shell, which is not mucilaginous, were im- bedded in a homogeneous jelly as a ground-work ; and this latter especially gave rise in time past to the theory of ' Intercellular Substance,' to which we shall recur. This be- haviour occurs in the tissue of some Fucaceae, and also in the endosperm of Ceratonia Siliqua (Fig. 41) ; cc are the outer layers of the wall of the cells a, which have become entirely converted into mucilage and rendered indistinguishable, their inner layer appear- ing as a strongly refractive shell. In the dry 'state the mucilaginous mass is almost horny; it swells up strongly in water with solution of potash ; with iodine and sulphuric acid it does not become coloured, but the sharply defined layer turns blue. In free-lying cells numerous layers of cell-wall may also form a mucilaginous shell, which is most beautifully developed in the spores of Pilularia (Fig. 35) and IVIarsilea. In the spore-fruit (sporan- gium) of these plants are certain masses of parenchyma, the cell-walls of which become mucilaginous on the inner side ; when dry the mucilaginous masses are firm and horny, but absorb so much water that they increase in bulk several hundred-fold, and burst the pericarp (Book II. Rhizocarpeae). On a similar transformation into mucilage of inner layers of cell-wall, while an outer, thin, and cuticularised shell retains its power of resistance, depends also the formation of the mucilage of linseed and quince-seed. The inner thickening-masses of the epidermis of the seed, transformed into mucilage, absorb the surrounding water with great force, swell violently, and, bursting the cuticle which is incapable of swelling, appear, in the pre- sence of a small quantity of water, as a hyaline layer enveloping the seed ; and, with more copious addition of water, become more and more diluted into thin mucilage. A similar process occurs in some other seeds, such as those of Teesdalia nudicaulis and Plantago Psyllium, in the seed-hairs of Ruellia, and the pericarp of Salvia. Gum-tragacanth consists of the cells of the pith and medul- lary rays of Astragalus creticus, A. Tragacantha, and other species, transformed into mucilage. When the walls of these cells become mucilaginous, and swell up on copious addition of water, they force themselves through slits in the stem as viscid masses, and dry up on the outside into a horny mass capable of swelling. Vegetable mucilage can, however, arise in other ways l. (f) Incombustible Deposits occur in every cell-wall. The presence of lime and silica can be directly proved ; but it can scarcely be doubted that potash, soda, magnesia, iron, sulphuric acid, &c., also occur in small quantities. The deposit of lime-salts and silica increases with age. The deposit may take place in two ways ; usually only extremely small particles of incombustible substance are deposited regularly between the mole- cules of the organic substance of the cell-wall ; and this may be recognised by the ash remaining behind after ignition in the organised form of the cell- wall (as a skeleton) ; but lim.e salts may also be contained in the cell-wall in the form of numerous very small crystals ; they then lie imbedded in the substance of the cell-wall itself, sometimes in the form of particular growths which project into the cell-cavity and are termed Cystoliths (cf. ^ect. 10). Fig. 41.— Section of the endosperm of Ceratonia SiliqJia. ^ Compare further, Frank: Ueber die anatomische Bedeutung und die Entstehung der veget. Schleime. (Jahrb. fiir wissen. Bot. V. 1866.) PROTOPLASM AND NUCLEUS. ?>! Skeletons composed of a substance ^ soluble in weak acids (generally thought to be lime), are obtained by combustion of very thin layers of tissue on glass or platinum-foil ; they occur so generally that it is unnecessary to adduce examples ; from entire vessels, I obtained, in the case of Cucurbita Pepo, beautiful lime-skeletons. Silica-skele- tons are obtained most abundantly from the epidermis-cells and from Diatoms ; but silicified cell-walls occur also in the interior of tissues (as leaves of Ficus Sycomorus, Fagus syl'vatica, Quercus suber, Deutzia scabra, Phragtnites communis, Ceratonia Stliqua, Magnolia gran'diflora, &c., according to Mohl-). The silicification does not generally affect the whole thickness of the cell-wall, but only an outer shell, as, for instance, in the case of epidermis-cells, the cuticularised portion only. In order to obtain fine skeletons, it is necessary previously to soak the removed epidermis or thin sections of it in nitric or muriatic acid, and then to burn them on platiniun-foil. I have found another method much more convenient : I place larger pieces of the tissue {e. g. of leaves of grass, stems of Equisetum, &c.) on platinum-foil in a large drop of concentrated sulphuric aeid, and heat over the flame ; the acid immediately turns black, a violent formation of gas follows ; the heat must be continued until only the pure white ash remains. This is soon effected by this means, whereas otherwise the reduction to ash is generally very tedious, and often does not afford an entirely colourless skeleton. (On the crystals sometimes deposited in the cell-wall, see infra sect, ii.) Sect. 5. Protoplasm and Nucleus^. — Now that the signification of the protoplasm as the peculiar living body of the cell has been sufficiently brought out, we need only add what is absolutely essential both as respects its chemical and physical nature, and its structure and movements. The protoplasm consists of a combination of (apparently different) albuminous substances with water and small quantities of incombustible materials (ashes). In most cases it also contains, as may be concluded on physiological grounds, considerable quantities of other organic compounds, belonging probably to the scries of carbo-hydrates and fats. These admixtures are distributed through its mass in an invisible form; but it not un- frequently includes visible granular formations of starch and fats, which at a sub- sequent period may either entirely disappear or may increase in bulk. Very commonly the rapidly increasing protoplasm, in itself colourless and hyaline, is rendered turbid by numerous small granules, consisting, probably, of small drops of oil. The protoplasm, as it is generally met with, ought therefore to be con- sidered as true protoplasm with varying admixtures of different formative materials (Metaplasm of Hanstein). The consistence of protoplasm varies greatly at different times and under different circumstances. It commonly appears as a soft, plastic, tough, inelastic, very extensible mass ; in other cases it is more gelatinous, some- times stiff, brittle (in the embryos of seeds before germination) ; but very commonly it gives outwardly the impression of being a fluid. All these conditions depend ^ The salts found in tlie ashes are partly products of combustion. Carbonic-acid salts may arise by the combustion of salts of vegetable acids. Since a strong red heat is necessary, easily volatile chlorides (potassium chloride or common salt) may disappear from the ash, &c. 2 H. von Mohl, Ueber das Kieselskelet lebender Pilanzenzellen, in Bot. Zeitg. 1861, no. 30 et seq, — Rosanoff, Bot. Zeitg. 1871, nos. 44, 45. ^ H. von Mohl, Bot. Zeitg. p. 273, 1844, and p. 689, 1855.— linger, Anatomic und Physiologic der Pflanzen, p. 274, 1855. — N.-igeli, Pflanzenphysiol. Untersuchungen, Heft I. Zurich. — Briicke, Wiener akad. Berichte, p. 408 et seq. 1861.— Max Schultze, Ueber das Protoijlasma der Rhizopoden imd Pilanzenzellen, Leipzig 1863. — De Bary, Die Mycetozoen, Leipzig 1864. — Hofmeister, Die Lehre von der Pflanzenzelle, Leipzig 1867. — Hanstein, Sitzungsberichte der niederrheinischen Gcsellschaft in Bonn, Dec. 19, 1870. 38 MORPHOLOGY OF THE CELL. essentially on the quantity of water it has absorbed. But, however great may be the quantity of water, and its consequent similarity to a fluid, the protoplasm is nevertheless jiever a fluid ; even the ordinary dough-like, mucilaginous, or gelatinous conditions of other bodies can only be very superficially compared with it. For the living and life-giving protoplasm is endowed with internal forces, and, as the result of this, with an internal and external variability which is wanting in every other known structure ; its active molecular forces cannot, in short, be conipared with those of any other substance \ The capacity which protoplasm has, in conse- quence of the forces which become manifested in it, of assuming definite external forms, and of varying these, as well as its capacity of secreting substances of different chemical and physical properties according to definite laws, is the imme- diate cause of cell-formation and of every process of organic life. The protoplasm of plants in a state of vital activity is generally very watery, and shows on one side an internal diff"erentiation of its substance into layers and portions diff'ering in their consistence and chemical nature ; on the other side it assumes definite outlines, and becomes bounded by surfaces of determinate, and mostly very variable, form. The internal differentiation of protoplasm is most commonly manifested by an outer, hyaline, apparently firmer, but mostly very thin layer, enclosing the inner mass, but in such a manner that the two remain in the most intimate con- tact. Every portion of a protoplasmic body immediately surrounds itself, when it becomes isolated, with such a skin (Hautschicht). Also in the interior a quantity of fluid sap, which permeates its substance throughout, invariably becomes separated in the form of drops (vacuoli) ; when the protoplasm is contained in a growing cell, these vacuoli increase in proportion as the cell grows, and the protoplasmic body becomes a sac filled with watery sap. One of the most common internal differentiations of the young protoplasmic body, while constituting itself into a separate individual, is observable in the formation of the nucleus. The substance of the nucleus is at first indistinguishable from that of the rest of the protoplasm, and its formation is essentially nothing but the collection of certain particles of protoplasm round a centre, which is also usually the centre of the whole protoplasmic body. Once formed, the nucleus (whose chemical nature, as far as observation goes, is altogether very much like that of the pro- toplasm) may become more sharply defined ; it may itself form a skin, and vacuoli and granular formations (the nucleoli) may become separated in it. But the nucleus always remains a part of the protoplasmic body; it is always imbedded in it ; very commonly it becomes again dissolved, after a short existence, in the protoplasm, i. e. its substance combines with it {e. g. in cells which frequently divide, as on p. 14: in the sacs of the Characese the nucleus disappears altogether when the streaming (Stromung) of the protoplasm begins). Another very common differentiation of the substance of the protoplasm consists in single portions of it becoming separated in a definite form and assuming a green colour, thus forming chlorophyll structures, which, like the nucleus, not only arise out of the protoplasm, but always remain as portions of the protoplasmic body. But since these require ^ For further details on this point, &ee Book III ; also my Handbook of Experimental Physiology, § 116. Leipzig 1865. PROTOPLASM AND NUCLEUS. ng a more minute investigation, they are only mentioned here ; the next section will be specially devoted to them. The external configuration of the protoplasm into a definitely formed body can be reduced to two cases : — either its single smallest particles are constantly grouping themselves concentrically around a common centre, or an internal motion takes place, which causes the protoplasmic body to become elongated in some one direction, and disturbs the centripetal arrangement. The former oc- curs commonly in the formation of new cells, the latter in their grow^th. The movements of the smallest particles of protoplasm which bring about its grouping and configuration in the formation and growth of cells, are generally so slow as not to be visible even when the cells are very highly magnified. Much quicker movements, even appearing rapid under a very high magnifying power, occur in cells already formed, more or less independently of their growth, and either preceding it (as in swarm-spores) or following it. As to the external appearance, the following kinds of movements of this nature may be distinguished : — (A) Move- ments of naked, membraneless protoplasmic bodies, (i) The sivimming of swarm- spores and spermatozoids ; this is characterised by the naked protoplasmic body, swarm - spore or spermatozoid, not changing its external form, while motile vibratile cilia, themselves probably thin threads of protoplasm, cause rotation round the longer axis, and at the same time a progressive movement in the water. (2) Amccba-movement ; — consisting of rapid changes of the external contour of naked protoplasmic structures, Myxoamoebae and Plasmodia, which, supported under water or in the air on a firm moist body, creep about as if flowing, extending and contracting ; while within both the principal mass and the appendages which proceed from it, 'streaming' motion occurs. (B) Movements of the protoplasm within the cell- wall ; this occurs after the protoplasmic body of the cell has formed a larger sap-cavity, and continues commonly after the grow^th of the cell has ceased until the end of its life. (3) Those movements are distinguished as Circulation when strings and bands, proceeding from the parietal protoplasm, run to that portion which envelopes the nucleus, and often stretch completely across the sap-cavity. A distinction is drawn between mass-movements of larger portions of protoplasm, and streaming movement of the substance of which they are com- posed ; the former consist in accumulation or diminution of the parietal layer, wanderings of the mass which contains the nucleus in difi"erent directions, and, dependent on this, of different groupings of the strings. Within these structures of the body of the cell itself streamings often occur, which are apparent from the movement of the enclosed granules, often in opposite direcdons within the same thin string. In the cells of lower and higher plants which are rich in protoplasm and sap but poor in granular contents, the circuladon is a widely distributed phe- nomenon, especially visible in the hairs. (4) The term Rotation is applied to those movements where the whole mass of protoplasm enclosing a cell-cavity circulates on the cell-wall as a thick current complete in itself, and carries along with it the grains and granules contained in it. This occurs in some water-plants, Characeae, Vallisneria, root-hairs of Hydrocharis, &c. (a) The protoplasm shows two conditions, which may be distinguished as the living and the dead; the former passes over into the latter by the most various chemical 40 MORPHOLOGY OF THE CELL. and mechanical processes ; the reactions of livhig protoplasm towards chemical reagents are essentially different from those of dead protoplasm, but this of course can only be perceived when the reagents do not at the same moment cause death. Solutions of different colouring matters, as aqueous solutions of the colours of flowers and the juices of fruits, especially also weak acetic extract of cochineal, have no power of colour- ing living protoplasm^; but if it has previously been killed, or if it has been deprived of its life-giving condition by continual action of these reagents, it absorbs a proportionately larger quantity of colouring material as a solvent ; the whole substance assumes a much more intense colour than the solution itself. Solutions of iodine in water, alcohol, potas- sium iodide and glycerine, act in a similar manner; they all cause a yellow or brown colouring of the protoplasm, which is more intense than that of the solution itself. If protoplasm is first heated with nitric acid, the excess of acid removed by water, and solution of potash added, it assumes a deep yellow colour ; saturated with a solution of copper-sulphate and then treated with potash, it becomes of a beautiful dark violet. Protoplasm containing but little water treated with a large quantity of concentrated English sulphuric acid, assumes a beautiful rose-red colour, without at first changing its form ; subsequently this colour and the form disappear together, the protoplasm dissolv- ing. Dilute solution of potash (sometimes also liquid ammonia), dissolves protoplasm, or at least destroys its form, and makes it homogeneously transparent. If, on the other hand, cells with protoplasm of characteristic form are placed in a concentrated solution of potash, the form itself remains for weeks, but disappears immediately on addition of water. All these reactions are collectively characteristic of true albuminoids, as caseine, fibrine, albumen ; and we are therefore justified in supposing that substances of this kind are always contained in protoplasm. If the protoplasm-sac in cells rich in sap is very thin, it acquires a greater power of resistance, and withstands the solvents mentioned for a longer or shorter time. In another respect also protoplasm behaves like albumin- oids ; by heating very watery protoplasm to above 50° G. it is killed, and becomes turbid and stiff, and gives the impression of coagulation ; alcohol and dilute mineral acids act in the same manner. The nucleus behaves towards all colouring substances, solvents, and coagulating agents in the same manner as living watery protoplasm, or it shows itself even more sensitive, especially in young cells ; in older cells however it may be less easily acted on. At the base of all protoplasmic structures there probably lies a substance which is colourless, homogeneous, and not visibly granular, to it alone the name Protoplasm ought perhaps to be applied, or at all events it ought to be distinguished as the founda- tion of protoplasm. The fine granules which are so often mingled with it, and which some used to consider an essential ingredient, are probably finely divided assimilated food-materials, which undergo a further chemical metamorphosis into protoplasm ; every intermediate form occurs from these more or less fine granules to the largest, which may be clearly recognised as fat and starch. Homogeneous protoplasm destitute of granules is found in the cotyledons of dormant embryos of Helianthus, and in the cotyledon- leaves of Phaseolus ; out of it chlorophyll is subsequently formed, and here the proto- plasm contains but very little water ; but the extremely watery protoplasm which rotates in the cells of Vallisneria is also destitute of granules ; nothing but nucleus and grains of chlorophyll can be recognised in it. In the development of the spores of Equisetum (Fig. 10) the finer granules separate repeatedly from the homogeneous protoplasm, and afterwards become again distributed through it. But in some cases the protoplasm is so loaded with granular and coloured materials, that the colourless hyaline original sub- stance can no longer be distinguished, as, for instance, in the ova of Fucus (Fig. 2), ^ In tlie same manner the protoplasm and nucleus in living cells with coloured sap are also colourless; in other cases, on the other hand, the protoplasm is tinged by a colouring matter soluble in water, which is not present in the cell- sap. (As in Floridecc and the flowers of Com- positoe, the last accordinc; to Askenasy.) PROTOPLASM AND NUCLEUS. 41 the zygospores of Spirogyra (Fig. 6), and in many spores and pollen-grains^. In the food-reservoirs of dry seeds {e.g. the cotyledons of peas and beans), the protoplasm itself is often collected into small roundish grains, between which lie the grains of starch; this condition of protoplasm will be further touched on hereafter. (b) Skin, Var.uoli, Mo'vement. Naked protoplasmic bodies, as the plasmodia of the Myxomycetes, some swarm-spores, e. g. of Vaucheria, allow the skin to be re- cognised, under sufficient magnifying power, as a hyaline edging; in the swarm- spores of Vaucheria it is evidently striated radially in the optical section, just as some cell- walls are ; Hofmeister (Handbuch, I. p, 25) found the same in the plas- modia of iEthalium. Probably this skin is nothing but the pure original substance l-IG. 42— />>(; protoplasm from an injured sac of I'auchcrta Urrestris, slowly euierginif in water, in dilTerent successive conditions, at intervals of about five minutes ; h the cell-wall of the ruptured sac ; i the part of the protoplasm which still remains in the sac ; a in fi, C, D, and F, a ball of protoplasm detaching itself, forming vacuoli, then dissolving (in F); d a. branchlet of the protoplasm from which the mass // is detached, this mass isolated in D, dissolved in F; c and d behave in a similar manner; G shows the further changes of the part c" \n F. An freshly escaped mass of protoplasm, rounded off into a sphere, the chlorophyll-grains lie all together in the inside ; hyaline protoplasm envelopes the whole as a skin. of the protoplasm itself free from granules, of which the whole body is formed ; only the parts which lie most in the interior are permeated by grains and granules. It follows that in the amoeba-like movements of the plasmodia the new processes are always at first formed of the skin alone ; it is only when they increase in size that the interior granular substance makes its appearance in them. This is more clearly the case in the masses of protoplasm that escape into water from the injured sacs of Vaucheria, which often instantly become rounded into globular bodies, but not unfre- quently show the amoeba-like movement of plasmodia for as much as half-an-hour or an hour (Fig. 42). This interpretation of the skin is not at all opposed to the fact that ^ J. Hanstein gives to the substances mingled with the true protoplasm and which undergo many transformations, the collective name of ' Melaplasm.' (Bot. Zeilg. p. 710, 1868.) 43 MORPHOLOGF OF THE CELL. it is denser than the inner and more watery substance. That the cohesion tn each pro- toplasmic body decreases from without inwards, follows from the easier mobility of the inner mass, which is especially the case with the plasmodia, and also from the formation of vacuoli, which clearly depends on the collection of a portion of the water present in the protoplasm round internal points, and the final formation of drops there, pre- supposing -that the cohesion is overcome at these points. The view here presented that the hyaline homogeneous original substance itself is formed on each free surface of motion of the protoplasm as a skin destitute of granules, entirely agrees with the sup- position that not only every vacuole in a solid protoplasmic body, but also every thread of protoplasm which penetrates the sap-cavity, and finally the inner side of the proto- plasm-sac which encloses the sap-cavity, is also bounded by a skin, even if it be so thin that it cannot be seen when strongly magnified \ If the protoplasm is not enclosed in a cell- wall, the vacuoli are usually small and not numerous ; if, on the other hand, a cell-wall is formed and if the cell grows rapidly, this is always accompanied by an increase in number and size of the vacuoli (Fig. i). This not unfrequently leads to a frothy condition of the protoplasm where the vacuoli are sepa- rated only by thin lamellae of that substance (Fig. 43, ^) ; but in other cases the inner protoplasmic mass of a cell breaks up into smaller portions, each of which encloses a large vacuole, which is surrounded by a thin membrane of protoplasm (Fig. 43, B, b). These are the 'sap-vesicles' (Saftbliischen) which so commonly occur, and which some- times enclose chlorophyll and other grains, and thus become similar to cells (not un- common in the flesh of berry-like fruits, and in tissues with mucilaginous juices). If the rapidly growing cell does not form new pro- toplasm, /. e. if its protoplasmic body is not correspondingly nourished, then, in propor- tion as the size of the cell and the amount of sap increase, the quantity of protoplasm decreases ; and not unfrequently it forms a thin sac not directly visible, lying between the cell-wall and cell-sap, clothing the former like thin tapestry, and becoming visible only by means of substances that remove the water, and loosen the protoplasm-sac (Primordial Utricle of Mohl) from the cell-wall by contraction (Fig. 43, C, p). The signification of this thin-walled protoplasm-sac, its production by increase in number and size of the vacuoli in an originally solid protoplasmic body, will no longer be doubtful to the reader after all that has been said in sects, i, 2, and 3, and by com- parison of Fig. I with Fig. 43. In younger cells, where the protoplasm forms a still thicker layer, or where it presents a net-work permeated by vacuoli, its substance, with the exception perhaps of the outermost layer lying on the cell-wall, appears to be always engaged in a 'streaming' movement, which is however usually very slow. In many mature and large cells this condition is permanent, when they do not serve for the storing up of assimilated materials, and when the protoplasmic body is sufliciently nourished. Fig. 43.— Forms of the protoplasm contained in cells. A and B of Zea Mais; A cells from the first leaf-sheath of a germinating plant ; B from its first internode ; C from the tuber of Heliaiithiis tuberosus, after action of iodine and dilute sulphuric acid; h cell-wall ; k nucleus ; p protoplasm. ' Cf, Hanstein, Die Bewegiingserscheinuiig des Zellkerns, u. s. Theinischen Gesellschaft z\\ Bonn, p. 224, 1870. Sitzungsberichte der niecler- PROTOPLASM AND NUCLEUS. 43 and does not, when the cell distends, contract to a mere thin membrane. If the whole mass of protoplasm withdraws to the cell-wall, enclosing a single large vacuole (the sap-cavity of the cell), all the particles of protoplasm, flowing in one direction, may form a continuous broad current encircling the cell (rotation), the direction of which is always such as to describe the longest course round the cell-cavity (Nagcli). Examples occur in Characeae, in many other submerged water-plants, as Vallisneria, Ceratophyllum, Hydrilleae, root-hairs of Hydrocharis ; the globular nucleus, when present (in Characeae it soon disappears), is carried along with the current. The protoplasmic body which encloses a large sap-cavity may, however, possess ridge-like prominences arranged in a net-work, the substance of w^hich flows in different directions ; by this means the nucleus may either, relatively, remain at rest, and, in a certain manner, form the centre of movement, cr it is carried along with it. Cases of this kind occur tolerably Fig. 44.— .7 stellate hair on the calyx of the young flower-bud o( Aithcen rosea; thicker portions of protoplasm lie on the proto- plasm-sac of each cell ; these are in the act of 'streaming' motion (indicated by the arrows). B epidermis (fj>) with the basal portion of a mature stellate hair, showing the structure of the wall (X550). frequently in the hairs of land-plants (as in the stinging hairs oiUrtica urens, sieWcXte hairs of Altbaa rosea). But the strings of protoplasm which show these currents may also penetrate the sap-cavity of the cell; not unfrequently {e.g. Spirogyra, hairs of Cucurbita) the nucleus then lies in its centre, enveloped by a mass of protoplasm ; the strings unite it with the protoplasm-sac w^hich clothes the cell-wall. These strings or threads, stretch- ing across the sap-current, may at first arise from the thin lamellae of protoplasm which in younger quickly growing cells still separate adjoining vacuoli; w^hen these finally flow together into a single sap-cavity, the thicker parts of these lamellae (Fig. i, B) may remain as strings, forming a more or less irregular net-work, which at first cor- responds, in posit:on and size, to the vacuoli that have now coalesced, but subse- quently, as the cell continues to grow, and in consequence of the internal movements 44 MORPHOLOGF OF THE CELL, of the \Ahole protoplasmic body, undergoes further distortions and entire change of form. But new strings also make their appearance ; ridge-like portions arise from the peripheral plasma, or even on the thicker strings, and finally become detached in such a way that the two ends of the new string remain united with the rest of the protoplasmic substance ; they do not grow up as branches with one free extremity. (Hanstein, /. c. p. 2 2 1.) In the same manner threads disappear; both ends, remaining in connexion with the rest of the protoplasmic body, coalesce with it. The strings form, together \vith the central masses of protoplasm which contain the nucleus and those which clothe the cell-wall, a connected system, single portions of which may change their position with respect to one another. Besides these displacements of large portions of the protoplasm of a cell endowed with circulation— in consequence of which the parietal protoplasm at any one spot accumulates or diminishes, and the mass of protoplasm in the cell-cavity which con- tains the nucleus wanders about, and alters the grouping and form of the strings to correspond to its own — under high magnifying power another form of movement then comes in view, which is undoubtedly of the same origin, although the exact mode is unknown. In the parietal plasma, in the mass which contains the nucleus, but most distinctly in the strings, the very small granules interspersed among the pro- toplasm, and generally also small grains of chlorophyll, are to be seen in ' streaming ' movement, which under high magnifying power may even appear very rapid. It must not, however, be overlooked that when the cell is magnified, say five hundred times, the rapidity of the movement is also apparently increased five hundred-fold. Within even a very thin string, the granules not unfrequently flow in opposite directions near one another. Granules of chlorophyll often appear to be in motion on the surface of thin strings; it may nevertheless be assumed with certainty that they also are enclosed in the substance of the string, but, being very prominent, are covered by only a very thin lamella of it. Those mass-movements of larger portions of protoplasm on which the various internal grouping of the protoplasmic body of the cell depends, may be compared to the displacements of the mass of the body which, in the case of naked Amoebae, change the external contour, and cause its creeping motion ; in the case of circulating proto- plasm the firm cell-wall hinders the external change of contour as well as the change of place of the whole ; but the large internal sap-cavity allows of similar displacements of larger portions in the interior. The ' streaming ' movement, which is visible by means of the imbedded granules, occurs in the creeping naked protoplasm of the Amoebae as well as in that enclosed in a cell-wall. (c) The Nucleus. That the nucleus, which is never absent from the Muscineae and Vascular plants, but more often from the Thallophytes, presents itself as a product of differentiation of the protoplasm, i. e. must be regarded as a formed portion of the pro- toplasm itself, is sufficiently evident, not only from its chemical behaviour (vide supra, under a), but also from its participation in the processes of cell-formation (cf. sect. 3); and this need not be further demonstrated. On the other hand, it must be made clear that, once formed, it constitutes a characteristically formed portion of the cell which, to a certain extent, has a mode of development of its own. At first the nucleus is always a homogeneous roundish body of protoplasmic substance ; subsequently its sur- face becomes firmer without its taking the form of a special skin ; in the interior arise usually two or three (sometimes more) larger granules, called Nucleoli, which, however, are often entirely wanting. The nucleus has, at the time of its origin in the young cell, usually already attained its permanent size, or nearly so ; its growth is never proportional to that of the cell ; in young tissue-cells (Fig. i) it usually occupies a large portion of the cell-cavity ; in fully grown cells its mass is increasingly small in proportion to that of the whole cell. Usually a further development remains also in the sharper bounding by a firmer outer layer and the formation of small vacuoli and nucleoli ; only rarely does it grow for a longer time ; more vacuole-fluid collects in the interior ; its substance may THE CHLOROPHYLL-BODIES. 45 become frothy; and it is also sometimes the case that it moves in a 'streaming' manner, and in the interior of the firmer enveloping layer a circulation sets up, as in a celP. The nucleus always remains enclosed in the substance of the protoplasm ; if this latter forms vacuoli or assumes the condition of circulation already described, the nucleus remains enveloped in a coating or in a thicker mass of protoplasm, which is connected M-ith the parietal protoplasm-sac by the lamellae lying between the vacuoli as well as by the current-threads. The nucleus apparently follows passively the displacements and wanderings of the portion of protoplasm in which it is enveloped; it also undergoes changes of form under the pressure and progress of the moving mass, which proceed under the eye of the observer. 'During the movement,' says Hanstein {I.e. p. 226) admirably, ' the bands of protoplasm are and remain very tightly stretched, so that the envelope of the nucleus is drawn out by them into sharp angles. It looks as if the nucleus (together with its envelope) \vere towed about like a ferry-boat by ropes stretched across. But since during this towing the bands themselves alter their direction and form, it is self-evident that the envelope of the nucleus must also change its form. But not only the envelope, but also the nucleus itself, does this. This latter is never spherical or of any similarly regular form during the time of its wandering, but is irregu- larly elongated, and usually in the direction of its motion at the time.' This change in the form of the nucleus may also be recognised from the displacement of the nucleoli within its mass. Sect. 6. The Chlorophyll -Bodies and similar protoplasmic Struc- tures I— Chlorophyll, the green colouring matter so generally distributed through the vegetable kingdom, is always united to definitely formed portions of the proto- plasmic body of the cells in which it is found ; these green-coloured portions of protoplasm may, in contradistinction to the colouring matter itself by which they are tinged, be designated Chlorophyll-bodies. Every chlorophyll-body consists then of at least two substances, the colouring matter and its protoplasmic vehicle ; if the former is removed by alcohol, ether, chloroform, benzine, or essential or fatty oils, the latter remains behind colourless. The colouring matter contained in each chlorophyll-body is itself only extremely small in quantity; after its removal the protoplasmic ground-work retains not only its form but also its previous volume. The latter is always a solid soft body containing extremely small vacuoli, in which the colouring matter is generally completely, though not always uniformly, distributed. Chlorophyll-bodies arise in the young cells by the separation of the protoplasm into portions which remain colourless and others which become green and sharply defined. The process may be supposed to take place by very small particles of a somewhat different nature originally existing in or being distributed through the pre- viously homogeneous protoplasm, then collecting at definite places and appearing as separated masses. The grains of chlorophyll which arise in this manner always re- main imbedded in the colourless protoplasm in a similar manner to the nucleus ; they ^ In young hairs of Hyoscyamus niger, according to A. Weiss in the Sitzungsberichte der kais. Akademie der Wissenschaften, Vol. LIV. Vienna, July 1866, 2 H. von Mohl, Bot. Zeitg. nos. 6 and 7, 1855.— A. Gris, Ann. des Sci. Nat. Ser. IV. Part VII. p. 179, 1857.— Sachs, Flora, p. 129, 1862; p. 193, 1863.— Sachs, Haiidbuch der Exper. Physiol, der Pflanzen, § 87, Leipzig 1863.— Hofmeister, Die Lehre von der Pflanzenzelle, § 44, Leipzig 1867. — Kraus, Jahrb. fiir wissensch. Bot. VIII. p. 131, 1871. 46 MORPHOLOGY OF THE CELL. are never in immediate contact with the cell-sap, but are enveloped on all sides by the colourless protoplasm. Their chemical and physical properties distinctly show that their colourless ground-work is a substance altogether similar to protoplasm. The chlorophyll-bodies consequently always behave as integral parts of the protoplasm ; and this is especially evident in the division of cells containing chlorophyll, in conjugation, in the formation of swarm-spores, &c. But the chlorophyll-bodies, when once formed, grow, and if they possess roundish forms they may be increased by division. Both appear always to depend on the growth of the collective proto- plasm-body in which they are deposited. It is only in the Algae that the forms of the chlorophyll-bodies show much variety ; in them it is frequently the case that the whole protoplasmic body, with the exception of an outermost layer, or of a little more than this, either appears homogeneously green [e.g. many swarm-spores, Palmellacese, gonidia of Lichens) or the chlorophyll-grains assume stellate forms {e. g. Zygnema criiciatum, Fig. 45), or they form several lamellae which have the appear- ance of a star when the cell is cut across (as in Closterium, &c.), or straight or spiral bands ~ ~ ~ {e. g. Spirogyra). But in most Algce and all VlG. 45-— ■'^ceW of Zyo-nei^ta criic/'ahim, withtwosteUnte jtt chlorophyll-bodies which are suspended in the interior of MOSSCS aUQ VaSCUlar plaUtS, tllC ChlOrOpliyll- the cell; they are united bj' a colourless bridge of proto- . plasm in whicli lies a nucleus ; the rays which form the bodlCS are TOUUdcd Or pOlygOUal maSSCS union with the parietal sac are already nearly colourless in the middle. In ea.h of the two chlorophyll-bodies lies a collcctcd arounQ Q, ccntrc, aud arc tcmied Grains of Chlorophyll. Generally a large number are contained in one cell, sometimes, however, only a few relatively large ones {e.g. Selaginella), and in one of the Hepatic^e of simplest structure (An- thoceros) only a single grain of chlorophyll is to be found, enclosing the nucleus ; this therefore, when the cells divide, itself also divides in a correspond- ing manner. With extremely few exceptions Grains of Starch arise in the homogeneous solid substance of the chlorophyll-bodies, and, where these have special forms, are distributed in definite places (cf. e. g. Fig. 5) ; in the ordinary chlorophyll-grains they arise in the interior in larger or smaller numbers. They are at first visible as points, gradually increase in size, and finally may so completely fill up the space of the chlorophyll-grain that the green substance is represented only by a fine coating on the mature starch-grain ; even this coating may, under certain circum- stances, disappear (as in old yellow leaves of Pisiun sativum, Nicotiana), and the starch-contents then lie in the cell (destitute of protoplasm) in the place of the chlorophyll-grains. Sometimes drops of oil also form in the interior of the chloro- phyll-substance {e. g. in the bands of Spirogyra) ; and here and there granular contents of an unknown nature are observed. All these structures which arise in the chlorophyll-bodies are, however, not constant portions of them ; their appearance and disappearance depend entirely on the light, temperature, and on other circum- stances ; the appearance of the chlorophyll-bodies themselves is also bound up with these conditions of life, to a description of which we shall not recur till Book III, where it will be shown that chlorophyll is one of the most important elementary structures, and that its contents are especially its products of assimilation. The THE CHLOROPHYLL-BODIES. 47 consideration of these and of numerous other purely physiological properties of chlorophyll must be deferred till then. Sooner or later, in the normal course of things, the chlorophyll-bodies are again absorbed ; this occurs in the most conspi- cuous manner at the time when the leaves of the higher plants are preparing for their fall; for instance, in the case of our native trees and shrubs, in the autumn. Here the whole mass of protoplasm, — and with it the chlorophyll-bodies from the cells of the leaves destined to fall, — is absorbed, and transferred to the perennial parts; the appearances which then present themselves are very different ; but finally there Fig. 46, — Transverse section tliroutrh the \>i3.( oi Sclacrinella inaquatifolia (X550). A in the middle, />'at the margin ; ch the grains of chlorophyll ; visib in them arc points, the small granules of starch ; en the lower epidermis eo the upper epidermis ; / the air-conducting intercellular space ; sp stomata. Fig. 47.— Chlorophyll-grains ol Funaria hygr07ttetrica (X550). A cell of a mature leaf, seen from the surface; the parietal chlo- rophyll-grains lie in a layer of protoplasm, in which the nucleus is also imbedded; the chlorophyll-grains contain starch grains (teft white). B single grains of chlorophyll containing starch ; a a young one, b an older one, b' and b" grains in the act of division ; c, d, e old chlorophyll-grains, the starch granules of which take up the space of the chloropliyll ; f a young chlorophyll-grain swollen up in water ; s; the same after longer action of the water ; the chlorophyll is destroyed, the starch-granules remaining behind. remain in the cells filled with water and often containing pointed crystals, a number of yellow glittering granules which have no similarity to chlorophyll ; if the falhng leaves are red, this depends on a substance dissolved in the sap; but in this case also the yellow granules are to be found. The presence of chlorophyll in tissues is not always to be recognised by the naked eye in the colouring of the organs. Sometimes the cells that possess chlorophyll themselves contain a red sap ; in other cases the green tissue of the leaves is covered by an epidermis provided with red sap (young plants of Atriplex hortensis) ; in this case, if the coloured epidermis be removed, the green tissue may be readily reco":nised. But in Al^se and Lichens we find that the chlorophyll- 4b MORPHOLOGY OF THE CELL. body of the cell itself contains, in addition to the green colouring matter, a red, blue, or yellow substance soluble in water ; the fresh chlorophyll-body appears then, by the admixture of the chlorophyll contained in it with these substances, verdigris- green (Oscillatoria, Peltigera canina, &c. ), a fine red (Florideae), or brown (Fucus, Laminaria saccharina), or buff (Diatomacese). (Cf. Book II. Algae). From this are to be distinguished those processes in which the originally green chlorophyll grains assume a red or yellow colour from transformation of their colouring material ; these, in reference to their physiological bearings, I have designated a degradation of chlorophyll. Thus the green bodies in the walls of the antheridia of Mosses and Characeae become, at the time of fertilisation, of a beautiful red; in ripening fruits {Lycium barbanwi, Solanwn pseiido-capsicwn, &c.), the change of colour from green to yellow and red depends also on a similar loss of colour of the chlorophyll-grains, accompanied by a breaking up into angular forms with two or three points (Kraus, /. c). Nearly related to the grains of chlorophyll are the vehicles of the yellow colouring materials to which many petals owe their yellow colouring {e.g. Cucurbita). The occasional blue {Tilland- sia amcetia) or brown and violet {Orchis Mon'o) bodies, are much further removed from this type, although they also have a basis similar to protoplasm which is tinged by a colouring material, in these cases soluble in water. (a) The Substance of the Chlorophyll-bodies is, irrespectively of the contents re- ferred to, destitute of those fine granules which are so generally distributed through colourless motile protoplasm ; in spite of their sharply defined form, they are very soft, and greasy M'hen crushed ; when they come into contact with pure water, vacuoli are formed, which at last, from their great distension, burst through the green substance as hyaline bladders ; young grains of chlorophyll may thus become converted into delicate bladders, in which the grains of starch remain ; old grains have a greater consistence. After extraction of the green colouring matter out of true chlorophyll-bodies, e.g. the bands of Spirogyra or grains of Allium Cepa, the remaining colourless ground-substance possesses greater power of resistance, is coagulated, and shows all the reactions of proto- plasm already mentioned. (b) The Origin of the Chlorophyll-bodies has, at present, only been directly observed in the granular forms; it can to some extent be compared with the process of free cell-formation; around given centres of formation within the protoplasm extremely small portions of it collect, forming sharply defined masses ; if the centres of forma- tion are at a considerable distance from one another, the chlorophyll-grains become round (as in hairs of Cucurbita) ; but if they lie close to one another and the grains are large, they are at first polygonal, as if they had been flattened against one another by pressure. The process then gives somewhat the same impression as the formation of numerous small zoospores in a sporangium of Achlya (Fig. 9, A) ; only that in this latter case colourless protoplasm always continues to lie between the green portion (parietal chlorophyll-grains of the leaves of Phanerogams). If a mass of pro- toplasm collects around the central nucleus during the formation of chlorophyll, the grains are often formed in its neighbourhood ; they may then revolve with the circu- lating protoplasm in the cell, or afterwards assume definite positions. In the filamentous Algae with apical growth {e. g. Vaucheria, Bryopsis), they form in the colourless protoplasm-body of the growing end of the sac, and then remain applied to the wall. In ripe spores of Osmunda regalis the chlorophyll surrounds the nucleus in the form of amorphous cloudy masses, which, however, separate on germination as oval grains, at first weakly afterwards sharply defined (Kny). In the chlorophyll-forming cells of the embryo-leaves of Phanerogams (cotyledons of the sunflower, primordial leaves of CRYSTALLOIDS. 49 Phaseolus, buds of the tubers of Helianthus tuberosus, &c.) a definitely formed hyaline protoplasm devoid of granules is to be observed, close to the cell-wall, which, as it developes, forms the grains of chlorophyll ; here the appearance is sometimes pre- sented as if the mass were cut up into polyhedral pieces. The formation of the grains of chlorophyll is not always contemporaneous with that of its colouring matter ; they may be at first colourless (as in Vaucheria or Bryopsis, according to Hofmeister) or yellow (in the case of leaves of Monocotyledons or Dicotyledons imperfectly exposed to light, or in the process of development), and may afterwards become green ; in the cotyledons of Coniferae the green colour appears contemporaneously with their origin even in the dark when the temperature is sufficiently high, as also in Ferns. The grains of chlorophyll, after assuming their green colour, grow by intussusception to many times their original size ; if they are parietal, their growth in length and breadth is gene- rally proportional to that of the corresponding piece of the cell-wall and of the proto- plasm-body in which they lie. But if the growth of the cell is very considerable, the growing parietal chlorophyll-grains divide ; this occurs by bipartition, a constriction arising which always penetrates more deeply in a direction vertical to the longest diameter, until the grain at length breaks up into two usually equal secondary grains. If it contained small grains of starch before the division, these arrange themselves round the centres of the newly formed grains. These processes are inferred from the increase of the number of grains on the one hand, and from the frequent occurrence of biscuit- shaped constricted forms on the other hand. After this bipartition of the chlorophyll- grains had been discovered by Niigeli in Nitella, Bryopsis, Valonia, and in prothallia, it was subsequently noticed in all the families of Cryptogams which form chlorophyll ; among Phanerogams also it appears widely distributed ; it was discovered by Sanio in Peperomia and Ficaria, subsequently by Kny in Ceratophyllum, Myriophyllum, Elodea, Utricularia, Sambucus, Impatiens, &c. In cells of the prothallium of Osmunda exposed to feeble light and containing but little chlorophyll, Kny states that moniliform rows of chlorophyll-grains arise by repeated bipartition, which, like the chains of cells of Nostoc, continue to elongate by intercalary divisions ; a branching of the rows takes place here also, in a manner similar to that which occurs in Nostoc ; single grains of chlorophyll increase' in size transversely, and produce branch-rows by division. (c) With reference to the Internal Structure of the chlorophyll-bodies, scarcely anything else can be said than that their outer layer often appears denser, and that the proportion of water in the substance increases towards the interior the cohesion decreasing, as is apparent from the formation of vacuoli. A perceptible differentiation into lamellae of different density crossing one another has, at present, only been once observed in old chlorophyll-grains of Bryopsis plumosa (Rosanoff). Sect. 7. Cry stalloids \ — A portion of the protoplasmic substance of the cells sometimes assumes crystalline forms ; bodies are formed which, bounded by plane surfaces and sharp edges and angles, possess an illusory resemblance to true crystals, even in their behaviour to polarised hght; on the other hand they are essentially distinguished from them by the action of external agents, and at the same time present significant resemblances to organised parts of cells. It is therefore legitimate to distinguish them by the term Crystalloids proposed by ^ Hartig, Bot. Zeitg. p. 262, 1856. — Radlkofer, Ueber die Krystalle proteinartiger Korper pflanzlichen und thierischen Ursprungs, Leipzig 1859. — Maschke, Bot. Zeitg. p. 409, 1859. — Cohn, Ueber Proteinkrystalle in den Kartoffeln, in the thirty-seventh Jahresbericht der Schlesischen Gesell- schaft fur vaterliind. Cultur, 1858, Breslau — N.'igeli, Sitzungsberichte der k. bayer. Akademie der Wissenschaften, p. 283, 1862. — Cramer, Das Rhodospermin (in the seventh volume of the Viertel- jahrsschrift der naturfoisch. Gesellschaft in Ziirich). — J. Klein, Flora, No. 11, 1871. E 50 MORPHOLOGY OF THE CELL. Nageli. They are usually colourless, but sometimes act as vehicles of colouring matters (not green) which may be removed from them. Their collective mass shows all the essential reactions of protoplasm, its power of coagulation and of taking up colouring matters, the yellow reaction with potash after treatment by nitric acid, as well as that with iodine. The solubility of different crystalloids varies greatly, as is generally the case with albuminoids. They are capable of imbibing water, and swell up enormously under the influence of certain solutions ; their outer layer possesses greater power of resistance than the inner more watery mass. Those crystalloids which have been most carefully examined consist of a mixture of two kinds of materials of different solubility ; the two are so combined that when the more soluble is slowly removed, the less soluble remains as a skeleton (Nageli). Their form is very different in different plants; they appear as cubes, tetra- hedra, octohedra, rhombohedra, and in other forms ; usually, however, their crystallo- graphic characters cannot be exactly defined, a consequence of their small size and of the inconstancy of their angles. In quickly growing organs of phanerogamic plants they are known only in Lathrcea squamaria ; more commonly they form in cells where large quantities of reserve-materials are collected which are only turned to use at a later period; the crystalloids themselves appear to be a form of protoplasmic structure especially adapted for a dormant condition (as potato-tubers, many oily seeds) ; they are seldom found in cells which contain sap (potato-tubers), more often in sapless, and especially in oily seeds. Crystalloids containing colouring matters are found in the petals and fruits. Sometimes they are formed only after the action of alcohol or a solution of sodium-chloride on the plants externally or internally (Rhodospermine). The crystalloids of potato-tubers are imbedded in the protoplasm ; those that are widely distributed in the tissues of LathrcBa squamaria are contained in great numbers in the interior of the nucleus; those found in oily seeds are generally enclosed in grains of aleurone. The Crystalloids discovered by Cohn in the tubers of the potato are convenient for observation ; they are found very abundantly in some kinds, in others less frequently, in the parenchyma- cells beneath the skin which contain but little starch, but tolerably deep in the tissue ; they lie enclosed in the protoplasm. Generally they are in the form of cubes (less often of derivative forms, as tetrahedra) of the most perfect form. Those found by Radlkofer in the nuclei of LathrcBa squamaria lie together in great quantities within each nucleus ; they have the form of thin square rectangular plates ; sometimes they appear to have rhombic or trapezoid forms ; Radlkofer thinks it most probable that they belong to the rhombic system. These crystalloids present themselves immediately to observation, and their relation to their environment is at once clear. The case is different with the crystalloids of oily seeds enclosed in grains of aleurone ; I shall recur to their properties, and will only mention here that the crystalloids of the brazil-nut are obtained in quantity by washing out the crushed oily parenchyma by oil or ether, the crystalloids settling down in the form of fine meal ; in sections through the tissue but little can be clearly made out. They were carefully investigated in the isolated state by Nageli; according to the manner in which they are seen they appear rhombohedral, octohedral, or tabular ; but it is uncertain whether they belong to the hexagonal or the klino-rhombic system. Dry crystalloids placed in water alter their angles about 2° or 3° ; in solution of potash they swell strongly and then alter their angles 15° or 16°. By weak acids and dilute glycerine a substance is extracted, and a CRYSTALLOIDS. 51 weak skeleton with firmer skin remains behind. The crystalloids in the endosperm -cells of Ricinus communis are, like all crystalloids, insoluble in water, and are very conspi- cuous when thin sections of the tissue are laid in water, which destroys the structures surrounding the crystalloid, and sets it free. They frequently take the form of octo- hedra or tetrahedra, less frequently of rhombohedra ; but the system is not certainly determined. The crystalloids of colouring matter were first detected by Nageli in an imperfect form in the petals of Viola tricolor and Orchis, better developed in the dried fruits of Solarium americanum ; in the latter case they form in the large cells of the flesh of the fruit clusters of a deep violet colour ; the separate crystalloids are thin plates, single regular rhombs, often with truncated angles, &c. According to Nageli it does not admit of a doubt that the crystalline form is the rhombic prism in a very abbre- viated tabular shape ; the hexagonal tables are composed of six simple ones. In pure water they remain unchanged ; alcohol extracts the colouring matter, as also do dilute acids ; both leave, after long treatment, a verv weak skeleton which is capable of swell- ing, while the whole crystal does not swell ; Nageli states that the crystalloid consists of a very small quantity of albuminous and a large quantity of another substance, with some colouring matter. Crystalloids of albuminous substance have also been found in red marine Algae (Florideae) and in one Fungus. Cramer observed the first case of this kind ; in specimens of the Floridea Bornetia seciindijlora which had lain a long while in solution of sodium- chloride, as well as in specimens prepared in alcohol of Callithamnion caudatum and semi- nudum he found hexagonal plates and prisms with all the properties of crystalloids, and coloured red by the expelled colouring matters of the Algae. They were found in the vegetative cells as well as in the spores. In sodium-chloride preparations of Bornetia octohedral crystalloids were found also, apparently belonging to the klino-rhomblc system ; they were colourless. In living plants of the same Alga, Cohn also dicovered colourless octohedral crystalloids which absorb the red colouring matter expelled from the pigment-grains. Within and without the cells of Ceramium rubrum preserved in sea-water with glycerine, klino-rhombic prisms formed, coloured red by the expelled pigment; they are clearly similar to the hexagonal crystalloids first observed by Cramer which appeared only after death, while the colourless octohedra are to be found in the living cells. Finally, in dried specimens of other Florideae, Griffithsia harhata, G. nea- politana, Gongoceros pellucidum, Callithamnion seminudum, Klein observed colourless, crystalloids of a different form. These formations may all be comprised in the name first given by Cramer, — Rhodospermine. In the sporangiferous filaments of'Pilobolus, Klein found also colourless octohedra of tolerably regular structure with the properties of crystalloids described above. Sect. 8. Grains of Aleurone (Proteine-grains^). — The reservoirs of ripe seeds, i. e. the endosperm and the cotyledons of the embryo, always contain ^ These structures were discovered by Hartig (Bot. Zeitg. p. 881, 1855), and described in detail but imperfectly (ibid. p. 257, 1856); further observations were furnished by Holle (Neues Jahrb. der Pharmacie, Bd. X, 1858) and Maschke (Bot. Zeitg. 1859). All these observations left undecided the relationship of the grains to the surrounding matrix ; it appeared in particular to be assumed that in oily seeds the latter consists of oil only. In the first and second editions of this book I op- posed this view, and pointed out that the matrix in the cells of oily seeds consists of a mixture of oil and albuminoids, or rather, of a very oily protoplasm ; on the other hand I fell into the error, partly in consequence of the use of diluted ether, of considering the aleurone-grains them- selves as a compound of albuminoids and oil. This error has been refuted by Dr. Pfeffer's recent researches. This very careful investigation was commenced in the Wiirzburg laboratory, where I had the opportunity of seeing numerous preparations which were decisive as to the principal question. Dr. Pfeffer had the kindness to communicate to me, before going to press, a detailed E 2 53 MORPHOLOGY OF THE CELL, considerable quantities of albuminoids together with starch and oily matter. If they contain much starch, as in the grasses, Phaseolus, Vicia, the oak, horse-chestnut, Spanish chestnut, &c., the albuminoid, which only contains very little oily matter, occupies the interstices; it consists of small or even minute granules, as shown in Fig. 48. In oily seeds, on the other hand, granular structures of roundish or angular form (Fig. 49) are found in place of the grains of starch, which also are sometimes not dissimilar to starch-grains in their appearance, surrounded by a more or less Fig. 48. — Cells of a very thin section through a cotyledon of the embryo in a ripe seed o{ Pisum sativum; the large concentrically stratified grains st are starch-grains (cut through) ; the small granules a are aleurone, consisting principally of legumine with a little oily matter; z'the inter- cellular spaces. Fig. 49.— Cells from the cotyledon of the ripe seed oi Lupin us varius ; A in alcohol containing iodine ; B after destruction of the grains by sulphuric acid ; z the cell-wall ; / the protoplasmic principal mass, containing but little oily matter ; y the aleurone- grains ; a drops of oil expelled from the principal mass by action of the sulphuric acid ; m empty spaces from which the aleurone- grains have been dissolved (x8oo). homogeneous matrix, which, as closer investigation shows, consists, according to the oiliness of the seed, of more or less oil combined with albuminoids. The grains themselves, on the other hand, consist, independently of certain enclosed matters, of albuminoids. In the grains of aleurone the albuminoid must be distinguished from the en- closed substances. The latter are either crystals of calcium oxalate, or they are non-crystalline, roundish, or clustered granules. Globoids. These are a double calcium and magnesium phosphate, in which the latter is greatly in excess. account of his labours for my use here ; what I have said above follows his views tolerably closely. There is, in fact, scarcely any one who combines, to the same extent as Dr. Pfeffer, the necessary skill in microscopic work with the chemical knowledge required in this excessively difficult work. I GRAINS OF ALEURONE. 53 Fig. 50.— Cells from the &ndo%ptrm oi Rtcinus com- mtifiis (xSoo). A fresh, in thick glycerine, B in dilute glycerine, C wanned in glycerine, D after treat- ment with alcohol and iodine, the aleurone grains are destroyed by sulphuric acid, the albuminoid remaining behind as a net-work. In the aleurone-grains the globoid may be recognised, and in (B, C) the crj'stal- loid. The whole albuminous mass (proteine) is now amorphous, and in that case not doubly refractive; or the greater part is developed into the shape of a crystalloid (sect. 7), which, together with the enclosed substances already named, is surrounded by a sparse amorphous envelope, constituting, together with the former, the grain of aleurone. (Fig. 50.) The crystalloids are all insoluble in water; neither alcohol nor water extracts anything from them. The grains destitute of crystalloids dis- solve in water entirely (as Pseonia), partially (as Lupinus), or not all (as Cynoglossum). But all dissolve completely in water containing only a trace of potash. With careful treatment there always remains behind a membrane surround- ing the grain, which behaves like coagulated albumen ; but it may be a yet unknown protein- aceous substance. With grains of aleurone con- taining crystalloids, there remains, after careful solution, a similar membrane, but the crystalloid itself also leaves behind a similar one ; this oc- curs also in the solution of globoids in acetic or hydrochloric acid, and reminds one of the similar behaviour of true crystals of calcium oxalate. The crystalline enclosures of calcium oxalate occur as clusters, clearly recog- nisable crystals, and needles, but are nevertheless not commonly met with. The globoids, on the other hand, are never absent from aleurone-grains ; when they occur together with crystals, it is almost always the case that globoids only are enclosed in one cell, crystals in another (as in Silybum uiarianum, and in all Umbel- liferse that have been examined). There occur how^ever exceptions ; and in Vth's vim/era it is even the case that a globoid forms itself around a crystal or a cluster of crystals. The globoids are soluble in all inorganic acids, and in acetic, oxalic, and tartaric acid, but not in dilute potash. The globoids, like the crystals, may occur in an aleurone-grain singly or in numbers ; in the latter case they are small, and the globoid-grains even too minute to be measured, but are then present in enormous numbers in one grain [e.g. Lupinus luteus, L. poljphjllus, Delphinium Requieni, &c.). Large globoids around crystals occur singly, the largest in the grape-vine. Pfeffer found crystals accom- panying crystalloids only in JEthusa Cynapium. The enclosed substances especially are most often absent from very small aleurone-grains. In some seeds there is in each cell one aleurone-grain distinguished from the others by its size (Solitar of Hartig), both when crystalloids are present and when they are absent (Elaeis, IMyristica, Vitis, Lupinus luteus) ; and a larger grain of this kind may also be distinguished by its enclosed substances. Thus in Lupinus luteus it possesses a tabular crystal ; the others only small and numerous globoids. In Silybum a cluster of crystals lies in one large grain, in the others a number of needle-shaped crystals. In other cases the enclosed substances are also similar, r^ MORPHOLOGY OF THE CELL. as is always the case with the globoids, which are especkilly larger in the large grain. The crystalloids are tolerably widely distributed, although the greater number of seeds are destitute of them. They are not, however, characteristic of families, but may be present or absent in members of the same family ; thus among palms, Sabal Adansonii is without, Elaeis guineensis provided with crystalloids ; in the same manner all Umbelliferse which have been investigated are deficient, with the excep- tion of JEthusa Cynapiuni, &c. In other cases ail seeds of the same family appear to contain crystalloids, as in the Euphorbiacese, among which in particular Ricinus offered the first example of fine crystalloids in the grains of aleurone. The matrix which surrounds the grains of aleurone in oily seeds is, as has been mentioned, always a mixture of oily matter and albuminoids, but the latter may be in very small quantities. Thus even in Ricinus and the brazil-nut, where the matrix appears to consist entirely of oily matter, the albuminous constituent is yet quite discernible, as is shown in Fig. 50, D; Pfeffer succeeded most readily by extracting with an alcoholic solution of calomel, and then colouring with aniline-blue dissolved in water. The matrix may be considered as the proto- plasmic mass of the cell, in which the water is replaced on drying by oil. But in addition it contains in the whole mass, not only insoluble albuminoids, but other substances also which are soluble in water containing potash in solution. This composition of the matrix, together with the solubility of the amorphous mass of the aleurone-grains in water, are the cause of the complete loss of form which the cell-contents of oily seeds immediately undergo in water (sections under the microscope). In order to recognise their structure it is necessary to place fresh sections in thick glycerine, or in alcoholic solution of calomel, in concentrated sulphuric acid, or in oil. The oily matter may besides separate out of the matrix in crystals, as Pfeffer has observed in the brazil-nut, Elaeis guineensis^ and the nutmeg. To the above may be added from Pfeffer's communication some explanations con- cerning the more difficult points. (a) The Mass of the grains of aleurone always consists to by far the greater extent of proteinaceous substances, with which only very small quantities of other vegetable sub- stances are usually or always mixed; these, nevertheless, are difficult of detection. This conclusion rests essentially on the following grounds : — all aleurone-grains are absolutely insoluble in alcohol, ether, benzol, and chloroform (that I formerly con- sidered them soluble in ether, was the result, as Pfeffer showed, of the ether con- taining a small quantity of water). All these reagents would dissolve oil (alcohol dissolves also glucose), if it were present, and would consequently also alter the appearance of the grain. There are grains insoluble in water {e.g. Cynoglossum officinale')', those soluble in water ^ pass over, on digestion with absolute alcohol in which corrosive sublimate has been dissolved, into an insoluble mercury-compound, out of which water dissolves nothing worth notice. Gum, pectinaceous substances, cane-sugar, and dextrine, do not, under this treatment, yield an insoluble compound. Of all widely distributed vegetable substances, only a proteinaceous substance can be mentioned which behaves in this manner towards corrosive sublimate. This may be recognised by reactions, of * On the causes of the solubility in water Pfeffer's exhaustive treatise which is immediately to ippear must be referred to. \ GRAINS OF ALEURONE. ^^ which the best in this case is boiling the mercury-compound with water. The modifi- cation of proteinaceous substance insoluble in dilute acids and alkalies is thus formed. (b) In proving that the aleurone-grains of oily seeds contain no oil, we have already seen that it must be present in the matrix. The doubt which arises from the first glance at sections of oily seeds, whether the great mass of oily matter can find space in the interstices of the grains, can be settled by calculation ; for if spheres (the grains may here be considered in this light) are so placed that they are enclosed in any number of equal cubes forming part of one great cube, 47*6 p. c. of the cavity remains; and if the spheres are distant from one another only about one-third of their radius, 697 p.c. ofthe cavity is left, and this is more than is sufficient in oily seeds to take up the oily matter. Immediate proof can be given of the existence of the oil in seeds which contain a certain amount of it by the appearance presented by the observation of dry sections ; if benzol is then added the intermediate masses are seen to disappear, while small quanti- ties of albuminoids always remain. Treated with alcoholic tincture of alkanet the matrix becomes of a deep blood-red colour if it contains a considerable amount of oil ; if the oily constituents of the seed are very small, the evidence cannot be obtained in this manner. If the oil is extracted from the sections of seeds by alcohol, and the grains of proteine then removed by solution of potash, a net-work remains behind in which the grains are replaced by cavities; on addition of acetic acid and iodine the net-work assumes a yellow-brown colour (Fig. 49, JS; 50, D). In most seeds this net-work is very beautiful, comparable, to a certain extent, to parenchymatous tissue ; in extremely oily seeds it often breaks up into fragments, the nucleus lying in it like a shrivelled ball. The threads of the net-work are composed of the insoluble proteinaceous materials of the matrix and of the enveloping membranes of the grains of proteine ; although a net-work may exist without the latter if grains have disappeared. (c) The Crystalloids of the grains of aleurone are, as has been said, insoluble in water ; they may therefore easily be isolated by treatment of fresh sections with water, the amorphous masses of aleurone dissolving, and the rest of the cell-contents being de- stroyed ; they then show all the reactions and the different forms of the crystalloids mentioned in sect. 7. But that they consist of two proteinaceous substances, and grow from within by intussusception, Pfeffer thinks he has good grounds for doubting. (d) If sections of the endosperm of the peony are treated with alcohol containing a small quantity of sulphuric acid, and if, after washing, they are placed in water, the sub- stance of the grains of aleurone (not containing crystalloids) is seen to be distinctly stratified ; but only a few firm and soft layers occur, the inner part of the mass is amorphous. PfeflTer's work should also be consulted here. (e) The De'velopment of the grains of aleurone is thus described by the investigator already so often named. — Their formation commences when the seeds attain the last condition of ripeness and the funiculus begins to become sapless ; in the very turbid emulsion which now fills the cells, the enclosed substances, especially the globoids, are already formed ; they are, even if not quite perfect, neaWy fully developed. Then, as the seed loses its water, the formation of mucilaginous masses commences, consisting of proteinaceous substances, which mostly already surround enclosed substances ; these mucilaginous bodies, usually nearly globular, continue to grow; their mutual distance thus decreases, and at -last the separation is complete ; the grains of proteine, still con- sisting of mucilaginous substance, are separated from the still turbid matrix, which be- comes clearer and clearer, while the seed becomes drier. Thus the previously spherical or ellipsoidal grains become more or less polyhedral, especially, as may easily be observed, in a few oily seeds which have generally bilt little matrix {e.g. Lupinus). While the formation of the grains of aleurone is beginning, the protoplasmic mass of the cell is only to be detected with difficulty in the turbid cell-contents ; yet, on remov- ing the oily matter by alcohol, it may be shown that it is present in the normal form ; sometimes in the copious matrix of some seeds the dried strings of protoplasm may be 56 MORPHOLOGY OF THE CELL. afterwards seen still extended. In Lup'inus luteus the crystal of calcium oxalate, which is afterwards enclosed by the largest grain, is already present in the cell-sap before the formation of the grains of proteine. Pfeffer was able to follow the development of the grains with remarkable ease in the peony ; in this case the seed is still, even when it has attained its full size, filled with large starch-grains, which become changed into oil only when fully ripe ; or even when the seed has been removed from the carpel before the reserve-materials have been completely introduced. The starch is not always, however, completely changed into oily matter. If the starch-grains in the seeds of the peony are imagined to be not completely transformed, and the intermediate mass, almost devoid of oily matter but very rich in proteinaceous substances, forms very small grains of pro- teine, we have what does actually occur in Phaseolus and in other seeds extremely rich in starch. There are, however, also seeds in which proteine and starch- grains occur in nearly equal quantities, but then always associated with oily matter. No argument can be founded on the turbid condition of the cell-contents and the softness of the growing grains of proteine, with respect to the manner of growth. Nevertheless it can mostly be affirmed with regard to ripe grains, that those situated farther towards the inside are softer, and that, consequently, on the application of very dilute reagents, they dissolve from within outwards. Different facts appear, never- theless, to show that no growth takes place by intussusception, as with the grains of starch. The origin of the grains of aleurone is simply a dissociation, which arises from loss of water by the seed, and, on germination, the cell-contents first of all returns more or less completely to the condition of a union of the matrix with the substance of the grains of proteine. Pfeffer followed out the formation of the crystalloids in Ricinus and Euphorbia sege- tum ; they arise nearly simultaneously with the globoids, at a rather early period, and both grow gradually, while the turbidity of the cell-contents at first somewhat increases. They mostly lie, even at an early stage, quite close to one another, but completely surrounded by the turbid mass ; the vacuoli which Gris (Recherches sur la germination, PI. I, Figs. 10-13) figures are the result of the very slight commencement of disorganisation of the cell-contents. The crystalloids are from the first sharp-edged, and, as soon as their size permits their form to be recognised, it agrees with that of the mature crystal- loids. The envelopment of crystalloid and globoid by amorphous coatings follows first, if the crystalloids are mature and the drying of the seed has commenced. With germination the crystalloids dissolve as well from without as from within, even after the envelope has first disappeared ; the enveloping membranes are for a time per- sistent, but gradually become invisible. The globoids also dissolve (no doubt in conse- quence of the acid reaction which the tissue assumes), and in the case of old seeds from the outside inwards. The grains of aleurone destitute of crystalloids next swell up and resume, on the germination of the seed, the form which they possessed in ripe but still watery seeds ; they then begin to mix with the substance of the matrix ; and thus sometimes a definite dissolution can be followed from without inwards ; but they often coalesce as mucilaginous masses. These changes occur with the first signs of ger- mination in the embryo ; formation of starch then also takes place simultaneously in the contents of the cells. Sect. 9. Starch Grains ^ — Plants which vegetate . under favourable cir- cumstances produce by assimilation a larger quantity of new formative organ- isable substance than they require or can employ at the time for the growth of the cells. These materials are stored up in some form or other in the cells them- ^ Niigeli, Die St"irkek6i-ner, in Pflanzenphys. Untersuchungen, Heft II, and Sitzungsber. der k. bayer. Akad. der Wissenschaften, 1863.— Sachs, Handbuch der Exp. Phys. Leipzig 1865, § 107. What I give here is essentially after Nageli's work. STARCH GRAINS. 57 selves, and only undergo conversion later. It has already been shown in the pre- ceding paragraphs how this happens with albuminous protoplasm-forming materials, and with oily matter. In far larger quantities another substance, in the most eminent sense organisable, Starch, is formed beforehand and stored up in an organised form in anticipation of future use. The starch always appears in an organised form as solid grains having a concentrically stratified structure, which arise at first as minute masses in the protoplasm, and continue to grow while lying in it ; if at a subsequent period they reach the cell-sap and cease to remain in contact with the protoplasm which nourishes them, their growth stops ^ Every grain of starch con- sists of starch, water, and of very small quantities of mineral substances (ash). The first is a carbo-hydrate of the same percentage composition as cellulose, to which it bears the greatest similarity of all known substances in chemical and morphological properties. The starch, however, occurs in each grain in two modifications : one more easily soluble, which assumes a beautiful blue colour with solution of iodine and addition of water (Granulose), and the other less easily soluble, which in its reactions comes nearer to cellulose (Starch-cellulose). At every point of a grain of starch both materials occur together ; if the granulose is extracted, the cellulose remains behind as a skeleton ; this skeleton shows the internal organisation of the whole grain, but is less dense or poorer in substance, and its weight amounts to only a small fraction of the whole grain (about 2-6 p. c). Since, then, the granulose greatly preponderates, and is present at every point of the grain, the grain shows, in the reaction with iodine, the blue granulose-colouring throughout its whole extent. The starch-grains have always rounded forms, and their internal organisation has reference to a centre of formation lying within themselves; the young small bodies appear to he always spherical ; but since their growth is scarcely ever uniform, their form changes into ovoid, lenticular, rounded polyhedral, &c. The internal organisation of the starch-grain is especially recognised by the diff'erent distribution of water in it (water of organisation). Every point of the grain contains water in addition to granulose and cellulose.- Most usually the amount of water increases from without inwards, and attains its maximum at a fixed point in the interior. With the increase in the proportion of water, the cohesion and density decrease, as also the index of refraction, on which partly depends the power of perceiving these properties. This change in the proportion of water is not, however, constant, but intermittent. To the outermost least watery layer succeeds a sharply defined watery layer, to this again a less watery one, &c., until the innermost less watery denser layer surrounds finally a very watery part, the nucleus. All the layers of a grain are disposed around this nucleus as their common centre, but every layer is not continuously developed around the whole nucleus ; in small spherical grains with few layers this is always the case, but when their number increases with growth, the number of layers increases most in * According to Hofmeister, the starch-grains in the milk-sap of Euphorbia appear to form an exception ; nothing however is known about their development ; the milk-sap (latex) always contains protoplasm-forming substances, albuminoids, which perhaps here also take part in the pro- duction of the starch-grains. 5o MORPHOLOGY OF THE CELL. the direction of most vigorous growth, which is continuous in a straight or curved line with the direction of least vigorous growth ; this line is called the axis of the grain ; it always passes through the nucleus. The growth of the grains of starch is accomplished exclusively by intussusception ; new particles of the formative material become intercalated between those already existing both in a radial and tangential direction, by which means the proportion of w^ater at particular places is at the same time changed. The youngest visible globular grains of starch consist of denser less watery substance ; in this is formed subsequently the central watery nucleus; in the latter a central part may become denser, and in this, when the increase in size has advanced sufficiently, a softer nucleus may again arise. It may however also happen, after a softer nucleus has arisen surrounded by a dense layer by differentiation of the original dense nucleus, that in the dense layer a new soft one may arise, and thus become split into two dense layers, the inner of which encloses the soft nucleus. The layers increase by deposition in thickness and circumference. When a layer has attained a definite thickness, it becomes differentiated by further growth into three layers. If it is a dense layer, watery substance becomes deposited in its middle, and there arises in the dense layer, which now splits into two lamellse, a less dense layer. But when a watery layer becomes sufficiendy thick, its middle lamella may become denser, and a new dense layer is formed between two lamellae of a less dense one. This process of splitting of the layers depends on their increase in thickness ; and since this itself is the most vigorous where the layers are intersected by the longer branch of the axis of growth, the splittings, i. e. the new formations of layers, ensue there most abundantly, least often on the opposite side of the nucleus, and may even entirely cease there. The layers of the more quickly growing side of the grain become, from bending round on the slowly growing side, constantly thinner, and finally disappear. Lenticular grains {e. g. in the endosperm of wheat) have a lenticular nucleus; their layers grow most quickly in the direction of the radii of a great circle concentric with it, and here most commonly split, the nucleus remaining central. If, on the other hand, the growth takes place in one direction {e. g. in the ovoid grains of the potato-tuber) the nucleus becomes eccentric, is further and further removed from the centre of gravity of the grain, and is in this case globular. In some ellipsoidal (in the cotyledons of peas and beans) or elongated grains, the nucleus is extended in the direction of the longest axis. It is very common for two nuclei to form in a small young grain; round each of them layers are formed, and the growth is strongest in the line of union. The distance of the nuclei from one another becomes continually greater ; thus a tension arises in the few common layers which surround both; this leads to the forma- tion of an inner fissure, which lies at right angles to the line of union of the two nuclei ; it is continued towards the outside, and the grain breaks up into two half-grains which may nevertheless adhere to one another. If this division occurs more often, perfectly compound grains arise, consisting of numerous secondary grains, the number of which may amount even to thousands {e. g. in the endosperm of Spinacia and Avena). Perfectly compound grains of from two to ten half-grains, with a mulberry-like STARCH GRAINS. 59 appearance, are extremely common in the parenchyma of quickly growing plants {e.g. seedlings of Phaseolus, stem of Cucurbita). Grains of this description are different in their origin from compound grains of the kind which occur in chlorophyll ; in this latter case a number of small grains exist in the first place, w^hich only touch and adhere to one another in consequence of increase of size. (Cf. Fig. 47, p. 47.) Partially compound starch-grains result when new nuclei and surrounding masses of layers are formed in one grain after each one has already formed several layers. The secondary grains appear therefore to be imbedded in the mass of- layers of the mother-grain. In this case also tension arises from the unequal growth of the common layers and of those belonging to each secondary grain, leading at length to the formation of fissures ; but these do not usually extend to the outside ; the secondary grains remain united. (a) T^he Groivth of grains of starch by intussusception must be inferred from the following considerations : — Supposing that the formation of layers occurs from without by deposition, grains would be found the outermost layer of which would be a watery one ; this, however, never occurs ; the outer- most layer is always the densest and least watery. According to this supposition the nucleus would also possess the properties of the youngest grains, M'hereas the nucleus is always soft, the youngest grains dense. The theory of apposition could only be brought in to explain the formation of the partially compound grains ; if we were to suppose that the common layers of a grain which is forming secondary grains had been subsequently de- posited around two or more previously iso- lated grains, then the common layers would have a different form, and the fissures in the interior of such grains remain unexplained. The theory of apposition, finally, is incom- petent to explain why, in secondary grains, the strongest growth always takes place in the line of union of their nuclei (Fig. 51). The possible hypothesis of a deposition of new layers from within would presuppose that the starch-grains were at least temporarily hollow bladders, w^hich has never been ob- served ; on this hypothesis, moreover, it can- not be explained how the phenomena arise which occur in the formation of 'half and secondary grains ; and the only hypothesis which can be accepted is growth by intussus- ception, namely in the direction of the sur- faces of the layers. The hypothesis of the growth of starch-grains by intussusception alone affords the simplest explanation of all phenomena ; and, after Nageli's researches, may be considered as a fully established fact. The formative material which penetrates from without into the grain once formed and there becomes deposited in the form of new particles of starch, is, of course, in solution ; but its chemical nature is not yet certainly known ; dissolved starch can never be found to exist in the plant, at least in those cells where active formation and growth of starch-grains has been observed. It is, however, probable that Fig. 51.— starch-grains from the tuber of a potato (xSoo). A an older simple grain; B a partially compound grain; C, D perfectly compound grains; E an older grain, the nucleus of which has divided ; a a very young grain, b an older grain ; c a still older grain with divided nucleus. iyiuis.iriiKji^\jKjx L/z- i nr^ \^ji,jul,. a solution of sugar contained in the protoplasm is the material out of which particles of starch are formed by further chemical and physical changes. The starch is easily changed into sugar by different agencies. From various facts i^e.g. the production of radial fissure-surfaces on drying), it must be concluded that the molecules of starch have not only a definite position in the direction of the radii, but are also arranged tan- gentially in a definite manner in each layer. A lamellar structure and the formation of areolae corresponding to this, appearing as a radial striation, has, however, been ob- served only occasionally and doubtfully. Growth by intussusception depends on the permeability of all parts of the grain to water and aqueous solutions. This again can only be explained by supposing that the substance of starch is not continuous, but consists of distinct invisibly minute particles, each of which possesses the power of attracting water, and surrounds itself with an aqueous envelope ; the particles of starch (molecules) are separated from one another by these aqueous envelopes ; the smaller the molecules in a given volume of a starch- grain, the more numerous are these envelopes, and the more watery the volume of starch under consideration. From this it results, on purely mechanical principles, that in this case the aqueous envelopes are thicker, that, on the other hand, as the mole- cules increase in size, they become thinner, and the molecules thus approach nearer one another. The watery layers therefore consist of small molecules which are sepa- rated by thick aqueous envelopes, the denser less watery layers of larger molecules with thinner envelopes. The internal organisation thus depends, in these cases, on a definite co-deposition of water and particles of starch ; the stratification of a starch- grain disappears, like that of a cell-membrane, as soon as the water is removed from it {e. g, by evaporation .or action of absolute alcohol, &c.), because the more w^atery layers then become similar to the less watery, and the difference of refractive power in the two ceases. In the same manner the stratification also disappears when the substance of the grain is rendered capable by chemical means (as weak solution of potash) of absorbing large quantities of water ; the denser layers absorb relatively more water ; they thus become similar to the more watery layers, and it is no longer possible to distinguish between them. Besides the abrupt differentiation of the proportion of water which is recognised in the form of stratification, there is also in every grain an increase from without inwards in the amount of water. This is partly ascertained by the refraction, partly by the regular decrease of cohesion from without inwards. If the water is removed from fresh starch- grains, they acquire cleavage-surfaces which cross the layers at right angles; in the interior a cavity is formed from which the fissures radiate ; these become narrower the further they penetrate outwardly ; they are widest* in the middle. From this it follows that on drying the greatest loss of water occurs in the interior, and that this regularly decreases towards the outside ; but it also follows at the same time that the cohesion of the layers is less in the tangential direction (at right angles to the cleavage-surfaces) than in the radial direction ; this points to the conclusion that within every layer the loss of water is greater in the tangential than in the radial direction. If the water be removed from a fresh starch-grain or from one saturated with water, it contracts ; the molecules contained in it approach one another when the layers of water between them become thinner. A similar change takes place if the granulose is removed from a grain ; the cellulose-skeleton of the grain which remains is, although saturated with water, much smaller than the intact grain. This possibly results from the fact that the molecules, now consisting only of cellulose, possess less attraction for water, and, having thinner envelopes, approach nearer; the cause may however also be that the number of molecules has diminished. (b) The Extraction of the Granulose of starch-grains, leaving behind a skeleton of cellulose, can be brought about in very different ways: — i. By maceration in saliva at an elevated temperature; in the starch of Canna indica the extraction, according to H. von IMohl, is slow at 35-40 C, but is completed in a few hours at 5o"-55 C. ; a lower STARCH GRAINS. 61 temperature suffices for wheat-starch, a higher is required for that of the potato. Nageli gives in general 4o°-47° G. 2. According to Melsens a similar extraction may also be effected by organic acids, diastase, and pepsin. 3. According to Nageli it can be accomplished also by very slow action of hydrochloric or sulphuric acid which has been so diluted with water that it does not cause the starch-grains to swell. 4. According to Franz Schulze, the granulose is extracted by a saturated solution of sodium chloride containing i p. c. of concentrated hydrochloric acid, at 60'' C. in two to four days ; the residuum, which does not perfectly show the organisation of the starch-grain, amounted, according to Dragendorff, to 5*7 p. c. in potato-starch, 2*3 p. c. in wheat-starch. These skeletons are not at all coloured by iodine (Nageli's preparation with sulphuric acid after one and a quarter year's extraction), or they become copper-red, and in places where the extraction was not perfect, bluish. They do not swell in boiling water. At 70° C. the whole starch-grain, according to Mohl, is dissolved in saliva : the ske- leton produced at 40°-55° G. is, however, not affected by saliva at 70°. Within the living cell the starch may be dissolved in very different ways ; pro- bably solution occurs mostly under the in- fluence of protoplasm or by the assistance of nitrogenous combinations in the cell-sap. Sometimes the solution begins, as in the extractions mentioned above, with the removal of the granulose, the cellulose re- maining behind ; but this often takes place only partially ; the extraction proceeds in single places from without inwards ; the ex- tracted places are coloured copper-red by aqueous iodine, the remaining mass blue ; then the grain breaks up into pieces, which finally are completely dissolved (as in the endosperm of germinating wheat, Fig. 52, 5). In other cases the solution begins also in particular places of the circumference ; the whole substance, however, gradually dissolves ; holes are formed, and finally the grain in these cases also breaks up into pieces (Zea Mais, Fig. 52, A). In the cotyledons of germinating beans, the so- lution of the ellipsoidal grains begins from within ; but before they break up into pieces the granulose is often so completely extracted that they assume with iodine a copper-red and in parts a bluish colour ; afterwards the whole is dissolved. In germinating potatoes and the root-stock of Canna lanuginosa, on the other hand, the solution of the grains proceeds from without inwards, removing layer after layer. Probably the same takes place here as when saliva is employed, whether the solvent acting slowly first extracts the granulose, or attacking it energetically dissolves the whole substance. Observations on germinating plants of the same species, developed at different temperatures, would possibly show corresponding differences. (c) Solubility, Snivelling. If grains of starch are crushed in cold water, a small Fig. 52. — A a cell of the endosperm of Zea Mai's, filled with crowded and therefore polygonal starch-grains : between the grains lie thin plates of dried fine-grained protoplasm ; small cavities and fissures are formed in the interior of the grains by drying ; a— ^ grains of starch from the endosperm of a germi- nating seed of maize ; B grains of starch (lenticular) from the endosperm of a germinating seed of wheat ; the commence- ment of the action of the solvent is shown by the more evi- dent appearance of stratification (x8oo). 6l MORPHOLOGY OF THE CELL. portion of the granulose is dissolved ; addition of iodine occasions precipitation of fine- grained blue pellicles ^ Starch-grains ground with fine sand give up an actual solution of granulose to cold water. Other fluids, as dilute acids, do not cause a solution of the starch, but rather a transformation into other substances (dextrine, dextrose), which then dissolve. Water of at least 55° C. causes swelling and formation of paste in the larger more watery starch-grains ; in smaller denser ones this begins, according to Nageli, at 65°. Heated in the dry state, at about 200° G. they are so changed that subsequent moistening causes swelling ; but the substance is by this means chemically changed ; it is transformed into dextrine. In the production of paste, the interior watery parts swell first, the outermost layer scarcely swells, it bursts and remains for a long time discernible by iodine as a pellicle, even after the breaking up of the inner parts into small particles. A similar action is occasioned by a weak cold solution of potash or soda ; the volume of a*grain may thus be increased one hundred and twenty-five fold, and so much fluid be absorbed that the swollen grain contains only 2-| per cent, of solid starch. Sect. 10. The Cell-sap. — The term Cell-sap may be understood in a wider or in a narrower sense. In the former sense it would express the collective mass of all fluids by which the cell-wall, the protoplasm-body, and all other organised structures of the cell are saturated, and would also embrace the fluids contained in the vacuoH of the protoplasm; in a narrower sense the latter only is ordinarily designated as cell-sap. In any case there are grounds for considering the compo- sition of the cell-sap as very variable, according as it has been imbibed by the protoplasm, the chlorophyll, the cell-wall, or the starch-grains of one and the same cell, or occurs as vacuole-fluid ; the latter may in general represent the reservoir out of which the organised absorbent parts of the cell supply their needs, but in which, on the other hand, the superfluous soluble products of assimilation and of transformation of material, and the food-materials that have been absorbed, also for a time collect. One constituent of the cell-sap, water, is always common to the vacuole-fluid and to that which saturates the organised structures. The share of the water of the cell-sap in the whole building-up of the cell has already been entered into sufficiently in detail. Its signification in the cell is a very manifold one ; it is at once the general solvent and the agent of transport of the food-materials within the cell ; the water itself enters in many ways into the chemical constitution of the substances produced in the plant ; its elements are essential for the production of assimilated substances ; for the formation of organised struc- tures, the cell-wall, the protoplasm-structures, and the starch-grains, it is indispen- sable (water of organisation) ; the growth of the whole cell-body depends imme- diately on the absorption of water, and on the accumulation of the cell-sap as vacuole-fluid (cf. Figs, i, 43, 44). The increase of size of rapidly growing cells is nearly proportional to the accumulation of the sap in them. The hydrostatic pressure which the vacuole-fluid exercises on the protoplasm-utricle and cell-wall co-operates in the conformation of the cell. The substances dissolved in the water of the cell-sap, partly salts absorbed from with- out, partly compounds produced in the plant itself by assimilation and transformation of ^ On the actual solubility of starch, see my remarks in my Ilandbuch der Experimental Physiologic, p. 410. THE CELL-SAP. 63 material, are, as such, not immediately the subject of morphological observation, to which we are for the time confining ourselves. Inuline ^ only, which is precipitated by the action of cold and desiccating agents from its solution in cell-sap in definite forms, and becomes visible in the interior of cells, need here be particularly mentioned. In the cell-sap of certain Algae (Acetabularia) and many Compositae (perhaps also in many other plants), Inuline, a substance closely related to starch and sugar, occurs. In sap obtained by pressure or boiling, it precipitates spontaneously after some time in the form of a white fine-grained pre- cipitate. From solutions it cry- stallises in the form of so-called sphere-crystals (Fig. 53, A), which consist of crystalline elements disposed in a radiate manner. Within the cells it may be made visible as a finely granular preci- pitate by drying or by rapid re- moval of water by means of al- cohol (Fig. 53, F). It is abun- dantly precipitated in the cells in the form of smaller sphere-crys- tals on dipping thin sections of the tissue in alcohol, becoming immediately visible on addition of water (Fig. 53, B). They are ob- tained much larger by laying whole Acetabularias or large pieces of tissues containing inuline (tubers and stems of Dahlia and Helianthus tuberosus) for a longer time in al- cohol or glycerine ; in the latter case a sphere-crystal very com- monly includes several cells of the tissue (Fig, 53, E), a proof that the crystalline arrangement is not necessarily destroyed by the cell-walls. Similar forms (as in Fig. 53, B) are formed when tissues containing inuline freeze, and they do not again become dis- solved in the cell-sap on thawing. Since the sphere-crystals consist of doubly refractive crystalline elements arranged radially, they show, under polarised light, the cross which occurs under such circumstances. They are not capable of swelling, are slowly dissolved in a large quantity of cold water, and quickly in a small quantity of warm water of from 50^-55° C. ; in solution of potash and nitric and hydrochloric acids they dissolve easily, the solu- tion always commencing from without ; by boiling in very dilute sulphuric or hydrochloric acid the inuline is immediately transform.ed into glucose. Solutions of iodine in alcohol or water penetrate into the fine crevices of the sphere-crystals, but occasion no special colour. Inuline-structures are easily and certainly recognised by these reactions, as Fig. S3.— Sphere-crystals of Inuline. A from an aqueous solution laid aside for 2i months ; at a the action of nitric acid is commencing. B cells of the root-tuber of Z)<7/j//« Wrtr/a^i'to; a thin section was placed for 24 hours in alcohol of 90 p.c, and was then dipped m water. C two cells with half sphere-crystals having their common centre in the middle of the separating cell-wall ; from an internode 8 mm. thick at the apex of an older plant of Helianthus titberosiis, which had remained for some time in alcohol. D fragment of a sphere-crystal. E a large sphere-crystal including several cells, from a larger piece of the tuber of Heiiauthus tuberosus, after lying for a longer time in alcohol. F Inuline after evaporation of the water from a thin section from the tuber of Helianthtis tuberosus (X500; E not so nmch). ^ Sachs, Bot. Zeitg. p. 77, 1864.— Prantl, Das Inulin, ein Beitrag zur Pflanzen-Physiologie. Preisschrift. Munich 1870.— Diagendoiff, MateriaUen zu einer Monographic des Inulins. Petersburg 1870. 64 MORPHOLOGY OF THE CELL. well as by their appearance. If masses of tissue containing much inuline (tubers of Inula Heknium and Heliantbus tuberosus, roots of dandelion and of other Compositae) are examined in the dry state, the parenchyma-cells are found to be filled with angular, irregular, shining, colourless fragments, which are seen in polarised light to be crystal- line, and may be recognised as inuline by the reactions above-named. If the ovaries and unripe fruits of the orange or citron are laid for some time in alcohol, concretions are found in their tissues, which completely resemble in form the sphere-crystals of inuhne ; but the chemical reactions and the degree of solubility show that they do not consist of this substance. Sect. ii. Crystals in the Cells of Plants \ — The crystalline forms de- scribed in sect. 7, in which albuminoids are sometimes found, though always mixed with other organic compounds, are not common phenomena, and must not be placed in the same category as the very abundant true crystals of lime salts now to be described ; from a morphological and physiological point of view the differ- ence is still more essential. Calcium carbonate occurs, where it has hitherto been observed in plants, not in the form of large crystals with clearly defined surfaces, but in that of finely granular deposits whose crystalline nature is recognised only by their behaviour to polarised light (illuminating in a dark field of view by a crossed Nicol) ; while their solubility in weak acids with evolution of bubbles of gas, characterises them (under the circumstances named) as calcium carbonate. It occurs thus, according to De Bary, in the form of roundish grains in the Plasmo- dium of the Physariae. In the epidermis-cells of the leaves of many Urticaceae (Ficus, Morus, Broussonetia, Humulus, Boehmeria, &c.), and in the stem of species of Justicia, stalked, club-shaped, stratified out-growths of the cell-wall are formed, by peculiar increase of thickness, projecting into the cavity of the cells. In the substance of these masses of cellulose ' are deposited clusters of very small micro- scopic crystals of calcium carbonate, which singly are scarcely or not at all distinguishable, and which, as their behaviour on illuminating with polarised light shows, are arranged in a radiate manner in each single cluster (group of crystals) around its centre.' (Hofmeister, /. c.) These structures are known as Cystoliths. The lime deposited in the cell-walls of many marine Algae appears to be still more finely divided ; their structure becoming in consequence stony and brittle. {Ace/a- bularia, Corallina, Melobesiaceae, &c.) All other crystals found in plants and hitherto accurately examined are shown, by their form where this is recognisable, and by their reactions, especially by their insolubihty in acetic acid, and their solubility without evolution of bubbles in hydro- chloric acid, to consist of Calcium oxalate. This salt is widely distributed, especially in the tissue of crustaceous Lichens, most Fungi and Phanerogams, and in the form of very small granules of crystalline structure, of clusters, of bundles of needles (Raphides), and often of large, beautiful individuals with perfectly formed crystalline surfaces. ^ Sanio, Monatsber. der Berl. Akad. p. 254, April 1857. — Hanstein, ibid. Nov. 17, 1859. — Gg. Holzner, Flora, pp. 273, 556, 1864, and p. 499, 1867.— G. Hilgers, Jahrbuch fiir wiss. Bot. VI, p. 285, 1867.— Rosanoff, Bot. Zeitg. 1865 and 1867.— Solms-Laubach, Bot. Zeitg. nos. 31-33, 1871. — Hofmeister, Lehre von der Pflanzenzelle, Leipzig 1867 ; cystoliths are treated of at p. 180. CRYSTALS IN THE CELLS OF PLANTS. 6^ In Fungi and Lichens the crystalline granules are commonly small and de- posited not in the interior of the cells, but on the outside of the cell-walls, and frequently in such large numbers that the tissue of hyphae becomes opaque and stiff in consequence. In some Lichens minute granules of calcium oxalate are deposited in the cell- walls of the dense cortical tissue {Psorosma lenh'gerum, De Bary). It is only exceptionally that crystalline deposits occur in the interior of the cells of Fungi, as, for example, in the form of radiate spheres (sphere-crystals) in the swellings of some of the hyphae of the mycelium of Phallus caiiinus ac- cording to De Bary. Little or nothing is known of the occurrence of calcium oxalate in most Algi^, in Muscineae, and in Vascular Cryptogams ; but it is found very abundantly in the tissues of most Phanerogams. In Dicotyledons it often occurs in the form of large beautifully perfect crystals in the cavities of cells {e. g. in the mesophyll and leaf-stalk of Begonia, and the stem and root of Phaseolus) ; clusters of crystals are, however, in this class much more commonly deposited in a nucleus of proto- plasmic substance («?. g. in the cotyledons of Cardiospermum Halicacabimi), where the separate crystals are completely formed only in the detached part. Sometimes also (as in the hairs of Cucurbita) small, beautiful, and perfectly developed crystals are seen enclosed in the circulating protoplasm. In Monbcotyledons, especially those allied to the Liliacese and Aroideae, the crystals of calcium oxalate occur mostly in the form of bundles of long very thin needles, forming the so-called Raphides, which lie parallel to one another in such a manner that they usually more or less completely fill- up the generally elongated cells. Needles of this kind are formed also in great quantities when the leaves of many woody plants change their colour and lose water by evaporation in the autumn, although absent during the period of vegetation. Where the crystals lie in the cavity of the cell, and this is usually the case with Angiosperms, they are commonly, perhaps always, coated by a thin membrane, which remains after the solution of the calcium oxalate, and must probably be considered as a coating of protoplasm. This is also the case, according to the older statements of Payen, even with the raphides, and according to the accurate observations and statements of others, also in the larger single crystals and clusters. In Dicotyledons calcium oxalate occurs apparently only rarely deposited in the substance of the cell-wall ; Salms-Laubach (/. c.) names different species of Mesembryanthemum [31. rhombeum, tigrinujn, laceru?7i, stramineiini, Lemanni) and Sempervivum calcareum, in which fine granules or (in the case of Sempervivu?7i) larger angular fragments of crystalline calcium oxalate are scattered through certain layers of the outer wall of the epidermis-cells of leaves. The occurrence of crystals of calcium oxalate in the substance of the cell-walls is, on the other hand, according to the same observer, of common occurrence in Gymnosperms. They generally consist of numerous small granules of unrecognis- able shape ; not unfrequently, however, also of well-developed crystals. In the bast-tissue of all parts of the stem deposits of this kind are found in the Cupres- sineae, Podocarpus, Taxus, Cephalotaxus, and Ephedra ; they are absent, on the other hand, from Phyllocladus In'chomanoides, Gingko biloba, Dammara australis, F 66 MORPHOLOGY OF THE CELL. and from all Abietineae that have been examined. The small angular granules or larger individual crystals are usually deposited in the soft lamella between the elements of the bast-tissue. Much more widely distributed even than in their bast, calcium oxalate occurs deposited in the cell-wall of the primary cortical paren- chyma of the branches and leaves of Gymnosperms, with the possible exception of some Abietineae ; here also the middle lamella of the common wall between each two cells is the place where the crystals are formed, as also in the bundles of thick- walled cells beneath the epidermis {e. g. Ephedra). The thick- walled often branched fibre-cells abundantly scattered through the parenchymatous tissues of Gymnosperms, the so-called ' spicular cells,' not unfrequently contain crystals deposited in their outer mass of layers ; these occur in unusually large numbers and great perfection in Welwitschia mirabilis. If the crystals are dissolved in hydrochloric acid, the empty cavities in the substance of the cell- wall retain com- pletely the form of the crystals, so that the unpractised observer thinks that he still sees them. Finally, fine granules are abundantly scattered through the thick- ened outer wall of the epidermis of Gymnosperms (Welwitschia, ' Taxus baccata, Ephedra, &c.) or, in other cases, well-developed small crystals (Biota orienialis, Libocedrus Doniana, Cephalotaxiis Fortunei, &c.). Connected with these deposits in the cell-wall itself are the clusters of crystals discovered by Rosanoff (Bot. Zeitg. 1865, 1867) in the pith of KePria japonica, Riciniis communis^ and in the leaf- stalk of different Aroideae (Anthurium, Philoden- dron, and Pothos), which, lying in the cavity of the cell, are united with the cell- wall by simple or branched threads of cellulose, and are even covered with a membrane of cellulose. The crystalline forms in which the calcium oxalate occurs in the cells of plants are extremely numerous, an immediate consequence of the circumstance that this salt crystallises in two different systems, according as it is combined with six or with two equivalents of water. The calcium oxalate containing six equivalents of water of crystallisation ^^ ^Cfi^-\-^ 2l(\\ crystallises in the quadratic system, the fun- damental form is an obtuse quadrate-octahedron (envelope-shaped) ; combinations of the quadratic prism with the obtuse octahedron are met with in abundance. The raphides, however, belong, as respects their behaviour in polarised light, according to Holzner, to the klino-rhombic system, in which calcium oxalate crystallises with two equivalents of water of crystaUisation y^ Q^QOg-f- 2 aq. j. The fundamental form of the numerous combinations belonging to this class is a hendyohedron ; it produces derivative forms which are very similar to calcspar (as, for instance, in the deposits in the cell-wall), and others very similar to calcium sulphate. The clusters of crystals may consist of individuals of one or the other system. On the physiological signification of calcium oxalate what is necessary to be said will be found in Book III. ch. 2. Here however a few remarks may be made on the directly recognisable relation of the crystals to the cells which produce them. When the crystals remain so small that their volume appears inconsiderable in rela- tion to that of the cell itself, this latter retains its usual character; it may possess protoplasm, nucleus, chlorophyll, and starch (as in the case of the hairs of Gucurbita or CRFSTALS IN THE CELLS OF PLANTS. 6 J the mesophyll of Begonia) ; when, on the other hand, a crystal, or a chister, or a bundle of raphides, or finally a mass of small crystals, nearly fills up a cell, no other constituent of definite form is usually present; it appears as if, in such cases, the cell is usually approaching a condition of rest or even of slow dissolution ; if at an earlier stage a larger mass of crystals has been formed in a cell, it often remains smaller and with thinner walls than its neighbours. The cells which contain raphides show loosened walls which easily swell, and the bundles of raphides are generally surrounded by a thick gummy mucilage. A similar reason also explains why the granules and crystals deposited in the cell-wall of Gymnosperms usually lie in a softened mucilaginous middle lamella or in the cuticularised layers of the epidermis ^ . ^ [Professor Mf'Nab gives (Journal of Botany, new series, vol. i. p. 33) for the composition of the potassium-chlorate solution : three grains of potassium-chlorate dissolved in two drachms of nitric acid of sp. gr. i-io. The preparation of ' Schultz's solution' is thus described by Schacht (The Microscope and its application to vegetable anatomy and physiology, translated by F. Currey, p. 43): Zinc is dissolved in hydrochloric acid; the solution is allowed to evaporate under contact with metallic zinc, until it attains the thickness of a syrup; the syrup is then saturated with potassium iodide, the iodine added, and the solution, when necessary, diluted with water. For the ' iodine-solution ' the same authority recommends one grain of iodine, and three grains of potassium iodide in one ounce of distilled water.^Ko.] "F a CHAPTER II. MORPHOLOGY OF TISSUES. Sect. 12. Deflnition. — In the widest sense every aggregate of cells which obeys a common law of growth (usually however not uniform in its action) may be termed a Tissue. Aggregates of this kind may originate in different ways. The cells concerned may be at first isolated, subsequently during their growth they may come into contact, and become so completely united at the surfaces of contact of their walls that the boundary surface between them becomes in- distinguishable. This hap- pens, e.g. in the sister-cells which have arisen by divi- sion in the mother-cells of Pediastrum,Coelastrum, and Hydrodictyon ; the sister- cells show in these cases within the mother -cell a ' creeping ' motion which lasts for a considerable time before they become connected into a surface (Pediastrum), or in the form of a sac-hke hollow net (Hydrodictyon), and form by their growth a tissue. In the same manner the sister-cells (endosperm) which arise in the embryo-sac of Phanerogams by free-cell- formation, unite with one another and with the wall of the embryo-sac itself, con- tinuing then to develop as a continuous tissue and to increase by division. In Fungi and Lichens the formation of tissue originates by the apical growth of juxtaposed thin filaments consisting of rows of cells (the hyphse), and different orders of branchlets of them ; each filament grows by itself, increasing the number of its cells by division, and branches copiously ; but this takes place in such a manner that the different hyphse undergo a similar development at definite spots on the whole body of the Fungus or Lichen ; thus arise surfaces, strings, hollow structures, &c., Fig. 'i^— Pediastrum granulaUiin (after A. Braun) (X400). A a disc consisting of cells grown together ; at g the innermost layer of a cell-wall is protruding ; it contains the daughter-cell* resulting from division of the green protoplasm ; at t are various states of division of the cells ; sp the fissures in the already empty cell-walls ; B the inner lamella of the mother-cell-wall which has entirely escaped (greatly enlarged) ; b contains the daughter-cells (g), these are in active creeping motion ; C the same family of cells 4j hours after its birth, 4 hours after the small cells have come to rest ; these have arranged themselves into a disc, which is already beginning to develop into one similar to A. DEFINITION. 69 ^vhich show a common growth, and yet consist of single elementary structures developing individually (Fig. 55). With the exception, however, of the instances named, and of some allied ones, the formation in the vegetable kingdom of many-celled bodies regulated by a common growth always arises from the tissue-cells which originate by the often repeated bipartition from common primary mother-cells, remaining from the very Fig. 56. — Epidermis (c) and subjacent cortical parenchyma of the hypocotyledonary segment of the sunflower, which r thickens quickly after completion of the division ; the darker, thicker cell-walls are the original ones, the thinner radial ones those most newly formed. The strong- tangential growth even of the epidermis-cells together with their cuticle is of special interest in this process. Fig. 55.— Part of a longitudinal section of a Gastromycete (Criicibulum vulgare), showing the course of the hyphse: their interstices are filled with a watery jelly, which has probably resulted from the conversion into mucilage of the outer cell-wall layers of the filaments. (For further details of the internal organisation, see Book II. Fungi. The drawing is partially diagrammatic, inasmuch as the hyphse are too thick for the small magnifying of the whole (about 35), and not so numerous as in nature.) commencement in connexion, in consequence of the manner of formation of the partition-wall; the cells are in these cases, at least originally, so united that they appear at an earlier stage like chambers in a mass which continues to grow uni- formly (Fig. 56). The two first-named kinds of tissue-formation may be distinguished as spurious from the latter or genuine form ; but there is no sharp boundary-line between them. In many cases, for example, the endosperm is only in its rudimentary state a spurious tissue, due to the amalgamation of originally isolated cells ; in its further development by cell- division it becomes a true tissue {e.g. Ricinus, &c.). The formation of tissue -surfaces occurs in the formation of the cortex of many Algae and of the genus Chara, by the development of single cell-filaments ; but in such a manner that by these means com- binations make their appearance which can no longer be distinguished from true tissues. 70 MORPHOLOGY OF TISSUES. Nageli and Schvvendcner (Das Mikroskop, II. 563 et seq.) may be consulted further on the growth of Acrochcetlcum pul'vereum, Stypopodium atomarium, Delesseria, Hypoglossumy and the leaves of Mosses 1. Sect. 13. Formation of the common wall of Cells combined into a Tissue^. — If the cell-wall between two adjoining cells is thin, it appears, even when very highly magnified, as a simple lamella ; and sometimes this is also the case when it has already attained a considerable thickness (in succulent parenchyma-cells). Usually it is only when the wall has attained some thick- ness that it can be seen that the one side of the partition-wall belongs to one, the other to the other adjoining cell. If stratification and differentiation into layers occur in a sufficiently thickened wall between two tissue-cells, a middle lamella always becomes discernible (Fig. 57, 7n), on which, right and left, the remaining cell-substance is superposed in the form of layers and shells, generally sym- metrically distributed, so that those on one side appear to belong exclusively to the one adjoining cell, those on the other side to the other (Fig. 57, i). The impression may thus be given to the observer as if the layers which are concentrically deposited around each cell-cavity formed the wall belong- ing to it alone, while the middle lamella belonged to a common matrix in which the cells are im- bedded ; or as if it were excreted from the neigh- bouring cells. Both views were actually held for a considerable time, and the middle lamella was then termed Intercellular Substance. If the older fragments of tissue represented in Fig. 57 are com- pared with the younger condition of the same, the thought at first suggests itself that the middle la- mellae may be the originally thin walls, on which the thickening-layers have been deposited on both sides inward by apposition ; this view has also found its defenders, by whom the middle lamell'a is dis- tinguished as the Primary Cell-wall. The remain- ing thickness is then correspondingly described Fig. 57.— Transverse section through thicken- ed cells with evident formation of central lamellse (w) ; i is always the whole of the superposed cell-substance ; / the cavity of the cell, from which the contents have been removed. A from the cortical tissue of the stem of Lycopodhim chajnacypai-issKs ; 5 wood-cells from the inner part of the wood of a young fibro-vascular bundle of the sunflower; C wood of Piniis sylvestris, St a medullary ray (xSoo). as secondary ; or if it is differentiated into cell- wall. two shells, as secondary and tertiary ^ On the formation of the cortex of Ceramiaceoe, see Nageli, Die neueren Algensysteme (Neuenburg 1847), and Niigeli und Cramer, Pflanzenphysiologische Untersuchungen. 2 H. V. Mohl, Vermischte Schriften botanischen Inhalts. Tubingen 1845, p. 314 et seq. — H. v. Mohl, Die vegetabilische Zelle, p. 196. — Wigand, Intercellularsubstanz und Cuticula. Braunschweig 1850. — Schacht, Lehrbuch der Anatomic und Physiologic der Gewachsc, I. p. 108, 1856. — Miiller, Jahrb. fiir wiss. Bot. V. p. 387, 1867. — Hofmeister, Lchre von der Pflanzenzelle. Leipzig 1867, § 31. FORMATION OF THE COMMON WALL OF CELLS. 71 The middle lamella is generally thin in lignified tissues, but strongly refractive, and formed of dense substance not capable of swelling; when the rest of the substance of the cell- wall has been dissolved in concentrated sulphuric acid, it remains (in fine transverse sections) as a delicate net-work; if, on the other hand, the cells are isolated by boiling in potash or nitric acid, solution of this middle lamella which resists sulphuric acid takes place, while in this case the rest of the cell-wall is preserved (as in all wood-cells and very many bast-cells). In other cases, as has already been mentioned in sect. 4, the middle layers of the partition-wall of adjoining cells are, on the contrary, converted into mucilage; the layer of cell-wall immediately surrounding each cell-cavity is dense, and appears like the entire cell-wall imbedded in a mucilaginous, swelling, weakly refractive matrix (the so-called intercellular substance) ; this occurs very com- monly in many Fucacese and in the endosperm of Ceraionia Siliqua (Fig. 41, p. 36). On a fine transverse section through the cambium tissue of a branch of Pinus sylvestris, the two phenomena here described may be seen simulta- neously; the wood-cells show the thin dense middle lamella, the young bast-cells appear deposited in a soft mucilaginous substance, which is especially thick between the radial rows of cells, and is interspersed with fine strongly refractive granules; but both forms of tissue arise out of the same young tissue (the cam- bium), the walls of which are simple thin lamellae, between which the cell-cavities themselves appear as so many compartments. Objects of this kind are well adapted to prove the correctness of the supposition that in general the formation of denser or softer middle lamellae depends only on a diflferentiation of the substance of the partition-walls during their thickening, a view which explains in a perfectly simple manner all the phenomena belonging to it, and agrees altogether with growth by intussusception. The thin entirely homogeneous lamella of cellulose which bounds the young cells never allows a separation into two lamellae to be recognised ; the bounds of the two cells are never marked by a fissure dividing the partition-wall. Nevertheless such a splitting of the still very thin lamella often takes place later locally, when the surface grows more quickly, as in the forma- tion of the intercellular space of the large-celled succulent tissue (parenchyma) of vascular plants, in the formation of stomata, &c. Fig. 58 shows some fully grown parenchyma-cells from the stem of Zea Mais in transverse section ; the cells were at first bounded by perfectly flat walls, which met nearly at right angles. As the growth increased, a tendency towards the rounding off of the polyhedral forms arose ; the unequal growth clearly leads to tensions which are compensated by the fact that on the line where one wall meets the other, the cohesion is destroyed in Fig. 58.— Transverse section through the succulent parenchyma of the stem of Zea Mais; gw common partition-wall of each pair of cells; z intercellular space caused by their splitting {X5S0). 72 MORPHOLOGY OF TISSUES. the interior of the substance of the cell-wall. Thus a fissure arises which, cor- responding to the relationship pointed out, assumes the form of a triangular prism with concave sides (Fig. 58, z). It becomes filled with air, and now becomes one of those intercellular spaces which very usually form in the parenchyma a continuous system of narrow channels. Not unfrequently the portions of the wall which confine the intercellular space grow rapidly, and thus it increases in width ; the cells assume irregular outlines, or appear star-shaped in transverse section, touching one another only at small portions of the surface (as in the parenchyma on the under side of many leaves of Dicotyledons, and the stems oi /uncus cffusus). In the middle also of the faces of the cell, when no other wall intersects them, splittings of the homoge- neous lamella may occur locally ; sometimes these are limited to narrowly circum- scribed places, which can then be recognised as shallow excavations in the otherwise Fig. 59. — Two rows of cells running in a radial direction /, //, /// and I, 2, 3) of the cortical parenchyma of the root of Sagittaria sagittafolia in transverse section ; a the protrusions, e the cavities between them (X about 350). Fig. 60.— From a tranverse section of the leaf of Pinus finaster ; h half of a resin-passage, to the left parenchyma-cells containing chlorophyll with fold- ings-in [/) of the cell-wall ; / pit-like formations (the contents of the cells contracted by glycerine, and containing drops of oil) (X800). homogeneous partition-wall. In other cases the splitting of the partition-wall into two lamellae takes place in such a manner that only single roundish places remain unsplit ; the separated pieces continue to grow rapidly by intercalary growth, and bag-shaped protrusions of the neighbouring cells arise which allow the originally unsplit fragments of cell-wall to be still recognised between them as partition-walls (Fig. 59). In other cases there follows on the partial splitting of the partition-wall a local growth of the two lamellae (or of only one) of such a character that a folding-in arises, which intrudes into the cell-cavity, as shown in Fig. 60, f. Finally, in some species of the genus Spirogyra, the septum between each pair of cells splits into two lamellae, each of which grows in a peculiar manner ; a protrusion is formed into the interior of the cell, which, when the adjoining cells separate, becomes turned inside out somewhat like the finger of a glove previously folded in. When the simple walls of cells united into a tissue split everywhere into two lamellae (the separation proceeding always from the original intercellular spaces) and become rounded off, a complete dissolution of the tissue takes place in this man- ner, and the tissue becomes a mere mass of isolated cells. This occurs in the FORMATION OF THE COMMON WALL OF CELLS. 73 flesh of many succulent fruits {e. g. in the snowberry in winter) ; and this separation can sometimes be artificially brought about by continued boiling in water (as in potato-tubers). The origin of the partition-walls in tissue-cells which increase by bipartition by no means requires the supposition that they were originally composed of two lamellae. In this case one would be led, by careful consideration of the properties of those tissues where numerous divisions follow one another and intercellular spaces afterwards arise, to extremely complicated hypotheses (which, moreover, also contradict growth by intussusception). Even in those cases where the union of the cells into a tissue arises from the amalgamation of originally separate cells (which are not sister- cells), the union of the cell-walls is so intimate that no boundary line can any longer be perceived ; and the formation of a middle lamella proves also in such cases ^ as does the formation of a middle lamella generally, that the hypothetical boundary-surface does not exist, and that the splitting of the homogeneous lamella is a consequence of different growth on its two sides. Both the manner in which the splittings of the homogeneous thin partition-walls arise, and also the formation of the middle lamella of thick walls, oppose the supposition of an originally double partition-wall in tissue-cells ^. The splitting of the partition-wall and the growth of its now separated lamellae lead to a variety of configurations in the interior of tissues which may be col- lectively included in the conception of the Intercellular Space. To this belong especially the large air-conducting channels in the tissue of many water and marsh plants (Nymphaeaceae, Irideae, Marsileaceae, &c.), and the formation of the cavity between the wall of the capsule and the spore-sac in the fruit of Mosses ^ Not unfrequently peculiar processes of growth of the adjoining cells unite in the origin of intercellular spaces. I will here only allude to three very diff'erent examples, the formation of stomata, the air-cavities of INIarchantia, and resin and gum pas- sages {vide infra). But in quite a diff'erent manner the behaviour of the partition-wall of two cells contributes to the production of air- or sap-conducting channels, which, like the air- or sap-conducting intercellular spaces, may form a continuous system in the col- lective mass of the substance of a plant. This happens by the partial or entire absorption of the partition-walls of adjoining cells, by which the cavities of long rows of cells of a tissue become connected ; and the single cells themselves become members of a bag-like or tubular structure. Unger has appropriately designated this Coalescence of Cells. Vessels of this kind (TracheVdes of Sanio) are formed in the wood of fibro-vascular bundles, in w^hich the protoplasm and cell-sap disappear, and they serve for conducting air. In the sieve-tubes in the bast- substance of the fibro-vascular bundles, on the other hand, the watery mucilaginous ^ For examples, see Hofmeister, Handbuch, I. pp. 262, 263. "^ Further detail of this subject is not possible here. I may here remind the reader only of the cleavage of crystals as an analogous case; the cleavage surfaces are determined by the molecular structure, but there is a wide difference between them and true fissures, however fine. 3 The wide air-canals in the stem of Equiseta, Grasses, species of Allium, Umbelliferoe, and Compositse arise, on the other hand, from the cessation of the growth of inner masses of tissue and their drying and bursting, while the surrounding tissues continue to grow. 74 MORPHOLOGY OF TISSUES. contents of the cells is not replaced by air ; the communication established between the cells of one row serves rather for a quicker movement of the succulent contents over greater distances. The Laticiferous Vessels must also be regarded as com- posed of coalesced cells ; they are the result of very early and complete absorption of the 'partition-walls of adjoining cells belonging to straight or much branched rows in the interior of different systems of tissues. Here however tubes produced by the coalescence of cells need only be opposed to intercellular spaces for the sake of contrast ; a more miiuite consideration will come better in connexion with the description of the systems of tissues. (a) ' Intercellular Substance'' and '■Primary Cell-^all.^ The hypotheses implied by these terms could only be entertained so long as it was supposed that the original thin lamellae between two adjoining tissue-cells were double ; and so long as it was be- lieved that the stratification of the cell-wall was brought about by apposition of new layers. The expression that the original thin partition-wall between two tissue-cells is a double lamella can only be understood in two senses ; either it means that the lamella consists of molecular layers, and that two of these contain between them the ideal boundary-surface of both lamellae belonging to the adjoining cells, or that there is an actual interruption of the molecular connexion, and from the first an actual cre- vice. The last supposition is inadmissible, since it does not rest upon observation ; it is besides contradicted by the detection of weak boundary lines between layers which nevertheless are molecularly united, and have no crevices between them. Thus in the layers of thick cell-walls and of starch-grains there are no crevices, and yet the bounds of the layers may be seen ; why then should the assumed actual crevice not be visible in the original partition-wall? If now the first alternative is assumed to be correct, and the composition from two lamellae considered as purely ideal, the question depends on a mere verbal controversy with reference to the intercellular substance ; for if the original homogeneous partition-wall, although consisting of molecular layers, is yet held together everywhere by molecular forces, and the supposed boundary surface is no interruption of the molecular structure, then the deposition of a different substance (intercellular substance) at the same place appears only as a process of ordinary growth by intussusception. The fact that the boundary line between previously separated cells disappears by subsequent coalescence proves that the outer molecular layers of cell- walls already formed may yet enter into molecular union. If in such cases a differen- tiated middle lamella is afterwards formed, this is the most striking evidence against the explanation of it as primary cell-wall. If an attempt is made further to construe on paper step by step the behaviour of a developing woody tissue, for example, while retaining the theory of the primary cell-wall, one is immediately involved in difficulties which do not arise on the supposition that the middle lamella is simply the result of subsequent differentiation of the cell-wall. (b) Addition to the Intercellular Spaces. With the origin of these spaces is very often connected, as has been mentioned, a peculiar development of separating cells, quite different from that of the rest of the tissue ; so that the intercellular space, together with its environment, represents, in a certain sense, a peculiar form of tissue or an organ for a definite purpose. The observation of some cases of this kind is well calcu- lated to show the beginner how, even in the domain of tissue-formation, morpholo- gically similar or equivalent processes lead to entirely different results from a physio- logical point of view. This subject will be treated in a more general and detailed manner in the third chapter, and in Book III. (i) The cleft of the stomata of the epidermis belongs also to the category of Intercellular Spaces, and its origin is peculiarly calculated to afford an insight into the mode of formation of an intercellular space. I have chosen the stomata on the leaves of FORMATION OF THE COMMON WALL OF CELLS. S Hyacinthus orientalis as an example. Figs. 61-64 are transverse sections perpendicular to the upper surface of the leaf; ee in all of them are the epidermis-cells, pp the paren- chyma of the leaf. The stoma (5) is formed of a smaller epidermis-cell which divides Figs. 61-63.— Development of tlic stoniata of the leaf o{ HjncuiiAus oj-ientalis, seen in transverse section (x8oo). FIG. 64. into two equal sister-cells by a wall standing vertically to the surface of the leaf; in Fig. 61, 5, this has just taken place; the partition- wall is formed ^ it appears as a very thin simple lamella, which soon attains greater thickness, and especially thickens more rapidly where it meets at right angles the wall of the mother-cell without and within (Fig, 62, ^). The thickening mass appears at first quite ho- mogeneous; afterwards an indication of stratifi- cation is to be observed, and the first trace of a separation of the still simple lamella into two lamellae (Fig. 62, B). In Fig. 63, /, the split- ting is already completed ; the growth of the separated lamellae now proceeds in a peculiar manner, so that a cleft arises narrower in the middle, wider without and within, which unites the intercellular space i (the air-cavity) with the external air (Fig. 64). It is worth mention that before the division of the mother-cell, an obvious not very thin cuticle has already overspread it together with the adjoining cells of the epidermis. This is especially to be recognised in the condition B, Fig. 62, while still continuous; by the splitting of the partition-wall into two lamellae it finally becomes ruptured (Fig. 63); and by the cuticularising of the outermost layer of the now separated lamellae the cuticle is afterwards continued over the surfaces of the cleft (Fig. 64). If the process of the formation of the stoma is followed up on a superficial view, it shows that the splitting of the partition-wall does not extend through its whole surface, but that a portion still remains above and below (taking the leaf in a vertical position) as a simple lamella (cf. sect. 15, Figs. 73-75). Both the Guard-cells (the cells which enclose the cleft) are not only distinguished from the other epidermis-cells by this peculiar mode of division and of growth ; they also differ from them by containing chlorophyll and starch. (2) In the family of Marchantieae belonging to the Hepaticae, the origin and struc- ture of the stomata (Fig. 65, B, sp) is much more complicated ; of this we must speak hereafter. Here it need only be pointed out that even before their formation the epi- dermis-cells have become detached from those lying beneath ; and in such a manner that the separating surfaces (seen from above) represent rhomboidal plates beneath the epidermis, which are marked off from one another by the walls of unseparated cells ^ I was unable to detect nuclei immediately before and for a consideiable time after the division. 76 MORPHOLOGY OF TISSUES. Fig. 65.— Transverse section through the hofizontal thallus of Marchantia j>olymo}-pha ; A central part, furnished on the under side with the leaf-hke appendages h, and the root-hairs h (Xso) ; B marginal part of the thallus, more highly magnified ; / colourless, reticulately thickened parenchyma ; o epidermis of the upper side ; chl the cells containing chlorophyll ; sp stomata ; j partition-walls between the broad intercellular spaces ; u lower epidermis with its cell- walls coloured dark. Fig. 66.— Sap-conducting intercellular passages in the young stem of ivy, in transverse section (XSoo) \ A,B,C show young passages at g, placed at the boundary of the cambium c and the soft bast wb ; h the wood ; D and E, at g, larger and older passages, lying at the boundary of the bast b and the cortical parenchyma (?■/). FORMS AND STSTEMS OF TISSUES. 77 (Fig. 65, B, ss). These intercellular spaces, which separate whole layers of cells each opening to the outside in its middle by a stoma, are destined to enclose the chlo- rophyll-containing tissue of these plants. The layer of cells which forms the bottom of the flatly extended intercellular space, after repeated divisions vertically to the sur- face, sends out protrusions upwards into the cavity ; these grow in a similar manner to many filamentous Algae, divide and branch and form grains of chlorophyll, while the whole of the rest of the tissue of these plants produces no chlorophyll. (3) The origin of resin and gum passages depends also on the formation of inter- cellular passages with a peculiar development of the cells which bound them. As I shall recur to other points in this structure, it is sufficient to refer to one example. Fig. 66 shows passages of this kind in the transverse section of young portions of the stem of the ivy. Conditions, such as B, C, show clearly that the intercellular space arises by the parting of four or five cells, and that these latter, distinguished by their turbid granular contents, increase by division. The formation of the much wider passages, D, E, is also to be referred to a similar subsequent increase and correspond- ing growth of the cells which surround the passage. By the growth of the cells which bound the intercellular passage, as well as by the manner of their division, by their contents, and by the circumstance that they excrete a peculiar sap into the passage, a structure of this kind appears as a differentiated part of the tissue, which is sharply marked off from its environment, and has a physiological significance of its own. Sect. 14. Forms and Systems of Tissues. — The whole mass of the cell- tissue which forms the body of a plant may be uniform or not ; in the first case the cells are all similar to one another, and their modes of union everywhere uniform. This case is rare in the vegetable kingdom ; and it is only the simplest forms that are constructed in this manner. Since in a homogeneous not differen- tiated tissue all the cells are alike, their union into a whole is physiologically and morphologically of very subordinate importance, because each cell represents the character of the whole tissue ; hence it not unfrequently happens in these cases that the cells become actually isolated and continue their life singly ; and such individuals are termed Unicellular Plants. Only a little higher are those which consist of an unbranched row of perfectly uniform cells, or of an arrangement of such into a surface or mass. When numerous and densely crowded cells form a mass of tissue, then it is usually the case that different layers of tissue develop differently; the body of the plant consists then of a differentiated tissue, or of different forms of tissue. In general their arrangement is determined by the fact that the whole mass of tissue has a tendency to become definitely bounded on the outside, so that there arises a differentiation of outer layers of tissue from the inner mass. But in the interior of the body enclosed by the epidermal tissues, fresh differentiations arise in the higher plants ; string-like arrangements of cells are formed, surrounded by fundamental tissue lying between them and the epidermis; these strings of tissue (vascular, fibrous or fibro-vascular bundles) usually follow in their longitudinal course the direction of the most vigorous growth which immediately precedes their differentiation. Not only the epidermal layer, but also the bundles and the fundamental tissue lying between them, are, however, usually not uniform among themselves ; the epidermal tissue itself is often differentiated into layers of different nature ; each bundle is also differentiated, but in a different manner and gene- rally in a still higher degree. In this manner arise in the higher plants, in the place 78 MORPHOLOGY OF TISSUES. of different layers of tissue, systems of tissue-forms, which may be designated simply 2LsSys/ems 0/ Tissue. We thus usually find an Epidermal System, a Fascicular System, and the system of the Fundamental Tissue between them (Fig. 67). But whenever a differentiation of tissues of this kind arises in a plant, it only takes place subsequently ; ori- ginally the whole mass consists of a growing portion of the plant (stem, leaf, root), always of a uniform 'Z/^^^^^M^i^O^MM^i^Z^^'yK' ' I \~Hiff tissue, out of which by diverse de- velopment of its layers these tissue- systems have their origin ; this tissue of the youngest parts of plants which is not yet differentiated may be termed, in opposition to the others, Primary Tissued (a) Within each tissue -system the cells may be formed and arranged in very different ways; their contents and their cell-wall may be differently de- veloped ; in each system they may be capable or incapable of division^. If the cells are pointed at the ends, and much longer than they are broad, and at the same time their ends penetrate between one another so that no inter- cellular spaces occur, then the tissue is termed Prosemhyma. If, on the other hand, the cells are arranged in rows, bounding one another with broad surfaces, thin- walled, not much longer than broad, and forming intercellular spaces, they form a Parenchyma. The two forms of tissue pass over into one another in many ways, and in the use of the term Parenchyma a painful uncertainty prevails in vegetable anatomy. There are forms of tissue which cannot be included under either of these two terms, if they are made to possess any definite signification ; as, e.g.^ the tissue of Fungi and Lichens, and even of Fucaceae. In parenchyma as well as in prosenchyma the cells may be thick or thin walled, lignified or not, the contents may be succulent or may consist of air. It would be convenient to generalise the term Sclerenchyma used by Mettenius, Fig. 67.— Transverse section of the stem oi Selasinella itiaqiialifoha. The cell-tissue, consisting of several layers of cells, has dark thick cell- walls ; the thinner-walled fundamental tissue envelopes three fibro-vas- cular bundles, separated from it by large intercellular spaces (/) (x8oo). ^ It may not be superfluous to remark here, in the meantime, that pith and cortex are neither forms nor systems of tissue, but altogether indefinite and undefinable ideas; we speak, e.g., of cortex in Thallophytes in quite a different sense to what we do in Vascular Plants ; the cortex of Mono- cotyledons is something different from that of Conifers and Dicotyledons; in the latter the cortex has quite a different signification in young and in older parts of stems. The same is the case with the pith. ^ An enumeration of the nomenclature of tissues would here be of no service. In elucidating the facts, as I do partly in the following paragraphs partly in Book II, I shall employ the technical terms as they are required by a consideration of the different objects and relationships. I keep, with a few deviations, to the terms and distinctions proposed by Nageli (Beitrage zur wiss. Botanik, Heft I, 1858). THE EPIDERMAL TISSUE. yg and to designate by it cells united in both a parenchymatous and prosenchymatous man- ner, when they are not only thickened, but also hard. We should then have Scleren- chyma in cork, in fundamental tissue (as the dark strings in the stem of Pteris aquilina, or the stone of stone-fruit), and in wood ; the indurated cells in the flesh of pears would also come under this name. In one word, by this term would be designated not a tissue-system, but only a physiological property of particular cells of a tissue-system \ If the cells of a tissue are all or mostly capable of division, it is a Generating Tissue (Meristem of Nageli) ; if they are not it is a Permanent Tissue. The primary tissue of the youngest parts of plants is always a IMeristem, and may be distinguished as Primary Meristem. In the older parts of plants portions of the tissue also remain merismatic or become so subsequently ; they may be designated Secondary Meristem. At one time this tissue was designated Cambium; but it is convenient to retain this word in its original signification for that merismatic layer in the tissue of older parts of plants by means of Avhich the increase in thickness of Dicotyledons and Conifers is accomplished. The arrangement may produce a simple row or line of cells, in contrast to a cell-surface where the cells form a lamella consisting of a single layer. If the cells are united in all directions, we have a Tissue with dimensions in three directions. When the latter is greatly elongated in one direction, and its growth proceeds especially at one or both ends, and it lies inside another tissue, it is a Fascicular Tissue ; the cells of such a tissue are usually elongated in the direction of its length, mostly prosenchymatous, and we then have Prosenchyma bundles. The most important form of these are the Fibro- 'vascular Bundles which are dispersed through the fundamental tissue of the higher Cryptogams and Phanerogams, whose cells are mostly elongated and partly prosenchy- matous, and are thus formed into vessels, /. e. long rows of lignified cells, the septa of which have been broken through. (b) The youngest parts of stems, extremities of roots, leaves, and other organs, consist almost entirely of Primary Meristem ; as they become more perfectly developed a separation may be recognised into layers of tissue and bundles, which represent the commencement of the tissue-systems ; within each system its different forms of tissue become gradually differentiated. When different tissue-systems in a mature condition are in contact, the history of development alone can often determine whether certain layers belong to one or the other system ; especially also because similar cell-forms occur in different systems. Thus, for instance, parenchyma and prosenchyma, scleren- chyma and secondary meristem may arise both in the fundamental tissue and in the fibro-vascular bundles ; in the layers lying beneath the epidermis it often cannot be determined whether they belong to the epidermal tissue or to the fundamental tissue which bounds it. In the same manner also different forms of glands, vesicular vessels, laticiferous ducts, resin and gum passages, occur in all three systems or in the funda- mental tissue and the fibro-vascular bundles. The forms of cells and tissues here named cannot be considered equivalent to these three tissue-systems ; they occur rather as constituents of different systems. Nevertheless, on account of their physiological peculiarities, I shall consider them together in a special paragraph, while the other more important forms of tissue will be treated of under the three systems. Sect. 15. The Epidermal Tissued — A differentiation into epidermal tissue and inner fundamental tissue can evidently only arise in plants and parts of plants ' Cf. Otto Buch, Ueber Sclerenchymzellen. Breslau 1870. '^ By the introduction of the idea of the Epidermal Tissue into general use, as I here employ it, a real want will, I think, be remedied in histology. In any case a series of histological facts, which have hitherto been treated of in a detached manner, will thus be brought under a common and higher point of view. 8o MORPHOLOGY OF TISSUES. which consist of a thick mass of tissue. In general the contrast of the two is the plainer the more the part of the plant concerned is exposed to air and light, underground and submerged parts showing it in a smaller degree ; in those des- tined to a longer term of life the formation of epidermis is usually also more perfect. The difference between epidermis and fundamental tissue can only be established by the outer layers of cells, whose morphological character is otherwise the same, becoming distinguished by the thickness and firmness of their cell-walls, and hence usually by being smaller than those which lie deeper inside. In this case a sharp boundary of the two tissues does not usually occur; the distinctions gradu- ally increase the more nearly the cell-layers approach the upper surface. This is usually the case, among Algae, with the Fucaceae and larger Floridese, with many Lichens and the fructification of Fungi ; even in the stem of Mosses the formation of epidermis is often indicated only in this manner. A further development of the contrast between epidermal and inner tissue arises when not only a sharp boundary lies between the two, but when an essentially different morphological development also distinguishes the epidermal from the inner tissue. In many Mosses and all Vascular Plants at least one outer layer of cells is to be distinguished in this sense as epidermal structure, and is here termed Epidermis. In true roots and many root- like underground parts of stems, as also in many submerged plants, it is generally only slightly different from the tissue lying beneath ; but in most parts of stems and leaves it shows an altogether peculiar development of its cells, giving rise to stomata and hair-formations of the most various kinds. In many leaves and parts of stems, the epidermis, after it has already become recognisable as a tissue of a peculiar kind (during or after the bud-condition of the organs concerned) undergoes cell-division tolerably late, by which it becomes divided into two or more layers. From this epidermis formed of several layers of cells (Pfitzer, /. hagnu77i cymbifoliunt (X900) ; x inner cells with colourless soft wallsj / cortical cells, becoming gradually narrower and thicker- walled towards the surface ; e e the epidermal layer ; / orifices through which the opposite cells communicate with one another. Fig. 71.— Piece of a radial transverse section through the sporangium oi Fitnaria hygrotnetrica (X300) ; e epidermis; the thick black streak at its circumference is the cuticle. (For further explanation of the fig. see Book II.) As in Mosses the formation of tissue attains especially a greater perfection in the sporangia, this is also the case in reference to the formation of epidermis ; the variously differentiated internal tissue of the capsule is surrounded by a highly developed true epidermis (sometimes provided with stomata) (Fig. 71). (b) The Epidermis'^. In Vascular Plants the epidermal tissue consists usually only of a single superficial layer of cells, the Epidermis. In its origin it always consists of a single layer ; but it sometimes becomes split into two or more layers by divisions ^ H. von Mohl, Vermischte Schriften hot, Inhalts, p. 260. Tubingen 1845. — F. Cohn, De Cuticula. Vratislaviae 1850. — Leitgeb, Denkschriften der Wiener Akad. XXIV, p. 253, 1865. — Nicolai, Schriften der phys.-okonom. Gesells. Konigsberg, p. 73, 1865. — Thomas, Jahrb. fiir wiss. Bot. IV, p. 33.— Kraus, ditto, IV, p. 305 and V, p. 83.— Pfitzer, ditto, VII, p. 561 and VIII, p. 17.— De Baiy, Bot. Zeitg. nos. 9- 11 and 34-37, 1871. THE EPIDERMAL TISSUE. 83 parallel to the surface which originate rather late during or after the bud-condition of the organs in question. In such cases the outermost may be distinguished, as the epidermis proper, from those which lie beneath, or the thickening-layers ; these latter generally consist of large thin-walled cells with contents as clear as water, for which reason Pfitzer terms them Aqueous Tissue. Epidermis of this kind, consisting of several layers, occurs in the leaves of most species of Ficus, in the stems and leaves of many Piperacese, and in the leaves of Begonia. In the roots also of some species of Crinum, the epidermis, at first simple, splits into several layers ; but this is much more striking in the aerial roots of Orchids and Aroideae, where these cell-layers afterwards lose their succulent contents and surround the substance of the root as an air-containing root-envelope (velamen). The Hypoderma is distinct in its development from the strengthening-layers which result by division from the originally simple epidermal layer, since it arises from the layers of the fundamental tissue covered by the true and simple epidermis. The cells of the hypoderma may also become developed as aqueous tissue like that mentioned above, and often to an enormous thickness ; this occurs in many Bromeliaceae and some species of Tradescantia. The hypoderma more often exists in the form of layers of very thick-walled often sclerenchymatous cells, whose origin has been proved to be from the fundamental tissue, not from the epidermis, at least in the case of Ephedra and Elegia, and is very probably so in other cases. While this scleren- chymatous hypoderma is especially frequent in Vascular Cryptogams (^. ^. Equisetum and Ferns), and in the leaves of Gymnosperms, a third form, the Collenchyma, occurs very abundantly in the leaf-stalks and succulent stems of Angiosperms, especially of Dicotyle- dons, its usually narrow but long cells being strikingly distinguished by the thickening- masses often deposited to a great extent in internally projecting longitudinal ridges at the angles, and swelling greatly with water or more powerful reagents (Fig. 21, B, p. 24). That the collenchyma originates from the fundamental tissue, and thus not from the epidermis, has been actually observed only in Euonymus latifoUusy Peperomia, Nerium, and Ilex, but is probable also in other cases. When in the sequel the term Epidermis is used without further remark, the ordinary simple layer, or the outermost when the epidermal tissue consists of several layers, is always to be understood. The cells of the epidermis, as also those of the strengthening-layers and of the hypo- derma, are in close contact on all sides ; intercellular spaces are formed only between the guard-cells of the stomata, through which the intercellular spaces of the fundamental tissue communicate with the surrounding air. This connexion without interstices is sometimes the only distinguishing mark of the epidermis, as in the submerged Hy- drillese, Ceratophyilum, &c. ; in other cases the formation of hairs helps to distinguish it, as in most roots, where the cells of the epidermis are otherwise similar to those of the fundamental tissue in contents and in the nature of their wall. But usually in the stem and foliar organs the epidermis is destitute of chlorophyll, starch, and espe- cially of granular contents, while in Ferns and in the water-plants mentioned above, as well as in other cases, the epidermis-cells contain grains of chlorophyll. Not unfre- quently the otherwise colourless cell-sap is tinged by a red substance. The form of the epidermis-cells in organs the development of which is chiefly in length, as roots, long internodes, and leaves of Monocotyledons, is usually elongated longitudinally ; in leaves with a broad surface it is mostly broadly tabular ; in both cases the side-walls are often curved in an undulating manner, so that the adjoining cells project into one another. The outermost lamella of the epidermis-cells is always cuticularised, and usually to the extent that cellulose is either not at all, or only with difficulty, to be detected in it. This true cuticle extends uninterruptedly over the boundaries of the cells, and is strongly contrasted with the subjacent layers of the epidermis. With preparations of iodine, with or without addition of sulphuric acid, the cuticle is coloured yellow or yellow- brown ; it is insoluble in concentrated sulphuric acid, but soluble in boiling caustic G 2 84 MORPHOLOGY OF TISSUES. potash. In submerged organs and roots it is very thin, difficult to be seen immediately, but rendered visible by iodine and sulphuric acid. The true cuticle is much thicker in aerial stems and leaves ; it may be obtained in them even in large lamellae by decay or solution of the subjacent cells in concentrated sulphuric acid. In many cases, and especially in stout leaves and internodes, the outer wall of the epidermis-cells lying beneath the cuticle is strongly often enormously thickened ; while the inner-walls re- main thin, the lateral walls are usually strongly thickened outwardly, becoming inwardly suddenly thinned. The thick portions of the wall are usually differentiated into at least two shells ; — an inner thin shell, immediately surrounding the cell-cavity, shows the reactions of pure cellulose, while the epidermal layers lying between it and the cuticle are more or less cuticularised, and the more so the nearer they lie to the cuticle. Not unfrequently these layers of cuticle extend downwards in the thick part of the side-walls, in which case the middle lam*ella sometimes behaves like the true cuticle, with which it is in contact on the outside. Like the isolated cells of the cuticle (pollen- grains, spores), the epidermis has also a tendency to form projecting lumps, knots, ridges, &c., but they almost always remain very insignificant-, and are best seen on a superficial view ; as, for example, in many delicate petals (cf. sect. 4, (e)). According to the recent researches of De Bary, particles of wax are deposited in the substance of the cuticular layers of the epidermis which cannot be seen on section, but separate in the form of drops when warmed to about 100° C. This deposit of wax (often combined with resin) is one of the causes which protect the aerial parts of plants from becoming moistened with water. But very frequently the wax extends in an unexplained manner over the cuticle, and becomes deposited there in different forms, forming the so-called bloom on fruits and some leaves, or as a continuous shining coating, which is reformed on young organs after being wiped off, and in ripe fruits of Benincasa cerifera (the wax-cucumber) appears again long after maturity. De Bary distinguishes four principal forms of this wax-coating. The bloom or gloss which is easily wiped off consists of small particles of two forms: — (i) of quantities of delicate minute rods or needles, e. g. the white-dusted Eucalypti, Acaciae, many Grasses, &c. ; or of granules collected into several layers, as in Kkinia jicoides and Ricinus communis ; these are aggregated wax-coatings. (2) Simple granular coatings consist of grains iso- lated or touching one another in one layer ; this is the most common form, e. g. in Iris pallida, Allium Cepa, Brassica oleracea, &c. (3) Coatings of minute rods consisting of thin, long, rod-shaped particles, bent above or even curl-shaped, and standing perpen- dicularly upon the cuticle, e. g. Eeliconiafarinosa and other Musaceae, Cannaceae, Saccha- rum, Beyiincasa cerifera, leaves of Cotyledon orbicularis. (4) Membrane-like layers of wax or incrustations ; (a) as a gritty glazing in Sempervivum, Euphorbia Caput-Medusce, T:huja occidentalis ; (b) as thin scales, in Cereus alatus, Opuntia, Portulaca oleracea, Taxus baccata ; (c) as thick connected incrustations of wax, which sometimes permit a finer internal structure to be recognised, similar to the striation and stratification of the cell-wall : Euphorbia canariensis, fruits of species of Myrica, stems of Panicum turgidum. On the stem of the Peruvian wax-palms, especially of Ceroxylon andicola, these incrus- tations attain a thickness of 5 mm. ; those on the stem of Chamcedorea Schiedeana are thinner, but of similar structure. According to Wiesner (Bot. Zeitg. p. 771, 1871), these flakes of wax consist of doubly refractive four-sided prisms standing perpendi- cularly close to one another. Hairs ^ are products of the epidermis ; they originate from the growth of single epidermis-cells, and are present in most plants in large numbers ; when they are ^ A. Weiss, Die Pflanzenhaare, in vols. IV and V of the Bot. Untersuchungen aus dem phys. Laborat. by ICarsten, 1867.— J. Hanstein, Bot. Zeitg. p. 697 et seq., 1868.— Rauter, Zur Entwickel- ungsgeschichte einiger Trichomgebilde. Wien 1871. [See also J. B. Martinet : Organes de secretion des vegetaiix. Ann. des Sci. Nat. Fifth series, vol. XIV, 1871.] THE EPIDERMAL TISSUE. 85 wanting in any part of a plant, it is termed glabrous. Their form is subject to ex- traordinary variation. The first indication of the formation of hairs occurs in the papillose protuberances of the epidermis of many petals, to which their velvety ap- pearance is due. To the simplest forms belong also the root-hairs which grow from the epidermis of true roots or underground stems {Pteris aquilhia, Equisetum, &c.) ; they are thin-walled bag-like protuberances of the epidermis-cells which lengthen by growth at the apex, or only branch exceptionally (as sometimes in Brassica Naptis). In Vascular Cryptogams their wall readily acquires a brown-red colour; their length of life is usually short, and when they die all trace of them disappears. In a similar manner the woolly hairs behave which appear early on the leaves and internodes of vascular plants, while still in the bud, especially Dicotyledons. On the unfolding of these organs they commonly fall off and disappear, as in the horse-chestnut, Rhododendron, and Aral'm papyri/era, where they form a felt easily wiped off from the freshly developed leaves ; in other cases they remain as a woolly coating, especially on the under-sides of leaves. In prickles the wall is mostly thicker, silicified, and hard ; they are shorter than the woolly hairs, pointed upwards, and a septum separates the prominence from the mother-cell. When two or more points endowed with a greater power of growth in their surface and apex arise on the free outer wall of unicellular hairs, branched forms result with continuous cavity. The papillose bulging of the epidermis-cells may become separated by a septum ; the hair then consists of a basal cell fixed in the epidermis and of a free hair-cell (as in Aneimia fraxinifolid) ; but the separated papilla may also become segmented by the formation of more or less numerous septa, when the hair grows considerably in length, and thus arise segmented hairs (as e.g. on the filaments of Tradescantia). Sometimes the segments form lateral shoots ; and thus arise tree-like branched structures with whorled or alternate branches {e.g. Ver- bascum Tbapsits, Nicandra physaloides). If longitudinal divisions occur in the segment- cells of the hair, or if the hair continues to grow by an apical cell which forms segments on two sides, flatly expanded hairs arc the result. To this form belong, for example, the so-called palcae of Ferns which sometimes entirely cover the younger leaves. Finally the divisions in the young hair may be so arranged that it presents at length a tissue, which on its part may again assume different forms, e.g. the pappus-like hairs of Eieracium aurantiacum and Azalea indica, the capitate hairs of Korrea and Ribes ianguineum. Very commonly the terminal cell of a segmented or the end of a solid hair (j. e. of one consisting of a mass of tissue), swells in a globular manner, and then usually forms a multicellular gland, while the cells of the head produce peculiar secretions. (On these Glandular Hairs cf. sect. 17, (b),) Not unfrequently the papilla which projects above the epidermis and is separated by a septum becomes divided^by vertical and radial walls, expanding in a disc-like manner, so that the head consists of a radially arranged disc of numerous cells; thus arise the peltate hairs, e.g. of Eleagnus, Hippuris, and Pin- guicula. Tufts of hairs arise when the mother-cell of the hair which belongs to the epidermis breaks up early into several cells lying close to one another ; each of which then grows independently into a hair, as is shown in Fig. 72, which is completed by Fig. 44, p. 43. Not unfrequently a luxuriant growth of the parenchyma takes place beneath the hair ; and this is imitated also by the epidermis ; the hair itself is then borne on a peg- shaped prominence or protuberance of the leaf or stem, and is often deeply implanted into it in its lower part ; as, for instance, in the prickles (stinging hairs) of the stinging- nettle. Thus also the prickles (climbing hairs) on the six projecting angles of the stem of the hop grow into a large basal protuberant mass of tissue, while the hair-cell grows in opposite directions into two sharp points. Such double-pointed unicellular hairs occur also on the under-side of the leaf of Malpighia urens ; they are 5-6 mm. long, fusiform, very thick-walled, and grow into the epidermis by their central part (without protuberance). In this case they easily become detached, and remain sticking in the H6 MORPHOLOGY OF TISSUES. skin of the hand which touches the leaf. (For further details on the Morphology of Hairs cf. sect. 22.) Fig. 72. — Development of the hairs on the calyx of a flower-bud oi Althu-a rosea (X300) ; A wh woolly-hairs of the inner side ; b and c glandular hairs in different stages of development ; at a (to the right) rudiment of a glandular hair ; ep always signifies the (still young) epidermis. The figures a.'\n A,^ (to the left) and "y (to the right below) show the first stages of development of the stellate hairs (or rather tufts of hairs), the subsequent condition of which may be compared in Fig. 44 (p. 43) ; at ^ a is the hair in longitudinal section ; /3 and y show the appearance seen from above ; the cells are rich in protoplasm ; the formation of vacuoli (->) in the protoplasm is beginning in y. T:he Stomata^ are always absent from the epidermis of true roots; on the other hand they are usually present on underground axial organs and leaves ; even on sub- merged parts they are occasionally found (Borodin, /. c.) ; but they are formed in the largest numbers on the aerial internodes and leaves, but are not altogether absent from the petals and carpels ; they are even formed in the interior of the cavity of the ovary (e.g. in Ricinus). They are most abundant where an active interchange of gases takes place between the plant and the surrounding air ; for, considered physiologi- cally, they are nothing more than the mouths of the intercellular spaces of the inner tissue which open in places externally between the epidermis-cells ; this is however always preceded by a peculiar development in a young epidermis-cell. Since the stomata do not arise till rather late, that is during or after the expansion of the internodes and leaves, their arrangement is partially dependent on the already elongated form of the epidermis-cells ; if these are greatly elongated in one direction and arranged in rows (as in Equisetum and the stem and leaves of many Monocotyledons, and Pinus), the stomata also appear arranged in longitudinal rows (the cleft lying in the direction of the axis of groAvth, the guard-cells right and left) ; if the epidermis-cells are irregular on a superficial view, curved, &c., the position of the stomata is more undefined and ^ H. von Mohl, Verm. Schriften bot. Inhalts, pp. 245, 252. Tubingen 1845. — Ditto, Bot. Zeitg. p. 701, 1856. — A. Weiss, Jahrb. fiir wiss. Bot. IV, p. 125, 1865. — Czech, Bot. Zeitg. p. loi, 1S65. — Strasburger, Jahrb. fiir wiss. Bot. V, p. 297, 1866.— E. Pfitzer, ditto, VII, p. 532, 1870. — J. Rauter, Mittheil. der naturwiss. Vereins fiir Steiermark, vol. II, Heft 2, 1870. — Borodin, Bot. Zeitg. p. 841, 1870. — Hildebrand, ditto, p. 1. — Ditto, Einige Beobachtungen aus dem Gebiete der Pflanzen- anatomie. Bonn 1861. THE EPIDERMAL TISSUE. 87 apparently irregular. The number of the stomata is generally extraordinarily great in the epidermis of organs containing chlorophyll ; A. Weiss counted on one square mm. in 54 species examined i-ioo stomata, in 38 species 100-200, in 39 species 200-500, in 9 species 4oa-5oo, and in 3 species 600-700 stomata. The origin of stomata is always the result of the formation of a mother-cell, first of all by division Figs. 73-75. — I'onu.Umn of the stoin.-ila of ihc loaf of Hyacinthtis nrientalis, seen from the surface (Xoi;)o) ; tlie preparations were made from leaves, wliich were at first 3-4 cm. long ; they were obtained simply by removing the epidermis ; e always signifies the cpidennis-cells, S the stoma. The order of development is Fig. 73, A, S' , S", S'" ; then Fig. 74, A, />, and finally Fig. 75. In Fig. 73, fi, a piece of cpidennis is represented with the mother-cells of the stomata already divided, after extraction of the protoplasm by solution of pot.ish and acetic acid. Preparations of this kind show most distinctly that the partition-wall never grows from without inwards ; it is either entirely absent or present along the whole surface. Fig. 75 shows the guard-cells after treatment with potas- sium odidc ; the protoplasm has contracted ; the stoma is not yet perfectly developed. of a young epidermis-cell, which is sometimes preceded by several preparatory divisions in it or in the adjoining epidermis-cells ; and this mother-cell becomes more and more rounded off, and the guard-cells of the stoma are produced from it by division. The variety of these processes up to the point when the opening itself appears, can hardly be explained in a few words ; I prefer therefore to describe some examples more minutely. One of the simplest is afforded by the development of the stomata on the leaf of Hyacinthus orientalis, which has already been depicted in transverse section in Figs. 61-64 (p. 75); these the reader must compare with Figs. 73-75, which represent the process seen from the surface. The preparation for the formation of the stoma is here very simple: — a nearly cubical piece of a long epidermis- cell (Fig. 73, A, S, S") is separated by a septum, and this is the mother-cell of the stoma. It is divided by a longitudinal wall (/*. e. by one lying in the direction of the axis of growth of the leaf, and standing at right angles to its surface) into two equal cells, which round themselves off as they grow. The manner in which the opening follows the partition-wall has already been described in Figs. 61-64, and can nov/ easily be understood with help of the superficial view in Fig. 74. In Equisetum Hmosum a similar appearance to that represented in Fig. 73 shows itself immediately after the first formation of the mother- cells of the stomata; but the mother-cell undergoes in these cases three divisions, first one obliquely to the right, then one obliquely to the left ; finally the tniddle one of the cells which originate in this manner is bisected by a wall standing at right angles to the surface. Four cells thus arise in one plane, of which the two outer ones grow more rapidly, while the inner are forced downwards and come to lie beneath them; the stoma then appears, when perfect, as if it had been formed according to the Hyacinthus type, in which each guard-cell has been again 88 MORPHOLOGY OF TISSUES. divided into an upper and a lower cell. But, according to Strasburger, this is not the case, the two pairs of guard-cells lie originally in one plane, and, strictly speaking, it is only the middle cell, — which is divided by a perpendicular wall, and the splitting of which forms the cleft, — that is to be considered as the mother-cell of the stoma ; the two oblique divisions by which the two lateral cells are formed that afterwards lie uppermost, must be regarded merely as a preparation for the formation of the mother-cell. Preparatory divisions of this kind occur in many Phanerogams ; one of the young epidermis -cells becomes the primary mother -cell of the stoma, and is divided successively in different directions by walls, which stand at right angles to the surface ; finally we have a cell surrounded by several cells formed in this manner, which afterwards forms the two guard-cells (as in Crassulacese, Begoniaceae, Cruciferse, Violarieae, Asperifolieae, Solanacese, Papilionaceae). In other plants, on the contrary, after the formation of the mother-cell of the stoma, which results from the division of a young epidermis-cell, divisions also take place in the adjoining epidermis-cells, so that the stoma is surrounded by a pair or by two decussate pairs, or by some other arrange- ment of epidermis-cells, which stand in relation to the stoma according to their origin and development; (as in Aloe socotrina, Gramineae, Juncaceae, Cyperacese, Alismaceae, Marantaceae, Proteaceae, Pothos crassiner'via, Ficus elastica, Goniferae, Tradescantia %ehrina). The origin of the mother-cell of the stoma in Plantaginaceae, Oenotherese, Silenese, Cen- tradenia, and many Ferns is of special interest in the mode of their cell-division. In these cases the mother-cells^ are so developed that from the young but already tolerably large epidermis-cell, a small piece is cut out on one side by a wall bent in a U-shape, the convexity of which faces the centre of the epidermis-cell, while its margins are applied to one of its side-walls. Not unfrequently, especially with Ferns {e.g. Asplenium bulbiferum, Pteris cretica, Cibotium Schiedei, &c.), preparatory cells are cut out in this manner from, the epidermis-cell before the period of the formation of the stoma-cell, out of which moreover the guard-cells are formed by simple longitudinal division. In consequence of the U-shape of the division -wall which separates the mother- cell of the stoma from the epidermis-cell, the former is half, or more than half, en- closed by the latter, when the epidermis is looked at from above. In some Ferns (and Silene?e) the wall of the mother-cell of the stoma is from the very commencement so strongly curved that it touches one side of the upper epidermis-cell only in one narrow band ; in Aneimia 'villosa it touches it only at one point, the curved partition-wall as seen from above appearing annular. In Aneimia densa and A. fraxinifolia the side- wall of the upper epidermis-cell does not anywhere touch the wall of the mother-cell of the stoma '^. At its commencement this cell has the form of a hollow cylinder, or, more exactly, of a truncated cone, the bases of which are portions of the upper and lower wall of the upper epidermis-cell ; out of the latter a cell is thus cut out like a piece out of a cork by a corkborer; the piece thus cut out is the mother-cell of the stoma, and thus arises the remarkable arrangement represented in Fig. 76, where, as may be seen, the two guard-cells are enclosed by a single annular epidermis-cell. Similar, but more complicated, are, according to Rauter, the arrangements in Nipho- bolus Lingua. By further growth of the guard-cells and of the epidermis-cells which surround them, different relative positions of the former to the surface may be brought about ; the guard-cells may, when mature, lie in one plane with those of the epidermis, or may be deeply pressed down and apparently belong to a deeper layer of cells ; sometimes they are on the contrary elevated above the surface of the epidermis. ^ Strasburger calls them ' special mother-cells.' I think it, however, better entirely to abandon this expression, the more so as its first introductitin in the formation of pollen depended on an obsolete view of the formation of the cell-wall (compare our description, pp. 32, 33). 2 Strasburger, Jahrb. fiir wiss. Bot. VII, p. 393. THE EPIDERMAL TISSUE. 89 The stomata of Marchantia may shortly be mentioned here in connexion with what has already been said on Fig. 65. After the formation of the air-cavities which are filled with outgrowths containing chlorophyll, one cell of the epidermis lying above the Fin. 76. — Supcrticial view of the stoma of Aneifniafraxitn- folia: \s\\.\\ the epiderniis-cclls surrouno) originates surrounded by four, six, or more guard-cells (Fig. 78, 5 and C, j/). Each of these cells is finally divided Fig. n%.— Marchantia polymorpha. Part of a young receptacle ; A vertical section, o epidermis, 5 partition-wall between the air-cavities with their chlorophyll-cells chl; ^ large parenchyma-cell; j/ stoma; B and C young stomata seen from above (X5So). by walls parallel to the epidermis-cell into 4-8 cells lying one above another, and the stoma becomes a channel surrounded by 4-8 or more rows of cells. 90 MORPHOLOGY OF TISSUES. (c) Cork, and Epidermal Formations produced by it'^ (Periderm, Lenticels, Bark). When succulent organs of the higher plants, no longer in the bud-condition, are injured, the wound generally becomes closed up by cork -tissue ; i. e. new cells arise near the wounded surface by repeated division of those which are yet sound, and these, forming a firm skin, separate the inner living tissue from the outer injured layers of cells. The walls of this tissue offer the strongest resistance to the most various agencies ; shnilar to the cuticular layers of the epidermis in their physical behaviour, flexible and elastic, permeable only with difficulty by air and water, they for the most part soon lose their contents and become filled with air. They are arranged in rows lying at right angles to the surface, of parallelopipedal form, and form a close tissue without intercellular spaces. These are the general distinguishing features of cork-tissue. It is formed not merely on wounded surfaces, but arises in much greater mass where succulent organs require an effectual protection {e.g. potato-tubers), or where the epidermis is unable to keep up with the increase of circumference when growth in thickness continues for a long period. In these cases, which occur but seldom in Monocotyledons {e. g. stem of Dracaena), but very generally in several-year-old stems and roots of Conifers and Dico- tyledons, the cork-tissue is formed even before the destruction of the epidermis ; and when this splits under the action of the weather and falls off, the new envelope formed by the cork is already present. The cork-tissue is the result of repeated bipartition of the cells by partition-walls, rarely in the epidermis-cells themselves, more often in the subjacent tissue. These partition-walls lie parallel to the surface of the organ ; here and there, where the increase of the circumference necessitates it, divisions also take place in a vertical direction, by which the number of the rows of cells is increased. From the two newly formed cells of each radial row (/. e. perpendicular to the surface of the organ) one remains thin-walled and rich in protoplasm, and in a condition capable of divi- sion ; the other becomes transformed into a permanent cork-cell. Thus arises usually parallel to the surface of the organ a layer of cells capable of division, which continues to form new cork-cells, the Cork-cambium or layer of Phellogen. In general this is the inner- most layer of the whole cork-tissue, so that the production of cork advances outwardly, and new layers of cork are constantly formed out of the phellogen on the inner surface of those already in existence. But, according to Sanio, it also happens at the commencement of the formation of cork that the formation of permanent cells proceeds centripetally, or an alternation of centripetal and centrifugal cell-formation takes place in t\iQ young cork- tissue. But sooner or later the centrifugal formation of cork always takes place with phellogen lying on the inner side, which follows from the circumstance that the tissues lying on the outside of completely suberised layers of cells sooner or later die. Usually the formation of cork begins first at single places of the periphery of lignified branches ; but gradually the phellogen forms a connected layer, from which new layers of cork are con- tinually pushed forwards outwardly. When in this manner a continuous layer of cork arises, steadily increasing from the inside, it is termed Periderm. The development and configuration of the cork-cells may change periodically during the formation of periderm ; alternate layers of narrow thick-walled and broad thin-walled cork-cells are formed ; the periderm then appears stratified, like wood showing annual rings (as in the periderm of Quercus Suber, Betula alba, &c,). In some cases the phellogen of the periderm gives rise not only to cork-cells, by which the periderm increases in thickness, but parenchyma- cells are also formed containing chlorophyll ; this always however happens in such a man- ner that only daughter-cells of the phellogen lying on the inner side (facing the substance of the wood) undergo this metamorphosis into permanent parenchyma-cells containing ' H. von Mohl, Vermischte Schriften bot. Inhalts, pp. 221-233. Tubingen 1S45. — J. Hanstein, Untersuch. iiber den Bau u. die Entwickelung der Baumrinde. Berlin 1853. — Sanio, Jahrl). fiir wiss. Bot. II, p. 39. — Merklin, Melanges biol. du Bulletin de TAcad. Imp. dcs sciences de St. Petersbourg, vol. IV, Feb. 26, 1864. THE EPIDERMAL TISSUE. chlorophyll. In this manner the green cortical tissue of some dicotyledonous plants becomes thickened by the layers of tissue proceeding from the phellogen, which Sanio terms the suberous cortical layer (Phelloderm). This occurs, for example, in two year or older branches of Salix purpurea and S. alba, the beech, &c. In such cases the phel- logen lies between the periderm and the phelloderm, the outer of its daughter-cells producing cork- cells, the inner phello- derm (Fig. 79). The layers of periderm which first undergo conversion into cork sometimes bear a very close resemblance to true epidermis, as, for instance, in first years' branches (August) of Pinus syh'estris, where, while the epidermis still remains, the cork-cambium is formed in the cor- tical parenchyma, and at first presents the appearance as if a second epidermis were formed with cells greatly thickened on the outside. As the epidermis is at first replaced by the periderm, so the periderm is afterwards replaced by the formation of bark when the increase in thickness continues long and vi- gorous. In larger woody plants, as oaks and poplars, the surface of one-year-old boughs is covered with epidermis, that of several-year- old boughs with periderm, that of the older branches and of the stem with bark^ The formation of bark depends on the repeated production of new lamellae of phellogen in the succulent cortical tissues of Conifers and Dicotyledons which continue to grow from within outwards. Layers of cells which can extend themselves through the most different tissues of the cortex, become changed into cork-cambium, which be- comes torpid after the production of thicker or thinner lamellae of cork, /. e. cork cut out, so to speak, from the Fu;. 79. — roriuation of cork in a one-year old branch o Ribes nigrinn; part of a transverse section; e epidermis, h hair, b bast-cells, pr cortical parenchyma distorted by the in- crease in thickness of the branch ; A' the total product of the phellogen f; /t the cork-cells arranged radially in rows formed from c in centrifugal order, pd phelloderm (parenchyma con- taining chlorophyll formed from c in centripetal direction) (Xsso). /. e. ceases to be active. These lamellae of cortex, scaly or annular pieces of the surface ; everything which lies outside them becomes dried up ; and since this process is con- stantly repeated on the outside of the stem, and the new lamellae of cork continu- ally intrench further on the growing cortical tissue, a layer, constantly increasing in thickness, of dried up masses of tissue becomes separated from the living part of the cortex, and this is the Bark. The process is very clear in the bark of the oriental plane which detaches itself in large scales ; and almost as clear in old stems of Pinus syl'vestris. Since the bark does not follow the increase in thickness of the stem, it splits in longi- tudinal crevices from the surface inwards, as in the oak, if the direction of weakest cohesion requires it ; in other cases it peels off in the form of horizontal rings from the stem (ring-bark), as in the cherry. The Lenticeh are a peculiarity of cork-forming Dicotyledons ; they appear before ^ A considerable increase of thickness is not always combined with the formation of periderm, as, e. g. in the sunflower and other annual stems. In Viscum, the epidermis always remains capable of development, and its thick cuticular layers render the protection of periderm super- fluous ; the formation of cork is also not a necessary consequence of vigorous increase of thickness ; the copper-beech and the cork-oak, for example, form only periderm. 9a MORPHOLOGY OF TISSUES. the formation of periderm in one-year-old branches as long as the cortex is still ' covered with uninjured epidermis; and are visible as roundish flakes. At the end of the first or in the following summer, the epidermis splits above the lenticel in the direction of its length ; it becomes changed into a more or less projecting wart, which is often divided by a central furrow into two lip-shaped rolls ; their upper surface is generally brown, their substance to a certain depth dry, brittle, and cork-like. With the further increase in thickness of the branch, the lenticels become extended in breadth and present transverse striae ; when afterwards cork or bark is formed, the splitting of the cortex in the lenticels commences, and they become indistinguishable (as in the silver poplar, apple and birch) ; by the scaling off of the bark they are of course removed. According to Unger, the lenticels arise only at those places of the cortex where stomata occur in the epidermis ; according to JMohl the inner cortical parenchyma pro- jects in a wart-like manner through the outer, and forms there a cork -tissue, which, on the formation of periderm, coalesces with the cork of the periderm ; as occurs also, for example, in young potato-tubers. The formation of cork on the lenticel lasts for a series of years, until the cortex which afterv/ards grows from within dies off on the outside, the periderm or bark-formations becoming interposed between the lenticels and the living part of the cortex. In many trees (as Crat^gus, Pyrus, Salix, Populus), where the formation of periderm begins from single points, and then becomes further extended in breadth, the lenticels are, according to Mohl, the points of departure. Sect. i6. The Fibro -vascular Bundles \ — The tissue of the higher Cryptogams and of Phanerogams is traversed by string-like masses of tissue, which in some cases develop by increase in thickness in such a manner that they lose externally the form of strings and present that of strong masses, retaining, however, in- ternally the corresponding structure. These are the Fibro-vascular Bundles. Very often they can be completely isolated with ease from the rest of the tissue of the plant. If, for instance, the leaf- stalk of Plantago major is broken across, they hang out from the paren- chyma as tolerably thick, flexible, elastic threads. In Pteris aqiiilina it is possible, by scraping off the mucilaginous parenchyma after removing the hard skin of the underground stem, to ex- pose them as strap-shaped or filiform very firm light yellowish bands (Fig. 80). From older foliage-leaves of trees, dry pericarps (as Da- tura), stems of Cactus, tSic, the fibro-vascular bundles are left, through the decay of the par- enchyma which surrounds them, as a skeleton Fig. Zo.— Pteris aqidlina. A transverse section of the underground stem (natural size) ; r brown hard epidermal tissue ; / soft mucilaginous parenchyma, rich in starch ; pr dark-walled sclerenchyma, forming two broad bands penetrating the stem ; ag fibro- vascular bundles running outside these bands of sclerenchyma ; ig others running within them. B the fibro-vascular bundle represented in A, iso- lated by scraping off the parenchyma ; it shows divi- sions and anastomoses ; the dotted lines u show the outline of the stem st, its forked branches st' and st", and a leaf-stalk b. ^ H. von Mohl, Vermischte Schriften, pp. 108, 129, 195, 268, 272, 285, 1845,— Ditto, Bot. Zeitg. p. 873, 1855. — Schacht, Lehrb. der Anat. u. Phys. der Gewuchse, pp. 216, 307-354, 1856. — Nageli, Beitrilge zur wiss. Bot. Leipzig 1858. Heft i.— Sanio, Bot. Zeitg. no. 12 et seq. 1863.— N.lgeli, Das Dickenwachsthum des Stammes u. die Anordnung der Gefassstriinge bei den Sapindaceen, Miinchen 1864. — Rauwenhoff, Archives Neerlandaises, vol. V, 1870. (Caractere et formation du liege dans les dicotyledons.) THE FIBRO-VASCULAR BUNDLES. 93 imitating more or less the form of the whole. Exceptionally beautiful and instructive skeletons of this nature are afforded by the stems of Tree-ferns, Dracaena, Yucca, Maize, &c., when their parenchyma is perfectly destroyed by long-continued decay, and only the epidermal tissue and the firm bundles in the interior remain. The beginner would do well in any case to prepare for himself preparations of this kind, or to examine them in collections ; they are, at least at first, extremely useful for a right comprehension of their structure. This is, however, the case only with lignified fibro-vascular bundles when they run isolated between soft parenchyma ; in some plants, on the contrary, the tissue of the bundles is even softer and more deli- cate than that of their environment {e. g. Ceratophyllum, Myriophyllum, Hydrilleoe, and other water-plants) ; in these cases they cannot of course be isolated. But in the older Hgnified stems and roots of Conifers and Dicotyledons, the fibro-vascular bundles are so densely crowded, and so developed by further tissue-formation, that at last very little or even nothing is left of the original fundamental tissue which separated them, and such stems consist almost entirely of fibro-vascular masses. Each separate fibro-vascular bundle consists, when it is sufficiently developed, of several different forms of tissue, and must therefore itself be considered as a tissue- system ; but different bundles, often in very large number, unite in most plants to form a system of a higher order. At present however we shall consider only the separate bundle. The fibro-vascular bundle consists at first of similar cells combined without intercellular spaces^- this form of tissue of the young bundle, which has not yet undergone differentiation, may be termed Procamhium ^. As it grows older, single cells of the rows forming the young bundle change into permanent cells of definite form (vessels, bast, &c.) ; from these points of origin the transformation of the procambium-cclls into permanent cells in the transverse section of the bundle advances until the cells are altogether changed into permanent cells ; or an inner layer of the bundle remains in a condition capable of further development, and is then called Cambium. In advanced age there are thus bundles devoid of and bundles containing cambium ; the former may be termed closed, the latter open ^. As soon as a procambium bundle has become transformed into a closed fibro- vascular bundle, all further growth ceases, as in Cryptogams, Monocotyledons, and some Dicotyledons. The open fibro-vascular bundle, on the other hand, continues to produce new layers of permanent tissue on both sides of its cambium, and thus the portion of the stem or root concerned continually increases in thickness, as occurs in woody Dicotyledons and Conifers ; the leaf-structures, however, of these ^ The young cells of the fibro-vascular masses are not always elongated and prosenchymatous ; in the roots, e.g. oi Zea Mais, the young tissue-cells which no longer divide and their neighbours are diagonally tabular or cubical. ^ Nageli calls the tissue of the young fibro-vascular bundles simply Cambium, and distinguishes by the same term the tissue, capable of further development, of the bundles which increase in thick- ness, which nevertheless ought to be distinguished from them. — Sanio terms the latter only Cam- bium, which I adopt. (Sanio in Bot. Zeitg. p. 362, 1863.) ^ This distinction was first made by Schleiden, but he incorrectly ascribed to Dicotyledons in general only open bundles ; his distinction of simultaneous and successive cannot be sustained ; all bundles become differentiated successively in transverse section. Sclileiden's simultaneous bundles of the higher Cryptogams belong to the closed description. 94 MORPHOLOGY OF TISSUES. plants possess closed bundles, or, if they are open, the activity of their cambium soon ceases. The different forms of tissue of a differentiated fibro-vascular bundle may be classified into two groups, which Nageli calls the Phloem- (Bast) and Xyle7n- (Wood) portion of the bundle. They are separated by the cambium if there is any. In many bundles the phloem is formed on one, the xylem on the other side of the procam- bium, and the development of both advances towards the centre of the bundle, where at length they meet. The phloem consists of succulent, generally thin-walled cells ; Fig. 8i.— Transverse section of a closed fibro-vascular bundle in the stem of maize (X550) ; fip the surrounding thin-walled parenchyma ; a outer side, i inner side (facing the axis of the stem) ; g g two large pitted vessels ; s spirally thickened vessel ; r isolated ring of an annular vessel ; / air-containing cavity, from splitting caused by growth ; •z/z- the cambiform or latticed cell-tissue which has passed over last into permanent tissue ; between it and the vessel j lie reticulately thickened and bordered pitted vessels ; the periphery of the whole bundle forms a firm sheath of thick-walled lignified prosen- chyma-cells. only the bast-cells, which are often absent, but very often massively developed, are usually greatly thickened (mostly however not lignified but flexible). These thin-walled succulent cells are either parenchymatous, or they are cambiform or latticed-cells, or finally sieve-tubes. The xylem-portion of the fibro-vascular bundle has mostly a strong tendency to thicken its cell-forms ; their walls become hard and woody ; in vessels and the bordered pitted wood-cells the contents disappear, and they henceforth conduct air. Woody parenchyma is also abundant, but in some cases the lignifying does not take place; the whole bundle is then soft and succulent, sometimes traversed only by single thinner bundles of lignified vessels and wood-cells THE FIBRO'VASCULAR BUNDLES. 95 (as in the roots of radish, tubers of the potato, &c.). The elements of the fibro- vascular bundles, as far as they consist exclusively of procambium, are prosenchy- matous or at least elongated in the direction of the axis of growth of the bundle. In open bundles there arise also in the cambium, with the increase of their thickness, horizontally extended rows and layers of cells disposed radially, by which the later- formed xylem- and phloem-layers of the bundle become arranged in a radial fan-like Fig. 82.— Transverse section of a fibro-vascular bundle in the mature elongated hypocotyledonary portion of the stem oi Ricinus cotnnutnis ; r cortical parenchyma; 7n parenchyma of the pith; * bast : y phloem portion with thin-walled cells; c cambium ; g g large pitted vessels ; t t smaller pitted vessels with wood-cells between them ; cb continuation of the cambium into the parenchyma lying between the bundles; the parenchyma-cells are repeatedly divided by tangential walls. (Between the cortex (r) and the phloem of the bundle lies a layer filled with compound starch-grains, the bundle-sheath, or starch-bearing layer.) manner ; these horizontal elements mostly assume the character of parenchymatous cells, and may be generally designated as rays ; within the xylem they are called xylem-rays, within the phloem, phloem-rays. The position of the layers of phloem and xylem in the transverse section of a bundle varies according to the class to which the plant belongs and the organ in which they are found ; in the open bundle in the stem of Dicotyledons and Conifers the former lies towards the circumference ^ the latter facing the axis of the organ ; between the two lies the cambium -layer (Fig. 82). But it sometimes occurs that a ^ In Dicotyledons bundles also occur exceptionally within the circle of wood proper (in the pith), where the phloem portion is surrounded by wood as by a sheath (in the rachis of the inflo- rescence of Ricinus) ; in Heterocentron roseum the medullary bundles have, according to Sanio, their 96 MORPHOLOGY OF TISSUES. layer of phloem is found in addition on the axial side of the xylem, so that the bundle possesses two phloem-layers, a peripheral, and an axial {e. g. in Cucurbitaceae and Nicotiana). In the closed bundles there occur, among Dicotyledons, considerable deviations from the typical position of the tissues ; among Monocotyledons these are still more conspicuous, especially if the sheath of lignified prosenchyma, which often occurs with them, is taken into account (see Fig. 8i). Among Ferns, Lycopodiaceae (with isolated bundles ^), and Rhizo- carpeae, the xylem lies in the centre of the transverse section, while the phloem forms a soft succulent sheath around it (Figs 67 and 83). Every one of its cell-forms may at one time or other be absent from a fibro-vascular bundle; bundles may occur without wood-cells, without vessels (very rarely), without true bast, &c. ; it is only the soft bast (the succulent thin-walled cells of the phloem) that is scarcely ever absent. All these variations may occur in the same fibro-vascular bundle in dif- ferent parts of its length, when this is considerable. The bundles of true roots are frequently (not always) without any true bast ; the termina- tions of the bundles which traverse the stem of Phanerogams are found in the leaves; there, as their thick- ness decreases, they lose all the ele- ments of the xylem except one or two spiral vessels, and finally these also ; the extreme ends of these bundles which traverse the mesophyll of the leaves often consist only of long narrow thin-walled succulent or of cambiform cells (Fig. 16, F, p. 21). If the fibro-vascular bundle is formed at the very earliest period within an organ which afterwards grows rapidly in length, then the elements which were formed before the increase in length (the innermost vessels and the outermost bast-cells) are the longest, since they participate in the whole increase of length of the organ ; the elements developed later, during the elongation, are shorter ; and those are shortest of all which arise after the increase of length of the whole organ has been Fig. 83.— a fourth of the transverse section of one of the large fibro- vascular bundles in the stem of Pteris aqtiilina, with a portion of the surrounding parenchyma,/; this is filled with starch (in winter) ; s spiral vessel in the focus of the elliptical transverse section of the bundle, sur- rounded by thin-walled wood-cells containing starch ; g- g the vessels tliickened in a scalariform manner, the structure of which is explained in Fig. 29 (p. 27) ; sp wide lattice-cells, between them and the xylem lies, in the winter, a layer of cells containing starch ; b bast-like cells, with thick soft wall ; sg the bundle-sheath ; between b and sg is a layer of cells containing starch. vessels in the 'centre ; they are completely surrounded by cambiform tissue ; in Campanula lattfoUa, according to the same authority, the bundles of the inner circle behave in the same manner as in Ricinus. (Cf. Bot. Zeitg p. 179, 1865.) * The bundle in the stem of Lycnpodimn chamiEcyparissii':, &c., is clearly a union of several fibro-vascular bundles THE FIBRO-VASCULAR BUNDLES. 97 completed ; this occurs in particular with the open bundles of Dicotyledons and Conifers. The development of the elements of a bundle always begins at single points in the transverse section, and extends from them in different directions ; and thus the permanent cells which arise one after another acquire different mature forms. In the open bundles in the stem of Dicotyledons and Gymnosperms the development usually begins with the thickening of single bast-cells on the peripheral side of the bundle ; somewhat later single spiral vessels (or annular- vessels) arise next the pith ; and while the development of the phloem proceeds centripetally, forming succes- sively and often alternately bast-cells, latticed cells and parenchyma, — annular and UJ "-1-^^, r^ T!| Fig. 84.— Longitudinal section of the fibro-vascular bundle of Ricinus, the transverse section being shown in Fig. 82; r cortical parenchyma ; g^s bundle-sheath ; tn parenchyma of the pith ; b bast-fibres ; / phloiim-parenchyma ; c cambium, the row of cells between c and / is afterwards developed into a sieve-tube. In the xylem portion of the bundle the elements are developed beginning from s gradually to ^ ; s the first narrow and very long spiral vessel, s' wide spiral vessel, both with a spiral band which can be unrolled ; / vessel thickened partly in a scalariform, partly in a reticulate manner ; h h' wood-cells ; / pitted vessel ; at q the absorbed septum; h" h"' wood-cells; f pitted vessel, still young; the pits at first show the outer border; afterwards the formation of the inner pore commences; at tt' t" in the wall of the vessel are observed the boundary- lines of the adjoining cells which have been removed. spiral vessels^ either separately or together, or reticulated vessels and eventually pitted vessels often alternating with wood-cells, arise centrifugally in the xylem (Fig. 84). In Conifera? only prosenchyma-cells with bordered pits (together with xylem-rays) are subsequently produced, so long as the stem or root grows. In Dicotyledons on the contrary, after the first year a combination of vessels and wood- prosenchyma, often mixed with wood-parenchyma, is annually formed. In trees with annual rings in the wood a periodicity may be remarked in the development of the xylem-cells ; and on this depends the stratification of the xylem into annual layers. Not unfrequently the phloem portion also shows a similar stratification. In the closed bundles of Monocotyledons the order of development in the first 1 These are formed only before the completion of the increase in length of the organ, to which the bundle belongs. ^8 MORPHOLOGY OF TISSUES. year is similar to that already described. In Fig. 8i, for example, the annular vessel r is first formed in the xylem portion, then the spiral vessel s, then advancing right and left the pitted vessels g g, and in the middle (advancing radially) the narrow pitted vessels. It sometimes occurs {e. g. in Calodracon, according to Nageli) that the formation of vessels advancing right and left encloses the pro- cambium, which afterwards passes over into latticed cells. In the leaf-stalk of Pteris aquilina the development of the xylem begins in the procambium bundles with an elliptical transverse section, by the formation of some narrow spiral vessels in the foci of the section ; then, following the longer axis, scalariform vessels are formed, first centrifugally then centripetally, until a compact woody mass is produced, elongated in transverse section ; around this the still remaining pro- cambium is transformed into latticed cells, sieve-tubes, and cambiform tissue, and partly (at the circumference) into bast-fibres (Figs. 83, 87, A). The fibro-vascular bundles of roots arise in a tissue which is differentiated out of the primary meristem of the apex of the root in the form of a solid (rarely hollow) cylinder. In this the development of vessels begins at two, three, four, or more points of the circumference, and advances radially inwards. If the pro- cambium is a solid cylinder, a diametral row of vessels (seen in transverse section) arises, or a star of three, four, five, or more rays, the youngest and broadest vessels lying next the axis ; between the starting-points of this formation of vessels bundles of phloem generally arise, and in roots which increase in thickness subsequently cambial tissue, which is then developed in centrifugal order, as occurs in the stem, vessels, and wood-cells ^ Forms of Cells. In the text I have indicated only the relative positions of the separate forms of tissue of the fibro-vascular bundle in their most important features ; some remarks will naturally follow on the forms of their cells ; but here also, in conse- quence of the numerous special modes of development, reference must be made to the special morphology of separate classes of plants in Book II. The cell-forms of the fibro-vascular bundles attain their most perfect and varied development in Dicotyledons ; the forms which occur in them may therefore be employed as a basis for the critical examination of the corresponding phenomena in other classes of plants. The Xylem-portion of the fibro-vascular bundle of Dicotyledons is composed of numerous cell-forms, which may be referred^ according to Sanio's careful researches, to three types. He distinguishes (i) Vascular, (2) Fibrous, and (3) Parenchy- matous. To the Vascular forms belong the ducts and the vascular wood-cells or Tra- cheides. This group of forms is characterised by their walls forming open orifices where two cells of the same form meet, so that their cell-contents soon disappear and air takes their place ; the thickenings show a tendency towards the formation of spiral bands, net-work, and bordered pits. True vessels (Figs. 27, 84) arise when the septa of cells whose form is similar, arranged in rows longitudinally over one another, are entirely or partially absorbed ; and thus long air-conducting tubes originate, consisting of many cells, distinguished from the adjoining wood-cells principally by their greater breadth. The septa may be placed horizontally or more or less ob- liquely ; and in general the mode of their perforation is directed accordingly ; hori- ^ Cf. Van Tieghem, Recherches sur la symmetrie de structure des plantes vasculaires, Paris 1871 THE FIBRO -VASCULAR BUNDLES. 99 zontal walls are often entirely absorbed, or they have large round cavities. The more oblique the septum, the more do the perforations take the form of narrow broad parallel fissures ; and the thickening-bands of the septum which remain present more or less the appearance of rungs of a ladder, while reticulated combinations of them are often formed. The scalariform septum is found, according to Sanio, not only in reti- culately thickened vessels and those with bordered pits, as was previously supposed, but also in spiral vessels {e.g. in Gasuarina, Olea, Vitis) where turns of the spiral band pass im- mediately into the scalariform markings. The loosening of the spiral band of the first-formed spiral vessel in stems and leaf-stalks of rapid growth, appears to depend solely on the loosen- ing of the band from the thin quickly-growing wall which is common to the vessel and the ad- joining cells. If the band could be unrolled owing to the absorption of this wall, the adjoin- ing cells must necessarily be opened. If the septa of the separate vascular cells are placed very ob- liquely, the latter assume a prosenchymatous appearance (Fig. 85), and the more this is the case the more does the vessel appear discon- tinuous. In the xylem of Ferns this is often car- ried to so great an extent that, after isolation of the single cells by maceration, it would be easy to believe that it is not the remains of vessels, but fusiform proscnchyma that is left (Fig. 29); but here also all kinds of transitions occur to the typical scalariform septa ^ Vessels with prosen- chymatous constituents now form the immediate passage to the vascular wood-cells (Tracheides). If the form of the cells is such that there is no longer any difference between the longitudinal wall and septum, which is possible only in deci- dedly prosenchymatous forms, then the perfora- tions of the cells which lie above and next one another are no longer different in form ; rows of cells no longer arise in an especially marked manner resembling continuous tubes, but whole masses of cells (bundles, &c.) are connected "with one another by means of open bordered pits. This occurs in an especially marked manner in the tracheides in the wood of Coniferae {vide Figs. 25, 26, p. 25). There is no other difference between these and true vessels ; for vessels when they have bordered pits behave in reference to the side-walls exactly like tracheVdes when they have open bordered pits (Fig. 27). The separate elements Fig. 85. — From the very young fibro-vascular bundle of a young leaf-stalk oi Scrofi/uilaria aquatica ; part of a spiral vessel surrounded by procambium ; two spirally thickened cells are in prosenchymatous appo- sition ; by the elongation of the leaf-stalk the turns of the spiral band now lying close to one another are drawn apart ; the spiral band becomes detached from the thin wall which is common to the vessel and to the adjoining cells, and so a spiral band is formed capable of unrolling. ' Cf. Dippel in the Amtlichen Bericht der 39. Vers, der Naturforscher u. Aerzte. 1865 (Giessen), Feb. 3, Figs. 7-9. Dippel's observations on Cryptogams and the whole description of the formation of vessels here given, their passage into Tracheides, and especially the fact that the air-conducting tracheal fonns have open bordered pits, — even when the parenchymatous constituents of a vessel are united not by large cavities, but by narrow fissures, &c. or are connected with one another (and are hence not closed cells, as Caspary thinks), — compel us to consider as erroneous Caspary's sup- position of the absence of vessels in Cryptogams and many Phanerogams. (Cf Caspary, Monats- herichte d. k. Akademie der Wissenschaften in Berlin, July 10, 1862.) lOO MORPHOLOGY OF TISSUES. of the vessels of F>rns composed of prosenchymatous cells (Fig. 29) may be correctly designated tracheides. The Fibrous cell-forms of the xylem are always prosenchymatous and fusiform, very thick in comparison with their diameter, with usually simple, but sometimes bordered pits, the pits small ; always without a spiral band ; and during the repose of vegetation containing starch. Next to the middle lamella of their partition-walls there more often lies an unlignified gelatinous thickening-mass which is coloured red-violet by Schultz's solution, resembling many bast-fibres ; these cells are generally much longer than the vascular forms. Sanio distinguishes here also two forms ;— the simple (Libriform), and the partitioned fibres ; the latter are distinguished from the former by their cavity being partitioned by several thin septa, while the common wall of the whole fibre is thick. These fibre-like cell-forms are found in the wood of dicotyledonous trees and shrubs in the most various intermixture with the vascular elements and the other forms to be named immediately. Whether wood-fibres occur in Cryptogams is at least doubtful. The Parenchymatous cell-forms of the xylem are widely distributed, and especially abundant when the woody substance of the fibro-vascular bundles attains a considerable thickness. They arise, according to Sanio, in the wood of Dicotyledons and Gymno- sperms by transverse division of the cambium-cells before their thickening commences. The sister-cells show this origin chiefly by the mode in which they are arranged; when completely developed they are thin-walled, with simple closed pits. Their contents in winter consists of starch, often associated with chlorophyll, tannin, and crystals of calcium oxalate. It also happens sometimes that the cambium-cells on the xylem-side of the bundle become transformed without transverse division into parenchymatous, thin- walled, simply pitted, conducting, elongated cells, which must also be considered as paren- chymatous forms of wood-cells'. To this last type are also to be referred the parenchy- matous elements in the xylem portion of the closed fibro-vascular bundles of Monocotyle- dons and Cryptogams ; but these thin-walled, mostly elongated, conducting cells do not in this case originate in the cambium (since this, according to the terms in customary use, is absent from the closed bundles), but immediately from the procambium of the bundle (Fig. 83, near S). Sometimes the wood-parenchyma resulting from the cambium of Dicotyledons (parenchyma of the xylem portion) attains a stronger development, while only a few vessels and tracheides are formed : this occurs in the thick napiform roots of the radish, carrot, beet, and dahlia, and in potato-tubers. The apparent pith of these organs corresponds, in its origin, to the woody substance of a dicotyledonous tree ; but the elements- of the xylem are not, or only slightly, lignified ; the succulent contents and the thin soft cell-walls scarcely give this xylem the appearance of an analogue of the ordinary woody substance, although there can be no doubt about this analogy. The Layers of Phloem of the fibro-vascular bundle show, when fully developed, similar celUforms to the xylem portion ; the sieve-tubes correspond to the vessels, the bast-parenchyma to the wood-parenchyma, the true bast-cells to the woody fibres. In the phloem as in the xylem, the different cell-forms may arise in the most various intermixture, sometimes in alternate layers, sometines irregularly. A very general cell-form in the phloem is the Cambiform, consisting of narrow, usually elongated, thin-walled, succulent cells which sometimes appear, in very thin bundles, to form the only constituent of the phloem. When this last is perfectly developed, regular latticed cells arise, which are not always to be easily distinguished from true sieve-tubes ; the formation of the latter has been already explained in Figs. 23 and 24. The perfora- tion of their older sieve-discs, especiahy on the septa, which may lie obliquely or transversely to the longitudinal rows of cells, can be easily proved by laying thin sections in concentrated sulphuric acid, especially if the preparation is saturated * Sanio applies to these cells the term ' Ersatzzellen.' THE FIDRO -VASCULAR BUNDLES. 01 with iodine-solution^. The cell-walls become dissolved, the protoplasmic mucilage remains behind coloured brown, and may be recognised in the form of fine strings of mucilage filling up the pores of the sieve-disc (Fig. 86, />). Those cells may pro- visionally (after Von IMohl) be called latticed-cells in which similar formations of wall are visible, even although the previous perforation of the narrow crowded pits (lattice) cannot be proved. To this category belong the so-called * Vasa propria' in the fibro-vascular bundle of Monocotyledons (Fig. 8i, nj), and the form of cells discovered by Dippel in Cryptogams, and called by him bast-vessels. (Dippel, /. c.) The latticed cells or sieve-tubes frequently have sieve- or latticed discs in their longitudinal walls also, when two cells of this kind are placed in juxtaposition side by side ; these discs are thinner portions of the cell-wall which show a fine puncturing or lattice-like thickening ; whether in these cases actual perforations also occur is still undetermined. These cell-formations (cambiform, latticed cells, sieve-tubes) may, in combination with the phloem-parenchyma in which they are imbed- ded, or which sometimes forms thicker layers, be included in the term Soft-bast, in opposition to the true bast which is sometimes entirely absent (as in Cucurbita), but in other cases is very abundantly developed (e. g. stem of Helianthiis tiiberosus, Tilia, &c.), and consists of elongated, prosenchymatous, fibre-like, flexible, tough, firm cells, usually greatly thickened. In Dicotyledons they are generally ar- ranged in bundles, frequently forming layers alter- nating with soft-bast (as in the grape-vine) ; but sometimes, especially in the later portions of the phloem, which are formed by the cambium, they occur also in separate fibres (as in the stem and tuber of the potato}. The middle lamella of the partition- wall of two fibres is generally lignified or cuticularised (not dissolving and turning yellow with iodine) when they are closely crowded ; but in other cases it forms a mucilaginous ' intercellular substance ' in which the cells (in transverse section) appear imbedded {e.g. the laburnum according to Sanio, Coniferae). The true bast-fibres of the phloem, like the fibres of the wood, may become partitioned by subsequent septa (as in the vine, occidental plane, horse-chestnut, Pelar- gonium roseiini, Tamarix gallic a, according to Sanio, I.e. p. iii). As the wood-cells are often found branched after isolation by maceration, so also are the bast-fibres, which frequently attain greater freedom at the expense of the surrounding soft tissue {Abies pectinata, according to Schacht). Sometimes the bast-cells are short and lignified when more decidedly thickened, and very hard (tuberous roots of Dahlia). In Apocynaceae {e. g. Vinca) the very long bast-cells are alternately wider and narrower, and also dis- tinctly striated (on laticiferous bast-cells 'vide infra). The true bast-cells of the Equi- setaceae, Ferns, and Lycopodiaceae (found by Dippel) are but little developed, the external thickening-layers of their walls being apparently generally mucilaginous ^ (or developed as intercellular-substance). Fig. S6.— Combinations of sieve-tubes, showing the perforation of the septa after solution of the cell-wall by sulphuric acid. A and B from the leaf-stalk of Cucurbita ; C from the stem of Dahlia. At A the cell-wall h h' is not yet com- pletely absorbed; s' the protoplasmic mucilage, o and « accumulation of it on the upper and under side of the septum ; / the strings of mu- cilage, which unite these accumulations and fill up the pores of the sieve-discs {cf. Figs. 23 and 24). ^ Cf. Sachs, in Flora, p. 68, 1863, and other proofs of the perforation in Hanstein, Die Milch- gefisse, Berlin, pp. 13 et seq., 1864. "^ There is no reason for describing, as is done by many writers, as bast the hypodermal fibres of Equisetum, the Lrown-walled prosenchyma in the fundamental-tissue of the stem of Tree-ferns and Pterin aqidlina, and other cell-formations which do not at all belong to the fibro-vascular bundles. 102 MORPHOLOGY OF TISSUES. All that has hitherto been said concerns only the elongated elements of the fibro- vascular bundle ; the radially extended elements (xylem- and phloem-rays) are a pecu- liarity of the open fibro-vascular bundles of Dicotyledons and Conifers. Sect. 17. The Fundamental Tissue (Grundgewebe). — By this name I de- signate those masses of tissue of a plant or of an organ which still remain after the formation and development of the epidermal tissue and the fibro-vascular bundles. The fundamental tissue consists very commonly of thin-walled succulent paren- chyma filled with assimilated food- materials ; but not unfrequently it is thick- walled ; sometimes separate portions assume the form of string-like tissues which consist of sclerenchymatous strongly lignified prosenchyma cells. The most various forms of cells and tissues may arise in the fundamental tissue as in the epidermal system and the fibro-vascular bundles ; a portion of the fundamental tissue itself may persist from the commencement in a condition capable of divi- sion, while the surrounding portion passes over into permanent tissue ; or special layers of the fundamental tissue, long after it has been transformed into perma- nent tissue, may again become subject to cell-division, and a generating tissue thus be produced, out of which originate, not only new fundamental tissue, but also fibro-vascular bundles {e.g. in Aloineae). In Thallophytes and many Muscineae the whole mass of tissue, with the exception of the outermost layer, which is often developed as epidermal tissue, may be considered as fundamental tissue ; but in these cases, in consequence of the ab- sence of the fibro-vascular bundles, this distinction has but little practical value. In Mosses with string-like formations in the stem it may appear doubtful whether these are to be considered as peculiar forms of the fundamental tissue or as very rudimen- tary fibro-vascular bundles. In Vascular plants, on the other hand, the independence and pecuHarity of the fundamental tissue, in contradistinction to the epidermal system and the fibro-vascular bundles, is at once apparent ; it here fills up the interstices of the fibro-vascular bundles within the space enclosed by the epidermal tissues. Where the fibro-vascular bundles are closed and show no increase in thickness (as in many Ferns), the tissue is frequently the most largely developed; where, on the other hand, closely crowded fibro-vascular bundles, by the development of cambium, produce in succession large masses of layers of wood and phloem (as in stems and roots of m.any Conifers and Dicotyledons), the fundamental tissue becomes a constantly less important portion of the whole organ. The disposition of the fibro- vascular bundles in stems is commonly of such a nature that the fundamental tissue is separated into an inner pith-portion, surrounded by the bundles, and an outer cortical layer enveloping the bundles. Since the bundles are not in contact laterally, or only partially so, there still remain between them portions of the fun- damental tissue which connect the pith with the cortex, and are 'termed Medullary Rays. If the fibro-vascular masses of an organ form an axial solid cyHnder, as occurs in some stems and in roots, the fundamental tissue is developed only as cortex. (a) Critical. The whole course of my description of the tissue-system requires the introduction of the idea of a ' Fundamental Tissue.' It has, in fact, long been required, since it was often necessary, in anatomical descriptions of the collective masses of the tissue which are neither epidermal nor fibro-vascular bundles, to distinguish them by some common term. Many writers employ the term Parenchyma in this sense in oppo- sition to the fibro-vascular bundles and the epidermis ; but this usage is not scientific ; THE FUNDAMENTAL TISSUE. lO:? the fibro-vascular bundles often contain parenchyma also, and -vice I'ersa, the fundamental tissue is not always parenchymatous but sometimes distinctly prosenchymatous. We have, moreover, to deal here not with forms of cells, but with the contrast of different systems of tissue, each of which may contain the most various cell-forms. I must com- pare somewhat more closely my description and use of terms with those of Nageli. It might be supposed that Nageli's Protenchyma is synonymous with my fundamental tissue- but this is not the case; the protenchyma of Nageli is a much more comprehensive idea' everything which I call fundamental tissue is protenchyma ; but all protenchyma is not fundamental tissue.^ Nageli ^ says, for example, that he would call the primary meristem and all parts of the tissue which arise immediately from it (/. e. only through the medium of secondary meristem, but not of cambium) Protenchyma (or Proten) ; the cambium on the other hand, and everything which directly or indirectly originates from it Epen- chyma (or Epen). When Nageli thus defined these terms, he was dealing with a de- scription of fibro-vascular bundles, and it is intelligible that he on this occasion included everything which does not belong to the fibro-vascular bundles under one common name (Proten). But our business is to give a uniform description of the various differentiations of the tissues of plants ; and there is no reason for bringing into prominence only the contrast between fibro-vascular and non-fibro- vascular masses (Epenchyma and Proten- chyma), and for considering as less important the other differentiations ; the proten- chyma of Nageli therefore splits up, according to me, into three kinds of equal value with his epenchyma. The primary meristem is as completely opposed to the fibro- vascular masses as to the epidermal and fundamental tissues, for the three systems of tissue arise by differentiation out of the still undifferentiated primary meristem. The conception of Proten, after the primary meristem has been eliminated from it, might be applied equally to the epidermal and the fundamental tissues ; but I see no reason which compels us to bring into prominence this contrast alone ; nature rather indicates that the differentiation between epidermal and fundamental tissues is as essential as that between fibro-vascular bundles and fundamental tissue. From all this it follows that primary meristem, epidermal tissue, fibro-vascular bundles, and fundamental tissue are concep- tions of equal value ; in each of the three differentiated tissues we find the most various forms of cells; and secondary meristem may also arise in each. Tn the fibro-vascular bundles the cambium is of this nature, the whole of the young epidermis is a generating tissue in as accurate a sense as the cambium ; if this latter forms vessels, wood, bast, &c., the former produces hairs, stomata, prickles, &c. The phellogen, belonging to the epidermal system, arises still more decidedly as a generating tissue ; finally even in the fundamental tissue a portion may persist for a considerable time as generating tissue, or may subsequently produce such a tissue [e. g. the meristem of the stems of Dra- caena), which brings about its increase in thickness and thus forms new fibro-vascular bundles. (b) Examples. The relationship of the three systems of tissue may be observed very simply and undisturbed by subsequent new formations in the foliage-leaves of Ferns and of most Phanerogams ; in these the fundamental tissue is generally the prevailing system, and is developed into different cell-forms. Isolated fibro-vascular bundles separated by the fundamental tissue traverse the leaf-stalk, and are distributed through the blade ; in the former they are generally surrounded by a broad-celled thin-walled parenchymatous fundamental tissue extended axially ; this also forms sheath-like envelopes around the stronger bundles of the blade, which are conspicuous on the under-side of the leaf as the Veins ; but the finer branches, and the finest of all, run through the so-called mesophyll, i. e. a peculiar form of the fundamental tissue distinguished by containing chlorophyll and by its thin cell-walls. Not unfrequently single cells of the fundamental tissue of the leaf- blade assume very peculiar forms {e. g. the larger stellate cells in the leaf of Camellia ^ See his Beitriige zur wissenschaftlichen Botanik, Heft i, p. 4. I04 MORPHOLOGY OF TISSUES, japonica, the erect post-like cells upon which the stomata of the leaves of Hakea are, as it were, supported). All these tissue-formations are enveloped by the epidermis, and fre- quently also by hypodermal tissues. In the carpellary leaves of Phanerogams there oc- curs commonly a more manifold differentiation of the fundamental tissue ; I will instance only the formation of the so- called stones of Drupaceae. The stone is here the inner layer of tissue of the same foliar struc- ture of which the outer layers form the succulent flesh of the fruit ; both are the fundamental tissue of the carpel, the former sclerenchymatous, the latter pa- renchymatous and succulent, both being traversed by fibro-vascular bundles. Equally clear is the structure in the stems of Ferns, among which the Tree-ferns and Pteris aquUina are of special in- terest, because the fundamental tissue occurs in them in two quite different forms ; its pre- ponderating mass consists, e, g. in Pteris aquiUna (Fig. 80) of a thin -walled colourless mucila- ginously succulent parenchyma, in winter rich in starch, in which there also run, parallel with the fibro-vascular bundles, filiform or strap - shaped lines of thick - walled prosenchyma- tous dark brown bundles of sclerenchyma. They have no- thing in common with the fibro- vascular bundles, but are only a peculiar form of the fundamental tissue which also often occurs elsewhere in Cryptogams in pro- senchymatous forms. The ten- dency to prosenchymatous de- velopment of the cells of the fundamental tissue occurs also especially in the stems of Lyco- podiacese. In Selaginella denti- culata (Fig. 87, y^) the axial fibro- vascular bundle is surrounded by a very loose parenchyma which forms large intercellular spaces ; this innermost portion of the fundamental tissue is enveloped by a thin- walled tissue without interstices, which shows itself on longitudinal section to be deve- loped prosenchymatously ; the cells are pointed above and below, and penetrate to a considerable distance between one another; towards the circumference they become gradually narrower and more pointed ; the outermost are dark-walled and form the epidermal system which gradually passes over into this fundamental tissue. In Lycopodium Fig. Z-j.—A transverse section of the stem oi Selaginella dentictdata; the fibro-vascular bundle is not yet fully developed ; the vessels are already lignified on both sides, but not yet in the centre; / air-conducting intercellular spaces in the parenchyma enveloping- the bundle ; towards b the part of the tissue corresponding to the bundle which bends outwards to the leaf. R transverse section of the mature stem of LycopodiH??t cha7nc?cyparisstis, the axial tissue- cylinder consists of densely crowded and coalescent fibro-vascular bundles ; the four parts of their xylem are quite separated, forming four bands on the trans- verse section, between and around which are found the narrower cells of the phloem. The phloem portions of the four bundles have coalesced ; between each pair of xylem-bundles is seen a row of wider cells, the latticed cells or sieve- tubes ; the narrow cells lying on the right and left edge of each xylem portion are spiral-vessel-cells (also in A). In the thick-walled prosenchymatous funda- mental tissue which envelopes the axial cylinder, is seen the dark transverse section, of a thin fibro-vascular bundle which bends outwards to a leaf; it con- sists almost exclusively of long spiral-vessel-cells (X about 90). THE FUNDAMENTAL TISSUE. 105 chamcpcy parts sus (B) the axial cylinder, which consists of several fibro-vascular bundles is surrounded by a thick layer of greatly thickened prosenchyma ; in the young stem the cells are similar to those of Selaginella ; but here also an enormous thickening adds to the prosenchymatous form of the cells of the fundamental tissue ; this is also enveloped by a layer of tissue, the cells of which are thin-walled and not prosenchymatous ; this layer is a descending continuation of the fundamental tissue of the leaves, which enve- lopes the stem everywhere and is itself covered by an evidently developed epidermis. (c) 'T/?e Cells and Tissues of the system of the fundainental tissue have not yet under- gone a comparative and comprehensive investigation, like those of the fibro-vascular bundles. Out of the very scattered material I select the following for the information of the beginner. Irrespectively of many altogether special phenomena, it is chiefly in connexion with the true epidermal tissue on the one hand and the fibro-vascular bundles on the other hand that the differentiation of the fundamental tissue takes place ; certain forms of this tissue occur as strengthenings, or at least as accompaniments of the epidermal tissue, and have already been described as Hypoderma ; other masses of tissue accompany the separate fibro-vascular bundles as partially or entirely closed envelopes or sheaths, which I term generally Bundle-sheaths. In the same manner the whole remaining internal space of the organ con- cerned is commonly filled up by other forms of tissue, which do not, as for the most part the two former do, occur in the form of layers, but in masses ; these I will designate simple Intermediate Tissue (Fiillge- webe). Each of these combinations of tissue may be composed of very different forms. The Hypoderma appears some- times as thin-walled succulent watery tissue (as m leaves 01 1 radeSCantiaand fig. SS.— Transverse section through the underground stem of Pterzs Bromeliacex ). I n Dicotyledons (stems T"^''-'^ ' '• '°°'-''^'''^ • . ^""""^'y '•"'^kened brown-waiied ceiis beneath ^ ^ ^ the epidermis ; y one lymg deeper and less strongly thickened ; a part and leaf-stalks) it commonly consists of the wall is seen in front ; j^cells of the deeper layers containing starch. forming the passage to the inner colourless parenchyma of the funda- mental tissue. Fig. 89.— Transverse section of the acicular leaf of Pinus Pinaster (x about 50) ; e epidermis; es hypodermal fibrous bundles ; sp stomata ; h resin-passages ; gb colourless inner tissue containing two fibro-vascular bundles. Fig. 90.— The left-hand corner of the previous figure mag nified (800) ; c outer cuticularised layers of the epidermis- cells ; i inner non-cuticularised layers ; C very strongly thick- ened outer wall of the epidermis-cells situated at the corner; ^zthe hypodermal cells ; g the central lamella: i' the stratified thickening-mass ; p parenchyma containing chlorophyll ; pr its contents contracted. longitudinally extended, narrow, and thickened in the angles by a mass capable of great swelling ; or the hypodermal fundamental tissue is developed in a sclerenchymatous 106 MORPHOLOGY OF TISSUES. manner, as in the stem of Pieris aqiiUina, or it occurs in the form of thick-walled but flexible fibres, and forming either layers and bundles (stem of Equisetum, leaf of Coniferae, Fig. 8.9), or in long isolated fibres, similar to true bast-fibres (leaf of Cycadeae). In all these cases the cells of the hypoderma are extended longitudinally ; but when it is re- quired in addition to produce very resisting layers, the cells often extend vertically to the surface of the organ, and, increasing greatly in thickness, form layers of closely arranged prisms, as in the pericarp of Marsilea and Pilularia and the testa of the seeds of Papilio- naceae. Isolated cells of the same kind are sometimes found in the hypoderma, as accompaniments of the stomata and air-cavities {e.g. in leaves of Hakea). The Bundle-sheaths are commonly formed of a single layer of cells, which is in close contact with and envelopes each separate fibro-vascular bundle (Fig. 83); or, when these are arranged in a circle in the transverse section of the stem, forms an envelope com- mon to the whole in contact only with the phloem-layers (Fig. 82). The longitudinal walls of these simple bundle-sheaths placed radially always show in transverse section a black point, in consequence of a peculiar folding of the wall. The walls of these cells are mostly thin, but lignified or otherwise altered ; in the thinner vascular bundles of Ferns on the side facing the bundle they are often much thicker and brown. In many Equiseta {e. g. E. hyemale) a continuous bundle-sheath runs along the inner side of the circle of the vascular bundle. In many Monocotyledons, especially Grasses and Palms, each fibro-vascular bundle, the xylem and phloem of which are soft-walled and deHcate, is surrounded by a layer, consisting of several strata of firm, long, lignified prosenchymatous cells (Fig. 81). Much stronger layers of brown-walled sclerenchyma accompany the vascular bundles in the stem of Tree-ferns. The axial fibro-vascular substance of all roots is surrounded by a simple bundle-sheath generally wath thin walls (Fig. 117). (On the bundle-sheaths cf. Caspary, Jahrbuch fiir wissen. Botanik. I. Hydrilleen. — Sanio, Bot. Zeitung, pp. 176 et seq., 1865. — Pfitzer, Jahrbuch fiir wissen. Bot. VI. p. 297.) The Intermediate Tissue consists of thin-walled succulent parenchyma with intercel- lular spaces which are absent from all other forms of tissue ; in the stem, however, of LycopodiaceaR and of many other Cryptogams the intermediate tissue consists of prosen- chyma, and this is then either thin-walled as in Selaginella, or thick-walled as in Lyco- podium. In so far as the intermediate tissue is parenchymatous, it may be termed simply parenchyma of the fundamental tissue or Fundamental Parenchyma. Two principal forms of this may be distinguished, which are nevertheless united by transitional forms, 1)1%. the colourless parenchyma which occurs in the interior of large succulent stems and tubers and in all roots and succulent fruits, and the parenchyma rich in chlorophyll which forms the superficial layers beneath the epidermal tissues of stems and fruits. In the foliage-leaves, when thin and delicate, it fills up the space between the upper and lower epidermis ; if they are very thick, as in species of Aloe, it forms only the super- ficial layers, while the inner mass of tissue is colourless parenchyma. Not unfrequently there occur in the fundamental parenchyma very peculiar isolated cells, groups of cells, bundles, or bands. For example, in the mesophyll of the leaves of Camellia (Fig. 16, P) branched thick-walled cells appear; similarly formed spicular cells occur in the parenchymatous tissues of Gymnosperms, and are especially abun- dant in Welwitschia ; the polyhedral stone-cells (sclerenchyma) in the flesh of pears are arranged in groups ; and a similar isolated or grouped arrangement occurs in the bark of many trees ; the brown-walled prosenchymatous sclerenchyma-cells in the fundamen- tal parenchyma of the stem of Tree-ferns and of Pteris aquilina appear arranged in the form of bundles and bands. The sclerenchyma in the carpel of stone-fruits (the tissue of the stone in Prunus, Cocos, &c.) forms closed massive layers. To this description must also be referred many peculiarly thickened cells which occur here and there in the parenchyma, as well as the fibrous cells of the anther-walls, if these do not rather belong to the epidermal system. (Further material will be found in Schacht, Lehrbuch der Anatomic und Physiologie der Gewebe, 1856 ; Thomas, Jahrbuch fiir wissen- THE FUNDAMENTAL TISSUE. 07 schaftliche Botanik, IV. p. 23; Kraiis, ditto, IV. p. 305, and V. p. 83; Borscow, ditto VII. p. 344.) (d) Nenv formations in the fundamental tissue. The collective fundamental tissue in the stem of the higher Cryptogams, in the stem of most Monocotyledons and of many Dicotyledons, as well as in all leaves, and in all roots not yet changed by growth in thickness, originates immediately from the primary meristem of these organs by further development, simultaneously with the fibro-vascular bundles and the epidermal tissues. In the stems and roots of many Phanerogams endowed with growth in thickness, it occurs, however, that within the fundamental tissue, either originally or subsequently, meristem is formed, out of which secondary funda- mental tissue, together with secondary fibro- vascular bundles, is then produced. This behaviour is seen very clearly in the stem of Dracaena, Aletris, Yucca, Aloe, Lomato- phyllum, and Calodracon \ In Dracaena and Aletris isolated fibro-vascular bundles are formed in the primary meristem of the apex of the stem, while the whole fundamental tissue which surrounds them and separates them from the epidermis is transformed into parenchyma, and passes over into per- manent tissue ; but after considerable time (in Aletris flagrans about 4-5 cm. below the apex of the stem, in Dracana reflexa, according to Millardet, as much as 17-18 cm. below the apex) a fresh formation of meristem begins in one of the cell- layers of the fundamental tissue which immediately surround the outermost fibro- vascular bundles ; the permanent cells con- cerned in it divide repeatedly by tangential walls ; and there arises (seen in transverse section) a girdle of meristem (Fig. 91, .v), the cells of which are arranged in radial rows. In this meristem new fibro-vascular bundles are produced ; one, two, or more adjoining cells of the transverse section dividing repeatedly by longitudinal walls in various positions. Out of the procam- bium-bundles which arise in this manner the cells of the fibro-vascular bundles pro- ceed immediately ; the intermediate meristem passes over likewise into permanent tissue, and indeed into strong-walled parenchyma, which now forms the secondary fundamental tissue between the secondary fibro-vascular bundles. Since the cells of the thickening- ring which face inwards pass over in centrifugal succession into permanent tissue, while the outermost divide repeatedly, the whole ring continually moves centrifugally, and leaves behind the new bundles and parenchyma-cells. In Yucca Millardet found the origin of the ring of meristem (thickening-ring) as little as 3mm. below the apex FiC. 91.— Part of the transverse section of a stem of Dra- crena (probably reflexa) about 13 mm. thick and i metre higli, about 20 cm. below the summit, e epidermis; k cork (peri- derm) ; r cortical portion of the fundamental tissue ; b trans- verse section of a fibro-vascular bundle, bending out to a leaf; ni the primary fundamental tissue (pith) ; g the primary bundles ; X the girdle of meristem in which very young fibro-vascular bundles are to be seen, while the older ones g have already partially or entirely passed out of it, its lower part becoming transformed into radiately arranged fundamental tissue [st). ' Compare Millardet's description, Sur I'anatomie et le developpement du corps ligneux dans les genres Yucca et Dracaena (Extrait des Mem. de la societe imper. des sciences nat. do Cherbourg, F. XI, 1865 ; and Nageli, Beitrrige zur wissen. Botanik. Heft i, p. 21). lUO mUKrnui^uisX ur i loour,.:). of the stem ; in Calodracon, according to Nageli, while the bundles and the fundamental tissue become differentiated at the apex of the stem, a ring of meristem remains over, which subsequently produces new bundles and secondary fundamental tissue. In Dicotyledons and Conifers similar phenomena arise still more frequently and with many compli- cations, the consideration of which I shall take up in Book II. Only one example may here be described, since it will serve to show the relationship of the fundamental tissue to the fibro-vascular bundles from a new point of view. In the hypocotyledonary segment of the stem of Ricinus communis there is found on transverse section, at the commencement of germination, a ring of generating tissue (Fig. 92, ^, x), by which the fundamental tissue VlG.g-z.—Ricznus commttnts ; transverse section through the centre of the hypocotyledonary segment at various stages of germination ; A after the appearance of the root beyond the testa of the seed ; B after the hypocotyledonary segment has attained a length of about 2 cm. ; C at the ead of germi- nation ; 7)1 pith, r cortex, a- generating ring of tissue (corre- sponding to Sanio's thickening ring) ; st medullary rays ; fv fibro-vascular bundles ; ch connecting bands of secondary meristem, producing xylem and phloem later, and forming true cambium. Fig. 93.—^ a part of Fig. 92, more highly magnified ; sg- spiral vessels ; ^j- bundle-sheath ; B the same ; true cambium is being formed by tan- gential divisions in the fibro-vascular bundle now isolated ; the other letters as in Fig. 92. Compare Fig. 82 (p. 95), which represents a part of Fig. 92 C, magnified to the same degree as Fig. 93. is divided into pith (w) and bark (r) ; at this time eight groups of narrow spiral vessels already indicate the differentiation of as many fibro-vascular bundles ; subsequently the generating tissue (5) becomes differentiated' into eight completely isolated fibro-vascular bundles (/t) and as many intermediate portions of parenchymatous fundamental tissue, which is in no way distinguished from that of the pith and the cortex (cf. Fig. 93, JS); the fibro-vascular bundles are now also separated by medullary rays. This condition, however, does not last long, for as soon as the segment of the stem has become longer and thicker, and the granular materials of the fundamental tissue are mostly consumed, repeated divisions by tangential walls commence in those portions of the medullary LATICIFEROUS AND VESICULAR VESSELS, ETC. IO9 rays whicli lie between the cambium-layers of each pair of adjoining bundles (Fig. 92, C, cb). A bridge of secondary meristem is thus, as it were, established between the cambium layers of the bundles ; and thus a closed ring of generating tissue is again formed, which also occasions the thickening of the portion of the stem, and may hence be termed a ' Thickening-ring ' ; but its origin is somewhat different to that in Dracaena and its allies. In them the thickening-ring has its origin entirely in the secondary meristem which was formed from the fundamental tissue, and the newly formed fibro- vascular bundles lie in the thickening-ring; here, on the other hand, the thickening- ring (C, cb) consists of cambium which lies in the vascular bundles, and of secondary meristem which proceeds from the fundamental tissue. Here, therefore, the thickening-ring passes through the fibro-vascular bundles ; but the fundamental tissue which generating the parts required to complete the ring between the bundles has itself only shortly before been formed from a generating tissue. Subsequently the cambium of the bundles constantly produces new xylem, the meristem between them does the same, and thus is formed a closed ring of xylem {i.e. a hollow cylinder), which continually increases in thickness; simultaneously the same thickening-tissue forms constantly towards the outside new layers of phloem. As soon as this takes place, all perceptible distinction ceases between the original cambium of the bundles and the in- termediate secondary meristem ; or a closed cambium-ring is formed. The fibro-vascular masses which are now constantly formed accumulate greatly, while the original funda- mental tissue diminishes more and more in mass. By the increase in size of the fibro-vascular substance in the segment of the stem, the epidermis and the cortical parenchyma become passively extended ; their cells grow rapidly in a tangential direc- tion ; but their original form is again restored as they become divided by radial walls ; and thus also division is subsequently brought about in the whole portion of the original fundamental tissue and the epidermis, by the processes which take place in the fibro- vascular substance. Fig. 56 (p. 69) represents these phenomena in the thickened hypo- cotyledonary segment of the stem of the sunflower ; the figure however is equally avail- able for Ricinus. Skct. 18. Laticiferous and Vesicular Vessels, Sap-conducting Inter- cellular Spaces, Glands. — Like oilier forms of cells and tissues, those of which we are speaking occur both in the fundamental tissue and in the fibro-vascular bundles, and even in the epidermal system ; and by a strict carrying out of the morphology of tissues these forms would also be considered as constituents of the three systems. If we nevertheless treat them both separately and together, the object is to place more conspicuously in the foreground their prominent physiological pecu- liarities. They show manifold transitions both to the forms of tissue of the system within which they lie, and among one another. The more simple vesicular vessels which occur especially in the parenchyma of the fundamental tissue of many Mono- cotyledons differ only by the greater length of the cells and by their union in rows from the surrounding parenchyma-cells ; when more mature the cells of these rows coalesce ; the septa become absorbed ; and thus longer tubes, mostly placed near the epidermis, are formed. From these to the true laticiferous vessels is only a step. They are also the result of the coalescence of rectilinear or branched anasto- mosing rows of cells. These canals, filled with milky sap, lie abundantly in the phloem-portion of the bundles, and accompany them through all parts of the plant, forming in it a continuous system. They occur also in xylem {e. g. Carica), where, originating from the coalescence of parenchyma-cells, they form an envelope round the vessels, and even penetrate into the cortex by means of the medullary rays ; in other cases again they form part of the fundamental tissue of the pith or cortex. no MORPHOLOGY OF TISSUES. Their walls are generally very thin when they arise from the coalescence of paren- chyma-cells which has already taken place in the primary meristem; they may, however, become thick, and it scarcely admits of a doubt that in many cases (as Apocynaceae and Euphorbiaceae) the bast-fibres themselves become transformed into laticiferous vessels ; according to Hanstein it is even probable that in some Aroidese vessels of the xylem assume the form and function of laticiferous vessels. The morphological signification of these organs may thus be very various ; physiologically they have this in common, that they contain dissolved and finely divided substances (emul- sions) which find in them open courses for rapid motion. The same object is, however, also obtained in the plant by the cells pouring out the substances they contain into specially formed intercellular spaces, which, like the laticiferous vessels, may form a connected system of channels in the plant. These also are produced sometimes in the parenchymatous fundamental tissue, sometimes in the xylem, some- times in the phloem of the bundles ; but they are easily distinguished from the former by the peculiar arrangement of the surrounding cells. The latex contained in them may be limpid, mucilaginous, or gummy {e.g. Araliacese), or there is mixed with it an emulsion of resin-forming materials (as in Umbelliferae) ; or the passage contains a resin-producing ethereal oil (as in ConiferEe), or other odoriferous and coloured fluids of oily nature {e.g. Compositae, Umbelliferae). Glands are distinguished from the latex-vessels hitherto mentioned by their not presenting channels or systems of channels, but being local formations. Separate cells or roundish groups, whose par- tition-walls frequently become absorbed, may take the form of glands, so that here again, by the process of coalescence of cells, arise receptacles for special substances (mostly strongly odoriferous, viscid, oily, or coloured). Glands may arise anywhere in the tissue ; and if they belong to the epidermis may discharge their secretions outwardly. (a) Laticiferous and Fesicular Vessels^ show, as has already been mentioned, such numerous and various transitions, that it would be desirable to be able to include them under a common term, such as Latex-sacs. The Gichoriaceae, Campanulacese, and Lobeliaceae possess very perfectly developed laticiferous vessels, belonging to the fibro-vascular bundles, which they accompany throughout the whole plant as reticulately anastomosing tubes, imbedded, in the case of the Cichoriacese in the outer, in that of the two other families in the inner phloem- layer. Their form is best recognised by boiling sections of these plants for some minutes in dilute solution of potash ; the reticulations are then clearly recognised in the trans- parent tissue (Fig. 94), and it is easy to separate them entirely in large pieces. In the Papayaccce (Carica and Vasconcella), the laticiferous vessels, on the other hand, run through the system of the fibro-vascular bundles; they, — /. e. the cells by the coalescence of which they- are formed — are repeatedly produced in layers from the cambium with the other elements of the xylem ; the pitted and reticulately thickened wood-vessels alter- nate with them. The branches of the laticiferous vessels envelope these in all direc- tions, and are sometimes firmly fixed to their surface ; but in addition horizontal branches of these bundles also penetrate the medullary rays ; and these terminate 1 J. Hanstein, Monatsberichte der Beii. Akad. 1859. — Ditto, Die Milchsaftgefasse u. verwandten Organe der Rinde. Berlin 1S64. — Dippel, Verhandlungen des naturwiss. Vereins fur Rheinland u. Westphalen. 22. Jahrg, vols. 1-9. — Ditto, Entstehung der Milchsaftgefasse u. deren Stellung im Ge- fassbiindelsystem. Rotterdam 1865.— Vogel, Jahrb. fiir wissen. Bot. vol. V. p. 31. LATIFICEROUS AND VESICULAR VESSELS, ETC. Ill towards the primary cortex in scattered ramifications or recurrent knots, as also in the pith if the stem is hollow. As in the last-named families, there is developed in the horizontal partition-walls which the pith-tissue forms at the origin of each leaf-stalk in the hollow of the stem, a rich reticulation of laticiferous vessels which penetrates across the horizontal partition-wall in countless ramifications and in several layers one over another, and connects the sacs of the medullary rays and of the whole wood- cylinder. In the Papaveraceae (Cheli- donium, Papaver, Sanguinaria) the lati- ciferous vessels are also very perfectly developed ; they are not here, however, as in the families just named, united in band-like groups, but they run mostly at a greater distance from one another, dispersed through the phloem and the surrounding parenchyma ; single ones appear also in the pith, but do not pene- trate into the xylem ; lateral out- growths and cross-anastomoses are found seldom in the stem, but abundantly in the leaves, and especially in the carpels in which close-meshed reticulations are formed in the parenchymatous funda- mental tissue (Unger) ; similarly also in the cortex of the root. In this family, especially in the root- parenchyma of Sanguinaria canadensis, the origin of the laticiferous vessels from the coalescence of cell-rows (absorption of the walls between adjoining cells), may, accord- ing to Hanstein, be proved ; imperfect unions occur in this case, in conse- quence of which the sacs appear bead- shaped. The richly developed system of the laticiferous vessels of the Urtica- ceae, especially of Ficus and Humulus, runs in the cortex in close proximity to the fibro-vascular bundles of the bast, in Ficus also in the pith, but not in the wood; but they are neither so abundant nor so evidently segmented as in the Papaveraceae, nor so regularly combined into a close-meshed net-work as in the Cichoriaceas ; they rather run within each segment of stem almost singly and uninterruptedly as uniform tubes, only occasionally putting forth a branch or uniting with another tube. In the nodes and leaves, on the other hand, they form numerous rami- fications, sometimes united into a network ; or small, fine, obtuse prolongations, as in the Cichoriacae. In the thicker leaves of many figs they are widely dispersed through the parenchyma, and extend to close beneath the epidermis. The laticiferous vessels of Eu- phorbiaceae are so far similar to these that they also belong to the branched description, and are abundantly distributed through the parenchyma of the fundamental tissue ; but they are distinguished by possessing thicker walls, and being similar, in transverse section, to the bast-fibres. Developed most abundantly in the neighbourhood of the bast-fibre-bundles, they sometimes entirely replace them {Euphorbia splendens) ; from Fig. 94. — A tangential transverse section through the phloem of the root oi Scorzonera. hisfiamca ; a number of laticiferous vessels anastomosing laterally among one another run through the paren- chymatous tissue ; B a small piece of a laticiferous vessel with the adjoining parenchyma-cells, more strongly magnified. ll'Z MORPHOLOGY OF TISSUES. them they run into the cortex and pith, forming, especially in the nodes of the stem and the cushions of the leaves, numerous ramifications. Still more similar to the bast-fibres are the laticiferous vessels of the Asclepiadae and Apocynaceae ; some are pointed at both ends ; sometimes also they have, like them, thickened and character- istically striated walls ; they are found sometimes actually in the place of true bast- fibres, sometimes united with them into one bundle (of the phloem), or surrounding them. In these cases it is therefore by the presence of the latex that the rela- tionship of these metamorphosed bast-elements to true laticiferous vessels is esta- blished; the more milky their contents, the thinner becomes the wall (Hanstein, /. c. p. 2 1). Together with these simple fibre-like tubes, branched and anastomosing ones are, however, also found, especially in the nodes of the stem, the pith, and the cortex {Nerium Oleander). In the Aroideae laticiferous vessels united into a network occur in the fibro- vascular bundles and the fundamental tissue ; but some genera, as Caladium and Arum, exhibit the peculiarity of laticiferous tubes running within the xylem, which, from their position and partly from their structure, must be considered as metamorphosed spiral vessels ; but in the fundamental tissue there also occur simple broad tubes similar to these. In the genus Acer the sieve-tubes are transformed into laticiferous vessels, as is inferred from their position in the phloem and the structure of their walls. The vesicular vessels discovered by Hanstein in species of Allium resemble sieve- tubes in form if not in position ; they contain (evidently at least in the bulbs of A. Cepa) latex, and in some other respects resemble the more simple laticiferous vessels of Dicotyledons. They consist of long broad cells which touch one another at their broad ends and there have sieve- like or latticed septa ; where two vessels are in lateral contact, the longitudinal walls have also a pitted structure similar to the sieve-tubes (Fig. 95); the perforation of the septa, /. e, the formation of open pores, is, however, doubtful in the species of Allium. These vesicular vessels permeate the scales of the bulb; at their base they anastomose, like those of the foliage - leaves and flower-stalks, into long nearly parallel rows, which are generally separated from the epidermis by 1-3 layers of cells. Similar rows are formed by the vesicular vessels of Amaryl- lideae (Narcissus, Leucojum, Galanthus) ; they re- semble, however, the laticiferous vessels in this, that the septa of the rows of cells become partially, sometimes entirely, absorbed ; but their latex is not milky, and contains numerous needle-like crystals of calcium oxalate (raphides). To these must be added numerous other structures in Monocoty- ledons which bear scarcely any other resemblance to laticiferous vessels ; in some genera of Liliaceae (Scilla, Ornithogalum, Muscari) the vesicular vessels often form shorter interrupted rows of cells, and in the bulbs themselves larger isolated paren- chyma-cells, similar to the former in containing raphides. That the cells containing raphides can, however, actually unite into tubes, which morphologically altogether resemble laticiferous vessels, is shown in the Gommelynaceae. Here rows of cells which are early distinguished from those which surround them by containing raphides arise in the young parenchyma of the fundamental tissue of the internodes and leaves ; they no longer divide ; while their neighbours continue to become shorter by septa, they remain Fig. 95.— Longitudinal section through the bulb- scale of Alliitni Cepa; e the epidermis ; c the cu- ticle ; / parenchyma ; sg the latex of the vesicular vessel coagulated by solution of potash ; ^r ^r its septum ; the longitudinal wall exhibits a pitted structure ; it separates the vesicular vessel, in this case very visible, from one lying behind it. LATICIFEROUS AND VESICULAR VESSELS, ETC. "3 longer, and their septa become absorbed, according to Hanstein, by the growth of the whole organ, by which the cells are extended. Thus long continuous tubes, filled with raphides of enormous length, arise out of the rows of cells of the fundamental tissue containing crystals. (b) The term Glands^ is applied to single cells or groups of cells which are strikingly distinguished by their contents from the surrounding tissue, especially when they contain odoriferous, strong tasting, coloured, oily, or resinous substances, which find no further use in changes connected with nutrition or growth. Usually the cell- walls also show certain differences from those of the adjoining cells, or they directly par- ticipate in the formation of the cavity and of the secretion which it contains, they them- selves becoming absorbed. A sharp boun- dary-line can hardly be drawn, especially between unicellular glands and single cells with peculiar contents {e.g. tannin, crystals) dispersed through the tissue. It is more sharply marked in those that are compound ; in them the mass of tissue which contains the products of secretion is usually sur- rounded by peculiarly developed layers, by which the whole is clearly marked oflf and individualised from the surrounding tissue ; while generally the proper gland-tissue, surrounded by it, is at length absorbed, and forms a cavity filled by the products of absorption of the cell-walls, and by the coalescing cell-contents. The secretion may collect in the interior of the gland itself, as oil of camphor in single cells of the parenchyma of the leaf of Camphora officinarum, oil of citron in the cavities of the large compound glands in the rind of the fruit of species of Citrus ; or it may be dis- charged externally, like the viscid excretion of the epidermis on the stem of Lychnis 'viscaria, the nectar of many nectaries, and the blastocoUa of the viscid hairy covering of many leaf-buds {•vide infra). Gkmds may be classified according to their position as internal (/. e. lying in the interior of the tissue), and superficial ; but doubtful instances occur. In both cases the gland may consist of a single cell or a group of cells. Instances of internal simple glands are the camphor-cells just mentioned, those of rhubarb containing chrysophane, the gum-cells of Cactaceae, orchis tubers (salep), and the crystalliferous cells whose cavity contains mucilaginous substances together with masses of crystals (sect. ii). Internal compound glands are, on the other hand, those that contain essential oils in the rind of the fruits of Citrus, as well as those covered only by the epidermis on the upper side of the leaves of Dictamnm Fraxinella. The former are to be recognised, even in the young ovary, as roundish groups of cells, the contents of which are distinguished by turbid protoplasm and small drops of ©il ; the walls of these cells soon swell, then become fluid, and thus form a spacious spherical space filled with mucilage and drops of essential oil suspended in it. The layers of cells that surround the cavity form an envelope, which marks it off sharply from the rest of the tissue. The formation of the internal glands of Dictamnus (Fig. 96 c) commences with only two cells, one of which belongs to the young epidermis, the other to the next layer of parenchyma ; the former ^ [See also J. B. Martinet, Organes de secretion des v^getaux : Ann. des Sci. Nat. 5th ser. vol. XIV.— Ed.] I Fig. 96.— Transverse section of the leaf of Psoralen hirta ; e e epidermis ; / parenchyma containing chlorophyll ; m recep- tacles of latex combining to form a gland. (After Hildebrand, I.e.) 114 MORPHOLOGY OF TISSUES'. furnishes on its part two layers of cells, the outer of which (d) forms a continuation of the epidermis, while the inner (c) contributes to the formation of the tissue of the gland, which, in its principal mass, originates by divisions of the two mother-cells of the gland (/> /») ; the enveloping layer of the gland is here scarcely developed, as is shown in Fig. 96 c, C. On the flower-stalks, bracts, and sepals of the same plant are formed large sessile or shortly stalked glands of somewhat ovoid form, bearing at their apex a single hair (Fig. 96 b). They always arise, as Rauter has shown, from a single cell of the young epidermis which divides first vertically then tangentially (Fig. 96, ^) ; thus two layers are formed, the outer of which represents a continuation of the epidermis, while the inner produces, by further divisions, the tissue of the Fig. 96 *.— Glandular hair of Dictamnus Fraxinella (after Rauter). A and B earlier stages of development ; C mature gland, with the hair at its apex. Fig. 96 c— Internal gland of Dictamnus Fraxinella (after Rauter). A and B early stages of development, C mature gland; a? the covering layer, developed as a continuation of the epidermis ; c and p mother-cells of the gland-tissue ; o a large drop of ethereal oil. gland {B) ; in the further course of development, the whole substance of the gland now becomes, as it were, forced outwards above the surface of the organ (C) ; and when, finally, the secreting tissue is absorbed, a cavity is formed filled with mucilage and drops of essential oil, and now surrounded only by the continuation of the epidermis. Whether the substance of the gland is to be considered as a portion of the hair which it bears must remain undecided ; it is also doubtful whether the gland should be termed an internal or an external one. Similar to glands in their origin are the gum-passages and gummy swellings of diseased stone-fruit. GregorieflF found the seat of the formation of the gum in them to be principally the soft bast of the fibro-vascular bundles which per- meate the fruit-pulp ; the cell-walls become absorbed after they have swelled up, indefi- nitely bounded cavities filled with gum are thus formed, which sometimes exude their con- tents externally through the flesh of the fruit when the production of gum is excessive. Under the head of Superficial Glands should perhaps be included all those which ETC. 115 discharge their secretion immediately outwards, as the groups of cells which secrete the nectar of many nectaries, e. g. those at the base of the petals of Fritillaria imperialis, and at the base of the ovary of Nicotiana. The superficial glands are represented frequently and in many different forms by glandular hairs, to which a large number of leaves and stems owe their viscid character, and many leaf-buds their gummy or balsamic coatings. Not unfrequently odoriferous viscid substances collect in the globular terminal cells or knobs of simple glandular hairs ; in other cases the odoriferous oily secretion penetrates through the cell-wall, and raises the cuticle in the form of bladders, collecting beneath it as a clear fluid, while the cells which produce it partially or entirely disappear, as in Salvia, Cannabis, and Humulus (the last on the perianth of the female flowers). We are indebted to a careful work of J. Hanstein's^ for an accurate knowledge of the glandular hairs on the leaf-buds of many trees, shrubs, and herbs. The parts of the bud are coated by a gummy substance, or one composed of gum-mucilage and drops of balsam, which he calls Blastocolla, while the glandular hairs which produce them he terms Golleters. These multicellular shortly stalked hairs springing from an epidermal cell may expand upwards in a strap-shaped manner (Rumex), or may bear cells arranged in a fan on a kind of mid-rib (Cunonia, Coffea), or may form spherical or club-shaped knobs (Ribes sanguineum, Syringa 'vulgaris) ; in Platamis acerifolia branched rows of cells occur, the roundish terminal cells of which become glandular. The colleters attain their full development at a very early period in the bud, when the leaf-structures and portion of the stem out of which they spring are still very young, and consist of tissue which is yet scarcely differentiated. They are borne especially by the enveloping scales of the leaf-buds (Aesculus), by the stipules which precede the leaves in development (Cunonia, Viola, Prunus), the ochreae (Polygonum), or the young leaves themselves (Ribes, Syringa). The secretion of the colleters is a watery mucilage in Polygonum, in the rest it is mixed with drops of balsam (resin). Gum-mucilage always arises from the conversion of a membranous layer lying beneath the cuticle of the col- leter, the substance of which swells on addition of water, and raises the cuticle in places into small bladders (Rumex), or detaches it continuously from the hair as a large bladder; finally the cuticle bursts, and the mucilage escapes and envelopes the buds ; the uninjured inner layer of cell- wall can, on its part, form a cuticle, beneath which a membranous layer again separates, and the process is repeated. Where balsam is also excreted, it may be recognised even in the cells of the hair ; but it appears outside the cell-wall in drops as a deposit in the mucilage, or forms the principal mass of the secretion. Frequently also the young epidermis itself between the colleters participates in these processes (Polygonaceae, Cunonid) ; or these latter are entirely absent, and the blastocolla is produced exclusively from the epidermis ; thus arises, for instance, the greenish balsam on the bud-scales and foliage-leaves of poplars. (c) Ihe Sap-conducting Intercellular Passages'-. It has already been explained in Fig. 66 (p. 76), that the 'resin passages' are intercellular spaces, arising usually from the separation of four cells; and they generally acquire a peculiar morphological character from the fact that the cells remain for a considerable time capable of division, and, obeying a common law of growth, form groups the arrangement of which may differ essentially from those by which they are surrounded. The development of the cell-walls is also different, as occurs especially in the resin-passages in the wood of Coniferae. Here the cells which surround the passage are originally like pitted tra- cheides ; but their walls remain thin and unlignified, their cavity enlarges, and their ori- ginal position is obliterated by their growth. The contents of the cells which enclose the passage are more or less like those of the passage itself, since they escape from the one into the other. In Helianthus and other Compositie it is a yellow or red intensely odoriferous oil ; in Umbelliferse a mixture of gum-mucilage and oily or resinous sub- ' Ueber die Organe der Harz- und Schleimabsonderung in den Laubknospen, Bot. Zeitg. 1868, no. 43 et seq. Compare the veiy instructive illustrations to this paper. 2 MuUer in Jahrb. fiir wissen. Bot. V. p 387, 1867.— Thomas, ditto, IV. pp. 48-60. I 2 ii6 MORPHOLOGY OF TISSUES. stances (gum-resin) ; in Conifcra? and Terebinthacece a clear bals-am, which hardens, on exposure to the air, into a firm resin. The resin-passages run mostly in straight lines, or follow the course of the fibro- vascular bundles ; apparently they only rarely anastomose. They so far resemble the simpler l^ticiferous vessels, that they may also form continuous system's running through the whole plant. When they occur in the parenchyma of the cortex and the pith which is formed from the primary meristem, they are mostly distributed at nearly equal distances through the transverse section of the stem, forming a circle ; when produced in phloem or xylem, they may recur periodically as elements of this system, and, so to speak, be formed in layers, i. e. in concentric circles, as, e, g. in the wood of Pinus, and in the phloem of Coussonia. The occurrence of these passages is limited to certain groups ; they are found in a high state of development in Coniferse and Gycadeee, Terebinthace;^, Umbelliferse, Araliaceae, and Composit?e. B Fig. 97-— Transverse section of resin-passages (ff) at the base of a first year's branch of Finns syl-vestris (XSSo). A, B, C passages lying in the peripherj' of the pith (s^^ spiral-vessels of a fibro-vascular bundle) ; at A the formation of a passage has not taken place, but the cells destined for its formation are there, their walls having become weaker ; D wood-cells (h) enclosing a group of resin-cells, not forming a passage (st a medullary ray) ; E part of the wood containing a resin-passage (g) ; next it wood- cells containing starch (a}?t), fonning in the wood a zone passing in a tangential direction from one passage to another. When the passages lie in a tissue which undergoes rapid growth in diameter, they not unfrequently attain a considerable size in this same direction ; as, e.g. in the primary cortex and leaf of Pinus (Fig, 60, >6), Cycas, &c. When, on the other hand, the growth of the tissue in diameter is inconsiderable, as in the wood of Pinus, the intercellular space which has become enlarged into a passage remains also small (Fig. 97, B, C, g). In the pith of the first-year's twigs of Pinus groups of cells are also found which resemble, in contents and form, the environing cells of resin-passages, but do not separate from one another, and thus do not form a passage. In this case the wood which is already formed prevents a subsequent extension of the pith in diameter, and thus the space is wanting which would be necessary to the formation of the intercellular space (Fig. 97, ^, D). THE PRIMARY MERISTEM AND TUE APICAL CELL, 11/ Sect. 19. The Primary Meristem and the Apical Cell ^ — At the growing ends of shoots, leaves, and roots, the forms of cell-tissue hitherto described do not yet exist ; here is found a uniform tissue, the cells of which are all capable of division, rich in protoplasm, with thin and smooth walls, and containing no coarse granules. This tissue is termed Primary IMeristem ; it is a meristem because all the cells are capable of division, and must be considered primary (rather, perhaps, proto-meristem) because it presents the primary condition of the tissue, out of which the different forms of the permanent tissue are successively formed by differentiation. If the structure of the plant is in general simple, as in Algae and Characeje, the cell-forms arising from the primary meristem only differ slightly from one another. If the plant belongs to a higher type, as in Vascular Cryp- togams and Phanerogams, from the uniform undifferentiated primary meristem proceeding from the growing apex layers of tissue of a different character first originate, within which, by further development of their cells (at a still greater distance from the primary meristem), the different cell-forms of the epidermal and fundamental tissue, as well as of the fibro-vascular bundles, finally arise. The differentiation takes place so gradually, and at such a different time in the various layers of the tissue, that no definite limitation of the primary meristem proceeding from the apex is possible. As growth proceeds at the end of shoots, leaves, and roots, portions of the primary meristem become gradually transformed further backwards into permanent tissue ; but the primary meristem is always again renewed by the production of new cells close to the apex. Nevertheless whole organs, the apical growth of which soon ceases, may at first consist entirely of primary meristem, which finally passes over altogether into permanent tissue, so that no primary meristem is left. Examples of ihis are furnished by the deve- lopment of the fruit of Mosses, of the sporangia of Ferns, and even of most leaves and fruits of Phanerogams. The terminal portion of an organ with permanent apical growth, consisting entirely of primary meristem, is termed the Pimcium Vegetaiionis ; not unfrequently (but by no means always) it projects as a conical elongation, and is in this case distinguished as the Vegetative Cone. The production and renewal of the primary meristem commence with the cells lying at the apex of the puncium vegciationis ; and, by the manner in which this happens, two extreme cases may be distinguished, which are however united by transitional forms. In the one case, the usual one with Cryptogams though not without exception, the whole of the cells of the primary meristem trace their origin back to a single mother-cell, lying at the apex of the punctum vegetationis and * Nageli, Die neueren Algensysteme. Neueuburg 1S47. — Cramer in Pflanzenphysiol. Unter- suchungen, Heft III. p. 21. Zurich. — Pringsheim, Jahrb. fiir wissen. Bot. III. p. 484. — Kny, ditto, IV. p. 64.— lianstein, ditto, IV. p. 238.— Geyler, ditto, IV. p. 481.— Miiller, ditto, V. p. 247.— Rees, ditto, VI. p. 209.— N.-lgeli und Leitgeb, in Beitriige zur wissen. Bot. Heft IV. Miinchen 1867.— J. Hanstein, Die Scheitehellgruppe im Vegetationspunkt der Phanerogamen (in the Festschrift der niederrh. Ges. fiir Natur- und Heilkunde. Bonn, und Monatsiibersicht of the same Society, July 5, 1S69).— Hofmeister, Bot. Zeitg. p. 441, 1870.— Leitgeb, Sitzungsb. der Wiener Akad. 1868 and 1869, and Bot. Zeitg. nos. 3 and 34, 1871.— Reinke in Hanstein's Botan. Untersuchungen, Heft III. Bonn 1871. i8 MORPHOLOGY OF TISSUES. called the Apical Cell. In some Cryptogams, on the other hand, and in Phanero- gams, there is no single apical cell of this character. Even when a cell lies at the apex, it is not, as in the former case, distinguished by its greater size; and, what is of greater importance, it cannot be recognised as the single original mother-cell of all the cells of the primary meristem, nor even of a definite layer. We may distinguish, therefore, between the Piincium Vegetaiionis with and without an Apical Cell. (a) Punctum Vegetaiionis with an Apical Cell. The formation of the primary meristem out of the apical cell may be brought about, as will be shown hereafter,' in different ways, but it generally results from the re- peated rhythmical division of each apical cell into two unequal daughter-cells. One of the two daughter- cells remains from the first similar to the mother- cell (the apical cell), and includes the apex; it is immediately enlarged by growth till it equals the pre- vious apical cell in size, and then again divides, and so on. This process produces the appearance as if the apical cell always remained intact ; and this has been assumed in ordinary language, although the apical cell existing at any time is only a daughter-cell of the preceding one. The other daughter-cell on the other hand appears from the first like a piece cut off from the back or side of the apical cell, generally in the form of a disc or angular plate, and is hence called the Segment^. In the simplest case the segment may, on its part, remain undivided ; and then the whole tissue which is produced from the apical cell appears in the form of a simple thread or row of cells, as in some Algae, Fungus-hyphae, and hairs. But gene- rally the segment is also again divided into two cells, each of which again breaks up into two, and this pro- cess is mostly repeated many times in the daughter- cells, until a more or less extensive mass of tissue is FIG. 9?.-A branch of the thaiiome of produccd from thc scgmcnt. The primary meristem fSX:Sd^^d[;::mS;ThiM^S! now consists of such portions of tissue. A very simple ceiiwliif''^'''"* ''"''' '""""'"'' case of this kind is shown in Fig. 98, where the * The portions of wall which enclose a segment-cell are different in their nature and origin, and behave differently in their subsequent growth. Each segment possesses two walls which were originally division-walls of the apical cell ; they are generally parallel to one another, and are called the Principal walls of the segment ; the older faces the base, the younger the apex of the organ. Another portion of the wall of the segment is a part of the outer wall of the apical cell ; it may be termed the Outer wall of the segment. \Miere the segments arise as transverse discs of an apical cell, the process is very complicated, from the segmentation taking place on two or three sides ; the segments have in this case also side-walls as well as the two principal walls and the outer wall, which intersect at oblique angles within and below. The side-walls are portions of the principal walls of older adjoining segments, which are always bounded by the youngest partition-wall of the apical cell, and this is at the same time the youngest principal wall. THE PRIMARY MERISTEM AND THE APICAL CELL. 1 19 apical cell, here very large {s), growing straight out from its base, is divided by septa (/", P), and thus forms the segments which lie in a row one over another ; but each of these last is again immediately broken up by a septum (//'', IP) into two disc-shaped cells, and in each of these there arise numerous small cells by the formation of vertical and afterwards horizontal walls (as may be seen in the figure), further back from the apex; and it is easily seen how the whole branch is built up of portions of tissue, each of which is composed of a single seg- ment. The same takes place on the lateral branchlets {x, y), which in this case arise originally from lateral protuberances from the apical cell. These processes are remarkably clearly seen in Stypocaulon, in the first place because only one row of segments is formed lying one over another, and in the second place be- cause the segments thems-elves are transformed into portions of tissue without at the same time growing, as is usually the case ; distortions often occur from the growth of the segments, which render difficult an investigation of the processes of division. Figs. 99 and 100 show us a case in which the apical cell is divided alternately right and left by oblique walls so as to produce two rows of segments attached to one another in a zigzag manner by their inner and lower sides, but separated to some distance in front; in the angle which the two youngest segments enclose lies the apical cell {s). Fig. 99 shows the end of a shoot of Metzgeria furcata in the act of bifurcation ; each fork ends in an apical cell {s) ; the segments and the masses of tissue which are formed from them are drawn just as they appear to the eye under the microscope in the superficial view of the flat strap-shaped shoot. But from the course of the cell-walls and the resulting grouping of cells around the apical cell the diagram represented in Fig. 100, A, is deduced, in which the distortions of the cell-walls occasioned by growth are omitted, and hence genetic relationships are represented more clearly. For further information Fig. 100, B, is added, which also represents diagrammatically the longitudinal section of the apical region, at right angles to the broad surface of the strap- shaped shoot. This longitudinal section bisects, behind the apical cell, the central nerve (Fig. 99, n, n), which consists of several layers of cells, while the lateral expansions of the shoot are only one layer in thickness. The origin of the tissue is now clear from the diagrammatic Fig. 100, A and B, if it is observed in the first place that the portions of the surface indicated by m, n, 0, p, and q are the segments of the apical cell (s) which were formed successively in the same order, so that 7)1 represents the oldest, q the youngest segment. From each segment a small piece is at first cut off behind by a wall oblique to the axis of the shoot; from the zigzag row of these inner divisions arises the mid-rib of the shoot, which attains a thickness of several layers of cells, each division first of all splitting up by a wall parallel to the surface of the shoot into two cells lying one over another ; each of these cells on its side again divides in the same manner. Divisions at right angles to the surface of the shoot (Fig. 100, B) are then also formed in the uppermost and undermost of the cells produced in this way; an outer small-celled layer (covering the upper and under side) becomes formed on the mid-rib, surrounding an inner bundle which consists of longer cells. While the posterior sections of the segment produce the tissue of the nerve, the tissue T20 MORPHOLOGV OF TISSUES. of the flat lateral portion (Fig. 99,,/;/') proceeds from the sections in front which face the margin of the shoot ; and this tissue is only one cell-layer in thickness, no division taking place in it parallel to the surface of the shoot. All the divisions in these marginal sections of the segment are, on the contrary, at right angles to the surface of the shoot, and are produced by the marginal section first of all breaking up into two cells lying close to one another (cf. Fig. 100, A, 0), each Fig. 99.— Apical region of a shoot oi Metzge7-ia fiircata in the act of dichotomous branching, looked at from the surface (after Kny). The shoots consist of a single layer of cells (ff), which is however penetrated by a mid-rib n n', three to six layers iis thickness. Fig. 100. — Diagrammatic representation of the segmentation of the apical cell, and of the first divisions in the segment of Metzgei-ia fiircata (after Kny). A apex seen from the surface ; B the same in vertical longitudinal section ; C an apex in the act of dichotomous branching ; a new apical cell is formed in the third-youngest segment. of which then forms several shorter cells by repeated bipartition, and these may again undergo further division according to the activity of the growth. In general the first divisions only of the segment are constant ; the further course of cell- multiplication is, according to the minute investigations of Kny, subject to many deviations. Since the tissue which is produced from the marginal sections assumes a prominent position during growth, it results that the apical cell lies, with the youngest segments, in a depression of the outline of the shoot ; and thus we have here a simple example of the depression of the punctum vegetationis in the tissue which grows more luxuriantly' around it, such as often occurs to a much greater extent in Fucaceae, Ferns, and Phanerogams. The differentiation of the tissue out THE PRIMARY MERISTEM AND THE APICAL CELL. 121 of which the shoot of Metzgeria furcata is built up does not attain a high degree; the perfectly developed cells of the margin and of the mid-rib are only slightly different from one another ; but it should be mentioned that this differen- tiation is brought about very early, even in the first division of the segment, so that the marginal tissue and the latest continuation of the mid-rib can be followed close up to the apical cell. Fig. loo, C, finally, affords an oppor- tunity of learning the mode of formation of a new apical cell out of a cell of the meristem, a case which occurs often enough in Mosses and higher Cryptogams ; while the thallome of Stypocaulon (Fig. 98) shows how the apical cell of the lateral shoot grows immediately from the apical cell of the principal process as a lateral protuberance, which is then cut off by a wall. In Metzgeria furcata, as is shown by the statements of Hofmeister, Kny, and Miiller, it appears that the origin of a new apical cell may be brought about in a different manner; Fig. 100, C, shows the case described by Kny. In the third-youngest segment ((?), which is formed from the apical cell {s), the customary separation into a nerve-mother-cell and a division belonging to the margin of the tissue has first taken place ; the latter then breaks up, as is usually the case, into two cells lying close to one another ; but the new apical cell is constituted by the appearance of a curved wall in one of these marginal cells of the second rank; and this wall comes into contact behind with the previous one, thus cutting out a wedge-shaped piece (2), which assumes at once the function of the apical cell of a new shoot. (We shall recur, in Chap. Ill, to this case of spurious dichotomy.) In the Equisetacex and many Ferns, the axis of the shoot terminates in a comparatively very large apical cell, which is bounded by four walls — an outer one overarching the apex, spherically triangular, and free, and three converging obliquely below and within, which form at the same time the upper principal walls of the youngest segment (Fig. loi, J, D); the apical cell has hence the form of a seg- ment of a sphere, or of a three-sided pyramid with spherical base turned upwards. The three plane principal walls of the apical cell are of different age; one is always the oldest, one younger, and the third the youngest. The next division- wall arises in the apical cell, and is parallel to the oldest wall ; a segment is formed bounded by tw-o triangular principal walls, an arched outer wall, and two nearly oblong side-walls^; after the apical cell has again grown to its original size a second division follows parallel to the next-younger principal wall, which is followed again, after fresh renewal of the apical cell, by a division parallel to the youngest principal wall. Three segments are now formed, placed somewhat like the steps of a winding staircase ; each is in contact with a principal wall of the apical cell; and in this manner the divisions are repeated; and since each segment takes in a third of a circuit of the winding staircase, the segments out of which the stem is bulk up all lie in three straight rows parallel to the axis, each embracing a third of the diameter of the stem. In Fig. 101, B and C, the segments are numbered /, //, ///, &c., according to the order of their formation, and are represented as they appear when the apex of the stem is seen from above and * These side-walls are pieces of the principal walls of the previously existing adjoining segments, as is seen in B and C. 122 MORPHOLOGY OF TISSUES. without (not in transverse section), or as if the arched surface of the apex were removed and spread out flat. If the segments are followed according to the order of their numbering, and the path thus described is indicated by a continuous line, a spiral is obtained, which is in reality an ascending spiral line, because each segment lies higher than the older ones, as is shown in Fig. toi, B, where, however, only two rows of segments are to be seen from without. The formation of tissue begins by each segment breaking up, soon after its production, into two Fig. ioi. — Apical regions of the stem of an Equisetum ; A longitudinal section of an underground very strong bud ot E. Telntatgia, in September (XSSo) ; B view of the apex from above (both from nature) ; C, D, E the same of E. ar-vense (after Cramer). Q diagrammatic ground-plan of the apical cell and of the youngest segment ; D external view of a slender stem-apex ; E transverse section through this from / to /) ; 5 is in all cases the apical cell, /, //, ///, &c. the segments ; i, 2, 3, &c. the division-walls in the segments in the order of their formation ; x, y, b, bs in A the first rudiments of leaves. equal plates, a division-wall springing up parallel to the principal walls, indicated in B, C, and 7? by i, i. Since in each of these two half-segments which lie one on another the further processes are almost exactly the same, it is necessary to keep in view only one half. Each half of the segment becomes divided first of all by a vertical curved wall, which meets internally a side-wall, externally the centre of the outer wall of the segment. Since three segments compose one section of the stem, and each half-segment breaks up in this manner into two cells, the section of the stem now appears as if composed of six cells or sextants, whose walls are placed nearly radially, forming a six-rayed star, as is shown in the transverse section THE PRIMARY MERISTEM AND THE APICAL CELL. 123 Fig. 10 r, I^. Hence the walls by which this division is brought about are called sextant-walls ; in C and D they are indicated by the figure 2. The sextant-cells are still further broken up by vertical walls into an outer larger and an inner smaller cell (Fig. 1 01, ^); and thus the foundation is laid of the two layers of tissue into which the primary meristem separates, viz. into an outer and an inner layer, as is clearly shown in Fig. loi, A. In the outer layer the divisions parallel to the })rincipal walls and in vertical radial direction at first preponderate ; in the inner layer the divisions are less numerous, so that the cells become more uniform in diameter. This inner mass of tissue, arising from the inner sections of the sextants, is the pith which splits as the stem developes, dries up, and thus causes its hollowness ; from the outer layer of tissue of the primary meristem are also formed downwards the cortex, the system of the fibro-vascular bundles, and later the epidermis \ The external conformation also of Equisetum is brought about by the outermost layer of the primary meristem, as has already been shown in Fig. 10 1, A, where the protuberances x, j>, 6, bs represent the rudiments of the leaves; processes to which I shall recur at length hereafter. Here it need only be mentioned that each three consecutive segments undergo at an early period a small vertical displacement, of such a nature that they form, at least with their outer surfaces, a diagonal belt, which becomes arched and is the origin of a leaf-sheath. As a final example of the formation of the primary meristem from an apical cell, we may now consider the processes that take place at the growing end of a Fern-root, with which the greater number of roots of Cryptogams agree in the main. riG. 102 —Apical region of .i Fern-root ; A longitudinal section through the end of the root of Pteris hastata; Ji transverse section through the apical cell and adjacent segments of the root of Asplenium FUix-fceniina (after Nageli and Lcitgeb). Fig. 102, A, shows the axial longitudinal section through a Fern-root, with the point turned upwards. From the apical cell v arises not merely the tissue of the substance of the root (", b'" arise as multicellular protuberances, which soon embrace the stem, and envelope it and the younger leaves like a sheath. In the axil of the third youngest leaf b" the youngest rudiment of a branchlet is visible as a roundish protuberance. KiG. 108. — Longitudinal section of the apical region of the pri- mary stem of the sunflower, immediately before the formation of the flowers; a the apex of the broad punctum vesetationis ; bb the youngest leaves ; r the cortex ; w the pith. VegetationiSf never from those parts of the stem which already consist of fully differentiated tissues. In Characece, Mosses, &c., before or during the first divisions of their segments the leaves become visible close beneath the apical cell, as protuberances, the outer portion constituting an apical cell, out of the segments of which are formed the leaves. In Vascular Cryptogams a many-celled vegetative cone often overtops the young- est rudiment of a leaf {e.g. strong Equi- setum buds, Salvinia, many Ferns and Selaginellae). In Phanerogams (Figs. 107, 108, 109) this is general; in them the rudiment of the leaf does not begin with an apical cell standing out beyond the vegetative cone, as in Cryptogams, but a roundish or broad cushion is formed, which from its very first origin consists of numerous small cells capable of division. (4) The Leaves are always Exogenous Formations, i. e. the rudiment of the Fig. 109.— Longitudinal section through the apical region of an upright shoot of Hippttris vulgaris ; s the apex of the stem ; b, b, b the leaves (verticillate) ; k A the buds in their axils, which all develope into flowers ; g^^r the first vessels (the dark parts of the tissue indicate the inner rind with its intercellular spaces). ' Since phenomena of this kind are confined to the flowers and inflorescence of Phanerogarp.":^ their consideration may for the time be postponed. 134 EXTERNAL CONFORMATION OF PLANTS. leaf never has its origin in the interior of the tissue of the stem, and is never covered by layers of tissue of the stem, like roots and many shoots. In Cryptogams it is usually one superficial cell (?'. e. superficial before the differentiation of the epidermis) ^vhich forms the foliar protuberance. In Phanerogams a mass of tissue bulges out as the rudiment of the leaf, consisting of a luxuriant growth of the periblem covered by dermatogen (sect. 9, Fig. 103). By this means the leaf is immediately distinguished from the hair in its most rudimentary state. The hair is an outgrowth of the epidermis ; but since in Phanerogams the primordial epidermis (dermatogen) covers the whole punctum vegetatiojiis above the leaves, hairs may also spring up higher in position than the youngest leaves from single dermatogen- cells (as in Utricularia according to Pringsheim). But in Cryptogams the dermatogen only becomes diff"erentiated after the formation of the leaf; and hence the hairs are always at a greater distance from the apex than the youngest leaves (Fig. 106); the superficial cell of the stem, which in Cryptogams becomes the apical cell of a new leaf, is not an epidermis-cell, since its origin dates long before the dififerentialion of the tissue into epidermis and periblem. (5) The Tissue of the Leaf is contimwus iji its formation with that of the Stem. It is impossible, histologically, to find a boundary line between the stem and the base of the leaf, although such a boundary line must be assumed ideally. If the surface of the stem is imagined to be continued through the base of the leaf, the transverse section thus caused is called the Insertioji of the Leaf. The con- tinuity of the tissue of stem and leaf is an evident consequence of the early origin of the leaf below the apex of the piinctiim vegetatioins before the differen- tiation of its tissue began. An inner mass of tissue is usually formed close be- neath the apex of the stem before the formation of the leaf, and to this mass may be applied in the case of Mosses, Equisetaceae, and other Cryptogams, the term Plerome, which Hanstein has proposed for Phanerogams (sect. 19). This takes no share in the origin of the leaf, and the continuity of its tissue is brought about by the outermost layers of the primary meristem, in which also the fibro- vascular bundles usually originate. When, however, the inner stem-tissue (ple- rome) is itself transformed into a fibro-vascular body, as in Hippuris (Fig. 109) (and notably in many Mosses), a continuity is subsequently brought about between the fibro-vascular bundles of the leaves and this innermost tissue of the stem (Fig. 109). When fibro-vascular bundles are formed in the stem having no con- nexion with the leaves, they are termed by Nageli ' cauline bundles'*; but in Pha- nerogams it is usually the case that each fibro-vascular bundle describes a curve beneath a leaf-insertion, one branch of which bends into the leaf, while the other branch runs downwards into the stem (Fig. 109,^^); the latter is called by Han- stein the inner leaf-trace, and the whole bundle is a ' common ' one, i. e. common to both stem and leaves ; both common and cauline bundles may run through the same shoot (as in Ferns, Cycadese, and Piperacccc). In the fully developed shoot the cortical layers of the stem, at least the outer ones, bend outwards into the leaf without obvious interruption, and form its fundamental tissue ; in the same manner the epidermis passes over continuously from the stem to the leaf. When the stem produces fibro-vascular bundles, the leaves are usually also provided with them ; they remain without vascular bundles only when they are arrested early in their LEAVES AND LEAF-FORMING AXES. I35 growth, and persist as small scales, as in Psilotum and in many small leaf-scales of Phanerogams. (6) T/ie Leaves usually groiv more rapidly in length than the shoot ivhich produces them does above their insertion (Figs. 106, 107, 108). If the leaves are formed quickly one after another, they envelope and overarch the end of the shoot, and thus form a Bud, in the centre of which lies the leaf-forming punctiim vegetationis. This bud-formation depends at the same time on the more rapid growth of the outer or under side of the leaves in their young state, by which they become concave on the inner (afterwards the upper) side, and adpressed upwards to the stem. It is only when perfectly developed, by the latest extension of their tissue, that the leaves turn outwards in the order of their age, and thus escape from their position in the bud. If the portions of the stem that lie between the insertions of the leaf undergo at the same time a considerable, and often very great extension, the leaves, when escaping from their position in the bud, become placed at a distance from one another, and a shoot results with extended internodes. In such cases the section of the stem in which the leaf-insertion lies usually undergoes a different development from the intermediate portions ; these zones are then termed the Nodes, the intermediate portions the Internodes or interfoliar portions {e.g. Characeae, Equisetaceae, Grasses). If the stem remains entirely undeveloped between the leaf-insertions, it possesses no proper free upper surface, and is endrely enveloped by leaf-insertions (as in Aspidium Filix-mas) ; but more commonly this is only apparently so from the internodes being very short, as in many palm-stems. The internodes may be present immediately after the first formation of the leaves, when the consecutive leaves or leaf-whorls appear at considerable distances in height from one another, as in Chara ^ and Zea (Fig. 107) ; or they may originate only after further development of the stem-tissue, as in IMosses (Fig. 106) and Equisetaceae, where each segment of the apical cell of the stem arches outwards and forms a rudiment of a leaf, so that the leaf-rudiments follow immediately one after another; and it is only by further differentiation that the lower portions of the segment become developed into the free portions of the surface of the stem, as is clearly shown in Fig. 106. The formation of a bud in the way described above is sus- pended when on the one hand the leaves are added very slowly one after another, and on the other hand -the stem grows rapidly in length between the youngest leaf-rudiments or even before the appearance of the youngest ; so that there is always only a slightly developed leaf near the apex, as in the underground creeping shoots of Pteris aqiiilina {vide Book II, Ferns). (7) Every leaf assumes a form different to that of the Stem which produces it, and to that of its lateral Shoots. This is usually so conspicuous that no further description is needed. Nevertheless one point must be mentioned which often causes difficulty to the beginner. It not unfrequently occurs that lateral shoots of certain plants present a great similarity to the foliage-leaves of other plants in form and phy- siological properties, as the flat lateral shoots which bear the flowers in Ruscus, Xylophylla, Miihlenheckia platyclada, &c. ; but the course of development shows » I consider in Chara, as in Mosses and universally, that the cortex belongs originally to the stem, and not to the leaf. J ^6 EXTERNAL CONFORMATION OF PLANTS. that these apparent leaves must, from their position, be lateral shoots, themselves producing leaves; and the leaves of these plants are usually of quite a different form from these leaf-like branches. The phrase 'leaf-like' has in these cases usually no distinct morphological, but only a popular meaning ; and what will be said under paragraph (8) may be applied here. The branches or leaf-forming lateral shoots arise in a very different manner in different plants ; but very commonly they have this in common with leaves, that their origin is equally in the primary meristem of the piincium vegetationis as lateral and exogenous outgrowths ; that they are formed, like the leaves, in acropetal succession ; and that the differentiation of their tissue proceeds continuously with that of the mother-shoot. They are distinguished, however, from the leaves of the same plant by the place of their origin, by their much slower growth, — at least at first (later they may overtake the leaves), — and by their relations in point of symmetry, of which we shall speak hereafter. The leadino- fact, however, is that the lateral shoot repeats in itself, by the formation of leaves, all the relations hitherto named between leaf and stem, and appears therefore as a repetition of the mother-shoot, although in other physiological rela- tionships it is very different. (8) The morphological concepHojis of Skin and Leaf are correlative; one cannot be conceived without the other ; Stem (Caulome) is merely that which bears Leaves ; Leaf (Phyllome) is only that which is produced on an axial structure in the manner described in paragraphs 1-7 ^ All the distinguishing characters which are applicable to the definition of Caulome and Phyllome express only mutual relationships of one to the other ; nothing is implied as to the positive properties of either. If we compare with one another all the things which we call leaves, without refer- ence to the stems to which they belong, we are unable to find a single charac- teristic which is common to them all and which is wanting in all stems. But that which is common to all leaves is their relation to the stem. Hence the ideas Phyl- lome and Caulome cannot be obtained by comparing with each other the positive properties of leaves and the positive properties of stems, or by laying stress on the points which they have in common and on those wherein they differ. But these ideas are obtained by observing exclusively leaves in their relation to the stem which produces them, and stems in relation to the leaves produced from them. In other words, the expressions Stem and Leaf denote only certain rela- tionships of the parts of a whole — the Shoot; the greater the differentiation, the more clearly are Stem and Leaf distinguished. The measure of the difference is usually arbitrary ; but if we confine ourselves to those plants to which the term leaf is applied in ordinary language, the distinction of leaves from stem depends on the relationships named in paragraphs 1-7 ; and in this sense certain lateral outgrowths in many Algae may be termed Leaves, and the axial structures which produce them Stems {e. g. Sargassum). But when the difference between the outgrowths and the axial structures which produce them is less, one or several of the relationships mentioned in paragraphs 1-7 disappear ; and it becomes doubtful ^ There are, for instance, thallomes strikingly similar to certain leaf-forms, as those of Lami- naria, Delesseria, &c. ; they are, however, not leaves, since they are not formed pn a stem as lateral structures. LEAVES AND LEAF-FORMING AXES. ^37 whether the expressions Leaf and Stem ought still to be used ; and when finally the similarity preponderates, the whole shoot is no longer called a Leafy Stem, but a Thallome. A branched thallome has the same relation to a leaf-bearing stem as a slightly differentiated to a highly differentiated whole. The differentiation of the external forms of the members of the shoot into Stem and Leaf is to a certain extent independent of the internal differentiation which brings about the different forms of tissue and the cell-divisions, as is shown in the com- parison of crosses and Characeae with Phanerogams. The internal segmentation may be reduced to a minimum of cell-divisions, or may altogether disappear ; in the latter case the single cell presents itself as a shoot, the lateral outgrowths of which behave as leaves, and the axial part as stem, as, for example, in Caulerpa amongst Algae. What has been already said as to the continuity of the tissue of the stem and leaf, and its origin from the primary meristem, must here be understood in an extended sense. In place of the primary meristem we have the pimctum vegetationis of a single cell continuing its growth at the apex, and instead of the differentiation of tissue the development of the older cell-wall and of its contents. Caulerpa consists of a single cell- utricle, which grows as a creeping stem and puts out lateral leaf-like protuberances and tubular hairs which perform the function of roots, the whole enclosing a continuous cell-cavity without partition-walls \ (a) The leaves, like the shoots, grow at first at the apex, /. e. at the free end opposite the place of their origin. This apical growth continues indefinitely in many thallomes and leaf-forming axes until checked by some external cause ; this is especially the case in the primary shoots of Fucacea", pleurocarpous Mosses, Characeae, the rhizome of Equisetaceac, ?>rns, the main stems of Coniferae and many Angiosperms. If the primary shoots themselves bear organs of reproduction, the apical growth generally ceases with their development, as in many acrocarpous Mosses, the fruit-stalks of Equisetaceae, the haulms of grasses which bear the inflorescence, and in all cases in Angiosperms where a primary shoot ends in a flower. The lateral shoots are usually of limited growth ; the growth frequently ceases without any external cause, and especially when they bear reproductive organs, become transformed into spines, or are very different in their shape from the primary shoot, as the horizontal lateral branchlets of many Coniferse, the leaf- like shoots of Phyllocladus, Xylophylla, Ruscus, &c. In by far the greater number of cases the apical growth of leaves ceases early, the apex itself becoming transformed into permanent tissue. In Ferns, however, the apical growth of the leaves usually continues, and in many genera is even unlimited, the apex of the leaf always remaining capable of development, and not becoming transformed into permanent tissue, as in Nephrolepis ; in Gleichenia, Mertensia, Lygodium, and Guarea, the growth of the apex of the leaf is, as in many shoots, periodically interrupted, and again renewed in each period of vegetation. (b) Besides the apical growth, there always exists however, both in stems and in leaves, an interstitial growth, the parts produced by the apical growth thus increasing in size and becoming further developed. The development of the internodes of the stem depends almost exclusively on this, as indeed is shown by the crowded position and the shortness of the internodes in the bud ; the interstitial growth generally appears at first very rapid, and the increase in size occasioned by it is often very considerable ; but it usually soon ceases, and the tissues become differentiated into unchanging permanent forms. Not unfrequently, however, a basal zone of internodes (as in Grasses, Equhetum * Cf. N.igcli, Zeitschiift fur wissenschaftliche Botauik und neiieie Algensysleme. I'^H EXTERNAL CONFORMATION OF PLANTS. hyeniale, &c.), and in many cases the base of the leaf also, remains for a long time in the condition of primary meristem, while the parts nearer to the apex, long since trans- formed into permanent tissue, have attained their full growth. In this manner a subsequent basal increase in length, often continuing for a long time, is occasioned in parts which have long ceased to grow above ; this occurs in a peculiarly marked manner in the long leaves of many Monocotyledons (Grasses, Liliaceae, &c.) which are sheath-like in their lower part ; and to a smaller degree in many Dicotyledons {e. g. Umbelliferae). Where, as in Ferns, and in a lower degree in many pinnate leaves of Dicotyledons, the apical growth long remains active, the basal interstitial growth usually soon ceases, and, vice 'versa, continues the longer the earlier the apical growth comes to an end. Two extreme cases may therefore be distinguished in leaves, although closely connected by intermediate forms ; the predominantly basifugal or apical and the predominantly basal growth. If the interstitial growth continues at one part of the surface of the leaf, and attains there a maximum which then decreases, a bag-like projection of the surface of the leaf is formed, which is termed a spur, such as occurs in many petals (as Aquilegia, Dicentra, &c.). (c) Before the tissues which are differentiated from the condition of primary mer- istem assume their definite forms, a rapid growth usually takes place in their cells, which is no longer accompanied by cell-division ; the size of the cells is not unfrequently increased by this means ten or even a hundred-fold and more. This process, which is mainly de- pendent on the rapid increase of the watery sap, may be termed Extension, in contra- distinction to the growth of the younger cells which is contemporaneous with their divisions and which always precedes the extension. On this extension depends the rapid unfold- ing of the parts of the bud, which had long before been formed in their main outlines, but had remained small. The buds very often remain a long time in a condition of rest, until a rapid unfolding of the leaves and internodes already formed suddenly takes place ; as, for instance, in the germination of many seeds, and in the persistent buds of many trees (Aesculus), bulbs (Tulip), and corms (Crocus, &c.), formed in the summer and germinating in the spring after long rest in winter. (d) The axis of length or growth of a member (as will further be shown in a special paragraph), is an imaginary line passing from the centre of the base to its apex. The entire growth both of leaves and of stems is usually most rapid in the direction of this line; they are therefore for the most part longer than they are wide or thick. In stems the growth is most often nearly equal along all diameters ; they assume therefore cylindrical, prismatic, or bulbous roundish forms. It is, however, sometimes the case that the growth in length advances much more slowly than that in diameter ; and then the stem becomes tabular or fiat, as in many bulbs, the corms of Crocus, and especially in Isoetes. It is only in the lateral shoots of higher plants with very limited growth that the internodes grow mainly in the directions of a plane which includes the axis of length, and thus become leaf-like, as in Ruscus, Xylophylla, &c. In leaves the growth usually preponderates in all the directions of a plane which cuts the stem transversely, and is mostly symmetrical right and left of a plane which in- cludes the axes of length both of the leaf and the stem ; the common form of leaves is therefore that of thin plates symmetrically divided into two longitudinal halves. There also occur, however, cylindrical and roundish tuber-like leaves, in \A7hich the growth has been nearly equally rapid in all diameters at right angles to the axis of the leaf {e. g. Mesembryanthemum echinatum). Sect. 22. Hairs (Trichomes) ^ is the term given in the higher plants to those outgrowths which arise only from the epidermis, i. e. from the layer of cells ^ Rauter, Zur Entwickelungsgeschichte einiger Trichomgebilde, p. 33. Vienna 1871.— Compare also sects. 15 and 19 (b). HAIRS (TRICHOMES). ^ OQ which always remain the outermost in roots, stems, and leaves, whether these outgrowths occur as simple utricular protuberances, row's of cells, plates of cells, or masses of tissue, or have the physiological character of woolly envelopes of the young leaves, root-like absorbing organs (Mosses), glands, prickles, or spore-capsules (Ferns). The hairs may originate from the primary meristem of the pimctum vegetationis, or from young leaves and lateral shoots, if an external layer of cells has already been differentiated as dermatogen, as in Phanerogams. But they may originate also in much older parts the tissue-systems of which have already become further differ- entiated, and which exhibit intercalary growth, because in such cases the epidermis long remains generative ; e. g. produces stomata and allows of cell-division. When hairs spring from the piuictiun vegctatioiiis, they are usually formed after the leaves, /. e. further from the apex than the youngest leaves ; but it also happens in Phanerogams that they are developed above the youngest leaves and nearer to the apex, the outermost layer of cells of the pwictum vegetationis having in this case already become differentiated as dermatogen (as in Utricularia according to Prino-s- heim). In ^Mosses and Vascular Cryptogams also, where the leaves become visible long before the differentiation of the external layers of tissue, the hairs do not show themselves on the surface of the stem till a later period and further from the apex. If the hairs arise in the neighbourhood of the apex of a punctum vegetationis or on a zone of interstitial basal growth (as in the sporangia of Hymenophyllacese), they may be arranged according to a definite law, which is not the case with hairs that spring from older organs, or at least not evidently so. Hairs are always strikingly different in their form from the leaves and lateral shoots of the same plant, although they sometimes bear a certain resemblance to these organs of other plants. The development in size of a single hair is usually extremely small compared to that of the member which produces it ; even the mass of all the hairs of a leaf, a root, or a stem, is generally quite inconsiderable com- pared to its weight. (a) The woolly and glandular hairs in buds are distinguished by a remarkably rapid growth ; they are often perfectly formed long before the parts of the bud unfold, but then they generally die off; the persistent hairs which remain during the life of the leaves are formed much more slowly, and are marked by a great variety of form. The root-hairs are formed at a considerable distance from the punctum 'vegetationis of the root, often 1-2 cm. from the apex, and mostly die off after a few days or weeks, so that the older parts of the roots of even annual plants are destitute of living hairs. The existence of these hairs is connected with the activity of the roots in the ground. The root-hairs which spring from the stems of Mosses are marked by a very long continued apical growth, and often by repeated branching. They consist of cells divided into rows by oblique septa, and, viewed physiologically, replace the root system of vas- cular plants. These root-hairs of Mosses are remarkably endowed with the generative principle, and behave in many respects like the Protonema, a means of propagation peculiar to Mosses; like it, they produce gemmae, w^hich, w-hen exposed to light, grow into leafy stems. If the root-hairs themselves come to the surface {e.g.h^ turning up a sod) they put out rows of cells rich in chlorophyll, on which also gemmae are produced. (b) Thallophytes, when they consist of a mass of tissue, also form true hairs, like Cormophytes ; but when the thallome consists only of one layer of cells, or, like Caulerpa J40 EXTERNAL CONFORMATION OF PLANTS. and others, is unicellular, one can no longer speak of an external layer corresponding to the epidermis ; and its hair-like outgrowths cannot therefore be considered as trichomes in the same sense as those of the higher plants. Nevertheless it is customary to speak in such cases also of hairs, when the outgrowths are thin and long, destitute of chlorophyll, and otherwise dissimilar to the thallus which produces them. On the other hand struc- tures occur in highly organised plants which are closely analogous to many forms of hairs in their physiological, and partly also in their morphological properties, but which differ from true hairs in not originating from single epidermis cells, but consist of massive outgrowths of the tissue which lies beneath the epidermis, remaining however covered by a continuation of it. Examples of such structures, which may perhaps be distin- guished by the term Emergences, are afforded, according to Rauter, by the prickles and glandular hairs of roses, and perhaps also of the various species of Rubus. Closely related to these are probably the warts, tubercles, and knobs on the surface of many fruits {e.g. of Euphorbiaceae, Ricinus). They resemble the leaves and branches of Phanerogams in their origin, but hairs in their later formation, and in their occurrence on stems as well as on leaves, and in their irregular disposition. For spines, which must not be confounded with prickles, cf. sect. 28. Sect. 23. The term Root^ is applied in botanical morphology, in contrast to its use in popular language, only to such outgrowths of the substance of the Fig. no. — Longitudinal section of the young primary root of the embryo of Marsilea salvatrtx; TVS the apical cell, 'uh', Tvh" , Ti'h'" the still simple root-cap ; x,y the last segments of the substance of the root ; z t intercellular spaces. Fig. III.— Longitudinal section of a somewhat older prhnarj- root of MarsUea salvatrix; ws apical cell; whl + Tvhl the first, whZ-ir-whi the second, wh^ the third layer of the root-cap ; each layer now consists of two divisions ; xy the youngest segments of the substance of the root ; o epidermis ; £^J' fibro-vascular bundles ; h the part of the root-cap which extends furthest back. plant as are clothed at their growing apex with a layer of tissue, the Root-cap already described in sect. 19. Roots do not form leaves or other exogenous ^ Niigeli und Leitgeb in Nageli's Beitriigen zur wissen. Bot. Heft IV, 1867. — Hofmeister, Morphologic der Gewebe, sect. 5. Leipzig 1868.— Hanstein, Botan. Abhandlungen, Heft I. Bonn 1870. — Dodel, Jahib. fiir wiss. Bot. VII, pp. 149 et seq. — Reinke, Wachsthnmgeschichte der Phanero- gamenwurzel in Hanstein's Botan. Untersuchungen, Heft HI. Tieghem : Ann. des Sci. Nat., 5th series, XIII, 1S71.] Bonn 1871. — [La Racine: Ph. van riAlRS (TRICHOMES). the surface of the c-gan from ^L■cl. he L p/oZ'^'l' ^°°' ''^^ ^^^'^ formed ,s usually covered ,vith thick layers of tissue t'j. '■°°^' "'''^" J«' "s further growth. Hence the roots are , va ' ;,"; " f '"''" "^™"''^ - character they are distinguished fron. all trichomesT, dT"" T"''™^' ^>' ^^'^'^'^ shoots. '="°'"'=s and leaves, and from most lateral vascufarbUr.'^ld'^herV'^": ''^ '"^"^ °^ "■''^" '-^ P— <^ b^ fi"- C.yp".og:^r" ""^ "■P^'=^^"'" "^^^^"^^ " ^W-- '» fi' "« p„„,a.y root of the e„,b,,o of Vascular 142 EXTERNAL CONFORMATION OF PLANTS. vessels being formed nearer to the circumference of the bundle, whereas the later bundles are always formed further inside, and hence centripetally in reference to the diameter of the root. Where bast-bundles occur, they arise in the cavities between the primary vascular bundles at the circumference of the fibro-vascular mass (Fig. 116, p. 146). Although roots are generally distributed among vascular plants the higher Cryptogams and Phanerogams, there occur even in these groups single species from which they are entirely absent ; thus among Rhizocarpeae the genus Salvinia, among Lycopodiaceae the genus Psilotum, among Orchideae Epipogiiim Gmelbii and Corallorrhiza imiata are destitute of roots ; the little Lemna ( Wolffia) arrhiza does not form roots, and is at the same time destitute of vascular bundles. With reference to the place of their formation roots are remarkably variable ; a root is usually formed even in the young embryo which proceeds from the fertilised ovule (but not in Orchidece). It appears at the posterior end of the em- bryonal stem, and may be termed generally the Primary Root, whether it remains weakly and soon dies, as in Cryptogams and Monocotyledons, or whether it continues to grow more vigorously, like all other roots, as in many Dicotyledons. But besides the primary roots, there are usually formed in addition a large number of Secondary Roots, or simply Roots (since they are a thousand times more nume- rous than the primary roots, and are also of much greater importance to the plant, it is superfluous to denote them by a special name, where the contrast to the primary root is not required). They arise in the interior of the primary or secondary roots, and on stems and leaf-stalks. The primary root with its secondary roots, or any root with its lateral roots, may be termed a Root-system. With the exception of many Dicotyledons with a persistent strongly developed primary root, the majority of roots spring from stems, especially when these latter creep, float, climb, or form bulbs and tubers. In Tree-ferns the stem is often densely covered wdth a felt of delicate roots throughout its whole length. In Ferns with densely crowded leaves in which no portion of the surface of the stem is left bare, the roots spring exclusively from the leaf-stalks, as, for example, in Aspiditnn Filix- mas, Aspkniiwi FiIix-/oe??ima, Ceratopteris thalictroides, &c. ; sometimes the fronds put out roots {e.g. Mertensia)^ When the stem possesses clearly developed nodes and internodes, the roots commonly proceed from the former; thus, e.g. exclusively from the nodes in Equisetacege, and most commonly so in Grasses. Observation of the nature of the tissues out of which the roots spring shows that they owe their origin either to the primary meristem, or to partially differentiated masses of tissue, or finally to a secondary meristem enclosed between layers completely differentiated. The primary roots of the embryos arise from quite undifferentiated primary meristem ; the lateral roots of Cryptogams, as Nageli and Leitgeb have shown, originate near the piinctum vegetalionis of growing roots, where the differentia- tion of their tissues first begins. With Phanerogams the same is the case, but stems may also produce roots near their punctum vegetalionis, w^here the differentiation of their primary meristem first commences ; this occurs in the case of the creeping ^ A leaf of Pha&eohis mnliiflorui cut off at the pulvinus and placed in water developed from the cortex of the intersected pulvinus an abundant root-system, and remained living for some months. HAIRS {TRICHOMES). 43 B stems of Rhizocarps and of Pieris aquilina. Roots are formed much further back wards from the puftcfum vegetationis , where the tissue is ah-eady completely differentiated from a secondary meristem, in older portions of the stem, and especially when mutilated, or when the environment is dark and damp. The order of development of the secondary roots is, according to Nageli and Leitgeb, dis- tinctly acropetal in the mother-roots of Crypto- gams, where they arise near the apex ; new roots are probably never formed in these plants between those already in existence in the mother- root. The same is, probably, always the case where roots are produced in the primary meristem or near the punctum vege- tatioiiis of the stem (as in Pilularia, Marsilea, Cereus, &c.). But even where their origin is fur- ther from the apex, as with the secondary roots in the primary root of Phanerogams and in many stems {Zea Mais, &c.), they generally appear in acropetal order; but by subsequent disturbance roots may arise adventitiously, /. e. in abnormal positions, as especially on older primary roots of Dicotyledons. Secondary roots usually make their appear- ance on the exterior of the fibro- vascular bundles ; the fibro-vascular bundle of the secondary root is then placed at right angles, or nearly so, to those of the mother-organ ; the cortex is then only in- completely continuous with that of the latter, the epidermis not at all so. The case is different in the primary roots of embryos, which are formed early and mostly so near the surface of the em- bryo that a complete continuity is possible in all the tissue systems between stem and primary root ; but in Grasses and some other Phanero- gams, the first root arises so deep in the interior of the embryonal substance that it is enclosed in the fully developed embryo of the ripe seed by a thick sac-like layer of tissue (Fig. 114, ws), which is ruptured on germination (Fig. 113, ws), and is known by the name of Root-sheath (Coleorhiza). Similar formations occur also in the first se- condary roots of the germinadng plants of Allium Cepa and occasionally elsewhere. But the secondary roots which are formed deeper Fig. 113.— Germination of maize in the order /, //, ///; A and B the embryo separated from /, in A seen in front, in B from the side ; iv the primary root ; -ws its root-sheath ; -w', w", -w'" secondary roots ; e the part of the seed filled with endosperm ; k the plumule ; sc scutellum of the embryo ; r r its open margins ; b b' b" the first leaves of the embryo-plant (natural size). in the tissue in other cases 144 EXTERNAL CONFORMATION OF PLANTS. simply break through the layers of tissue which cover them, and then project from a two-lipped open chink. The typical form of roots is filiform and cylindrical ; their section is circular when they are not compressed from without. It is only when the roots undergo a subsequent increase in thickness, and serve as store-houses, as in many Dicotyledons and Monocotyledons, that the original fili- form shape is changed into the fusiform or into tuberous swellings (as in turnips, tuberous roots of Dahlia, Bryonia, Aspho- delus, &c.). Roots rarely form chlorophyll {e. g. in Menyanthes), and even then only in small quantities ; usually they are quite colourless, not only when^they grow in the ground, but also in water or air. A subsequent basal growth appears never to occur in roots as it does in many leaves and internodes when once the re- gions near the apex have been transformed into permanent tissue. Interstitial growth behind the apex often continues, however, for a long time (in Lycopodiacese accord- ing to NageU and Leitgeb). The extension of the tissue commences immediately behind the terminal part of the root which has been formed of primary meristem — an arrangement by which the elongation of the roots in the ground is essentially assisted. Fig. 114. — I^ongitudinal section of the grain of Zea Mais (X about 6); c pericarp; ?t remains of the stigma; fs base of the grain ; <^ hard yellowish part of the endosperm ; e7Li whiter less dense part of the endosperm ; jcscutelluin of the embrj'O ; jj its point ; e its epidermis ; k plumule ; iv (below) the primary root ; 7US its root-sheath ; lu (above) secondary roots springing from the first internode of the embryo-stem it. (a) The primary root of the embryo of most Phanerogams gives the impression of being entirely superficial, as if its apex were the actual posterior termination of the embryonal stem ; but its first origin is endogenous ; for the posterior termination of the embryo is connected with the ' pro-embryo ' in Phanerogams, and the primary root is, at its first origin, covered by this. (A more exact account of this, according to the most recent researches of Hanstein on the formation of the embryo will be given in Book II, on the Characteristics of Phanerogams.) There was formerly some doubt as to the endogenous origin of the primary root of Ferns and Rhizocarps; but when it is observed that the root is not constituted as such until the apical cell has thrown off the first layer of the root-cap, it is evident that in this case also the apex of the new root lies from the first inside the embryonal tissue. (Compare the illustrations of the embryos of Ferns and Rhizocarps in Book II.) (b) ^he Formation of Lateral Roots in a mother- root commences — as Niigeli and Leitgeb have proved in the case of Cryptogams, and Reinke in the case of Phanerogams, — in a layer of tissue which must be considered the outer layer of the plerome (or procam- bium), and is called Pericambium. In Cryptogams the secondary roots originate in acro- petal succession from definite single cells of the pericambium, which lie before the vascular bundles, oblique divisions arising in them by which the three sides of the new apical cells that lie behind are marked off; a transverse division follows immediately, by which the first layer of the root-cap of the new lateral roots is separated. The apical cell of the secondary root formed in this manner and already provided with a root-cap, forms new HAIRS (TRICHOMES). H5 segments, from which the cap arises, as has already been shown in sect. 19, Fig. 102, p. 125. The roots of Lycopodiaceae do not produce any lateral roots, they branch instead dichotomously at the apex (Fig. 130, p. 161). In Phanerogams, the commencement of a lateral root is indicated by the splitting of several cells of the pericambium of the mother-root by tangential walls, so that the pericambium is divided there into two layers (Fig. 115, ^). The outer layer is im- mediately formed into dermatogen (d), which afterwards forms the layers of the root-cap by tangential divisions; while the outer layer of cells which proceeds from the young derma- togen always constitutes a layer of the root-cap (C/j). Theinneriayerof cells, resulting from the splitting of the pericam- bium (^, n n), which faces the vessels of the bundles of the mother- root, then also splits again into two layers (B) ; and further longitu- dinal and transverse di- visions follow, by which the primary meristcm of the young root is formed. This soon di- vides into three parts; a basal part by which the young root remains in connexion with the vas- cular bundle of the mother- root (D, m m), and an anterior mass of tissue which becomes differentiated into pericambium and plerome (D, pp). While the young root lengthens in a direction transverse to the axis of the mother-root, somewhat obliquely downwards, it compresses the cortical tissue (D) ; the innermost layer of cortex {A-D, r) resists disorganisation longest, and, at least at first, follows the growth of the young root, sur- rounding it in a sheath-like manner till it is destroyed. Finally, therefore, the young root lengthens and its apex protrudes through the cortical tissue of the mother-root. In stem-formations lateral roots arise either from the interfascicular cambium (e. g. in Impatiens parinflora immediately above the soil in the primary stem), or from the outermost phloem-layer of the fibro-vascular bundles, which is more commonly the case. These layers of tissue then behave like the pericambium of a mother-root {e. g. Veronica Beccabunga, Lysimachia nummiilaria, the ivy, according to Reinke). (c) Whilst the formation of the root-cap, as has already been shown in sect. 19, continues from the apex of the root, its outermost layers of tissue pass over into permanent tissue ; the cells retain simple forms, but their walls become thicker, and swell up in the outermost cell-layers of the cap, become gelatinous, and thus cause the apex of the root to appear viscid ; finally they die and become detached. L Fig. 115.— Mode of formation of the lateral roots in a mother-root of Trapa 7iatans {after Reinke). A the pericambium (ir) bounded by the innermost cortical layer splits into der- matogen (if) and an inner layer n, which in B is already ajjain divided. C young root enclosed in the tissue of the mother-root ; R r cortex of the latter; tt the pericambium of the mother- root from which the secondary root has been formed ; h the first layer of its root-cap, d its dermatogen. D secondary root in a further stage of development, surrounded only by the innermost cortical layer r of the mother-root; // its periblem, in the middle of the ple- rome ; fft tn the tissue that supplies the connexion with the mother-root. 146 EXTERNAL CONFORMATION OF PLANTS. In aerial and underground roots the root-cap is closely attached to the substance of the root by its oldest layers which generally extend backwards ; in the roots of Lem- naceae, Stratiotes, and some other plants which float on the water, it forms a loose sheath which envelopes the body of the root high up, and is only fixed below to the apex of the root. (d) We have already, in sect. 19, touched on the manner in which the primary meristem of the apex of the root is differentiated in Angiosperms into three layers, the Dermatogen (primordial epidermis), the Periblem (primordial cortex), and the interior tissue or Plerome. In roots which remain slender, like those of Cryptogams and of many Phanerogams, the whole of the plerome is transformed into an axial fibro-vascular cylinder, in which two, three, or more vascular bundles arise. The vessels are formed first of all near the periphery of the bundle, at two, three, or more points of the section, and afterwards further towards the interior, until the rows of vessels meet in the middle, and form a diametral row, or a star of three, four, or six rays\ The lateral roots appear on the outside of the bundle in front of the primary ves- sels. Between the vascular bundles bast-bundles are formed (although not always) somewhat later in the peripheral gaps of the axial fibro-vascular cylinder. On originally thicker roots of Phanerogams the axial cylinder or plerome becomes again differentiated ; its central portion becomes parenchymatous and forms a pith (Fig. 116, m), while vascular and bast-bundles arise only in the peripheral portion. When a root is able to increase subsequently in thickness, like the napiform primary roots of Dicotyledons, there is formed in the thickening-ring x (Fig. 116) be- tween the vascular bundles g and on the inside of the bast-bundle b, a secondary meristem, a true cambium, which on its part behaves exactly like the cambium of a stem capable of subsequent increase of thickness ; it produces inwardly in a centrifugal di- rection xylem, outwardly bast, especially phloem. Fig. 116 shows at ^ the transverse section of the primary root of a bean before the increase in thickness has begun ; and at B after the growth has continued for some weeks; between the thin pith m and the primary cortex pr a four-rayed woody substance has been formed; the four intermediate ' medullary rays' correspond to the original woody bundles gg which have not continued to develope centrifugally ; the primary bast-bundles are still visible in B, i? ; but in addition, the cambium has produced a layer of phloem with secondary bast-fibres /?'. The strong primary root of the maize also produces, as has already been shown in Fig. 104, sect. 19, by the differentiation of the plerome, a pith-like substance m, surrounded by a fibro-vas- cular hollow cylinder .v, in which are formed vessels and elongated wood-cells. Bast- cells are here not so clearly visible as in Phaseolus, or not at all. Fig. 117 shows the transverse section of the same root, somewhat higher than the longitudinal section in Fig. 112. No subsequent increase in thickness takes place in this case, nor is such active cambium formed in the fibro-vascular bundle as in Phaseolus. These are only some of the ^" S k Fig. 116.— Transverse section of the primary root of Phaseolus multifloriis (slightly mag- nified). A a few centimeters above the apex of the root; B higher up on a much older root ; pr primary cortex ; ni pith ; x thic ken- ing ring ; gggg primary vascular bundles ; bbbb primary bast bundles; b' secondary bast ; k cork. In the thin embryo-roots of Triticum and other Grasses, an apparent central (axial) vessel is formed. HAIRS (TRICHOMES). 147 of the tissue of the root, which it is desirable to of the formation of pith which is usually entirely simpler cases of the differentiation mention here especially on account absent from slender roots. Generally also the pith dis- appears in thicker roots when they become more slender as they increase in length; the hollow cylinder of fibro - vascular tissue ends in a solid bundle. (e) The roots are ge- nerally clearly distinguished from the leaf- forming shoots by the characteristics mentioned above ; there occur, however, a few tran- sit ion al forms •^vhich show that roots can become di- rectly transformed into leaf- bearing shoots, as in Neottia nidus-a'vis, where older lateral roots of the stem throw off their root- caps and form leaves be- neath the apex (Reichen- bach, Irmisch, Prillieux, Hofmeister). On the other hand, leaf -forming shoots cease to produce leaves, as in many Hymenophyl- laceae (according to INIct- tenius) ; these leafless growing shoots form root-hairs, and assume the habit of true roots (whether they actually form a root-cap is doubtful); in these species true roots are wanting. In Psilotum triquetrum Naigeli and Leitgeb have shown that the apparent roots are only underground shoots, on which more or less evident traces of leaf-formation may be recognised ; they are similar to true roots in function and in the formation of tissue, but have no root-cap, and, when they come to the light above ground, continue to grow in the manner of ordinary leaf-shoots. In the Selaginella: also, the same investigators have shown the presence of leafless shoots (root-bearers) which grow downwards, and do not form root-caps until they touch the ground (cf. Book II. Lycopodiaceae). We thus see that transitional formations between roots and leaf-shoots are found even in highly differentiated plants. But even in Algae the thallus is often fixed to its substratum by organs of attachment which may be compared with roots in their habit and in many functional properties, not only in the case of the large Fucaceae and Laminariae, but also in the unicellular Vaucheria and Caulerpa. In reference to the confirmation of the Theory of Descent brought forward at the conclusion of this work, it is of great importance to know that members differing to the greatest extent morphologically and physiologically are connected by transitional forms, and that, especially in the branched thallomes of Algae, starting points are to be found for all the differentiations of the higher plants. Distinctions which, in the ramifications of the Alga-thallus, are only of a weak, undefined, and rudimentary character, increase more and more in the higher plants ; points which can be sharply defined in the latter become indistinguishable when we are considering the more simple Thallophytes. The more L 2 FIG I from the ^ s bundl 17. — Part of a transverse section of the primary root oi Zca Mais not far apex ; e epidermis with its strongly swollen outer walls vd; p the cortex ; ; sheath ; m the pith ; a- the thickening ring in whicli lie the vessels i^^. 148 EXTERNAL CONFORMATION OF PLANTS. the attempt is made to set up sharply defined ideas for single forms, the more does one become convinced that all definition, all limitation, is arbitrary, and that Nature presents gradual transitions from the indistinguishable step by step to the distinct, and finally to the opposite. Sect. 24. Various Origin of Equivalent Members ^ — (i) The different members of a plant spring out of one another ; the member produced may there- fore be similar (homogeneous), or dissimilar (heterogeneous) to the member which produces them. In the former case the formation of new members is ordinarily termed Branching. A root, for instance, branches in the production of new roots, a shoot in that of new shoots, a thallonie in that of new thallomes ; in the same sense the production by a leaf of lateral leaf-structures must also be considered a case of branching. On the other hand the stem produces also leaves, roots, and hairs ; leaves not unfrequently produce leaf-forming shoots, sometimes roots, generally hairs ; leaf-forming buds may also arise from thallomes (as in Mosses), and from roots. Since, therefore, members morphologically dissimilar — stem, leaf, root, tri- chome — do not differ absolutely, but only in degree, the difference between Branch- ing and the production of dissimilar new parts, between homogeneous and- hetero- geneous growth, must be conceived not as an opposition, but only as a gradually increasing differentiation of the members which grov/ out of one another. (2) New members may originate either by lateral budding or by dichotomy. Lateral budding occurs when the producing member, after its previous increase in length at the apex (the axial structure), forms outgrowths de/ow it, which, at their first origin, are weaker than the portion of the axial structure which lies above them. Dichotomy (rarely Polytomy), on the other hand, is caused by the cessation of the previous increase in length of a member at the apex, and by two (or more) new apices arising at the apical surface close to one another, which, at least at first, are equally strong, and develope in diverging directions. Lateral budding may form structures which are similar or dissimilar to the axial structure ; and thus there arise, by lateral budding from the stem — leaves, roots, hairs, branches; from the leaf — leaflets, lacinioe, lobes, hairs, sometimes leaf-forming shoots, or even roots. Dichotomy, on the contrary, never produces structures which are dissimilar to the producing structure ; the divisions of a root produced by dichotomy are both roots, those of a leaf-forming shoot both leaf-forming shoots, those of a leaf both leaf-structures ; dichotomy hence always falls under the conception of Branch- ing in the above narrower sense. Dichotomous branching is very common among Thallophytes, especially Algae and the lower Hepaticae ; among Phanerogams it occurs only exceptionally ; among Vascular Cryptogams it appears to occur in Ferns {e.g. the leaves of Platy- cerium alcicorne) ; but it is the only mode of branching in all shoots and roots of Selaginellae, Lycopodia, and in the roots of Isoetes. (For further details of lateral branching and dichotomy see the conclusion of this section and sect. 25.) ^ Compare the literature mentioned in the previous sections, and in addition, H. von Molil, Linngea, p. 487, 1837. — Trecul in Ann. des Sci. Nat. vol. VIII. p. 268, 1847. — Peter-Petershausen, Beitrage zur Entwickelungsgeschichte der Biutknospen. Hameln 1869. — Braun and Magnus, Ver- handlungen des Bot. Vereins der Provinz Brandenburg 187 1 (on Oaniopsis). [Warming: Recherches sur la ramification des Phanerogames. Danish with French abstract. Copenhagen 1872.] VARIOUS ORIGIN OF EQUIVALENT MEMBERS. 1 49 (3) The origin of lateral members, whether similar or dissimilar to the pro- ducing member, is either exogenous or endogenoics. It is the former when they are formed by lateral outgrowth of a superficial cell or of a mass of cells including the outer layers of tissue, as in the case of all leaves and hairs and most normal leaf- forming shoots. A member is of endogenous origin when it is covered, even when in a rudimentary condition, by a layer of the tissue of the producing member, as in all roots, all lateral shoots of Equisetaceae, and in adventitious buds. (4) Lateral members of any kind are almost always formed in considerable numbers on the axial structure which produces them, and even repeatedly one after another, because the producing structure continues to increase in length, and the conditions are repeated along its length for similar equivalent outgrowths. Thus the stem, so long as it continues to grow at the apex, produces leaves, hairs, often even roots, and generally lateral shoots in great numbers, one after another ; roots usually form in succession many lateral roots, branching leaves usually several laciniaj. If the apical growth ceases early, the number of the lateral members is also limited ; thus the short primary stem of Welwitschia mirahilis produces only two leaves. When the increase in length of the stem is very slow, the formation of lateral shoots from it is sometimes altogether suppressed, as in Isoetes, Botrychium, and Ophioglossum. (5) An axial structure may produce the lateral members which are equivalent to one another in such a manner that either only one always arises at the same level or several ; in the first case the members formed in succession are termed solitary, in the second case all the similar members arising at one level form a whorl or verticil. Leaves often occur in whorls, shoots less frequently, roots oc- casionally (in the primary roots of Phanerogams). Within the same whorl the members may arise either simultaneously, as the petals and stamens of many flowers, the whorl of foliage-leaves of many Phanerogams; or the members of a whorl may be successive, as those of Characeas and Salvinia. A whorl is a true one when the level of the axial structure is originally such, as occurs in both the last- named plants and in many flowers ; spurious whorls, on the other hand, are such as are formed by displacement and unequal growth of the axis, as in the Equi- setaceae, where leaves, roots, and shoots arise from transverse zones (nodes) which are themselves formed by displacement of three segments of the stem ^ (6) Similar and equivalent lateral members usually arise on the common axial structure in acropetal or basifugal order, i. e. the younger a member is the nearer it is to the apex ; counting from below upwards the members occur in the order of their age. The lateral members which are formed from the pimctum vegetationis of an axial structure sufficiently near the growing apex are apparently always acropetal; but the order is disturbed when lengthening at the apex ceases and new formations occur at the primary meristem below, as in many flowers. The lateral members formed at a greater distance from the growing apex of the axial structure are sometimes, but not always, acropetal. Since branching and ^ The three segments, which form the contour of the stem, stand at first at different heights, but arrange themselves, as Rees has shown, in a transverse zone (node), which forms outwardly a circular cushion, the rudiment of the leaves (cf. Book II. Equisetaceae). j^O EXTERNAL CONFORMATION OF PLANTS. the formation of lateral members out of the punctum vegetationis occur in nearly all plants, and by their regular repetition at definite points of the grow- ing axis determine the external form of the plant, they may be considered as normal, in opposition to the adventitious production of members which takes place at the older parts of the axial structure at a distance from the apex and without definite order. Such new formations are of equal importance in the external form of the plant, and though adventitious are often of great import- ance in a physiological point of view. Adventitious shoots are generally formed in the interior near the fibro-vascalar bundles of the shoot, leaf, or root, and are therefore endogenous ; but it does not follow from this that all endogenous shoots are adventitious. All the shoots of Equisetum are endogenous in their origin, but are not adventitious, since they are produced in the primary meristem below the apex of the mother-shoot, and in a perfectly definite order. It is equally incorrect to call all roots adventitious although they arise in the interior of the stem leaves or roots. They are adventitious only when they occur in older parts ; when they arise close to the growing point of a mother-root or a stem, they are arranged in strictly acropetal order, and are for that reason not adven- titious. When a member grows on a basal zone and produces lateral members from it, they may be arranged in basipetal order, i. e. the younger a lateral member is the nearer it wdll be to the base, as the sporangia on the columella of Hymeno- phyllacese (according to Mettenius), or the lacinige of the leaves of Myriophyllum. (7) When in the higher plants a new individual is formed which is destined for permanent and independent vegetation, a leaf-forming axis is first constituted, that is, a shoot on which roots, hairs, and lateral shoots then arise. In all vascular plants this first shoot (the primary stem) arises immediately out of the sexually pro- duced embryo ; it appears therefore as if the externally unsegmented embryo is to be considered as itself a primary shoot-axis ^ In Mosses, on the other hand, the sexually produced embryo becomes transformed into the so-called Moss-fruit, a structure without leaves, roots, or branches, the sole function of which is the formation of spores. A new Moss-plant is, on the contrary, constituted by the formation of a leaf-bearing shoot out of a branch of the alga-like Protonema, which branches, strikes root (by root-hairs), and is independently nourished. The shoot first produced, which developes the rest of the shoots and roots, is termed the primary shoot, and its portion of the stem the primary stem, when it is more strongly developed than its lateral shoots, as in most Ferns, Cycadeae, Coniferae, Palms, and Amentaceae. The primary shoot produces lateral shoots of the first order, these again lateral shoots of the second order, and so on. Nevertheless it often happens that lateral shoots of any order become independent, take root, and become detached from the primary shoot ; they then assume all its peculi- arities, and may equally be considered as primary shoots. But it also happens that the primary shoot itself is arrested at an early period, while new orders of shoots proceed from it which gradually become stronger, as in many bulbous and tuberous plants. Shoots which become detached from the mother-plant in a but slightly developed condition, and then continue to grow by independent nourish- ^ Compare what will be found under Rhizocarpeoe and Angiosperms in Book II. VARIOUS ORIGIN OF EQUIVALENT MEMBERS. 151 ment, while they repeat the peculiarities of the primary shoot, are called Gemmse or Bulbils ; they are often adventitious shoots ; but bulbils may be shoots of normal origin, as those of many species of Allium. Now that we have already spoken of the origin of leaves, hairs, and roots, and entered sufficiently into detail on the more important points (sects. 20, 21, 22), it only remains to go a little further into the various modes of origin of leaf-forming shoots. (a) The Formation of Leaf-forming Axes from Ihallomes (without the medium of a germ-cell) occurs only in the Muscineae, especially in Mosses. From their spores, but VlG.iiB.—Afm'um hormon (xso); -wiv parts of root-hairs of mature plants which have put out protonenia-threads n n when the sod has been inverted in damp air ; on these are formed the leaf-buds A' A". also from the root-hairs and other parts, are developed Conferva-like segmented filaments (Fig, 118, «/z) growing at the apex, and branching. These often continue to grow for a long time, obtaining their nourishment independently, and sooner or later produce short lateral branches, generally at the base of longer branches. The apical cell, in which elsewhere segmentation of the filament is always produced by septa, is in them divided by oblique walls, and a usu- ally triangular pyramidal apical cell of the stem is thus formed, the oblique segments of which at once dcvelope into leaves ; and thus shortly stalked leaf-buds arise (Fig. 118, AT AT), which at once take root by root- hairs, and become developed into inde- pendent Moss-stems. (b) In many Ferns leaf-shoots arise from Lea'ves, and especially when branch- ing of the stem seldom or never take place, as in Aspidium Fi/ix-mas, Jlsplenium Filix- foemina, Pteris aquilina, &c. In these species the buds spring singly out of the lower parts of the leaf-stalk at a greater or less height above its insertion. In other species it is the lamina which mostly produces nume- rous buds, generally in the axils of the laci- niae, as in Asplenium decussatum (Fig. 119), A, Bellangeri, A. caudatum, Ceratopteris thalictroides, or on the surface of the leaf itself, as in Aspknium furcation, &c. In all these cases the buds produced on the leaves Fig. iig.—AsJ>lemtfm decussatum; middle part of a mature leaf; its mid-rib st bears the laciniae //; at the base of one the bud K is formed, which has also already put out a root (natural size). 152 EXTERNAL CONFORMATION OF PLANTS. are exogenous in their origin, and those on the leaf-stalks of the first-named species arise at an early period, while the leaves are still very young, out of single superficial cells 1. These shoots take root while they still remain in connexion with the mother- leaf, but sooner or later become detached (in Aspidium FUix-mas and Pteris aquilina often only after some years, when they have already acquired considerable strength, and the base of the mother-leaf finally dies off and decays). In Phanerogams buds generated on the leaves also occur, although much more rarely. The best known are those which are formed abundantly in the indentations of the leaves oiBryophyllum calycinum ; according to Hofmeister''^ they arise before the complete unfolding of the leaf as small masses of primary parenchyma in the deepest parts of the incisions of the leaf. In the aquatic Utricularia ^vulgaris weak shoots arise, according to Pringsheim^, mostly in the neighbourhood of the axils of the divisions of the leaf; in both cases these shoots are of exogenous origin. Nothing is known of the develop- ment of the buds produced on the leaves of Atherurus ternatus or Hyacinthus Pouzolsii (Doll, Flora von Baden, p. 348). (c) Adventitious shoots springing from Roots are ahvays endogenous ; they arise in the neighbourhood of the fibro-vascular bundles or in the cambium, as in Ophioglossum, Epipactis m'lcrophylla, Linaria 'vulgaris, Cirsium arvejise, Populus tremula, and Pyrus Ma/us (according to Hofmeister). (d) Adventitious Buds arise moreover in an endogenous manner under peculiar cir- cumstances from older detached leaves or pieces of stem and root, especially when kept damp and in darkness. On this depends the propagation of many plants in gardens, as of Begonias from leaves, Marattias from their thick stipules, &c. Adventitious buds also sometimes appear in considerable quantity in old stems of woody plants, in the cushion which projects between the bark and the wood, especially if the stem is cut off above the root. The branchlets which break out in old stems of Dicotyledons and Mono- cotyledons are, however, often not true adventitious shoots, but old dormant ' eyes ' which have been left behind, having been formed at an earlier period as normal exoge- nous axillary buds, when the stem itself was still in the bud-condition ; they had become enveloped by the bark as the stem increased in thickness, and carried on a feeble ex- istence, until placed in a condition for active growth by a favourable accident, as the removal of the stem above them (Hartig). (e) In the genus Isoetes the leaf-forming shoot arises exclusively from the fertilised germ-cell or embryo, and forms neither normal lateral buds out of the stem nor any from the leaves or roots, nor any kind of adventitious buds. (f) ^he Normal Formation of Lateral Shoots from the primary meristem of the punctum 'vegetationis of the mother-shoot is endogenous only in Equisetacesc, elsewhere it is always exogenous. The Equisetacea: stand in this respect quite alone in the vegetable kingdom ; with the exception of the weak primary shoot which is developed out of the embryo, all their lateral shoots are of endogenous origin (Fig. 120, KK')] they are developed out of a cell in the interior of the tissue of the stem near to the punctum uege- tationis somewhat later than the youngest leaf-cushions, and afterwards break through the base of the older leaf-sheaths. With this exception all normal lateral shoots produced at the vegetative cone of the bud or in its neighbourhood (in the bud) are, like the leaves, exogenous'. (g) The lateral shoots which normally arise below the growing apex of a mcther- shoot are always arranged acropetally, like the leaves, with which they exhibit various relationships as to position, age, and number. ^ Hofmeister, Beitriige zur Kenntniss der gef. Kryptogamen. II. Leipzig 1S57. ^ Hofmeister, Allgemeine Morphologic, p. 42.1. 3 Pringsheim, Zur Morph. der Utriculaiien ; in Monatsb. der k. Akad. der Wissen. Berlin 1869. * [Leitgeb has recently described the endogenous formation of branches amongst the Hepaticce : Ueber endogene wSprossbildung bei Lebermoosen. Bot. Zeitg. 1872. — Ed.] VARIOUS ORIGIN OF EQUIVALENT MEMBERS. 15.3 (a) The numerical relationship of the lateral shoots to the leaves which are formed along with them on the same axis is so far variable that the number of the former may be either equal or unequal to that of the latter. If the number is unequal, a greater number of leaves than of branchlets usu- ally arises on the same axis; in jNIosses, Ferns, Rhizocarpeae, Gycadeae, and Coni- ferae a much larger number. A branch- let may arise whenever a perfectly defi- nite number of leaves has been formed, as in many Mosses and some Ferns, or the formation of a branchlet results when the increase in length of the primary shoot and the formation of its leaves ceases for a time, as in the genus Abies, and is subse- quently renewed. When the leaves stand in whorls, the number of the lateral shoots may be equal to that of the members of the whorl, as in Equisetaceae, or it may be smaller, as in Characese. Only rarely is the number of branchlets larger than that of the leaves, as in some Monocoty- ledons and Dicotyledons, where two or more lateral buds often arise side by side above a leaf (Fig. 122), or one above another (as in Aristolochia Sipho, Glcdit- schia, Szc). In most Monocotyledons and Dicotyledons the number of the lateral branchlets (with the exception of the flower-shoots) is, at first, equal to that of the leaves ; but usually only a much smaller number attain a higher development. (;y) A relationship in position exists be- tween the origin of the leaves and the normal lateral shoots of a common mother-shoot, since a constant arrangement is found of leaves and shoots in each species and often in whole classes of plants, the shoots being always produced either below, beside, or above the leaves. The lateral shoots arise below the leaves (according to the acute investigations of Leitgeb^), probably in all Mosses, as well as in the Hepaticae Radula and Lcjeunia; the shoot springs (as shown in Fig. 106, 2) out of the lower part of a segment of the stem the upper part of which has developed into a leaf. In Fontinalis this occurs below the median line (the symmetrically dividing plane) of the leaf, in Sphagnum laterally below one half of the leaf. According to the same observer, the lateral shoots arise in place of a half-leaf beside the remaining half, in many Hepaticae of the section Jungermannieae (Frullania, Madotheca, Mastigobryum, Jungermann'ia tricho- phylla; Leitgeb, Bot. Zeitg. p. 563, 1871). If each tooth in the leaf-sheaths of an Equi- sctum is considered to be a leaf, the buds originate at the side of the leaves and between them, for they break through the leaf-sheaths between the median lines of the teeth. Pig. 120 — HquisetHfn arvense ; longitudinal section of an underground bud in March ; ss the apical cell of the stem ; b — 96 its leaves ; A' A'' two endogenous lateral buds exposed by the cut. Tlie youngest rudiments of buds are to be found, however, at b", and they have probably begun to be formed even at a greater height (X50). ^ Leitgeb, Beitriige zur Entwickelungsgeschichte der Pflanzenorgane in Sitzungsber. der kais. Akad. der Wissen. zu Wien, Bd. 57, 1SO8, and Bd. 59, 1869; and Bot. Zeitg. no. 34, 1871. See also more in detail, Book II. Mosses. 54 EXTERNAL CONFORMATION OF PLANTS. In the Characeoc, Monocotyledons, and Dicotyledons, the normal lateral branchlets spring out of the axil of the leaf, /. e. above the leaves, in the acute angle which the leaf forms with the stem (Figs.121,123). Usually only one is formed above the middle of the Fig. 121.— Longitudinal section of the apical region of a shoot of Clematis apiifolia; s apex of the stem; bb leaves ; gg the first traces of spiral vessels, bending out uninterruptedly from the stem to the leaves. Fig. 122.— Bulb of Miiscari botrioides; one of the lower bulb-scales is thrown back, in order to show the numerous buds standing side by side in its axil. insertion of the leaf, or 2-3 one above another; some- times several are formed side by side above the middle and right and left of it, as in the bulbs of Muscari (Fig. 122), and the flowers in the axils of the bracts of species of Musa. Such branchlets are called axillary shoots; in Monocotyledons and Dicotyledons the branching is, with few (and usually doubtful) exceptions, axillary. (y) If we except some inflorescences in Phane- rogams, the general rule that The normal Lateral Shoots appear later than the youngest Leai^es deter- mines the relationship in age ^. This is the case in Characea^, Mosses (according to Leitgeb), Equisetaceae, and the vegetative shoots and most inflorescences of Phanerogams, as is shown in Figs. 106, 107, 109, 120, 121, 123. In Mosses it is clear that the youngest branches stand at a greater distance from the apex of the stem than the youngest leaves. In the other groups named a different relationship would be possible ; but in these cases it may also happen that the youngest lateral buds are almost always at a greater distance from the apex than the youngest leaves ; or, in other words, the youngest leaves stand between the youngest lateral shoot and the apex. If the position of the branchlet is axillary, it would also be possible that when the formation of leaves ceases, the youngest axillary shoot would be observed to stand above the youngest leaf ; but this would not prove that it was in this case formed earlier. Fig. 123. — Apical region of a primary shoot of Dictajn>t!is Fraxi>ie//a, seen from above; J apex of the primary shoot; b b b the young leaves ; A: /i: their axillary buds, the two young- est leaves have not yet axillai-y buds. ^ Hofmeister indeed maintains the contrary (AUgem. Morphologie, p. 911, 1868). Since that, however, the Mosses have been shown by Leitgeb to be exceptions; and I constantly find in vegetative shoots and many inflorescences of Phanerogams young leaves above the youngest axillary buds. DEVELOPMENT OF THE MEMBERS OF ONE BRANCH-SYSTEM. 155 If the formation of leaves is very feeble, as in the inflorescence of Grasses and some Papilionaceae (^Amorfha fniticosa), the lateral shoots may become visible earlier than the leaves in the axis of which they stand ; the same is the case, according to Hofmeister, in Casuarina, Dianthus, Orchis Morio, and Salix (in the inflorescence or on vegetative shoots). In Cruciferae, finally, the floral axes and the branches of the raceme spring from the primary shoot with- out any formation of bracts preceding or following (Fig. 124). But since in by far the greater number of Phanerogams the normal branching of the shoots is always axillary and subsequent to the formation of leaves, the above-named exceptions may, according to the principles of the theory of descent, be considered of little importance, since the leaves concerned (the bracts or leaves with buds in their axils) have lost their physiological signification, become useless, and at length entirely disappear. In such cases, the morpho- ,^:^-:::^Z^^:T:::: :^^:'^ logical character which is peculiar to a whole group of of the inflorescence; the flower buds shoot " '■ «-> 1: o„t beneath it (in whorls of four) ; the plants is usually altered m particular cases. youngest are still simple leafless protuber- (5) The fact that lateral shoots arise far most fre- quently at a greater distance from the apex of the stem than the youngest leaves, distinguishes them sufliciently from dichotomous branchings which must always of necessity arise above the youngest leaf. But even when the formation of leaves is observable later than the branching, as in the inflorescences of Gramineae, or is even completely suppressed as in Crucifenie, it is still impossible to confound lateral with dichotomous branching, if, as in these cases, the vegetative cone greatly overtops the youngest lateral apex, and continues to grow in a straight line (Figs. 107, 109). Still more distinctly conspicuous is the distinction between lateral branching and dichotomy when the generating stem-axis ends in a broad flat apical surface, as in the young capitula of Compositse. Here the lateral shoots (the flowers) are so small relatively to the mother- shoot, and from the first at so great a distance from its apex (the centre of the apical surface), and placed so uniformly on all sides of it, that in this case the mother- shoot must be regarded as the independent centre of all new formations. The idea of dichotomy supposes that the mother-shoot ceases as such, and that two shoots, at first at least equally strong, continue growth in length in diverging directions in its place. If it is desired to include lateral branching from the punctum •vegetationis and dicho- tomy of the apex under one common term, in order to distinguish them from the lateral formation of branches from older portions of the stem, leaves, or roots, the expression 'Terminal Branching commends itself, which I have already employed in this sense in the first edition of this book. Sect. 25. Different capacity for Development of the members of one Branch-system '. — Systems of members bearing the same name originate by branching ; out of a root a root-system originates, out of a shoot a shoot-system ; when a leaf branches, we get a pinnate, digitate, divided, lobed, or incised leaf, &c. It becomes therefore necessary to examine the more important relationships of form of such a system, if we for the time take into account only the relative size and capacity for development of the branches of the various orders. We may here leave adventitious branchings entirely out of consideration ; for it appears clear from » Nageli iind Schwendener, Das Mikroskop, p. 599. Leipzig 1867.— Hofmeister, Allgemeine Morphologic der Gewebe, § 7. Leipzig 1S68.— Kaufmann, Bot. Zeitg. p. 886. 1869.— Kraus, Medic. -Phys. Soc. in Erlangen. Dec. 5, 1870, 1 ^6 EXTERNAL CONFORMATION OF PLANTS. these observations that they play no essential part in the whole external confor- mation ; we have therefore only to do with the branchings which arise at the end of a growing shoot, leaf, or root, or with terminal branchings. The terminal branches may be referred (as has already been shown in sect. 24, div. 2) to two principal forms, dependent on the origin of the branching by dichotomy or by lateral shoots ; branch-systems of the first kind we will call simply Dichoto??iies, of the second kind Monopodia. A Dichotomous Branch-system, according to the definition given in sect. 23, is the result of the cessation of the growth at the apex in the direction previously taken, and its continuation in two diverging directions at two newly constituted apical points, which arise close beside the previous one ; as is very clearly shown in Fig. 125 ^ We will distinguish the two newly formed branches by the term Bifurcations, the member which produces them we will call the Base of the bifurcation. From the nature of the case every base can only bi- furcate once ; but every branch may again become the base of a new bifurcation ^. A Mofiopodiiim arises when the gene- rating structure, following the direction of its previous axis of growth, continues to grow at its apex, while lateral structures of the same name grow beneath it in acropetal succession, the longitudinal axes of which are placed in an oblique or transverse direction to that of the generating member. The generating member, since it continues to grow during the branching, may form numerous lateral members ; for all these it is the common base ; hence the name Monopodium (Figs. 109, 113, 124). Every lateral branch may again branch in the same manner, and thus itself become a monopodium of the second order. Just as the dichotomy may consist of numerous bifurcations, so may a monopodium consist of several orders of monopodial branching. These definitions refer only to the rudimentary condition of the branchings, Fig. 125.— Dichotomy of the thallus of Dictyota dkhotovia after Nageli) ; the order of development is according to the letters A — E ; the letters t — z indicate the segmentations of the apical cell before it dichotomises ; i is the division-wall by w hich the dichotomy commences ; 2—6 the segments of the new apical cells. ^ Since we have to give here a narrower application to the term Direction of Growth, it will be necessary to compare the following section on Direction of Growth and Symmetry. 2 In Cryptogams with apical cells it may be thought that dichotomy must necessarily be brought about by longitudinal division of the apical cell. When the segments arise by transverse division this is actually the case, as is shown in Fig. 125; but when the segments are arranged in two or three rows, this would necessitate that the dichotomising wall proceeding from the apical surface of the apical cell should bisect its posterior angle, and thus have a position which is apparently universally avoided in cell-division. It is nevertheless possible that a true dichotomy may take place without this. Suppose the old apical cell, immediately after the formation of a new one near it, were to change the direction of its longitudinal growth, so that thus both apices diverge from the previous direction of growth ; the old apical cell then represents the apex of a new direction of growth. From this it seems to me that we are peculiarly well able to arrive at the distinction between dichotomy and monopodium. Mutatis mutandis this is also true of Phanerogams which have no apical cell. DEVELOPMENT OF THE MEMBERS OF ONE BRANCH-SYSTEM, 157 or the bud-condition of the branch-system. Not unfrequently the original cha- racter is maintained in their further growth, not only in dichotomous but also in monopodial systems. The two bifurcations develope, in the case of dicho- tomy, with equal strength and branch uniformly ; in the case of a monopodium the primary axis continues to grow more strongly than all the secondary axes, and branches more luxuriantly. But it is very commonly the case that in a dichotomous system single bifurcations grow more weakly, or that in a system which starts on a monopodial plan some of the lateral axes, soon after their formation, grow more strongly and branch more luxuriantly than the primary axis. In such cases the original character of the branch-system becomes less and less evident as it de- vclopes ; and it ma}' happen that systems originally dichotomous have subsequently the appearance of monopodia, and vice versa. It is therefore impossible to infer, without further evidence, the original form of a branch-system from its mature condition. It cannot be inferred from a maturely developed system whether it originated in dichotomy or in lateral branching. It will therefore be desirable to make here a simple classification of the most important changes which a branch- system undergoes during the development of its members. (i) The Dr^elopmctit of Dichotomous Systems may take place either in a forked or a sympodial manner ; I call a S}stem forked when at each bifurcation the two branches develope with equal strength, as in Fig. 126, A. The dichotomous system is developed sympodially when at each bifur- cation one branch developes more strongly than the other ; in this case the base of each successive bifurcation forms apparently a primary shoot, on which the weaker branches appear as lateral shoots (Fig. 126, B, C). The apparent primary shoot, which in fact consists of the bases of consecutive bifurca- tions, may on this account be termed a Pseud- axis or Sympodium ^ Thus in B (Fig. 126) the sympodium is composed of the left-hand branches ///; in C of the alternate left and right-hand branches / r, I r. Whether the case represented in B, which, on account of its similarity to certain monopodial systems, may be termed a Helicoid (bostrychoid) Dichotomy, actually occurs is doubtful (probably however in the leaf of Adia?itum pedatiwi). On the other hand the development represented in Fig. 126, C, is common in shoots of Selaginella, and, on account of its resemblance to some monopodial systems, may be termed a Scorpioid (cicinal) Dichotomy-. Fig. 126.— Diagram of the various modes of develop- ment of a dichotomy ; A one developed by bifurcation ; B a helicoid ; C a scorpioid dichotomy. * In opposition to the view expressed in my first edition, I now consider it more convenient to apply the term Sympodium only to the pseud-axis itself, and not to the whole branch-system ; and the same in the originally monopodial systems. - On Dichotomous Inflorescences cf. Book II. Phanerogams. 358 EXTERNAL CONFORMATION OF PLANTS. (2) An originaUy monopodial hranch-systejn may develope in a racemose or cymose manner ; and the cymose development may be either apparently dicho- tomous (or even apparently polytomous) or sympodial \ (a) A racemose system occurs when, with a monopodial origin, the mother- shoot, which is originally stronger, continues also to develope more strongly than all the lateral shoots, and when each lateral shoot of the first order behaves again in the same manner in respect to its lateral shoots of the second order, and so on. This occurs very clearly, for instance, in the stems of most Conifers (especially Pinus, Araucaria, &c.) and in the compound leaves of Umbellifers. (b) The cymose development of a monopodial system, or a Cyme, depends on the fact that each lateral shoot, at first weaker, begins from an early period to grow more strongly than the mother-shoot above its point of origin ; and, in consequence of this, also branches more vigorously than the mother-shoot, the growth of which then usually soon ceases. Two principal forms of Cyme may be distinguished, according as a Pseud-axis (Sympodium) is formed or not. (a) When two, three, or more lateral shoots arise beneath the growing end of each shoot, which develope in different directions more strongly than their mother- shoot, the growth of which soon ceases, a false Di- chotomy, or Trichotomy, or Polytomy arises. Fig. 127 represents the forma- tion of a false dichotomy ; the shoot / produces the shoots //', //'', originally weaker, but soon growing more strongly, while the growth of 1 ceases ; the same takes place with ///' and Iir\ False dichoto- mies of this kind, which occur abundantly in the inflorescences of Phanerogams, are termed by Schimper Dichasia. But instead of two lateral branches growing out in opposite direcdons, three or more shoots standing in a true or spurious whorl may develope more strongly than their mother-shoot, and thus arises an umbrella-shaped or umbellate system, such as is developed in a typical manner in the inflorescences of our native Euphorbias ; a system of this kind may be called a Cymose Umbel. (/3) The sympodial development of an originally monopodial system occurs when one lateral shoot always developes with greater vigour than the portion of its mother-shoot which lies above its origin, as is shown, e. g., in Fig. 128, A, where the lateral shoot 2-2 grows more strongly than the part 2-1 of its mother-shoot, and so on. Usually the portions of all the shoots which lie below their lateral branches develope more strongly than the terminal portions, as is shown in the ^ Here also I deviate from the terminology of the first edition ; not because that was incorrect, but because, by so doing, a greater facility is attained in the mode of expression. Fig. 127.— Diagram of a false dichotomy ; the Roman numerals indicate the order of development of the shoots of the system. DEVELOPMENT OF THE MEMBERS' OF ONE BRANCH-SYSTEM. 159 figure by the thicker lines ; the terminal portions (indicated by thin lines) often die off early; the thicker basal portions of the different ramifications which pro- ceed from one another then commonly place themselves in a straight line, and have the appearance of a connected whole, like a primary shoot to which the terminal portions of each separate order of shoots are attached like lateral branches ; the apparent primary shoot of the system is called the Sympodium or Pseud-axis. The latter consists, in Fig. 128, B,e.g., of the pieces between 1-2, 2-3, Fig. i;8.— Cymose branchings represented dia^ammatically ; ^, B scorpioid (cicinal) cyme; C dichasium ; D helicoid (bostrj'- choid) cyme; the numerals indicate the order of succession of the lateral shoots which spring from one another i. 3-4, 4-5; the weaker terminal portions of the respective branches r, 2, 3, &c. are bent sideways. A comparison of Fig. C with A shows that between a sympodially developed and a spurious dichotomous system there is only this one point of difference, that in the latter each branch produces not only one but two stronger lateral branches. If in C one of the branches is imagined to be suppressed alternately left and right, the form A results, which then is easily transformed into B. Sympodial systems appear in two different forms, according as the lateral shoots, the basal portions of which form the pseud-axis as they are continuously developed, arise always on the same side or on different sides. If the sympodial ramification takes place always on the same side, e.g. always to the right, as in Fig. 128, D, or always to the left, the whole system is called a Helicoid Cyme (or bostryx) ; if, on the other hand, each branch which continues » [Some difficulty will probably be felt with regard to Fig. D, which stands for a helicoid cyme in the text, but which is also identical with the scorpioid cyme of descriptive botany, and corresponds to the specific name ' scorpioides ' given by Linnaeus to several plants in which it occurs. The term scorpioid was introduced by A. P. De Candolle (Organographie, i. 415), to express a unilateral cyme the undeveloped portion of which is usually rolled up. This is the characteristic inflorescence of the Borraglnacea^, amongst which Myosotis has long been distinguished as ' scorpion grass' on this account. Bravais (Ann. de Sci. Nat. 2^. ser. vii. 197) distinguished the helicoid cyme, which he defined as having the successive flowers ranged in a spiral round the pseud-axis, while in the text above they are all placed in the same plane. Bravais amended De Candolle's definition of the scorpioid cyme by pointing out that the flowers are in two rows parallel to the pseud-axis. — Ed.] i6o EXTERNAL CONFORMATION OF PLANTS. the system arises alternately right and left, as in Fig. 128, A, B, the system is a Scorpioid Cyme (or cicinus). If in these cases we have to do with leaf-forming shoots with a spiral arrangement of the leaves, a more exact definition of the terms right and left becomes needful. It is then necessary to imagine a median plane drawn through the longitudinal axis of each shoot and through that of its immediate mother-shoot; then in the helicoid cyme each following median plane always stands either right or left of the preceding one, following the course of the leaf-spiral; in the scorpioid cyme, on the other hand, the consecutive median planes stand alternately right and left. (a) In Thallophytes and thalloid Hepaticee, dichotomy is very widely prevalent; monopodial branchings also occur in the most various degrees of development. The dichotomies are unusually clear and generally developed in a forked manner among Algae, especially in the Dictyotese, and species of Fucus (in particular F. serratus). In some there occurs a tendency towards sympodial development of dichotomies, but usually only at a late period ; so that in those which branch the dichotomy can be clearly recognised even with the naked eye. The same is the case, among Hepaticae, in the Antho- cerotese, Riccia, Marchantia, and Metzgeria (Fig. 129), where the flat extension of the thallus or thallus-like stem arises between the young branches, first of all as a protuberance (/" y"), which however cannot be considered as a continuation of the shoot, since it has no apical cell or mid-rib ; subsequently this protuberance disappears, as mf"^. Decidedly monopodial (lateral) ramifications are particularly clear in filamentous Algae, when the apical cell remains unbranched, and lateral branches grow only out of the individual cells (segments of the filament) ; as in Cladophora, Lejolisia, 7! offia'nale: A the apical region of the primary- stein ; its vegetative cone is seen at s, its youngest leaf at b; b' youngest leaf but one with the pinnation commencing ; C, bs the apex of the \&ia.i\/,fi,f'' the leaf-branchesof the first order ; c» tVir/^^ r\r- 9i\ia riorsepals J have been removed, the posterior ones left; //petals ; j; W^^)y ^^^^ '" ^"^ LUrCC- Of TIVC- stamens, the posterior ones already large, the front ones not yet even in ,^ortArl <^o1\;^ac r»f tnnct PVranArrKTQmc a rudimentary state ; c the carpel or rudiment of the fruit. partCQ Caiy CCS 01 mOSl r naUCrOgamS. Fig. 136. — Apical region of a shoot of Coriarta viyrtifolia ; at .^ in transverse section, S in longitudinal section ; s apex of the stem ; d b leaves in pairs, i. e. in decussate whorls of twos ; k axillary bud ; g yot vessel. igest ^ Roper, Linnoea, p. 84. 1827. — Schimper-Braun, Flora, pp. 145, 737, 748. 1835. — Biavais, Ann. des Sci. Nat., vol. VII. pp. 42, 193. 1837. — Wichura, Flora, p. 161. 1844. — Sendtner, Flora, pp. 201, 217. 1847. — Brongniart, Flora, p. 25. 1849 — Braun, Jahrb. fiir wissen. Bot. I. p. 307. 1858. — Irmisch, Flora, pp. 81, 497. 1851. — Hanstein, Flora, p. 407. 1857. — Schimper, ditto, p. 680. — Buchenau, Flora, p. 448. i860. — Stenzel, Flora, p. 45. [86 = -Numerous papers by Wydler, e. g. Linngea, p. 153, 1843 ; Flora, 1844, 1850, 1851, 1857, 1N59, 1S60, 1863 ; and elsewhere. — Hofmeister, Allgemeine Morphologie der Gewebe, §§ 8, 9. Leipzig 1868. [Haughton, Manual of Geology. — RELATIVE POSITIONS OF LATERAL MEMBERS. \6^ The lateral members are, on the other hand, isolated or scattered when each member stands on a different zone of the axis. If the surface of an axial structure (which sometimes is quite ideal, as in Aspidiiim Filix-mas, &c.) is imagined to be continued through the base of each lateral member, the section forms its Pla7ie of Insertion. An imaginary point in this is considered its organic centre, but does not usually correspond to its geometrical centre ; this point may be termed the Point of Insertion (cf. sect. 27). A plane which bisects a lateral member symmetrically, or divides it into two similar halves (sect. 28), and contains the axis of growth of the lateral member as well as that of the axial member, passes through the point of insertion, and is called the JSIedian Plane of the lateral member in question. If members are so arranged at different heights on an axis that their median planes coincide, they form a straight row or Orthostichy ; generally there are two, three, or more orthostichies on an axial structure, and the members are then said to be recti- serial. If there are no orthostichies, /. e. if the median planes of all the members intersect one another on an axis without coinciding, their arrangement is solitary. The size of the angle which the median planes of two members of the same axis enclose is their Divergence ; it is expressed either in degrees or as a fraction of Fig. 138— Diagrams of a slioot witli tlie leaves arranged sinj,'ly with a uniform divergence of ^. Fig. 139 —Diagram of the flower-stallc o^ Paris quadrifclia; II whorl of the large foliage-leaves beneath the flower; ap outer, ip inner pcrianth-whorl ; <»tz outer, i.x inner stamens ; in the centre is the rudiment of the fruit consisting of four carpel- lary leaves. the circumference of the axis, which is then supposed to be a circle, although in fact this is not usually the case. In order to represent the divergences clearly, they may be drawn on the horizontal projection of the vertical axial structure, in the manner represented in Figs. 138 and 139. The transverse sections of the axial structure which bear the lateral members, in this case leaves, are denoted by concentric circles, and in such a manner that the outermost circle corresponds to the lowest, the innermost to the highest transverse section. On these circles, which thus represent the relative ages from without inwards according to their succession in the acropetal development of the axis, the places of the members are denoted by points, or the forms of the planes of insertion themselves may be approximately indicated, as in the figures. On such a projection or diagram the median planes Ellis, Mathematical Tracts.— A. Dickson, Trans. Royal Soc. Edinb. vol. XXVI. p. 505.— Chauncey Wright, Mem. Amer. Acad. vol. IX. p. 37Q.— H. Airy, Proceedings Royal Society, vol. XXI. p. 176. — Beal, American Naturalist, 1873, vol. VII. p.449] i68 EXTERNAL CONFORMATION OF PLANTS. of the members appear as radial lines, indicated in Fig. 138 by I-V. Since in this case several members stand upon each median plane, they are arranged in orthostichies ; and these again are so placed that they divide the circumference into five equal parts. But if the members are considered in reference to their age, as indicated by the figures i-ii, it is seen that the divergence between i and 2 is f, as also is that between 2 and 3, between 3 and 4, and so on. The diver- gences are therefore all equal, or the members have in this case on the same axis the constant divergence f. In Fig. 139 the members are arranged in a quaternary whorl ; on each circle or section there stand in this case four similar members with the divergence \ ; but the successive whorls are so placed that the median planes of one whorl exactly bisect the angle of divergence of the preceding and following whorls ; the whorls here alternate, and all the members are arranged in eight orthostichies. If, on the other hand, two whorls stand one over the other in such a manner that their members fall into the same median planes, or cover one another, they are said to be superposed. Thus, for instance, the staminal whorl is superposed on that of the corolla in Primula ; and in the primary roots of Pha- seolus, Tropaeolum, Cucurbita, and other Dicotyledons, there not unfrequently occur superposed whorls of lateral roots. When alternating whorls have only two members, the position of the members is said to be decussate, as in Fig. 136, a very common case with leaves. If it is required to represent the divergences not merely on an axis but on an axial system, such as a system of leaf-forming shoots, by a horizontal projection, Fig. 140.— Diagram of a weakly plant of Euphorbia Jielioscopia; crthe cotyledons; /, / the first, i— lo the later foliage- leaves ; numbers 6—10 form one whorl ; at ^ / in the centre is the terminal flower of the primary shoot ; R II the terminal " flower of one of the five axillary shoots ; ///, ///, /// the leaves of three axillary shoots of the second order. it may be done on the same principle, as is shown in Fig. 140. Each system of concentric circles contains the members (in this case leaves) of an axis ; the RELATIVE POSITIONS OF LATERAL MEMBERS. 169 lateral axes (here axial shoots) are interposed between the "insertion of the respective leaves and their stem-axis. If the axial members are greatly shortened, the view (from above) of an axis, with its lateral members, often itself supphes the diagram ; as, for instance, in the leaf-rosettes of Crassulaceae, and in most flowers. In other cases a transverse section through the bud enables the observer to examine the divergence of the leaves; but in many other cases the relative positions are more obscure, and can only be ascertained by careful examination. In addition to the study of the history of development, peculiar methods, depending on geometrical principles, are often necessary in order to represent the relative positions correctly and at the same time clearly. There are also circumstances in which it is desirable, instead of representing the relative positions on a horizontal projection, to project them on the unwound surface of the axial structure, the latter being considered as a cylinder the surface of which is supposed to be flattened out. The transverse sections of the axis lying one over another are denoted on this surface by straight lines on which the positions of the members are drawn. Among the diff"erent arbitrary constructions which may be attempted on paper, for the purpose of comparing the relative positions of the members of an axis, or of reducing them to short geometrical or arithmetical expressions, the fol- lowing is of peculiar interest, and has been specially employed to denote the re- lative positions of the leaves and lateral shoots of the stem : — A line is imagined proceeding from any one of the older members in such a direction that, travers- ing the axis towards the right or the left, it includes the points of insertion of all the successive lateral members according to their age ; the horizontal projection of this line is called the Gaieiic Spiral ; in reality it is a helix ^ running round the stem more or less regularly. The importance of this construction has been very" much overrated, and it has been employed where it is not only inapplicable to the eluci- dation of the history of development, but even where it has no longer even a geometrical meaning, and no longer assists a conception of the relative positions, but even makes it more difficult and complicated. When we are dealing with solitary leaves or shoots standing on the axis in three, four, five, eight, or more directions, and when the diver- gences are not too variable, the construction of the genetic spiral is of excellent service for a ready understanding of the position of the leaves (Fig. 141); and a more exact Fig. 141. — Transverse section through the convolu- tion of the leaf-sheaths i— 6 of Snbal ambractilifera, in the centre of the section of a young Jeaf-blade. The arrangement of the leaves 'S a ^ divergence. If the numbers i— 6 are united by a line, the genetic spiral is obtained. ' If the spiral winds from right to left, the right edge of the leaves is called the kathodic, the left (ascending) edge the anodic ; the reverse in the spiral of an opposite direction seen from without. i-o EXTERNAL CONFORMATION OF PLANTS. knowledge of the peculiar'properties of this ideal line may under these circumstances be of great use to morphology. In some cases it may be applied with advantage even to the relative position of whorls. But in a large number of cases other constructions appear much more natural, since they afford an easier explanation of the relative positions, and are more in accordance with the phenomena of growth. The construction of the desired genetic spiral is altogether impossible where the leaves are formed in simultaneous whorls ^ as the petals, stamens, and carpels of most flowers, or even in successive whorjs where the members proceed from one point of the axis, and are formed in advancing order right and left, as in Characeae and the flowers of Reseda (Fig. 137). In the successive whorls of Salvinia natans the construction of a genetic spiral would be equally impossible. Fig. 142, B, shows the diagram of the stem ^ of this plant with three con- secutive three-leaved whorls ; in each of these the leaf iv is formed first, then the leaf Z^, and finally the leaf Z.^. If an attempt be made to con- struct the spiral, it must pass from w over L^ across to Z^, then again in the same direc- tion over IV across to L^ ; the figure thus formed is a circle, in which the diver- gences of successive leaves are very difi'erent. If we now pass to the next whorl, the line pro- ceeds in a spiral direction to the next leaf w \ but then, to retain the genetic succession in the second whorl, the line must be continued in an opposite direction ; and this is repeated with every new whorl. It is evident that no clear conception can be obtained in this forced manner, and the whole construction appears alto- gether superfluous, since it is required by no feature in the history of develop- ment. The stem of this plant is constructed, as Pringsheim has .shown, of two rows of segments (G, H, J, K, &c., in Fig. 142, A)^ which arise alternately right and left fro*m the apical cell. Even before the formation of the leaves each segment undergoes various divisions, and in this manner sections of the stem are formed which alternately perform the functions of nodes and internodes of the stem. Each nodal section consists of the anterior half of an older segment and the posterior half of a segment next younger iK age, as shown in the figure. An internode is formed of a whole segment of one row and of two half-segments of the other Fig. H2.—A the veijetative cone of the stem o( Sa/vinm natajis, regarded diagranimatically and looked at from above; xx projection of the plane which divides it vertically into a right and left half; the segments are indicated by stronger outlines, their divisions by weaker lines; the succession of the segments is denoted by the letters F—P; B diagram of the stem with three whorls of leaves, its ventral side indicated by vv; w the first-formed floating leaf; L\ the aerial leaf formed next; /,•> the second aerial leaf of the same whorl formed last of all between the two first (after Pringsheim). ' Many writers employ even in such cases the conceptions borrowed from a spiral arrangement, considering arbitrarily as of successive origin the members of the whorl which arise simultaneously, by which means the path to more accurate knowledge is stopped. RELATIVE POSITIONS OF LATERAL MEMBERS. 171 row. Cells of the nodal sections occupying clearly-defined positions produce the leaves in the ord^r stated. This development furnishes no evidence that the leaves are formed in spiral succession ; the bilateral structure of the stem shows rather that a spiral construction is in this case altogether inadmissible. The same may be shown to be the case in Marsilea, where the creeping stem bears on its upper side two rows of leaves, while the under-side forms roots; the leaves borne on the upper side may in this case be united in the order of their age by a zigzag line broken right and left, which does not anywhere touch the leafless under-side of the stem, and corresponds also in its course to the bilateral structure of the stem. The spiral construction appears also to be meaningless in all those cases where it is indifferent whether the spiral be carried right or left. This is the case where the members are placed in two rows, with a constant divergence of |-, and are thus arranged alternately in two orthostichies lying exactly opposite to one another, as is the case with the branchings of many thallomes {e.g. Stypocaulon, Fig. 98), the leaves of Grasses, the lateral shoots of Tilia, Ulmus, Corylus, &c. In all these cases of decidedly bilateral construction the genetic spiral may be imagined just as well and with the same divergence ascending right or left, by which of course it loses its importance for any morphological conclusion as much as if one sup- posed it to change its direction from leaf to leaf. In upright free-growing axes with solitary leaves arranged in three, four, five, or more directions, the spiral construction appears especially natural ; and this also agrees with the symmetrical relationships of plants, of which more will be said hereafter, as well as with the fact that the spiral construction proves to be opposed to nature in bilateral structures, especially in creeping or climbing stems, and in lateral branches. In those cases in which the spiral construction may be employed natur- ally, I. e. in the least forced manner, to elucidate the relative positions of the members, two cases may be distinguished, according as the divergences are very unequal and change abruptly, or are nearly or quite equal to one another or only change gradually. In the first case the members appear to be arranged irregularly and without order, as the foliage-leaves on the stem of Fritillaria imperialis (Fig. 143), the flowers on the rachis of the raceme of Triglochin palustre, and in many Dicotyledons. When the change of divergence on the same axis is abrupt, it may also appear more natural to represent the position of the leaves by two homodromal spirals instead of one, as in many species of Aloe, where the shoots commence with leaves arranged in two rows and then pass over into complicated divergences, which lead finally to rosettes of leaves radiating on all sides. This occurs, e. g. in Aloe ciliaris, laii/olia, brachyphylla, lingua, nigricans, and Serra. Fig. 144 shows the transverse section of a shoot of the last-named species; the first six leaves are arranged exactly alternately m two rows with a constant divergence \ ; at the 7th leaf this arrangement is suddenly changed ; instead of being placed over 5, its position is between 5 and 6 ; but the 8th leaf ex- hibits the divergence \ from the 7 th ; the 9 th again changes the divergence, instead of being placed over 7, it is between 7 and 6 ; the loth leaf again di- verges about \ from the 9th; and so it goes on. The leaves 7-15 are evidently arranged in pairs, the pairs being 7, 8 ; 9, 10; n, 12; 13, 14; each pair 72 EXTERNAL CONFORMATION OF PLANTS. consists of two alternate (/. e. not opposite) leaves, the divergence of which is alike -J- ; but the pairs themselves diverge from one another by smaller fractions. -Diafjram of a flower-stalk of Fritillaria inipe7-ialis, sliowiiif,' the diverg-ences of the first twenty-four foHag-e-leaves ; the relative lengths of the internodes are indicated by the larger or smaller distances between the circles. If it is desired to unite all the leaves from 1-15 by a genetic spiral, an abrupt alteration of the divergence would be obtained within it. The relative positions are shown, however, more simply and clearly if, keeping in view the bilateral origin of the shoot, two spirals are constructed, each of which commences from one of the origmal orthostichies, and, so to speak, continues it in a spiral curve ; the one contains all the leaves with an even number, the other those with an uneven number ; the tw^o are homodromal, runnino- in the same direction round the stem. The bilateral origin of the shoot may be followed in this manner up to -Transverse section of a shoot of .l/oi;' St RELATIVE POSITIONS OF LATERAL MEMBERS. 17.3 the rosette of leaves of the older shoot radiating on all sides. Similar relation- ships appear to exist in Dracaena and in some Aroideoe. At first sight such kinds of phyllotaxis appear as if the leaves were placed in two lines which have become changed by the torsion of the stem ; but this hypothesis seems in this case scarcely admissible. If we now turn, in conclusion, to those cases which clearly gave rise in times past to the erroneous hypothesis that the primary law of phyllotaxis is a universal spiral arrangement, we find the leaves placed singly, their divergences almost or quite equal or gradually passing over into some other value corresponding to the second case named above of spiral arrangement. In these cases the spiral con- struction affords a simple expression of the law of phyllotaxis; the only thing required is to name the constant angle of divergence ; — according as this is ^, ], i, f, I, &c., the phvllotaxis is termed simply one of ^, i, i, and so on. It is usual in such cases for the divergence not to remain constant for all the members of an axis ; shoots which form numerous leaves mostly begin with more simple arrangements, as .J, and then pass over into more complicated ones, an arrangement being considered more complicated when the numerator and denomi- nator of the fraction of divergence are larger. When the divergences between lateral members placed solitarily with spiral arrangement are equal, they must then also stand in straight rows, the number of which is expressed by the denominator of the angle of divergence. If, for instance, the divergence is a constant one of f , as in Fig. 145, there are eight orthostichies, the 9th member standing on the same median plane as the ist, the loth as the 2nd, the nth as the 3rd, and so on. In a f phyllotaxis, in the same manner, the 6th member stands over the ist, the 7th over the 2nd, and so on. In some cases the orthostichies are very obvious, as, for instance, in Cacti with prominent angles, the angles corresponding to the ortho- stichies of the spirally arranged leaves, which, however, in this case mostly re- main undeveloped. In verticillate leaves also the straight rows are mostly conspicuous if the shoot is looked at from above, as, for instance, in the decussate two-leaved whorls oi Etiphorbia Lathyris, and the cactus-like E. canarieiisis. When the members of a spiral phyllotaxis with a constant angle of divergence stand sufficiently close to one another, spiral arrangements are easily seen and followed to the right and left which more or less conceal the genetic spiral. These rows are called Parastichies, and are particularly clear in the cones of species of Pinus, the leaf-rosettes of Crassulaceae, the flowers of the sunflower and other Compo^itee, and in the spadices of Aroide^. They may be seen in every spiral phyllotaxis with a constant divergence, and can always be made clear in the dia- gram, or when the arrangement is represented on an unrolled cylindrical surface. Fig. 145.— Diagram of a shoot in which the leaves have a cuiistant phyllotaxis of tj. 1/4 EXTERNAL CONFORMATION OF PLANTS. The considerations of these constructions leads to definite geometrical rules, by means of which the genetic spiral can be easily deduced from the parastichies \ It is evident that the constructions hitherto mentioned can only be more or less convenient aids to an understanding of the actual principles of the arrangement of leaves. But in order to obtain, with their assistance, a deeper insight into the processes of grow^th themselves of which these principles are the result, it is necessary to follow up the history of development, and in every single case to ask the question, what circumstances are the cause of a new member being formed just in this place and nowhere else. It may be well, therefore, to bring forward here some of the points which must be considered in reference to this view. (r) The first question to consider is always the permanence of the order of succession which may occur at the time of the origin of the lateral members. (2) Attention must be paid not only to the lateral deviation or divergence, but also to the longitudinal distance at which a new member is formed at the punctum vegetationis above the members next pre- ceding it. The longitudinal distances of the youngest lateral structures of a punc- iiim vegetationis from one another are usually very small ; there is often no va- cant space to be distinguished between them ; the planes of insertion of the youngest members are in contact. This circumstance may, on the one hand, assist in the determination of the place where the next member must be produced; but on the other hand may give occasion, as the development of the axis proceeds with its crowded lateral members, to com- pression and distortion, by which the ori- ginal arrangement is altered. (3) By the increase in length of the common axis, members w'hich were at first closely crowded become placed at a considerable distance from one another; others, in consequence of slower growth, remain closely packed, so that a different distribution occurs in different parts of the stem (as in the leaf-rosettes and flower- stalks of Crassulaceae, Agave, Aloe, &c.). In the same manner the angle of divergence frequently becomes changed from the more rapid growth in thickness of the axial Fig. 146. — Diagrammatic representation of the orthostichies of a i phyllotaxis ; A before and B after the torsion of the stem. Each of the orthostichies /, //, /// is indicated by a double line ; the genetic spiral is single ; where it crosses the orthostichy, the leaf-insertions are indicated by fignres. ^ As the treatment of the subject is only of value to those who are practically concerned with phyllotaxis, I must refer to the detailed description in Hofmeister's Allgem. Morphologie, § 9. RELATIVE POSITIONS OF LATERAL MEMBERS. 1 75 Structure on one side than on the other ; and still more commonly by torsion around its own axis of growth. By such torsions lateral members, arranged at first exactly in straight rows, become displaced so that the orthostichies appear as if wound spirally round the axis. This occurs, for instance, according to Nageli and Leitgeb, in the root-systems of Ferns, Equisetaceae, and Rhizocarps, as well as in the three-rowed phyllotaxis of the Moss Fontinalis antipyretic a, according to Leitgeb. But the most striking example is furnished by the stem of the screw-pine Pandanus utilis ; in the bud, the numerous and already strongly developed leaves stand, as is shown by the transverse section, in three perfectly straight lines with the phyllotaxis ^ ; but, as the development of the stem advances, it undergoes so severe a torsion that the three orthostichies are transformed into three strongly curved spiral lines running round the stem. In these and similar cases the change in the relative positions caused by the torsion of the axial structure is easily and certainly established. But when the positions are so related to the apex of the axial structure that the angle of divergence cannot be accurately estimated by an apical view from above, it must remain uncertain whether the position of the mature members is unchanged or has been altered by lateral displacement and torsion of the axis. A displacement, for instance, of about 9" of the circumference of the axis would be sufficient to alter the divergence from f to ^, a similar dis- placement of 1-3"' would change the divergence from y% to ^\. When the phyl- lotaxis is very complicated and the number of the longitudinal rows very large, extremely small and almost inappreciable distortions are sufficient to destroy the original arrangement and to bring into existence altogether different systems of parastichies. This observation is of interest so far as it makes it seem doubtful whether certain complicated phyllotaxes are always due to the original arrangement of the members \ (4) It must be observed whether the position of newly-formed members or their later transformation shows any relation to the direction of the force of gravity, of the light which falls upon them, or of any pressure acting from without^. The effect of the force of gravity is that primary shoots which are in the main upright put forth radiating leaves on all sides ; while such as have a decidedly horizontal growth, in which a rooting under-side is contrasted with an upper-side, usually show an arrangement of leaves on the latter in two rows, or one which is divided into two equal halves by a plane cutting the stem longitudinally ( 3-"^ their length varies from twice to 100 times as much. They ivide only longitudinally by elongating to double their normal length and subsequently pinching in ; ley never branch. The cells resulting from the division either separate or remain attached in lains. By the swelling up of their cell-membranes they may form a jelly-like mass or colony ?;oogl8ea). Most Bacteria present a motile and a motionless condition ; their movements in the )rmer are extremely various. The systematic place of these organisms is at present purely provi- onal. E. R. Lankester has shown (Quart. Journ. Micr. Sc. 1873, p. 408), from the investigation F a peach-coloured species which made its appearance in water containing decomposing animal re- tains, that the series of forms distinguished by Cohn cannot be maintained as distinct, and that they lust either be regarded as 'a series of steps in the ontogenesis of a specific form, or they are a number f phases or " form-species " of a Protean organism.' Lister (Quart. Journ. Micr. Sc. 18 73, pp. 393-4) elieves that he has demonstrated the origin of Bacteria from a Fungus, a species of Bematuim. On le other hand, Cohn has remarked the surprising resemblance of the microspores of an interesting >scillarian Alga Crenothrix to Bacteria, although he is disposed to think that there is no genetic Dnnexion between the two. (See Quart. Journ. Micr. Sc. 1873, p. 163.) — Ed.] ^ De Bary, Flora, 1863, pp. 553 et seq. — Thuret, Observations sur la reproduction de quelques [ostochines : Mem. de la Soc. Imp. des Sci. Nat. de Cherbourg, vol. V. Aug. 1857. ALGM. place, by which the threads come to consist of ,pvp™i . c „ of division contain a homogeneous or granu ar proIoDl sm f K, '^k ""'^ '^"^ ^^P^' brown colour, consisting of a minture^of ct r'o;h7 ^h th: b^f cT'" "■" '^"'^ already mentioned or with a yellow one Tn n«.iL^ ! colouring substan thread are alike; the thread Ll is c^^indric^ „d ;hetV'r"'"K^'' '""^ ^^"^ °' short transverse discs. In the other ^enera the thread, ,"' *' ^PP«-ance spherical or ellipsoidal and of two different forms h JIT ' "r"''°™' ">^ ^^ capable of division; between them occur a ™ 'ter df """^ber are green a, thread, colourless cells of considerable sl^ Lf tf'^'f^' «■• « the end of tl The Nostochine. live in ^'atrTr Ire conl '"?''"'/''''"*"' '''^ ""^'^^^V^' walls, where they form ge.atin^lsIcTrat onTo Ist'TheToir^ ^°t' ^' IS known only in a few genera of this division '^ ■•eproductK Nostoc, which may be considered the tvne nf ia-„.* of a large number of moniliform threads i„.e ^°^*^''^^^' <=o>«'st=. when matur bedded in a glutinous iellv and Zs nnit^H .'"f™T'" """"^ ""'= ''"°">^'- «"d in New colonies are, accor*n'g to Thurerf '!, '°L°"'f "^ ^ ^P*^'^"''^^"^ '^^^fi-'^d f"™ the old colony b;cores oft ned b I'atr The' nort' T'^k' 7"— ^he jelly < the heterocysts become detached sen" rUe from'^th n ^l "■'''* '^'"« ''^'^^'^ while the heterocysts themselves rema^irth i^v Aft 'I.'k '"■"«"™ ""="'"'™' increase the number of their eel thrthen . '°"u""" '" '''°"«="^ ^"^ " contact with those of the next oJ/ndthTr'' "k'T '''"" '^™ '^™''"=" ^^"^ " Nostoc-filament. Indi,idtnrcols nn!, „ u "*"'' """' '"'° '' ''"^'^ '^""« cysts. In the meintin e 'e .. ' 'PP'''"'">' ^^"h""' m definite law, become hetero- the original,; ™:^Ti,'tbTtanr'::T"''' "'''"' ""^ '"^'"^"' '= ^'^^'^P^"' »^ continuous h^crease .^XSlTdi^rs^Tthrelir^'^^ ''' '^ °^ ^ -^'^ ^>- R.wl^l!7/°^ 'T ''^^*P'"'"' °f Hlvularie^ has been observed by De Bary the thread docs nT' '"k^'T '°"'"*=" '° " riding-whip. The pointed end of of the t™sverse dl'™"' "' "'^ '""g'tudinal growth and the increase in number FructLaton takes olaT' ", ™' /"''"'''" ''°-"™'-'^^ - f- - the basal cell, ructmcat.on take s place nearly simultaneously in most of the fi laments of a colony. lower slTo^fTelh Xrt/r"''"',""^'"" by Janczewski to enter the young stomata on the tTclTn B L^ia Pent n T? 7 " ' '"« "■"' '" ''''"''' ^""^ '" *^ "«"- ""ifferent Hepa- gemi"of thTs; sne t 'n r"'' T- ''"""^' '"' """^ e^"=^^"^ t==" ""='d--'d endogenous thewll' '^u ; ",. J™""""^' P™^-<=<3 'heir true nature. Nostoc also establishes itself in If th/Sr f r ! ■? . ' ''"■=' of Sphagnum. The entrance of Scytone,ne=B into the parenchyma manner The ,: *^°'>:''^°"°"= ?'»'• ^--era, is brought about, according ,„ Reinke, i„T differ nt WsTf ^'^T" ""' P"f "'=h>--<=<>"^ °f *e circumference of the stem, themselves covered by and ,T PJ^^"t>™;' "' <'f ?<='y filled with colonies of the Alga. (Bot. Zeitg. 1872, pp so l\\ I J''° ^""- '^'' ^"- ^'"- '^7'- P- 3°«. ""d Q«"t. Journ. Micr. .Sc. ,8,;, p ,60 1 [Archer has described the occurrence of -spores' in Nostoc paludosum which were alwavs placed singly between Ihe heterocysts. Quart. Joum. Micr. Sc. 1S72, p. 367._Ed.] ^ i6 THALLOPHYTES. The cells which lie immediately above the basal cell form a resting spore ; it becomes thicker, and at the same time lo to 14 times as long as thick, of cylindrical form and with rounded ends, and now forms, so to speak, the handle of the whip-shaped filament ; its contents become denser, and darker from numerous granules, without, however, losing the bluish-green colour, and it surrounds itself with a compact firm membrane or sheath. At the commencement of the winter the cultivated plants disappear, only the Spores together with their sheaths remain behind, and commence germinating in January. The cylindrical cell divides first of all into 4, 6, 8, or 12 shorter cylindrical cells ; the biparti- tion is then repeated in all the cells through several generations, until the filament which arises in this manner from the spore numbers from 120 to 150 cells. The cells have already begun to be rounded off, and the filament has become moniliform; as it lengthens it splits the envelope of the spore, or raises up its upper part like a cap, while the lower end of the filament remains in the sheath. With its increase in length the filament decreases in breadth. When it has attained double the length of the sheath, it escapes completely from it, and the terminal cells become pointed. The filament then splits up into from 5 to 7 pieces about equal in length and in the number of their cells ; the pieces place themselves close to one another, until they form a bundle or tuft ; then each piece begins to transform itself into a whip-shaped Rivularia-filament ; one terminal cell be- comes the basal cell ; at the other end of the filament the cells elongate into an articulated hair. Various deviations from these normal processes occur however not unfrequently. The tuft of threads proceeding from a spore now forms a young mass of Rivularia, the threads of which are already imbedded in jelly. The multiplication of the filaments of a young growing mass takes place by apparent branching ; /'. e. one of the lower cells becomes a new basal cell ; the piece of filament lying between it and the old basal cell developes into an independent filament, which places itself beside the mother-filament. With respect to colour, habitat, and mode of life, as well as the tendency to form gelatinous envelopes, the Chroococcacese agree with the Nostocaceae ; the diff'erence lies in their cells not being united into filaments. In Synechococcus, Gloeothece, and Apha- nothece, the cells of all the generations elongate and divide in the same direction, and would form filaments if they did not separate from one another. In Merismopoedia the generations of cells divide alternately in two directions, flat plates consisting of one layer being thus formed. In Chroococcus, Gloeocapsa, and Aphanocapsa, the division takes place alternately in three directions, roundish families arising which are finally amorphous ^ (Fig. 154). The mass of layers of the softened gelatinous walls of the mother- cell surround the daughter-cells which proceed from it with their gelatinous envelopes which are also stra- tified, and thus form systems of layers enclosed in one another. The relations of growth now pointed out in the case of Nostocaceae and Chroococcacese are repeated, in all essential particulars, in some other groups of very simple Algae, the cells of which contain pure chlorophyll. The peculiar bluish- or brownish-green colour which the Nostocaceae share with the Chroococcaceae, is caused by a mixture of true chlorophyll with phycoxanthine and phycocyanine ; the phycocyanine is diffused from dead or ruptured cells, and thus produces, for example, the blue stains on the paper round herbarium specimens of Oscillatorieae. If the plants are crushed and an extract made with cold water, a solution^ is obtained of a beautiful colour which is blue in trans- mitted and blood-red in reflected light. If the crushed plants, after extraction of the Fig. 154.— Mode of cell-division Chroococcaceae. ^ Niigeli, Gattungen einzelliger Algen. — Braun, Verjiingung, p. 139. — Ray Soc. Bot. and Phys. em. 1853, p. 131. 2 Cohn, Archiv fiir mikr. Anat. von Schultze, III, p. 12. — Askenasy, Bot. Zeitg. 1867, no. 29. ALG^. 217 blue colouring-matter, are again extracted with strong alcohol, a green solution is obtained, which 'may be split up, as Millardet and Kraus have shown, into chlorophyll and phycoxanthine, if a large quantity of benzine is shaken up with it ; this takes up the green chlorophyll and forms when again at rest an upper layer, while the lower alcoholic layer retains the yellow phycoxanthine^. The Hydrodictyeae are a small group of Algae to which belong without doubt the genera Hydrodictyon and Pediastrum, and, probably also according to Pringsheim some others whose cycle of development is not yet known. Their cells contain pure chloro- phyll, and are distinguished by forming a large number of swarm-spores, which, when they come to rest, unite into a single family ; this is tabular in Pediastrum ; in Hydrodictyon it forms a wide-meshed sac-like net. They also produce in addition smaller swarm- spores which go through a long period of rest, and, in their further development, give rise to an alternation of generations. From the researches of A. Braun^ and Pringsheim'', the following process of development occurs in the case oi Hydrodictyon utriculatufn, which. lives in stagnant or slowly flowing fresh water. In the mature state the thallome of this plant is a sac-like net several inches long ; the individual cells, united only at their ends and forming four- or six-cornered meshes, are cylindrical and some lines long ; all the cells of a net are sister-cells, formed simultaneously from one mother-cell. The mature cells have a firm compact w^all, clothed with a thick layer of chlorophyll-green protoplasm, and enclosing cell-sap. In reproduction, the green protoplasmic sac in some cells of the net splits up into large naked daughter-cells, whose number reaches from 7000 to 20,000; but in other cells into smaller ones numbering from 30,000 to 100,000. Only the first or larger ones form new nets at once ; they move first of all with a trembling motion within the mother-cell for about half an hour, and then form a daughter- net, which becomes free by absorption of the mother-cell-wall, and, under favourable conditions, attains its full size in three or four weeks, the separate cells elongating 400 or 500 fold. The smaller swarm-spores, on the contrary, leave the mother-cell and disperse, remaining in motion often for three hours. They are oval, and are fur- nished at the hyaline end with two long cilia ; when finally at rest, they are spherical, and surround themselves with a firm cell- wall. In this state they may remain dried up for months, if protected from light. After remaining several months at rest, these spores begin slowly to grow, and a vacuole is formed in the green protoplasm. At first from T2() to T^o "ini. in size, they attain a diameter of -in mm. Their contents split up by successive divisions into two or four portions, each of which forms a large swarm- spore. After a few minutes they come to rest, each large swarm-spore constituting a peculiar polyhedral cell, the angles of which grow out into long horn-like prolonga- tions. These polyhedra make a considerable growth ; their protoplasm becomes parietal and encloses a large sap-cavity ; it finally splits up again into swarm-spores which move about with a creeping motion for 20 or 40 minutes within the inner layer of the wall of the polyhedron which protrudes from it as a hernioid sac; they then come to rest, and form a hollow net. These nets formed from the polyhedra consist of only from 200 to 300 cells, but otherwise behave in the manner described above. In some polyhedra smaller and more numerous swarm-spores are formed, but these also unite into a net. The Volvocineee* are, during the whole of their vegetative period, continually in 1 Millardet and Kraus, Comptes Rendus, LXVI, p. 505. [See also Sorby, Proc. Roy. See. XXI, .P- 457-] 2 A. Braun, Verjiingung, p. 146.— Ray Soc, Bot. and Phys. Mem. 1853, pp. I37. 190- 3 Pringsheim, Men. der konigl. Akad. der Wiss. zu Berlin. Dec. 13, i860. [Quart. Joum. Micr. Sc. 1862, pp. 54, 104.] * Cohn, Ueber Bau und Fortpflanzung von Volvox glohator in Berichte iiber die Verhand. der schlesischen Ges. fur vliterland. Cultur, 1856 (also in the Comptes Rendus, vol. XLIII, Dec. i, 1856, and Ann. des Sci. Nat. 1857, p. 323).— Cohn, Ueber Chlamydococcus und Chlamydomonas (Protococcus), 2l8 THALLOPHYTES. motion, interrupted only by certain periods of repose ; the motion, as is usually the case with swarm-spores, being caused by two cilia. They are, ho\^'ever, distinguished from swarm-spores by the cells — which either live isolated (Chlamydomonas, Chlamydo- coccus), or are united into angular and tabular (Gonium) or spherical families (Volvox, Stephanosphaera, Pandorina) — being surrounded, while in motion, by a mem- brane of cellulose, through which the cilia project free into the water, and produce by their vibrations the rotating and progressive movement of the single cells or of the whole family. This hyaline envelope of cellulose lies either in close contact with the green primordial cell (Chlamydomonas), or is separated from it by a colourless space (water?), from which fine threads of protoplasm run from one to the other, as in Stephanosphaera (Fig. 155, FII). As an example of the history of development, we may choose Stepha- nosphara plwvialis (after Cohn and Wichura in Leopold. Akad. vol. XXVI, p. i). This Alga occurs occasionally in rain-water in the hollows of large stones. When fully mature, Stephanosphaera (Fig. 155, X, XI} is a hyahne ball ('envelope-cell') in which, Fig. 153. — Stephanosphc^ra pht-jialis (after Cohn and Wichura). standing vertically to its horizontal diameter, lie eight (or more) chlorophyll-green primordial cells; these are fusiform (Fig. 155, XI), and attached to an equator of the envelope-cell at both their ends by branched threads of protoplasm. These primordial cells, derived from one mother- cell, form a family which rotates on the axis at right angles to the plane passing through them all. Out of each cell of a family of this kind is produced, so long as the conditions of vegetation (light, warmth, and water) are favourable, a new family which begins to be formed in the evening and is matured during the night. Each cell divides in succession into two, four, or eight cells, lying in the same plane and forming a disc divided into eight parts ; they secrete a common envelope and develope their cilia. The cells separate from one another and their common envelope detaching itself become spherical ; and thus eventually eight young families are found moving in circles within their mother-cell, until they are freed Berichte der schles. Ges. 1856. — Cohn tind Wichura, Ueber Stephanosphcera pluvialis: Nova Acta Acad, nat. curios, vol. XXVI. p. i. [Quart. Joum. Micr. Sc. 1858, p. 131.] — Pringsheim, Ueber Paarung der Schwarmsporen in Monatsber. der Berliner Akad. Oct. 1869. — De Bary, Bot. Zeitg. 1858, Supple- ment, p. 73. [See also on Volvox, Carter, Ann. Nat. Hist. 1839, vol. III. pp. 1-20. — Williamson in Phil. Soc. Manch. IX. p. 321 ; Trans. Micr. Soc. 1853, p. 45 ; Busk, Trans. Micr. Soc. 1853, p. 31. —On Stephanosphoera : Cohn, Ann. Nat. Hist. 2nd ser. X. pp.321, 401; Archer, Quart. Journ. Micr. Sc. 1865, pp. 116, 185.— On Protococcus: Cohn, Ray Soc, Bot. and Phys, Mem. 1852.— On Pandorina, Henfrey, Trans. Micr. Soc. 1856, p. 49.] ALG^. 219 by its rupture. Several generations of families endowed with motion are formed in this manner one after another. The succession of these generations is sometimes interrupted by the formation of Micro-gonidia ; i.e. of small swarm-spores, which, resulting from repeated division of the primordial cells of a family, become completely isolated and dispersed. They are furnished with four vibratile cilia and each, secreting a cell-wall, finally passes into a roundish resting-cell, the fate of which is still un- known (Fig. 155, XIT, XIII, XIF). The succession of generations of motile families is brought to an end by the formation of resting-cells. The separate primordial cells of the last motile family lose their cilia, and surround themselves with a firm closely- adherent cell-wall ; they then resemble the cells of Protococcus, and are Hke the resting-cells which proceed from the small swarm-spores of Hydrodictyon. They accu- mulate at the bottom of the water, and there grow into larger green balls (Fig. 155, /), the colour of which passes over, when mature, into red. Only when these resting-cells have remained dry for a long time are they in a condition, when again moistened, to develope gradually generations endowed with motion ; their contents divide into two, four, or sometimes eight parts, which, after absorption of the cell-wall, set up a motion as swarm-spores with two cilia (Fig. 155, 77, 777, IF). In the course of the day they surround themselves with a separable cell-wall, and in this condition the unicellular swarm-spores (Fig. 155, F, FI, FII) resemble those of the genus Chlamydococcus. After some hours each of these swarm-spores divides into two, four, or eight daughter-cells, which, lying in one plane, secrete a common cell-wall, and each developes two cilia ; they then separate from one another, but remain enclosed in the common spherical cell- wall which has now separated itself from them^. After absorption of the mother-cell- wall the new family endowed with motion thus becomes free (Fig. 155, FII b, Fill, IX, X), grows in the course of the day, and forms in the night eight new similar families. After Cohn and Carter had already detected phenomena in certain Volvocineae (Volvox and Eudorina), which pointed to sexual union, Pringsheim has recently proved this to be the case with certainty in Pandor'ma morum, one of the commonest species. The sixteen cells of a family of Pandorina are closely crowded together, and surrounded by a thin gelatinous envelope out of which the long cilia project. The asexual multipli- cation results from the formation of a new sixteen-celled family in each cell of the mother-family ; and the sixteen daughter-families become free by the absorption of the gelatinous envelope of the mother-family. The sexual reproduction is brought about in the same kind of way though with some points of difference ; the gelatinous envelopes of the young families become softened, and the separate cells are thus freed and move each with its two cilia ; these free swarm-spores are of very variable size, rounded and green at the posterior end, pointed hyaline and furnished with a red corpuscle in front, where they bear the two cilia. Among the crowd of these swarm-spores may be seen some which approach in pairs as if they were seeking one another. When they meet, their points come in contact, and they coalesce into a body at first biscuit-shaped, but gradually contracting into a ball ; in this ball the two corpuscles are still to be seen, the hyaline part is relatively large, and both pairs of cilia are still present ; but these all soon dis- appear. These processes last for some minutes. The green ball which results from the conjugation is an Oospore, and germinates only after a long period of rest. If the oospores, when dried up and of a red colour, are placed in water, the development begins after twenty-four hours ; the exospore breaks up, as in Hydrodictyon ; an inner membrane swells up like a bag, and allows the protoplasmic contents to escape in the form of a swarm-spore (more rarely after division into two or three). These swarm- spores which proceed from the oospore surround themselves with a gelatinous envelope, split up by successive divisions into sixteen primordial cells, and thus form new Pando- rina-families. » [Archer has described, /. c, pp. 7, 8, a remarkable amceboid phase which the primordial cells of Stephanosphtera undergo. — Ed.] 220 THALLOPHYTES, The CoNjUGAT-s: \ a family of Algae rich in genera and species, are distinguished by the occurrence of reproduction by Zygospores, in addition to the simple multiplication of cells by division ; swarm-spores are not formed. In one section, comprising the Meso- carpeae and the Zygnemeae, the cells remain united, and form long unbranched threads, the cells of which are cylindrical, and only occasionally, w^here they are in contact with a firm surface, produce lateral root-like ramifications, as organs of attachment. In the Desmidieae the mature cells consist usually of two symmetrical halves often separated by a constriction ; the division takes place in this constriction or, at all events, symme- trically, each half becoming completely developed into a perfect cell. The external contour of these cells is very various ; and since their divisions always take place parallel and in the same plane, as in the previous group, they form, when the cells are attached to one another, filiform rows ; but very commonly they split up and live singly. A comparison of the unicellular Desmidieae with those which possess a filiform arrange- ment and with the Zygnemeae, shows clearly that it is a matter of subordinate importance whether cells live singly or united, so long as they are similar to one another. Each single cell of Spirogyra, like an isolated cell of Closterium, &c., constitutes an individual. The cells of the Conjugatae are distinguished by the most various configuration and the most beautiful arrangement of their masses of chlorophyll ; it occurs in parietal spiral bands (Spirogyra), axile plates (Mesocarpus), a pair of radiate bodies (Zygnema), or plates arranged into stars (Closterium), &c. The zygospores are always resting-cells, gerrriinating only after a long period of rest, even not till the next year. The formation of the zygo- spores results, in the Zygnemeae, from a strong contraction of the protoplasmic sub- stance in the manner shown in Fig. 6 (p. lo), although with some modifications in the diff'erent genera. In the Mesocarpeae the conjugating protuberances unite in a similar manner, but the zygospore is formed by the accumulated contents of the canal becoming separated on both sides from the mother-cells by division-walls ; and the central piece of the conjugating apparatus thus individualised is the zygospore'^. In the Desmidieae the zygospore is produced in the same manner as in the Zygnemeae; it developes either one, two, or four new cells, each of which splits up into two equal daughter-cells capable of division. FIG. 156.— Germination o{ Spirogyra jugalis (after Pringsheim, Flora, 1852, no. 30) ; / a resting zygospore ; // commencement of its germination ; /// the young plant further developed from a zygospore, which had been enclosed in the cell C, the conjugating apparatus being still visible ; e outer cell-vvfall of the spore ;y yellowish brown layer of the cell-wall ; g the third and innermost layer of the cell-wall of the spore, which forms the germinating filament ; w iu' the first septa of the germinating filament, the posterior end d growing into a narrow appendage. The genus Spirogyra (as an example of the Zygnemece) has already been described and figured several times in sect. 3 of Book I; the additional Fig. 156 will suffice, ^ A. de Bary, Untersuchungen iiber die Familie der Conjugaten. Leipzig 1858. ^ [Two kinds of conjugation are distinguished ; transverse, in which the cells belong to different filaments ; longitudinal, in which they are parts of the same filament. See also Hassall, /. c. ; Witt- rock on Mesocarpea:; Quart. Journ. Micr. Sc. 1873, p. 123.— Ed.] ALGM. 221 together with Figs. 5, 6, and 15 (pp. 10 and 17), to give an idea of the process of development of these plants. Among the Desmidieae ^ we may choose as an example for closer observation Cosmarium Botrytis (after De Bary, /. c). The cells live isolated, and are symme- trically bisected by a deep constriction (Fig. 157, X), and are also compressed at right angles to the plane of constriction {I, a) ; in each half-cell are two grains of starch and eight discs of chlorophyll, which curve and converge in pairs running from two centres to the wall. The multiplication of the cells by division is brought about by the narrowest part of the constriction elongating a little, when the thicker outer layer of the cell- wall opens by a circular fissure ; the two halves of the cell hence appear separated from one another and united by a short canal, the wall of which is a con- tinuation of the inner layer of the walls of the half-cells. A septum soon appears in the piece which unites them, by which the cell is divided into two daughter-cells, each Fig. in.— Cosmarium Botrytis (after De Bary, /. c). (I-III X390, Il^-A' X190). of which is a half of the mother-cell. The septum, at first simple, splits into two lamellae, which immediately become convex towards one another (JX, h)\ each daughter-cell now possesses a small rounded outgrowth which grows gradually and assumes the form of a half-cell, so that each daughter-cell now again consists of two symmetrical pieces [X). While the wall is undergoing this growth, the discs of chlo- rophyll of the old halves grow into the newly-formed halves of the cell. The two grains of starch of the old half-cells elongate, become constricted, and each divides into two grains; of these four grains two pass over into the new half-cell, and all four again arrange themselves in the original symmetrical manner. Conjugation takes place between cells lying in pairs in a crossed position enclosed in thin jelly (Fig. is7, I)- Each of the two cells emits from its centre a conjugating protuberance (/, c) which meets the other; these protuberances are formed by a delicate membrane which is a continuation of the inner layer of the cell, the firm outer layer of which is split {I,c). Both protuberances swell up into a hemispherical bladder while in contact with one another until the separating wall disappears, and the contents unite in the broad canal thus formed; the protoplasm becomes everywhere loosened from the cell- wall, and contracts into a spherical form. The united protoplasm appears as if [See also Ralfs, British Desmidiese, 1848.— Aixher in Pritchard's Infusoria.] 222 ^H^- LLOPHYTES. surrounded by a delicate gelatinous wall (i/,/) by the side of which lie the empty cell- walls (//, e, b). The zygospore now becomes rounded into a ball ; its wall forms, as it matures, three layers, an outer and an inner colourless layer of cellulose, and a middle firmer brown layer. This stratified cell-wall grows out at several points into spiny protuberances which are at first hollow and afterwards solid, each of them producing at the end a few small teeth (///). The starch-grains of the conjugating cells become transformed into fat in the zygospores. Germination commences by the protrusion of the colourless inner layer through a wide split in the outer layer {IF); the thin-walled sphere thus set free considerably exceeds the zygospore itself in size. In the contents of this sphere {F) may be recognised two masses of chlorophyll surrounded by fatty protoplasm which might have been distinguished even before their escape from the external layer of the zygospore. The contents now contract and become surrounded by a new wall (F) from which the older wall detaches itself as a thin vesicle. After some time the protoplasm becomes constricted by a circular furrow, and splits into two half-balls, each of which contains one of the two chlorophyll-grains {FI). Each half-ball remains for a time naked and again constricts itself; but this time the constriction does not advance to the centre ; the body changes its form in other respects also, and each half of the germinating cell now appears as a symmetrically divided Cosmarium-cell {FII), which surrounds itself with a wall of its own. The planes of the constrictions of the two cells derived from the zygospore cut the dividing plane of the zygospore itself at right angles; they themselves also stand at right angles to one another, and therefore lie crossed in the mother-cell. In each of these the contents now arrange themselves in the manner above described ; the mother-cell-wall is absorbed and the new cells separate from one another. All these processes of germination are completed in one or two days. The new cells, whose outer wall is smooth, now divide in the usual manner, but the newly grown halves are larger and rough on the outside {Fill, IX, X) ; \\\e four daughter- cells of the two cells produced from the zygospore are therefore of two different forms ; two have the halves equal and two unequal ; the latter constantly produce by division one with equal and one with unequal halves ; the former only cells with equal halves. The Diatomacese^ (Bacillarieae) extremely rich in species follow naturally after the Desmidieae ; in particular they are allied to the Conjugatae, their processes of develop- ment coinciding with the conjugation of the latter, or at least bearing a certain resemblance to it^. They also resemble the Desmidiege in the configuration of their cells, in the manner of division, and in the mode of completion of the daughter-cells. Like the Desmidieae, the similar cells of the Diatoms may be united into threads, or may live entirely isolated. The tendency of the Diatoms to secrete a thin jelly in which they live socially is found also in the Desmidieae, although less strongly dis- played. In the same manner the movements of Diatoms are not altogether dissimilar to those of the Desmidieae, and even the silicification of the cell-wall, which is very strong in the former, is found, though to a smaller extent, in Closterium and other Desmidieae; and the fine sculpturing of the silicious shell also finds an analogue, although in a coarser form, in the cell-wall of some Desmidieae. The Diatoms are the only Algae, except the Conjugatae, in which the chlorophyll occurs in the form of discs and bands, but in some forms it is also found in grains, and the green colouring matter is concealed, like the chlorophyll-grains in Fucaceae, by a buff- ^ Luders, Ueber Organisation, Theilung und Copulation der Diatomeen, Bot. Zeitg. 1862, no. 7 et seq. — Millardet and Kraus discuss their colouring-matter in Compt. Rend. vol. LXVI. p. 505; and Askenasy in Bot. Zeitg. 1869, p. 799. — Pfitzer in Heft II, of the Botanische Abhand- lungen edited by Hanstein. Bonn 1871. [Quart. Journ. Micr. Sc, 1872, 1873.] ^ [Thwaites first discovered the conjugation of the Diatomacese, Ann. Nat. Hist. 1847, vol. XX; see also Carter: Ann. and Mag. Nat. Hist. 1856. — Schmitz, Quart. Journ. Micr. Sc., 1873, p. 145. — Smith, Synopsis of British Diatomacese. — Ed.] ALiG/E, QiQi'i coloured substance, Diatomine or Phycoxanthine. One of the most prominent pecu- liarities of the Diatoms consists in their silicified cell -wall being composed of two separated halves or valves of unequal age, of which the older one is pushed on to the younger like the lid of a box. When the cell begins to divide, the valves separate from one another, and after the division of the contents into two daughter-cells, each of them forms a new layer on their plane of division which is adjusted by its turned-in margin (the girdle) to the girdle of the old valve of the mother-cell ; this latter extends, like the lid of a box, over the newly formed valve; and the two valves of the two daughter-cells lie next one another. Since, according to Pfitzer, the silicious valves, which also contain some organic matter, do not grow, it is clear that the new cells must always become smaller from generation to generation. When they have thus attained a certain minimum size, large cells, the Auxospores, are suddenly formed ; the contents of the small cells, leaving the silicious valves which fall away from one another, increase either simply by growth or by both conjugation and growth. After this the auxospores surround themselves with new valves. Since the large auxospores are of somewhat different shape to their smaller mother-cells and primary mother-cells, the first result of their division must necessarily be cells of a different form and with unequal halves, as in the Desmidieae (Fig. 157). The origin of the auxospores has been more exactly followed out only in a few cases. It would appear that they are formed in very different ways, from two or from one mother-cell, simply or in pairs, with or without conjugation ; they are alike only in so far that their size greatly exceeds that of the mother-cell. Diatoms are found in enormous numbers at the bottom both of the sea and of fresh water, and attached to the submerged parts of other plants. Besides the ordinary rotation of protoplasm in their interior, they also exhibit a creeping motion by means of which they crawl over hard bodies or push small granules along their surface. This occurs only in a line drawn along the length of the cell- wall, in which Schultze^ supposes crevices or holes through which the protoplasm protrudes ; and this, although not yet actually seen, probably occasions the creeping motion. The genus Vaucheria- must be considered somewhat more in detail, as the best- known representative of a larger group, the Siphoneae, which, in the mode of growth of their thallus, are nearly related to the Conjugatae, but whose mode of reproduction is not yet sufficiently known. The thallus of Vaucheria consists of a single sac-like cell, variously branched, often several inches or a foot long, containing no nucleus, and developing on damp shady earth or in water. The fixed end is hyaline and branched in a wavy manner ; the free part contains within the thin cell-wall a layer of protoplasm rich in chlorophyll-grains and drops of oil, and enclosing the large sap-cavity. This part of the thallus forms one or more main branches or stems which again branch beneath their growing point (j) ; only in F, tuberosa the branching is also dichotomous ; though commencing monopodially the lateral branches often develope sympodially. Besides the occasional multiplication by the separation of branches or the regeneration of separated pieces of the thallus, reproduction is also brought about by asexual spores and by sexually produced oospores, and in such a manner that a long series of asexual generation usually proceeds from the latter, until at last sexual plants arise from asexual spores ; but the sexual generation can also produce spores as well as oospores. The spores may be produced in very different ways, from the simple detaching of the end of a branch to the formation of zoospores. In T. tuberosa lateral branches (sometimes also forked branches) swell up to a considerable size through becoming filled with cell-contents, separate at the base, and put out one or more germinating tubes. In T. geminata the end of a branch swells up to an oval shape ; its accumulated contents become separated by a septum ; it contracts, and * [See Pop. Sci. Rev. 1866, p. 395.] " Pringsheim, Ueber Befruchtung und Keimung der Algen. Berlin 1855, and Jahrbuch fiir wissen. Bot. II. p. 470.— Schenk, Wiirzburger Verhandl. vol. VIII. p. 235.— Walz, Jahrbuch fur wissen. Bot. V. p. 127. — Woronin, Bot. Zeitg. 1869, nos. 9, 10. 224 THALLOPHYTES. forms a cell-wall of its own. These spores do not fall out ; they either become freed by the decomposition of the sporangium-wall, or they fall off together with the sporangium ; some days after their formation they put out one or two germinating-tubes. The spores of F. hamata are formed in the same manner, but after their formation the sporangium splits at its apex, the spore slips out with a jerk and remains at rest, to germinate the next night. In several other species (as V. sessilis, sericea, a.nd piloboloides) true zoospores are formed. The preparations for them are similar to those in the last case ; but the contents of the branch which has swollen into a zoosporangium do not become sur- rounded by a cell-wall, but contract, showing in the interior one or more vacuoli, and then escape as a naked cell from a fissure at the apex of the branch (Fig. 158, j4, sp'). The escaped primordial cell contains numerous grains of chlorophyll surrounded by a layer of colourless protoplasm, and is everywhere covered by delicate densely crowded short cilia. Their vibratile motion causes a movement of the large ellipsoidal zoospore (as much as ^mm. long) about its longer axis, which sometimes, however, (as in V. sericea,^ lasts only for ^ to i§ minute. In F. sessilis the rotation begins, as I have Fig. i^B.—Vattcheria sessilis (X about 30). distinctly seen, during the escape from the sporangium ; and if the opening is too small, the zoospore splits into two pieces; both become rounded off; the outer piece rotates and swims out, while the inner piece rotates within the sporangium. As soon as the zoospores come to rest, the cilia disappear, and a cellulose-wall is produced (Fig. 158, B). The formation of the zoosporangia generally begins in the night ; the spores escape in the morning, and their germination commences the next night. The spore puts out either only one or two tubes (C, X)), or it forms on the other side at the same time a root-like organ of attachment {E, F, «u;). The Sexual Reproduction is brought about by oogonia (female cells) and antheridia (male cells). Both originate as twig-like protuber- ances from a branch or stem (Fig. 159, yl, B), sometimes even on the germinating tube of the zoospore (Fig. 15S, F, og, b). All the species of Vaucheria are monoecious, and the two kinds of sexual organs are mostly found very near together. The Antheridia usually arise ^ as the terminal cell of a branch by its transverse division, and contain very little * In the Vaucheria synatidra discovered by Woronin living in brackish water 2-7 small horns (antheridia) arise on the large ovoid terminal cell of a two-celled branch (Bot. Zeitg. 1869, nos. 9, 10). ALG7E. 225 or no chlorophyll (Fig. 159, B, a). From one part of the protoplasm of this antheridiiim- cell are formed the numerous spermatozoids, very small long bodies with two cilia (D). In several species the antheridia are curved like horns {F. sessilis, geminata, and terrestris); in others they are straight (F. sericea) or curved sacs {F. pachyderma). The Oogonia arise near the antheridia as thick protuberances (Fig. 159, A, B, og) filled with oil and chlorophyll. They swell up into an ovoid form (usually oblique), and finally the dense contents are separated by a septum from the base of the branch {F, osp). The green and granular mass collects in the centre of the oogonium ; colourless proto- plasm accumulates at its mouth, and withdraws from the cell-wall ; the cell-wall Fig. i^q.—Vaucheria sessilis; A, B origin of the antlieridium a on the branch b, and of the oogfonium og; C an open oogonium expelling a drop of mucilage si; D spermatozoids ; E the spermatozoids collected at the mouth of the oogonium ; i^, « an empty antheridiuni ; osp the oospores in the oogonium (A, B, E, F from nature, C, D after Pringsheim). suddenly opens and swells up into a jelly, and at this moment the contents are trans- formed into an oosphere, contracting at the same time. In some species (as F. sessilis) a colourless drop of mucilage is expelled from the mouth (C, si). At the same time that the oogonium opens, the antheridium also bursts and allows the spermatozoids to escape ; these press through the thin mucilage, on which they collect {E\ reach the oosphere, mingle with it, and disappear. The oosphere appears then to assume at once a sharp outline, and a double cell-wall may soon also be detected. The oosphere has transformed itself into an oospore; its chlorophyll assumes a red or brown-red colour, and the cell-wall thickens, so that three layers may generally be. distinguished in it (Fig. 159, F, osp, the oospore in the oogonium). The formation of the oogonia and antheridia begins in the evening, and is completed the next morning ; fertilisation is accomplished between 10 and 4 in the day. In their processes of vegetation several other genera approach very near to Vaucheria, especially Botrydium. The young plant is (according to A. Braun : Rejuvenescence, p. 128) a spherical Protococcus-like cell ; subsequently a hyahne prolongation is formed below which branches like a root and penetrates the earth, while the upper part swells up into an ovoid vesicle, in which the protoplasm forms, with the chlorophyll-grains, a parietal layer. From this arise, after the growth is matured, a number of zoospores Q 226 THALLOPHYTES. which are set free by the wall of the mother-cell becoming gelatinous and finally de- liquescing. This is evidently a more simple mode of growth than that of Vaucheria. A higher degree of branching than in this latter is found in Bryopsis, which is also unicellular. This genus also forms on one side root-like organs of attachment, on the other upright much-branched stems (several inches in height) with unlimited apical growth ; small branches with limited apical growth are formed on them in two rows or spirally, which clothe the stem like leaves, and after they have detached them- selves from it, fall off; while in them are formed the numerous motile zoospores^. (A. Braun, /. c.) The branching of a single large cell is carried still further in the genus Gaulerpa, which forms creeping stems growing at the apex with descending branched rhizoids and ascending leaf-like branches^. The growth of a unicellular thallus takes place in still another manner in Acetabularia. Here the plant, one or two inches high, has the form of a slender Hymenomycetous Fungus, the stem of which forms a rhizoid below and bears a pileus above, consisting of a disc of closely crowded rays, which are themselves radial branches of the stem. This ends above in the form of a boss ; at the base of the radial branches surrounding the boss stands a circle of umbel- lately branched articulated hairs. In the rays of the pileus are formed the asexual spores (the cell-sap contains inuline). Finally JJdotea cyathiformis must be mentioned here. This species forms a stalked leaf-like thallus, the stalk | inch, the thallus from | inch to 2 inches long and broad, its thickness from y^ to -^ line. As seen externally it seems to consist of a cellular tissue, in reality it is composed of a regular collocation of branched sacs, which, forming two cortical and one medullary layer, are all ramifica- tions of one cell (Nageli, Neuere Algensysteme, p. 177)^ The FuCACE^ comprise, in the narrow limitation proposed by Thuret-^, a few genera of large marine Algae, the thallomes of which, often many feet long, have a greenish-brown colour and a cartilaginous consistency. They are fixed to stones or other bodies by a branched attachment-disc. The thallomes branch dichotomously, and the further development is also frequently forked, but in other cases sympodial, as in Fig. 160. The ramifications, irrespectively of later displacements, all lie in one plane. The tissue consists at the surface of small closely-crowded cells ; in the interior it is laxer, and the elongated cells are often connected into articulated threads. The cell-walls often consist of two clearly distinct layers, an inner thin, firm, compact layer, and an outer gelatinous one, capable of swelling greatly in water, which fills up the interstices of the cells, and has the appearance of a more or less structureless 'intercellular substance'; it is clearly the cause of the slimy character which the Fucacese assume after lying for some time in fresh water. The granular cell-contents have been but little investigated ; they appear to be mostly brown, but contain chlorophyll which is concealed by other colouring materials ; from dead plants cold fresh water extracts a buff-coloured sub- stance •^. The tissue often becomes hollowed out internally into large cavities containing ^ Nageli, Neuere Algensysteme. Neuenburg, 1867. 2 Zeitschrift fur wissensch. Bot. von Nageli u. Schleiden, 1844, I. p. 134 et seq. ^ [The remarkable fossil plant from Canada of Devonian age, Prototaxites Logani, was probably an enormous Siphonaceous Alga; see W. T. Thiselton-Dyer, Journ. of Bot. 1871, p. 252, and Carruthers, Monthly Micros. Journ. 1872, p. 160. — Ed.] * G. Thuret, Ann. des Sci. Nat. II. 1854, p. 197. ^ In a recent paper (Comptes Rendus de rx\cad. des Sci. Feb. 22, 1869) Millardet showed that from quickly-dried and pulverized Fucaceas an olive-green extract is obtained by alcohol, which, shaken up with double its volume of benzine and then allowed to settle, produces an upper green layer of benzine containing the chlorophyll, while the lower alcoholic layer is yellow and contains phycoxanthine. Thin sections of the thallus, completely extracted with alcohol, contain also a reddish-brown substance which in fresh cells adheres to the chlorophyll-grains, and can be extracted by cold water, more easily when the dried Fucus has been previously pulverized. Millardet calls ALGM. 22' air which are forced outwards and serve as swimming bladders. The thallus has not as far as I know, been further minutely examined ; the outer conformation especially has been but little investigated from a morphological point of view. (Gf. Nageli, Neuere Algensysteme.) The mode of sexual reproduction is far better known through the labours of Thure and Pringsheim. The antheridia and oogonia are formed in spherical hollows (Con Fig. i6o.— Funis platycarpus (after Thuret) ; A end of one of the larger branches (natural size) ; ff fertile branchlets ; B transverse section of a receptacle ; rfthe surrounding epidermal tissue ; a the hairs projecting- from the mouth ; b hairs in the interior; c oogonia, e antheridia (cf. Fig. 2, p. 3). ceptacles) which make their appearance in large numbers and densely crowded at th ends of the longer forked branches or of lateral shoots of peculiar form. These re ceptacles are not formed in the interior of the tissue, but as depressions of the sur face which become walled in by the surrounding tissue, and so overgrown that a length only a narrow channel remains, opening outwards. The layer of cells whicl clothes the hollow is thus a continuation of the external epidermal layer of the thallus and since the filaments which produce the antheridia and oogonia sprout from it, thes( latter are, morphologically, trichomes. Some species are monoecious, /. e. both kind of sexual organs are developed in the same receptacle, as in Fucus platycarpus (Fig 160); others are dioecious, the receptacles of one plant containing only oogonia, thosi of another plant only antheridia {e. g. Fucus vesiculosus, serratus, and nodosus, Himanthal'u this reddish-brown substance phycopheeine. (Compare further the interesting treatise of Rosanofi Observations sur les fonctions et les proprietes des pigments de diverses Algues, in Memoires de 1 Soci^t6 des Sci. Nat. de Cherbourg, vol. XIII. 1867 ; and Askenasy, Bot. Zeitg. no. 47, 1869.) [Se also Sorby, Proc. Roy. Soc. 1873, vol. XXI. pp.445' 454' 461-] Q 2 128 THALLOPHYTES. lorea). A number of hairs which grow in the receptacles among the sexual organs are long, slender, articulated, but unbranched, and project in F. platycarpus out of the mouth of the receptacle in the form of tufts (Fig. i6o, B). The Antheridia are produced as lateral ramifications of branched hairs. Each antheridium consists of a thin-walled oval cell, the protoplasm of which splits up into numerous small spermatozoids ; these are pointed at one end, each furnished with two motile cilia ; in the interior they contain a red point. The formation of the Oogonium begins with the papillose swelling of a parietal cell of the receptacle ; the papilla is separated off by a septum, and divides, as it grows in length, into two cells, a lower, the pedicel-cell, and an upper, which forms the oogonium ; this swells up into a spherical or ellipsoidal form and becomes filled with dark protoplasm. This protoplasm of the oogonium remains undivided in some genera (Pycnophycus, Himanthalia, Gystoseira, Halidrys), and the whole contents of the oogonium thus form an oosphere ; in others (Pelvetia) it splits into two, four {0%othaUia imlgaris), or eight pieces (Fucus). Fertilisation takes place outside the recep- FlG. i6i.—Fua/s vesiailosus (after Thuret) ; A a branched hair bearinsj antheridia ; B spermatozoids ; / an oogonium, Og^ after the contents have divided into eight portions (oospheres), surrounded by simple hairs (/) ; // commencement of the escape of the oosphere ; the membrane (a) has burst ; the inner membrane i is ready to open (the two together constitute an inner layer of the cell-wall of the oogonium ; /// oosphere surrounded by spermatozoids ; IV, V, gemiination of the oospore (5X33o, all the rest Xi6o). tacles. The oospheres are expelled, surrounded by an inner membrane of the oogonium, and escape through the opening of the receptacle ; the antheridia at the same time become detached, and collect in numbers before the mouth of the receptacle when the fertile branches are lying outside the water in moist air. When they again come into contact with the sea-water, the antheridia open and allow the spermatozoids to escape, the oospheres at the same time escaping from the envelope which still surrounds them, and which is then seen to consist of two separated layers (Fig. i6i, 11). The spermatozoids collect in numbers around the oospheres, become firmly attached to them, and when their number is sufficiently great, their movement becomes so energetic that they impart to the very large oosphere to which they are attached a rotatory motion which lasts for about half an hour. Whether the spermatozoids force themselves into the oosphere Thuret leaves undecided ; but analogy with the processes observed by Pringsheim in Vaucheria and CEdogonium scarcely admits of a doubt that one or several of them mingle their substance with that of the naked ball of protoplasm. A short time after these pro- cesses are completed, the fertilised oosphere or oospore surrounds itself with a cell-wall, fixes itself to some body or other, and begins, without any period of rest, to germinate, and, lengthening at the same time, undergoes first of all a transverse division followed h] numerous other divisions. The mass of tissue thus formed puts out from the part or which it rests a root-like hyaline organ of attachment, while the thick free end forms th( growing apex (Fig. i6i, IF). The development of a fertile thallus from the oospon has not yet been observed ; and the whole cycle of forms of the Fucace^ has there fore not yet been certainly determined ^. The CEdogoniese - include at present only the two genera CEdogonium and Bulbo chaete, a few species of which are common in stagnant fresh water, fixed by an organ o attachment at the lower end to solid bodies, mostly the submerged parts of other plants The thallus consists of unbranched (CEdogonium) or branched (Bulboch^te) rows o cells, which multiply by intercalary growth, while the terminal cells readily elongab into hyaline bristles. The longitudinal growth of the cylindrical cells is caused by th( formation of an annular cushion of cellulose inside the cell, close beneath its uppe: septum ; the cell-wall ruptures at this place circularly ; the ring of cellulose thei stretches, and a broad transverse zone is thus intercalated in the wall of the cell. Th( process is constantly repeated immediately beneath the older very short upper piec( of the cell, so that these pieces, forming small projections, give to the upper enc of the cell the appearance of consisting of caps placed one over another, while th( lower end of the cells appears to be enclosed in a long sheath (the lower old piec( of cell-wall). This lower part of an elongated cell is always separated by a septum fron the upper cap-bearing piece (Fig. 17, p. 22). In Bulbochaete the growth of all th( shoots, even of the first which proceed from the spores, as far as it is connected witl cell-multiplication, is limited to the division of their basal cell ; so that the cells of eacl shoot must be considered at the same time as basar cells of the lateral shoot whicl stands upon them. The cells contain chlorophyll-grains and nuclei in a parietal layei of protoplasm. The Reproduction of the CEdogonieae takes place by asexual swarm^ spores and by oospores produced sexually. Both are formed, like the spermatozoids in the cells of the filaments. An alternation of generations takes place in the fol- lowing manner. From the oospores which have remained at rest for a considerabk period several (usually four) swarm-spores are immediately formed, which produce asexual, /. e. swarm-spore-forming plants, from which again similar ones proceed, unti the series of them is closed by a sexual generation (with formation of oospores) ; bul the sexual plants produce swarm-spores as well. The sexual plants are either monoeciouj or dioecious ; in many species the female plant produces peculiar swarm-spores (Andro- spores), out of which proceed very small male plants (dwarf males). Several generative cycles or only one may be completed in a vegetative period. The Swarm-spore \i formed in an ordinary cell of the filament (sometimes even in the first cell, Fig. 162, E) b) the contraction of its whole protoplasmic substance ; it becomes free from the mother- cell, the cell-wall splitting by a transverse slit into two very unequal halves (as in the division of the cells) (Fig. 162, A, B, E). It is at first still surrounded by a hyaline mem- brane, which however it also breaks through. The swarm-spore is encircled at its hyaline end — the anterior end during the swarming — by a crest of numerous cilia. This end lies laterally in the mother-cell, and, after the movement ends, becomes the lower attached end which grows out into a rhizoid. The dir-ection of growth of the new plant is thus at right angles to that of the mother-cell. The Spermatozoids are very similar in form to the swarm-spores, but much smaller (Fig. 163, D, 2); their motion, due to a crest of cilia, is also similar. The mother--cells of the spermatozoids are cells of the filament, but shorter and not so rich in chlorophyll as the vegetative 1 [Thuret divided the olive-coloured sea-weeds (Melanosporea?) into two groups, of which the Phceosporece (Laminaria, &c.) are distinguished by possessing zoospores, the Fucaceoe being desti- tute of them. — Ed.] 2 Pringsheim, Morphologic der CEdogonieen in Jahrb. fiir wissen. Bot. vol. I. [Ann. des Sci. Nat. 1856, vol. V. p. 251.— Carter, Ann. and Mag. Nat. Hist. 1858, vol. I. pp. 29-39.] 30 THALLOPHYTES. cells ; they lie either singly or in groups (sometimes as many as twelve) above one another in the filament. In most species each mother -cell of this description (antheridium- cell) divides into two equal special mother-cells, each of which produces a spermato- zoid ; they escape by the splitting of the mother-cell (as in the case of the zoospores) (Fig. 163, D). The androspores from which the dwarf male plants arise are produced from mother-cells similar to those which give birth to the spermatozoids (without formation of special mother-cells). After swarming they fix themselves to a definite part of the female plant, on or near the oogonium, and after germination produce at o«ce the antheridium-cells, and in them the spermatozoids (Fig. 163, A, B, m, m). The Fig. 162.— Development of the swarni-spores of CEdogonium (after Pringsheim). A, B their origin from an older filament ; C free swarm-spore in motion ; D commencement of its germination ; E a swarm-spore formed out of the entire con- tents of a germinating plant (X350). Fig. 163.—^ CEdogonittm ciliatum (X2S0) middle part of a sexual filament with an antheridium (m) at the upper end, and two fertilised oogonia (og) by the dwarf male plant ?«; R oogonium at the moment of fertilisation; o the oosphere, z the spermatozoid in the act of forcing its way in, m dwarf male plant ; C ripe oospore ; D piece of the male filament of O. ge7nell2pcirzini, z spermatozoids. E branch of a hybernated plant oi Bidbochate intertnedia, with one oogonium still containing a spore, another in the act of allowing it to escape; in the lower part an empty oogonium. F the four swarm-spores resulting from an oospore ; G swarm-spores from an oospore come to rest (after Pringsheim). Oogofiium is always developed from the upper daughter-cell of a vegetative cell of the filament which has just divided, and immediately after the division swells up into a spherical or ovoid form. In Bulbochaete the oogonium is always the lowest cell of a fertile branch. This is not opposed to the law of growth above-mentioned, inasmuch as the mother-cell of a branch fulfils at the same time the function of its basal cell ; the oogonium of Bulbochaete is never the first cell of a branch, since this is always developed as a bristle. The oogonium becomes at first more completely filled with contents than the remaining cells ; immediately before fertilisation the protoplasm contracts and ALGM. 231 forms, as in Vaucheria, the oosphere, in the interior of which the grains of chlorophyll are densely crowded. The part of the oosphere which faces the opening of the oogonium consists simply of hyaline protoplasm. The opening of the oogonimn is produced in a variety of ways. In some species of Qildogonium and all of Bulbochaete its wall has an oval hole in its side, out of which the colourless part of the oosphere protrudes in the form of papillae, and takes up the spermatozoids. In some species of CEdogonium (Fig. 163, A, B), on the other hand,- the oogonium-cell splits, as when the swarm-spores are escaping ; and the otherwise straight row of cells of the filament thus appears as if broken at this spot. In the lateral crevice appears some colourless mucilage, which the observer can actually see take the form of an open beak-like canal (Fig. 163, 5, a), through which the spermatozoid enters. It mixes with the hyaline part of the protoplasm of the oosphere while it melts away. Imme- diately after fertilisation the oosphere sur- rounds itself with a membrane, which after- wards, like its contents, assumes a brown colour ; but in Bulbocha!te the contents of the oospore thus formed is of a beautiful red colour. The oospore remains enclosed in the membrane of the oogonium, which separates from the neighbouring cells of the filament and falls to the ground, where the oospore passes its period of rest. When it awakes to new activity, the oospore does not itself grow into a new plant ; but in Bulbochxte, where this process has been observed, its contents divide into four swarm-spores, which escape together with the inner skin of the oospore, and after this latter is dissolved, swim about. After becoming stationary each grows into a new plant'. The Coleoclisetee 2 are small (about 1-2 mm.) fresh-water Algae, chlorophyll- green and constructed of branched rows of cells, attached in standing or slowly run- ning water to the submerged parts of other plants {e.g. Equisetum), and forming circu- lar closely-attached or cushion-like discs Their chlorophyll assumes the form of parietal plates or of larger lumps ; and the name of the genus Coleochsete (sheath- hair) is due to the circumstance that certain cells of the thallus form lateral colourless bristles fixed in narrow sheaths (Fig. 164, J, F). If the phenomena of growth of the different species are compared, two extreme cases are seen, united by Fig. 164.—^ an asexual plant of Coleochate soluta (x^^o) B a piece of a similar disc ; the letters a-g indicate the sue cessive dichotomous branchings of the terminal cells (after Pringsheim). ^ [Confervacese are a group of green filamentous Algae. They are reproduced by zoospores (Cladophora, Chroolepus). Colin has described the sexual reproduction of Sph" ''°™'" '^'^'''ain por- siace=e they are, like the tetZores n 0^.1 ""^ ''^"" °' ''"^' '" «><= M^'^be- overarching of ihe surround ng t,s ue 'xhe rounH- 7""" "'"' ^''^ f°™^'* "^ ">« do not swar™, but are move/ air/pasliv:,,.;"; the ^"''^ '"^ "° ^'"^ ^"'' fert;!;strfi:sf o^a-rs™^^^^^^^^^^^^ f r-'') -""'•- ^- '- cording to Bornet and Thuret ' Thl t "f" ' '^"'""' '""''*"■'■'■''"'> ac- an'lp^jS-brtl'tusTowfrtT^n ;iri^^^^^^ ^5°^- "^ ^ -^" ^'^^ ^^^ --ping filament with a root-hair and the creeping sten,'its apicalce" si^ua e/at ^and it -r T^'l" ^ k^' ^ ' '"^"'' <™°"--°-) P'-t ; .. a root-hair of row of cells being however no indie ved-/; trlhn ^P'^!f-''^'b""=hesbearmg the sexual organs; a« antheridia. the axial the cystocarp ; Ja spore escaped f on, tt X-stT.^^T ^ ^' '"^^ °^ '^^ ^"^"^ ' °' "'<= ^^""^ ^"-^"^^ ^ ^^ ^"^ en^-^'°P- °f H- /' P ^e-^^aped from the cystocarp; Can empty cystocarp, its envelope consisting of rows of cells. ^0 ^^ich no longer grows, and a broader cell which splits up by longitudinal walls into nve cells, one central (axial) and four peripheral. One of the latter, the one turned away rrom the mother-filament, becomes coloured and filled with strongly refractive proto- plasm, and then divides by septa into three cells lying one over another, and composing ^ Bornet and Thuret (I.e.) discovered these remarkable processes tnchophore had long before been accurately described by Nageli. The structure of the 30 THALLOPHYTES. the Trichophorc. The uppermost of these cells elongates into a hair-like continuation, the Trichogyne {B, tg, where the septa are not shown), which grows up beside the apical cell (t) of the fertile branch. The three other peripheral cells divide, after the fertilisa- tion of the trichogyne, and develope into articulated branches, which grow upwards close to one another and form the peculiar ' Pericarp' (C) of Lejolisia. The spores arise in the centre of this pericarp as outgrowths of the central cell, the cells of the trichophore not participating in their formation. The trichophore is pressed aside by the cystocarp (B, h, tg)j and hence at a later period the trichogyne occupies a lateral position. The relative positions of the parts of the cystocarp are more clearly shown in the representation of Herpotbamnion hermaphroditum copied from Nageli (Fig. 167). On a primary branch st (in A) arises a branch a, bearing an antheridium {an) and a young cystocarp. The antheridium consists of an axial row of cells which is a prolongation of the branch, and of the very short branches which shoot from its members and bear the mother-cells of the spermatozoids, the whole being surrounded by a mucilaginous mass. The female branch first of all forms the lower cells b^ c, and ends in the apical cell i ; Fig. 167. — Herpotha7nnio7i heymaphrodition. A a branch with the rudiment of the cystocarp y and an antheridium an. fi the mature cystocarp after fertihsation (after Nageli in the Sitzungsberichte der k. bayer. Akad. 1861). the last cell between c and i forms the cystocarp, being divided into four peripheral and one axial cell by longitudinal divisions ; of the former the one facing the observer {g) and a lateral one on the left are shown ; another peripheral cell has become transformed by transverse divisions into a row of cells, which form the trichophore (/), and the tricho- gyne t. In Fig. 167, B, fertilisation has already taken place, the branch a does not here bear an antheridium. The cell c corresponds to the cell c in A, and the apical cell / with /in A\ the axial cell d corresponds to the one lying behind ^ in ^ ; the tri- chogyne, 2^, and the cells of the trichophore lying beneath are still visible. From the two lateral cells which in A lie next the trichophore, the masses of spores, g, h, have arisen by the formation of very short branch-systems ; beneath the cell c filaments shoot from the pedicel-cell a, and form the envelope of the cystocarp. It is clearly seen in both examples that, as the result of the fertilisation of the trichogyne, the masses of spores are produced not by this organ, bu:; by neighbouring cells which lie deeper and do not in any way belong to the trichophore, but which originate in the same way, and that the formation of the envelope of the cystocarp is also a consequence of fertilisation. The fertilisation of the trichogyne takes place more immediately in the Nemalieae to which Batrachospermum belongs^. In them there is no trichophore, but ^ [Sirodot (Compt. Rend. May and June 1873) has found that the spores of Batrachospermum produce a Chantransia from which again the Batrachospermum is developed. — Ed.] ALGm. 237 the trichogyne replaces it. The lower swelling itself produces after fertilisation (ac- cording to Bornet and Thuret) the cystocarp, numerous articulated branches sprouting from it and forming a spherical ball (the Glomerulus), and the terminal members of this produce the spores ; while beneath the trichogyne enveloping branches also arise (for further details on Batrachospermum see Solms-Laubach in Bot. Zeitg. nos. 21, 22, 1867). Thuret and Bornet found the most complicated and remarkable process of fertilisation in Dudresnaya. Here the cystocarps arise on altogether different branches from the trichophore ; after the long spiral trichogyne at their base has been fertilised, branches shoot out from beneath it, which grow across to the fertile branches ; each fertile branch has a spherical apical cell, and the ' tube connecteur ' applies itself closely to this cell and afterwards continuing its growth becomes successively united with several other fertile branches. At the points of union the articulated 'tube connecteur' coalesces with the apical cell of the fertile branch, and the wall of both disappears. The part of the ' tube connecteur ' which has thus conjugated swells up and becomes filled with proto- plasm, which is separated by a wall and now produces the cystocarp. The ' tubes connecteurs' thus convey the fertilising power from a trichogyne to the other fertile branches, and produce cystocarps by conjugation with them. The act of fertilisation itself consists, in all Florideae, of a conjugation of the roundish spermatozoid with the trichogyne ; /. e. the spermatozoid comes into contact with the trichogyne, the wall becomes absorbed at the spot, and the contents of the spermatozoid pass over into the trichogyne. This process of fertilisation takes place in the Nemahe^e at the base of the trichogyne itself; in the Ceramiaceae and others in adjoining cells; in Dudresnaya in altogether different branches by means of the 'tube connecteur.' While the simpler processes in the Nemaliea? may be compared with corresponding processes in ColeochcTte, the origin of the cystocarp of Lejolisia and Herpothamnion and of the more robust Plorideae reminds one of the origin of the receptacle produced by fertilisation in the Pczizse and Erysipheae among the Fungi ^ ' [Porphyrese. Janczewski (M.'ni. de la Soc. Nat. dc Cherbourg, vol. XXI. p. 345, 1872, Ann. des Sci. Nat. 1873, vol. XXII) describes the reproductive organs oiPorphyra leucosticta and P. laciniata. In the former the frond, consisting of a single layer of cells, produces octospores by the division of the contents of marginal cells. The octospores are set free by the softening and deliquescence of the mother-cell-walls and of the septa between them. When free they are destitute of a cellulose investment, and move by slow contractile changes of shape, only, however, very rarely putting out short pseudo- podia. The octospores finally come to rest, develope a cell-wall, and germinate. The spermatozoids are developed in cells like those which produce octospores ; there are usually however sixty-four from each mother-cell ; they are spherical when free, destitute of a cell-wall, and without any mobility. Occasionally a portion of the contents of a mother-cell is converted into octospores, and the rest into spei-matozoids. Porphyra laciniata differs from the preceding species in being dioecious. The segmentation of the contents of the mother-cells producing octospores is not, however, fully carried out. The antheridial mother-cells only produce thirty-two spermatozoids. The protoplasmic contents of the cells in the Porphyrece are coloured violet by iodine solution (with KI). The endochrome is a mixture of chlorophyll and phycourythrine. Porphyrece appear to be connected with the Floridese through the Dictyotex, all three agreeing in the immobility of their spermatozoids and spores (disre- garding the amoeboid movements of the latter), but distinguished by their female organs, which are quite distinct in the Dictyotex from those of the Floridece, and perhaps do not require fertilisation ; while they are absolutely wanting in the Porphyrece, as far as our present knowledge extends.— Ed.] 2'^H THALLOPHYTES. CLASS IL F U N G p. The structural element from which the thallome of Fungi is built up consists of cellular filaments destitute of chlorophyll, endowed with apical growth, only rarely branching dichotomously, more usually abundantly by lateral shoots. These ele- mentary constituents of Fungi are called HyphcB. It is only in a single group of Fungi — forming the transition from the Siphoneae among the Algae to the typical Fungi — the Phycomycetes, that the hypha consists of a single undivided cell ; in all other cases it is divided by transverse septa. The Hypha is thus usually a branched row of cells destitute of chlorophyll wdth a growing apical cell which divides transversely ; intercalary transverse divisions, however, also occur in the cells. In the simplest forms, which have been termed Haplomycetes, including however mere conditions of development of higher forms, the whole thallome con- sists of a single hypha usually very much branched. The massive compact sub- stance of many Fungi is formed by the aggregation of numerous hyphae having a common growth ; the larger Fungi are, without exception, examples of this. The hyphae either run parallel to one another, or their numerous ramifications are inter- woven in the most various modes. If these textures are very dense and the joints of the hyphse therefore short and thick, and of a polyhedral form from pressure on opposite sides, the mass assumes the form of a parenchymatous tissue, the origin of which from hyphae justifies its appellation of Pseudo-parenchyma. It is especially developed on the surface of larger Fungi as an epidermal system. When the substance of a Fungus consisting of a number of hyphae grows in length forming an apex at one point, this — as follows from what has been said — ' can never take place by means of otte apical cell, but a certain number of hyphae reach to the apex, where each lengthens by apical growth but in unison with its neighbours. If the substance of such a Fungus spreads out in the form of a disc growing at the margin, this is occasioned by the hyphae proceeding from a centre lengthening radially and ramifying laterally in proportion to the growth of the circumference. Ramification rarely occurs in Fungi of this kind ( ripe oospore ; E,F,G formation of swarm-spores from oospores ; z endospore (after De Barj') ( x 400). Peronospora simple spores, in others (as P. infestans), and in all species of Gystopus, they are not immediately capable of germination, but when in contact with water, as for instance drops of dew or rain, develope several zoospores (Fig. i68, C, Z), -E, i^). In some species cf Gystopus the terminal member of each row of conidia is capable of is in fact one of true conjugation. Where there are no lateral branches fertilisation would seem to be required by means of antherozoids. Max Cornu believes that the supposed spermatozoids discovered by Pringsheim really belong to an endophyte of the group ChytridinecB, which have often been iden- tified with organs of the plants they infest. He thinks it probable that the spermatozoids closely resemble the zoospores in appearance, and have been overlooked in consequence. This is confirmed by the process of fertilisation in Monoblepharis, the spermatozoids are half the size of the zoospores, but of the same form. They creep with amoeboid movements over the wall of the oogonium and fertilise the oosphere by blending with it, (See also Archer, Quart. Journ. Mic. Sc. 1867, p. 121. —Ed.] ^ De Bary, I.e. p. 176, and Ann. des Sci. Nat. 4th series, vol. XX. FUNGI. 245 at once forming a filament in germination. The zoospores of Peronospora infestans are firmly attached, after swarming, to the cuticle of its host, surround themselves with a thin membrane, and penetrate by a small hole the outer wall of the epidermis (Fig. 168, i/, j/>), through which the germinating filament penetrates into the epidermis-cell with the whole of the protoplasm of the zoospore, and then, again piercing through the wall of the epidermis-cell, reaches the intercellular space. The zoospores of Cystopiis candidus are firmly attached near the stomata and push their germinating filaments into its orifice, and thus find their way at once into the intercellular space ; but unless the spores have been sown upon the (green) cotyledons of the host {Lepidium sati'vum^ Cap- sella) they do not develope a mycelium. When the mycelium has once been formed in the parenchyma of the host, it continues to grow in it, and finally often spreads through the whole plant, putting out its conidia-bearing branches at various places in the stem, leaves, or inflorescence. In this manner the (unicellular) mycelium of P. infestans, for instance, can even hibernate inside the tubers of the potato, to undergo further development into germinating filaments in the following spring. The sexual organs of the Peronosporeae are developed in the interior of the tissue of their host. Spherically dilated ends of branches of the mycelium shape themselves into oogonia (Fig. 169, A, og\ in each of which an oosphere is formed out of a definite portion of the protoplasm {B, os). From another branch of the mycelium a branchlet grows towards the oogonium, swells, and becomes closely attached to it ; and the thicker part becoming separated by a septum (just as takes place with the oogonium itself), developes into an Antheridium. As soon as the oosphere is formed, a fine branch of the antheridium (5, ««) reaches it by penetrating the membrane of the oogonium. After fertilisation the oosphere be- comes surrounded by a coat which thickens and forms an external rough dark-brown layer (the Exospore) and an inner one (the Endospore). These oospores remain dor- mant through the winter and then germinate ; in the case of Peronospora Falerianella, they form a mycelium directly on moist ground ; those of Gystopus, however, produce zoospores, the endospore (/) forces itself like a bladder out of the ruptured exospore (Fig. 169, F), and then bursting, the zoospores (G) are set free, which behave in exactly the same manner as those produced from conidia. (3) Among Mucorini\ Rhizopus nigricatis {Mucor stolonifer') may be specially men- tioned. It infests dead or dying parts of plants, especially fleshy fruits, which quickly decay in consequence of its attacks. The mycelium is from i to 3 cm. long, and forms stolon-like filaments, which are closely attached to the substratum by root-like branches that afterw^ards become septate, while each of the ascending branches 2 to 3 mm. in height bears a sporangium. The ends of these branches swell up into a sphere and become filled with protoplasm ; the septum which separates this swelling from the branch becomes arched convexly into the cavity of the sporangium, in which numerous small spores now arise. These are set free by the giving way of the wall, and ger- minate only upon a substratum capable of nourishing them (not in pure water) ; putting out at once a germinating filament. They can, however, preserve their germinating power for months, if kept dry. When the mycelium has produced a number of spo- rangium-bearing filaments, the fomiation of the Zygospores begins beneath the white felt- like texture formed by it ; where two of the firm mycelium-filaments touch, each puts out a swelling which is in close contact with that of the other. Both grow in this manner to a considerable size, and assume the form of a club ; a septum then forms in each, by which the thick end is cut off as a conjugating cell. One of the two conjugating cells, which are in contact for some distance, is smaller than the other ; the wall which separates them is then absorbed ; and the two cells coalesce into a single cell (the ^ De Bary und Woronin, Beitrage zur Morph. u. Phys. der Pilze, p. 25. Frankfort 1866.— On Pilobolus crystalli?ms, cf. Cohn, Nova Acta Acad. Nat. Curios, vol. XV, pt. i. p. 510. [Klein in Pringsheim's Jahrb. vol. VIII. — Van Tieghem and Le Monnier, Ann. des Sci. Nat. 1873, vol.XVlL] 246 THALLOPHYTES. Zygospore), which then increases (to i mm.) and takes the form of a sphere flattened by the two supporting-cells. The exospore is thick and of a blue-black colour. The formation of zygospores takes place in May, June, and July, on stone-fruit and berries, and takes twenty-four hours for its completion. The germination of the zygospores has been observed in another genus, Sporidinia grandis {Mucor Syzygites), which infests fleshy Fungi. In this case tBiey form a filament, on which is developed a system of sporangia with asexual spores ; these then produce a mycehum which forms first zygospores and then again asexual spores. An alternation of generations thus takes place ^. II. The Hypodermic 2^ The best known species of this order, Puccinla graminis, belonging to the family Uredineae, may be taken as its type. Its development not only shows a distinct alternation of generations (although no sexual organs are as yet known), but also in combination with it, the hetercecism which occurs also in some other Fungi, but is not elsewhere so clearly defined. De Bary has given the term Hetercecism to that peculiarity by which one generation of a parasitic Fungus is developed exclusively on one host, or only on those which belong to a particular group, while another stage of development of the same species occurs only upon a different host. On the leaves of BerberLs 'vulgaris are found in the spring yellowish swollen spots, where dense masses of mycelial filaments are interposed between the parenchyma-cells (Fig. 170, ^ and /, the felted mycelium, lying between the cells, being indicated by dots). In these swollen spots are found two kinds of fructification, the Spermogonia, which are produced somewhat earlier, and the j^cidia. The spermogonia (Fig. 170, J, j/») are urn-shaped receptacles surrounded by a layer of mycelium as by an envelope ; hair-like threads which clothe the cavity protrude in the form of a brush from the opening of the spermogonium, penetrating the epidermis of the leaf; the bottom of the spermogonium is covered with short mycelial branches, from the ends of which are detached numerous very small spore-like bodies, the Spermatia. The second form of ^ [On the Mucorini, the Memoirs of Brefeld (Botanische Untersuchungen iiber Schimmelpiize, Leipzig 1872), and Van Tieghem and Le Monnier (Ann. des Sci. Nat. 1873, vol. XVII, and Quart. Journ. Micr. Sc. 1871, pp. 49-76), should be consulted. The following particulars are extracted from the last cited memoir. The mycelium of the Mucorini always originates from an asexual spore. The zygospores never give rise in germinating to a mycelium, but always produce, as in other Fungi, and as also in Muscineae, an asexual reproductive apparatus. The mycelium is at first always destitute of partitions ; later, as the protoplasm disappears, septa make their appearance irregularly. The filaments occasionally anastomose ; they may be wholly immersed in the nutrient medium or partly aerial. The mycelium of some species {e. g. Chaetocladium), which are normally nonpara- sitic, have also the capacity of fixing themselves on the mycelium of other species and living para- sitically. All the Mucorini develope sporangia upon aerial extensions of their mycelium, in which asexual spores originate by division of the protoplasm. In some genera {e. g. Thamnidium) these sporangia are of two kinds, but the spores they contain are similar. Peculiar asexual spores (Chlaray- dospores) also arise by local condensation of the protoplasm within the mycelium and in different positions. A single large echinulate or tuberculate chlamydospore may be formed within the extre- mities of all the branches of the aerial hyphae ; and this may for a long time be the only mode of reproduction exhibited (Mortierella). Zygospores arise from a true conjugation. They have been observed in six genera : Sporodinia (Ehrenberg, 1829), Rhizopus (De Bary, 1866), Mucor {M.ftisiger, Tulasne, 1866, M. Mticedo, Van Tieghem and Le Monnier, Comptes Rendus, Apr. 8, 1872), Phyco- myces (Van Tieghem and Le Monnier, 1872), Cha^tocladium and Piptocephalis (Brefeld, 1872). Before germination the zygospore requires a certain period of dryness and rest. After again becom- ing moist it pro,.—Agaricus cainjrestris (natural size). beneath the lamellae to the margin of the pileus, into which their hyphae are continued. When at length the pileus' extends horizontally from the elongation of the tissues, the membrane (volva) becomes detached from its margin, and hangs from the stem like a ruffle. (Compare also Fig. 68, p. 8i, Boletus Jlavidus .^ The hymenium, as has already been mentioned, covers the surface of the lamellae- form peg-shaped or tubular projections of the under-side of the pileus. A trans- verse section of the latter across the hymenium gives, in all three cases, nearly the same figure, as is seen in Fig. 174, drawn from ylgaricus campestrh. ^ shows a piece of the disc of the pileus cut transversely, b the substance of the pileus, / the lamellae ; in 5 a piece of a lamella is more strongly magnified to show the course of the hyphae. The substance of the lamella, called the Trama (/), consists of rows of long cells, which FUNGI. 251 diverge from the centre right and left to the outside, where the cells of the hyphae are short and round, and form the sub-hymenial layer {sh in B and C). From these short cells spring the club-shaped cells (^), densely crowded and at right angles to the surface of the lamella, forming together the hymenial layer {B, bj). INJany of these remain sterile, and are called Paraphyses, others produce the Spores and are the Basidia. Each basidium produces in this species only two, in other Hymenomycetes usually four spores. The basidium first of all puts out as many slender branches (/) as there are spores to be formed ; each of these branches swells at the end, the swelling in- creases and becomes a spore (/', /"), which falls from the stalk on which it was placed, leaving it behind (/"'). On the formation of the tissue of this group only one further remark need be made ; that in the receptacle of some Agaricineae {e.g. Lactarius) some of the much-branched hyphae are transformed int3 laticiferous ves- sels, from which large quantities of latex flow out when injured. (2) The Gasteromycetea agree with the previous group in the mode of formation of their spores (eight spores are often produced on a ba- sidium) ; but their receptacles are always angiocarpous. The hymenia are formed in the interior of the receptacle, which is at first usually spherical, or at any rate is not di- vided externally into distinct parts. The spores are disseminated by means of remarkable dificrentiations of the different layers, the growth of par- ticular masses of tissue, or the simple bursting of the outer layer (the Peridium). The nature of these processes, which are extremely vari- ous in their external appearance, may be understood from two ex- amples. The first example, Cruci- bulum imlgare \ is selected from the beautiful Nidularieae -. The my- celium forms a small white crust of branched hyphae, which creep over the surface of wood. In the middle of the crust the filaments are interwoven into a roundish body, the rudiment of the receptacle ; this grows by the intercalation of new branches of the hyphae, and gradually assumes a cylindrical form. The outer threads form at an early stage yellowish-brown branches, which are again branched and directed outwards, forming a dense covering of hair. While the receptacle is becoming changed into a cylinder, a large number of brown threads shoot out from it externally to this (Fig. 175,(7, r/"), which form a firmly-woven layer, the outer peridium, and on the outside a dense mass of Fig. iT\.—A/grat'ictis campestris ; structure of the hymenium ; A, B slightly magnified ; C a part oi B (X3S0). The protoplasm is indicated by fine dots. Compare Sachs in Bot. Zeitg. 1855. [See also Tulasne, Annales des Sci. Nat. 1844, vol. I, p. i.] 2''i2 THALLOPHFTES. erect hairs. The walls of the hyphae of this part assume a dark colour, but the inner tissue remains colourless (Fig. 175, ^); its apex increases in breadth, the hairs separate from one another, and the outer peridium ceases to exist at the apex (Fig. 176, ap). In the meantime the differentiation of the tissue commences in the interior of the Fungus, ^^'hich is at first formed of densely-woven much-branched hyphae, enclosing amongst them a considerable quantity of air which gives the whole a white appear- ance. Certain portions of the air-containing tissue become mucilaginous and freed from air ; between the threads is formed in some places a hygroscopic transparent jelly, while in others none is produced. The conversion into mucilage begins first below the surface of the white nucleus (Fig. 175, v^), and its outer layer is thus trans- formed into an inner peridium which is a colourless sac protruding from the dark outer peridium, and composed chiefly of branches of hyphae running longitudinally upwards (Figs. 1 76 and 177, //>). While this differentiation is proceeding from below upwards. riG. 175. — Crucihulum viilgare; A, B, C m longitudinal section (slightly magnified) ; D the entire plant nearly mature (natural size). " / Fig. 176. — Crucibiihint vulgare; longitudinal section through the upper part of a young receptacle (x about the same as Fig. 175, B). The section is seen by transmitted light ; the dark parts in the interior are those where air occurs between the hypha; ; at the light parts a transparent mucilaginous substance free from air has formed between the hyphjE. The light parts of this figure are dark in the previous one. small mucilaginous areolae form at certain points in a deep layer of the white air-contain- ing nucleus, also proceeding from below upwards, like all the succeeding differentiations (Fig. 175, B, and Fig. 176). The formation of mucilage advances at the same time from the inner peridium inwards, and leaves round each of the mucilaginous areolae a border of air-containing tissue (Fig. 176), which afterwards developes, by the dense interweaving of its branched hyphae, into a firm envelope consisting of two layers, in which the muci- laginous areola lies. For want of a better term, it may be called the Sporangium. While the centre of the Fungus is becoming changed into mucilage, the sporangia grow into lenticular bodies ; a mucilaginous point has appeared at an early stage on the lower and outer part of each sporangium, and forms its umbilicus. From it a denser bundle of threads runs downwards to the peridium, the umbilical bundle (Fig. 176, «, and Fig. 177, ns) ; this is itself surrounded by a conical bag {t) which surrounds the bundle like a loose sheath. This sheath eventually becomes mucilaginous ; the bundle runs upwards into the mucilaginous depression of the umbilicus, where it is resolved into its threads which are now more loosely connected. The mucilaginous tissue in the interior of each sporangium disappears, leaving a lenticular space similar in form to the sporangium itself; FUNGI. 253 and from the inner layers of the hyphae of the sporangium branches now arise which are directed inwards and form the hymenium. Each sporangium is therefore clothed on its inner surface by a hymenial layer formed of paraphyses and basidia ; each of the basidia produces four spores on short stalks. As the Fungus matures, the upper part of the peridium becomes stretched and flat, forming the Epiphragm, it afterwards ruptures and disappears, and the Fungus thus opens into a cup. The mucilage which sur- rounds the sporangia dries up, and the sporangia now lie free in the cup formed by the peridium, held by their umbilical bundles, which, when moistened, may be drawn out into long threads. If we imagine the sporangia more nume- rous and more closely packed and with less dense walls, we obtain an explanation of the roundish cell-like loculi which occur in the receptacles of other Gasteromycetes (as Octa- viania, Scleroderma, &c.). Still more remarkable are the changes produced in the Phalloi- desB by internal dilTerentiation of the tissues ; but of these only the most important points can be illus- trated in the case of Phallus impu- d'lcus. Here also the young recep- tacle, formed on the underground perennial mycelium which consists of thick threads, is at lirst a homo- geneous convolution of threads, in which the differentiation begins and advances during growth. When the body has attained the size and form of a hen's or even a goose's Q^'g^ a longitudinal section gives the appear- ance represented in Fig. 178. The tissue consists at this time of dif- ferent portions which may be classi- fied into four groups — (i) The Peridivmi, composed of an outer firm, thick, white membrane (a), of an inner white, firm, but thin mem- brane (/■), and of an intermediate thick layer of mucilaginous hyphae (^) (the gelatinous layer). (2) The Spore-forming apparatus or Gleba (j/»), bounded on the outside by the inner peridium (/'), on the inside by a firm thick layer {t) from which walls project outwards united in a honeycomb manner dividing the gleba into a number of chambers. In these chambers the fertile branches of the hyphse are found in great numbers, and on their basidia are formed four or more spores ; so that, when ripe, the dark-green gleba appears to consist almost entirely of spores. (3) The Stem {st\ formed of air-containing tissue hollowed into a large number of very narrow chambers ; it is hollow, that is, its axial portion is transformed into a deli- quescent jelly ; the canal thus formed is open above in some individuals, in others it is closed by the inner peridium. (4) The Gup («) forms a low broad column of firmer tissue, the outer part running upwards into the inner peridium, and sending up at the same time a layer which becomes softer between the stem and the inner membrane of Fig. xii .—Crucibultim vulgare; longitudinal section through the upper part of the right side of the mature receptacle, showing the course of the filaments ; for the sake of clearness tlie number of fila- ments has been reduced and their thickness increased. '-5M 1 n/\L,jjUi-nr i tL^. the gleba (/); the base of the cup is continuous with the outer firm peridium. In this state the spores ripen ; but for the purpose of their dissemination a great elon- gation of the stem {st^ takes place ; the peridium is ruptured at the apex, the gleba becomes detached from the inner peridium, this latter splitting at ^, and the mem- brane t becoming detached below. The gleba is by this means raised up high above the peridium on the apex of the stem, while the stem attains the height of from 6 to 12 inches. This elongation is brought about by the widening of its chambers, which give the mature stem the appearance of a coarsely porous sponge ; it increases in thickness in proportion to its increase in length. The spores now drop off the gleba in masses, the sporiferous hyphae deliquescing into thick tena- cious mucilage ; till at last nothing remains of the gleba but the membrane (/) with its honeycombed walls, which depends like a frill from the apex of the stem, and is called the Pileus. The peculiari- ties in the detail of these processes exhibit the greatest variety in different species of the Phalloi- deae, which may be investigated in Corda, /. f ., and De Bary, I.e. p. 84. IV. The AscoMYCETES comprise a greater variety of forms than any other order of Fungi. Commencing with very simple forms comparable to some unicellular Algap, as Endomyces, Saccha- romyces, and Exoascus, they ascend to the truffles, morells, and Sphaeriaceae with receptacles formed of great masses of hyphae, the internal and external FIG. i78.-Longitudinal section of a nearly ripe gtrUCtUrC of which is SO VariOUS that a COmprC- plant of Phallus inipudictis immediately before the ^ elongation of the stem (.i the natural size) ; a outer hcusivc dcSCriptioU of them is impOSSiblc. The layer of the peridium; ^4> its gelatinous layer ; zinner , j^ • i- i i • i ^^ ,^ i-rr peridium; st the stem of the pileus/ not yet eion- commou charactcristic by which all thcsc different gated, covered by the white honeycomb-like ridges ; f^^^c j,_p rf>nnprf-P(i is thp n<;PVlinl formation of sp the dark-green mass of spores (gleba) ; h hollow lorms are connectCQ IS inc ascxuai lormaiiou or cavity of the stem, filled with watery jelly ; n the cup sporcs iu thc iutcrior of sacs by frec-cell-formation. in which the base of the stem remains after its elon- gation ; x the place where the inner peridium be- ThC AsCOSpOrCS, hOWCVCr, bclOUg Only tO OUC comes detached by the elongation of the stem ; 7n „„„„^„4.r^„ ;„ j.i,« ^.,^1^ „r J„ „1„^„,„„4- ^C „ „„^ mycelial thread. generation m the cycle or development or a spe- cies ; for in large sections of the Ascomycetes there occur in addition Stylospores of a very different nature. The course of develop- ment generally shows in these cases a greater variation within a single species than occurs among the Hymenomycetes ; and in many cases an alternation of generations has already been recognised, in so far as the receptacles in which the ascospores are produced owe their origin to a conjugation or sexual union which takes place on the mycelium (as in Erysiphe, Peziza, Ascobolus, Eurotium, &c.). Want of space compels me to limit my special descriptions to examples of only a few families of the order. (i) The simplest forms of the Ascomycetes are the Yeast-fungi or Ferments of the genus Saccharorayces \ which cause the alcohohc fermentation of the saccharine juices of plants (must, cider, &c.), of beer or of artificial solutions which contain sugar in addition to nitrogenous substances (albuminoids or ammonia-compounds) and mineral substances which form the food of plants. These Fungi consist of small roundish or ellipsoidal cells, which grow in fluids, and, in nourishing themselves, cause their de- composition, with formation of alcohol, carbonic acid, and other substances. Each ^ Max Rees, Botan. Unters. iiber die Alkoholgahrungspilze. Leipzig 1870. [Compare also Huxley on Yeast, Contemp. Rev. Dec. 1871 ; Pasteur, New Contributions to the Theory of Fer- mentation, Comptes Rendus, 1872, pp. 784-790, and Quart. Journ. Micr. Sc. 1873, p. 351.] FUNGI. ^55 yeast-cell produces similar new cells by the protrusion of small projections at first re- sembling warts, which soon attain the form and size of their mother-cell, and sooner or later become detached from the narrow points of union. Usually they remain for a time united, and thus form combinations of shoots which may perhaps be considered as branched hyphae with short, roundish, easily detached segments. When the supply of nourishment is less abundant, — for instance, when the yeast is grown on cut slices of potato, turnip, Jerusalem artichoke, or carrot, the yeast-cells grow to a more considerable size, their protoplasmic contents produce, by free-cell-formation, from i to 4 roundish spores, which, when placed in a fermentable fluid, immediately form new yeast-cells by branching and the detachment of the terminal cells. The fermentation of beer is produced by Saccharomyces Cereuisicp, which occurs in two (cultivated) varieties, as yeast of the lower fermentation, which take place between 4° and 10° C, and as the yeast of the higher fermentation, which takes place at higher temperatures. The fermen- tation of wine and cider is caused by S. ellipsaideus, conglomeratus , exiguus, Pastorianus, and apiculatus, which are formed, together with other Fungi, on the surface of fruits, and thus find their way into the expressed juiced (2) The Tuberaceee form, like most Gasteromycetes (with which they may easily be confounded by the beginner), roundish, tuberous, bodies usually underground and often surrounded by the copiously branched mycelium. Nothing is known of the first appear- ance of the receptacle from the mycelium, and the development of the mycelium from the spore has also not been followed ; no other kinds of spores than the ascospores have been met with. The receptacle is always angiocarpous. It consists, when mature, of an outer more or less thick pcridium, in which an inner and an outer layer are usually distinguishable, the latter often provided with beautiful protuberances, and of a tissue of hyphae enclosed within it, on branches of which the asci are formed. A very simple structure is shown in Hydnobolitcs. The receptacle here consists of a tissue formed of densely woven hyphae, in which are everywhere imbedded numerous spore-mother-cells placed upon the branches of the hyphae. Only the super- ficial layer of tissue, consisting of a fine down of sterile hypha^^, forms a kind of peri- dium. In Elaphomyces, where the pcridium is firm and more highly developed, a mass of slender hypha: with long cells springs from its inner side in every direction ; here and there these are united more densely into larger discs and bundles projecting inwards ; but there is no gleba divided into closed chambers. The cavities left in the tissue formed of slender filaments are everywhere loosely filled by the hymenial tissue, which consists of hyphaj 2 or 3 times thicker, formed of shorter cells, much bent and woven into balls, and bearing the asci on the ends of their branches. When ripe the whole hymenial tissue dissolves into jelly and disappears, while the mass of slender filaments remains as a delicate Gapillitium between the loose dust consisting of spores. In another group a sterile matrix may be distinguished in the interior with a number of groups or nuclei of hymenial tissue imbedded in it, in which are again imbedded a number of asci springing irregularly from the ends of the branches. In Balsamia there is a thick pcridium, and the interior is divided into many narrow curved air-containing chambers by means of thick plates of tissue which spring from the peridium, like the partition-walls of the Hymenogastreee among the Gasteromycetes. To this is also related the genusTuber; but the chambers clothed with the thick hymenium are very narrow, and much curved ^ [Protomyces, P. macrosporm, infests the foliage of some species of Umbelliferoe. Its myce- lium is coloured blue by Schultz's solution and produces spherical asci, which enclose great numbers of minute spores. These spores conjugate in pairs, and the zygospore emits a germinatmg filament, which penetrates the epidermis of the host, and developes a new mycelium producmg a new series of asci. See De Bary, Beitriige zur Morph. der Pilze, i Heft.— Ed.] 2 This and what follows is after De Bary, Morph. u. Physiologic der Pil: e, p. 91.— Compare also Tulasne, Fungi Hypogcei. Paris 1857. 2 --6 THA LL OPIIYTES. and branched. The section of a truffle shows a dark matrix (the fertile tissue), in which run two kinds of branching veins ; — the one opaque and destitute of air, com- posed of the main branches of the fertile hyphae, spring from the inner surface of the peridium ; the other white and air-conducting are prolonged to its outer surface. Hyphae of the adjacent tissue begin to grow into these last-mentioned cavities from an early period, and have a white appearance in consequence of the presence of air filling up the spaces between them. The peridium of truffles is a strong shell consisting of pseudo-parenchyma, the outermost cell-walls of which are usually of a brown or black colour. The Asci of the Tuberaceae are globular ; and the spores, furnished with spines or honeycomb-like projections of the exospore, arise in indefinite numbers during a considerable period, and are without nuclei. The formation of spores shows some peculiarities, \Ahich are described by De Bary {I.e. p. io6; cf. also Tulasne, Fungi Hypogaei)\ (3) The Pyrenomyeetes^ usually produce in their asci, which are mostly long ' and club-shaped, eight spores formed simultaneously ; they are not unfrequently septate. The asci are formed in the interior of small flask-shaped or roundish receptacles, which are here termed Perithecia. The contents of the perithecium are at first a deli- cate transparent tissue containing no air, which afterwards becomes compressed by, the asci and paraphyses. These spring from a hymenium which clothes the wall of the perithecium or includes only its basal portion. The perithecia are either open from the first (in Sphoeria typhina), or they are at first closed and afterwards form an orifice clothed with hairs, through which the spores escape (as in Xylaria) ; or finally the perithecium is ruptured to admit of their dissemination {e.g. Erysiphe). In one series of forms (Sphaeriae simplices, Pleospora, Sordaria, &c.) the perithecia originate singly or in groups from the filamentous inconspicuous mycelium ; in» others (as Clavi- ceps), a so-called Stroma is first formed, /. e. a pillow-shaped, cap-shaped, arborescent, or cup-shaped receptacle, in which the perithecia usually arise in large numbers (Fig. 811). Besides the ascospores in the perithecia, other forms of spores are also pro- duced by separation from the ends of filaments ; 'vi^.. (i) Conidia (also septate) on filiform receptacles which spring from the mycelium or the stroma (Fig. 180, c) \ (2) Stylospores, essentially like the conidia (simple or septate), formed in the interior of conceptacles which are termed Pycnidia ; and (3) Spermatia, formed in masses in depressed receptacles (Spermogonia), usually very small, often bacilliform or bent, appa- rently not capable of germination, and similar in their origin to the conidia and stylo- spores. The diff'erent forms of spores do not usually appear at the same time either on the same mycelium or the same receptacle ; generally first conidia, then spermogonia, then pycnidia, finally perithecia, although each member of the series (except the peri- thecia) may be absent. According to the most recent investigations of De Bary, Woronin, and Fuisting, it is probable that the perithecia of the Pyrenomycetes are always the result of a develop- ment caused by a peculiar sexual union not unlike that of the Florideae. At present this has only been observed with certainty by De Bary in the genera Eurotium and Erysiphe ; but in other genera very dissimilar in other respects to these, tnz. in Sordaria and in Sphoeria Lemannea, Woronin found similar processes of development on the ^ [Onygenacese are developed on animal substances, as feathers, horns, hoofs, hair, &c. The form of the general receptacle is that of a small round-headed nail. Externally it is smooth and the peridium is brittle, filled with branched threads producing asci at different points, which are soon absorbed, setting free the sporidia. See Berkeley, Outlines of Cryptogamic Bot. p. 272; Tulasne, Ann. des Sci. Nat. 1844, vol. I. — Ed.] 2 Tulasne, Selecta Fungorum Carpologia. Paris 1860-65. — Woronin and De Bary, Beitriige zur Morph. u. Phys. der Pilze, 3rd series (on Sordaria, Eurotium, Erysij^he, «S<:c.). Frankfort 1S70. — Fuisting, Bot. Zeitg. 1868, p. 179. FUNGI. ^57 mycelium, although the conjugation itself was not observed and the mode of origin of the asci remained doubtful. It is nevertheless certain that the receptacles of the last- named Pyrenomycetes are developed from an apparatus similar to the sexual organ of Eurotium and Erysiphe ; and earlier statements of Fuisting contain at least indications that in other Pyrenomycetes also the perithecia may be the result of a sexual process. Since the development of the Fungi belonging to this section undergo important modifications in the different genera, a comprehensive description would be altogether wanting in lucidity. I prefer therefore to explain the most important points in two very different examples. One of the simplest of the Pyrenomycetes is Eurotium repens (Fig. 179); and but very slightly dilFering from Eurotium is Aspergillus glaucus, the history of whose de- FlG. 179— Development of Eurotium "efens (after De Barj'). A small portion of a mycelium, with the conidia-bearing^ liypha; c and young ascogonium as; B tlie spiral ascogonium as with the antheridium /; C the same, beginning to be surrounded by the threa' lon- gitudinal section of the upper paj-t of A ; C transverse section through the sphacelia, w« its mycelium, b the branches from whicli the conidia are detached, 7v the wall of the ovary ; D germinating conidia, forming secondary conidia jr. {A, B, C after Tulasne; D after Kiihn). ' De Bary, Ueber die Fruchtentvvickelung der Ascomyceten, • Leipzig 1863, p. 11. — De Bary und Woronin, Beitr;;ge zur Morpliologie u. Physiologic der Pilze, and series, pp. i and 82, Frankfort 1866.— Tulasne, Annales des Sci. Nat. 5th series, vol. VI. p. 247. 1866.— Janczewski, Bot. Zeitg. 1871, no. 18. S 2 26o THALLOPHYTES. lays the surface of the folded pileus ; in Peziza it clothes the concavity of the cup, which is either flat and sessile (Fig. 182) or stalked. The hymenium consists of paraphyses and asci, in which eight spores are usually formed simultaneously ; the paraphyses generally appear earlier, but are finally crowded out by the asci. The spores sometimes possess nuclei, but are sometimes destitute of them (Fig. 182). The Disco- mycetes agree, however, with the Pyrenomycetes — from which they are principally distinguished by their gymnocarpous receptacles — in the occurrence of spermogonia, pycnidia, and conidia, as forerunners of the ascospores. In Peziza Dur'mana two kinds of receptacles have even been observed, one with larger ascospores, which put out germinating filaments, the other also with ascospores, which, however, form a pro- mycelium from which minute spores are detached. The variety of structure is further increased by the fact that many species produce sclerotia. Peziza Fuckeliana is a peculiarly interesting example. Its sclerotium is developed, according to De Bary, in Fig. 181. — Claviceps purpurea ; A a sclerotium forming a receptacle d (ergot) ; B longitudinal section of upper part of a receptacle, cp the perithecia ; C a perithecium with the surrounding tissue (greatly magnified) ; cp its orifice, hy hyphae of the pileus, sh epidermal layer of the pileus ; D an ascus ruptured and allowing the spores to escape (after Tulasne). the tissue of decaying vine-leaves in autumn and winter ; if it is placed when fresh, or after having been for some time at rest in the dry, upon the surface of damp ground, it begins after about twenty-four hours to put forth conidia-bearing hyphae, and these prove to be identical with Botrytis cinerea. If, on the other hand, the sclerotium is buried beneath the surface of the soil to the depth of i cm., it does not put out conidia- bearing hyphae of this kind, but produces, on the contrary, in the summer following its production, stalked trencher-shaped little cups, the ascus-bearing receptacles. Sclerotia sometimes again arise from the germinating filaments of the ascospores without any pro- duction of conidia. In other cases the mycelium which grows luxuriantly in the vine- leaves puts out Botrytis-threads at the same time that the formation of sclerotia takes place ; from the germinating filaments of the conidia (of the Botrytis) De Bary always saw Botrytis again produced first of all, and its mycelium probably also forms sclerotia. Like the perithecia of the Pyrenomycetes, the receptacles of the Discomycetes arise from a peculiar act of sexual union which takes place on the mycelium, so that the FUNGI, Q,6l mycelium is the sexual, the receptacle the asexual generation. This has indeed up to the present time been directly observed only in a series of the smaller species of Peziza and Ascobolus, but may well be assumed to exist also in the rest of the Disco- mycetes. In Peziza cojijluens, the species in which the sexual reproduction of the Ascomycetes was first discovered by De Bary in 1863, the process is as follows, according to De Bary's and Tulasne's exhaustive researches: — The mycelium of P. conjiuens grows on the ground ; branches arise at particular points of its hyphae which are directed - .,.Tmr r .., „ , ^ 7. Fig. 183.— Conjugating apparatus oi Peziza cok- nueits (after Tulasne, very strongly magnified) ; in B is shown the commencement of the formation of hypli« h, the result of fertilisation, from which the receptacle is developed. Fig. \Z'2.—rcziza convexula; A vertical section of the whole plant (X about 20) ; h hymenium or layer in which lie the asci ; ^^ the tissue of the Fungus, surrounding the hymenium like a cup at its margin q; at its base fine filaments proceed from the tissue, which penetrate into the soil ; B a small part of the hymenium (X about 50c) ; sh sub-hjnnenial layer of densely interwoven hyphse ; a-/ asci, with intermediate slender para- physes, in which are red granules. upwards and again branch abundantly; at the end of the branchlets the organs of conjugation or fertilisation are produced in large numbers close together, forming rosettes. The terminal cells of the stronger branchlets swell up into ovoid vesicles (Fig. 183, a\ which put out a usually crooked prolongation (/). From another cell of the same branch lying beneath this vesicle grows a club-shaped branchlet, the Antheridium, the apex of which (i) unites with the prolongation just mentioned. After this has taken place, a number of fine hyphse {h) shoot out of the filament which 262 THALLOPHYTES. bears these organs, and these surround the rosette of the organ of conjugation, enclosing it in- a dense felt. This felt forms the substance of the receptacle ; upon its upper side densely crowded hyphae immediately rise up to form the hymenial layer ; finally the receptacle forms a Peziza-cup, which possesses somewhat the form represented in Fig. 183, and produces the ascospores in its hymenium. Woronin observed similar phenomena in P. granulosa and scutellata. In these species branches consisting of three or more cells arise from the mycelium ; the terminal cell swells out into a globular or ovoid form, without, however, putting out a prolongation ; from the cell lying beneath it arise two or more slender filaments which attach themselves closely to the former. By this means the conjugating apparatus is densely enveloped in numerous hyph^ which originate beneath it ; and from them is developed the fruit-cup. In Ascobolus pulcherr'imus the structure which corresponds to the structure af\x\ Fig. i 83 consists of a vermiform body, which Tulasne calls the Scoledte. It is a branch of the mycelium, consisting of a row of short cells which are much broader than those of the mycelium. The adjacent threads put out small branches or Antheridia, the terminal cells of which attach themselves firmly to the anterior part of the scolecite. It is sub- sequently covered over, together with this fertilising organ, by branched hyphae which spring from the neighbouring mycelium ; and a ball is thus formed in the middle of which lies the scolecite ; and this finally grows into the fruit-cup. To these observa- tions of Woronin, Janczewski^ has recently added the additional important fact that in Ascobolus fiirfuraceus, where the processes agree in other respects with those of A. pulcherrimus, the tissue of the cup, together with the paraphyses, proceeds from the branches of hyphge which envelope the conjugating apparatus, and that, on the other hand, the asci are derived from a central cell of the scolecite. This cell puts out a number of filaments which penetrate between the meshes of the tissue of the receptacle, ramify extensively between the bases of the paraphyses, and there form the sub-hymenial layer, out of which the asci spring and grow up among the paraphyses. By this it is demonstrated that the scolecite corresponds to the ascogonium of Eurotium (and generally of the Pyrenomycetes) ; and it is to be expected that a structure similar to the female fertilising apparatus (Fig. 183, af) will hz proved to precede the formation of the asci of Peziza conflue7ts. The similarity of these processes to the formation of the reproductive organs of Floridese, which I have already pointed out in the earlier editions of this book, was also recognised by De Bary. The chief difference lies in this — that in the Floridece, instead of the antheridia, cells endowed with passive motion which detach themselves from the plant conjugate with the female organs of reproduction. The ascogonium (or the scolecite), on the other hand, is comparable to the trichophore in all the essential points by which both are at once distinguished from the oogonia of other Alga? and Fungi. (5) Lichens^. From the most recent researches of Schwendener^, there can no longer be any doubt that the Lichens are true Fungi of the section Ascomycetes, but 1 [Annales des Sci. Nat. 1872, vol. XV, p. 198.] 2 Tulasne, Memoire pour servir a I'histoire organographique et physiologiqiie des Lidiens (Annales des Sci. Nat. 3rd series, vol. XVII). — Schwendener, Untersuchungen iiber den Flechten- thallus (in Niigeli's Beitriige zur wissensch. Botanik. i860 and 1862. — Schwendener, Laub- u. Gallertflechten (Niigeli's Beitrage zur wissensch. Botanik. iS68). — Ditto, Flora 1872, nos. 11-15. [Quart. Journ. Micr. Sc. 1873, p. 235.] ^ [The views of Schwendener have been corroborated by Bornet in an elaborate memoir pub- lished in the Ann. des Sci. Nat. 1873, vol. XVII. He also put them to a synthetical test by sowing the spores of Parmelia pariet'na upon Protococcus. About the fifteenth day the hyphae were well developed and ramified. Wherever they met isolated cells of Protococcus or groups of them, they attached themselves either directly or by means of a lateral branch. They did this to the Protococcus only, neglecting altogether the other bodies which were mixed with it. Similar results FUNGI. 263 distinguished by a singular parasitism. Their hosts are Algae, which grow normally in damp places but not actually in water, and belong, moreover, to very various groups (rarely Confervaccce, frequently Chroococcaceae and Nostocacete, more often Palmellaceae sometimes Chroolepus). The Fungi themselves (Lichen-forming Fungi) are not found in any other form than as parasites on Algae; while the Algae which are attacked by them, and which, when combined with the Fungus, are called Gonidla, are known in the free condition without the Fungus. When the species attacked by the Lichen- fungus is a filamentous Alga, and the development of the hyphal tissue is only moderate (as in Ephebe and Coenogonium), the true state of the case is at once clear ; and as Lichens of this kind have become better known, the suspicion has frequently arisen that they are in fact only Algae infested by Fungi. In the Gollemaceae also attention has frequently been drawn to the identity of the gonidia with the moniliform filaments of Nostocaceae ; but in this case the nourishing Alga usually undergoes considerable changes of habit, at least in its external contour, from the influence of the parasitic Fungus, like Euphorbia Cyparissias from its parasitic iEcidium. But the greater number of Lichen-fungi prefer as hosts the Chroococcaceae and Palmellaceae which grow as stains and incrustations on damp ground, the bark of trees, and stones. The separate cells and groups of cells of these Algae become so involved by the tissue of the Fungus, that they are at last only interspersed here and there in the dense hyphal tissue, or appear in it as a peculiar layer (the gonidial layer). The growth and multiplica- tion of these Algae, which thus become entirely enclosed by their parasites, is not hindered, but their development is disturbed in other ways. When, however, they are freed from their enclosing Fungus-tissue, their normal development proceeds, and in a few cases even the formation of zoospores takes place in them, a fact first observed by Famintzin and Baranctzky, but incorrectly explained. It is to Schwendener's know- ledge of the facts, the result of researches extending over many years, that the correct interpretation is due in these cases of the relationship borne by the Lichen-forming Fungus to the gonidia, /. e. to the Alga which it attacks \ After these preliminary remarks the following description will be intelligible to the beginner. It is transferred, with but slight alterations, from the first edition of this book. We will consider first the Lichen as a whole, as it comes under observation, the nourishing Alga being distinguished as an elemental form of the thallus under the name Gonidia; and will afterwards discuss the nature of the nourishing Algae more in detail. The Thallus of Lichens is commonly developed in the form of incrustations which cover stones and the bark of trees, or penetrate between the lamellae of the epidermis of woody plants, and then expose only the receptacles above the surface. These Crus- taccous Lichens, as they are termed, have become so completely united in their growth were obtained when the spores of Biatora musconnn were sown upon Protococcus. Spores of Parmelia sown separately ramified much less and developed no chlorophyll ; Protococcus, on the other hand, during the same period remained unchanged and put out no hyphse. Tulasne, however, sowed the spores of Lichens and believed that he twice detected the formation of gonidia upon the hyph?e (Ann. des Sci Nat. 1852, XVII, pp. 96-98). De Bary indeed described the green gonidium as originating by the expansion of a short lateral branch of the hypha into a globular cell, which is shut off by a septum and assumes a green colour; once formed, it increases independently by division, and a number of the gonidia eventually lie without stipites in the interstices of the Lichen-tissue (Morph. u. Phys. der Pilze, pp. 258, 263-265). Berkeley also believes that the gonidia originate from the hyphce, having had ' a good opportunity of ascertaining their development from the threads of the mycelium in specimens developed within' the vessels of pine wood' (Introd. to Crypt. Bot. p. 373). For a careful resume ol all the recent literature of the subject by Archer, see Quart. Journ. Micr. Sc. 1873, p. 217. In this country Bentham has criticised Schwendener's view (Address to Linn. Soc. May 23, 1873), and Thwaites and Berkeley have also expressed their dissent (Gard. Chron. 1873, p. 1341). — Ed.] ^ A few additional historical notes will be found at the end of this section. 204 TIIALLOPHYTES. to their substratum, at least on the under side, that they cannot be detached completely from it without injury to the thallus (Fig. 184, A, B, C). The crustaceous Lichen-thallus passes over, through various gradations, into that of the Foliaccous Lichens ; the latter Fig. 184.—^. B Graphis ele^ans, a crustaceous Lichen growing on the bark of the holly ; A aatural size, B slightly magnified ; C Pertusaria U'ul/eni, another crustaceous Lichen (slightly magnified). Fig. 185.— a piece of the foliaceous thallus of PelH- S-era horizontalis ; a the apothecia; r the rhizines (natural size). Fig. 186. — Collema pjilposiaii, a gelatinous Lichen (slightly magnified). forms flake -like expansions often curled, which can be completely detached from the ground, stones, moss, bark, &c., which support them, since they are attached to it only in places by a few organs of attachment, the Rhizines. The foliaceous thallus often Fig, iBy.—A Usnea harbata, a fruticose Lichen (natural size) ; B Sticta pjilmonacea, a foliaceous Lichen (natural size) seen from beneath ; a apothecia, /"the attaching disc of A , by which the Lichen becomes attached to the bark of a tree. attains considerable dimensions, in the large species of Peltigera and Sticta as much as a foot in diameter, and from | to i mm. in thickness, and then generally assumes a circular form; at the growing margin it forms rounded indented lobes (Fig. 185 and Fig. 187, 5). FUNGI. 26 A third form of the Lichen-thallus, also united with the previous one by transitional forms, is shown in the Fruticose Lichens, which are attached only at one spot and with a narrow base, and rise from it .,^ .^ _ , in the form of small much-branched shrubs. The branches of the thallus are either flat and ligulate, like the lobes of many foliaceous Lichens, or slender and cylindrical (Fig. 187). In Cladonia and Stcreocaulon we have not so much a transition from the foliaceous to the fruticose thallus as a combination of the two, a folia- ceous expansion of small size being first formed, the cup-shaped or fruti- cosely- branched thallus afterwards rising from this. The thallus of Lichens can be dried so as to be pulverised without losing its vitality. When saturated with water it has generally a leathery consist- ence, is tough, elastic, and flexible ; but a large number of genera, which are remarkable also in other ways, are slimy and gelatinous in this condition. These Gelatinous Lichens, as they are termed, form cushion-like masses with an undulated surface, and in their growth are sometimes more like the fruticose, sometimes more like the foliaceous Lichens. A typical form is shown in Coilema, Fig. 186. The disposition of the gonidia and hyphae in a thallus may be such that these two structures appear about equally mingled (as in Fig. 189), and the thallus is in this case Fig. 188. — Transverse section throisgh the foliaceous thallus oi Stutn/u- lijrinosa (X500) ; o cortical or epidermal layer of the upper side ; n of the under side ; rr rhizines or attaching fibres, springing from the epidermal layer and therefore trichonies ; 771 the medullary layer, the hyphce of which are seen cut, sonic transversely, some longitudinally. The upper and under cortical layers also consist of hyphre, which however are nuich thicker, coiisist of shorter cells, and are united without inter- stices, forming a pseudo-parenchyma ; g the gonidia (their light-green masses of protoplasm are coloured dark) ; each gelatinous envelope en- closes several gonidia produced by division. FIG. 1S9.— Vertical section of the gelatinous thallus of LeJ>to,cM»t scotinum (X500) ; an epidermal layer clothes the interior tissue, which consists mainly of shapeless and colourless jelly in which lie the coiled chains of gonidia; some of the larger cells of the chains are left white; between them run the fine hyphae. called Homoomerous; or the gonidia are crowded into one layer (as in Fig. 191), by which the hyphal tissue is at the same time separated according to circumstances into an outer and inner or an upper and under layer ; the thallus-tissue is then stratified, and such Lichens are termed Heteromerous (Figs. 188 and 191). 266 THALLOP BYTES. The mode of growth, branching, and external structure of the Lichen-thallus may either be determined by the gonidia, the hyphae being concerned only in a secondary degree in the construction of the substance, or it may happen that the hyphae determine the form and mode of growth, while the gonidia have only a secondary share in the forma- tion of tissue. The first mode occurs in only a few Lichens ; the other mode of growth is the more common, and is that of the typical Lichens, especially of those that are heteromerous. In some homoomerous gelatinous Lichens (as Fig. 189) it appears doubtful whether the change in the external form proceeds more from the gonidia or from the hyphae. This relationship, which, although both morphologically and physiologically important, has not hitherto had sufficient attention paid to it by lichen- ologists, will be made sufficiently clear by an examination of Figs. 190 and 192. In Fig. 190.— a liranch of the tliallus of Ephebe pitbesccjis ( x 550). Pig. 191. — Usnea harbata; A longitudinal section of a slender branch, soaked in potash-solution ; B transverse section of an older thallus-stem with the basal portion of an adventitious (or soredial) branch sa (X300) ; s apex of the branch, r the cortex, x the axial medullary bundle, m the loose medullary tissue, g the gonidial layer. Fig. 190 is shown the longitudinal section of a branch of Ephebe pubescens ; the large gonidia are left dark, and the very fine hyphae are indicated at h. The branch increases at the apex by longitudinal growth and by transverse division of a gonidium [gs), which is here the apical cell of the branch. The cells produced from the apical gonidium afterwards divide parallel to the longer axis of the branch ; still later divisions are formed in different directions, and thus groups of gonidia arise at some considerable distance from the apex of the branch. The fine hyphae are represented in our figure as reaching to the apical cell ; in other cases they come to a termination at a considerable distance beneath the apical gonidium. There are also only a few single hyphae which follow the longitudinal growth of the branch ; these grow within the gelatinous envelope which is evidently derived from the gonidia. At a considerable distance from the FUNGI. 267 apex of the branch the hyphae first put forth lateral branches which penetrate between the single or grouped gonidia, forcing their way through the deliquescent mass of their gelatinous cell-wails. In this manner the whole form of the branch, its growth both in length and thickness, is determined by the gonidia; the hyphas, from their small number and their fineness, produce scarcely any essential alteration either in the external form or the internal structure of the branch. This is clearly shown also in the origin of the lateral branches of the thallus of Ephebe pubescens. One of the exterior gonidia lengthens in a direction vertical to the axis of the parent-branch, and becomes the apical cell of the lateral branch, producing at the same time new cells by transverse divi- sions, as is shown in Fig. 190, a. Branches of the hyphae which run into this cell turn in the same direction, and behave, in relation to the new apical cell, in the manner described above with respect to those of the primary branch. In a manner similar to Ephebe pubescens, Usnea barbata, a fruticose Lichen, also forms a much-branched fruticose thallus. The branches of the thallus here also elongate by apical growth (cf. Fig. 191, A); but this is not brought about, as in Ephebe, by the gonidia, nor by a single apical cell. The hyphae at the end of the branch which are nearly parallel and approximate at the apex, elongate each by the apical growth of its terminal cell, and thus produce in common the apical growth of the branch ; this is m FIC. 192.— Verti-al section of the gymnocarpous apotlicciuin n{ Aitaptychia ciliaris (X about 50) ; h the Iiymciiiimi, j' sub- hymenial layer and excipuUiiii ; all the rest belongs to tlie thallus ; -,n its medullary layer, r its cortex, g its gonidia ; at ^i" the thallus forms a cup-shaped rim round the apotheciuin. followed further backwards by an intercalary growth, the result of the intercalary elongation of the hyphae and of the formation of new branches in different directions. The hyphae lie so close together near the apex that they form a compact mass without interstices ; it is only at some distance from it that the hyphal tissue is differentiated into a very dense cortex of fibres interwoven on all sides, an axial bundle of densely- crowded threads running in the direction of length, and a looser layer (the medullary layer) furnished v.-ith air-containing interstices. The point below the apex where this differentiation of the hyphal tissue begins is also that of the point of commencement of the gonidial layer, which consists of small roundish green cells, collected in small groups in consequence of their increase by division. But these groups themselves lie in a layer between the medullary and cortical layers {cf. Fig. 191, B, the transverse section). Below the growing apex of the branch of the thallus there are only single gonidia, by the division of which the cells in the gonidial layer subsequently increase. It is evident therefore that in Usriea barbata the growth in length and thickness and the internal differentiation of the tissue depend entirely on the hyphae, and that the gonidia behave like foreign bodies in the hyphal tissue ; the formation of new branches proceeds also from the hyphae and not from the gonidia. The branching may be dichotomous ; and in this case the apical cells of the hyphae converge towards two nearly adjacent points, and then continue to grow in corresponding directions, so that the two equal branches 268 THALLOPHYTES. form an acute angle. Adventitious branches arise laterally below the apex of the thallus, the cortical fibres forming at a particuFar point a new apex and subsequently growing outwards. Gonidia are also to be found below the new apex, while the base of the branch sends out medullary fibres and an axial bundle into the primary branch, so that the homologous forms of tissue of the two are continuous. The growth of Usnea may be compared, irrespectively of subordinate points, to that of the so-called stroma of the Xylariae ; the formation of the gonidia is a subordinate element in the structure of the whole. In some crustaceous Lichens the thallus possesses in general no defined contour, and no external differentiation takes place ; the thallus appears as a somewhat irregular aggregation of masses of gonidia traversed by hyphse. In other crustaceous Lichens (as Sporastatia Morio, Rhizocarpon suhconcentricum^ Aspic'ilia calcarea, &c.), the thallus forms lobed discs which increase by centrifugal growth at the margin ; the growing margin consists altogether of hyphal tissue, in which, further inwards masses of gonidia appear at a few isolated spots and gradually spread ; the cortical tissue is indented at the circum- ference of the spots where the gonidia are formed. Isolated scaly pieces of a true Lichen-thallus then arise on a fibrous substratum called the Hypothallus^. 7he Formation of the Spores of Lichens takes place in receptacles termed Jpothecia, similar to those of the Discomycetes, or in other cases to those of some Pyrenomycetes. They are formed in the interior of the tissue of the thallus, and only appear above its surface at a later period, when they either expand their hymenial layer to the air (Gymno- carpous Lichens), or allow the spores to escape outwards through an orifice (Angio- carpous Lichens). In all Lichens without exception the apothecium and all its essential parts derive their origin exclusively from the hyphal tissue ; it is the Fungus alone that produces the receptacles ; the nourishing Algee, /. e. the gonidia, take no part whatever in it, or only in a secondary manner in so far as the thallus-tissue together with its gonidia grows like a wall round the apothecium and to a certain extent envelopes it (as shown in Fig. 192), or grows luxuriantly beneath the apothecium and raises it upon a kind of stalk above the surrounding thallus. The only exception to this endogenous origin of the apothecium occurs in Coenogonium and similar forms, where it is impossible, because the hyphae form only a very thin layer round the filamentous Alga which per- forms the part of gonidia. These forms serve to show with especial clearness, as w^e know from Schwendener's researches, that the receptacle of Lichens belongs exclusively to the hyphal tissue. The history of the development of the apothecium is a branch of the inquiry attended with great ditTiculty, and in more than one point is still obscure^. It originates, in heteromerous Lichens, beneath the cortical layer, in the lower part of the gonidial zone, or, in some crustaceous Lichens, in the deepest part of the thallus in immediate contact with the substratum; in homoomerous gelatinous Lichens and in Ephebe it arises beneath the surface of the thallus. The commencement of the gymnocarpous apothecium is, in heteromerous Lichens, a very small roundish ball of confused interwoven hyphae, on the outer side of which a tuft of very delicate hyphse — the first paraphyses — rises at a very early period. The most external hyphal investment of this ball, and therefore surrounding the tuft of paraphyses and opening above (outvv-ards), is termed by lichenologists the Excipulum. The further growth of the rudiment of the apothecium is now occasioned by the increase in size of the excipulum by the formation of new fibres, w^hile new paraphyses are intercalated among those already formed and outside the tuft, the extension of the apothecium being the immediate result of the fresh forma- tion of these bodies. The growth is first completed in the centre of the apothecium ; at the outside it continues longer, often even after the appearance of the apothecium ^ See Schwendener, Flora, 1865, no. 26, 2 What follows is taken from De Bary's account of his oynH researches, and fiom those of Schwendener and Fuisting. FUNGI. 25^ above the surface of the thaUus. The mother-cells of the spores, the asci are formed according to Schwendener and Fuisting, in a peculiar manner. ' Even in the young ball, and among the first rudiments of the paraphyses, thicker hyphse are to be seen inter- woven among the rest, rich in protoplasm, undivided by septa, and with numerous ramifications ; the upright ends of the branches of these hyphae which penetrate between the ends of the paraphyses, develope into club-shaped asci ; they may hence be termed Ascopborous hyphcE. They are very readily distinguished from the paraphyses by their membrane being coloured blue by iodine after treatment with potash-solution, while that of the paraphyses remains colourless. They disappear at a very early period from the lower part of the rudiment of the apothecium, and remain only in one narrow layer which runs parallel to the upper surface of the apothecium, and extends below the lower ends of the ripe asci. In this layer they ramify in a centrifugal direction in proportion as the margin of the excipulum grows, and send out new asci among the new paraphyses. The first asci appear in the centre of the apothecium ; and Schwendener states that no genetic connexion exists between the ascophorous hyphae and those from which the. paraphyses are derived ; the two form separate systems but interwoven into one another \' The layer in which the ascophorous hyphae run is called the Sub- hymejiial Layer; the hymenium itself consists of the paraphyses and the asci taken together. The term Hypcthecium is given to the mass of fibres lying beneath the sub- hymenial layer, and is often strongly developed through subsequent growth ; it consists of hyphae the branches of which end in the hymenium as paraphyses, and of the remains of the primary ball ; when mature, it can scarcely be distinguished from the excipulum. The growing apothecium bulges more and more, and finally breaks through the layer of thallus which covers it ; the hymenium and the margin of the excipulum appear above the surface of the thallus, or the part of the thallus which surrounds the excij)ulum rises and grows with it forming a bowl-like rim. Among the medullary hyphcC which surround the apothecium a number of gonidia subsequently appear in many Lichens, so that a gonidial zone runs beneath the apothecium. In Peltigera and Solorina even the young apothecium is expanded flat, its paraphyses project vertically towards the surface of the thallus, and the layer of thallus which covers them is finally lifted like a thin veil. In Bxomyces, Calycium, &c., the basal portion of the hypothecium is deve- loped into a long stalk which elevates the apothecium. The apothecium of Angiocarpous Lichens is so similar in its mode of development and in its mature state to the pcrithccium of the Xylariae, that there is no need to give an exact description of it. The club-shaped asci of Lichens are similar in every essential point to those of the Pyrenomycetes and Discomycetes ; their wall is often very thick and capable of swelling; the spores (Fig. 193) arise simultaneously, as in those Fungi, by free-cell-formation, while a considerable portion of the protoplasm often remains unused in their production. The normal number of spores is eight, although sometimes only 1-2 (in Umbilicaria and INIegalospora), 2 or 3 or from 4 to 6 (in several Pertusariae) ; in Bactrospora, Acaro- spora, and Sarcogyne on the other hand their number amounts to some hundreds in one ascus. The structure of the spores is very various, but in general similar to that of the Ascomycetes ; very commonly they are septate and multicellular ; the exospore is usually smooth and often of various colours. The spores are set at liberty by moisture penetrating the Jiymenium ; they are suspended in the fluid which fills the ascus, and are expelled together with the fluid by ^ From the newly discovered processes in the formation of the reproductive organs of the Pyrenomycetes and Discomycetes, especially from the most recent statements of Janczewski on Asco- bolus furfuraceus {cf. pp. 256 and 261), it may be assumed that the tubular hyphx of the sub-hymenial layer arise from a yet undiscovered ascogonium or scolecite; and that thus the apothecium of Lichens is the result of a sexual process in a similar manner to the peiithecia of the Pyrenomy- cetes and the fruit-cups of Peziza and Ascobolus. THA LL OPHYTES. the rupture of its apex. This expulsion is probably caused by the lateral pressure of the swollen paraphyses and the property of swelling possessed by the membrane of the ascus itself. The germination of the spores of Lichens takes place by the endospore of each spore-cell putting out a filament which ramifies and creeps upon the damp substratum on which the spore is placed. The origin of the first gonidia has never been observed after the dissemination of the spores ; but Tulasne sometimes found groups of gonidia at Fig. 193 —Vertical section of a small portion of the apotheciuin of A^iaptychia ciliaris (x 550) ; m the medullary layer of the thallus; y the hypothecium, together with the sub-hymenial layer; / the paraphyses of the liymenium, tlieir upper ends of a brown colour ; among them are the asci in various stages of development ; in i are the young spores not yet septate, in 2-4 the spores more fully developed ; the protoplasm in which the spores are imbedded is contracted by the drying up of the Lichen before the preparation was made. a later period upon the web of hyphae derived from the spores ; and even small rudiments of a thallus were observed ; but the genetic connexion of the gonidia with the germinating filaments has not been made clear \ The mode of germination of the very large spores of some genera, Megalospora, Ochrolechia, and Pertusaria, differs from that of all the rest. They are simple, not septate, and densely filled with drops of oil (Fig. 194, A, B). Each spore puts out from different parts of its circumference in germination a great number, even as many as a hundred, germinating filaments. The formation of each begins with the appearance in the endospore of a cavity widening from within outwards, * [Tulasne believed that he twice detected the formation of gonidia upon the hyphoe : Ann. des Sci. Nat. 1852, vol. XVII, p. 96.— Ed.] FUNGI. 271 which becomes surrounded by a very delicate membrane and grows outwards in the form of a filament (Fig. 194, -^, B). Besides the apothecia with ascospores capable of germination, Spermogonia are also generally present in Lichens, as in Ascomycetes ; they generally occur on the same thallus as the apothecia. There are cavities in the thallus (Conceptacles), globular, flask- shaped, or sinuous, and densely clothed and almost filled with sterigmata ; from these sterigmata the spermatia are detached in very large numbers, and escape through Fig. 194.— Lichen-spores germinating; A longitudinal section of a spore oi Pertusaria communis after lying 34 hours in glycerine, s the first start of the germinating filaments ; B spore of Pertusaria leioplaca with a number of germinating filaments (after Ue Bary, X390) ; C germinating septate spores oi Solorina saccata (after Tulasne). a fine orifice in the spermogonium. Sometimes also conceptacles are found in which larger structures, more like spores, are detached from the sterigmata; receptacles of this kind are called Pycnidia. The signification of both is at present unknown. Besides the spores, most Lichens also possess organs of a very great reproductive power, the Soredia. This term is applied to single gonidial cells or groups of gonidia which, surrounded by a weft of hyphae, are pushed out of the thallus, and are able, without any further process, to grow into a new Lichen-thallus. The soredia are produced from the thallus in the non-gelatinous Lichens as a fine powder, forming sometimes dense pulvinate masses (as in Usnea, Ramalina, Evernia, Physcia, Parmelia, Pertusaria, &c.). In the heteromerous thallus the soredia appear in the gonidial layer ; single gonidia, or some- times several together becoming woven over by branches of hyphae which cling closely to them and form an envelope of fibres. The gonidia divide repeatedly, and each daughter- cell is again woven over. This process is often repeated, the soredia accumulate in great numbers in the gonidial zone, and finally rupture the cortex. After escaping in ;: THALLOPHYTES. this manner, the soredia can still further multiply outside the thallus ; but under favour- 's oO^°oHc. Fig. ige^.—A—D soredia of Usnea barbata; A a simple sorediuni, consisting of a gonidium covered with a web of hyphre; B a soredium, in which the gonidium has multiplied by division ; C a group of simple soredia, resulting from the penetration of the hyphae between the gonidia : A E germinating soredia ; the hyphas are forming an apex of growth and the gonidia are multiplying ; a-c soredia of Physcia parictma ; a with an envelope of pseudo-parenchyma ; 6 the envelope producing rhizines; c a. young thallus formed from a soredium (after Schweiidener, X500). able conditions either a sin;?le soredi Fig. 196.— Various Lichen-gonidia ; A of Roccella tiiictoria, i^-g"' in the act of multiplication, g g united with branches of hyphee h; B of Everjiia di-varicata united with a branched hypha ; C of Usnea barbata in the act of division, at h united with a hypha ; D chain of gonidia of Lichina pygmcea (after Schwendener). of growth of hyphcT and gonidia um or a mass of them grows out at once into a new thallus (Fig. 195). Schwendener states that in Usnea barbata this may occur while the soredia are still included in the mother-thallus ; soredial branches, as they are termed, are thus pro- duced. We may now turn to the consideration of the other elemental form out of which, in addition to the Fungus-hyphae, the thallus of Lichens is constructed, the Gonidia. It has al- ready been shown that these are nothing but Algae which are attacked and surrounded in their growth by Ascomycetes, and serve as hosts to them, the capability of assimilating in- organic materials being wanting on the part of their parasites. Every Lichen-forming Fungus chooses a particular Alga, just as other parasites like the Hypodermiae mostly infest only particular hosts. The peculiarity of the parasitism of the Lichen-fungi lies in the fact that they are not attached to their host externally at any one par- ticular spot, and do not penetrate into the cells themselves, but become woven round them, and thus enclose them in their hyphal tissue. Com- plete unions of growth however sometimes take place, single hyphae becoming closely attached to the cell walls of particular Algae (or gonidia) (Fig. 196, jrl g, Bg, Cg), a phenomenon which led at one time to the assumption that the gonidia are themselves products of the hyphae, the branches of which swell up in places to a glo- bular shape, and form chlorophyll. The opposite view was also at one time held, that the hyphae grow out of the gonidia (as, for in- stance, by me in Bot. Zeitg. 1855, on Gollema). But these very rare phenomena may now be more easily explained as simple unions , and bv no means stand in the way of the FUNGI. 273 view that the gonidia are true Algae, since the proofs of this are over- whelming. Reviewing the views of older lichenologists, which may be found represented in detail in the writings cited below of Baranetzky and Schwendener ; it may be mentioned, first of all, that De Bary (Handbuch der phys. Bot. vol. II. p. 291) had already arrived at the following alternative with respect to gelatinous Lichens, as Ephebe and similar forms: — 'Either these Lichens are the perfectly developed fructifying states of plants the incompletely developed forms of which have hitherto been placed among Algae under the names Nostocaceae and Chroococcaceae, or these last are typical Algae which assume the forms of Collema, Ephebe, &c. Certain parasitic Ascomycetes penetrate them, extend their mycelium into the growing thallus, and often form an intimate attachment with those of its cells which contain phycochrome (as Plectospora, Omphalaria). In the latter case these plants may be called Pseudo- lichens.' From the last sentence of this quotation it follows that E^ Bary would not apply the latter alter- native at all events to the heteromerous Lichens. Soon afterwards Famintzin and Baranetzky, and then the latter alone, published researches on the further changes which the gonidia of Lichens undergo when they are set free by the decomposition of the hyphal tissue in water\ Baranetzky comes to the conclusion that: — 'The gonidia of the heteromerous chlorophyll-containing Lichens (as Physcia, Evernia, Cladonia, &c.), as well as of the heteromerous forms which contain phycochrome {e.g. Peltigera), and of the gelatinous Lichens (such as Collema), are capable of carrying on an entirely inde- pendent life outside the Lichen-thallus. When set free the Lichen-gonidia appear to expand their cycle of life ; and thus the independently vegetating gonidia of Physcia, Evernia, and Cladonia produce zoospores.' He also found that all the ball-like masses of Peltigera-gonidia were afterwards transformed so as to become extremely like the interstitial cells of a Xostoc ; and he did not doubt that this was their permanent condition. ' Some, perhaps many, of the forms hitherto described as xA.lgae must be considered as independently vegetating Lichen-gonidia, such as, for the present, the forms Cystococcus, Polycoccus, and Nostoc' The researches of Schwendener, carried on in part earlier in part simultaneovisly and later, and conducted in the most careful manner, led to the opposite conclusion, that the gonidia are in fact Algae which are more or less disturbed in their manner of life by the Fungus which becomes parasitic upon them. He first stated in the frankest and clearest manner that this was his opinion with respect to all Lichens in his treatise ' Ueber die Algentypen der Flechtengonidien' (Basel 1869). In this memorable \vork, which settled for the future the systematic position of the Lichens atnong the Ascomycetes, he gives a review of those genera of Algae which were up to that time known as hosts of Lichen-fungi, in other words, as playing the part of gonidia : — I. Algce ~.vith Blue-green Contents {NostochinecB). Name of Group of Al^^.x. Lichen on which they occur as Gonidia. (i) Sirosiphoneae . . Ephebe, Spilonema, Polychidium. (2) Rivularieae . . Thamnidium, Lichina, Racoblenna. (3) Scytonemeae . . Heppia, Porocyphus. (4) Nostocacese . . Collema, Lempholemma, Leptogium, Pannaria, Peltigera. (5) Chroococcaces . Omphalaria, Euchylium, Phylliscum. 1 Memoires de 1" Acad. Imp. des Sci. de St. Petersbourg, 7th series, vol. XL no. 9, Jind Melanges biologiques tires du Bulletin de I'Acad. Imp. de St. Petersbourg, vol. VI. 1867.— [Ann. des Sci. Nat. 5th series, 1867, vol. VIII. pp. 137-144.]— Also Itzigsohn, Bot. Zeitg. 1868, p. 185. T /4 THALLOPHYTES. II. j4lg(B avith Chlorophyll-green Contents. Name of Group of Algae. Lichen on which tliey occur as Gonidia. (6) Confervaceae . . Coenogonium, Cystocoleus. (7) Chroolepideae . . Graphideae, Verrucarieae, Roccella. (8) Palmellaceae . . Many fruticose and foliaceous Lichens : e. g. Cystococcus humicola on Physcia, Cladonia, Evernia, Usnea, Bryopogon, and Anaptychia^ Pleurococcus, on Endocarpon and various crustaceoiis Lichens. Since anatomical and analytical research has led to this view of the nature of Lichens, the next step must be to complete synthetically the proof of its correctness by sowing the spores of the Lichen-fungi on or near those Algas which serve as their gonidia, and to induce their germinating filaments and the hyphae which proceed from them to invest the Algae ^. If in this manner a true Lichen-thallus is obtained, each fresh case in which this is successful would furnish a new proof of the correctness of Schwendener's theory. This synthetical method has already been adopted by Reess with the greatest success ; for he has succeeded in growing in this manner the Lichen-thallus of Collema glaucescens both from germinating spores of the same species and from Nostoc lichenoides^. SUPPLEMENT. MYXOMYCETES^ Under this head is included a numerous group of organisms which in many respects differ widely from all other vegetable structures, but, in the mode of formation of their spores, stand nearest to Fungi, on which account we may treat them as a supplement to that class. The .]\Iyxomycetes are remarkable in no ordinary degree from the fact that during the period of their vegetation and assimi- lation of food they do not form cells or tissues. The protoplasm, which in all other plants is also the general motive power of the phenomena of life, remains in them during the whole of this period perfectly free, collects into considerable masses, and assumes various shapes from the internal force residing in it, with- out becoming divided into small portions which surround themselves with cell- walls (or become cells). It is only when the protoplasm passes into a condition of rest in consequence of being surrounded by unfavourable conditions, or when it concludes its period of vegetation by the formation of the reproductive organs — its internal and external movements ceasing at the same time — that it breaks up into small portions which surround themselves with cell-walls, and which even then never form a tissue in the proper sense of the term. 1 See Schwendener in Nageli's Beitrage, &c. 1868. Heft VL pp. no, 11 1. 2 [See Bomet, Annales des Sci. Nat. 5th series, 1873, vol. XVII.— Treub, Bot. Zeitg. 1873, pp. 721-727.] 3 Monatsberichte der konigl. Akad. der Wiss. zu Berlin. Oct. 1871. Compare further, Schwen- dener, Flora, 1872, nos. n and 12. [Quart. Journ. Micr. Sc. 1873, p. 235 ; compare Archer, Quart. Journ. Micr. Sc. 1872, p. 367.] * De Bary, Die Mycetozoen, in the Zeitschrift fiir wiss. Zool. vol. X. 1859 ; separate 2nd edition, Leipzig, 1863; (this work is the leading authority for the whole group). [Ann. Nat. Hist, i860, vol. V. p. 233.]— Cienkowski in Jahrb. fur wiss. Bot. vol. III. pp. 325, 400.— Oscar Brefeld, Ueber Dictyostelion mucoroides, Abhandlungen der Senkenhergischen Gesellschaft, vol. VII. Frankfort 1869, MYXOMYCETES. 275 The Myxomycetes live upon decaying and rotting vegetable substances, such as tan, rotten stems, and the like. While endowed with motion they either creep over the surface of the substratum, or live in hollows and pores in its interior; but for the purpose of reproduction they always come to the surface. When a Myxomycete is entering the reproductive condition, the whole of its protoplasm (the Plasmodium) becomes transformed into sporangia or large receptacles. In most Myxomycetes the sporangia have the form of round, longish vesicles, sessile or stalked, one or more millimetres in length; less commonly they form horizontal tubes either cylindrical or flattened. The structure of the walls of these sporangia is similar to that of ordinary cell-walls ; they sometimes exhibit similar thickenings and stratification; they are colourless, or violet, brown, red, or yellow, according to Fig. 197, -A Plasmodium o( Didymium leticopus (after Cienkowski, X3S0) ; B a closed sporangium ot Arcyria incarnata C after rupture of the wall / and extrusion of the capillitiuni cp (after De Bary, x 20). the species. In some, as Licea and Cribraria, the cavity of the sporangium is entirely filled with small spores ; but generally the sporangium contains, besides the spores, a structure called the Capillitium, consisting sometimes of small thin-walled lubes anastomosing reticulately which are attached to the wall of the sporangmm {e.g, Physarum); while in Arcyria (Fig. 197, O the wall of these tubes is furnished with thickenings of an annular or wartlike or varying shape, projecting on the outside. The capillitium of the genus Trichia consists of separate long fusiform tubes not united to one another; their wall is thickened spirally like the spiral cells of higher plants. In the Stemoniteae the pedicel which bears the sporangium is continued into it and forms the so-called Columella, from which the branches of the capillitium spring and anastomose reticulately. When the spores are being disseminated, the rupture of the wall of the sporangium and the expulsion of the T 2 276 THALLOPHYTES. spores are assisted by the capillitium ; its fibres, which in the unripe condition are irregularly folded, becoming straight and tense as they dry. After the rupture of the sporangium-wall, the capillitium becomes exposed in many cases as an extremely beautiful net- work (Fig. 197, C). The structure of the receptacles of JEthalium, Spumaria, and Didymium is different. Those of J^thalium (the so-called * flowers of tan') are cake-shaped, not unfrequently a foot in length and breadth, and over an inch thick, but more often smaller and closely adherent to the substratum (generally tan). The cake is surrounded by a brittle skin several milli- metres in thickness, which is at first bright yellow, afterwards brownish, and spreads itself over the substratum. The inside of this cake is composed of a dark-grey easily-pulverised mass (the spores) penetrated by yellow veins, and consists of tubes M'hich are interwoven in all directions and united into a net-work, but which otherwise possess exactly the structure of the sporangia of Physarum, not excepting the capillitium. The cortex consists of densely interwoven irregular 'bundles or of tubes connected in a peculiar manner, which contain within the membrane an immense number of calcareous grains (which are also found in the sporangia), as well as a yellow pigment. The ^thalium-cake is therefore, according to De Bary, a tissue of the tubular sporangia of a Physarum surrounded by a calcareous cortex. The receptacles of Lycogala epidendron bear a resemblance to those of some Gasteromycetes (the Lycoperdacese). They are surrounded by a papery cortex consisting of two layers. The inner layer is a homogeneous stratified light-brown membrane, the outer one is much thicker and consists of a weft of branched hollow fibres, the walls of which have cleft-shaped dots or are reticulate thickenings projecting externally. A number of fibres of this kind penetrate into the interior, piercing through the inner membrane, and form the capillitium. On the surface of the structure are firm large vesicles filled with granular contents. The spores of the Myxomycetes, which fill up all the interstices of the capillitium, resemble, from the reticulated ridges or tubercles on their surface, the spores of some Tuberaceae and Gasteromycetes; but sometimes they are smooth. ^ The spores are capable of germinating immediately after -tBeir dissemina- tion ; but when kept dry they preserve this power for years. When a spore is saturated with water it opens, and the whole of its protoplasmic contents escape as a roundish naked mass ; but after some minutes it assumes another form, becomes long and pointed at one end, where it is provided with long cilia; it- has, in fact, developed into a swarm-spore, which is either endowed with rotatory motion or creeps along changing its form like an Amoeba. These swarm- spores multiply by division. But on the second or third day a new process begins; the swarm-spores cease dividing and unite, two or more: of them coalescing- — after they have gone over into the Amoeba-form — into a homogeneous protoplasmic" substance, also endowed with an Amoeba-like motion^ the Plasmodium. This increases, constantly absorbing into itself more swarm-spores -and coalescing with:' other Plasmodia. These plasmodia now creep along the surface of the nourishing" substratum (Fig. 197, A)^ and the movements are essentially similar to those of' the protoplasm which circulates in the large cells of plants, but are freer and more' varied. This movement in position or 'creeping ' is caused by arm-like protuberances MVX O MYCE TES. 2/7 from the margin of ihe plasmodium, which increase by the protoplasm flowing into them from behind. These arms or branches coalesce laterally, anastomose, and form new projecting arms. When this has proceeded for a considerable time in the same direction, the whole plasmodium is found to have changed its position. In addition smaller tentacle-like arms, into which the inner granular protoplasmic substance does not penetrate, are also put out and again drawn in from the outside. Finally a streaming motion takes place in the interior of the larger arms or of the plate-Hke expansions of the plasmodium, the direction of which is constantly changing. The motile substance in the interior is granular and more watery; but the circumference of the plasmodium is formed of a hyaline layer (the marginal layer) destitute of granules and apparently denser, which is sometimes also surrounded by a layer of mucilage ; this latter is not protoplasm, and is left behind during the creeping like the slime of a snail. The greater number of plasmodia are colourless, many yellow (as jEthaliiim septiciwi) or reddish yellow ; some are very small, scarcely visible to the naked eye, others, when mature, attain a size of some square inches, and those of ^thaliiun septiciwi sometimes collect on the surface of the tan into masses the size of the hand or larger, and one-half to one inch thick ; and, in this state, may form spherical or Clavaria-like upright bodies, which however consist of soft cream- like protoplasm. In this condition the ^Ethalium is able to creep away from the tan upon plants even several feet high, and accumulate above on the leaves. The Plasmodia take up hard foreign substances which they enclose ; and De Bary supposes that they obtain food in this way. The quantity of the ab- sorbed materials seems, however, too small for this purpose ; the residue of them are afterwards again expelled, especially when the plasmodium passes over into the cellular condition. The swarm-spores may, under unfavourable conditions, be again transformed into cells, surrounding themselves with a cell- wall {Microcysts), When dry they remain in this state for months in a vital condition, and when placed in water revert to the motile form. Young plasmodia form, under similar conditions, ' solid- walled cysts,' dividing themselves into pieces of unequal size which become rounded off and surrounded by cell-walls. When the weather is moist and warm the Plasmodium again creeps out of these cysts. The mature plasmodia finally arrive at a state of rest, forming bodies which De Bary calls Scleroiia, when the tempe- rature and amount of moisture decrease. The plasmodium first of all draws in its arms and forms a sieve-like plate or mass of small irregular nodules, and the whole substance breaks up into a large number of roundish or polyhedral cells 4V to -jV mm. in diameter. The body thus formed is wax-like, gritty and dry. When placed in water the cell-walls again become absorbed, and the sclerotium reverts to the condition of a motile plasmodium. When the mature plasmodia have lived for some time and been in motion on the surface of the substratum, they assume a firmer consistency, and after the net-like mass has collected, it either forms a cake, as in ^Ethalium, or puts forth outgrowths directed upwards, but always soft, of the form of the future sporangia. A firm membrane is formed on the outside, the capillitium and the spores in the inside. If the plasmodium contains lime, it is deposited in the form of granules ■^ CHAKACE.E. in the capillitium or in the wall of the sporangium. These processes are mostly completed in some hours ; in ^Ethalium one or two hours is generally sufficient to transform the still motile plasmodium into the organs of reproduction ; the water contained in the plasmodium is partially expelled in the fluid state, the remainder subsequently evaporates. GROUP IL CHARACE/E CLASS III. C H A R A C E yE. The Characese are submerged aquatic plants, rooting in the ground and growing erect, attaining a height of from ^^ metre to a metre, and containing abundance of chlorophyll. They are very slender, forming stems and leaves only |- to 2 mm. in thickness. With an alga-like habit, they possess a delicate structure, though sometimes attaining greater firmness from the deposition of lime on their surface. They live gregariously, mostly in crowded tufts at the bottom of fresh-water ponds, ditches, and streams ; they may grow in deep or in shallow^, in stagnant or in quickly flowing water ; and are either annual or perennial. In the greater number of species, which are distributed over all quarters of the glo-be, there prevails nevertheless so great a uniformity that they may all be arranged into two genera with some transitional forms ; while, on the other hand, they are so different from all other classes of plants that they must be erected into a special group by the side of the Thallophytes and Muscineae. Among the Thallophytes they would approach most nearly to certain groups of Algae, but diff'er from all the members of that group in the form of their antherozoids ; and in this respect resemble the Muscineae, from which again they diff'er entirely in the structure of the antheridia and of the female organs of reproduction, as we>ll as in that of their organs of vegetation. ' A. Braun, Ueber die Richtungsverhiiltnisse der Saftstrome in den Zellen der Charen in Monats- l)eiichte der Berliner Akad. der Wiss. 1852 u. 1853. — Pringsheim, Ueber die nacktfiissigen Vorkeime der Charen, in Jahrb. f. wissen. Bot. 1864, vol. III. — NTgeli, Die Rotationsstromung der Charen, in dessen Beitrogen zur wissen. Bot. i860, vol.11, p. 61. — Thuret, Sur les antheridies des cryptogames, Ann. des Sci. Nat. 1851, vol. XVI. p. 19. — Montagne, Multiplication des charagnes par division, ditto, 1852, vol. XVIII. p. 65. — Goppert u. Cohn, Ueber die Rotation in Nitella flexilis, Bot. Zeitg. 1849. — De Bary, Ueber die Befruchtung der Charen, Monatsber. der Berliner Akad. May 1871. [For additional Bibliography, see Lindley, Vegetable Kingdom, 3rd edit. p. 28.] CHARACE.E. 279 From the central cell of the fruit of Chara^ the sexual leaf-forming plant is not immediately developed, but a Pro-emhryo precedes it, which attains only small dimen- sions and consists of a single row of cells with limited apical growth. The stem of the Leaf-bearing Sexual pla?it springs from a cell which lies at some distance from the apex of the pro-embryo and grows in a direction nearly at right angles to that of its axis. The unlimited apical growth of the plant depends on an apical cell (Fig. 199, C, t) from which segments are cut off by septa. Each segment immediately divides again by a septum into two superposed cells, the lower one of which ig) always grows without further division into a long internode (frequently 5 to 6 cm. in length) ; the upper one scarcely lengthens, but is first divided in half by a vertical wall, and each half then divides by further successive septa so as to form a whorl of peri- pheral cells. From the node thus constituted the leaves are developed, each from a peripheral cell, and the normal lateral branches, which always originate from the axil of the first or of the two first leaves of the whorl. The leaves of such a whorl, from 4 to 10 in number, repeat in a modified manner the develop- ment of the stem, but their apical growth is limited. After the formation of a definite number of segments, the apical cell ceases to divide and grows into the terminal cell of the leaf which is usually painted (Fig. 199, A, h"). From these leaves lateral leaflets may arise in a similar manner to that in which the leaves themselves have been formed from the stem ; and the leaflets may again in turn produce others of a higher order. The successive whorls of a stem alternate, and in such a manner that the oldest leaves of the whorl, in the axils of which the branches stand, are arranged on a spiral line winding round the stem. Each internode also usually under- goes a subsequent torsion in the same direction. The lateral shoots, of which in Chara one is always developed in the axil of the oldest, in Nitella one in the axil of each of the two oldest leaves of the whorl, repeat the primary stem in all respects (Fig. 210, p. 287). It has already been mentioned that the leaves undergo a segmentation similar to that of the stem ; they also consist at first of very short internodes which are afterwards greatly elongated (Fig. 199, B, y), and are separated by inconspicuous transverse plates or nodes. From these the leaflets arise in whorls the members of which are formed in succession, but they are directly superposed one above another, and do I-IG. 198. — Chara fragilis ; sp germina- tinfj spore; i d q pi together form the pro- embryo (// is segmented, which is not clearly indicated in tiie drawing) ; at d are the rliizoids iv ; w' the so-called primary root; ^ the first leaves (not a whorl) of the second generation or leaf-bearing plant (after Pringsheim, x about 4). This has not yet been observed in Nitella. 2^0 CHARACE.E. not alternate like the whorls of primary leaves (Fig. 200,'C-E, /3). Each leaf begins with a node (the basal node ^), by which it is united with the stem-node, just Fig. 199.— Longitudinal section tlirough tlie biul of Cliara fragiLis ; in A the contents of the cells have been removed ; in B the fine-grained substance is protoplasm, the larger granules are chlorophyll ; the formation of vacuoles is shown ; in C the contents of the cells have been contracted by iodine solution (X500). Fig. 200.— Leaves oi Chara fragilis ; a terminal cell, b penultimate cell of a leaf; z internodal cell; w cells of the leaf- node ; y" mother-cell of a leaflet and of its basal node ; from it arise v and u (the uniting cell), br the basal node which produces four simple cortical lobes and jS the leaflet. A and C in longitudinal section, B an entire young leaf, external view, with the 'stipule' J and its descending cortical lobe of the stem sr ; D external view of the middle part of cin older leaf, though still young ; E transverse section of a leaf-node, of the same age as D. like each leaflet with its primary leaf. These basal nodes are the points of origin of the formation of the cortex which, in the genus Chara, covers the internodes * The cell x in Fig. 199, A, may however be considered also as the first intern ode of the leaf; then the nodes of the stem would consist only of the middle plate m, which is bisected by a longi- tudinal wall. A comparison with Muscineoe and Vascular Cryptogams leads however also to the supposition that the whole group of cells ;c S r" r" which proceeds from y belongs in common to the stem and leaf: CHARACE.E. 281 of the stem, but is wanting in Nitella. From the basal nodes of each leaf one cortical lobe which is morphologically individuaHsed runs downwards, and one upwards^ (Fig. igg, B,r, r\ /' and Fig. 201). In the middle of each internode there fore as many ascending cortical lobes as there are leaves in the whorl meet with the cortical lobes that ascend from the next 'whorl below. The number of the latter is, however, smaller, ^because the leaf in the axil of which the lateral shoot arises does not form an ascending lobe. The cortical lobes are in close con- tact laterally, and form a closed envelope round the internode, the ascending and descending lobes dove-tailing in a prosenchymatous manner. The forma- tion of the cortex takes place so early that the elongating internode is covered Fig. eot.— Development of the cortex of the stem of Chara fragilis ; A a very young internode of the stem with the cortical lobe r still consisting of one cell ; B—D its further development ; r r signifies in all the figures the cortical lobes that ascend from the lower, r' r' those that descend from the upper leaves ; i>v the apical cell of each cortical lobe ; ^^ its inter- nodal cells, n m n the commencement of the formation of the node ; Dc the central cell of a cortic2iI node ; .S signifies in all the figures the unicellular 'stipules' which spring in pairs from the base of the leaves. by it from the first, the lobes keeping pace with its extension in length and thickness. Each lobe continues to grow, like the stem, by means of an apical cell, which becomes segmented by horizontal septa ; out of each of the segments cortical internodal and nodal cells are formed by repeated divisions. The latter divide, by successive septa, into an inner cell, in contact with the internode of the stem, and three outer cells, the middle one of which commonly grows into the form of a spine or knob, resembling a leaf. The outer lateral cells of the cortical node, on the other hand, foll'owing the elongation of the internode itself, grow into longer tubes, so that each cortical lobe consists of three parallel rows of cells, the middle row however containing alternately short and long (inter- nodal and nodal) cells. The cortex of the leaves is derived from the leaflets, and its formation is much simpler (Fig. 200, C-E, br). From the basal nodes of Chara other foliar structures also arise, both on the inner and outer side of the base of the leaf (Fig. 199,6'), which Braun calls Stipules ; they are always unicel- lular, and are sometimes very short, somethnes elongated. * The first internode of every branch and leaf becomes covered with a cortex derived only front! the descending cortical lobes of the next node above. ;82 CHARACE.E. The nodes are the part from which all the lateral members of the Characese originate. The root-like structures or Rhizoids spring from the outer cells of the lower nodes of the primary shoot, and consist of long hyaline sacs growing obliquely downwards, and elongating only at their apex. They are formed by the outgrowth of flat cells at the circumference of the node, and are therefore at- tached to it by a broad base; but these bases of the' stouter rhizoids themselves divide still further, giving rise, especially at the upper margin, to small flat cells from which slender rhizoids are developed. The rhizoidal tubes are segmented by only a few septa which lie far below the growing apex, and have at first an oblique position. The two adjoining cells abut one another like two human feet placed sole to sole. The branching always proceeds only from the lower end of the upper cell (Fig. 202, B) ; a swelling is here formed which becomes cut off" by a wall, and by further division produces several cells which grow into branches ; these therefore stand on one side like a tuft. The tubular cells com- posing the rhizoids attain a length of frorn several millimetres to more than two centimetres, with a thickness of from ^^ to j\y mm. The Vegetative {asexual) Reproduction of Characeae always proceeds at the nodes, and has three modifications: — (i) Nodular formations called Bulbils which occur in Chara stelUgera. They are isolated underground nodes with greatly abbreviated whorls of leaves of beautiful regu- larity ; their cells are densely filled with starch and other fornmtive materials; new plants are pro- duced from shoots laterally developed. (2) The Branches with naked base of Pringsheim. These are formed on old hibernating or on cut nodes of Chara in the axils not only of the oldest but also of the younger leaves of a w^iorl, and are in fact only slightly diff"erent from the normal branches, the greatest difference being in the partial or entire absence of the cortex of the lower internode and of the first whorl of leaves. The cortical lobes which descend from the first node of the branch often become detached from the internode and grow free, curling upwards, while the leaves of the lowermost whorl often do not form nodes. (3) The Pro-embiyonic Branches. These spring, together with the last, from the nodes of the stem, but are essentially different from the branches, and have a similar structure to the pro-embryos which proceed from the spores. Like the last, they have only been observed in Chara fragilis (by Pringsheim). A cell of the node becomes elevated, and growls into a tube, and its apex becomes separated by a septum. In this growing terminal cell further divisions take place, till the 'apex of the pro-embryo' which proceeds from it consists of a row of from three to six cells. Beneath the apex of the pro-embryo (Fig. 203, C, a, b) the tube swells, and the distended part becomes separated by a septum as a cell, which Pringsheim calls the * bud -rudiment,' (Fig. 203, C, I-IG. 2U2.— Rhizoids of Chrra fragilis ; A end in process of development ; B a 'joint,' the lower part of the upper cell is branching (after Prings- heim, X 240). The arrows indicate the direction of the currents of protoplasm. CHARACE^. 283 including the parts from v to d). This cell is now divided by two oblique walls into three cells, the middle one of which (q) lengthens into a tube (like an inter- node), while the upper and lower ones remain short. Out of the lower cell is afterwards formed a root-producing leafless node (Fig. 203, d, and Fig. 198, d), while the upper one, M'hich lies between the apex of the pro- embryo a b and the elongated cell q becomes the axis of the new generation. It becomes arched on one side outwards, and divides in succession into the cells /, //, ///, and V. Each of the cells /,. //, and /// becomes transformed by divisions into a disc of cells or transitional node, three of which thus stand over one another without intermediate internodes. Their lateral cells grow right and left, and form imperfect leaves of different lengths. The cell which lies outermost (Fig. 203, C, v) now begins to un- dergo a series of divisions, corre- sponding to those of a normal leaf- bearing shoot. It is, in fact, the mother-cell and at the same time the first apical cell of the new generation, i.e. of the sexual leaf-bearing plant which arises from the pro-embryo. The dis- placement indicated in Fig. 203, C, subsequently causes the apex of the pro-embryo to be pushed to one side ; and since this apex has the appearance of a simple leaf uncovered by cortex, the further development of the lateral leaves which spring from the cells /, //, and ///, brings about an appearance as if these different leaves together formed a whorl ; and the bud of the lateral shoot thus comes to stand ap- parently in the centre of this pseudo- whorl (Fig. 203, A). If the structure which springs from the germinating spore is now compared with the pro-embry- onic branch, the perfect homology cannot fail to be observed which Pringsheim pointed out in the parts that will be found indicated by the same letters in Figs. 198 and 203; but the pro-embryo of the spore has in addition a small node at the opening of the spore from which a rhizoid, sometimes called the primary root of Chara, springs (Fig. 198, w'). The Sexual Reproduction of the Characeae results from organs which, in their development and definite form do not, in the present state of our knowledge, FIG. 20-^.— Chara fragiUs ; A an entire pro-embryonic branch; i the lowermost colourless cell below the root-node ; rfroot-producing leafless node ; q the long cell proceeding from the middle cell of the bud-rudiment; // apex of the pro-embryo; g the pseudo-whorl of leaves, v the bud of the second generation of the leaf-bearing plant ; B upper part of a young pro-embryonic branch ; z', d, q as before, b apex of the pro-embryo ; /, //, /// the young leaflets of the transitional node, v the bud of the leafy stem ; C still younger pro-embryonic branch ; i, d, q, b as before ; /, //, /// the cells out of which the transitional nodes arise, v apical cell of the stem-bud (after Pringsheim, iSxijo). ^«4 CHARACE.Ji. correspond to those of Thallophytes, nor to those of Muscinese or Vascular Cryptogams. To the male organs or Antheridia of Characeae the term Globules is generally given ; while the female organs, which are neither oogonia nor trichogynes nor archegonia, are usually called Nucules ^ Globules and nucules stand on each side of the leaves ; the globules are always the metamorphosed terminal cell of a leaf or lateral leaflet ; the nucules spring, in the monoecious species, close beside them from the basal node of the same leaflet (Chara), or from the last node of a primary leaf crowned by a terminal globule (Nitella). The nucule therefore stands, in the monoecious Nitellae beneath, in the Charae above of beside the globule. In the dioecious species, these relationships of position of course fail ; but the morpho- logical significance and position remain unchanged. Both kinds of organs may first be examined in the mature state. The Globules are globular bodies, ^ ^o i mm. in diameter, at first green, then red. The wall consists of eight flat cells, four of which, situated around the distal pole of the ball, are triangular, while the four situ- ated around the base are quad- rangular and become narrower below ; each of these cells forms a segment of the shell of the ball, and they are hence called Shields. When unripe their inner face is covered with green grains of chlorophyll, which, in the ripe state, are of a red colour. Since the outer face is destitute of these c:rains, the outside of the ball appears clear and transparent (Fig. 204, -c4). From the lateral edges several folds of the cell- wall penetrate towards the middle of each shield, which gives them the appearance of being lobed in a radiate man- ner. From the middle of the inner face of each shield a cylindrical cell projects in- wards, nearly to the centre of the hollow globule ; these cells are called the Maniibria. The flask- shaped cell which supports the globule also penetrates into the interior between the four lower shields ; at the central end of each of the eight manubria is a roundish hyaline cell, the Head ; and these twenty- FlG. 204. — Chara /ragilis ; A middle part 01 a leaf* with a globule a and a nucule S, c its crown ; j8 sterile lateral leaflet ; j3' large leaflets by the side of the nucule ; /3" the bracteoles, springing from the basal node of the nucule (X about 50). R a young globule a wth a still younger nucule SK; w the nodal cell of the leaf, n the union-cell between it and the basal node of the globule ; / cavity of the internode of the leaf; br cells of the leaf covered with cortex (X350) (cf. Fig. 201). * [' Spore-biids ' of Sachs, 'ovum-buds' of De Kary.] CHARACE.E. ^«5 five cells form the framework of the globule. Each head bears in the centre six smaller cells (secondary heads), and from these grow four long slender whip-shaped filaments, which, being coiled round and round, fill up the interior of the globule (Fig. 205, B). Each of these filaments (the number of which amounts to about 200) again consists of a row of small disc-shaped cells (Fig. 205, D, F, F), numbering from 100 to 200. In each of these 20,000 to 40,000 Fig 206. — Nitella flexilis ; A fertile branch (natural size) ; i internodc, b leaves ; B upper part of a fertile leaf b with the node K ; on the node are two lateral leaves n b, and two very young nucules S; a a globule ; C older leaf with two leaflets, a ripe globule a, and two unripe nucules S ; D a lialf-ripe nucule more strongly magnified. Fig. 2o%.—.\i(ella Jltxitis; A an almost ripe giolniie at the end of the primary leaf, by its side two lateral leaflets, i neutral lines ; the arrows indicate the direction of the currents of protoplasm ; R a manubrium with its head and the whip-shaped filaments, in which the antherozoids arise ; C end of one of the you/ig filaments ; D middle part of an older one ; E of one still older ; F ripe antheridial filament with antherozoids C (C — G X 550). cells is formed an antherozoid, a slender spiral thread, thickened behind, and bearing at its pointe'd end two long fine cilia (Fig. 205, G). When perfectly ripe, the eight shields fall apart, their spherical curvature becoming diminished ; the antherozoids leave their mother-cells and move about in the water. This breaking up appears generally to happen in the morning, and the antherozoids are in motion for some hours, till evening. The mature Nucule, when ready for fertilisation, is a longer or shorter prolate spheroid ; it is placed upon a short pedicel, visible externally only in Nitella, and con^ sists of an axial row of cells, closely surrounded by five tubes which are coiled round it spirally. The whole must be considered as a metamorphosed shoot. The pedicel corresponds to the lower internode of a shoot; it bears a short nodal cell, from which the five enveloping tubes spring as like a whorl of leaves. Above the 2S6 CHARACEM. nodal cell rises the peculiarly developed apical cell of the shoot, very large as com- pared to the other parts, and ovoid. At its base, immediately above the nodal cell, an inconspicuous hyaline cell is separated at an early stage in Chara ; in Nitella a somewhat disc-shaped group of similar cells takes its place, which have been termed by Braun ' Wendungszellen.' The large apical cell of the nucule is filled with a number of drops of oil and grains of starch as well as with protoplasm ; it contains pure hyaline protoplasm only in its apical region (the apical papilla). The enveloping tubes, which contain a quantity of chlorophyll, project above the apical papilla and bear the Croivn, consisting in Chara of five larger, in Nitella of five pairs of smaller cells, which have already been separated at an early stage from the enveloping tubes by septa. Above the apical papilla and beneath the crown, which forms a compact lid, the five enveloping tubes form the neck which encloses a narrow cavity, the apical cavity ; above the papilla this cavity is of an obconical figure narrowing upwards, the five divisions of the neck projecting and forming a kind of diaphragm, through the central very narrow opening of which the union with the upper roomy part of the apical cavity is effected. This is closed above by the crown ; but, at the time of fertilisation, it opens externally by five clefts between the five divisions of the neck of the sac ; and through these clefts the antherozoids penetrate into the apical space filled with hyaline mucilage, to find their way into the apical papilla of the oosphere, where the cell-wall is apparently absent. After fertilisation the chlorophyll-grains of the envelope become reddish-yellow, the wall of the tubes which lie next the oosphere increase in thickness, become lignified, and assume a black colour; and thus the oosphere, now transformed into an oospore, becomes surrounded by a hard black shell with which it falls off, to germinate in the next autumn or spring. The Characeae are distinguished by the size of their cells, and by the simple relations of the individual cells to the structure of the whole substance. All the young cells contain nuclei, which at first always lie in the centre of the protoplasm that fills up the whole cell ; each bipartition of a cell is preceded by the absorption of the nucleus and the formation of two new nuclei. As the cells grow vacuoles form in the protoplasm which finally coalesce into a single large vacuole (the sap-cavity). The protoplasm, now clothing the wall as a thick layer, commences its rotatory motion which always follows the longest direction of the cell ; the nucleus about this time becoming absorbed, while grains of chlorophyll are formed. With the growth of the whole cell these grains also grow and multiply by repeated bipartition ; they adhere to the inner side of the outermost thin stationary layer of protoplasm, and take no part in the rotation of the layers which lie further inwards. The rotating protoplasm becomes differentiated, as the cell grows, into portions some very watery and others less watery and denser, the former appearing as hyaline cell-sap in which the latter float in the form of roundish larger or smaller lumps. Since these denser bodies are passively swept along by the rotating clear protoplasm, as may be seen from their tumbling over one another, the appearance is presented as if the cell-sap caused the rotating motion. Together with the denser lumps of protoplasm of less regular form, there are also a number of bodies of globular shape covered with delicate spines, consisting also of protoplasm. The current, as Nageli has shown, is most rapid next the stationary parietal layer, and becomes CHARACEM. 287 gradually slower towards the interior ; hence the spheres and globules which swim in the thin rotating protoplasm tumble over one another, because they become immersed at different spots in layers of different rapidity. Dependent on the direc- tion of the current, the grains of chlorophyll are arranged in longitudinal rows on the stationary layer, and are deposited so thickly that they form a stratum ; they are absent only at the neutral lines (Fig. 20^, A, i). These neutral lines mark the position where the ascending and descending portions of the rotating protoplasm of a cell run side by side in opposite directions and neutralise each other, and where therefore there is no motion. The direction of the rotatory motion in each cell stands in a regular relation to that of all the other cells of the plant, and hence to its morpho- logical structure, as has been shown by A. Braun. With regard to the various processes of development, I will here describe only those of the Globules and Nucules. Globules. The order of development of the cells has already been exhaustively de- scribed by A. Braun in the case of Nitella symcarpa and Chara Baueri ; it agrees with that of Nitella flexilis and Chara fragilis. In Nitella the terminal cell of the leaf becomes the globule ; the oldest leaf of a whorl first forms its globule, the others follow according to their age ; the globules are recognisable even in the earliest state of the whorl of leaves. In Fig. 207, A^ is shov/n the longitudinal section through the KiG. 207.— Development of the antheridia oi Nitella flexilis. In R, C, and D the protoplasm has been contracted by glycerine. apex of a branch, / being its apical cell ; its last-formed segment has already been divided by a septum into a nodal mother-cell K and an internodal cell lying beneath it ; beneath this lies the node with the last whorl of leaves ; b is its youngest leaf, bK the basal node of the oldest leaf which already consists of the segments 7, //, ///; a is the terminal cell of this leaf which becomes transformed into the globule. While the globule is becoming developed, the leaf also undergoes still further changes which must be first considered. The segment III becomes the first internode of the leaf, 7/ becomes a node from which are developed the lateral leaflets nb \x\ C and 7). The cell 7 divides into two (C, 7), the lower of which remains short, while the upper grows into a flask-shaped cell (Fig. 207, 7),/, and Fig. 208). The globular mother-cell of the globule (Fig. 207, A, a') first of all divides into two hemispheres b.y a vertical wall passing through the axis of the leaf; these are divided into four segments by a vertical wall at right angles to the first ; in each of the four quadrants a third division takes place horizontally and at right angles to the two last ss CHA RACEME. walls ; and the globule now consists of four lower and four upper octants of a sphere. Contraction by glycerine clearly shows that each of these divisions of the protoplasmic body is completely effected before the appearance of the cellulose-wall (Fig. 207, 5); the second division even takes place before the wall has arisen between the two first- formed halves ; and the four quadrants may be made to contract without any wall being visible between them. In Fig. 207, B, the third division has also taken place, the second ver- tical wall is already formed, and the two quad- rants there visible are already divided ; but no horizontal wall has yet appeared. In Fig. 207, u4, a, are shown the eight octants in perspective together with their nuclei. Each octant now breaks up first of all into an outer and an inner cell (Fig, 207, C) ; the latter is again divided in all the eight octants (D), so that each octant now consists of an inner, a middle, and an outer cell (Z), /, m, e). Up to this time the globe re- mains solid, and all the cells lie close to one another ; but now commences an unequal growth, and w^ith this the formation of inter- cellular spaces (Fig. 208). The eight outer cells (£■) are the young shields, the side-walls of which show even at an earlier period the radial infolding already mentioned. They grow more strongly in a tangential direction than the inner cells, the outside of the globe increasing more rapidly than the inside ; the middle cells (w), which form the manubria, remain attached to the centres of the shields, but are separated from one another by the tangential growth of the shields ; they grow slowly in the radial direction ; the innermost cell / of each octant is rounded off and becomes the head. The celiy in Fig. 207, D, now also grows quickly, and forces itself between the four low^er shields into the interior of tlie ^lobe ; it becomes the flask-shaped cell, upon the apex of w^hich rest the eight heads. In Fig. 208 this condition of the globule is shown in longitudinal section ; where the walls of the heads bound the intercellular spaces which have now been formed and are filled with fJuid ; they put out branches (c) which become septate, and again ramify ; and these branches elongate by apical growth and also become septate. The lowermost cells swell up into a roundish shape, and form the secondary heads, upon which stand the whip-shaped filaments, consisting of the discoid cells which are the mother-cells of the antherozoids. (Compare Fig. 208 with Fig. 205, B.) The globules of Chara fragilis are produced by metamorphosis of those leaflets which form the innermost row on a leaf, and in fact, as is shown in Fig. 210, the develop- ment advances downwards to the primary leaf. The succession of cells and the mode of growth show no noteworthy difl^eren-ces from those of Nitella ; the flask-shaped pedicel is here placed on a small cell wedded in between the cortical cells, the central cell of the basal node of the leaflet, which Braun asserts to be present also in sterile leaves, where however I have not succeeded in finding it. Antherozoids. The whip-shaped filaments in which the antherozoids arise, do not grow merely at their apex, but have also an intercalary growth. This is shown by the elongated cells in the middle of young filaments, each with two nuclei, between which no division-wall has yet been formed (Fig. 205, C). The longer the filaments become, the more numerous are their divisions, until at length the individual cells have the appearance Fig. 208. — Antheridium o{ Nitellaflexilis in a furtlier stage of development (X about 500). CHARACEM. 289 of rather narrow transverse discs. The further development of the contents of these mother-cells of the antherozoids progresses backwards from the end of the filament • the antherozoids are formed in basipetal order in each filament. At first the nucleus of each mother-cell lies in its centre, later it places itself in contact with the septa ; the nuclei then disappear, and their substance becomes mixed with that of the protoplasm, which now forms a central discoid mass in the mother-cell, surrounded by a hyaline fluid (Fig. 205, £■). From this is formed the antherozoid, in addition to which, when it is mature, there is no granular protoplasm ^ The antherozoids begin to rotate even while within their cell, and escape out of it after the rupture of the antheridium ; the filiform antherozoid has in Nitella 2 or 3, in Chara 3 or 4 coils; the posterior thicker end contains a few glistening granules. 7be Dei'elopment of the Nucules has already been described in detail by A. Braun ; I have also studied it in Nitella Jlexilis and Chara fragilis. In Nitella Jiexilis it springs from the node of the leaf beneath the globule (Fig. 206, B and C) ; its origin is much later than that of the latter. Fig. 209, A, represents a very young nucule, the pedicel of which (/>) bears the smaller nodal cell with the five rudiments of the envelop- ing tubes (/j) (two only are shown here in longitudinal section). Above the nodal cell lies the apical cell (j) of the shoot, which represents the nucule. B represents a iurther stage of development, in which the first of the cells, designated by A. Braun the ' Wendungszelle,' has already made its appearance, and two septa have also appeared on the upper part of each envelop- ing tube ; these upper short cells are raised up by the intercalary growth of the tubes, above the apical cell, and form the crown ^ in C and D. The lowest of the cells of the crown each forms a prolongation projecting inwards and downwards, as shown in Fig. 209 C and Z), so that the whole nucule resembles a ' lobster-pot.' The spiral torsion of the enveloping tubes does not begin till a later period ; the coils become gradually flatter while the apical cell of the nucule increases considerably in size and developes into the oosphere (Fig. 206). The development and fertilisation of the nucule of Chara has recently been described in detail by De Bary in the case of C fcetida. Here also it consists, from an early stage of its development, of an axial row of three cells, and five others consisting each of two cells which form an envelope round it. The lowermost cell of the axial row is the nodal cell, the second remains small and colourless, and corresponds to the first 'Wendungszelle' in Nitella. It becomes in this case also, as De Bary's drawings show, separated by a somewhat oblique septum at the base of the apical cell (now the third of the axial row). Originally almost hemispherical, the apical cell grows first of all into the form of a narrow cylinder, and then becomes ovoid; it is provided, until it attains its full size, with a thin very delicate cell-wall ; drops of fat and starch grains accumulate in its protoplasm. Its apex however remains free, and forms a Fir.. 209. — Development of the nucule of Kitella Jlexilis (x about 300); -.- ' Wendungszellen.' * Compare the opposite view of Schacht, Die Spermatozoiden im Pflanzenreich, 1864, p. 30. U u CHARACEM. transparent tinely granular terminal papilla, the receptive portion ; the apical cell of the nucule has therefore become transformed into an oosphere. The five enveloping tubes are from the first in close contact with the apical cell or oosphere; after each has become divided by a septum about half way up, the uppermost of the cells thus separated also become closely united with one another above the oosphere. This closing of the envelope takes place at least in Charaf(Etida, before the ' Wendungszelle' has separated from the oosphere. The five upper cells of the envelope are at first as long as the five lower ones, and the septa which separate them lie about half way up the oosphere. As it now increases in size, the five lower ones become elongated into long tubes, which are at first straight but afterwards wind spirally round the oosphere. The five upper cells form the crown, which is elevated some distance above the apex of the oosphere. Between the crown and the apex of the oosphere the enveloping tubes grow inwards and increase in breadth, so that together they form, above the apical papilla of the oosphere, a thick diaphragm open only in the middle, by which a narrow space lying below the crown is separated from a still narrower one above the oosphere. The cells of the crown form a closed cover above the upper space ; the upper and under space are united through the narrow opening in the diaphragm. De Bary found a similar struc- ture in Nitella. As soon as the nucule attains its full size, the small space above the diaphragm enlarges, while the tubes between the diaphragm and the crown grow longer. This piece of the envelope, which only attains its full size at a later period, _De Bary calls the Neck. The sacs now separate laterally from one another, forming five clefts below the crown and above the diaphragm. Through these clefts the antherozoids force their way in great numbers into the apical space, which is filled by a hyaline mucilage. That one or more of them even find their way into the oosphere is rendered the more certain by the fact that about this time its papilla is protected by a very weak cell-wall or has none at all, as is shown by the small pressure required to expel its contents into the apical space. It may therefore be considered as demonstrated that the apical cell of the nucule is actually the oosphere of Gharaceae. A. Braun's description of the morphological value of the nucule of Chara is fully confirmed by our Fig. 210, A. It is necessary to explain, in the first place, that this is the lower part of a young fertile leaf oi Chara fragilis, together with the contiguous piece of the stem, and an axillary bud represented in longitudinal section ; m is half of the nodal cell of the stem, i its upper, i its lower internode, sr a descending, y an ascend- ing cortical lobe ; sr the cortical lobe of the lower internode which descends from the leaf, rK a node of it ; i" the first internode of the axillary bud which rests upon the cell n that unites the nodal cell m with the basal node of the leaf. The leaf shows its three lower internodes, 2, a, «, still rather short ; they eventually attain from 6 to 8 times this length. Between them are the nodal cells w, nv ; v, v are the cells which unite the leaf-node with the basal node of the leaflet ^ on the outer side of the leaf ; a the corresponding cells on the inner side of the leaf; l?r the cortical lobes of the leaf, two of which go upwards and two downwards from each leaflet /S; the lowermost internode of the leaf is however covered only by descending lobes ; by the side of one of them stands the stipule s. x, x are the cortical lobes which descend on the inside of the internodes of the leaf, where the leaflets are transformed into globules, a, a ; the ascending cortical lobes of the leaf are here absent, because one nucule always springs from the basal node of each leaflet. (Compare with this Fig. 204, A and B.) In reference to the origin of the nucule, A. Braun says {I.e. p. 69) that it springs from the basal node of a leaflet just as the branch does (in Chara fragUis from the basal node of a globule which stands in the place of a leaflet). As in the leaf which subtends a branch the ascending cortical lobes are wanting, so also in the leaflet which bears the nucule the cells forming the ascending portion of the cortex are also wanting. As it is the first leaf of the whorl on the stem that produces a branch in its axis, so it is also from the first (inner) leaflet of the whorl on the leaf that the nucule originates. The basal node of the globule in C. fragUis has, according to A. Braun, not four CHARACE.^. 291 peripheral cells, as in sterile leaflets, but five ; an upper odd one which is first formed, two lateral ones which follow, and two lower ones which are formed last of all. Of these five cells only the two lower ones are developed into cells which form the cortex (of the leaves), the upper one, wanting in the sterile basal nodes, is the mother-cell of the nucule • but the two lateral ones are developed into leaflets which stand laterally between the globule and nucule {cf. Fig. 204, /3"); these latter Braun calls Bracteoles. The mother-cell of the nucule now grows out of the axil of the globule, and divides FIG. ij^i.—Charafra^ilis; A lower part of a fertile leaf, a lateral bud springing from its axil ; B lower part of a sterile leaf without an axillary shoot (in longitudinal section). itself by a septum into an upper outer terminal cell and a segment which in its turn is broken up into two discs by a w^all paraflel to the previous one (Fig. 210, A, SK). The lower cell does not divide any further, it forms the concealed pedicel of the nucule, and corresponds to the first internode of a branch ; but the upper one has the character of a nodal cell ; it is divided by tangential walls into a zone of five outer and one inner cell (SK') ; the former are the rudiments of the enveloping tubes, which are therefore morphologically leaves. u 2 :yZ MUSCINE/E. GROUP III. MUSCINEyE. The Hepaticse and Mosses, which are comprised under the term IMuscinese, are distinguished by a sharply defined Alternation of Generations. From the germinat- ing spore is developed either immediately a sexual generation rich in chlorophyll and self-supporting (as in most Hepaticae), or a confervoid thallus is first formed (the Pro-embryo or Protonema), out of which the sexual generation grows as a lateral shoot (as in some Hepaticae and all Mosses). In the female sexual organ of this first generation there arises— as a new generation the result of fertihsation — a structure of an entirely different form, which is destined exclusively for the produc- tion of asexual spores. Without being organically united to the previous generation, this structure is nevertheless nourished by it, and appears, when observed externally, simply as its fruit ; it is hence called indifferently Fruit or Sporangium. Since however it is an organism of an altogether peculiar kind, it may be desirable to give it a special name, which shall at once exclude any false analogy ; I propose therefore to call it the Sporogoniiim. The Sexual Gefieratio7i of Muscineae which is produced directly from the spore or with the intervention of a pro-embryo, is either a flat leafless thallus, as in many Hepaticse, or a slender leafy stem, often much branched. In both cases, which are united by gradual transitional forms ^ a number of root-hairs are usually formed, which fix the thallus or the stem to the substratum. In some cases this vegetative structure scarcely attains a length of i mm., but in others as much as from lo to 30 cm. or even more, and ramifies copiously. In some of the smallest forms its term of life is Hmited to only a few weeks or months ; in most it may be termed unlimited, since the thallus or the leaf-bearing stem continually grows at its apex or by a process of renewal (Innovation), while the oldest parts die off behind. In this manner the branches become finally independent plants ; and this, as well as the multiplication by gemmae, stolons, detached buds, the transformation of hairs into pro-embryos (in IMosses), &c., serves not only to increase enormously the number of individuals formed by the asexual method, but is also the immediate cause of the social or cespitose mode of growth of these plants. Many IMosses in particular, even those which only rarely fructify, may in this manner form dense masses extending over considerable areas (as Sphagnum, Hypnum, Mnium, &c.). ^ From the great similarity of the true leafless thallus of some Hepaticas to the thalloid stems of others furnished with leaves on the under side, it will be convenient to use the term ' thalloid forms' for both; the term including both a true thallus {e.g. Anthoceros) and also a thalloid stem (as in Marchantia). MUSCINEM. 20^ The sexual organs are Antheridia and Archegonia. The mature Anther idiwn is a body with a longer or shorter stalk, of a spherical, ellipsoidal, or club-shaped form, the outer layer of its cells forming a sac-like wall, while each of the small and very numerous crowded cells enclosed within it developes an antherozoid. The anther- ozoids are freed by the rupture of the wall of the antheridium at the apex ; they are spirally coiled threads thicker at the posterior and tapering to a fine point at the anterior end, at which are placed two long fine cilia, the vibrations of which cause their motion. The female organs, which since the time of Bischofif have been called Archegonia, are, when in a condition capable of being fertilised, flask-shaped bodies bulging from a narrow base and prolonged into a long neck. The w^all of the ventral portion encloses the central cell, the protoplasm of which, contracting and rounding off, forms the oosphere. Above this begins a row of cells which passes through the neck in an axial direction, and is continued as far as the cells which form the so-called ' Stigma.' The cells of this a,xial row become broken up before fertilisation, and transformed into mucilage which finally swells up and forces apart the four stigmatic cells. In this manner an open canal is formed, which leads down as far as the oosphere, and enables the antherozoids to enter it. This mucilaginous axial row of cells occurs also in the archegonium of Ferns, but in Rhizocarpese and Lycopodiacex is reduced to a single rudimentary cell. The great diversity in the origin of the sexual organs of Muscinese is of extreme importance. In the thalloid Hepaticae these organs arise below the growing apex from the superficial cells of the thallus or of the prostrate thalloid stem, or on specially metamorphosed branches (as in the Marchantieae) ; in the foliose Jungermannie^e and in the Mosses not only the antheridia but also the archegonia may be formed from the apical cell of the shoot or from segments of it ; in this case they may take the place of leaves, or of lateral shoots, or even of hairs. Thus the antheridia appear as metamorphosed trichomes in the axils of the leaves of Radula, as metamorphosed shoots in Sphagnum, as apical structures and also as metamorphosed leaves in Fontinalis. In the same manner the first archegonium of the fertile shoots of Andraea and Radula arises from the apical cell, the later ones from its last segments ; and this is probably the case in Sphagnum. Antheridia and archegonia are usually produced in great numbers in close proximity ; in the thalloid forms of the Hepaticse they are generally enveloped by later outgrowths of the thallus ; in the foliose Jungermanniese and in Mosses several archegonia are commonly surrounded by one envelope formed of leaves which is termed the PerichcEtium ; in Mosses a male flower (sometimes a her- maphrodite one) is usually formed in this manner, while the antheridia of the Jungermanniese and of Sphagnum stand alone. Very commonly, especially in the foliose kinds, Paraphyses, i. e. articulated threads or narrow leaf-like plates of cells, are formed in the male and female flowers by the side of the sexual organs. Besides the envelopes just named, there is also often in Hepaticse (but not in Mosses) a so-called Perianth, which grows as an annular w^all at the base of the archegonium, and finally surrounds it as an open sac. The Asexual Generation or Sporogonitwi, arises in the archegonium from the fertilised oosphere or oospore. It first developes by repeated cell-divisions into an ■1)4 MUSCINEM. ovoid embryo, growing at the end turned towards the neck of the archegonium, viz. the apex. Its final form is very different in different sections. In its lowest type (in Riccia) it is a globe, the outer cell-layer forming the wall, while all the inner cells become spores. In all other cases the sporogonium becomes differ- entiated externally into a slender stalk or Seta which penetrates into the bottom of the archegonium and even into the underlying tissue, and a Capsule (Urn or Thecd) turned towards the neck of the archegonium, in which the spores arise. Together with the spores, long cells thickened by spiral bands, the FAaters, are also produced in most Hepaticae. The internal differentiation of the spore-capsule is, in addition to this, very varied, and attains a very high degree of complexity, especially in the Hepaticee. While the sporogonium is developing, the ventral portion of the archegonium also continues to grow ; its cells increase rapidly in number, and it thus becomes broader, enclosing the young sporogonium, and, in this condition, is termed the Calyptra. Its behaviour supplies distinctive characters for the larger groups. In the lowest Hepaticae (Riccia) the sporogonium remains always enclosed in the calyptra; in the higher Hepaticae it protrudes only after the ripening of the spores, its stalk elongating suddenly, and the capsule protruding from the ruptured calyptra for the purpose of disseminating the spores, the calyptra surrounding the base of the seta as a cup-like membranous structure. In the typical Mosses, on the other hand, the young sporogonium first assumes the form of a greatly elongated fusiform body, which, even before the development of the capsule, exerts a strong upward pressure upon the calyptra, which becomes detached at its base, and is raised up by the young sporogonium in various forms ; the seta penetrates deep down into the tissue of the stem, by which it is surrounded as a sheath ( Vag inula). The spores of the Muscineae arise i7i fours ; the mother-cells — w^hich had previously been united into a tissue with the surrounding cell-layers, but had become isolated even before the formation of the spores — show a rudimentary division into two previous to complete division into four. The number of the mother-cells and the place where they are produced in the sporogonium depends essentially on the internal differentiation of the latter. The ripe spores show a thin cuticle (the Exospore) provided with small excrescences, which is ruptured on germination by the inner layer of the cell- wall (the Endospore). Its contents consist, in addition to colourless protoplasm, of grains of chlorophyll, starch, and oil. The Differentiation of the Tissues of Muscineae is very various, and more con- siderable than in Algae, but less so than in Vascular Cryptogams. Fibro-vascular bundles are not found ; only in the stem and leaf- veins of the more perfect Mosses is an axial bundle of elongated cells differentiated, which may be con- sidered as a slight indication of the fibro-vascular system. The Marchantiese, on the other hand, show on the upper side of their thalloid stems, and the Mosses on their thecae, a distinctly differentiated epidermis, which usually also forms stomata. The cell-walls of the Muscineae are generally firm, often thick, tough, and elastic, and in this case frequently of a brown, bright red, or violet colour. The tendency towards the formation of jelly and mucilage, so general in the Thallophytes, is not found in the Muscineae, with the exception of certain processes in the mother-cells MUSCINE.E. ^„- of the spores. Various forms of thickening are not uncommon, especially in the spore-capsule, as in the spiral bands of the elaters of Hepaticse, and the formation of the epidermis and peristome of the thecse of Mosses. Classification of Muscinece. The sexual generation is developed from the spore, generally after the previous formation of a pro-embryo. It is the longest-lived of the two generations, and constitutes the self-supporting vegetative structure of these plants, presenting either a flat dichotomously branched thallus, or a thalloid stem, or a filiform stalk furnished with two or four rows of leaves. True fibro-vascular bundles are not produced. The archegonia and antheridia are, except in the simplest thalloid forms, stalked multicellular bodies usually free, but sometimes buried in neighbouring masses of tissue from the subsequent growth of these latter. The central cell of the ventral part of the archegonium produces the oosphere by rejuvenescence of its proto- plasmic body into a primordial cell. The antherozoids are spirally coiled threads with two cilia on the anterior pointed end. The asexual generation or sporogonium arises from the oosphere within the actively growing ventral part of the archegonium, which becomes developed into the calyptra. The sporogonium is nourished by the sexual plant; it has therefore no independent existence, and appears externally as an appendage to it. It is usually a stalked cap- sule, in which (with the exception of Archidium) a number of cells are always deve- loped into the mother-cells of the spores ; and from these the spores are formed by division into four after bipartition has commenced but has not been completed. (i) Hepaticce. The sexual generation arises either directly from the spore or with the intervention of a small inconsiderable pro-embryo. It is developed as a flat dichotomously branched thallus or a thalloid stem, or finally as a filiform stalk furnished with two or four rows of leaves. This vegetative structure is usually broadly expanded and clings closely to the ground or to some other substratum ; even when the stems grow erect there is still an evident tendency towards the formation of an upper (dorsal) and an under (ventral) surface. The mode of growth is hence always distinctly bilateral. The asexual generation or sporogonium remains surrounded by the calyptra until the spores are ripe ; the calyptra is usually at length ruptured at the apex, and remains at the base of the sporogonium as an open sheath, while the free spore-capsule projects above its apex, to allow the escape of the spores. The mother-cells of the spores arise either from the whole of the cells except those of the single layer which forms the wall of the capsule, or the intermediate cells commonly become developed into elaters. (2) Mosses. The sexual generation is developed from the spore with the intervention of a pro-embryo consisting of branched rows of cells and often vegetating for a con- siderable time independently, even when it has already produced leafy stems by lateral budding. The vegetative body is here always a cormophyte, a filiform stem furnished with leaves in two three or four rows, usually without any definitely indicated bilateral structure, and generally branched in a monopodial, never in a dichotomous manner. The asexual generation or sporogonium is only at first formed in the calyptra, after- wards this is usually ruptured below (at the vaginula), and raised up by the apex of the sporogonium, which covers it like a cap. The capsule, which is now first developed, produces the spores from an inner layer of tissue, while a large inner mass of tissue remains sterile, and forms the Columella. The wall of the capsule is covered by a distinctly differentiated epidermis, the upper part of which usually becomes detached from the lower part (the Urn) in the form of a cover, in order to allow the escape of the spores. 296 MUSCINl/E. CLASS IV. H EPATIC^\ (i) The Sexual Generation is developed, in some genera, directly from the germinating spore, its first divisions resulting in the formation of a cellular lamina or a mass of tissue which fixes itself by root-hairs and produces the thallus by growth at its apex, as in Anthoceros and Pellia. In other cases the embryo which results from the divisions of the spore first forms a narrow ribbon-like lamina of cells, the apical cell of which becomes subsequently the apical cell of a stem, and the segments form leaves, as in Jungermarinia bicuspidaia (according to Hofmeister). Or again, the bud of a leafy stem springs immediately from the spore {Frullania dilatata). In other cases, on the other hand, a pro-embryo is formed; the endospore which grows out into the form of a tube produces a short articulated filament, on which the rudiments of the thallus are formed as lateral shoots, in a manner similar to the leaf-buds of Mosses on the protonema {e. g. Aneura palmata, Marchantia). In Radula the spore produces first of all a cake-shaped plate of cells, from which the first bud of the leafy stem shoots laterally (Hofmeister). The vegetative structure of Hepaticse is always formed in a distinctly bilateral manner; its free side, turned towards the light, is differently organised from that which faces and often clings closely to the substratum and is not exposed to light. In the greater number of families and genera the vegetative structure is a broad, flat, or curled plate of tissue, varying in length from a few millimetres to several centimetres ; and is either a true thallus without any formation of leaves, as in Anthoceros, Metzgeria, and Aneura, or lamellaeform outgrowths arise on the under or shady side, which at the same time produces root-hairs ; and these outgrowths may be looked on as leaves. For the sake of having a common expression for these forms extremely similar in habit, they may be comprised in the term Thalloid^, ^ Mirbel, Ueber Marchantia, in the Mem. de I'Acad. des Sci. de I'lnst. de France, vol. XIII, 1835.— G. W. Bischoff, in Nova Acta Acad. Leopold. Carol. 1835, vol. XVII. pt. 2.— C. M. Gottsche, ditto, vol. XX, pt. I. — Gottsche, Lindenberg u. Esenbeck, Synopsia Hepaticarum, Niirnberg, 1844. Hofmeister, Vergleich. Untersuchungen, 185 1. — [On the Germination, Development, and Fructifica- tion of the Higher Cryptogamia : Ray Society, 1862.] — Kny, Entwickelung der laubigen Lebermoose ; Jahrb. fiir wiss. Bot. vol. IV, p. 66, and Entwickelung der Riccien, ditto, vol. V, p. 359. — Thuret, in Annal. des Sci. Nat. 1851, vol. XVI (Antheridia). — Strasburger, Geschlechtsorgane u. Befruchtung bei Marchantia ; Jahrb. fiir wiss. Bot, vol. VII, p. 409. — Leitgeb, Wachsthumsgeschichte der Radula complicata; Sitzungsber. der Wiener Acad. 1871, vol. LXIII. — Ditto, Bot. Zeitg. 1871, no. 34, and 1872, no. 3. — A portion of what is said about the apical growth of Jungermanniere is derived from communications by letter from Leitgeb. ^ [The term ' thalloid' is here, as on p. 292, preferred to the one in more general use ' fron- dose.' — Ed.] HEP A TIC2E. 297 in contrast to the Foliose Hepaticce belonging to the family of Jungermanniece, the vegetative structure of which consists of a small slender filiform stem, bearing distinctly differentiated leaves (Jungermannia, Radula, Mastigobryum. Frullania, Lophocolea, &c.). Between the thalloid and foliose forms of this family are some which present various stages of transition (as Fossombronia and Blasia). The Leaves of all Hepaticce are simple plates of cells, in which even the mid-rib usual in the leaves of Mosses is always wanting. In most of the thalloid forms the growing apical region of each shoot (Fig. 211, s) lies in an anterior depression, produced by the more rapid growth in length and breadth of the cells which are derived right and left from the seg- ments of the apical cell, while the masses of tissue which lie behind the apical cell in the central line of the shoot grow more slowly in length. Within this depres- sion the terminal branching of the shoot also takes place ; the branches originate from the youngest segments of the apical cell, which, from their position in the FIG. 211 —Mttsgeriafurcata ; the right-hand figure seen from the upper, the left-hand figure from the under side ; fH the mid-rib; s,s',s"t\ie apical region ; y.y wing-like expansion formed of a single layer of cells; f'f'fi" its de- velopment by branching (X about 10). depression and their powerful growth, push aside the apex of the primary shoot, and form with it a fork (dichotomy). In the angle between the two bifurcations the permanent tissue increases more rapidly, and forms, so long as the two forks are still very short, a projection (Fig. 2\i,f\f") which overtops and separates its apical regions, but which, when the forks are longer, is in turn overtaken by them, and now appears as an indented angle of the older fork (/). The fiHform stem of the foliose Jungermanniese, on the other hand, ends in a bud as a more or less prominent vegetative cone, with a strongly arched apical cell. In this case also the lateral branches spring from individual mother-cells, which, however, do not origi- nate from the youngest segments of the apical cell, but lie even at an early period below the apex ; the branching is therefore, from its commencement, distinctly monopodial. We shall speak, under the separate sections, of the form of the apical cell, which forms two, three, or four rows of segments ; as well as of the origin of the -:g. MUSCINEJE. leaves and lateral shoots, since Leitgeb's researches show that great morphological differences occur in the different genera. For the same reason very litde of a general character can be said, in addition to what has been mendoned above, on the habit and anatomical nature of the vegetative structure, \vhich must therefore be considered under the separate families. The Asexical Propagation of Hepaticse is often brought about by the dying off of the thallus or stem from behind, the shoots thus losing their connexion and becoming independent. Adventitious shoots, arising in the thalloid forms from cells of the older marginal parts, become detached in a similar manner. The propa- gation by Gemmae is very common and characteristic ; not unfrequently a number of cells of the margin of the leaf of foliose Jungermannieae {e. g. in Madotheca) simply detach themselves as gemmse ; in Blasia, on the other hand, as well as in INIarchantia and Lunularia, peculiar cupules are formed on the upper side of the flat shoots exposed to the light, which are flask-shaped in Blasia, broadly cup- shaped in Fig. 212 — Marchantia polyiiiorpha ; A,B younjj siioots; C the two shoots which result from a g-ennna, with cupules; vv the depressed apical region; Z) a piece of the epidermis seen from above ; sp stomata on the rhomboid plates (./-C'X slightly ; D more strongly). Fig. 213. — Development of the gemmae Marchantia. Marchantia, crescent-shaped and deficient on one side in Lunularia. From the bottom of these cupules shoot out hair-like papillae, the apical cells of w'hich be- come transformed into a mass of considerable size constituting the gemma. (See Figs. 212, 213.) From the two depressions which lie right and left on the margin of the lenticular gemma (Fig. 213, VI) spring the first flat shoots (Fig. 212, B, C), when the gemmae have fallen out of the cupule and lie exposed to light on damp ground. The Sexual Organs are formed, in the thalloid forms, on the upper side exposed to light ; in Anthoceros in the tissue of the thallus itself (endogenous) ; in the other thalloid forms from cells M'hich project Hke papillae and are of definite origin in reference to the segments of the apical cell. In the Marchantieae branches of a very peculiar shape, which have a tendency to shoot upright from the flat stem, are formed, producing the antheridia on the upper, the archegonia on the HEP A TiCm. 299 Fig. 214. — Anterior margin of the young antheridial disc of Marchaittia polymor- pha ; r the growing margin ; a, a, a the antheridia in different stages of develop- ment ; s/i the stomata above the air-cavities between the antheridia (after Hofmeister, X300). under side, and thus forming inflorescences distributed monoeciously or dioeciously. There is a general tendency in the thalloid Hepaticae for the sexual organs to be depressed into hollows by overarchings of the surrounding tissue, and often opening externally only by a narrow mouth. An example of this is given in Fig. 214. In the foliose Jungermannieae the origin of the antheridia and archegonia is very various, and they are also enveloped in different ways. Further reference will be made to this in describing the different families. The Anther idium consists, in the mature state, of a pedicel surmounted by a globular or ellipsoid body; in those which are imbedded in the tissue the former is usually short, in the free forms it is long, and composed of from one to four rows of cells. The body of the antheridium consists of a wall formed of a single layer of cells containing chlorophyll ; the whole of the space enclosed by it is densely filled by the mother-cells of the an- therozoids ; their escape is occasioned by the ac- cess of water and separation of the cells of the wall at the apex ; sometimes, as in Fossombronia, these cells even fall away from one another. The small mother-cells of the antherozoids which escape in great numbers, separate in the water ; the an- therozoids become free, and have the appearance of slender threads curved spirally from one to three times, and provided at the anterior end with two long very fine cilia, by means of which they move in the water with a rotating motion. Usually they drag after them at the posterior end a small delicate vesicle, the origin of which Strasburger traces to the central vacuole in the protoplasm of the mother- cell, in the periphery of which the antherozoid has been formed. The succession of cell-divisions in the formation of the antheridia has been shown by the researches of recent observers to present great diversities in the different genera; they agree, however, in the antheridium always making its first appearance as a papillaeform swelling of a cell from which it is separated by a septum. This papilla thus detached again divides into a lower and an upper cell, the former of which produces the pedicel, the latter the body of the antheridium (parietal layer and mother-cells of the antherozoids). There is also some doubt as to the succession of cell-divisions in the formation of the archegonia, since the observations of Leitgeb on Radula and those of Kny and Strasburger on Riccia and Marchantia do not entirely agree. An ultimate agreement may however be expected, since, on the other hand, Leitgeb's history of the development of the archegonia of Radula coincides with that studied by Klihn and Schuch in the Mosses ^ It is certain that the archegonium, like the ^ [Janczewski has made a series of comparative researches into the development of tlie archego- nium of Muscinetc, Bot. Zeitg. 1872, p. 869 et seq. — Ed.] ;oo MUSCINEM. antheridium, makes its first appearance as a simple papilla, which, in the case of the first archegonium of an inflorescence of Radula, is itself the apical cell of the shoot. This papilla is detached by a septum, and is divided by a second septum into two cells, the lower one of which produces the pedicel, the upper one the ventral portion and neck of the archegonium. The lower cell undergoes numerous transverse and longitudinal divisions into several rows of cells. In the upper cell, in the case of Radula, Leitgeb states that there arise three (Kny and Strasburger assert in the case of Riccia and Marchantia four) somewhat oblique longitudinal walls, by which three outer cells are formed; these, on their part, enclosing an inner axial cell which overtops them. This latter is divided by a septum into an upper and a lower cell (see Fig. 214 bis, B). The lower cell is the central cell of the arche- gonium ; the upper one subsequently divides cross-wise, and forms the apical stigmatic cells of the neck. While the three (or four) outer cells produce the wall of the ventral portion and neck of the archegonium by transverse and sub- sequently by longitudinal divisions — the whole thus increasing in height and dia- meter — the central cell divides into a lower and an upper cell ; the former produces the oosphere by contraction and rounding off of its protoplasm ; the upper one lengthens within the growing neck, and forms the axial row of cells, the conversion of which into mucilage forms at length the canal of the neck. (2) The Asexual Generation or Spo- rogoniuni arises and is entirely formed within the growing ventral portion of the archegonium, which from this time is termed the Calyptra. The sporogonium does not anywhere unite in its growth with the surrounding tissue of the vegetative structure of the sexual generation, even when its pedicel penetrates into its tissue. The external form and internal structure of the sporogonium are very different in the different groups. In the Anthoceroteae it is when mature an elongated two- valved pod projecting from the thallus. In the Riccieae it is a thin-walled ball entirely filled with spores, and, together with the calyptra, depressed in the thallus. In the Marchantieae it is a shortly-stalked ball enclosing elaters as well as spores, and, after it has broken through the calyptra, bursting irregularly or opening by a circular fissure and detaching an operculum. In the Jungermannieae it ripens even within the calyptra, but breaks through it and appears as a ball borne upon a long slender stalk ; the receptacle consists, as in the Marchantieae and Riccieae, when ripe, of a single layer of cells, but separates cross-wise into four lobes, to which the elaters remain attached. The elaters are, as in the Marchantieae, long fusiform cells, the delicate colourless outer layer of which is thickened within by from one to three brown spiral bands. Fig. 214 bis.— First stafje of development of the archesjo- nium of Andrenfa (after Kiihn) ; A terminal arrhejronium arising from the apical cell of the shoot ; b b the younjjcst leaves ; B after the formation of the central cell and stig- matic cell; C transverse section of the young ventral portion. HEP A TICM. 301 The sporogonium also originates in different ways. The fertilised oosphere is always first divided in the archegonium into two cells, the upper of which, facing the neck, forms the growing apical cell; but this divides in very different ways in the different groups: — in Anthoceros by oblique walls inclined in four directions ; in the IMarchantiese and Ricciese by walls inclined alternately in two directions ; while the sporogonium of the Jungermanniese contains, even in its very earliest stage, four apical cells lying beside one another like octants of a sphere, FlC. 215.— Later stage in the development of the archegonia and origin of the sporogonium of Marchantza poly- morpha; /,//, young archegonia ; III, II', after absorption of the axial row of cells of the neck ; ^' when ready for fer- tilisation ; K/ -/'/// the cells of the mouth of the neck a- relaxed after fertilisation; the fertilised oosphere y shows its first divisions. In these figures si is the lowest cell of the axial row in the neck which is last converted into mucilage; e in I-IVl\\e central cell, in A' the unfertilised oosphere ; // in V-VII the perianth in process of development ; /^ the unripe sporogonium in the ventral portion of the archegonium which has developed into the calyptra ; a neck of the archegonium ; f wall of the sporogonium ; st its stalk ; inside the sporogonium are the young elaters arranged in rays, among them the spores. (/-F///X300, IX about 30.) which divide simultaneously by horizontal septa. When the young sporogonium has in this manner attained its destined height, and partially even at an earlier period, a number of divisions of different kinds take place in the segments of the apical cell, by which the structure is completed. The wall of the sporogonium also becomes differentiated from the tissue from which the mother-cells of the spores are to arise ; if elaters are formed they originate from the same tissue, the cells ceasing to divide transversely at an earlier period and remaining long, while ]0 2 M use IN EM. the intermediate cells become rounded off and give rise to the mother-cells of the spores (Hofmeister). The mode of division into four of the mother-cells of the spores also varies. Those of Anthoceros form at first two, and afterwards four, new nuclei (in addition to the primary nucleus), which are arranged tetrahedrally ; the division-walls advance from without inwards, by which means the spherical mother- cell breaks up into four spores. In Pellia and Frullania, on the other hand, the division of the mother-cells commences by four protuberances arranged tetra- hedrally, which at length separate by constriction ; each contains a nucleus, and they form as many spores ; in Pellia the spores immediately again divide several times, and thus give rise to the sexual generation. Fig. -zit.— Anthoceros Icmis (after Hofmeister); A a branched thallus ; B longitudinal section of a shoot (X40); an antheridia beneath the layer of superficial cells ; C longitudinal section through the apical part of a shoot ; ar rudiments of archegonia (X500) ; D ar fertilised archegonium in the longitudinal section of a shoot, with rudimentary sporogonium consisting of two cells ; E multicellular rudimentary sporogonium ; A' in 5 a colony of Nostoc settled in the tissue of the thallus. The Hepaticse are usually divided into five families, viz. : — 1. Anthoceroteae, 2. Ricciese, 3. Monocleae, 4. Marchantieae, 5. Jungermanniece, of which the first four include only thalloid forms, the fifth both thalloid and foliose genera. I. Anthocerotese. Anthoceros IcB'vis 2L\\d punctatus, which grow in summer on loamy ground, develope a perfectly leafless flat ribbon-like thallus, its irregularly developed ramifications forming a circular disc ; the regularity of the (dichotomous) branching is disturbed by the adventitious shoots, which proceed from the margin of the thallus, and, in A. punctatus, also from the upper surface. The thallus consists of several layers, and the apical cells of the branches which lie in the anterior depressions are divided by walls inclined alternately upwards and downiwards (Fig. 216, C). In each of the cells of the thallus, the upper layer of which does not become differentiated into an epidermis, only one mass of chlorophyll is formed, surrounding the nucleus. On the under side of the thallus, Janczewski states that stomata are formed close behind the HEPA TIC.E. 303 growing margin, through which filaments of Nostoc frequently penetrate, forming roundish balls in the tissue of the thallus (Fig. 216 5), which were at one time considered to be endogenous gemmae. The antheridia and archegonia arise apparently without any definite arrangement in the interior of the upper side of the thallus. The formation of the antheridia commences by a circular group of cells of the outer layer separating from the subjacent tissue and thus producing a broad intercellular space, several of the low^er bounding cells of which, after some vertical divisions, rise up in the form of papillae, and form the antheridia, the position of which is represented in Fig. 216, B, an, their mode of formation in Fig. 214. It is only w^hen the grains of chlorophyll in the walls of the antheridia have assumed a yellow colour and the antherozoids are mature, that the roof of the cavity is ruptured, the antheridia opening at their apex and allowing the antherozoids to escape. The archegonia are formed in a manner still more different from that of all other Hepaticae (Fig. 218, C, ar). A row of cells perpendicular to the surface, resulting from the divisions of an upper segment of the apical cell of the shoot, becomes /^> Fig. 217. — AtUhoceros l^FZ'is ; s£ the young sporogoniuni ; L the involucre (after Hofmeister, Xiso). filled with protoplasm ; the lowest cell of this row swells and becomes the central cell of the archegonium. While this cell is growing and rounding off, the other cells of the row become absorbed; the canal of the neck (Fig. 216, D, ar) which conducts to the interior is thus formed, surrounded by six rows of cells. After fertilisation, the oosphere is first divided by an oblique wall ; in the upper ceM, which becomes the apical cell, other walls are formed inclining alternately right and left; but the walls after- wards arise in four alternating directions. While the immature sporogonium is thus becoming transformed into a multicellular body enlarged below (Fig. 216,^), the sur- rounding tissue of the thallus divides repeatedly and grows into an involucre which is arched upwards and through which the elongating sporogonium afterwards pushes its way. The sporogonium, which had hitherto consisted of homogeneous tissue, now be- comes differentiated; the cylindrical Columella, consisting of from 12 to 16 rows of cells, is formed, its cells being elongated in an axial direction, while those of the sur- rounding layer become divided by horizontal walls, and form the mother-cells of the spores and elaters. The outer four or five layers of cells form the wall of the sporo- 304 HEP A TIC IF.. gonium. Those cells of the layer surrounding the columella which are to become elaters' undergo either one or several additional vertical divisions ; the elaters are in this case transverse rows of cells in which no spiral bands are formed. The intermediate cells become rounded off and isolated progressively from the apex to the base of the spo- rogonium ; and after they have still further increased in size, the division commences into four spores arranged tetrahedrally. The sporogonium extends and forms a pod some 15 or 10 mm. in height, the brown wall of which splits into two valves from above downwards. 2. The family Monocleae appears, according to the 'Synopsis Hepaticarum,' to contain transitional forms between the Anthoceroteae and the Jungermanniese. The long sporogonium has a longitudinal dehiscence and no columella ; and the sexual gene- ration is either thalloid or foliose. ^. The E-iccieee form a flat dichotomously branched but thalloid stem, floating in \vater or rooting in the ground, the apical cells of w^hich, lying close to one another in the anterior depressions of the branches, are stated by Kny to become multiplied by vertical longitudinal partitions, and segmented by walls inclined upwards and down- FIG 218 —Ricciaglciiica; A vertical longitudinal section through the apical region ; s apex, * leaves, a yoiinsr anthe- ridium, a' older antheridium already surrounded by involucral tissue w ; B rudiment of an antheridium a already overarched ; C young antheridium a in longitudinal section (after Hofmeister, X500). \vards\ On the upper side a distinct epidermis is differentiated, but without stomata, and beneath this lies the green tissue often provided with air-cavities, which is de- rived from the upper segments of the apical cells ; the under side is provided with a single longitudinal row of transverse lamellae, which, resulting immediately from the lower segments of the apical cell, must be considered as leaves. Afterwards they split length- wise and form two rows ; between them arise a number of root-hairs wnth conical thick- enings projecting inwards. The archegonia and antheridia are formed on the upper side from young epider- mal cells which grow^ into papilla;, and are overarched, in consequence of their mode of development, by the surrounding tissue (Fig. 218). This involucre sometimes forms an elevated neck above the sessile antheridia. The archegonia project, at the time of fertilisation, above the epidermis ; subsequently they are arched over, and develope from 1 In a letter on the apical growth of Blasia, Leitgeb shows that this Liverwort possesses only one four-sided apical cell. He remarks: — 'I entertain no doubt that in Hepaticse also, which, according to Kny, have a row of apical cells (Pellia, Riccia), only one apical cell is really present, which divides as in Blasia. The deception may result from the lateral segments forming their first divisions in the same manner as the apical cell, by the formation of segments standing fore and aft. This led to the conclusion that the observer had before him in fact a row of apical cells.' HE PA TIC.E. 3^5 their fertilised oosphere the globular sporogonium with a wall consisting of a single layer of cells, and entirely filled with spores, without elaters. The spores are set free by the decay of the surrounding tissue. 4. The Marchantiese have all a thalloid stem extended flat upon the ground • it is ribbon-like, dichotomously branched, possesses a mid-rib, and is always composed of several layers ; the under side produces a number of hairs with conical thickenings Fir,. 219. — Ricciii c^-lauca ; yl apical region in vertical longfitudinal section ; ar archegonium ; c oosphere (Xs6o) ; B the unripe sporogonium s^ surrounded by the calyptra, which still bears the neck of the archegonium ar (X300, after Hofineister). projecting inwards placed upon a spiral constriction of the internal cavity (Fig. 220 bis), and also two rows of leaf-like lamellae, like the Riccieae. The upper side is covered by a very distinctly differentiated epidermis, penetrated by large stomata of peculiar form. Each of these stands, in jNIarchantia, Lunularia, &c. in the centre of a rhombic plate; these plates are parts of the epidermis which overarch large air-cavities, from the bottom of which the cells containing chlorophyll spring in a conferva-like manner, while the rest Fig Qfio—Marchantia polymorfha ; A a horizontal branch t with two ascending branches which bear antheridial receptacles /z?< ; A' vertical section through an incompletely developed antheridial receptacle ini and the part of the thalloid stem a from which it springs ; b b leaves ; h root-hairs in a channel of the antheridial receptacle ; o o opemngs of the hollows in which the antheridia a are placed ; C a nearly ripe antheridium ; st its pedicel ; w the wall ; D two antherozoids tium hornuni. with leaf-forming buds A'; ■w-w the root-hairs of an inverted sod, from which shoot protonema-filaments « n (xgo). The apical cell of the stem is wedge-shaped in Schistostega and Fissidens, and produces two straight rows of alternating segments ; in the rest of the Mosses it is a three-sided pyramid, with the bottom surface turned upwards (Fig. 106, p. 132). Each segment of the apical cell arches outwards and upwards as a broad papilla ; this is cut off by a longitudinal wall (which Leitgeb calls a foliar wall), and developes, by further divisions, into a leaf, while the lower inner part of the segment produces, by further divisions, part of the inner tissue of the stem. Since each segment now forms a leaf, the phyllotaxis is determined by the position of the consecutive segments. In Fissidens two straight rows of alternate leaves are thus formed ; in Fontinalis three straight rows with the divergence \, the segments themselves lying here in three straight rows with the ^ arrangement, because each newly formed transverse wall is parallel to the last but three (both belonging to one segment). In Polytrichum, Sphagnum, Andreoea, &c., on the other hand, each new primary wall encroaches on the ascending side with regard to the leaf-spiral ; the transverse walls of each segment are not parallel ; the segments themselves do not lie, even 3^4 MUSCINEM. when first formed (without the assistance of any torsion of the stem), in three straight rows, but in three parallel spiral lines winding round the axis of the stem one above another; and the consecutive segments and their leaves diverge at an angle which, from what has been said, must be greater than ^ ; the phyllotaxis is •f, I, and so on \ The primary meristem of the stem situated beneath the punctum vegetationis which passes over into permanent tissue usually becomes differentiated into an inner and a peripheral mass of tissue, which are not generally sharply defined ; the cell- walls of the peripheral and especially of the outermost layers are usually strongly thickened and of a bright red or yellowish red colour ; the cells of the inner funda- mental tissue have broader cavities and thinner walls more slightly or not at all coloured. In some INIoss-stems this differentiation goes no further than into an outer skin consisting of several layers and a thin-walled fundamental tissue {e. g. Gymno- stovium rupestre, Leucobryiim glaucum, Hed- wigia ciliata, Barbiila abides, Hylocoiniuni splendens, &c., according to Lorentz) ; while in many other species a central bundle of very thin-walled and very narrow cells is formed in addition (Grimmia, Funaria, Bar- tramia, Mnium, Bryum, and others)^. In Polytrichum, Atrichum, and Dawsonia alone decided thickenings of the cell-walls take place in the central bundle and in such a manner that numerous groups of cells,*origin- ally thin-walled but each group itself sur- rounded by a thick wall, form the bundle. In Polytrichuvi commune there are found also similar thinner extra-axial bundles. Some- times bundles of thin-walled cells run from the base of the leaf-veins obliquely downwards through the tissue of the stem as far as the central bundle, which Lorentz regards as foliar bundles [e. g. in Splachniim luteum, Voitia nivalis, &c.). If it is borne in mind that in some vascular plants fibro-vascular bundles of the most simple structure occur, and the similarity of the cambiform cells of true fibro-vascular bundles to the tissue of the central and foliar bundles in Mosses is considered, these latter may without doubt be held to be fibro-vascular bundles of the simplest kind. As has already been mentioned, the leaf originates from the broad papillose bulging of a cell of the stem which is separated by a longitudinal partition ; a lower Fig. 228.— Transverse section of the stem of Bryi roseum, with root-hairs iv (X90). * If the position of each fourth division of the apical cell is kept in view, it gives the impression as if the apical cell rotated slowly on its axis, producing, at the same time, leaf-forming segments, (Compare on this subject the work of Leitgeb mentioned above, Lorentz's work, Hofmeister's Mor- phologic, p. 194, and MuUer, Bot. Zeitg. 1869, pi. VIII.) ^ It is stated by Lorentz that the pedicel of the sporogonium is always provided with a central bundle of this kind. M0S6'ES. 315 (basal) part of this cell is however concerned in the formation of the outer layers of tissue of the stem. The apical part of the papilla constitutes the apical cell of the leaf; it forms two rows of segments by partitions perpendicular to the surface of the leaf. The number of the segments thus formed, in other words, the terminal growth of the leaf, is limited, and the formation of tissue from the cells thus formed advances downwards, ceasing finally at the base. The whole of the tissue of the leaf is sometimes (as in Fontinalis) a simple layer of cells ; but very commonly a vein, i. e. a more or less broad bundle, is formed from the base towards the apex, dividing the unilamellar lamina into right and left halves, and consisting itself of several layers of cells. The vein is sometimes composed of uniform elongated cells, but more often different forms of tissue become dif- ferentiated in it,^ among which are often formed bundles of narrow thin-walled cells similar to the central bundle of the stem, and these are sometimes continued to it through the external tissue of the stem as foliar bundles (9^ Lorentz, /. c\). The shape of the leaves of Mosses varies from almost circular through broadly lanceolate forms to the acicular ; they are always sessile and broad at their insertion ; usually densely crowded ; only on the stolons of some species, the pedicels of the cupules of the gemmoe of Aulacomnion and Tetraphis, as well as at the base of some leafy shoots, do they remain small and remote (cataphyllary leaves). In the neigh- bourhood of the reproductive organs they usually form dense rosettes or buds, and then not unfrequently assume special forms and colours. In Racopilum, Hypo- pterygium, and Cyathophorum, there are two kinds of leaves, a row of larger upon one side, and a row of smaller leaves upon the other side of the stem. The leaves are not branched, but entire or toothed, rarely slit. In some kinds peculiar outgrowths are formed upon the inner or upper surface of the leaves ; in Barhida abides^ articulated capitate hairs. The lamina, which in other cases expands right and left from the median plane, is, in Fissidens, expanded in the median plane itself, proceeding from an almost sheathing base. The tissue of the leaf is, with the exception of the central vein, usually homogeneous and composed of cells containing chlorophyll, which sometimes project above the surface as mamillae ; in the Sphagnacese and Leucobryum the tissue is differentiated into cells of definite position, some containing air, others sap. The mode of branching of the stem of Mosses is apparently never dichotomous, but also probably never axillary, although connected with the leaves. Even when the branching is copious the number of lateral shoots is nevertheless usually much smaller than that of the leaves ; in many cases the lateral branches are definitely limited in their growth, leading sometimes to the formation of definite ramified systems similar to pinnate leaves (Thuidium, Hylocomium). When the primary shoot produces reproductive organs at the summit, a lateral shoot situated beneath it not unfrequently displays a more vigorous growth, continuing the vegetative system; and by such innovations sympodia are formed. It sometimes happens that stolons, that is shoots either destitute of or furnished with very small leaves, creep on or beneath the surface of the ground, elevating themselves at a later period as erect leafy shoots. The mode of branching is very various, and is closely con- nected with the mode of life. The morphological origin of the lateral shoots has been carefully investigated by Leitgeb in the case of Fontinalis and Sphagnum, and Ji^ MUSCINE.E. admirably described. Since these two genera belong to very different sections, the results obtained in this case may be considered as of general application to the whole class. They agree in the fact that the mother-cell (which is at the same time the apical cell) of a branch originates beneath a leaf from the same segment as the leaf (Fig. io6, p. 132). In Fontinalis the branch arises beneath the median hne of the leaf; but in Sphagnum beneath its cathodal half. In consequence of the further development of the mother-shoot, the lateral shoot in Sphagnum appears at a later period to stand by the side of the margin of an older leaf; and this is probably the explanation of the earlier statement of Mettenius that in Neckera Fig. 229,— .-^ young plant of a Barbula in with the root-hairs h, to the growing ends of which particles of earth have become attached ; at / a superficial root-hair is putting out branches containing chlorophyll, in other words a pro- tonema ; at -^ a tuberous bud is growing from an underground branch of the root-hairs ; B this bud more strongly mag- nified (^X2o; ^X30o). complanata, Hypmnn triquetrum^ Racomitrium canescens, and others, the lateral shoots stand by the side of the margins of the leaves. When the shoot arises beneath the median line of a leaf, and the leaves are arranged in straight rows, the further growth of the stem may cause it to seem as if the shoot originated above the median line of an older leaf, in other words as if it were axillary. Leitgeb states that articulated hairs arise in the genera named in the axils of the leaves, or perhaps more correctly at the base of the upper surface of the leaves. The dimensions attained by the leaf-bearing axes and axial systems of Mosses show a wide range. In the Phascaceae, Buxbaumia, and others, the simple stem MOSSES. r> I H is scarcely i mm. in height ; in the largest species of Hypnum and Polytrichum it is not unfrequently 2, 3, or more decimetres in length, and, if belonging to more than one axis, even longer owing to the formation of innovations and sympodia (Sphagnum). The thickness of the stem is less variable; yL nim. in the smallest, it scarcely exceeds i mm. in the thickest forms. For this reason, however, its dense tissue, coloured externally, is very firm, often stiff, always very elastic, and capable of offering long resistance to decay. The Root-hairs (Rhizoids) play an extremely important part in the economy of Mosses. It is only in the otherwise very abnormal section of the Sphagnacese that they are very sparsely and poorly developed ; in most other forms they occur in large numbers at least at the base of the stem, often clothing it completely with a dense reddish-brown felt. Morphologically the rhizoids are not sharply distinguished from the protonema ' ; and it will be seen further on that they, like it, are capable of forming new leafy stems. They arise as tubular protuberances from the superficial cells of the stem, elongate by apical growth, and are segmented by oblique septa ; at the growing end the wall is hyaline, and particles of earth become attached to it in the ground ; subsequently these fall off ; the wall becomes thicker and brown ; the latter being the case also with the aerial root-hairs. The cells con- tain a considerable quantity of protoplasm and drops of oil (Fig. 229, B)\ behind the septa branches proceed from the cells, often disposed in tufts, and in this case the individual threads are very slender. In many Mosses the root-hairs branch very copiously in the ground ; they often form a dense inextricable felt ; a felt of this kind may even arise above ground as a dense turf, and may serve as a soil for future generations. In Atrichum and other Polytrichaceoe, the stouter rhizoids coil round one another like the threads of a rope, the branches which proceed from them doing the same, and only the last and finest ramifications remain free. The Vegetative Reproduction of Mosses is more copious and varied than is the case in any other section of the vegetable kingdom. It presents the peculiarity that the production of a new leaf-bearing stem is ahvays preceded by the formation of a protonema, even when the propagation takes place by gemmae. Exceptions are afforded only by the few cases in which leaf-buds become detached and commence immediately to grow\ In describing the different cases in detail, the first point that must be brought prominently forward is that both the protonema which proceeds from the spore itself and the leafy stems which spring from it are capable of reproduction of different kinds. The original protonema is so far an organ of reproduction that it may produce upon its branches a smaller or larger number of leafy stems in succession or simultaneously ; sometimes the individual cells of the protonema- branch separate from one another after they have become rounded off into a spherical form, acquire thicker walls, and become for a time inactive (as in Funaria hygrometrica), forming, probably, at a later period again protonema-filaments. A * The rhizoids appear to be distinguished from the protonema only by the absence of chlo- rophyll and by their tendency to grow downwards ; the protonema developes certain branches as rhizoids ; and the rhizoids may, on their part, develope single branches as a protonema growing upwards and containing chlorophyll. ; I N MUSCTNE.W. secondary prolonema may, however, be formed from any root-hair when exposed to light in a moist atmosphere. It is not known whether, under such circum- stances, the apical cell of the stouter rhizoids can itself undergo the change ; but it is certain that the separate cells of the root-hairs form branches which behave in exactly the same manner as the protonema which proceeds from the spore form chlorophyll, and produce new leafy plants {c/. Fig. 226 and Fig. 229, A, p). In some species (Mnium, Bryum, Barbula, &c.) it is sufficient to keep a turf of moss damp for some days and turned downwards, in order to produce hundreds of new plants in this manner. Some apparently annual species, e.g. of Phascum, Funaria, and Pottia, persist perennially by means of their root-hairs ; the plants disappear completely from the surface of the ground from the time that the spores become ripe till the next autumn, when the root-hairs again produce a new protonema, and upon this new stems arise. A similar production of gemmae from the roots occurs also, according to Schimper, in the felted protonema of some species of Polytrichum [P. nanum and aloides) on the slopes of hollow roads, and on that of Schisiostega osmimdacea in dark hollows. The root-hairs may also immediately produce leaf-buds, and behave, in this respect, exactly like the protonema. When the buds arise on underground ramifications of the root-hairs (Fig. 229, B) they remain in a dormant state, as small microscopic tuberous bodies filled with reserve food-material, until they chance to reach the surface of the ground, when they undergo further development {e. g. Barbula murali's, Grimmia pulvinata, Funaria hygrometrica, Trichostomum rigidujn, Atrichum). The aerial root-hairs may, however, not only produce a protonema containing chlorophyll, but also leaf-buds without its intervention ; and Schimper cites the remarkable fact that in Dicrajium iinduJaiwn annual male plants are formed in this manner on the perennial clods of the female plants, and fertilise the latter. Even the leaves of many Mosses produce a protonema, their cells simply growing, and the tubes thus formed becoming segm.ented. This occurs in Ortho- trichum Lyelli and obtusifoliiim ; in 0. phyllanthu77i club-shaped tufts of protonema with short cells arise at the apex of the leaves ; and the same phenomenon occurs in Grimmia trichophylla, Syrrhopodon, and Calymperes. In Oncophoriis glaucus a dense felt of interlacing protonema-filaments is formed at the summit of the plant where the reproductive organs are produced, which arrests its further growth, and hence produces at a later period new clumps of young plants. In Buxbaumia, especially B. aphylla, the marginal cells of the leaves form a protonema enveloping them as well as the stem with its filaments. Lastly, even detached leaves, if kept moist, may emit a protonema, as for instance those of Funaria hygrometrica. GemmcE, which, like those of the Marchantieae, are stalked fusiform or lenti- cular cellular bodies, occur in Aulacomnion aiidrogynum at the summit of a leafless elongation of the leafy stem {Pseudopodia) ; in Tetr aphis pellucida enveloped by an elegant cup composed of several leaves, out of which they subsequently fall. These latter then put forth protonemal filaments, which produce first of all a flat pro- emhryo; and upon this finally new leaf- buds arise (Figs. 230, 231). Finally the deciduous branch-buds of Bryum annotinum may also be considered as organs of reproduction ; as also, according to Schimper, may the branches of MOSSES. 319 Conomiln'um julianiim and CincUdotus aquaticus, which hkewise have the power of detaching themselves. The Sexual Orgajis of Mosses usually occur in considerable numbers at the end of a leafy axisS surrounded by enveloping leaves often of peculiar shape, and mixed with paraphyses. A compound structure of this kind may, for the sake of brevity, be called a * Flower.' The flower of Mosses either terminates the growth of a primary axis (Acrocarpous Mosses), or the axis is indeterminate, and the flower is placed at the end of an axis of the second or third order (Pleurocarpous FiC. ly^.— Tetrnphis fiellucirla; A a plant producing fremni.Te (natural size) ; A' the same, magnified ; y the cup in which the genimsc are collected ; C longitudinal section through the sum- mit of the plant, b the leaves of the cup. K the gemmae in various stages of development ; the older ones are forced off their stalks by the later growth of the younger ones, and forced over the side of the cup; D a mature gemma (xsoo), consisting at the margin of one, in the centre of several layers of cells. Fig. -z-^T.—Tetraphis feUucida ; A, 6 a. gemma, detached from its stalk at a, the protonema-filament xy has been formed by tlie growth of a marginal cell of the gemma, and the flat structure/ as a lateral outgrowth from the protonema ; this has also put out root-hairs w.iv'.tu" (xioo); B,/> a flat pro-embryo from the base of which a leaf-bud A' and root-hairs w, w' have sprung ; the base of the pro-embryo often puts out a number of new flat pro-embryos before a leaf-bud is formed. Mosses). Within a flower either both antheridia and archegonia are produced (bisexual flowers), or it contains only one kind of sexual organ, and the flowers may then be either monoecious or dioecious. Sometimes the male flowers appear on smaller plants with a shorter duration of life (as Fiinaria hygrometrica, Dicranum undulatum, &c.). In external appearance the bisexual are similar to the female The male branches of Sphagnum form an exception {vide infra). ]20 MUSCINEM. flowers, while the habit of the male flowers is altogether difl"erent. In the former the archegonia and antheridia occur either close to one another at the smnmit of the stem in the centre of the envelope {PerichcEtium), either in two groups, or separated by peculiar enveloping leaves, and the antheridia stand in the axils of these arranged in a spiral, surrounding the central group of archegonia. The form of the perianth is, in the female and bisexual flowers, that of an elongated almost closed bud, formed by several turns of the leaf-spiral. Its leaves are similar to the foliage-leaves, and become smaller towards the interior, but grow all the more vigorously after fertilisation. The male perianth {Perigonium) consists of Fig. 232.— Longitudinal section 01 the summit 01 a verj- small male plant oi Futiarm hy,srroniefrica ; a a young, h a nearly ripe antheridium ; c paraphyses ; d leaves cut thrrugh the mid-nb ; e leaves cut through the lamina (X300). broader firmer leaves, and is of three different forms ; usually it is bud-shaped, and resembles that of the female flower, but is shorter and thicker, its leaves often coloured red, and decreasing in size towards the outside ; flowers of this type are always lateral. Those shaped like capitula are, on the contrary, ahvays terminal on a stouter shoot and globular, their leaves are broad, sheathing at the base, thinner and recurved at the upper part ; they become smaller towards the interior, and leave the centre of the flower, with the antheridia, free ; these flowers are some- times borne on a naked pedicel, a prolongation of the stem (Splachnum, Tayloria). Finally, the discoid male perigonia consist of perianth-leaves which are very difl'erent MOSSES, 321 from the foliage-leaves ; they are broader and shorter, expanded horizontally at the upper part, delicate and of a pale green, orange, or purple colour ; they are always smaller the nearer the leaf-spiral approaches the centre; the antheridia stand in their axils (]Mnium, Polytrichum, Pogonatum, Dawsonia). The paraphyses stand between or by the side of the sexual organs ; in the female flowers they are always articulated filaments ; in the male flowers filiform or spathulate, and consisting, in the upper part, of several rows of cells. T/ie Antheridia are, when mature, stalked sacs with a wall consisting of a sino-le layer of cells containing grains of chlorophyll, which however, in the ripe state, assume a red or yellow colour. In the Sphagnacece and in Buxbaumia the antheridia are nearly spherical, but in all other ^Mosses of an elongated club shape. In the Sphagnacece they open in the same manner as in the Hepaticae ; in the other orders by a slit across the apex, through which the antherozoids still enclosed • ,;, in their mother-cells are discharged as a thick mucilaginous jelly. The interstitial mucilage dis- solves in water, and the antherozoids escape from their mother-cells and swim about free. The careful investigations of Leitgeb show that the morphological significance of the an- theridia is very various. In Sphagnum the mother-cell of the andieridium arises in exactly the same place in which a shoot would otherwise be formed, i.e. from the segment of the axis of the antheridial shoot which lies beneath the cathodal half of the leaf; the antheridia may in this case be considered as metamorphosed shoots. In Fontinalis, on the other hand, their morpholo- gical significance varies within the same flower ; the one first formed is the immediate prolonga- tion of the axis of the shoot, arising from its apical cell ; the succeeding ones are developed from its last normal segments, and therefore resemble leaves in their origin and position ; the last antheridia, finally, exhibit the morphological characters of trichomes, both in their variable number, their development as cells of the epidermis, and the want of definiteness in their place of origin. According to Kiihn, Andresea behaves in precisely the same way as Fontinalis. The mother-cell of the antheridium of Fontinalis is constituted as an apical cell forming two alternating rows of seg- ments ; in forming the oldest and terminal antheridium the apical cell changes from a triseriate to a biseriate segmentation. These segments are next divided by tangential walls in such a manner that the transverse section (which meets two segments) of the young organ shows four outer and two inner cells; the wall of the antheridium, one cell in thickness, arises from the former by further division; the small-celled tissue which produces the antherozoids from the latter. Y Fig. 233. — Fnnaria hygrometyfca ; A an anthe- ridium bursting, a the antherozoids (X3S0) ; R the antherozoids more strongly magnified, b in the mother-cell ; c free antherozoid of Polytrichum {X800). 322 MUSCINEM. Andrecea behaves also very similarly in these respects; the primary mother-cell of the antheridium appears as a papilla and is cut off by a septum; the lower cell produces a cushion-like support; the upper cell is again divided by a septum into a lower cell from the divisions of which the tissue of the stalk is formed, and an upper cell out of which the body of the antheridium arises ; the formation of the latter takes place in the same manner as in Fontinalis. In Sphagnum the long stalk originates by transverse divisions of the growing papilla which produces the antheridium, the transverse divisions then dividing again in a cruciform manner. Fig. 234.— First stage of development of the archego- nium of Andreaea (after Kiihn) ; A terminal archegonium arising from the apical cell of the shoot ; b b the youngest leaves; 5 after the formation of the central cell and stig- matic cell ; C transverse section of the young ventral portion. FIG. 235. — Finiaria hyzrojnetrica ; A longitudinal section of the summit of a weak female plant (xioo), a archegonia, b leaves ; B an archegonium (X550), b ventral portion with the central cell, h neck, w mouth still closed ; the cells of the axial row are beginning to be converted into mucilage (the preparation was made after lying three days in glycerine) ; C the part near the mouth of the neck of a fertilised archegonium, with dark red cell-walls. The terminal cell then swells, and becomes divided by oblique walls of somewhat irregular position ; a tissue is thus formed, which, at a subsequent period, consists also of a wall formed of a single layer of cells and an inner very small-celled tissue which produces the antherozoids. The Archegonium consists when mature of a massive, moderately long base, which supports a roundish ovoid ventral portion ; above this rises a long thin neck, generally twisted on its axis. The wall of the ventral portion, which consists, even before fertilisation, of a double layer of cells, passes up continuously into the wall of the neck consisting of a single layer of cells formed of from 4 to 6 rows (Fig. 235). Together they enclose an axial row of cells, the lowest of which, ovoid and lying MOSSES. 323 in the ventral portion, produces the oosphere from its protoplasmic substance by rejuvenescence, while the axial cells which lie above it become mucilaginous be- fore fertilisation. This mucilage forces the four apical cells (stigmatic cells) of the neck apart, and thus opens the canal of the neck, allowing the antherozoids to penetrate to the oosphere. Fig. 235, B, shows the row of cells of the canal at the period when disorganisation is beginning, and when the stigmatic cells of the neck are still closed. In reference to the morphological significance of the arche- gonia, Leitgeb has already shovrn that at least the first archegonium of Sphagnum arises immediately from the apical cell of the female shoot ; more recently Kiihn found that in Andrea^a the first is formed from the apical cell, the succeeding ones from its last segments, in the same manner as the antheridia of the same genus, and those of Radula and Fontinalis. According to preparations which Schuch obtained in the laboratory at Wiirzburg, the first archegonium arises also in typical Mosses from the apical cell of the shoot. The order of succession of the cells in the construction of the archegonium has been studied in detail by Kiihn in the case of Andresea. According to his observations it is in the main similar to that stated by Leitgeb in the case of Radula, although there is a striking discordance in the statements in reference to the forma- tion of the neck and of the row of canal-cells. In Fig. 234 is shown at A the origin of the first archegonium of Andreaea from the apical cell of the shoot ; a septum {?/i m) has already separated the ovoid mother-cell, and a second oblique wall {a a) has divided this into a lower and an upper part ; the former produces, by further divisions, the stalk or base of the archegonium ; from the upper part proceed its neck and ventral portion. While this apical cell is increasing considerably in size, and especially in height, three oblique longitudinal walls (Fig. 234,^, i, 1...2, 2...3, 3) next arise successively, by which a central cell is formed, arched and broader above, and surrounded by a three-celled wall {cf. Fig. 234, C, in transverse section). A septum now separates the upper part of the central cell like a lid, while the lower part is completely enclosed by this and the lateral ones. So far the statements of Kiihn agree with those of Leitgeb in the case of the archegonium of Radula ; but while, according to the latter, the central cell produces both the oosphere and the axial row of canal-cells, the upper ones forming only the stigmatic cells of the neck, and the three lateral ones the wall of the ventral portion and of the neck, Kiihn states, on the contrary, that the upper cell continues to grow as the apical cell, developing successively new stages consisting each of three lateral cells sur- rounding a central canal-cell. Since, how^ever, Kuhn's drawings may be reconciled with the statements of Leitgeb in the case of Radula, it may, perhaps, be assumed that a fresh series of observations would show that after the separation of the first stigmatic cell the axial row is formed entirely from the central cell, the wall of the ventral portion and of the neck entirely from the three first lateral cells. A nearer agreement would thus be indicated not only with the Hepaticae, but also with the higher Cryptogams ^ ^ [According to Janczewsld (Bot. Zeitg. 1872, p. 869) the archegonia of Mosses possess an apical growtli which is wanting elsewhere in Cryptogams. — Ed.] Y 2 324 MUSCINEM. (3) The Sporogonium, which results from the fertilised oosphere, attains, in Sphagnum, almost perfect development within the actively growing ventral portion of the archegonium, which becomes transformed into the calyptra ; but in all other Mosses the calyptra is torn away from the vaginula at its base, by the elongation of the sporogonium, usually long before the development of the spore-capsule, and (except in Archidium and its allies) is raised up as a cap. The neck of the archegonium, the walls of which assume a deep red-brown colour, still for some time crowns the apex of the calyptra. The sporogonium of all Mosses consists of a stalk (the Seta)^ and the spore-capsule {Theca or Urn); but the former is very short in Sphagnum, Andreaea, and Archidium, longer in most other genera, and with its base planted in the tissue of the stem, which, after ferti- lisation, grows luxuriantly beneath and beside the archegonium, forming a sheath- like protection, the Vagmida. The un- fertilised archegonia may frequently be seen on the exterior slope of the vagi- nula, since only one archegonium is usu- ally fertilised in the same flower, or it is only the one first fertilised that perfects its oospore. The capsule has in all Mosses a wall consisting of several layers of cells with a distinct epidermis which sometimes possesses stomata; the whole of the inner tissue is never used up in the formation of spores, even when, as in Archidium, it is subsequently supplanted by them ; a large part of the central tissue remains as the so-called Columella, and it is at the circumference of this that the mother-cells of the spores are formed. The structure of the mature capsule, and espe- cially the contrivances for dispersing the spores, are, however, so different in the various principal sections of Mosses that it will be better to consider them more closely separately, and the more so because by this means we shall at the same time arrive at the distinctive characters of the larger natural systematic groups. In the mode of origin of the sporogonium there is, as might be expected, less variety. The oospore is first of all clothed with a cell-wall, continues to grow considerably, and is then divided by a (horizontal ? or) slightly inclined wall. Hofmeister asserts that in Bryum argenteum the upper cell (that facing the neck of the archegonium) is again divided once or twice by horizontal septa before the first oblique division, while in Phascum, Funaria, Andreaea, and Fissidens, this oblique septum is formed immediately after the first horizontal one. The apical Fig. 236. — Ftinaria hygronietrica; A origin of the spo- rogonium ff in the ventral portion b b oi the archego- nium ; (longitudinal section X500) ; B, C different further stages of development of the sporogonium y and of the calyptra c; h neck of the archegonium (x about 4°)- MOSSES. 325 cell now forms two rows of segments by partition-walls inclined alternately, and these segments are next divided by radial vertical walls, followed by further numerous transverse divisions. By this process the young sporogonium growing at its apex is transformed into a multicellular body which is usually fusiform, the lower end not participating in the growth in length. A swelling of this lower end, such as usually occurs in Hepatic^, takes place also in Sphagnum and Archi- dium. The apex of the sporogonium now becomes inactive, and beneath it the capsule is formed from a spherical, ovoid, cylindrical, or frequently unsymmetrical swelling which originates, in the typical IMosses, only after the elongation of the fusiform or cylindrical sporogonium, and after the raising up of the calyptra. The internal differentiation of this mass of tissue, at first homogeneous, forms the various tissues which compose the capsule of ]\Iosses, and especially the mother-cells of the spores which first of all become isolated and then divide each into four spores. The contents of the mother-cell begin to divide into two, but this bipartition is usually not completed, the division into four taking place at once. The preparation for the formation of spores takes place simultaneously everywhere within the same capsule. The ripe spores are roundish or cubical, surrounded by a thin finely granulated exo- spore, which is of a yellowish, brownish, or purple colour. Besides protoplasm, they contain chlorophyll and oil. In Archidium, where only sixteen are formed in each capsule, they are about | mm. in size, in the highly developed Dawsonia scarcely 75-J^ mm. (Schimper). . When kept dry the spores often retain their power of germmation for a long time, but when moist they frequently germinate after a few days, those of Sphagnum after two or three months. The time necessary for the formation of the sporogonium varies greatly in the different species, but is usually very long in comparison with the small size of the body concerned. The Pottieie blossom in summer, and ripen their spores in the winter ; the Funariae are perennially in blossom, and have constantly sporogonia in all stages of development, occupying for its completion probably 2 to 3 months ; Phasciim cuspidatum developes in the autumn from its perennial underground proto- nema, and ripens its spores in a few weeks before the winter. The bog Hypna, on the other hand {H. giganteum, cordi/oh'um, cuspidatum, nitens, &c.), blossom in August and September, and ripen their spores in June of the next year ; they often require ten months for the development of their sporogonia. H. cupressiforme bears in autumn at the same time flowers and ripe spores, and hence requires one year. The same length of time is required by Philonotis, and by some species of Bryum and some of Polytrichum which blossom in May and June \ Mosses may be distributed naturally into four parallel orders: — 1. Sphagnaceae, 2. Andreaeaceae, 3. Phascaceae, 4. Bryaceae (True Mosses). Of these the first includes a single genus, the second and third only a few ; the fourth all the remaining extremely numerous genera. The first three groups recall, in many respects, the Hepaticae ; even the series of true Mosses commences with some genera Klinggraff, Bot. Zeitg. i860, p. 344- 326 MUSCINE^. which still resembles that class ; the lowest forms of all the groups exhibit many resemblances which are wanting in the most highly developed. We have therefore four diverging series. I. The Sphagnacese^ include only the single genus Sphagnum. When the spores germinate in water, a branched protonema is developed, on which the leaf-buds imme- diately appear laterally (Fig. 237, C). On a solid sub-stratum, on the other hand, the short protonema forms first of all a branching flat pro-embryo (Fig. 238), on which (as in Tetraphis) the leaf-buds appear. The leafy stems produce root-hairs only in the young state, the abundant protonema of true Mosses is entirely wanting. The stem, as it increases in strength, produces laterally, by the side of every fourth leaf, a branch, which, even at the very earliest period, is again much divided ; tufts of branches arranged regularly thus arise which form a compact mass at the summit of the stem, but lower Fig. 238. — Sphaipinrn actttifoHinii ; the flat pro-enibryo /r with a young leafy stem 711 (after Schimper, x about 20). Fig. 237. — Sphagnmn aciUifolhatt ; A a large spore, seen froin the apex ; B a small spore ; C a protonema n it' resulting from the spore s; pr rudi- ments of young plants (after Schimper). down are more divergent. The separate branches develope in different ways; one is produced each year beneath the summit after the ripening of the fruit, and developes in a similar manner to the primary stem, growing up along with the prolongation of the latter, so that each year a false dichotomy takes place on the stem. These inno- vations afterwards become separated by the slow decay of the plant advancing from below, and constitute independent plants. Some of the branches of each tuft, however, turn downwards, become long, slender, and finely pointed, and are closely applied to the primary stem, forming a dense envelope around it ; while other branches of each tuft turn outwards and upwards. The leaves spring from the stem and the branches from a broad base, and are usually arranged with a ^ W. P. Schimper, Versuch einer Entwickelungsgeschichte der Torfmoose. Stuttgart 1S58 (with many beautiful plates). MOSSES, 327 tongue-shaped or apiculate, and, with the exception of the first on the young stem, are composed of two kinds of cells arranged regularly. The young leaf necessarily consists of homogeneous tissue ; but as the development progresses the cells of the veinless lamina become differentiated into large broad cells about the shape of a long lozenge, and into narrow tubular cells, running in the midst of the former, bounding them, and united with one another into a network ; they are, as it were, squeezed in among the larger ones. The larger cells lose the whole of their contents, and hence appear colourless ; their walls show irregular narrow spiral bands with the turns some distance apart, as well as large dots, each of which has a thickened edge, while the part of the cell-wall which closes the dot is absorbed. Large holes usually circular are thus formed in the cell-wall of the colourless cells. The intermediate tubular narrow cells retain their contents, form grains of chlorophyll, and thus constitute the tissue of the leaf, FIG. zyj.-Sphagnum acnti/olium ; part of the stem below the apex ; a, a tlie male branches, b leaves of the primary stein ; ch perichcCtial branch with old still closed sporogonia (after Schimper X5-6). the entire area of which is, however, smaller than that of the colourless tissue (Fig, 240). The stems consist of three layers of tissue, the innermost of which is an axial cylinder of thin-walled colourless cells elongated in a parenchymatous manner; it is enveloped by a layer of thick-walled, dotted, firm (lignified?) prosenchymatous cells, with their walls coloured brown. The epidermal tissue of the stem, finally, con- sists of from I to 4 layers of very broad thin-walled empty cells, which, m S, cjm- bifolium, possess spiral threads and round holes similar to those of the leaves (r/ Fig. 70, p. 82). These colourless cells, both those of the leaves and of the epidermal layer of the stem and of the branches, serve as a capillary apparatus for the plant, through which the water of the bogs in which it grows is raised up and carried to the upper parts; hence it results that the Sphagna which always grow erect are 328 MUSCINEM. penetrated with water to their very summits like a sponge, even when their tufts stand high above the surface of the water. The Archegonia and Antheridia of Sphagnum arise on the fascicled branches, as long as they are still near the summit of the primary stem and belong to the terminal tuft. The time of flowering is mostly in autumn and winter, but is not exclusively confined to these periods. The antheridia and archegonia are always distributed on different branches, sometimes even on different plants, and in this case the male and female plants- form large distinct patches. When the primary stem does -not con- tinue to grow during the development of the sporogonia in dry weather, growth still takes place subsequently at the terminal tuft ; but when the supply of water is great and vigorous increase of length takes place, the fertile branches diverge from one another, and are consequently found lower down on the stem ; and the sporogonia and older male inflorescences are thus removed to a distance from the summit, although at the time of flowering they stand near it. The branches which bear the antheridia are generally Fig. 240. — Sphagnum aatti/olium ; A a portion of the surface o the leaf seen from above, cl the tubular cells containing chlorophyll, y the spiral bands, / the holes in the large empty cells ; B transverse section of a leaf, cl the cells that contain chlorophyll, Is the large empty cells. Fig. 241. — Sphao-7iii7n acHtifoliinn; A a male branch, with the leaves partially removed in order to show the antheridia a; .6 an open antheridium (verj' strongly magnified) ; C a free motile an- therozoid (after Schimper). conspicuous externally by their imbricated leaves forming beautiful densely crowded orthostichies or spiral parastichies ; the leaves are generally yellow, bright red, or especially dark green, and can hence be easily recognised (Fig. 239, « a). The an- theridia stand, on the mature shoot, by the side of the leaves ; they are never terminal, and are found only in the middle part of the male branch, one standing beside each leaf ; the male branch may therefore continue to grow at the summit, and become an ordinary flagellate branch. This position of the antheridia, and still more their roundish form and long pedicel, causes the Sphagnaceae to resemble some Junger- mannieae ; the mode in which they open (Fig. 241) recalls the Hepaticse even more than the true Mosses. The archegonia arise at the blunt end of the female branch, the upper leaves of which form a bud-like envelope ; but the young perichaetial leaves are still contained within this at the time of fertilisation, although they afterwards become further developed. The archegonia are exactly like those of the rest of the MOSSES. 329 Mosses ; several of them are usually fertilised in one perichuetium, but only one perfects its sporogonium. This development occurs within the perichsetium ; the summit of the branch then begins to rise, grows out into a long naked receptacle, and elevates the sporogonium contained in its calyptra high above the perichcetium. This so-called Pseudopodium must not, however, be confounded with the seta of other jMosses. At Fig. 242, 5, is shown in longitudinal section the nearly ripe sporo- gonium developed within the calyptra. Its lower part forms a thick base imbedded in the end of the pseudopodium wTiich is transformed into the vaginula. The origin of the spore-mother-cells is a cap-shaped layer of spherical cells beneath the apex of the spherical theca ; the part of the inner tissue which is found beneath it forms a low nearly hemispherical column, which is in this case also termed the Columella, although it is distinguished from the colu- mella of true Mosses by not reaching to the apex of the theca. The mode of the formation of the spores from the mother- cells resembles that of true Mosses ; but there occur, besides the ordinary (large) spores, also smaller spores in special smaller sporogonia, which owe their origin to a further division of the mother-cells (r/". Fig. 237, E). The theca opens by the detachment as a lid of the upper segment of the ball, which is sometimes more strongly convex. The calyptra, which closely surrounds the growing sporogo- nium as a fine envelope, is ruptured irre- gularly. 2. The Andreaeaceae * are small ccspi- tose Mosses which are very leafy and much branched ; their very shortly stalked theca is elevated, as in Sphagnum, above the perichffitium on a leafless pseudo- podium. The long apiculate theca raises up the calyptra in the form of a pointed cap, as in the true Mosses, while the short seta remains buried in the vaginula. The body of the young sporogonium becomes differentiated into a parietal tissue con- sisting of several layers which surrounds the simple layer of the spore-mother- cells without any intermediate cavity, and a central mass of tissue, the colu- mella ; in the same manner as in the Sphagnaceae the layer of cells which produces the spores is bell-shaped and closed above, the columella terminating beneath it. The ripe theca does not open by an operculum, but by four longitudinal slits at the sides ; four valves are thus formed united at the apex and at the base, which are closed in damp, but open in dry weather. 3. The Phascaeeae are small Mosses, the short stems remaining attached to the protonema until the spores are ripe ; they may be considered as the lowest form of the following group, to which the genus Phascum forms the transition. They are, however. Fig. 242. — A, P, SphagJiiim acidifoliitfn ; A longitudinal section of the female flower, ar archegonia, ch perichsetial leaves still young, jv the last perichretial leaves or perigynium ; B longitudinal section of the sporogonium sg, the broad base of which sg' remains in the vaginula v, while the capsule is sur- rounded by the calyptra c, upon this is the neck of the arche- gonium ar, ps the pseudopodium ; C Sphag7uim sqiiarrosmn, ripe sporogonium sg with its lid d and ruptured calyptra c, qs the elongated pseudopodium growing from the perichsetium ch (after Schimper). J. Kuhn, Zur Entwickelungsgeschichte der Andreaeceen, Leipzig 1870. 330 M us CINE M. all distinguished by their theca not opening by an operculum, but allowing the escape of the spores only by its decay. While in the genera Phascum and Ephemerum ^ the internal differentiation of the theca corresponds essentially to that of true Mosses, although more simple, the genus Archidium displays a more considerable deviation, and as an interesting transitional form may be examined a little more closely 2. The very short pedicel of the sporogonium swells, as in Sphagnum and Hepaticae ; the roundish theca ruptures the calyptra laterally, without raising it up as a cap. Archidium agrees with the true Mosses in the formation in the theca of an intercellular space running parallel to its lateral surface, which separates the wall from the inner mass of tissue. The latter appears as a colum.n continuous at the foot and apex with the wall of the theca. But while in the true Mosses a layer of cells parallel to this inter- cellular space produces the spore - mother- cells, it is here only a single cell lying eccentrically in the inner mass of tissue that becomes the primary mother-cell of all Fig. ■2\'^.— Archidium phascoides ; A longitudinal section of the young sporogonium, showing the mother-cell m of the spores ; B longitudinal section through the young sporogonium with its calyptra and vaginula, /base of the sporogonium, w wall of tlie theca, i intercellular space, c columella, h hollow out of which the spore-mother-cells have fallen, v vaginula, st stem; b leaves, a neck of the archegonium. After Hofmeister (X200). Fig. ■2i,^.\— Archiduim phascoides ; longitudinal section through a nearly ripe sporogonium, iu its wall, sp its spores, V the vaginula, b leaves of the stem. After Hofmeister ( X 100). the spores (Fig. 243, A). It swells considerably, and supplants the other cells, until it lies free in the hollow of the theca ; it then divides into four cells, each of which produces four spores. The wall of the primary mother-cell remains entire, while the sixteen spores grow, and fill up the whole of the theca, the inner cell-layer of which is also absorbed (Fig. 244). 4. In the BryaceaB or True Mosses the sporogonium is always stalked, and the pedicel is usually of considerable length. The pedicel {Seta) is cylindrical, obtusely pointed below, and firmly implanted in the vaginula; the theca always opens by the detachment of its upper part as a lid {Operculum) ; the operculum is either simply and smoothly detached from the lower part of the theca, or a layer of epidermal cells term.ed the Annulus is thrown off in consequence of the sw^elling of their inner walls, and the operculum in this way separated from the theca. Most commonly, after the operculum has fallen off, the margin of the theca appears furnished with appendages of very regular and elegant fonn arranged in one or two row^s; the separate append- ^ J. Miiller, in Jahrbuch fiir wiss. Bot. 1867, vol. VI. p. 237. ^ Hofmeister, in Bericht der konigl. Sachsisch. Gesellsch. der Wiss. 1854, April 22, MOSSES. 33^ ages are termed Teetb or Cilia, the whole together the Peristome; if the peristome is wanting, the theca is said to be gymnostomous. The theca is at first a solid homo- geneous mass of tissue ; the differentiation of its interior begins with the formation of an annular intercellular space which separates off the wall of the theca consisting of several layers of cells ; but the wall remains attached above and below to the colu- mella. The intercellular space is traversed by rows of cells which stretch across from the wall of the theca to the inner mass of tissue ; they resemble most nearly proto- nemal filaments, or those of Algae, but have been formed by simple differentiation of the tissue of the theca. They contain grains of chlorophyll like the inner cell-layers of the wall. The outer layer of the wall of the theca is developed into a very character- istic epidermis strongly cuticularised externally. The third or fourth layer of cells of the inner mass of tissue, which is thus separated from the annular air-cavity by two or three layers of cells (forming the spore-sac), produces the mother-cells oif the spores. They are first of all distinguished by being densely filled with protoplasm, in which Pig. 24S.—Fu>iafia hyi^^roinetrica ; A a young leafy plant.;' with the calyptra c; B a plant jC with the nearly ripe sporoj^o- nium, s its seta, y the theca, c the calyptra ; C longitudinal sec- tion of the theca bisecting it symmetrically ; rf operculum, a annulus, / peristome, c c' columella, h air-cavity, s the pri- mary mother-cells of the spores. Fig. 246. — Mouth of the theca of Fontinalis (intipyretica ; afi outer peristome, 2 inner peri- stome. After Schimper ( x 50). Hes a large central nucleus, and are attached without interstices to the surrounding tissue in a parenchymatous manner. From their division proceed the spore-mother- cells, which are isolated by the deliquescence of the cell-walls, and then swim in the fluid contained in the spore-sac, till they form the spores by repeated division. The Spore-sac is the term given to those layers of cells by which the large air-cavity is separated from the spore-mother-cells. It seems convenient to consider the layers which bound the spore-cavity on the axial side (Fig. 247, /) also as a part of the spore-sac ; its cells contain on both sides starch-forming grains of chlorophyll. The inner large-celled tissue, which contains but little chlorophyll, and is thus surrounded on all sides by the spore-sac, is distinguished as the Columella. The spore-sac is ruptured by the casting off of the operculum, but the columella remains dried up, and in Poly- trichum there remains also a layer of cells, the Epiphragm, attached to the points of the teeth of the peristome, and covering the opening of the theca. We must now examine somewhat more closely the origin of the Peristome. In those genera w-hich, like Gymnostomum, do not form a peristome, the parenchyma which fills up the inner space of the operculum is homogeneous and thin-walled ; when 33^ MUSCINEJE. the theca is ripe, it contracts and dries up at the bottom of the operculum, which is formed essentially only of the epidermis ; or it remains attached to the columella and forms a thickening at its summit, which projects over the opening of the theca; or again it forms a kind of diaphragm, which closes the mouth of the theca after the casting off of the operculum (Hymenostomum). The transition to the genera provided with a true peristome is furnished by Tetraphis. In this genus the firm epidermis of the upper conical part of the theca falls oif as the operculum, while the whole of the internal tissue of the operculum, the two outer layers of which are thick-walled, splits across into Fig. 247. — h'linaria hy^'ronietrica transverse section through the spore- sac ; A, su the primary mother-cells ; B, stn the spore-mother-cells not yet isolated; a outer side, i inner side of the spore-sac (XSSo). Fig. 248. — Development of the spores of Fjniaria hygrometrica observed in very thin glycerine ; A mother-cells, at a still united, at b and c the separation has commenced ; B isolated mother-cells clothed with cell-walls ; at f expelling the protoplasmic contents ; C mother-cells with indication of the commencement of the bipartition of the contents ; D the contents have divided into four lumps of protoplasm, still surrounded by the primary cell-wall, but they themselves are naked ; E the spores enveloped by cell-walls ; F ripe spores (X550). Fig. 249.— Various states of division of the mother-cells of the spores oi Fiotayia hygrometrica, observed in water, the progress of development indicated by the letters a—i. four valves. These are also termed by systematists a peristome, although their origin and structure are widely different from that of the true peristome in other genera. For, except in the Polytrichaceae, neither the teeth nor the cilia consist of cellular tissue, but only of thickened and hardened parts of the walls of a layer of cells, which is separated by some layers of thin-walled cells from the epidermis which forms the operculum ; the latter layers, as well as the dehcate parts of the former, becoming ruptured and disappearing, while the thickened parts of the wall remain after the casting off of the operculum. This will be rendered clear by an example. Fig. 250 represents a part of the longitudinal section which bisects the theca of Funaria MOSSES. 333 hygronietrica symmetrically, corresponding to the part in Fig. 245, C, designated a ; ^ ^ is the reddish-brown epidermis strongly thickened on the outside ; at the part where it bulges its cells are of a peculiar shape, forming the ring or annulus ; se is the tissue Fig 250. — Futt.tria hy^rotnetrica ; part of a longitudinal section of an unripe theca. Fig. I'^x.—Funaria hygronietrica ; part of a transverse section through the operculum. FIG. 252.—^ longitudinal section of the theca of Poly- trichum piliferunt (after Lantzius-Beninga, X15) ! B the transverse section (x about s) ; ™ wall of the theca, cii operculum, c c columella, p peristome, ep epiphragm, a a annulus, i t the air-cavities penetrated by alga-like cel- lular filaments, s spore-sac, containing the primary mother- cells of the spores, st the seta, the upper part of which forms the apophysis ap. lying between the epidermis of the theca and the air-cavity h ; the large-celled tissue p is the prolongation of the columella within the cavity of the operculum ; at S are seen the uppermost spore-mother-cells ; directly above the air-cavity h rises the layer of cells 334 MUSCINEm. which forms the peristome ; its walls {a\ which face outwards, are strongly thickened, and of a bright red colour ; the thickening is continued also partially along the septa ; the longitudinal walls which lie on the axial side of the same layer of cells [i) are also coloured, but less strongly thickened. In Fig. 251 is shown further a part of the transverse section through the basal part of the operculum ; r r are the epidermal cells placed immediately above the annulus, forming the lower edge of the operculum; a and i the thickened parts of the layer of cells concentric with the operculum, which form the peristome. A section near the apex of the operculum would show, instead of the broad thickening-masses /*, z", i" , only the middle part of the inner wall, but more strongly thickened. If now it is supposed that when the theca is ripe the annulus and the operculum fall off, the cells p and those which he between a and e (Fig. 250) disappear, and that the thin pieces of wall between a, a, a", and between i, i', i", in Fig. 251, are also]^ destroyed, then the thick red pieces of wall alone remain, forming sixteen pairs of tooth-like lobes pointed above, crowning the edge of the theca in two concentric circles. The outer row are termed Teeth, the inner row Cilia. The thickened cells at t, Fig. 250, unite the base of the teeth with the edge of the theca. According as the layer of cells which forms the peristome consists, in transverse section, of a larger or smaller number, and according as one or two thickened cells are formed within each one of these cells, the number of teeth and cilia varies ; it is always however a multiple of four, generally 16 or 32. In many cases the thickening at i is wanting; the peri- stome is then simple, and formed only of the teeth of the outer row. The thickenings at a are very commonly much stronger than is the case in Funaria, and the teeth there- fore stouter. The thickened parts of the wall may also partially or entirely coalesce laterally with one another ; and then the parts of the peristome either above or below form a membrane ; in this case the teeth appear split from one another above, and the endostome (the inner peristome) is composed of a lattice-work of longitudinal or trans- verse ridges instead of cilia (Fig. 246). A great variety is met with here, which may easily be understood by the beginner when he has obtained a clear idea of the principle. The inner and outer sides of the teeth of the peristome are hygroscopic to a different degree ; hence, as the amount of moisture in the air varies they bend inwards or out- wards, or sometimes in a spiral whorl, as in Barbula. The genus Polytrichum, to which the largest and most highly developed Mosses belong, differs from the other genera in several points in the structure of its theca. The teeth of the peristome are composed not simply of single pieces of membrane, but of bundles of thickened prosenchymatous cells ; these bundles are horseshoe-shaped ; the branches of two adjoining bundles directed upwards form together one of the 32-64 teeth. A layer of cells uniting the points of the teeth (Fig. 252, ep) remains, after the casting off of the operculum and the drying up of the adjoining cells, as an epiphragm stretched across the theca. The spore-sac is, in some species {e. g. P. piliferum), separated from the columella by an air-cavity, which is penetrated, like the outer air-cavity, by conferva-like rows of cells. In most species the seta is swollen beneath the theca, forming the Apophysis, a phenomenon which is repeated in a somewhat different manner in the genus Splachnum, where this part is sometimes expanded transversely as a flat disc. VASCULAR CRYPTOGAMS, 335 GROUP IV. VASCULAR CRYPTOGAMS. Under this term are included in one group the Ferns, Equisetacese, Ophio- glossacese, Rhizocarpeoe, and Lycopodiacese. As in the Muscineas, the process of development is divided into two generations which are extremely different both morphologically and physiologically. From the spore proceeds first of all a sexual generation ; from its fertilised archegonium is produced in the second place a new plant, which does not form sexual organs, but in their place a number of spores. In the Ferns and Equisetacece these spores are all alike ; the Rhizocarpese and Lycopodiacea?, on the contrary, produce two kinds of spores, large and small, Macrospores and Microspores. The Sexual Generalion which is developed from the spore always preserves, in Vascular Cryptogams, the form of a thallus ; it never attains, as in the more highly developed Mosses, to a differentiation into stem and leaf, but remains small and delicate, and closes its life with the commencement of the development of the second generation. It appears, therefore, externally as a mere precursor of further develop- ment, as a transitional structure between the germinating spore and the variously differentiated second generation. Hence the name ProthaUium has been given to this first or sexual generation of Vascular Cryptogams. If now the five classes are considered in the order mentioned above, the remarkable fact appears — and it is one of great importance in comparing them with the group that follows — that, in proceeding from the Ferns to the Lycopodiacese, the development of the prothallium becomes continually simpler and its morphological differentiation less pronounced. In the Ferns and Equisetaceae the prothallium resembles the thallus of the lowest Hepaticse. These prothallia sometimes continue to grow for a considerable time ; they contain a large amount of chlorophyll, and form numerous root-hairs. After they have thus attained sufficient vigour by inde- pendent nourishment, they produce the Archegonia and Antheridia, usually in considerable numbers. A tendency to become dioecious is then manifested in these prothallia, although they proceed from similar spores ; both kinds of sexual organs being, however, often produced on the same prothallium. In the Rhizocarpeas and Lycopodiaceae, on the other hand, the separation of the sexes is already prefigured by the two kinds of spores, the Macrospores being female, in so far as they develope a very small prothallium, which produces exclusively archegonia, or sometimes only a single one. The female prothallium of the Rhizocarpese is a small appendage of the macrospore, formed in its interior but afterwards developed externally although nourished by it ; in Selaginella and Isoetes, which belong to the ^^6 VASCULAR CRYPTOGAMS. Lycopodiacece, the prothallium is developed in the spore itself, filling it up with a mass of tissue, the archegonia becoming exposed only by the splitting of the cell-wall of the spore. The microspores of this section produce the antherozoids after a previous endogenous formation of cells, which must be regarded as a rudi- mentary prothallium. The Archegojtia of Vascular Cryptogams are, like those of the Muscineae, masses of tissue, consisting of a ventral part which encloses the oosphere, and of a neck, usually short and composed of four longitudinal rows. The two groups differ in the fact that in Vascular Cryptogams the tissue of the wall of the ventral part is formed from the prothallium itself; and the ventral part of the archegonium is therefore enclosed in the tissue of the sexual generation, the neck only projecting beyond it. The neck and central cell arise from an epidermal cell of the prothallum ; the protoplasm of the central cell divides in this case also into two unequal portions ; the lower larger one becomes by rejuvenescence the oosphere, while the upper small portion, the canal-cell, penetrates between the rows of cells of the neck and becomes converted into mucilage (after having, in the case of Ferns, produced, according to Strasburger, at least an indication of an axial row of cells). The mucilage thus produced in the neck finally swells up considerably, drives apart the four apical cells of the neck, and is expelled; an open canal is thus formed, leading from without to the oosphere ; the expelled mucilage appears to play an important part in the conduction of the ' swarming' antherozoids to the opening of the neck. FertiHsation is always effected by means of water, which determines the opening of the antheridia and archegonia, and serves as a vehicle for the antherozoids. The advance of these latter as far as the oosphere, and even their entrance into and coalescence with its protoplasm, has been directly observed in the different groups. The Antherozoids are, like those of the IMuscinese, spirally coiled threads usually wdth a number of fine cilia on the anterior coils. In the cases hitherto observed they arise from the peripheral part of the protoplasm of their small mother-cells, a central vesicle of protoplasm, containing starch-grains, remaining behind, which, adhering to a posterior coil of the antherozoid, is often dragged along by it, but is detached before its entry into the archegonium. The mother-cells of the antherozoids arise, in Ferns and Equisetacese, in the antheridia, which project free from the prothallium as roundish masses of tissue ; but in the Ophioglossacese are imbedded in the pro- thallium. Among Rhizocarpeae, Salvinia forms a very simple antheridium proceeding from the microspore, while the IMarsileaceae and Selaginelleae produce their anthero- zoids within the microspore itself; but in the latter only after a few-celled mass of tissue has been formed in it which must be considered as a rudimentary prothallium (Millardet). The Asexual Generation which produces Spores arises from the oospore or fertilised oosphere in the archegonium. In Ferns, Equisetaceae, and Rhizo- carpeae, its earliest divisions, the rudiments of the first root, the first leaf, and the apex of the stem, can be recognised, while at the same time a lateral outgrowth of its tissue, called the Foot, commences at the bottom of the ventral part of the arche- gonium, and draws from the prothallium the first nourishment for the young plant. The ventral part of the archegonium at first grows vigorously (except apparently VASCULAR CRYPTOGAMS. 037 in the Selaginellese), enveloping the embryo, until this latter finally protrudes free, leaving, however, for some time, the foot still attached to it as a nutritive organ. This process offers an unquestionable analogy to the formation of the calyptra of the IMuscineae. While, however, the spore-producing generation of the Mus- cineae remains a mere appendage of the sexual plant, appearing, in a certain sense, as its fruit, the corresponding generation of Vascular Cryptogams developes, on the contrary, into a conspicuous, highly organised, independent plant, which frees itself at a very early period from the prothallium, and obtains its own nourishment. It is this asexual generation which is called, in ordinary language, simply the Fern, Equisetum, &c. ; it always consists of a leafy stem, usually pro- ducing a number of true roots ; roots may, however, occasionally be entirely absent, as in some species of Hymenophyllum, and in Psilotum and Salvinia. In many cases, especially in Ferns, Equisetaceae, and (especially the extinct) Lycopodiaccae, the spore-producing generation attains great dimensions with an unlimited term of life ; only a few species are (like Salvinia) annual. The Leaves are either simple, unsegmented, or variously branched (Ferns, Ophioglossaceae). There does not, however, occur so great a variety in the forms assumed by the leaves in the same plant due to metamorphosis as in Phanerogams. The Roots usually arise in acropetal succession on the stem (or on the leaf- stalk in some Ferns), and branch monopodially or dichotomously ; they always remain nearly uniform in size, the first root never attaining the dimensions of a tap-root, as in many Phanerogams. The Differentiation of the Systems of Tissue attains a high degree of perfection for the first time in this group of plants. The epidermis, fundamental tissue, and fibro-vascular bundles are always clearly distinct, and are composed of cells of various forms. The fibro-vascular bundles are closed; their phloem usually sur- rounds the xylem of each separate bundle like a sheath. The Bra?iehi?ig of the Stem is very different in the different classes of Vascular Cryptogams, and will be considered hereafter; it may be remarked here that axillary branching probably does not occur in the same sense in which the term is applied to Phanerogams. The Production of the Sporangia is, in most cases, evidently a function of the leaves ; in a few cases (as Pilularia) this mode of origin is, however, still doubtful. In their form and mode of envelopment by neighbouring organs the sporangia show considerable differences ; but within each class their characters are very constant. It is clear from what has now been said that the sporangium of Vascular Cryptogams is equivalent, from a physiological but not from a morphological point of view, to the sporogonium of Mosses. This latter forms by itself the whole of the asexual generation of Mosses ; while the sporangium of Vascular Cryptogams is a relatively small outgrowth of a foliar structure of the asexual generation which consists of stem, leaf, and root. The mode of origin of the mother-cells of the spores is also different from that in the Muscineae, though the spores themselves are produced in the mother-cells in a manner more like that which occurs in Muscinese. The spore-mother-cells of Vascular Cryptogams also become isolated from their combination into a tissue, and divide into four spores, an indication of a division into two generally preceding that into four. The distinction between z 33« VA SCULA R CRYPTOGAMS. macrospores and microspores in the Lycopodiacese and Rhizocarpe:^ is manifested only after the division into four of the mother-cells, which were previously alike in the case of both kinds of spores. Vascular Cryptogams form a group connected with one another by very obvious bonds of relationship, but may be divided into five parallel and diverging series or classes. In the formation of a prothallium the Ferns and Equisetacese show a marked affinity with the lowest stages of development of the Muscinese. The Rhizocarpeae and Lycopodiaceae diverge in this respect greatly from these classes, and in their mode of sexual reproduction form a transition to Phane- rogams, — from Spore-plants to Seed-plants, as will be shown when treating of the general characteristics of the latter. The proof that what is termed the Moss-fruit, i. e. the sporogonium, is, from its position in the alternation of generations, the equivalent of the entire leafy and rooting spore-producing plant of Vascular Cryptogams, was brought forward by Hofmeister as long ago as 1851 (Vergleichende Untersuchungen, p. 139^). In connexion with the relationships pointed out by him between the Lycopodiaceae and Coniferae, this discovery is one of the most fertile in results that has ever been made in the domain of mor- phology and classification. The researches of Pringsheim and Hanstein on the develop- ment of Rhizocarps, carried out with great acuteness and deep penetration, those of Nageli and Leitgeb on the roots of Vascular Cryptogams, and of Cramer on the apical growth of the stem of Equisetacese and Lycopodiaceae (with which the more recent labours of Rees made under Nageli's superintendence agree), have not only contributed to a more accurate knowledge of this group of plants, but have especially cleared up the fundamental morphological facts. Since the appearance of the first edition of this book, our knowledge of the alternation of generations has been enriched by Millardet's discovery of the male prothallium in Selaginella ; and the labours of Millardet, Stras- burger, and Kny have resulted in a more complete acquaintance with the development of the sexual organs and of the process of fertilisation itself in its details. The following systematic review will serve as a preliminary introduction to the group of Vascular Cryptogams : — The sexual generation is developed from the spore, and is a thalloid structure of small size; the archegonia have their ventral part imbedded in this prothallium; the antherozoids are spirally-coiled threads, generally furnished with a number of cilia at their anterior pointed end. The asexual generation, resulting from the fertilisation of the oosphere in the archegonium, produces the spores, and is difterentiated into stem, leaves, and roots. The branching of the stem is not axillary ; its tissue is differentiated into epidermis, fundamental tissue, and closed fibro-vascular bundles; the sporangia are products of the leaves ; the mother-cells of the spores arise from a central cell or from a mass of sporangial tissue, and form the spores by division into fours after previously showing a tendency towards bipartition. I. Isosporous Vascular Cryptogams. Only one kind of spore is produced ; the prothallium vegetates for a considerable time independently of the spore, and produces antheridia and archegonia. ^ [On the Germination, Development, and Fructification of the Higher Cryptogamia, and on the Fructification of the Coniferae, by W. Hofmeister; translated by F. Currey; Ray Soc. 1862, V- 434-] VASCULAR CRYPTOGAMS. ^^g (i) Filices or Ferns. Prothallium above-ground, green, monoecious; branching of the stem probably at first dichotomous ; exogenous adventitious buds formed on the leaves ; the sporangia are trichomes of the leaves, which are stalked, usually large and branched, and are characterised by the long continuance of their apical growth. (2) Equisetaceee. Prothallium above-ground, green, monoecious or dioecious; branching of the stem exclusively by endogenous verticillate lateral buds; leaves very simple, verticillate, forming sheaths; the sporangia are produced in groups on the margin of metamorphosed leaves, and constitute a terminal fructification. (3) Opbioglojsacecs. Prothallium underground in the two known cases, not green, monoecious; the stem has apparently no provision for branching; the leaves have a stalked lamina and a sheathing base ; the sporangia are produced on a branch of the leaf, and constitute a spike or panicle. II. Heterosporous Vascular Cryptogams. INTacrospores and microspores are produced; the macrospore produces the female prothallium and nourishes it, the prothallium never becoming independent ; the micro- spores produce a rudimentary prothallium which does not become free, and in which the antherozoids are formed. (4) Rbizocarpecp. The female prothallium protrudes from the cavity of the spore, and remains attached by its lower side to the macrospore; its size is less than that of the spore ; the sporangia are produced in numbers in the interior of hollow receptacles (sporocarps), and produce either a single macrospore or a number of microspores ; the sporocarps are appendages of the leaves. (5) Lycopodiacea;. INIacrospores are known to occur only in two sections, the Selaginellcas and IsoetCcT ; the prothallium in these cases fills the cavity of the macrospore, and only the part which bears the archegonia protrudes; the terminal branching of the stem is dichotomous, or there is no provision for branching (Isoetes); the sporangia are produced singly on the upper side of the leaves near their base ; the macrosporangia produce a few macrospores, the microsporangia a large number of microspores. z 2 340 VASCULAR CRYPTOGAMS. CLASS VL FE RNS^. The Sexual Generation or Prothallium is a thalloid structure containing chlo- rophyll and obtaining its nourishment independently ; its development presents striking resemblances to that of the simpler Hepaticai, and to a certain extent even to the formation of the pro-embryo of some IMosses. It produces simple tubular unarticulated root-hairs, and finally antheridia and archegonia. Its develop- ment and the duration of its life may embrace a considerable space of time, espe- cially when the archegonia are not fertilised. When the spores germinate, which usually does not take place till a con- siderable time after dissemination (but in Osmunda after only a few days) the cuti- cularised exospore, generally provided with ridges, bosses, spines, or granulations, splits along its edges ; the endospore, which now protrudes and is not unfrequently already divided by septa, produces the prothallium, either immediately, as in Os- munda, or after the preliminary formation of a filamentous pro-embryo, which pre- sents in Hymenophyllaceje certain resemblances to that of the Andreasaceae and of Tetraphis among IMosses. The development of the prothallium has been more exactly investigated only in the Hymenophyllacese, the Polypodiaceae, and also in Osmunda and Aneimia ; and the considerable diff'erences which have thus been established necessitate separate descriptions. In the Hymenophyllacese the contents of the spore are divided, even before germination, into three cells meeting in the centre ; in some species of Trichomanes small cells are cut off at three points of the circumference, while a large central ^ H. von Mohl, Ueber den Bau des Stammes der Baumfarne (Verm. Schriften, p. loS). — Hof- meister, Ueber Entwickelung imd Bau der Vegetationsorgane der Fame (Abhandlungen der konigl. Siichs. Gesells. der Wissen. 1857, vol. V). — Ditto, Ueber die Verzweigung der Fame (Jahrb. fiir wissen. Bot. vol.111, p. 278). — Mettenius, Filices Hort. Bot. Lipsiensis (Leipzig 1856). — Ditto, Ueber die Hymenophyllaceen (Abhandlungen der konigl. Sachs. Ges. der Wissen. 1864, vol. Vll). — Wigand, Botanische Untersuchungen (Braunschweig 1854). — [On the Germination, Development, and Fruc- tification of the Higher Cryptogamia, &c. Ray Society, 1862, pp. 128-266.] — Dippel, Ueber den Bau der Fibrovasalstrange in the Berichte deutscher Naturforscher u. Aerzte in Giessen, 1865, p. 142. — Rees, Entwickelung des Polypodiaceensporangiums (Jahrb. flir wissen. Bot. 1866, vol. V. p. 5). — Strasburger, Befruchtung der Farnkrauter (Jahrb. fiir wissen. Bot. 1869, vol. VII, p. 390). — Kny, Ueber Entwickelung des Prothalliums und der Geschlechtsorgane, in the Sitzungsberichte der Gesellschaft naturforschender Freunde in Berlin, Jan. 21 and Nov. 17, 1S68. — Kny, Ueber Bau und Entwickelung des Farnantheridiums (Monatsberichte der kais. Akad. der Wissen. Berlin, May 1869). — Kny, Beitrage zur Entwickelungsgeschichte der Farnkriiuter (Jahrb. fiir wissen. Bot. vol. VII. p. I). FERNS. 341 cell remains undivided. The cells develope into germinating filaments, bursting the exospore in three directions ; these filaments then grow at their apex, and become segmented by septa; only one of them however generally attains a more decided development, the others soon assuming the form of hairs. In Hyfiieriophyllum tim- bridge?jse the former frequently developes finally into a cellular plate ; but in other species it forms a much-branched conferva-like protonema, on which flat prothallia 2 to 6 lines in length and |- to i^ in breadth are formed as lateral shoots. Each cell of the filament may give rise to a branch which is given off behind the anterior septum, and is at once separated by another septum. Some of these branches continue to grow Hke the mother-shoot indefinitely, others end in becoming hairs; a larger number are transformed into flat prothallia, but most develope into root-hairs. Here and there the rudiment of a filamentous branch becomes con- verted into an antheridium, or even into an archegonium. At the apex of the flat prothallia spherical cells arise in Trichornanes mcisiim on marginal flask-shaped cells : these must probably be considered as organs of propagation ; but the mar- ginal cells of the flat prothallia may develope into root-hairs and new protonemal filaments, and also into new flat shoots. The root-hairs are mostly short, with brown walls, and produce at their end lobed attaching-discs or branching tubes. In the Polypodiaceoe and Schizaeacese the endospore developes into a short articulated filamentous pro-embryo, at the end of which, even at an early stage, a more or less considerable increase in breadth takes place ; a plate of tissue is thus formed consisting at first of only one layer, which soon assumes a broadly cordate or even reniform shape, and has its growing apex situated in an anterior depression. Its apical cell forms two rows of segments right and left, by walls which are per- pendicular to the surface, and from their further divisions the flat tissue is produced. The power of rejuvenescence of the apical cell is, however, limited ; it ends in the formation of a septum by which a new apical cell is formed, which then divides by longitudinal walls, and thus forms a row of apical cells lying side by side which occupies the bottom of the depression of the prothallium-disc, in the same manner as in the thallus of Pellia. The root-hairs are all lateral structures, springing in large numbers from the under- side of the posterior part of the pro- thallium ; among them are the antheridia, which in this case are only rarely mar- ginal. The archegonia are also produced on the under-side, but on a cushion behind the anterior depression formed of several layers; in Ceratopteris several cushions are formed bearing archegonia. Osmunda (examined minutely by Kny, and compared with the preceding, /. c.) is distinguished in the first place from the Polypodiacese and Schizaeaceae by the absence of the pro-embryo. The endospore undergoes divisions at the very com- mencement of germination, which form a plate of tissue of which a posterior cell is converted, as in Equisetaceae, into the first root-hair. The succeeding root-hairs arise from marginal ceils and on the under-side of superficial cells of the prothallium, the apical growth of which follows a similar course to that of Polypodiaceae. The mid-rib consisting of several layers is characteristic of Osmunda, penetrating the ribbon-like prothallium from the posterior end to the apex, and producing a large number of archegonia on both sides. The antheridia spring partly from the margin, partly from the lower surface with the exception of the mid-rib. 342 VASCULAR CRYPTOGAMS. Like many thalloid Hepaticae, the prothailia of Ferns also produce adventitious shoots from single marginal cells ; this happens with especial profusion in Osmunda, where the adventitious shoots become detached, and play the part of vegetative organs of reproduction. The prothailia show a tendency to be dioecious, which is manifested in the fact that all the spores from a sporangium sometimes produce prothailia bearing antheridia only (as in Osjiiunda regalis) ; while in other cases the archegonia appear later and in smaller numbers, and are fertilised by the antheridia of younger prothailia. The Antheridia are, speaking morphologically, trichomes; they are produced in the same manner as the root-hairs, as outgrowths of the marginal or superficial cells of the prothailia ; in the Hymenophyllaceae they are also produced on the protonemal filaments. The projection is usually separated from the mother-cell by a septum, and swells up spherically at once or after the formation of a pedicel. In some cases the mother-cells of the antherozoids are formed at once in this globular cell ; but it usually undergoes still further divisions ^ in consequence of which the wall of the antheridium consists of a single layer of cells surrounding the central cell. The cells of this wall form grains of chlorophyll on their inner face, while the central cell of the antheridium divides further into the mother-cells of the antherozoids, which, however, are not numerous. The dehiscence of the ripe antheridium is the con- sequence of a rapid absorption of water in the parietal cells, which swell up violently and compress the contents of the central cell till the antheridium is ruptured at the apex. The antherozoid- cells thus escape, and out of each of them is set free an antherozoid coiled spirally three or four times. The finer anterior end of each antherozoid is provided with a number of cilia ; the thicker posterior end often drags with it a vesicle furnished with colourless granules, which subsequently falls off and remains at rest, while the filament alone continues in motion. Strasburger states that this vesicle is formed from a central part of the contents of the mother-cell, the parietal protoplasm of which forms the filament and its cilia. The vesicle is hence properly not a part of the antherozoid ; it is only attached to it, and swells up strongly in water by endosmose, as is shown in Fig. 253. The Archegonium arises from a single superficial cell of the pro thallium, which is at first only slightly arched and is divided by a wall parallel to the upper surface. The lower of the cells thus formed is the central cell of the archegonium : the upper ^ These divisions take place in a very remarkable manner. In the hemispherical mother-cell of the antheridium oi Aneimia hirta, an arched wall arises, by which it is divided into an inner hemi- spherical cell, and an outer one which covers the former like a bell ; the latter is then split up by a transverse annular wall into an upper lid-like and a lower hollow cylindrical cell. The same thing occurs in Ceratopteris ; in other cases, as in Asplenium alatum, a funnel-shaped wall is formed in the hemispherical mother-cell of the antheridium, and with the end of the funnel above the wall of the mother-cell ; the upper part of this is cut off by a level septum as a covering cell ; two, or even three, funnel-shaped walls may be formed in succession, so that the parietal layer of the antheridium consists of two or three superposed funnel-shaped cells and a covering cell (as in Fig- 253). The mode of formation of the antheridium-wall is quite different in Osmunda, where it consists below of two or three cells, upon which rest several of the upper cells which result from the division of the stigmatic cell (Kny, /. c). FERNS. 343 and outer one produces by further divisions the neck, which, when mature, consists of four rows of cells meeting in its axis. A layer of cells is formed by division of the cells surrounding the central cell, corresponding to that of the wall of the ventral part of the archegonium of Muscineoe. The further changes which take place within the central cell, and the formation of the canal of the neck, are described by FiC. 253 — Amlicridia >.>( Adiaiiluin Cnpillits-l'eneris (X 550), in loiijjitiKlinal optical section ; / not yet ripe ; // the antherozoids already mature ; /// the antheridiuni burst, the parietal cells greatly swollen radially, the antherozoids mostly escaped ; / prothallium, a antheridiuni. s antherozoid, b the vesicle containing starch- grains. Strasburgcr and Kny in the works already mentioned, in accordance with my earlier observations; so that the drawing, Fig. 255, given in my first edition, can be re- tained ; it is completed by Fig. 254, borrowed from Strasburger, which represents a younger condition of development. The contents of the central cell are divided ^ A Fig. 254.— Young archegonia oi Pteris serriilata (after Strasburger) ; the oosphere, h h the neck, k the canal-cell. into two unequal portions; the larger and lower one (Fig. 254, A, e) is at first broad, almost discoid, and afterwards becomes round ; it is the oosphere. The other portion {k), which is at first smaller, grows in between the four rows of cells of the neck, forcing them apart ; it thus forms a canal filled with mucilaginous protoplasm, in which a row of nuclei arises, but without the corresponding cell- 344 VASCULAR CRYPTOGAMS. divisions taking place. The substance of this Ca?ial-cell, as it is termed, is finally completely converted into mucilage, swells and forces the apical cells of the neck apart, escapes, and remains collected before the opening of the neck. The anthero- zoids are retained by this mucilage and collect in large numbers before the archego- nium ; a number force themselves into the canal of the neck, often finally stopping it up ; a few reach the oosphere, force themselves into and disappear in it. The entrance takes place at a lighter spot of the oosphere facing the neck, which is termed the Receptive Spot ^ (compare the oogonia of Algae). After fertilisation the neck closes. The Asexual Generatmi or Fern (as it is popularly termed) is developed from the oospore or fertilised oosphere of the archegonium. At first the sur- FlG. 255.— Archegonia oi Adiantian Capillns- Veneris ( X 800) ; A, B, C, E in longitudinal optical section ; D in transverse optical section ; A,B,C before, E after fertilisation ; h neck of the archegonium, st mass of mucilage, e oosphere ; E, e the two-celled embryo (observed after lying one day in glycerine). rounding tissue of the prothallium keeps pace with the increase of the oospore, so that this latter remains for some time enclosed in a protuberance springing from the under surface, until the first leaf and root break through. The first processes of division of the oospore are, as Hofmeister has shown in the case of Pieris aqidlina and Aspidium Filix-mas, not entirely alike in diff'erent species. It is certain that the first division-wall of the oospore is transverse to the longitudinal axis of the prothallium, and inclined to it obliquely; as shown in Fig. 255, E, its inclination is the same as that of the neck of the archegonium. It is also certain that each * Strasburger states that the act of fertilisation may be observed especially clearly in Cera- topteris ; the forcible entrance of the antherozoids as far as the oosphere had previously been seen by Hofmeister. FERNS. 345 of the two daughter- cells is at once divided again by transverse septa, so that the segmented oospore or embryo now consists of four cells placed as quadrants of a sphere, and which are bisected by a longitudinal section. In Fig. 256 these first transverse divisions are indicated by thicker Hues, the embryo being seen in longi- tudinal section. The explanation of the figure points out the interpretation which w/i FtG. 256.— Vertical longitudinal section of the embryo of Pferis aquilina (after Hofmeister, Entwickelung iind Bau dcr Vegctationsorgane der Fame, p. 607) : the thicker lines are sections of the first three division-walls by which the embryo is divided into four cells. The lower anterior cell forn)s, according to Hofmeister, the leaf 5 at the apex of the stem st; from the lower posterior cell is produced the root, stu being its apical cell and wh its root-cap. In Pteris, the foot y is formed from the two upper of the first four cells. In Aspidiian Filix-mas, the same author states that these processes diverge still further from those in the Rhizocarps. Hofmeister gives to the first four cells of Pieris aquilifia, which the reader may compare with the corresponding development of Salvinia and Marsilea; but it must not be forgotten that the embryo of the Fern lies, so to speak, on its back. Al- though it is impossible in this place to go into a more minute description, it is ,still necessary at least to point out the resemblance between the embryo of Ferns and that of Rhizocarps. Fig. 257.— .-/« Capillus-Vencris ; vertical longitudinal section through the prothallium // and the young Fern £: k root-hairs, a archegonia of the prothallium, * the first leaf, w the first root of the young plant (X about 10). FIG. ■2-^9,.—Adiantuiii CapUlus-Veneris ; the pro. thallium // seen from below with the young Fern attached to it ; b its first leaf; w' iv" its first and second roots; /z root-hairs of the prothallium (X about 30). If we neglect for the moment the points which are still doubtful in the signi- ficance of each of the first four cells of the embryo, it is certain that one of them which is inferior^ and posterior becomes the mother-cell of the first root, and that the apical cell of the stem lies immediately in front of and above the base of the leaf. * The terms posterior, anterior, superior, inferior, refer also to the prothallium, the apex of which is turned in front, and its archegonia-bearing surface downwards. ;4'-> VASCULAR CRYPTOGAMS. and that the upper part of the embryo between the apex of the stem and the base of the root becomes transformed into a special organ, the Foot, by which the young plant attaches itself to the tissue of the prothallium, in order to draw nourishment from it, while the first roots and leaves are being put out. This foot or apparatus for obtaining nourishment, which I consider a lateral structure, is called by Hofmeister the first axis of growth, or primary axis of the Fern ; the leaf-bearing axis arises upon it as a lateral shoot. But on this point also I consider, in opposition to the views of this distinguished morphologist, that the analogy with the processes described by Pringsheim in Salvinia must not be lost sight of; I must refer to the description of the origin of the embryo in the archegonium given under the Rhizocarpese. The first parts of the stem and the roots and leaves, which are now developed in succession from the embryo, are very small, and remain so ; those which are formed later are gradually larger. The leaves become constantly more complex in form, and the structure of the stem more intricate as the new additions to it increase in diameter. The first parts of the stem, like the first leaf-stalks, contain each only one axial fibro-vascular bundle ; the later ones a larger number. In this manner the Fern continues to gain strength, not by subsequent increase of size of the embryonic structures, but by each successive part attaining a more considerable size and development than the preceding ones; until at length a kind of stationary con- dition is arrived at in which the newly-formed organs are nearly similar to the preceding ones. The following observations refer especially to this mature condition of Ferns. The mature Fern is, in some Hymenophyllacese, a small delicate plant, not much exceeding in dimensions the larger IMuscinese ; in other sections the fully grown plants attain the size of considerable shrubs ; some species, natives of the Tropics and of the Southern Hemisphere, assume even a palm-like habit, and are called Tree-ferns. The stem creeps on or beneath the ground (as in Polypodium and Pteris aquilijia), or climbs up rocks and stems; in some it ascends obliquely {e.g. Aspidiiwi Filix-mas)\ in Tree-ferns it rises up vertically in the form of a column. The roots are usually very numerous ; in Tree-ferns the stem is often entirely covered by a dense mantle of them. They arise on the stem in acro- petal succession ; sometimes close to the growing apex of the stem (as in Pteris aquilind). When the internodes remain very short, and the stem is entirely covered with the bases of the leaves, the roots arise, as in Aspidium Filix-mas, from the leaf- stalks. In many Hymenophyllacese which have no true roots, branches of the stem assume a root-like structure. In creeping and climbing species the leaves are sepa- rated by distinct internodes which are sometimes very long ; in thick, ascending, and vertical stems, the internodes are usually undeveloped, and the leaves so crowded that no free portion of the stem remains uncovered, or only a very inconsiderable one. The leaves of Ferns are usually characterised by a circinate vernation, and they only unrol in the last stage of their growth ; the mid-rib and the lateral veins are curved from behind forwards. The forms of the leaves are among the most perfect in the whole vegetable kingdom ; they manifest an enormous variety in their size, the lamina being usually deeply lobed, branched, or pinnate. In com- parison with the stem and the slender roots they are mostly very large, and some- times attain extraordinary dimensions, even a length of from lo to 20 feet (as in FERNS. 347 Pteris aqiiilina, Cibotium, and Angiopteris). They are always stalked, and continue their growth at the apex for a long time ; the leaf- stalks and the lower parts of the lamina are often completely unfolded while the apex is still growing (as in Nephrolepis). This apical growth is not unfrequently interrupted periodically {vide infra) ; in Lygodium the leaf-stalk or the rachis even resembles a twining stem growing for a long period, the pinnae presenting the appearance of leaves. The amount of metamorphosis of the leaves is, notwithstanding, very inconsiderable ; on the same plant the same forms of leaves, mostly foliage-leaves, are constantly repeated ; scale-like leaves occur on underground stolons (^. g. in Struthiopieris geniiatiica), and in many cases the fertile leaves (those which bear sporangia) assume special forms. Such differences as occur in most Phanerogams are not found in the development of the leaves of one plant ; Platyceriwn alcicome must, however, be mentioned, as having the foliage-leaves alternately developed as broad plates closely applied to the -supporting surface and as long dichotomously branched ribbon-shaped tIG. 359. — Pteris aqiiilitta, a part of the under^rouiKj stem witli leaves and bases of the leaf-stalks (reduced about one-half) ; / older portion of the stem bearinjj the two bifurcations // and //', ss the apex of the weaker branch // ; beside it the youngest leaf-rudiment 8 ; 1-7 the leaves of this branch, one being developed in each year; 1-5 the leaves of earlier years, which have already died off at some distance from the stem ; 6 the leaf of the present year with unfolded lamina, the stalk having beencut off ; 7 the young leaf for next year; at the apex of the stalk is the lamina still very small and entirely clothed with hairs. The leaf-stalk / bears a bud II a, which has developed a leaf b that has already died off. The more slender filaments are roots. All the parts shown in the figure are underground. erect leaves. Amons^ the various forms of trichomes of Ferns those termed PalecB are especially striking, from their great numbers and from being frequently flat and leaf-like ; the younger leaves are generally entirely covered and concealed by them. After these preliminary particulars, we may now turn to a consideradon of the mode of growth of the separate organs. The growing end of the stem sometimes far outruns the point of attachment of the youngest leaves, and then appears naked, as in Polypodium vulgare, P. sporodo- carpitm, and other creeping Ferns, as well as in Pteris aquilina, where, according to Hofmeister, it frequently attains in old plants a length of several inches without bearing leaves. Mettenius states that in many Hymenophyllaceae leafless prolon- gadons of the axis of this kind have been taken for roots. In other cases, on the contrary, especially in Ferns with an erect growth, the increase in length of the stem is much slower, its apex remaining enclosed in a leaf-bud. The stem generally ends in a flat apex ; sometimes, as in Pteris, it is even imbedded in a funnel-shaped ^^■^ VASCULAR CRFPTOGAMS. elevation of the older tissues (Fig. 261, £). The apex of the stem is always occupied by a clearly distinguishable Apical Cell, which is either divided by walls alternately inclined, and then resembles, when viewed from above, the transverse section of a biconvex lens; or it is a three-sided pyramid, with a convex anterior surface and three oblique lateral surfaces, which intersect behind. The oudines of the segments, which are in the first case in two, in the second case in three rows, or arranged with more complicated divergences, soon disappear in consequence of numerous cell-divisions and of the displacement caused by the growth of the masses of tissue and leaf-stalks surrounding the apex. The apical cell, for in- stance, of Pieris aqiiilina, is wedge-shaped, the segments on the horizontal stem forming a right and a left row ; the edges of the apical cell face upwards and downwards (Fig. 260). The same is also the case, according to Hofmeister, in Niphohohis dmiensis and rupestn's, Polypodium aureum and piinctulatum, and Plaiy- ceriuni alcicorne. In Polypodium vulgare he states that it is sometimes •wedge-shaped, sometimes pyramidal with three faces ; the last-named form occurs also in Aspidium Filix-mas, Marattia cicutafolia, &c. As a rule it may for the present be assumed that Pig, 260.— Apical view of the end of the stem of Pteris aquiltna ; y the apical cell of the stem ; x the apical cell of the youngest leaf ; h h hairs which cover the apical region surrounded by a cushion of tissue. creeping stems with a bilateral development have a wedge-shaped apical cell, upright or ascending stems with radiating rosettes of leaves one that is a three-sided pyramid. The further relationships of the segments of the apical cell of the stem to the origin of the leaves and to the building up of the tissue of the stem itself, are still but litde known in detail. It cannot be doubted that each leaf results from a single segment only, and that this segment-cell is devoted from an early period to the formation of the leaf, but it appears doubtful whether the segments always form leaves, and if not what proportion of sterile segments precedes one from wdiich a leaf is developed. The phyllotaxis of Ferns sometimes corresponds to the rectilinear arrangement of the segments of the apical cell. Thus the distichous arrangement of the leaves of Pieris aquilina, Niphoholus rupeslris, and of some species of Polypodium, corre- sponds to the biseriate segmentation of the apical cell of the stem. But where the phyllotaxis is complicated and spiral and the apical cell a three-sided pyramid, as occurs in Aspidium Filix-mas^ the same processes may take place as in those Mosses FERNS. ^^g which have their leaves arranged in many rows with a triangular apical cell, such as Polytrichum\ The Terminal Branching of the stem which occurs in all Ferns Hofmeister con- siders to be dichotomous. The branches arise very near the end of the stem, and are, at least at first, like the primary stem, so that the branching is a bifurcation. That the branches are independent of the leaves is inferred by this writer from the fact that the ends of the stem of Pteris aquihna, which are leafless and often several inches long, regularly fork. These forks are, in this and in many other cases, not axillary ; and where, in other Ferns, they appear axillary, we must assume, with Hofmeister, that the forking has taken place immediately in front of a youngest leaf, and that the fork which stands before the leaf developes to a smaller, while the other (a prolongation of the primary stem) does so to a greater extent. Thus, in other words, the apparent axillary branching of some Ferns must be considered as a consequence of the sympodial development of dichotomous ramifications which take place in the plane of insertion of the leaves. The branching at the end of the stem does not usually take place in the same plane as the insertion of the leaf imme- diately preceding, and the branch then stands laterally on the stem beside the leaf. To this class belongs, according to INIettenius's description, the extra-axillary branching of those Hymenophyllaceae which have their leaves in two rows. That which dis- tinguishes Ferns from Phanerogams with axillary branching, especially Angiosperms, is the rarity of terminal branching. While in the latter every leaf-axil, at least in the vegetative region, bears a bud, even the apparently axillary branches of creeping Ferns with long internodcs occur mostly only at great distances, being wanting in a number of intermediate leaves. In those Ferns where the growth of the stem is slow and the apical region of considerable size, especially in erect species like Aspi- dium Filix-7iias and Tree-ferns, the terminal branching of the stem is reduced to a minimum, or is entirely absent, or occurs only in abnormal cases. The formation of new shoots from the bases of leaf-stalks must be distinguished from the normal terminal branching of the stem. These have nothing to do genetically with the stem, any more than the formation of adventitious shoots from the lamina of the leaves {vide infra). The Development of the Leaf is decidedly basifugal and apical, the further growth being also basifugal. The leaf-stalk is first formed ; at its apex the lamina begins subsequently to show itself; its lowest parts are formed first, its higher parts in basifugal succession. The extraordinary slowness of this growth is very re- markable, finding its parallel only among the Ophioglossaceae. In old plants of Pteris aquilina the formation of the leaf commences fully two years before its un- folding ; at the commencement of the second year only the leaf-stalk is as yet in existence, about one inch high, its growth having taken place up to this period from an apical cell which is divided by oblique walls in alternating directions ; in the summer of the second year the lamina arises for the first time at the apex of this rod-like body, and may be found hidden in the form of a minute disc beneath the long hairs. It immediately bends downwards at its apex, and hangs See Hofmeister, Allgemeine Morphologic, p. 509; and Bot. Zeitg. 1870, p. 441. 3rp VASCULAR CRYPTOGAMS. down like an apron from the apex of the stalk (Fig. 261, B, C, B). Its growth now proceeds underground, so that it does not begin to unfold till the third spring, M^hen it is raised above ground by the elongation of the leaf-stalk. The whole of the leaves of a rosette of Aspidiiim Filix-mas have been in course of formation two years before their unfolding ; the leaf-stalk is in this case also formed in the first year, and the first formation of the lamina takes place on the oldest leaves of the young rosette. The basifugal apical growth of the lamina of Fern-leaves is however most conspicuous when it continually advances for a considerable time without attaining a definite conclusion while the lower parts of the lamina have long been fully developed, as in Nephrolepis. The periodical interruption of the apical growth of the lamina already mentioned occurs in many species of Gleichenia and Mertensia, where the development of the leaves remains stationary above the first pair of Fig. 261. — Pteris aqiiilijia; A the end of a stem st, the apex lying at ss; by its side at b is the rudiment of a leaf, bs the stalk of a leaf in the second year, at h its lamina enveloped by hairs, K a bud at the back of the leaf-stalk, 7u roots ; B a young- leaf in the second year, bs its stalk, / its small lamina with the hairs removed ; C longitudinal section of a similar leaf, connected witli the transverse section of the stem st, bs and / as in ^; D the lamina of a leaf in the second year seen in front, i. e. on the upper side (X about 5) ; the first divisions have begun to be formed ; E horizontal longitudinal section of a fork of the stem, ss s's the two apices, a a brown epidermal tissue, b b brown sclerenchyma, g fibro-vascular bundles. {A, B, C natural size.) pinnae (and when the pinnation is compound is often repeated in the several orders of branching) ; so that the apex, forming apparently a bud in the fork, either remains altogether undeveloped, or is only incompletely developed in a succeeding period of vegetation, and then again in the same manner. This intermittent development of the leaves may apparently extend over many years (see Braun, 'Rejuvenescence,' p. 114). According to Mettenius, the lamina of some Hymeno- phyllaceae is capable of unlimited development, and is annually renewed. In Ly- godium the primary branches of the lamina remain also in a bud-like condition at the end after the formation of each pair of pinnae of the second order, while the rachis of the leaf grows without limit and resembles a twining stem. The branching of the lamina of Fern-leaves is not unfrequently forked in the mature state, as in Platycerium, Schizaea, &c. ; but Hofmeister refers also the pinnate forms to dichotomous branching at the commencement, which becomes sympodial with further development, a right and left fork being alternately weaker in its growth. FERXS. 351 and forming the lateral pinna? ; while the branches, the growth of which is favoured form the rachis of the leaf or of the branch of a leaf '. I'he Fortnatio?i of Adventitious Buds, which do not result from the terminal branching of the stem, is, in Ferns, connected with the leaves. These buds make their appearance on the leaf-stalk or on the lamina itself. The shoots of Pteris aquilijia which spring from the leaf-stalk (Fig. 261) stand at the back of the Fig. ^2.—.ls/>id!H»i Filixmas; A longitudinal section through the end of a stem, v the apical part of the stem st, b b leaf-stalks, *' a young leaf still rolled up, the rest enveloped in long palere, ^ fibro-vascular bundles ; B a leaf-stalk of the same plant broken off, bearing at .* a bud with several leaves, iv a root of this bud ; C a similar leaf-stalk cut through lengthways, bearing a root at lu and a bud at h; D end of a stem with the leaf-stalks cut off with the exception of the youngest leaves of the terminal bud in order to show the arrangement of the leaves, the spaces between the stalks b b are filled with numerous roots which themselves all spiing out of the stalks; E end of a stem the cortex of which has been peeled off in order to show the net-work of fibro-vascular bundles i^; F a mesh of this net-work slightly magnified, showing the basal portions of the bundles which pass into the leaves. individual leaf-stalks near the base; in Aspidium Filix-inas (Fig. 262) they arise at a moderate height above the insertion, usually on one of the lateral edges of the leaf-stalk. In both cases Hofmeister states that they are formed on the young ^ It must be observed here also that Hofmeister applies the term 'dichotomy' in a much wider sense than is usually done. New examinations of a large number of species are greatly to be desired, both in reference to the formation of leaves and to the terminal branching of the stem. ;-i VASCULAR CRYPTOGAMS. leaf-Stalk even before the first appearance of its lamina, and before the differentiation of its tissue. A single superficial cell of the leaf-stalk is the mother-cell of the new shoot ; and as the surrounding tissue of the leaf-stalk grows like a wall around them, they may, as in Pteris, be placed in a deep depression, where they sometimes remain dormant for a long period. Even when the leaf has long died away, the leaf-stalk still remains succulent above the bud, and filled with food-materials ; and in Aspidium Filix-mas vigorous stems are not unfrequently found with a number of leaves at their posterior end still attached to the leaf-stalk of an older stem. In some cases, as in Struthiopteris germanica, the buds produced on the leaf-stalks develope into long underground stolons furnished with scale-leaves, which become erect at the end and unfold a crown of foliage-leaves above ground. In Nephro- lepis undulata they swell at the end into tubers. Adventitious buds spring from the lamina, especially in many species of Asplenium ; in A./urcaium, e. g., often in large numbers from the middle of the upper surface of the pinnae; in A. deciissatiim from the base of the pinnse (or axillary on the mid-rib ?). Ceratopteris thaliciroides not unfre- quently produces buds in the axils of all the divisions of the leaves, which, especially when the detached leaf is laid upon damp ground, germinate rapidly, and grow into vigorous plants. According to Hofmeister, these buds also spring from superficial cells of the leaf. The long pendulous leaves of some Ferns touch the ground with their apices, root, and sometimes also put out new shoots from these points {e.g. Chry sodium flagelli/erum, Woodwardia radicans, &c.). The Roofs. During its growth the stem is usually constantly forming new roots in acropetal succession, which, in the creeping species, become at once fixed to the substratum. In Pteris aquilina the new roots appear close behind the apex, and, both in this species and in Aspidium Filix-mas, they also proceed from the adventi- tious buds of the leaf-stalk while still very young. It has already been mentioned that, in the last-named species, when the mature stem is completely covered by leaf-stalks, all the roots spring from them and not from the stem. In Tree-ferns especially the lower part of the erect stem is entirely covered by slender roots, which grow downwards, forming an envelope several inches thick before they penetrate the soil, and thus give a broad base to the stem although it is there really much more slender ; but in the upper part there are also a great many roots. In small plants they are very slender ; on large plants they attain a diameter of from I to 3 mm. ; they are cylindrical, generally covered with a number of root-hairs which form a kind of felt, and are of a brown or black colour. The history of the growth of Fern-roots has been studied by Nageh and Leitgeb^ The apical cell is a three-sided pyramid, with a convex equilateral base. The segments or layers of the root-cap detached by convex septa parallel to the base first separate into four cells placed crosswise, so that those of successive layers alternate by about 45° ; each of the four cells of a layer then splits up into two external and one internal (central one), so that the layer is now formed of four internal cells arranged in a cross, and of eight external cells. Further divisions may then follow ; * Sitzungsber. der bayr. Akad. der Wiss. Dec. 15, 1865. Compare with what follows the diagram of a root given under the Equisetaceoe, which serves in the main also for Ferns and Rhizocarps ; also in addition p. 123. FERNS. 363 the central cells of the layer grow more quickly in an axial direction, and may become divided by transverse septa, by which the layer is made to consist of two or more strata in the middle. The formation of a layer is generally followed by that of three root-segments before a further new layer is formed ; these segments, corresponding to the faces of the three-sided apical cell, lie in three straight longi- tudinal rows. Each of these triangular tabular segments includes a third of the circumference of the root, and is first divided by a radial longitudinal wall into two unequal portions. The transverse section of the root now shows six cells, three of which meet in the centre, while the other three do not reach quite so far. Each of these six cells is then divided by a tangential wall (parallel to the surface) into an inner and an outer cell ; the inner ones form the fibro-vascular bundle, while the six outer cells form the rudiment of the cortex. If the root becomes thick, the six cortical cells divide by radial walls ; if it remains slender, this division does not take place. The six or twelve cortical cells are now divided by a tan- gential longitudinal wall, and the fibro-vascular bundle is enclosed by two layers of cells, the outer of which forms the epidermis, the inner the fundamental tissue of the cortex. The epidermis usually continues to consist of one layer only, dividing only by walls vertical to the surface ; but in some Ferns {e. g. Polypodium, Blech- num, and Cystopteris) the layer of epidermal cells is doubled. The layer of cells between the epidermis and the central bundle becomes double, an outer and inner cortex resulting from further divisions. In most Ferns, however, the distinction between the two layers cannot be made out in the fully grown root ; thotigh in some the inner cortex consists of thick-walled long cells, the outer cortex of thin- walled short ones. The Fibro-vascular Bundles consist at first, as has been mentioned, of six cells in transverse section ; these are each divided simultaneously by a tangential wall into an outer tabular and an inner cell. From the further divisions of the outer cell proceeds a tissue which Nageli and Leitgeb call Pericambium, and the cells of which are characterised in the fully developed root by their thin walls and by their granular and mucilaginous contents. They are broad, but short. From the six inner cells proceeds the prolongation of the true fibro-vascular bundle ; they divide in all directions, the divisions advancing in centrifugal succession ; the peripheral cells are considerably smaller, after the completion of the division, than the inner ones. The formation of vessels begins with their production at two or three points of the circumference lying diametrically opposite one another on the inner side of the pericambium ; it proceeds either at first right and left (tangentially), or centri- petally, a diametral row of vessels being thus formed. In slender roots this may proceed no further than the production of the first vessel ; in thicker roots one or more broad vessels lie in the centre, which only become woody at a later period. The peripheral cells and the narrow ones that lie between the vascular bundles form the phloem-layer of the bundle by the thickening of their walls. The roots of Ferns branch in a monopodial manner only ; the lateral rootlets arise in acropetal succession on the outside of the primary vascular bundle, and are therefore usually arranged in two rows, rarely in three or four. The mother-cells of the lateral roodets belong to the innermost layer of the cortex, and are separated from the vascular bundle of the primary root by the pericambium ; the roodets originate very near J.54 VA SC ULA R CR YP TOGA MS. the apex, when the vessels are not yet in existence. Adventitious lateral rootlets (arising behind those already formed) do not occur. The mother-cell of a lateral rootlet first of all forms its three-sided pyramidal apical cell by three oblique divi- sions ; the first la)er of the cap being then formed from it. When two primary vascular bundles arise in a lateral rootlet, they lie right and left in reference to the primary root. The cortex of the primary root is simply penetrated, no root-sheath being formed. The fibro-vascular bundles are always formed singly and in the axis of the root, even in very slender filiform stems, as in those of Hymenophyllacese, and in the young plants of larger species. When the stems of the latter and their leaf-stalks Fig. 26^.— Pteris aquilinn ; A transverse section of the stem, r its brown sheath (the layer of sclerenchyma beneath the epidermis), / the soft colourless paren- chyma of the fundamental tissue ; ig inner fibro- vascular bundles ; ag upper broad primary string of the outer bundle ; B the separated upper fibro- vascular bundle of the stem st and of its branches st' and st", b bundles of the leaf-stalk, ii u outline of the stem (natural size). Fig. 264.— a quarter of the transverse section of a fibro-vascular bundle from the stem of Pteris aquilina, with the adjacent parenchyma P containing starch, sg the bundle-sheath, b the layer of bast, sp the large sieve-tubes, g g the large vessels thickened in a scalariform manner, 5 a spiral vessel surrounded by cells containing starch (X300). become thicker with increase of growth, a network of anastomosing bundles is formed in place of the central bundle, presenting, in typical cases, a wide-meshed hollow cylinder, by which the fundamental tissue of the stem is separated into an outer cortical layer and an inner medullary portion (Fig. 262, A and E). Not un- frequently, however, isolated scattered bundles also arise in addition ; thus in Pteris aquilina two strong broad cauline bundles are formed within the medullary portion (Fig. 263, A, I'g), and in Tree-ferns a number of filiform bundles are scattered through it which enter into the leaf- stalk through the meshes of the primary bundle ^ ' For a more special description see Mettenius on Angiopteris, in Abhandlungen der konigl. Sachs. Gesellsch. der Wiss. 1864, vol. VI. FER.XS. 355 The primary bundles which form ihe cylindrical network already mentioned are mostly ribbon-shaped, broad, and, in the case of Tree-ferns, commonly have their margins curved outwards. From these margins spring the more slender filiform bundles which enter the leaf-stalk, and are more numerous in proportion to its thickness. These may also coalesce laterally into plates of different forms (as in F/en's aquilina), or may run separately side by side. The leaf-stalk always cor- responds to an opening of the meshes of the cylinder of the primary bundle. The thick bundles which run through the stem appear to be all cauline. Hof- meister found in Ptcris aguilhia that they exhibit the same distribution on the leafless elongated ends of the stem as on its leafy parts, a proof that the distribution does not depend on the leaves, as in Phanerogams. The end of the bundle may even be followed up to near the apical cell of the stem, in places where the nearest leaf-stalks have not yet begun to form bundles. The fibro-vascular bundles of Ferns are, like those of all Vascular Cryptogams, closed ; they consist of a mass of xylem, completely enveloped by a layer of phloem. Besides a few narrow spiral vessels, lying in the foci of the elliptical transverse section, the xylem consists of vessels with bordered pits which usually resemble transverse clefts (scalariform vessels), their ends being mostly obliquely trun- cated, or fusiform and pointed. Between the vessels lie narrow thin-walled cells, which contain starch in winter. The phloem, in addition to cells similar to those last named, contains wide sieve-tubes or latticed cells, and at the circumference narrow, bast-like, thick-walled fibres. The whole bundle is usually enclosed by a distinct sheath of narrower cells (vascular bundle-sheath) ; the latter often, but not always, have the walls which face the bundle strongly thickened and of a dark reddish-brown colour. The Fimdamcntal Tissue of the stem and of the leaf-stalks consists, in some species (as Polypoditim aureian and vulgare, and Aspidhun Fih'x-mas), entirely of thin-walled parenchyma ; in otherg (as Gleichenia, species of Pteris, and Tree-ferns), string-like, ribbon-shaped, or filiform portions of the fundamental tissue become dif- ferentiated, the cells of which undergo great thickening, and become brown-walled, hard, and prosenchymatous. JVIettenius aptly terms them sclerenchyma. In the stem of Pteris aquiliiui (Fig. 263, A) two thick bands of sclerenchyma of this descrip- tion (/>;-) lie between the inner and outer fibro-vascular bundles, and fine threads of sclerenchyma appear on the transverse section of the colourless parenchyma as dark points. In other cases (as in Polypodium vaccijiiifolium and in Tree-ferns), dark layers of sclerenchyma, the nature of which was in these cases first correctly recognised by H. von Mohl, form sheaths round the fibro-vascular bundles. The outer layer of the fundamental tissue of thicker stems and leaf-stalks lying beneath the epidermis, is often dark brown and sclerenchymatous, forming a hard firm sheath, as again, for instance, in Pteris aquilina (Fig. 263, A, r) and Tree-ferns. In order to facilitate, in spite of this firm coat, the communication of the outer air with the inner parenchyma which is rich in assimilated food-materials, it is, in Pteris aquilina, interrupted along two lateral lines, where the colourless parenchyma rises to the surface. In Tree-ferns, on the other hand, according to H. von Mohl, depressed cavities appear on the enlarged base of the rachis of the leaf, where the sclerenchyma is replaced by a loose and pulverulent tissue. A a 2 ^r^6 VASCULAR CRYPTOGAMS. It may be mentioned here in addition, as an isolated histological peculiarity, that in Aspidhmi Filix-mas, according to Schacht, roundish stalked glands occur in the fundamental tissue of the stem, which I have also noticed in the green paren- chyma of the leaves, and on the pedicels of the sporangia of the same Fern (Fig. 266, C, d). The lamina of the leaf consists in Hymenophyllaceas only of a single layer of cells, as in Mosses ; in all other Ferns it is formed of several layers. Between the upper and under epidermis lies a spongy parenchyma containing chlorophyll, the Mesophyll, penetrated by the fibro-vascular bundles which form the venation of the leaf. The epidermis of Fern-leaves is distinguished by containing chlorophyll, and by the peculiarities of the stomata already spoken of in the part of this work relating to tissue (Fig. 76, p. 89). The course of the veins is very various ; sometimes they run branching dichotomously at acute angles, or spreading like a fan upwards and sideways, without anastomosing and without forming a mid-rib ; more often the undivided lamina, or a division of the lobed, incised, or pinnate leaf, is penetrated by a distinct median vein though but slightly projecting, from which spring more slender branches, which themselves again ramify monopodially or in a forked manner, and run to the margins. The finer veins frequently anasto- mose like those of the leaves of most Dicotyledons, and divide the surface into areolae of characteristic appearance. The Trichomes of Ferns are produced in a great variety of positions. True root-hairs, simple unarticulated tubes, arise, not only on the roots themselves, but also on underground stems and on the bases of leaf-stalks (as in Pteris aquilina and Hymenophyllaceae). On aerial creeping stems and on the leaf-stalks the numerous usually brownish or dark-brown flat multicellular hairs, the Palece, occur, soon becoming dry, often entirely enclosing the buds, and attaining a length of from I to 6 cm. (as in Polypodium, Cibotium, &c.). Long strong bristles are sometimes found on the lamina (in Acrostichum crimtum), and very often fine, delicate, articulated hairs. Finally, the sporangia themselves are trichomes. The Sporangia of Ferns are, from a morphological point of view, trichomes of the leaves. They arise from epidermal cells, and are usually stalked capsules, the wall of which, when mature, consists of but a single layer of cells. A ring of cells belonging to the wall of the capsule and running across or obliquely or lengthwise is generally developed in a peculiar manner, and is then termed the Aniitdus. By its contraction when dried up the capsule bursts (at right angles to the plane of the annulus). Sometimes, instead of the annulus, a terminal or lateral group of the cells of the wall of the capsule is developed in a similar manner. The sporangia are generally combined into groups, each group being termed a Sortis ; the sorus contains either a small definite number or a large indefinite number of sporangia, and among them also very commonly some slender articu- lated hairs, the Paraphyses. The whole sorus is very generally covered by an excrescence of the epidermis, the true hidusium ; in other cases the false indusium consists of an outgrowth of the tissue of the leaf itself, and is then composed of several layers, and even has stomata; or the covering of the sorus is simply the result of the margin of the leaf being recurved or rolled over it. In Lygodium each separate sporangium is covered by a pocket-shaped growth of the tissue of the leaf FERNS. 357 like a bract ^ Sori are not usually formed upon all the leaves of the mature plant ; sometimes groups of fertile and sterile leaves alternate in regular succession, as in Striithiopteris germanica. In some cases the sori are uniformly distributed over the whole of the lamina, in others they are connected with definite portions of it. The fertile leaves may be in other respects like the sterile ones, or they may be strikingly different from them ; and this difference is not unfrequently occasioned by the partial or entire failure of development of the mesophyll between and near the fertile veins ; the fertile leaf, or the fertile part of the leaf, then ap- pears like a spike or panicle furnished with sporangia {e. g. Osmunda, Aneimia). The sporangia generally arise from the epidermis of the veins of the leaf, and especially on the under side of the lamina ; but in the Acrostichacece they spring both from the veins and from the mesophyll ; in Olfersia they cover both surfaces of the leaf at the sides of the mid-rib, or in Acrostichum only the under side. When, as is usually the case, the veins are the only parts that bear the sporangia, the fertile veins may be like the sterile ones, or may undergo a variety of changes at the spots where they bear the sori; they may be swollen into a cushion (forming a receptacle), or they may project beyond the margin of the leaf, as in the Hymenophyllacese. The sorus may be FIG. 265.— Under side of a lacinia of a leaf of Aspidiwn Filix-nias, with eight indusia i (X2). Fig. ■ifA.—Aspidium. Filix-ntas. A transverse section of a leaf with a sorus consisting of the sporangia s and the indusium ii; right and left in the mesophyll of the leaf are two small fibro-vascular bundles, the sheath of which shows the dark brown thickenings on the walls that face inwards. B a young sporangium, its annulus standing vertically to the plane of the paper, r its apical cell ; in the interior four cells are seen resulting from the division of the central cell ; C lateral view of a nearly ripe sporangium, rr its annulus, d the stalked gland peculiar to this species ; within the sporan- gium are seen the young spores already formed. seated on the end of a vein, which then frequently puts out two branches in the angle of which is placed the sorus, or it may be formed on the back and below the ends ^ Athough these points of structure are employed in systematic botany as characters of families, their morphology is at present but little known. A history of the development of the sori of Ma- rattia, Kaulfussia, and Dan^a, consisting of so-called united sporangia, is an especial desideratum. 35^ VASCULAR CRYPTOGAMS. of the veins ; or the sorus may run for a considerable distance by the side of the veins. Sometimes the fertile veins run close to the margin of the leaf, in other cases close to the mid-rib of the lamina. The Developmeftt of the sporangium ^ is accurately known only in the Poly- podiacese ; it arises there from a papillose outgrowth of one of the epidermal cells from which the sorus originates. Rees has shown that before the formation of the sporangium the epidermal cell concerned has been already divided cross-wise ; the papilla is cut off by a septum, another septum arising, after further elongation, in the mother-cell of the sporangium thus formed; the lower cell forms the pedicel, the upper cell the capsule of the spor- angium. The pedicel is usually trans- formed, by intercalary transverse divisions and longitudinal walls, into three rows of cells ; the nearly hemispherical mother-cell of the capsule is next transformed, by four successive oblique divisions, into four plano-convex parietal cells and a tetra- hedral inner cell ; in the former further divisions follow perpendicular to the sur- face, while the inner cell again forms four tabular segments which are parallel to the outer parietal cells. These inner parietal cells also divide perpendicularly to the surface of the capsule, the wall of which thus consists of two (or, according to Rees, of three) layers. The cells of the outer parietal layer from which the annulus is to be formed are further divided by parallel walls perpendicular to the surface of the sporangium and to the median line of the annulus, until the prescribed num- ber of cells of the annulus is reached ; these cells then project above the surface of the capsule. While the tetrahedral central cell is now producing by successive bipartitions the mother-cells of the spores, the cells of the inner parietal layers are absorbed, and the cavity of the sporangium is considerably enlarged by this means and by the superficial growth of the outer parietal layer ; so that the mass of mother-cells (according to Rees there are always twelve), floats entirely free in the fluid that fills the sporangium (Fig. 266). For the further peculiarities ex- FlG. 267.— Development of the sporangium of Asple- niinn Trichonianes ; the order of succession according to the letters a-i; in i the annulus r is shown ; the other figures are seen in optical longitudinal section, EUid the an- nulus is perpendicular to the paper (X550). * When the first sporangia are ripening, all stages of development of the younger ones may be found in the same sorus side by side. FERNS. 359 hibited, reference must be made to the work of Rees already quoted ; the illustra- tions, Figs. 266 and 267, had already been drawn on the wood when his more detailed investigations were pubHshed, and confirm his statements on all essential points. It is impossible, as has already been maintained, that the mother-cells of the spores can arise by free cell-formation. Each mother-cell is, in Aspidium Filix-mas (Fig. 268, /), provided with an evident nucleus; after its absorption (//), two new large clear nuclei arise (///), between which an evident line of separation is some- times to be seen. After the absorption of these nuclei which show the commence- ment of a bipartition, four new smaller nuclei appear {IV), the mother-cell splitting up into four spore-cells ( T), the relative position of which varies (as is shown in Figs. VI, VII, and VIII), The spore now becomes clothed with its cell-wall, which is differentiated into an endospore consisting of cellulose and a cuticularised brown exospore furnished with ridges {IX), and chlorophyll is formed within the spore. The spores of many Polypodiaceae are distinguished by the long period during which they retain their power of germination, and by the slowness of this process ; those of Hymenophyllacese often begin to germinate while still in the sporangium. Fig. 268.— Development of the spores oi Aspidium Filix-mas (XS50). Ihe Systematic Classification of P'erns, as generally given in handbooks, is based arti- ficially on the form of the mature sporangium for the families, and of the sorus for the genera ; only those groups which have already been repeatedly meniiioned are accurately known. It appears certain that the Hymenophyllaceae contain the lowest forms most nearly allied to the Muscineae ; the genetic relationship of the other families with the Hymenophyllaceae and with one another has not yet been ascertained; but the Hy- menophyllaceae probably form the starting point for two or more series of families. IMettenius (Filices Horti Botanici Lipsiensis) distinguishes the following families, which I adopt with some alteration in the arrangement : — I. Hymenophyllaceae. 2. Gleicheniacese. • 5. Marattiacese (vide infra). 3. Schizaeaceae. 6. Cyatheaceae. 4. Osmundaceae. 7. Polypodiaceae. In the characters of the families which follow, I have adopted the diagnoses of Mettenius, but at the same time give prominence to a few facts which may serve to complete the morphological statements already made. I. Hymenophyllacese. The sporangia have an oblique or transverse complete annulus ; and therefore burst with a longitudinal slit ; they are formed on a prolon- gation of the fertile vein (the Columella), projecting beyond the margin of the leaf, 360 VASCULAR CRYPTOGAMS. which is surrounded by a cup-shaped indusium. The antheridia and archegonia are formed for the most part on the surface of the prothallium, and chiefly from its marginal cells. The archegonia are borne on a cushion formed of several layers of cells. The mesophyll of the leaves usually consists of a single layer of cells, and is then necessarily destitute of stomata, which do however occur in Loxsoma on the leaf, which then consists of several layers. The stem is generally creeping and mostly very slender, and furnished with an axial fibro-vascular bundle. True roots are not present in all the species where they are absent, the stem itself is clothed with root-hairs : a large number of species of Trichomanes are described by Mettenius as rootless, and in these cases the ramifications of the stem assume a deceptive root-like appearance. The develop- ment of the axes precedes by a long space that of the leaves; several internodes have usually completely ended their growth while the leaves belonging to them are still very small; and these apparently (or actually?) leafless shoots often branch further to a great extent. The formation of the tissue of these families shows also many pecu- liarities, concerning which reference must be made to Mettenius (Hymenophyllaceae, /. f .). The fertile end of the veins of the leaf projecting beyond its margin, or the colu- mella, elongates by intercalary growth, and the newly formed sporangia are, in a corre- sponding manner, produced in basipetal succession. They are arranged in a spiral line on the columella. The sessile sporangia are biconvex, and are attached to the colu- mella by one of their convex surfaces. The annulus projecting in the form of a cushion which separates the two convexities is usually oblique, and divides the circumference into two unequal portions. In Loxsoma the sporangia are pear-shaped and distinctly stalked. Paraphyses occur only in a few species of Hymenophyllum. 2. The GleicheniaceaD have sessile sporangia with a complete transverse annulus, and hence a longitudinal dehiscence. The sori are dorsal, without indusium, and mostly formed of a few, sometimes of only three or four, sporangia. The innovation of the lamina of the leaf has already been mentioned. The leaf-stalk is not articulated. 3. SchizasacesB. The ovoid or pear-shaped sporangia are sessile or shortly stalked; the annulus forms an apical cap-like zone, and is complete and circular, and the dehiscence is therefore longitudinal. The leaf-stalk of all the species contains only one fibro-vascular bundle. In Lygodium the climbing leaf-stalk is indefinite in its growth ; its primary branches end in a lamina which is not circinate, and in L. tenue is trans- formed into a leaf-stalk with indefinite growth. The two pinnae at the base of each primary branch of the leaf have a flatly expanded lamina definite in its growth. The fertile segments are spicate, and each bears on its under side two rows of sporangia, each of which is placed in a pocket-shaped outgrowth of the tissue of the leaf. To this order belong also Schizaea and Aneimia. 4. Osmundaceae. The sporangia are shortly stalked, unsymmetrically rounded, and furnished on one side, instead of the annulus, with a pecuharly developed group of cells beneath the apex; on the other side they split longitudinally. In Osmunda the fertile leaves or leaf-stalks are contracted, that is, their mesophyll is not developed ; in Todea they resemble the sterile ones. 5. Cyatheacese. The sporangia have a complete, oblique, eccentric annulus, and transverse dehiscence; the indusia are variable or absent; the sorus generally on a strongly developed receptacle. The leaf-stalk is usually not articulated, passing gradu- ally into the stem. The genera Alsophila, Hemitelia, and Cyathea, include species with columnar stem (Tree-ferns), and large, often compoundly pinnate, leaves. 6. Polypodiacese. The sporangia have a vertical (/. e. longitudinal) incomplete annulus, and therefore split transversely. Mettenius distinguishes five sub-divisions of this family, which contains the largest number of species of any: — FERNS. 25 1 (a) Acrostiche(E. The sori cover the surface and veins of the under side or of both sides, or are placed upon a thickened receptacle which stands on the vein. There is no indusium. (Acrostichum, Polybotrya.) (b) PolypodiecB. The sori occupy either the whole length of the veins, or special anastomosing branches of it, or the back or thickened end of a vein. They are naked, or with a lateral indusium. (Polypodium, Adiantum, Pteris.) (c) Asplen'iecE. The sori are unilateral on the course of the veins, and are covered by a lateral indusium, or rarely without any; or they extend at their apex over the back of the veins, and are covered by an indusium springing from it ; or they occupy special anastomosing branches of the veins, and are unilateral and covered by an indusium free on the side of the vein. The leaf-stalk is not articulated. (Blechnum, Asplenium, Scolopendrium.) (d) Aspidiea. The sori are dorsal on the veins, covered with an indusium, or terminal and without indusium. (Aspidium, Phegopteris.) (e) DavalliecB. The sori are terminal on a vein or at a fork, and are furnished with an indusium ; or are placed on an intramarginal anastomosing bend of the veins, and covered with a cup-shaped indusium, free at the outer margin. (DavaUia, Nephrolepis.) The Marattiace£B, hitherto included among Ferns, must, from the earlier state- ments of Russow and the more recent investigations of Luerssen (Habilitations- schrift, Leipzig 1872), be separated from them, and classed with the Ophioglossaceae (and Equisetaceae), in consequence of the entirely different mode in which their spor- angia are formed. The large sporangia of Marattia are placed singly on lateral veins of the pinnae, to which they are attached by a narrow ridge-shaped base (pedicel). Two longitudinal rows of loculi contain the spores, which are not formed, as in the true Ferns, from a single primary mother-cell (central cell), but from a mass of tissue composed of primary mother-cells, filling up the loculus. The single loculus of the sporangium of Marattia corresponds so far to the single sporangium of Ophioglossum. A nearer affinity to the Ophioglossaceae may be indicated by the stipular struc- tures of the Marattiaceae, which, while entirely foreign to Ferns, exhibit a certain resemblance to those of the Ophioglossaceae. [The classification of Mettenius given above will serve as a guide to the principal morpho- logical differences between the various types of Fern structure. The student may however consult Hooker and Baker's Synopsis Filicum (London, 1S68) for a systematic arrangement more in accord- ance with our present extended knowledge of species, though still, no doubt, artificial. — Ed,] 36- VASCULAR CRVPTOGAMS. CLASS VII. EQUISETACE^^ The Sexual Generation or Prothallium. The spores of the Equisetaceae which have just attamed the ripe condition (they retain their power of germination only for a few days), show, when sown in water or on damp soil, the preparatory phases of germination after only a few hours. In the course of some days the prothallium becomes developed into a multicellular plate, the further growth of which then proceeds very slowly. The spore, which contains a nucleus and grains of chlorophyll, increases in size as soon as germination commences, becomes pear-shaped, and divides into two cells, one of which is smaller with scarcely any except colourless contents, and soon developes into a long hyaline root-hair (Fig. 269, /, //, ///, w), while the anterior and larger cell includes all the chloro- phyll-grains of the spore which multiply by division. This cell produces by further divisions the prtmary plate of the prothallium, which increases by apical growth and soon branches {III- VI). The process of multiplication of the cells is therefore apparently extremely irregular; even the very first divisions vary; sometimes the first wall in the primary apical cell which contains chlorophyll is but little inclined with respect to the longitudinal axis of the young plant (in E. Tel- viaicia the axis sometimes dichotomises) ; in other cases, on the contrary, this cell developes into a longish tube, the apical part of which is cut off by a septum (occa- sionally in E. arvense). The further growth is brought about by one or more apical cells dividing by septa, and longitudinal walls are subsequently formed in the seg- ments in an order very difficult to determine. Ramification takes place by the bulging out of lateral cells, which then continue their growth in a similar manner. The chlorophyll-grains increase simultaneously by division in the cells. The young ^ G. W. Bischoff, Die kryptogamischen Gewachse (NiiiTibeig 1828). — W. Hofmeister, Vergl. Unters. (185 1). — Ditto, Ueber die Keimung der Equiseten (Abh. der konigl. Slchs. Gesell, d. Wiss. 1855, vol. IV. p. 168). — Ditto, Ueber Sporenentwickelung der Equiseten (Jahrb. fiir wiss. Bot. vol. III. p. 283). — [Germination, Development, and Fructification of the Higher Cryptogamia (Ray Society), pp. 267-306]. — Thuret (in Ann. des Sci. Nat. 1851, vol. XVI. p. 31). — Sanio, Ueber Epidermis und Spaltoffnungen des Equis. (Linngea, vol. XXIX. Heft 4). — C. Cramer, L'ingenwachsthum und Gewebebildung bei E. arvense und sylvaticum (Pflanzenphys. Unters. von Niigeli und Cramer, 1855, vol. III.). — Duval- Jouve, Hist. Nat. des Equisetum (Paris 1864). — H. Schacht, Die Spermatozoiden im Pflanzenreich (Braunschweig 1864). — Max Rees, Entwickelungsgeschichte der Stammspitze von Equisetum (Jahrb. fiir wiss. Bot. 1867, vol. VI. p. 209). — Milde, Monographia Equisetorum, in Nova Acta Acad. Leop. Carolina, 1867, vol. XXXV. — N^geli und Leitgeb, Entstehung und Wachsthum der Wurzeln (Beitr. zur. wissen. Bot. von Nageli, Heft IV. Munchen 1867). — Pfitzer, Ueber die Schutzscheide (Jahrb. fiir wissen. Bot. vol. VI. p. 297). EQUISETACE^. 363 prothallia are, in E. Telmakia, usually narrow and ligulate, and consist of but a single layer of cells. The older prothallia are, both in this and in other species, branched in an irregularly lobed manner ; one of the lobes takes, sooner or later, the lead in growth, becomes thicker and fleshy, consisting of several layers of cells, and puts forth root-hairs from its under side. The prothallia of the Equisetaceae are, in general, dioecious. The male pro- thallia remain smaller, attaining a length of a few millimetres, and produce archegonia only in exceptional cases on shoots of later origin (Hofmeister). The female prothallia are larger (as much as i inch) ; Hofmeister com- pares them to the thallus oi Anthoceros purictatus^ Duval-Jouve to a curled endive-leaf. Duval- Jouve states that the antheridia appear about five weeks after germination, the archegonia much later. These statements refer especially to E. arvejise, limosum, and palustre ; according to the same writer, the prothallia of E. Telmateia and sylvaticum are broader and less branched ; those o{ E.ramosissinmm and variegatinii slenderer and more elongated. The Antheridia arise at the end or margin of the larger lobes of the male prothallium. The apical cells of the enveloping layer of the anlheridium contain but little or no chloro- phyll ; they separate from one another on the addition of water (like those of Hepaticae), to allow the escape of the antherozoids, which are- still enclosed in vesicles and number from 100 to 150. The hindermost and thickest of the two or three coils of the antherozoid, which is larger in this class than in all other Crypto- gams, bears an appendage on the inner side which Hofmeister terms an undulating Float, Schacht a thin-walled vesicle of protoplasm, and which contain granules of starch and sap (com- pare with Ferns and Lycopodiaceoe). The Ai'chegonia are developed from single cells of the anterior margin of the thick and fleshy lobes of the female prothallium. As the tissue of the prothallium be- neath them continues its growth, the archegonia come, as in Pellia, to stand on its upper surface. The mother-cell of the archegonia, after it has become much curved, divides by a wall parallel to the surface of the prothallium ; the lower of the two daughter-cells, which is entirely sunk in the tissue of the pro- thallium, becomes the central cell; from the outer one is formed the neck, con- sisting, at a subsequent period, of four parallel rows of cells. The four upper cells become very long ; the four middle ones remain shorter ; the four lower Pig. 269.— First stage of development of tlie prothallium of Eguiseium Telmateia ; tu the first root-hair; / rudiment of the prothallium. The order of development follows the numbers I—l'I {X about 200). 3^4 VASCULAR CRYPTOGAMS. ones scarcely elongate at all, and contribute, by their multiplication, like the cells of the prothallium which surround the central cell, to the formation of the wall of the ventral part of the archegonium, which consists of one or two layers. The oosphere is produced in the central cell, the contents of which it gradually displaces. The four upper long cells of the neck curve radially outwards, when the canal of the neck is being formed, like a four-armed anchor \ Immediately after fertilisation the canal of the neck closes, the oosphere, the nucleus of which disappears (and Fig. 270. — A male prothallium of EqiUsetian arvense with the first antheridia a (after Hofmeister, x 200) ; B — E anthero- Eoids of E. Telmateia (after Schacht). Fig. 271.— Lobe of a highly developed female prothallium of Equisetttm arvense cut through vertically (after Hofmeister, (X about 60) ; a « a two abortive and one fertilised archegonium, h root-hairs. which has now become the oospore), enlarges, and the cells of the wall of the ventral part of the archegonium which surrounds them begin rapidly to multiply. Development of the Asexual Generation of Equisetum The formation of the embryo from the oospore is the result of divisions, the first of which is inclined to the axis of the archegonium, and is followed, according to Hofmeister, in each of the two cells by a division-wall placed perpendicularly to the first. The embryo appears ^ Recent investigations are wanting from the point of view taken in Ferns and Rhizocarps. From analogy, however, the existence of a ' canal-cell ' may be inferred here also. EQUTSETACEM. 3^5 to be composed of four cells arranged like die quarters of a sphere. The same author states that the foot, which he terms in this case also the primary axis, arises from the lower quarter, the rudiment of the first shoot from one of the lateral ones, turning upwards immediately afterwards and producing as the rudiment of the first leaf a projecting girdle, w^hich then grows out into three teeth (Fig. 272 B). The first root now {?) arises from an wner cell of the tissue. It may here be remarked that this observadon of Hofmeister's would establish, on the one hand, an essential difference between the mode of formation of the first root in Equi- setaceae and in other Vascular Cryptogams ; while, on the other hand, the origin of the first leafy axis from one of the quarters of the embryo corresponds to the behaviour of Ferns and Rhizocarps, and hence does not agree with the other pro- cesses of growth of the Equisetaceae, since in all of them the other shoots are developed from inner cells of the tissue. Duval-Jouve maintains, in fact, in oppo- sition to this view, that the first leaf- bearing axis has a lateral origin in the interior of the already muUicellular em- bryo, so that even the first shoot of Equisetum would be of endogenous origin. The unaccountable errors of this writer on the subject of apical growth render his statements of but little value in contrast to those of Hofmeister ; the question is however, in any case, deserving of further in- quiry. The first leaf-bearing shoot grows upwards, and forms from ten to fifteen internodes with sheathing leaves end- ing in three teeth. It soon produces at its base a new stronger shoot with four-toothed sheaths (as in I^. arvense, praiense, and variegatum^ according to Hofmeister), which in turn gives origin to new generations of shoots, developing constantly thicker stems and sheaths with a larger number of teeth. Somedmes the third or one of the succeeding shoots penetrates downwards into the ground, forming the first perennial rhizome, which again produces from year to year new underground rhizomes and ascending leafy shoots. In order to facilitate the understanding of the Mode of Growth of the Stem and Leaves, it is necessary to glance in the first place at their structure in the mature state. Every axis of an Equisetum consists of a series of joints (internodes) usually hollow and closed at their base by a thin septum. Each internode passes upwards into a leaf-sheath embracing the next internode, the sheath being split at its upper margin into three, four, or usually a larger number of teeth. From each tooth of the sheath a fibro-vascular bundle runs vertically downwards into the internode as far as the next node, parallel wdth the other bundles of the same internode ; at the lower Fig. 272. — Development of the embryo oi Equzsetum arvense (after Hofmeister); A archegonium cut through vertically with the embiyoy (X 200); H embryo further developed and separated, b rudiment of a leaf, s apex of the first shoot (X 200) ; C vertical section of a plate of a prothalliuni //, with a young (asexual) plant, w its first root, bb' its leaf-sheaths (X 10). '^f^ VA SC ULA R CR YP TOGA MS. end each bundle splits into two short diverging arms, by which it unites with the two neighbouring bundles of the next lower internode, where they descend into it from their sheath-teeth. The joints of the stem and their leaf-sheaths therefore alternate; and since in each joint the arrangement of bundles, leaf-teeth, projecting longitudinal ridges, and depressions or furrows, is exactly repeated in the transverse section, the different parts of a joint always correspond to the intervals between the homologous parts of the next upper and next lower joint. If the internode has projecting longitudinal ridges on its surface, one of these always runs down- wards from the apex of each leaf-tooth parallel with the others as far as the base of the internode ; between each pair of leaf-teeth commences a furrow or channel, which also continues as far as the base of the internode. The projecting Fig. itz a.—Eqnisetnm Tebnateia; A piece of an upright stem (natural size), i t' intemodes, k its central cavity, I lacunre of the cortex. 5 leaf-sheath, :: its apex, a a' a" the lower intemodes of young leaves ; B longitudinal section of a rhizome (X about 2), k septum between the cavities h A, .4' fibro-vascular bundle, / lacunae of the cortex, S leaf- sheath ; C transverse section of a rhizome (x .about 2), i> and / as before ; D union of the fibro-vascular bundle of an upper and lower internode z i\ A' the node. ridges lie on the same radii as the fibro-vascular bundles, each of which contains an air-canal; the depressions or furrows lie on the same radii as the lacunae of the cortical tissue (which are sometimes wanting), and alternate with the fibro- vascular bundles. The branches and roots spring exclusively from within the base of the leaf-sheath; and as this forms a whorl, the branches and roots are also verticillate. The branches are all of endogenous origin ; they arise in the interior of the basal tissue of the leaf-sheath, upon radii of the stem which alter- nate with the fibro-vascular bundles, and thus also with the teeth of the sheath. A root may arise beneath the bud of each branch ; both break through the leaf- sheath at its base. All the joints of the axis agree in these respects, however they may be modified as underground rhizomes, tubers, ascending stems, leafy branches, or sporangiferous axes. EQUISETACE^. 36: The end of the stem enveloped by a large number of younger leaf-sheaths terminates in a large apical cell, the upper wall of which is arched in a spherical manner, while below and at the side it is bounded by three almost plane walls. The apical cell has therefore the form of an inverted triangular pyramid, the upturned basal surface of which is a nearly equilateral spherical triangle. The segments are cut off by walls which are parallel to the oblique sides of the apical cell, that is, to the youngest primary walls of the segment ; the segments, disposed FIG. 273.—^ ongitudinal section of the end of a stem in an under^ound bud of Equisetum Tehnatein ; S apical cell, xy first indication of the girdle from which the leaves are subsequently formed, b b s. more advanced and distinctly marked foliar girdle, bs the apical cells of a strongly projecting foliar girdle, rr rudiment of the cortical tissue of the internodes, gg rows of cells from which the leaf-tissue and its fibro-vascular bundle proceed, zzthe lower layers of cells of the segment which take no part in the formation of leaves (from nature) ; B horizontal projection of the apical view of the end of a stem of E. Telmateia ; s apical cell, /— Kthe successive segments, the older ones still further divided; C horizontal projection of the apical view of E. a-rvetise; D optical longitudmal section of the end of a very slender stem ; E transverse section of the end of a stem after the formation of the vertical and first tangential walls. (C, D, E, after Cramer; the Roman numerals indicate the segments, the Arabic numerals the walls formed in them in the order of their succession ; the letters the primary walls of the segments.) in a spiral \ arrangement, lie in three vertical rows. Each segment has the form of a triangular plate with triangular upper and under walls, rectangular lateral walls lying right and left, and an outer rectangular wall which is curved. Each segment is first divided— as was shown by Cramer and Rees and confirmed by myself— by a wall parallel to the upper and under surfaces into two equal plates lying one above another, and consequently each half the height of the undivided segment. Each 368 VASCULAR CRYPTOGAMS. half-segment is then again halved, in the most usual case, by a vertical nearly radial wall. The segment now consists of four cells, two of which lie one above the other and reach as far as the centre, but the other two do not because the vertical wall is not radial but intersects one of the lateral walls of the segment (the anodal w-all) (Fig. 273, E). Divisions now take place without any strict rule in the four cells of each segment parallel to the primary and the lateral walls ; and tangential divisions also soon make their appearance, by which the segment is split up into inner and outer cells, in which further divisions afterwards take place. The former produce the pith, which is soon destroyed as far as the septum at the base of each internode by the expansion of the stem; the latter produce the leaves and the entire tissue of the hollow internodes. The segments are, as has been men- tioned, disposed originally in a spiral with i arrangement ^ and since each segment without excep- tion (as in Mosses) produces a leaf or what corresponds to a part of a leaf-sheath, the leaves of Equise- tum must also be inserted on a spiral. This does, in fact, some- times occur when the growth is abnormal ; but when the growth is normal, a small displacement takes place at a very early period, of such a nature that the three seg- ments which form a cycle always become arranged into a disc trans- verse to the stem, their outer sur- faces thus forming an annular zone or girdle. According to Rees, to whom this observation is due, the three segments of each cycle are formed in rapid succession, while a longer time elapses between the formation of the last segment of the preceding and that of the first of the succeeding cycle. Thus by the un- equal growth of the segments in longitudinal direction each cycle of segments or turn of the spiral produces a whorl, which therefore, strictly speaking, is a pseudo- whorl, because resulting from subsequent displacement. Each whorl of segments now forms a leaf-sheath, and the corresponding internode or joint of the stem. The above-mentioned divisions take place in the three segments during their arrange- ment into a transverse disc, each segment becoming converted into a mass of cells Fig. 274.— Left half of a radial longitudinal section beneath the apex of an underground bud of Hqm'setitm Tehnateia in September : pK lower part of the vegetative cone, // b" b'" leaves, bs their apical cells, r" r'' r'" the cortical tissue of the corresponding internodes ; vi m pith, vvv thickening ring, g g layer of cells from which the fibro- vascular bundles of the leaf-tooth arises. EQUISETACE.E. 3^9 consisting- of from four to six layers. As soon as the transverse zone is formed, the formation of the leaves commences by the growth of the outer cells of the segments. They form an annular wall ; one of the upper transverse cell-layers of the whorl of segments projects outwardly, forms the apex (the circular apical line) of the wall (Figs. 273, 274, ds), and those of its cells which lie most on the outside (the apical cells) divide by walls inclined alternately towards and from the axis. The circular apical line becomes more and more elevated, and thus the annular wall becomes a sheath enveloping the end of the stem. This same layer, of which the outermost cells form the apical line of the annular wall, produces in the interior of the sheath a tissue in which the fibro-vascular bundles of the leaf-sheaths arise. The lower transverse cell-layers of the whorl of segments grow only slightly outwards Fig. 276. — External view of three teeth of a young leaf-sheatli of Eq%usetnni Telmateia. Fig. 275. — The same as Fig. 274, but at a greater distance from the apex, showing a further advance of the differentiation of leaf- sheath and internode ; r r cortex of the upper, r' r' r' cortex of the lower internode, e e the inner, e' e' the outer epidermis of the leaf-sheath, ^^ the foliar portion of the fibro-vascular bundle, g" g" g^ '^ts descending portion belonging to the internode ; the first annular vessel is formed at their point of meeting. and upwards, become divided by vertical and afterwards rapidly by transverse walls, to produce the tissue of the internode, which passes gradually into that of the leaf. A vertical layer of this tissue forming a hollow cylinder (Fig. 274, v v) is distinguished by numerous vertical divisions ; it forms a ring of meristem (or thickening-ring in Sanio's sense), in which the vertically descending fibro-vascular bundles of the internode are formed. These bundles form the prolongations of those of the leaf- teeth, which they meet, as shown in Fig. 275, g g\ at an obtuse angle, and coalesce to form curved ' common ' bundles. The layers of cells which lie outside this ring of meristem that gives rise to the bundles produce the cortex of the internode, in which air-conducting canals soon arise. Even at an early period B b 370 VASCULAR CRYPTOGAMS. tlie first rudiments of tlie slieath-teeth appear as protuberances at regularly dis- tributed points, each of them ending in one or two apical cells (Fig. 276) \ The Equisetaceae are the only class of plants the Branching of which depends exclusively on the formation of endogenous lateral buds. These are formed in the tissue of the youngest foliar girdle at points alternating with the sheath-teeth long before the differentiation of the fibro-vascular bundles. The position of the spot where they originate has not yet been precisely determined; it is probably a cell of that layer from which the fibro-vascular bundles originate. Hofmeister was the first to show that each bud proceeds from a single cell of the inner tissue ; and although I have myself never seen it in a unicellular condition, I have found rudimentary branches composed^ of only two or four cells ; and these showed that even the first three divisions of the mother- cell of the branch are inclined in three directions in such a manner that a triangularly pyramidal apical cell is produced; and the first three divisions thus form the first three segments. Lateral buds of the rhizome of E. Tel- mateia and E. arvense, late in the autumn or early in the spring, usually show in longitudinal section all the stages of development of endogenous buds. After they have formed several foliar girdles and their apex is covered by a firm envelope of leaf-sheaths, they break through the base of the parent leaf-sheaths. They may also remain dormant for a long period, as is shown by the circumstance that buds break out when the underground nodes of ascending stems are exposed to the light. It may be assumed that there is always as large a number of buds in a rudimentary condition as there are sheath-teeth. On the erect leafy stems of E. Telmateia, E. arvense, and other species, they all attain complete development, and produce the numerous slender green leafy shoots of these species ; in other species the development of the branches is more sparing; some, as E. hyemale, usually form no aerial lateral shoots at all except when the terminal bud of the stem is injured, and then the FIG. 277. — Longitudinal section through an underground bud of EquisetKin aruense ; ss apical cell of the stem, b-'ib the leaves; K K' two buds; the horizontal lines across the stem indicate the position of the septa (diaphragms). ' On the original number and subsequent increase of the sheath-teeth, &c,, compare Hofmeister and Rees, /. c. EQUISETACEM. 371 node next below produces a shoot. Branches do not usually make their appear- ance on rhizomes in the form of complete whorls, but in twos or threes; but on the other hand they are more vigorous and become either new rhizomes or ascendino- stems. Since in the cases first mentioned the buds arise like the leaves in strict acropetal succession, it may be assumed that where the production of shoots is only induced at a later period by accidental circumstances, the buds have up to that time remained dormant in the interior. The Rools arise in whorls, each immediately below a bud ; but they may also often be suppressed, and may be developed, according to Duval-Jouve, even on aerial nodes, by humidity and darkness. Their development has been studied by Nageli and Leitgeb (/. c.) ; in its earliest stages, which are represented dia- grammatically in Fig. 278, it resembles essentially that of Ferns. The cortex is differentiated into an inner and an outer layer; the former forms air-conducting ^ .? s / r c Fig. 278.— Diagram of the succession of cell-divisions in the apex of the root of F.qtiisetnm hyevtale (after Nageli and Leitgeb) (this diajfrani will serve also in the main for Ferns and for Marsilea). A longitudinal section ; B transverse section at the lower end of ^; hhh the primary walls, sss the walls of the sextant-segments, indicated in ^ by the figs. /—-l'/-'/, kl7nnpW\& layers of the root-cap, all the further divisions being omitted \ c c \x\ the interior of the root indicates the cambium-walls by which the primary fibro-vascular bundle is divided from the cortex of the root, e the boundary-wall between the epidermis o and the cortex (epidermal wall), r r boundary-wall between the outer and inner cortex (cortical wall), i, 2, 3, the successive tangential walls by which the inner cortex is divided into several layers, the radial divisions being omitted. intercellular spaces, at first arranged, like the cells themselves, in radial and concentric rows, and afterwards combining by the rupture of the cells into a large air-cavity surrounding the central fibro-vascular bundle. As the fibro-vascular bundle of the root developes, (seen in transverse section,) each of the three primary cells which alone of the six reach the centre is first of all divided by a tangential wall, so that the rudiment of the vascular bundle now consists of three inner and six outer cells. The six outer cells produce a cambial tissue in which the formation of vessels begins, commencing from two or three points of the circumference and advancing towards the interior. Last of all one of the three inner cells forms a broad central vessel ; and phloem is produced in the circumference of the vascular bundle. The branching of the root is, as in Ferns, strictly monopodial or acropetal ; but since there is here no ' pericambium,' the lateral roots arise in contact with the outer vessels. B b 2 372 VASCULAR CRFPTOGAMS. The Sporangia of Equisetaceae are outgrowths of peculiar!}' metamorphosed leaves, and are generally formed in numerous whorls at the summit of ordinary shoots or of those specially destined for this purpose. Above the last sterile leaf- sheath of the fertile axis an imperfectly developed leaf-sheath is first of all produced (Fig. 279, a), a structure corresponding in some degree to the bracts of Phanerogams. The development of this structure is sometimes more sometimes less leaf-like ; foliar girdles are formed above it in acropetal succession beneath the growing end of the shoot, projecting however but slightly, as in the ordinary formation of leaves of Equisetum. A larger number of protuber- ances project from each of these girdles, corresponding to the teeth of the ordinary leaf-sheaths ; and thus several whorls of hemispherical projecdons are formed lying closely one over another, which, increasing more rapidly in size at their outer part, press against one another, and thus become hexagonal, the successive whorls alternat- ing; while the basal (inner) portion of each protuberance remains slender, and forms the pedicel of the hexagonal peltate scale. The outer surface of these scales is tangential to the axis of the spike ; on its inner side, facing the axis, arise the sporangia, five or ten in number on each scale. In the early stages of development each single sporangium has the appearance of a small blunt multicellular wart, the internal tissue of which ^ produces the spore-mother-cells which become isolated, while of the three exterior cell-layers which at first envelope it only the outermost finally remains as the wall of the sporangium or spore-sac. The mother-cells of the spores, connected together in groups of fours or eights, float freely in a fluid which fills the sporangium and is interspersed with granules. The processes that take place in the mother- cells up to the time of the formation of the spores have already been described in detail in Chap. I (see Fig. 10, p. 14). It was there shown how the division into four of the mother-cells is preceded by a bipartition which is at least indicated in Equi- setum, in a manner analogous to the corresponding process in Ferns. The ripe sporangium opens by a longitudinal slit on the side which faces the pedicel of the Fig. 279. — EgJtisetum Tehnateia ; A upper part of a fertile stem with the lower half of the spike (natural size), b leaf-sheath, a the annular 'bract,' x the pedi- cels of peltate scales which have been cut off, y transverse section of the rachis of the spike ; B peltate scales in various positions (slightly magni- fied) ; st the pedicel, s the peltate scale, s^ the spo- rangia. ' The formation of the spore-mother-cells from a single original central cell which occurs in Ferns and Rhizocarps, has been contrasted by Russow with that of Equisetum (compare p. 358). EQUrSETACEM. 373 peltate scale. The very thin-walled cells of the wall have previously formed spiral thickening-ridges on the dorsal, annular ones on the ventral side of the sporangium, arising, according to Duval-Jouve, in the case of J^. limosinn, with extraordinary rapidity immediately before the dehiscence. The development of the spores of Equisetum, after they have made their appearance as naked primordial cells by the division into four of their mother-cells, shows the pecuharity of a successive formation of distinct coats. Each spore forms first of all an outer non-cuticularised coat capable of swelling, which, splitting subsequently into two spiral bands, forms the so-called Elaters, a second and third coat soon afterwards making their appear- ance within it. All three lie at first closely one upon another like successive layers of a single coat; but when the spore is placed in water, the outer one, even at this period, swells up strongly and becomes detached from the others (Fig. 280, B). The three coats may be easily distinguished even in the quite fresh spore when placed in distilled water (/!), (in the case of E. limosum), the outer one (i) being colourless, the second (2) light blue, and the third (3) yellowish. As riG. 280.— Development of the spores of l-.qnisetum limosuin (x8oo) ; A unripe spore with three coats just placed in water ; B the same after two or three minutes in water, the outer coat having become separated, a large vacuole is seen by the side of the nucleus ; C conunencement of the formation of the elaters on the outer coat c (= i in Figs. A and B) ; D, H the same stage of development in optical section after lying twelve hours in glycerine, e the outer coat ; 2, 3, the inner coats separated from one another ; F the outer coat split into spiral elaters, coloured a beautiful blue by Schultz's solution. the development advances, the outer coat is separated like a loose investment from the body of the spore (C, d, e), and at the same time its division into elaters is first indicated. The optical longitudinal section shows that the spiral thickening- bands of this coat are separated only by very narrow spaces of thin membrane [D, E) ; these at length entirely disappear, and, when the surrounding air is dry, the thicker parts separate from one another as spiral bands, forming when unrolled a four-armed cross; they are united by their centre, and attached there to the second coat. It is probably this spot which may be recognized even in the unripe spore 'in the form of an umbilical thickening {11 in A and E). In the fully developed elaters an external very thin cuticularised layer may be distin- guished. They are extremely hygroscopic ; when the air is damp they are rolled round the spore, but when dry are again unrolled. When this alternation takes place rapidly (as when lightly breathed on under the microscope), the spores are set in active motion by the bendings of the elaters. If spores, the outer coat of which 3 74 VASCULAR CRYPTOGAMS. has not yet become split up into elaters, but \vhich already show the corresponding differentiations {D, E), are allowed to lie for some time in glycerine, the spore contracts considerably, surrounded by its inner coat, while the second cuticu- larised coat raises itself from the former in folds. The inner coat is differentiated into an outer granular cuticularised exospore, and an inner endospore of cellulose. Very little need be said about the Classification of Equisetaceae, as all existing forms are so nearly related to one another that they may be included in a single genus, Equi- setum. Even the Equisetaceae of earlier geological periods, the Calamites, show, in the little that is still discernible of their organisation, the closest agreement with existing forms. The Habit of the Equisetaceae is, like their morphological structure, of a very characteristic kind. In all the plant is perennial by means of creeping underground rhizomes, from which ascending aerial shoots rise annually, mostly lasting only for one period of vegetation, less often for several years. The sporangiferous spikes appear either at the summit of these axes, which are at the same time the organs of assimi- lation, or on special fertile shoots which, when destitute of chlorophyll and unbranched, die after the dissemination of the spores {E. ar-vense and Telmateia'), or throw off the terminal spike and act as vegetative shoots {E. syhmticum and pratense). The fertile axes are developed from the underground internodes of the erect vegetative axes; they remain during the summer, in which the latter are unfolded, in the bud- condition beneath the ground, but during this period either develope their sporangi- ferous spikes so far that in the next spring nothing is necessary except elongation and the dissemination of the spores {E. ar'vense, pratense, Telmateia, Sec), or the spikes attain their full development only in the spring after the elongation of the axes which bear them {E. limosum). The habit of the aerial shoots is determined especially by the number and length of the verticillate usually very slender lateral branches; in some, as £. hyemale, trachyodon, ramosissimum, and imriegatum, they are generally entirely \vanting ; in others, as E. palustre and limosum, rather few, in others again, as E. ar~ I'ense, Telmateia, and syl'vaticum, they are developed in large numbers. The height of this leafy stem is in our native species mostly from i to 3 feet ; in E. Telmateia^ where the ascending axis of the sterile shoots is colourless and destitute of chloro- phyll, it attains a height of 4 or 5 feet and a thickness of about | inch ; while the green slender leafy branches are even in this case scarcely | line thick. The tallest stems are produced by E. giganteum in South America, as much as 26 feet high, but only about the thickness of the thumb, and are kept in an upright position by neighbouring plants. The Galamites were as lofty, and as much as i foot thick. The rhizomes mostly creep at a depth of from 2 to 4 feet beneath the ground, and extend over areas 10 to 50 feet in diameter ; but are also found at a much greater depth. They prefer damp, gravelly, or loamy soil, their thickness varying from i to 2 lines to as much as | inch or more. The surface of the internodes of the rhizome is, in some species, as E. Telmateia and syl'vaticum, covered with a felt of brown root-hairs, which also clothe the leaf-sheaths of the underground part of ascending stems, a peculiarity which reminds one of Ferns. In some species, as E. limosum and palustre, the surface is smooth and shining, while in others it is dull. The ridges and furrows of the aerial stems are usually but little deve- loped on the underground stem.s ; sometimes the rhizomes are twisted. The central canal of the internodes is sometimes wanting in the rhizomes ; but the lacunae of the fibro-vascular bundles (carinal canals) and those of the cortical parenchyma (vallecular canals) are always present ; the air which the tissues require and which is not found in the usually very compact soil is carried by these canals from the surface to the under- ground organs. As in the case of the spikes, the formation of the branches of the leafy stems has already commenced entirely or at least for the greater part in the preceding year in the underground bud, so that in the spring the internodes of the EQUISETACE.E. 375 ascending axis have only to extend and the slender lateral branches to unfold, as may be seen with especial ease in E. Tdmateia. All the more important cell-formations and the processes of morphological differentiation thus take place underground ; the aerial un- folding has for its main purpose only the dispersion of the spores and assimilation in the leafy shoots, by the exposure of the cortex, which contains chlorophyll, to light. The rapid growth of the upright stems in the spring is brought about especially by the simple elongation of the internodal cells already formed, although permanent intercalary growth of the internodes sometimes also takes place, and especially at their base within the sheaths. The tissues often remain there for a long time in the young state, and in E. hyemale the internodes, still short and lighter in colour after passing through the winter, push themselves out of their leaf-sheaths ; the shorter they were before the winter, the more they elongate afterwards. Special Organs for Fegetatiir Propagation, like those of Mosses, are not found in the Equisetaccce any more than in Ferns ; but every part of the rhizome, and the under- ground nodes of ascending stems, are adapted for the production of new stems. In some species some of the underground shoots swell up into ovoid (£. ar'vense) or pear- shaped {E. Telmateia) tubers about the size of a hazel-nut; Duval -Jouve states that these occur also in E. paliistre, syh'aticum, and littorale, but in other species {E. pratense, limosion, ramosissitnum, 'variegatu>7i, and hyemale) they have not yet been observed. The tubers are produced by the rapid increase in thickness of an internode at the end of which is situated the terminal bud ; this may repeatedly form tuberous internodes so that the tubers become moniliform, or they may dcvelope simply as a rhizome, or sometimes a central internode of a rhizome is developed in a tuberous manner. The parenchyma of these tubers is filled with starch and other food-materials ; they may apparently long remain dormant and form new stems under favourable circumstances. Among the Forms of Tissue of the Equisetaceae the epidermal system and the funda- mental tissue are in particular developed in a great variety of ways. The fibro-vascular bundles, which in Ferns are so thick and so highly organised, especially in their xylem- portion, appear to be less developed in the Equisetacea? ; they are slender, the lignification of the xylcm-portion very slight (as in many water and marsh plants) ; the firmness of their structure is chiefly due to the epidermal system with its highly developed epidermis, and to the hypodermal fibro-vascular bundles. What follows has special reference to the internodes ; the leaf-sheaths are usually similarly constituted in their lower and central parts ; at the teeth the tissue is simpler and more uniform. The Epidermal Cells are mostly elongated in the direction of the axis, and are arranged in longitudinal rows separated by transverse or slightly oblique walls; the boundary-walls of the adjoining cells are often undulating. The epidermis of the underground internodes is almost always destitute of stomata, and consists of cells with either thick or thin M-alls, usually brown, which, in some species, as E. Telmateia and arirnse, develope into delicate root-hairs. The epidermis of the deciduous sporangi- ferous stems of the species just named is similar to that of the rhizome and without stomata; and the same is the case with the upright colourless sterile stem of E. Tel- mateia. In all the aerial internodes which contain chlorophyll, the leaf-sheaths, and the outer surface of the peltate scales, the epidermis possesses numerous stomata which always lie in the channels, never on the ridges, and are arranged in longitudinal rows either single or lying close to one another. On the ridges the epidermal cells are long, in the channels between the stomata shorter. All the cells, even those of the stomata, have their outer walls strongly silicified, and exhibit very often on their outer surface protuberances of various forms, which are also and indeed peculiarly strongly silicified. These protuberances resemble fine granules, bosses, rosettes, rings, transverse bands, teeth, and spines; on the guard-cells they usually occur in the form of ridges, running at right angles to the orifice. The guard-cells are generally partially overreached by the neighbouring epidermal cells. The mature stoma appears to be formed of two pairs of guard-cells lying one over another; Strasburger asserts that these four cells 3/6 VASCULAR CRYPTOGAMS. arise from one epidermal cell, and lie at first side by side at the same level. Only at a later period the two inner ones (the true guard-cells), become pressed inwards and overreached by the tv^^o outer ones which grow more rapidly. Bundles or layers of firm thick-walled cells (Hypodermal Tissue) are of common occurrence beneath the epi- dermis of rhizomes, of upright stems, and of their leafy shoots (with the exception of the deciduous sporangiferous stems). In the rhizomes they form a continuous stratum of brown-walled sclerenchyma consisting of several layers ; in the aerial inter- nodes they are colourless and are developed with especial prominence in the projecting ridges. The Fundamental Thsue of the internodes consists in the main of a colourless thin- walled parenchyma occurring only in the rhizomes, the deciduous sporangiferous stems, and the colourless sterile axes of JE. Telmateia. The green colouring of the other shoots is caused by layers consisting of from i to 3 strata of parenchyma containing chlorophyll (the cells lying transversely). This green tissue lies especially beneath the furrows, cor- responding to the stomata, and forms on a transverse section ribbon-shaped masses concave outwardly ; in the slender leafy branches, where the ridges sometimes cause the transverse section to have a stellate outline {e.g. E. arvense') the tissue containing chlorophyll is in excess. The vallecular canals, which correspond to the furrows, arise in the fundamental tissue by separation and partially by rupture of the cells ; they may be absent from the slender leafy branches. The Fibro-'vascular Bundles are arranged, in a transverse section of the internodes, as in Dicotyledons, in a circle, each corresponding to a ridge of the surface, between the cortical canals but somewhat nearer the centre. In the axis of the sporangiferous stems, where the diaphragms are wanting, they run in the same manner, and bend out singly into the pedicels of the peltate scales (as in the sheath-teeth). The bundles of a shoot are all parallel to one another ; each bundle is the result of the coalescence of two portions ; one of these belongs to the leaf-sheath and developes in the median line of one of its teeth from below upwards ; the other portion developes in the internode itself from above downwards. At the angle where the two portions meet, the form- ation of tissue begins in both, and thence advances in opposite directions; the lower end of each bundle unites by two lateral commissures with the two next alternate bundles of the next lower internode ; and the Equisetaceae have therefore only ' common ' bundles. In transverse sections these bundles resemble the fibro-vascular ones of Mono- cotyledons, especially of Grasses ; the first-formed annular, spiral, or reticulated vessels belonging to the inner side, together with the thin-walled cells which separate them, are subsequently destroyed, and a canal (carinal) remains in their place traversing the whole length of the fibro-vascular bundle on its inner side. Right and left of this lie on the outside a few not very broad vessels thickened reticulately ; external to the canal lies the phloem-part of the bundle, formed of a few wide sieve-tubes and narrow cambiform cells, and at the circumference of a few thick-walled narrow bast-like cells. These are all enveloped by a prosenchymatous tissue. A vascular-bundle-sheath, as it is termed, either surrounds each bundle or (varying with the species) runs continuously outside the circle of all the bundles ; sometimes (as in E. hyemale, &c.), a similar layer of tissue is found on the inner side of the circle of bundles as well. [Professor W. C. Williamson is led by a study of the internal organisation of Calamites and Calamodendra ^ to the conclusion that in England at least we have but one group of these fossil plants. When young their vascular zone, separating a medullary from a cortical parenchyma, ^ [On Fossil Equisetaceae, see Williamson, Mem. Lit, Phil. Soc. Manch. 3rd ser, vol. IV, pp. 155-183 ; Ditto, Trans. Roy. Soc. vol. CLXI, pp. 477-510. — Coemans, Joum. Bot. 1869, pp. 337- 340. — Dawson, Ann. Nat. Hist. 4th ser. vol. IV, pp. 272-273. — Grand'Eury, Ann. Nat. Hist. 4th ser. vol. IV, pp. 124-128; Compt. Rend. vol. LXVIII. — M^Nab, Joum. of Bot. 1873, pp. 72-80. — Ed.] EQUISETACEM. 377 was scarcely more than a thin ring of longitudinal canals, each of which had a few vessels at its outer border In this state the structure of the plant presented a close resemblance to that of a recent Equisetum. But as the plant grew in size, new vessels were added to the exterior of the pre- existing bundles, so that each of the latter became the starting-point of a woody wedge which con- tinued to grow peripherally until it assumed large dimensions. In some specimens these wedges measure fully two inches between the canal marking their medullary angle and their peripheral or cortical base. Each wedge is composed of vertical radiating laminae of barred or reticulated vessels separated by cellular rays. The medullary portion became fistular, as in the recent Equisetaccee at an early age, and the fistular cavities becoming filled with sand or mud, the very thin layer of medullary cells which remained did not prevent the sand from moulding itself against the inner angles of the vertical woody wedges, and thus produced the longitudinal grooves so characteristic of the casts commonly seen in collections. In such specimens most of the vegetable elements dis- appeared during fossilisation, and what remained in the shape of a thin film of coal moulded itself upon the medullary cast, and gave to the specimens the appearance of having had corresponding grooves upon their outer bark surfaces. No single example of a specimen of which the internal organisation is preserved — and we now possess these in great numbers— sustains this conclusion. Wherever the true bark is preserved it exhibits an outline indicating a smooth surface. The longitudinal woody wedges of each internode alternated their arrangement at each node ; the wedges of one internode becoming vertically superposed on the larger cellular masses separating the wedges in the nodes above and below. At each node an irregular verticil of vascular bundles left the vascular zone to supply some peripheral organs, probably branchlets ; but besides these diverticula, in one large group an exceedingly regular verticil of canals, with circular or oblong sections, proceeded from the central fistular cavity through the woody zone to the bark. One of these canals occupied the uppermost end of each of the large cellular rays which separated the vascular wedges of each internode. In the common fossils these canals are indicated by a very regular verticil of small round or oblong impressions, which some writers have erroneously asso- ciated with roots, and others with vascular bundles going to leaves or branches. But they never contained any vascular tissues whatever. Of the leaves of Calamites we have no knowledge, although some have identified them with those of Asterophyllites and Sphenophyllum. Professor Williamson has only obtained one example of a fruit which he can with confidence identify with Calamites {Volhnannia Daxvsoni, Williamson, in Mem. Lit. Phil. Soc. Manch. 3rdser. V. p. 28). It is a strobilus the structure of the axis of which corresponded most closely with that of a young Calamitean shoot. At each node it has a curiously perforated disk fringed with numerous peripheral bracts. From each disk there projects vertically upwards a ring of slender sporangio- phores, around each of which were clustered three or four sporangia full of spores. These sporangia are so compactly compressed that a transverse section of this fruit presents the appearance of a compact mass of spores, amongst which the outlines of the sporangia are traceable with difficulty. Whilst he has failed to find any tnie stem in which the surface of the bark was fluted, that of the intemodes of this fruit was undoubtedly so. He has not obtained more than half-a-dozen examples of Calamite-stems in which the outer bark is preserved. Nearly all the specimens are absolutely decorticated. Hence we cannot speak with certainty as to what may have been the condition of the surface of the bark in many of these plants. The flutings of the fruit-bark do not, like those seen in the carbonaceous film covering the common casts, correspond in number and position with those caused by the woody wedges, since two vascular bundles are located in each projecting ridge of the axis of the former structure, instead of one as in the latter. Mr. Carruthers believes the fruits figured by Mr. Binney, Professor Schimper, and himself, under the several names of Calaiiiodendron commune, Calamostachys Binneyana, and Volhnannia Binneyi (Joum. of Bot. 1867, pp. 349-356), to belong to Calamites; and he further regards the spores as having been furnished with elaters similar to those of Equisetum. Professor Williamson is unable to agree with either of these conclusions. He considers the supposed elaters to be merely fragments of the torn mother-cells of the spores, and that the affinities of these fruits are with the Lycopodiacere rather than with the Equisetacece. — Ed,] 37^ VASCULAR CRYPTOGAMS. CLASS VIII. OPHIOGLOSSACE.^^ The Sexual Generation. The prothallium is at present known only in Ophio- glossum pedimculosum and BoUychium Lunaria. In both cases it is developed underground. It is destitute of chlorophyll, and forms a parenchymatous mass of tissue which, according to Mettenius, has at first, in the species first-named, the form of a small round tuber, out of which is subsequently developed a cylindrical vermiform shoot, which grows erect underground, is rarely and slightly branched, and elongates by means of a single apical cell. When the apex appears above Fig. iZi.—Botrychhim Lunaria; A longitudinal section of protliallium (Xso), ac an archegoniiim, an an anthcri- diuin, iv root-hairs; B longitudinal section of the lower part of a young plant dug up in September (X20) ; st stem, b b' b" leaves (after Hofmeister). ground and becomes green, it forms lobes and ceases to grow. The tissue of this prothallium is differentiated into an axial bundle of elongated, and a cortex of shorter parenchymatous cells, and the surface is clothed with root- hairs. With a transverse diameter of 4 to i^ lines, it attains a length of from 2 lines to 2 inches. The prothallium of Boirychiu??i Lunaria is, according to Hofmeister, an ovoid mass of firm cellular tissue, the greatest diameter of which does not exceed h line, and is often much less (Fig. 281, ^). It is light brown externally, yellowish white internally, and provided on all sides with sparse moderately long root-hairs. These prothallia are monoecious ; each one produces a number of antheridia and archegonia, which are distributed with tolerable uniformity over the ^ Mettenius, Filices horti botanici Lipsiensis. Leipzig 1856, p. 119. — Hofmeister, Abhandlungen der konigl, Sachs. Gesellsch. der Wissens. 1857, p. 657. — [On the Gennination, Development and Fructification of the Higher Cryptogams, Ray Soc. 1862, pp. 307-317.] — On the probably near rela- tionship between this class and Marattiaceee, see p. 361. OPHIOGLOSSA CEX. 379 ..hole Of its upper surface, .ith the exception, in 0. palunculoswn , of tne srnall •mary tuber ; In Botrych.um it is the lower side which ch.efly bears anther.d.a. The lntl.n,iia are cavities in the tissue of the prothallium covered externally by a few layers of cells, and in Ophioglossum only slightly project.ng beyond the surface. n this .venus the mother-cells of the antherozoids originate by repeated dwjs.ons ,L on: or two cells of the inner tissue (covered externally by one or two layers of Tens) ; they form a mass of tissue of roundish form. The antherozo.ds are liar n form to those of the Polypod.ace*, but larger; they escape through a ■ t\ ^th natiij-al size) ; -w roots, st stem, hs leaf-stalk, . . u „r ,l,e antheridium. The Archegonia are apparently narrow opening ur the cover of the ''"^f^^^^^^,,. Cryptogams. Mettenius developed in a similar manner to *-^^ "^^ "^^^ J ^ f ^f two ^ells, a superficial saw in Ophioglossum mstances m wh ch ^^J ^^ ^^^^^^ ,,, ,,„tral cell, cell and one lying below it; this latter, ne covering cells of the the former producing the necU ^^ 'f /^tTaTL p ouTdwhich are then central cell arranged in the form of ^ >■-; " \^^^ ,„„,i,,„g of two or transformed, by further divisions, mto four vertical row 380 VASCULAR CRYPTOGAMS. more cells, and thus form the neck. The wall of the ventral part which surrounds the central cell is formed by divisions of the cells of the prothallium which surround it; the ventral part is therefore completely imbedded, and only the neck, which is usually very short, projects above the surface. Mettenius asserts that in Ophio- glossum a prolongation of the oosphere (probably a canal-cell, as in Ferns and Rhizocarps), penetrates into the lower part of the neck. The Asexual Generation. The first divi- sions of the oospore are not known; but the mode of formation of the embryo differs, as may be concluded from the circumstances al- ready named, from that of Ferns. Mettenius states that in Ophioglossum pedunculosum the end of the embryo which faces the apex of the pro- thallium developes into the first leaf, while the opposite end produces the first root. Unlike what occurs in Ferns, the concave upper side of the first leaf faces the neck of the archego- nium ; the rudiment of the stem (which Met- tenius terms the 'primary rudiment of the em- bryo '), lies nevertheless on the side of the embryo which faces the base of the archego- nium. Hofmeister, on the other hand, makes the following statement with regard to Botry- chium : — ' The position of the embryo with re- spect to the prothallium differs widely from that which occurs in the Polypodiaceae and Rhizo- carpese ; Botrychium approaches in this respect those Vascular Cryptogams the prothallium of which, like that of Ophioglossaceae, is destitute of chlorophyll (Isoetes, Selaginella). The piinc- iinn vegetationis of the embryo lies near the apical point of the central cell of the archego- nium; the fi.rst roots arise beneath it, near the base of the archegonium ' (/. c. p. 308). The processes of growth of the mature plant have not yet been ascertained with as much cer- tainty as in other Vascular Cryptogams. In Ophioglossuni vulgatum and Botrychium Lunaria the erect stem, buried deep in the earth and growing very slowly in length, appears never to branch. Even the comparatively thick roots rarely branch, and it is not known whether the branching is then monopodial or dichotomous. The flattened apex of the stem, surrounded by the insertions of the leaves, is buried deeply in the leaf-sheaths, and shows, in Ophioglos- sum vulgatum, according to Hofmeister, a three-sided pyramidal apical cell as seen from above. The leaves have a sheathing base, and each is completely enclosed in Fig. 283. — Longitudinal section through the lower part of a mature plant of Botrychuem Lic- naria. st stem, ^^' fibro-vascular bundles, iv a young root, b apex of the stem, * b' h'' h"' the four leaves already formed, b'" the one unfolded during the present year : b' shows the first indi- cation of the branching of the leaf; in b'' this has advanced further ; m is the median line of the sterile lamina, having already its lobes right and left which are not shown ; y the fertile lamina with the young ramifications, on vvliich the sporangia will be produced (x about lo). OPHTOGLOSSACE.E. 3«' the next older one, as shown in Fig, 283 in the case of Botrychium Lunaria. lii Ophioglossum the relative positions of the parts at the end of the stem are still more complicated, from the fact that the rudimentary leaves, while completely enclosed one ^vithin another, produce ligular structures which grow together so completely that each leaf appears as if enclosed in a kind of chamber formed by the cohesion of the ligular parts of leaves of different ages, recalling a similar structure in Marattia. These cohesions however leave an opening at the apex of each chamber ; the apex of the stem is therefore ex- posed to the air through a narrow canal (Hofmeister). As soon as the leaves have attained a certain age, each leaf bears a collection of sporangia, which form a branch springing from the axial side of the leaf (in O. palmatum two or more such 'fertile segments' are formed). In the genus Ophioglossum both the outer sterile and the fertile branch of the leaf are unbranched or only lobed {0. palmatum) \ in the genus Botrychium both are again branched and in parallel planes (Fig. 282, A and B). The earlier hypothesis of a cohesion of the two leaf- stalks of a fertile and of -a sterile leaf is at once nega- tived by the history of development (Fig. 283), and would lead to very complicated theories of stem-branch- ing, of which we have no evidence. The history of de- velopment rather indicates, as Hofmeister first showed, that the receptacle originates on the inner side of the leaf. In the mature state the fertile leaf-branch eiUier separates from the sterile (green) one at the base or at the middle of the lamina {0. pendulum)^ or the two branches of the leaf appear as if separated deep down to their origin (6^. bergianum), or, finally, the fertile branch springs from the middle of the leaf-stalk {^Botrychium rutafolium and dissectinii). The Spora?igia of the Ophioglossaceae are so essen- tially different from those of Ferns and Rhizocarps that these plants cannot, for this reason, be arranged in either of these classes ; whether they differ as greatly from those of the Equisetacege and Lycopodiacese is yet to be proved by the history of their development. They agree with the sporangia of all Vascular Cryptogams in the one point of belonging to the leaves. The history of their development is not yet accurately known ; but from the half-ripe states which I have been able to examine in B. Ltmaria and 0. vulgatum, it is evident that the sporangia cannot be products of single epidermal cells, as in Ferns and Rhizocarps, but that their origin more resembles that of the pollen- sac of the anthers of many Angiosperms. Each sporangium is, in Botry- chium, an entire lobe of a leaf, the inner tissue of which produces the mother-cells of the spores. A longitudinal section through the unripe so-called spike of 0. vulgatum (Fig. 284) shows that the outer layer of the wall of the sporangium is a continuous Fig. 284. — Longitudinal section through the upper part of a spike of Ophioglossian vttlgaUan : s its free apex, sp the sporangia! cavities, r the part where they burst transversely; ^ the fibro- vascular bundles (x about lo). ■^(S2 VASCULAR CRYPTOGAMS. prolongation of the epidermis provided v/ith stomata and covering the whole of the fertile branch of the leaf. At the places where the lateral transverse line of de- hiscence subsequently appears in each sporangium, these epidermal cells are elon- gated radially, and the whole layer exhibits an indentation at first scarcely per- ceptible. The spherical cavities which contain the masses of spores are imbedded in the tissue of the organ, and are therefore entirely surrounded by its parenchyma ; this is found also in several layers on the outer side w^here the transverse fissure subsequently arises. The middle part of the parenchyma is penetrated by three fibro-vascular bundles which anastomose with one another into long meshes, and send out a bundle transversely between each pair of sporangial cavities. The course of development is the same in Botrychium, if the separate sporangiferous branches of the panicle are compared with the spike of Ophioglossum. The sporangia are similarly placed on them in two rows and alternate ; only they project further because the tissue between each pair of sporangia is but slightly developed. In specimens of both genera preserved in spirits the young spores still connected together in fours, are found imbedded in a colourless, granular, coagulated mass of jelly, which in the living plant clearly corresponds to the fluid in which the spores of other Vascular Cryptogams float before they are ripe. The spores are tetrahedral ; in Botrychium they are provided, even in a very early state, with knob- like projections on the cuticularised exospore. Among the Forms of tissue of the Ophioglossaceae, the prevailing one is parenchy- matous fundamental tissue. It consists, especially in the leaf-stalk, of long, almost cylindrical, thin-walled succulent cells with straight septa and large intercellular spaces ; in the lamina the latter are, in O. 'vulgatum, very large, and the tissue spongy. In O. 'vulgatum and B. Lmiaria, the epidermal tissue nowhere possesses special hypodermal layers ; a well-developed epidermis with numerous stomata on the upper and under side of the leaves immediately covers the outer layers of the fundamental tissue. The fibro- vascular bundles of 0. njulgatum form, according to Hofmeister, a hollow cylindrical network in the stem, on w hich the leaves are arranged spirally, with a § phyllotaxis ; each of the meshes of this network corresponds to a leaf, and gives off to it the foliar bundles from their superior angle. , The whole of the tissue which fills up the meshes of the network is frequently transformed into scalariform vessels, so that considerable lengths of the stem then form a closed hollow cyhnder; this sometimes occurs on one side only. The leaf-stalk is penetrated by from 5 to 8 slender fibro-vascular bundles, which, in transverse section, are arranged in a circle, and between which the fundamental tissue presents wide lacunae. Each of these bundles has on its axial side a strong fascicle of narrow reticulately thickened vessels, a broad fascicle of soft bast (phloem) lying on their peripheral side. In the sterile lamina the slender bundles branch copiously and anastomose into a network ; they run into the mesophyll wiiich contains chlorophyll, without forming projecting veins. The slender stem of B. Lw naria has the same structure as that of Ophioglossum ; its vascular bundles appear to be only the lower ends of the foliar bundles (Fig. 283). In each leaf-stalk, which has a conical hollow below obliterated above, arise two broad ligulate bundles, which split above, below where the leaf divides into the fertile and sterile laminae, into four narrower bundles. Each of these latter consists of a broad axial fascicle of tracheides thickened in a scalariform or reticulated manner, which is enveloped by a thick layer of phloem. This layer shows an inner stratum of narrow cambiform cells, while the outside is formed of soft thick-walled bast-like prosenchyma (as in Pteris and other Ferns). In the lobes of the sterile lamina the bundles split repeatedly dicho- tomously, and run through the mesophyll without forming projecting veins. RHIZOCARPE.E. j^. Ha/fit and Mode of Life. The number of leaves which appear each year is small and constant in the species; thus O. ini microsporangia, ma macrosporangia. ^.00 VASCULAR CRYPTOGAMS. their outer gelatinous envelope, which surrounds the exospore, swelling up. Their size being thus increased, they glide out, and escape into the surrounding water, where the germination of Marsilea sal-vatrix begins and completes its course with extraordinary rapidity. With a favourable summer temperature, antherozoids and archegonia ready for fertilisation are formed within 12 or 18 hours. Hanstein was the first to de- scribe these processes accurately ; I have myself repeatedly seen them in sporocarps, for which I am indebted to him^ To him also we owe the knowledge of a similar though in many respects different process in Pilularia glohuUfera. In this species the sporocarps lie on or beneath the ground ; they burst at the apex into four lobes, and exude a tough hyaline mucilage which escapes only w^hen the sporocarp is buried in the earth, forming a round drop which continues to increase in size for some days. In this drop of mucilage the macrospores and microspores rise to the surface and germinate, the drop of mucilage melting away only after fertilisation has been accomplished. The fertilised macrospores remain lying on the ground, and are temporarily fixed to it by the root-hairs of the prothallium, until the first true roots of the young plant penetrate into the ground. CLASS X. LYCOPODIACEyEl The Sexual Generatmi of Lycopodiaceae is, up to the present time, known only in the genera Isoetes and Selaginella; in these large female and small male spores are produced, as in the Rhizocarpese. In the genus Lycopodium only the early stages of germination are known, and that only in one species; Hke Tmesipteris and Psilotum, it possesses only one kind of spore, which corresponds externally to the microspores of the first-named general These differences would ' [See Hanstein in Ann. des Sci. Nat. 1863, vol. XX, pp. 149-166.— Ed.] 2 Hofmeister, Vergleichende Untersuchungen, 185 1. — [Germination, Development, and Fructifi- cation of the Higher Cryptogamia, Ray Soc. pp. 336-399.] — Mettenius, Filices horti bot., Lips. 1856. — Cramer, Ueber Lycopodium Selago in Niigeli and Cramer's Pffanzen-phys. Unters. Heft 3, 1855. — Hofmeister, Entwickelung der Isoetes lacustris in Abhandl. der konigl. Sachs Gesellsch. der Wis- sensch. vol. IV, 1855. — De Bary, Ueber die Keimung der Lycopodiaceen, in Berichte der naturf. Gesellsch. zu Freiburg-in-Br. 1858, Heft IV.— N:igeli u. Leitgeb, Ueber Entstehung u. Wachsthum der Wurzeln, in Nageh's Beitrage zur wissensch. Bot. Heft IV, 1867. — A. Braun, Ueber Isoetes in Monatsber. der Berl. Akad. 1863.— Milde, Filices Europe et Atlantidis, Leipzig 1867. — Mettenius, Ueber Phylloglossum, Bot. Zeitg. 1867. — Millardet, Le prothallium male des crypt ogames vascu- laires, Strassburg 1869.— Juranyi, Ueber Psilotum, Bot. Zeitg. 1871, p. 180. — Pfeffer, Entwickelung des Keims der Gattung Selaginella in Planstein's Botanische Abhandlungen, Heft IV, 187 1. 3 [J. Fankhauser (Bot. Zeitg. 1873, pp. 1-6) has described the hitherto unknown prothallium of Lycopodium, which is underground and destitute of chlorophyll. In September he found it more or less preserved and still attached to young plants less than three inches high, growing in moss in a damp wooded locality near Langenau in Emmenthal. He describes it as a yellowish white irregu- larly lobed structure, furnished sparingly with small root-hairs. The under side is comparatively LYCOPODIACEM. 40: be sufficient to divide the Lvcopodiacece into two classes, and to include the genera L}copodium, Psilotum, Tmesipteris, and the less known Phylloglossum in the highest class of higher Cryptogams, if the difference were an actually existing one ; but at present it rests only on an insufficient knowledge of these genera, which are otherwise closely allied to Selaginella in the mode of formation of their tissue, in the dichotomous branching of their stem and root, in the nature of their leaves, and in other characters. We must therefore, until our knowledge is more complete, consider all these genera as members of one class. The Microspores of Isoetes and Selaginella do not produce the mother-cells of the antherozoids immediately from their contents, as was formerly thought. To the treatise of Millardet mentioned in the foot-note we owe our knowledge of the fact, Fin. 301. — Germination of the microspores of Isoetes lacustrts (after Millardet). A and C microspores seen on the ri^ht side. A' and Don the ventral face ; A and /> show the fonnation of the antheridium, 65 its dorsal cells, ^^ its ventral cells, C and D the fonnation of the antherozoids, /3 and 5 have disappeared; 7/ is the vegetative cell (prothalliiini of Mil- lardet) ; and a—d x 5S0, c andy X 700). so important in connection with the relationship of the higher Cryptogams to the Gymnosperms, that at the period when the microspores are ripe, their contents are smooth, while the upper has numerous grooves and protuberances. In these grooves the antheridia and archegonia are situated. A vertical section through the prothallium shows that the cellular structure is formed of three regions ; the uppermost, in which the antheridia and archegonia are developed, consists of thin-walled cells poor in cell-contents ; the cells of the middle layer are rather smaller, and filled with dark granular contents rich in fatty matter ; those of the lowermost region are somewhat elongated parallel to the surface, and their contents are turbid and finely granular. Starch does not appear to be present in any part of the prothallium. The antheridia are filled with innumerable antherozoid-mother-cells ; the antherozoids are only slightly twisted and are stout compared with those of Selaginella. The archegonia were not observed, but the position they would occupy was indicated by that of the gei-minating plants, and it seems probable that they are not sunk completely in the tissue of the prothallium. In general only one embryo is produced from each prothallium, but it appears that a second may be produced from a second prothallium when the first is abortive. The reproduction of Lycopodium appears, therefore, to bear the greatest resem- blance to that of Ophioglossaceee. Berkeley remarks (Introd. to Crypt. Bot. p. 549) that Ophio- glossece ' are plainly connected with Clubmosses by Rhizoglossum, a Cape genus which has precisely the habit of Phylloglossum (Lycopodiacea?), consisting of a few subulate leaves and a pedunculate spike of sporangia.' — Ed.] Dd 402 VASCULAR CRYPTOGAMS. transformed into a mass of tissue consisting of but few cells. One of these cells remains sterile, and may be considered a rudimentary prothallium ; while from the others originate the mother-cells of the antherozoids, and these may therefore be looked on as a rudimentary antheridium. The microspore of Isoetes laaistris breaks up, after hibernation, into a very small sterile cell and a large one comprising the whole of the rest of the contents (Fig. 301 A — C). The former {v), cut off by a firm wall of cellulose, does not undergo any further considerable changes ; the latter, on the other hand, splits up into four primordial cells without cell-walls, of which the two ventral ones produce each two antherozoid-mother-cells, and therefore four in all. Pfefifer has confirmed the statements of Millardet that in Selaginella, long before the spores escape from the sporangium, a small sterile cell is first of all separated by a firm wall, while the other large cell breaks up into a number (6 to 8) of primordial cells (Fig. 303 A — D). He found, however, their arrangement different in Selaginella Mariensii and caulescens from that which Millardet described in the case of 6". Kraussi'ana, a variation which seems immaterial when compared with similar differences in the antheridium Fig. 202.— Isoetes lacustris (after Hofmeister) ; A macrospore, two weeks after its escape from the sporangiuin, rendered transparent by glycerine (X6o) ; R longitudinal section of the prothallium four weeks after the escape of the macrospore, a archegonium (X40). of Ferns. The essential difference between the results of the two observers con- sists in this : — that, according to Millardet, only two of the primordial cells produce the mother-cells of the antherozoids, which then, increasing in number, cause the absorption of the rest of the primordial cells, and fill up the spore; while Pfeffer found, in his species, that all the primordial cells underwent further division, and con- tributed to the formation of the antherozoids. As to the latter they were both in accordance. In Isoetes the antherozoids are long and slender, attenuated, and splitting up at both ends into a tuft of long slender cilia; in Selaginella they are shorter, thick behind, finely drawn out in front, and divided there into two long fine cilia. In the perfectly mature condition the antherozoids are rolled up into an elongated helix or into a short spiral. The mode of their formation in the mother- cells is the same in both genera, and agrees in essential points with that of Ferns. A cell-nucleus is not present at the time when the antherozoid is first formed ; the contents of the cell are perfectly homogeneous ; the antherozoid originates from a shining scarcely granular mass of protoplasm which encloses a vacuole, the cilia at one end being formed first, and the spiral body becoming differentiated from before backwards by a kind of splitting of the protoplasm. The antherozoid is LFCOPODIACE.E. 403 originally curved spirally round the central vacuole ; this latter, surrounded by a fine membrane, not unfrequenlly remains attached to the posterior end of the antherozoid after it has escaped, and is carried along by it. The movement does not last longer than five minutes in the antherozoids of Isoetes, in Selaginella from one-half to three-quarters of an hour. From the commencement of germination till the complete maturity of the antherozoids there is, in Isoetes, an interval of about three weeks; the same period from the dissemination of the spores is neces- sary in Selaginella. The JMacrosporcs produce the female prothallium, which is an endogenous struc- fV^:^ Fig. 303.— Germination of Selaginella (after Pfeffer) ; /—///, S. Martensii, A—D, S. caulescens; I longitudinal section of a macrospore filled with the prothalliinn and ' endosperm,* d the diaphragm, ee' two embryos in process of formation ; //a young archegoniuni not yet open ; /// an archegonium with the oospore fertilised and divided once ; A a microspore showing the primordial cells ; B C different views of these divisions ; D the mother-cells of the antherozoids in the perfect anther- idium. ture in a still higher degree even than is the case with Rhizocarps. In this respect and in the mode of its development, it shows a still greater resemblance to the tissue that fills up the embryo-sac of Gymnosperms, and even of Angiosperms. In Isoetes the cavity begins to be filled with cellular tissue a few weeks after the escape of the macrospores from the decaying macrosporangium ; the cells of this tissue are all at first still naked (without cell-wall) : they appear to become enclosed in firm cell- walls only when the whole cavity of the endospore is filled with them (Fig. 302). In the meantime the endospore thickens, becomes differentiated into layers, and D d 2 404 VASCULAR CRYPTOGAMS. assumes a finely granular appearance, phenomena which, as Hofmeister insists, are exhibited in like manner in the embryo-sacs of Coniferge. The spherical pro- thallium now swells up, the three convergent edges of the exospore burst length- wise and thus form a three-rayed fissure, where the prothallium is covered only by the membranous endospore ; this also peels off, and softens, finally exposing the corresponding part of the prothallium. At its apex appears the first arche- gonium ; if this is not fertilised, several others are subsequently formed at its side. In Selaginella, even when the macrospores are still lying in the sporangium, the apical region is found to be clothed with a small-celled meniscus-shaped tissue which is probably formed, during the ripening of the spores, by the division of an accumulation of protoplasm. This tissue afterwards produces the archegonia, and is therefore the true prothallium ; but a few w^eks after the dissemination free cell- formation begins beneath it in the spore-cavity, finally filling up the whole cavity, and forming a large-celled tissue, which Pfeffer, supported by considerations with which I also agree, compares to the endosperm of Angiosperms, and, following this analogy, calls by the same name. At the period of fertilisation and of the form- ation of the embryo, the macrospores of Selaginella contain, therefore, both a pro- thallium and an endosperm. The formation of the archegonia begins even before the rupture of the exospore, which occurs in this genus in the same manner as in Isoetes. The first archegonium originates at the apex of the prothallium ; the others arise, whether the first is fertilised or not, in centrifugal succession on the free parts of the prothallium. In both genera the archegonium originates by division of a superficial cell parallel to the surface ; the outer of the two new cells divides into four cells placed crosswise, each of which splits by an oblique division into two, one lying over the other ; in this way the neck is formed, consisting of four rows, each of two cells. The low^er of the first two cells is the central cell, the protoplasm of which separates into an upper smaller and a lower larger portion ; the former is the canal-cell, which penetrates between the two rows of the neck which separate from below, becomes converted into mucilage, and finally breaks through the neck, while the lower portion of protoplasm becomes rounded off and forms the naked oosphere (Fig. 303, //)• Finally, in Lycopodium iniindatiim ^ the germination of the spores has been observed by De Bary. The endospore stretches, and protrudes as a nearly spherical vesicle from the exospore which is split into three deep lobes ; it is divided by an upper partition-wall into a hemispherical basal cell which does not undergo any further changes, and an outer cell which continues to grow as the apical cell and forms two short rows of alternating segments by walls inclined alternately in two opposite directions. Each segment is broken up by a tangential wall into an inner and an outer cell, so that the prothallium consists finally of four short cells forming an axial row, surrounded by two rows of lateral cells, and by the basal and the apical cell. De Bary was unable to follow the further stages of development ; and it is therefore still impossible to form a judgment on the true nature of this structure. [See, however, supra, p. 400.] The Asexual Generation. The mode of formation of the embryo is, as has been said, known only in Isoetes and Selaginella. The first division of the oospore LVCOPODIACE^. 405 differs from that of Ferns and Rhizocarps, taking place perpendicularly to the axis of the archegonium. According to Hofmeister, each of the two cells first formed is divided in Isoetes in a plane at right angles to that of the first division, the relation of which to the first root, the first leaf, the stem, and the foot of the embryo, requires yet further elucidation. The formation of the embryo of Selaginella has recently been investigated in detail by Pfeffer. From an elongation of the upper half of the oospore is formed the Suspensor, a body which is wanting in all other Cryptogams, but universally present in Phanerogams, and through which Sela- ginella consequently approaches flowering plants. The suspensor seldom remains a simple cell; a smaller or larger number of divisions usually takes place in its lower part (Fig. 304, A-D). The embryo itself originates from the lower half of the oospore, which must itself be considered as the primary apical cell of the stem, and the suspensor as its first segment. By the elongation of the suspensor and tlie compression and absorption of the surrounding cells, the mother-cell of the embryo is forced into the endosperm, in which the embryo now undergoes Ji 5 I' M' Fig. 304.— Fonnation of the embryo of Stua^infUa .Vayte>isii {After Pfeffer) ; ^/, /? lower part of the suspensor with the first much-ilividci.1 scfjnients of the ciiibrj-o and the apical cell s of the future stem ; M the first leaves ; C apical view of the same ; D the apex seen from above in the act of forming two new apical cells, right and left ; /, //, /// the primary walls of the primary apical cell ; /'— K//' the longitudinal walls by which the two new apical cells are formed. further development, as in Phanerogams. In the mother-cell of the embryo two segments are in the meantime cut off by two oblique walls ; out of each pro- ceeds an embryo-leaf (cotyledon), and a longitudinal half of the hypocotyledonary segment of the stem ; the foot and root originating besides from the older segment. Between the two segments in front lies the two-edged apical cell of the stem (Fig. 304, A, B). While the two segments are becoming transformed by a number of cell-divisions into masses of cells, of which an inner mass very soon separates itself as the procambium of the axial bundle and a peripheral mass as dermatogen and periblem, a swelling is produced laterally beneath the first leaf, forming the foot ; by its increase the stem is forced over to the other side (that of the younger segment) ; so that the apex comes to lie horizontally, and afterwards is even directed -upwards (Fig. 303 /) ; and finally the bud, with its first leaves, the cotyledons, grows out 'upright from the apical part of the macrospore when the embryo begins to increase in length. The first root is formed a considerable time 4o6 VASCULAR CRYPrOGAMS. afterwards between the foot and the suspensor. It is lateral, and its apical cell is formed from an inner cell of the older segment ; but the first layer of its root-cap originates from the splitting into two layers of the overlying dermatogen ; the later layers of the root-cap arise from the apical cell of the root itself. It has already been mentioned that in Pteris and Salvinia the position of the apical cell of the growing stem is placed at an angle of about 90° with respect to that of the embryo. Something of the same kind occurs in Selaginella ; the apical cell which lies betw^een the rudiments of the first two leaves is divided by walls in such a manner that a four-sided apical cell is formed (Fig. 304 C, D), the segments of which arise in decussate pairs. In the fifth or sixth segment a second four-sided apical cell is now formed by a curved wall with the convexity turned towards the primary apical cell, so that a longitudinal section through the two apical cells cuts at right angles the common median line of the first leaves, and that of the original two-edged apical cell. Each of the two four-sided apical cells now developes into a branch of the first dichotomy ; but neither of the segments continues to grow in the direction of the hypocotyledonary segment ; the division, therefore, takes place immediately above the first leaves or cotyledons. The four-sided apical cells of the two rudimicnts of shoots are, however, soon transformed into two-sided apical cells each forming two rows of segments \ The first formation of all the organs and the first dichotomy always take place before the protrusion of the embryo from the spore. The External Differentiation is very various in the different genera of Lycopo- diaceae, if the habit of the mature plant is taken into account ; but they agree in a few points of great morphological importance. The leaves, different as they appear in other respects, are always simple and unbranched, and are penetrated by only one fibro-vascular bundle ; the branching at the end of the shoots and roots is always dichotomous, and the dichotomies succeed one another (with the exception of the older states in Selaginella) in planes at right angles. The roots of Lycopodiacese are, at present, the only ones knowm to dichotomise in the whole vegetable king- dom^. The difference of habit depends especially on the relative size of the leaves and on the different rapidity of the growth of the stem in length. One extreme is afforded in this respect by the genus Isoetes, with its extremely short un- branched stem, growing scarcely at all in length but much in breadth, its dense rosettes of leaves, which are of considerable and often of very great length and the number of which is often very large, and its numerous roots ; the other extreme occurs in Psilotum, where the stem regularly dichotomises, remains slender, grows much in length, but forms only very small leaves and no true roots at all. In Selaginella and Lycopodium the leaves are not large but nevertheless strongly developed, and the repeatedly dichotomising branches are densely covered with leaves, and produce numerous roots in acropetal succession. Very different from * The newwedge-shaped apical cells of the two first branches lie parallel to the primary apical cell of the embryo, as also do the apical cells of the succeeding branch. The second and succeeding planes of dichotomy therefore cross the first at a right angle, but only at the outset ; since, by a twisting of the first branches, their dichotomies come into the same plane as the first. ^ According to Reinke, however, some adventitious roots of Cycadecc do dichotomise. LVCOPODIACEM. 407 these genera in appearance is Phylloglossum, a small Australian plant only a few centimetres in height, which puts out a stem from a small tuber, and produces a rosette of a few long leaves and one or more lateral roots, then lengthens into a slender scape, and bears above a small-leaved spike of sporangia. The plant is renewed by lateral adventitious shoots, consisting of a tuber and a leafless rudiment of a bud; and in this respect resembles our native Orchideae. The S/em is distinguished in Isoetes, as has already been mentioned, by its extraordinarily small growth in length, with which is connected, in this as in other cases, an absence of branching ; no internodes ^ are formed, the leaves with broad bases of insertion constituting a thick rosette, without leaving between them any surface of the stem bare. The upper region of the stem, which is furnished with leaves, has the form of a shallow funnel, depressed in the centre or apex (Fig. 305). .vv/|y^i- V- FIG. 305.— Longitudinal section of Isoetes lacustris at right angles to the fork of the stem ten months old (after Hofmeister)> 5 stem, b\. — *8 leaves, rl—rlO roots (X30) ; the ligula of the two developed leaves is shaded. The increase in thickness, the long continuance of which distinguishes the stem of Isoetes from that of all other Cryptogams, is brought about by a layer of meristem lying inside, surrounding the central vascular body, and continually producing new layers of parenchyma on the outside. This takes place especially in two or three directions, so that two or three corresponding masses of tissue are formed, slowly dying oif on the outside, between which lie as many deep furrows meeting on the ventral side of the stem. From these a large number of roots are produced in rows in acropetal succession. In Selaginella, Lycopodium, Tmesipteris, and Psilotum, the stem remains slender, but lengthens rapidly by a great number of dichotomies, and forms distinct The same occurs in Ophioglossacese and the short tuberous Cactacese. 4o8 VA SC ULA R CR YP TO GA MS. internodes. In Selaginella the end of the stem rises above the youngest leaves as a slender cone ; in Lycopodium it is blunt and flat. The branches of the dichotomies grow with equal vigour in Psilotum, and often also in Lycopodium ; but in the latter genus and in Selaginella some of the branches develope into primary stems or branches, v/hich either assume a creeping position as rhizomes or an ascending one as aerial stems. In Selaginella a tendency prevails to sympodial scorpioid develop- ment of the dichotomous systems of branches (see p. 157) which not unfrequently leads to the system of abundantly branched shoots developed bilaterally in one plane attaining a definite outline, and a corresponding resemblance to a compoundly pinnate leaf. The small size of the leaves in these genera causes the general habit to be mainly dependent on the development of the systems of branches. The Leaves of Lycopodiacese are always simple, unbranched, penetrated by only a single fibro-vascular bundle, terminating in a simple point, and ending, in Selaginella and Lycopodium, in a fine awn. The largest leaves occur in Isoetes, where they attain a length of from 4 to 60 cm. They are in this case divided into a basal part or sheath, and an upper part or lamina. The sheath does not entirely embrace the stem, but rises in a somewhat triangular form from a very broad insertion, and is acuminate ; it is convex behind and concave in front, where there is a large depression, the Fovea, con- taining the sporangium ; the margin of this depression rises in the form of a thin mem- branous outgrowth, which, in many species lies above the sporangium and envelopes it, the Velum. Above the fovea and separated from it by a 'saddle,' lies a smaller depres- sion, the Foveola, the lower margin of which forms a lip, the Labium, while from its bot- tom an apiculate membranous structure, the Ligule, with a cordate base, is prolonged beyond the foveola (Fig. 306, A). The lamina of the leaf, containing chlorophyll, into which the sheath passes above, is narrow and thick, almost cylindrical, but flattened in front, and penetrated by four wide air-canals, which are divided by septa. This form is exhibited by the fertile leaves of all the species of Isoetes ; a rosette of such leaves is produced annually ; but between each pair of annual whorls is formed a whorl of imperfect leaves, which consist, in /. lacustris, of only a small lamina, but in the terrestrial species are destitute even of this, and may therefore be considered as scale-like hypsophyllary leaves (phyllodes). The leaves of Sekginella are never more than a few millimetres in length, and are usually cordate at the base with a narrow insertion, acuminate, and from lanceo- Fig. 306. — A longitudinal section through the base of a leaf of Isoetes lacustris with its microsporangium miitWX unripe ; B longitudinal section of the lower part of a young sporangium (X300) (after Hofmeister). LYCOPODIACEJE. 409 late to ovate in form. In the greater number of species the sterile leaves are of two different sizes, the ventral leaves attached to the under or shaded side of the obliquely ascending stem are much larger than the dorsal leaves on the upper side exposed to the light (Fig. 307, A). Both kinds taken together form four longi- tudinal rows {vide infra). On its upper side and near the base each leaf bears a ligule ; the point of attachment of the sporangium is below this on the fertile leaves. The fertile leaves form a quadrangular terminal spike, are uniform in size, and usually of somewhat different form from the sterile ones. This difference is more striking in those species of Lycopodium (Z. clavatum, &c.) which form a terminal spike of sporangia, the leaves of which are usually yellow or at least not green, and broader and shorter than the sterile foliapre- leaves. In other species however (Z. Selago, &c.) the sporangia are seated in the axils of the ordinary foliage-leaves, without forming an externally distinguishable spike. The form of the leaves of Lycopodium, although always simple, is also very various in the different species, in some cases resembling the aci- cular leaves of Conifers, in others broad, and always spreading on all sides. In Psilotum all the leaves are rudimentary, very small, membranous, and scale-like, even the fibro-vascular bundle is wanting in them ; on the underground shoots of these plants, which assume a root-like appearance (true roots are altogether wanting, vide infra), the formation of leaves is still more completely suppressed, and is often only recognizable by the arrangement of the cells near the punctum vegetaiionis. Tmesipteris, which is allied to Psilotum, possesses, on the other hand, large strong leaves. The Phylloiaxis is either spiral or de- cussate. In Isoetes the rosettes are arranged spirally, with the divergences '^, jVj 2T' if> the fractions becoming more complicated the larger the number of leaves that are annually formed. In Lycopodium the arrangement is also spiral ; and the number of orthostichies is frequently considerable ; but not unfrequently the leaves form in this genus pseudo-whorls in spiral succession, which appear as decussate pairs (Z. complanatum) or as alternating whorls of numerous leaves, as in Z. Selago, where the forked branches begin with pseudo-whorls of three leaves, but then produce others with four and finally five leaves. In the species of Selaginella which have their leaves arranged in four rows, each dorsal and ventral leaf form together a pair, whose median plane, however, does not intersect that of the next Fig. 307. — Sela^utelia i>i(£qiialifolia ; A fertile branch (one-half natural size) ; B apex in longitudinal section bearing microspores on the left, macrospores on the right (magnified). 410 VA SC ULA R CR YP TO GA MS. pair at right angles but obliquely, an arrangement which is often clearly seen on old shoots of -S*. Kraussiana. The Apical Growth of the stem takes place, in Isoetes, Selaginella, and Psilotum, by means of an apical cell. That of hoetcs lacustris is, according to Hofmeister, two-edged when the stem has two furrows; in the species with three furrows it is a three-sided pyramid. In young plants the leaves stand accordingly in the first case in two, in the second case in three rows ; but later the phyllotaxis becomes more complicated and spiral, indicating perhaps that in the older stem the primary walls of the segments are arranged in regular succession, in the same manner as in those Hepaticse which have a three-sided apical cell and a complicated phyllo- taxis. In those species of Selaginella which have the leaves in four rows, the apical cell of the stem is, according to Pfeffer, two-edged (Fig. 308, A, B). The two rows of segments here form an elevated vegetative cone, at the base of which the rudiments of the leaves first appear at the height of the fourth or fifth Pig. 30S.— Apex of the stem of Selaginella Martensii (after Pfeffer) ; A longitudinal section of the end of the stem with the first rudiment of the leaves ; R apex of the stem seen from above ; C dichotomy of the apical cell seen from the side ; D the same seen from below. The primary walls of the segments are denoted by darker lines ; the segments themselves are numbered with Roman figures. segment. The two edges of the apical cell are directed upwards and downwards (on the obhquely ascending shoot). The relationship of the leaves to the seg- ments has not yet been entirely made out. The tw^o leaves of each pair arise ob- liquely ; one above, the other below, and alternately right and left ; where the pairs cross obliquely, each embraces about a fourth of the circumference of the stem. Divisions then take place which are directed obliquely upwards and downwards, and a row^ of apical cells is thus formed, by means of which the growth of the leaf is continued (Fig. 308, A). The dichotomy of the shoot is caused by a second two-edged apical cell being formed from the youngest segment (Fig. 308, C, D). The two shoots which are thus formed grow right and left of the previous direction of growth, and all the successive dichotomies take place in one and the same plane. In Psilotum triquetrum the root-like underground shoots have been investigated by Nageli and Leitgeb in relation to their apical growth. They found a small three-sided apical cell, the divisions of which however advance (as in Polytrichum LFCOPODIA CE.E. ^ 1 j^ and Sphagnum), in the anodal direction, and thus produce rows of segments arranged spirally. In Lycopodiwn clavatum, finally, the same authorities thought they recognised a small apical cell, but were uncertain whether it was two- or four-faced. Pfeffer, on the other hand (as he informs me in a letter), did not find an apical cell in either Z. clava/ufji, annotiniim^ or ChamcEcyparissus ; and Cramer's experience was the same with Z. Selago. The dichotomy begins in this case by two small-celled papillae rising on the flat apex of the shoot, and growing up into the two shoots of the dichotomy. The gemmce or bulbils of Z. Selago, which subsequently fall off, are probably products of the leaves, not of the stem ; they are apparently axillary. It appears however to result from Cramer's description and drawings that they spring from the basal part of the leaf itself— at least this is indicated by the circumstance that the vascular bundle does not spring from the cauline but from the foliar bundle. The additional circumstance that sporangia are developed on the earlier leaves of a year's growth, bulbils on the later ones (the branch continuing to grow for years without dichotomising), appears further to justify the supposition that the bulbils occupy morphologically the same position as the sporangia, which in Lycopodium unquestionably originate from the leaves, and are not axillary. The Roots of Lycopodiacece show very remarkable morphological peculiarities ; they are the only roots at present known the branching of which is (apparently or actually) dichotomous ; the successive dichotomies lie in planes crossing at right angles. A second peculiarity are the Rhizophores of Selaginella and the root-like shoots of Psilotum. All these phenomena have been investigated by Nageli and Leitgeb (/. c). Psilotum triqucirum is a plant perfectly destitute of roots, forming however a number of underground shoots which serve the purpose of roots and are extremely similar to them. On the shoots of the rhizome which approach the surface of the ground may be detected with a lens minute leaves of a whitish colour and acicular shape ; the deeper root-like shoots have a blunter end, on which no trace of leaves can be detected, even with the lens. While the anatomical structure of the super- ficial shoots corresponds to that of the true stem of these plants, in these deeper shoots the vascular bundles are united into an axial group, as in true roots. The shoots which bear visible rudiments of leaves may turn upwards, become green and transformed into ordinary foliage-shoots, while the root-like shoots, which are more slender, may also turn upwards, become thicker, and assume the appearance of the ordinary superficial rhizome-shoots. In this point therefore they differ at once from true roots, but still more in the absence of a root-cap. They terminate in an apical cell, which forms oblique segments alternating in different directions. The most important point, however, is that these shoots really possess rudiments of leaves which consist of only a few cells and do not project above the surface, but remain concealed in the tissue. They are best recognised in longitudinal section, when they are seen to consist of an apical cell and from two to five cells with the characteristic arrangement of leaf-cells. Similar rudimentary leaves consisting of but few cells occur also on the ordinary rhizome- shoots, where, however, they do not undergo further development, especially when the end of the shoot appears 412 VASCULAR CRYPTOGAMS. above ground. The root-like shoots branch like the ordinary ones ; a cell is cut off by an oblique wall from one of the youngest segments, and forms the apical cell of the new shoot. All the species of Selaginella possess true roots ; but in some, as S. Martensii and Kraussiana, they arise on a structure which Nageli calls the Rhizophore, and which has no root-cap. In S. Kraussiana the rhizophores spring from the dorsal side of the stem, nearly at the base of the weaker fork of each dichotomy, curl themselves round it, and then grow downwards ; it is only rarely in this species that two of these organs arise near one another. ^. Martensii, on the other hand, forms at each fork two rhizophores, one on the dorsal and one on the ventral side (the plane which passes through them is perpendicular to the plane of dichotomy), but usually only the ventral one undergoes further development, while the dorsal generally remains in the form of a small protuberance. The rhizophores arise very near the piindum vege/ationis, probably at the same time as the branches of the dichotomy ; unlike the roots, they are exogenous structures which, when young, possess a distinct apical cell. This is probably two-edged, but soon ceases to form new segments, the further growth being effected by intercalary divisions of the segments and elongations of the cells w^hich proceed from them. After the cessation of the apical growth, the end of the still very short rhizophore swells up into a spherical form ; its cell-walls become thicker, and in the interior of the swelling the first rudiments of the true roots originate, which however do not break through until the rhizophore has attained such a length by intercalary growth that its swollen end penetrates into the ground. The cells of this terminal part become disorganised and deliquesce into a homogeneous mucilage, through which the true roots penetrate into the ground. The rhizophores, as Pfeffer has shown (in S. Martensii, i7icBqualifolia, and levigata), are often transformed into true leafy shoots, which at first show some deviations from the normal structure in their first leaves, but afterwards continue to grow as normal shoots, and even produce sporangiferous spikes. In Selaginella Kraussiana, cuspidata, and some other species, there are no rhizophores, but the roots spring immediately from the places nearest the ground where the stem forks and dichotomises, like the rhizophores of S. Martensii, even before they reach the ground. These roots are also formed very early, near the pundum vegetationis, probably at the same time as the branches of the stem. The roots which spring immediately from the stem, as well as those which proceed from the rhizophores, branch dichotomously, and in such a manner that the planes of the successive dichotomies cross one another at right angles. The branchings of the roots follow one another very quickly, and at the end of the mother-root are densely crowded ; the apical cell is difficult to detect, but is probably, like those of the stem and of the rhizophore, two-edged. It soon ceases to form segments ; the increase of length of each fork of the root takes place therefore almost exclusively by intercalary growth. Similar phenomena are observable in the roots which proceed from the furrows of the stem of Isoetes, and which dichotomise three or four times in planes at right angles to one another. Nageli and Leitgeb failed to find in them any apical cell distinguished by its form or size, although they considered the exist- ence of a two-edged apical cell probable. In Ljropodium davaium the roots spring L rCOPODIA CE.E. ^I 3 from the ventral side of the creeping stem without any fixed rule ; they fork when they have attained a length of 3 or 4 centimetres, but probably not until they come in contact with the ground. Their plane of dichotomy stands (as in Se/a~ ginella la'igata and cuspidata) at right angles to the longitudinal axis of the stem (in Isoetes it is, on the contrary, parallel to it) ; the succeeding branches are either also dichotomous, or sympodial ; in the latter case the real or apparent lateral branches appear distributed either in decussating pairs or singly with a divergence of \ or \. The position of these lateral roots has not been correlated with the arrange- ment of the fibro-vascular bundles of the mother-root. This, together with the circumstance that the young ramifications are densely crow^ded at the end of the mother-root, appears to exclude the supposition that the branching is monopodial ; and in any case the process is more like that which occurs in Selaginella and Isoetes. Above ground these roots are of a bright green colour. It is very difficult to prove the existence in them of an apical cell ; yet Nageli and Leitgeb conclude that one is present, having the form of a four-sided pyramid, the segmentation of which would influence the peculiar position of the root-branches. The Sporangia exhibit considerable diff'erences in the difi"erent genera of the class, both in their position on the fertile branch, and in their development and mature form. But they agree in a single sporangium being always formed in the axil of a leaf; and they are distinguished by their size from those of all other Cryptogams. The sporangia of Isoetes arc sessile in the fovea of the leaf-sheath, to which they are attached by their dorsal line (Fig. 306 A). They are unquestionably products of the leaves ; the outer leaves of the fertile rosettes produce only macro- sporangia, the inner ones only microsporangia, the former containing a large number of macrospores. Both kinds of sporangia are imperfectly segmented by threads of tissue {Trabeailcc) which cross from the ventral to the dorsal side. The sporangia do not dehisce, but the spores escape by the decay of the wall. In Selaginella the sporangia are shortly stalked roundish capsules, their origin being still doubtful, whether from the base of the leaf or from the stem itself, per- haps variable in the different species. The macrosporangia contain usually four, less often two or eight macrospores. In the division of Articulatse the lowermost sporangium only of a spike produces macrospores ; in the other divisions there are several macrosporangia. The remaining genera have, as has been mentioned, only one kind of sporan- gium, the contents of which bear a greater resemblance, in Lycopodium to the microspores of Selaginella, in Psilotum to those of Isoetes. The sporangia originate, in Lycopodium, from the leaf itself ; they consist, as in Selaginella, of only one compartment, and split into two valves at the apex or on the anterior surface. In Psilotum. the sporangia are described as trilocular, and as placed in the axil of a bipartite leaf; Juranyi's recent researches seem, however, to show that three spo- rangia are here seated round the end of a short branch, which at the same time forms below them two leaflets on the outer side. In Tmesipteris the elongated sporangium is seated on a stalk (shoot .?) bearing two leaves right and left of it. The history of development of the sporangia is still incomplete in many points. It is important to observe at the outset that Hofmeister considers that the sporan- gium of Lycopodiacece arises from a single cell of the leaf or stem, and that he 4J4 VASCULAR CRYPTOGAMS. traces back also the origin of the spores in Selaginella to a single primary mother- cell, which is the central cell of the sporangium. All that is known in addition about Lycopodium and Psilotum is that the first point is at least not universal, while Russow, 1 think rightly, has doubts about the latter. In Isoetes the sporangia originate, according to Hofmeister, from the leaves in their very earliest stage. A single cell produces the mass of tissue, of which two outer layers of cells (Fig. 306 B) become the wall of the sporangium, while strings of cells running transversely form the trabeculae. The numerous cells which lie between the latter still remain united into a tissue, increase in number, and form the Fig. 309. — Development of tlie sporangia and spores oi Selaginella ijiaqjtalifolia ; the order of succession is indicated by the letters A—D ; A and B serve for all the sporangia. C and D for the microsporangia only ; H division of the mother- cells of the microspores, h four nearly ripe spores ; in ^, C and D, a, b, c are the three layers of the wall of the sporangium, d the primary mother-cells (A, B and E X 500 ; C and D X 200). mother-cells of the spores ; they finally become isolated and rounded oif. The spores are formed by a repeated bipartition of these spore-mother-cells in planes at right angles to one another. In Selaginella again, according to Hofmeister, the sporangium springs from a mother-cell which belongs to the periphery of the stem. In later states the spor- angium is inserted in the axil or even in the base of the leaf. As in Isoetes, the fibro-vascular bundle of the leaf runs beneath the sporangium, without sending a branch into it {cf. Psilotum, infra). I was unable, even in the youngest which came under my observation, to recognise a central cell which could be considered the mother-cell of the spores ; on the contrary, even in very young sporangia a separation LVCOPODIACEyE. 415 of the tissue could be observed into a central mass and a wall consisting of three layers; the cells of the former soon become isolated and rounded off, and if a micro- sporangium is under observation, they all divide, after previous indication of a bipartition (Fig. 309 E, e,/) into four spores arranged in a tetrahedron, which retain this arrangement until they are ripe {g, h). In the macrosporangia, on the contrary, one of these mother-cells grows more vigorously, divides, and produces the four macrospores, while the rest of the mother-cells remain undivided ; but, at least in S. in(2qualifolia, continue to exist for a considerable period by the side of the much larger macrospores. These latter also retain until their dispersion their primitive position at the corners of a tetrahedron which they owe to the division of the mother-cell. Weakly macrospores are very commonly to be found in otherwise normal spikes of sporangia. The three cell-layers of the wall of the sporangium continue to exist until the spores are ripe, while in the case of Ferns the inner layers, as we know, are destroyed during the formation of the spores. The youngest rudiments of sporangia which I could detect in Lycopodiiim Cha??icpcypan'ssus — but which I have fre- quently examined — have the appearance of broad protuberances of the upper side of the young leaf, at first very flat, and in this case it is quite clear that they do not belong to the axil of the leaf nor to the stem itself; the fibro-vascular bundle of the leaf passes beneath them, and it appears as if in this case the sporangium is not produced from a single superficial cell. In the youngest, and even in older states, where it already projects as a flat segment of a sphere, the epidermis of the leaf is continuous over the sporangium, constituting its parietal layer. While the sporangium becomes more and more protuberant, this layer undergoes numerous divisions at right angles to the surface. Even in the youngest stages there can be recognised, beneath the swelling of the epidermis, a layer of cells, out of which, as the growth of the protuberance advances, a spherical group of large cells is formed, which divides in all directions to form the mother-cells of the spores. The processes appear to be still the same when the sporangium has grown to a considerable size and is almost spherical in radial section ; at that time a tangential division is seen here and there in the parietal layer, which, in the mature state, clearly consists of at least two layers. Older stages of development have not come under my notice; what I have here stated was deduced from the observation of some longitudinal sections of very young spikes preserved in glycerine. In Psilotum the short branches on which the apparently trilocular sporangia arise appear as papillce on the vegetative cone, which, according to Juranyi, possess, as well as the vegetative branches, a three-sided apical cell. A bundle from the Fig. 309 *. — A nearly ripe macrosporangium o( Se/a- gmella iiMquali/olia ; the fourth spore which lies be- hind is not indicated (Xioo). 1 6 VASCULAR CRYPTOGAMS. tibro-vascular bundle of the mother-shoot runs into these papillae, without, however, reaching more than half their height. The two small leaves of this fertile shoot, which at one time were thought to be a bipartite leaf, originate separately on the papillae, and unite only at a later period. The papilla itself consists, even at a rather late period, of a homogeneous tissue which becomes separated, in a similar manner to the anthers of Phanerogams, into parietal layers and three groups of spore-mother-cells. Three loculi are thus formed, protruding strongly outwards "^ Fig. 310. — A transverse section of the.stem oi Selagi7iella denticiilata, the central vessels of the bundle not yet Hgnified ; B transverse section of the stem oi Lycopodiiivi Chamcecyparissus (X150). and separated by longitudinal walls and by an axial mass of tissue. These three loculi I consider to be as many sporangia which are formed round the summit of the fertile shoot, through which the axial fibro-vascular bundle ascends. The Systematic Classification of Lycopodiaceae can only be regarded as provisional until the mode of germination of the remaining genera is accurately known. As we have seen, they may in the meantime be arranged into two groups : — A. LycopodiecB ; with only one kind of spores. Lycopodium, Tmesipteris, Phylloglossum, Psilotum. B. Selaginelleas, with two kinds of spores. Selaginella, Isoetes. LYCOPODIACEM. 417 As to the Forms of Tissue in Lycopodiaceae\ it may be remarked that the fibro- vascular bundles which penetrate the stem belong exclusively to it or are ' cauline.' They may be followed in the procambial condition close beneath the apical cell to the apex of the stem and the youngest leaves. This I have found to be the case in Selaginella inaqiialiforia and Martensii and in Lycopodium Chayyicecyparissus ; and, according to Nageli, the fibro-vascular bundle of Psilotum is also cauline, since no branches pass from it into the leaves (Nageli, Beitr. p. 52). Proceeding downwards from the apex of the stem, it is seen that the leaves which are already more developed each form a pro- cambial bundle which applies itself to that of the stem. In the angle where they meet the formation of spiral vessels begins, and advances downwards into the stem, outwards Fir., -ji I. —Transverse section of tlie stem of Sfla^inella inc^quali/olia (X150). into the leaf. In their procambial origin part of the fibro-vascular bundles of Lyco- podium and Selaginella are therefore cauline, and part foliar; but the formation of the first spiral vessels takes place as if they were 'common' {cf. Equisetum). The first spiral vessels of the cauline bundle arise near its edges; the formation of the wider vessels, which are thickened in a scalariform manner, proceeds from them in a centripetal direction as seen in a transverse section. This occurs in different ways ac- cording to the nature of the cauline bundle, which is very simple in Selaginella denticulata ^ On the development of the tissues in the roots, especially on the eccentric position of the fibro-vascular bundles in those of Isoetes, compare NageH and Leitgeb, Beitrage zur wissensch. Bot. 1867, Heft IV. E e 1 I N VASCULAR CRYPTOGAMS. (Fig. 310 A), Kraussiana, and Martensii : and in these cases it has, in transverse section, an elongated elliptical form. The first narrow spiral vessels arise nearly in the two foci of the ellipse ; from these points two alternating rows of much v»'ider scalariform vessels proceed inwards and become very slowly lignified, until a vascular band formed from a double row of woody vessels lies within the fibro-vascular bundle. The outer much narrower elongated cells of the bundle do not become woody ; they form the phloem, the outermost peripheral layer of which consists of much wider cells. In Sela- ginella inaqualifolia (Fig. 311), three fibro-vascular bundles lie parallel and side by side in the stem, each resembling the single bundles of the species mentioned before. In Lycopodium ChamcBcyparissus (Fig. 310 5) a fibro-vascular cylinder occurs in the stem; four parallel transverse bands of xylem lie in it, each of which consists of a double row of wide scalariform vessels, with narrow spiral vessels also right and left at its ends. Each of these transverse bands corresponds in all respects to the single fibro-vascular bundle of Selaginella ; the whole of the cylinder in the stem of Lycopodium is therefore made up by a coalescence of four fibro-vascular bundles. In the same manner the whole of the densely lignified tissue which fills up the interstices of the fibro-vascular bundles and forms an envelope to them, is the result of a coalescence of as many layers of phloem, each fibro-vascular bundle being enveloped by its own layer. Between each pair of transverse bands of xylem lies also a row of wider cells, which may be recognised on longitudinal section as sieve-tubes ; the periphery of the whole of the phloem is also formed of v/ider cells. It is therefore beyond doubt that the axial cylinder of L. Cham(£cy parts sus consists of several coalescing parallel fibro-vascular bundles^. If the three bundles in the stem of S. inoequalifolia are imagined laid side by side and to have united in growth laterally, they would present a precisely similar structure. In Lycopodium Selago there is a similar axial cylinder, but the groups of vessels do not in this case form transverse bands, but a more complicated figure in a transverse section ; otherwise the arrangement agrees completely with that of L. ChamcEcyparissus {cf. Cramer). In L. cla'vatuyn two bands of vessels lie in the tranverse section of the axial cylinder, and between them a diametral row of wider cells (sieve-tubes). The outer fibro-vascular bundles are curved like a horseshoe : from the concavity which faces outwards a group of vessels projects : between the three arms so formed of each of these horseshoe-shaped outer bundles lie again two rows of wider cells (sieve-tubes), while all the rest of the phloem Vv'hich fills up the space between the fibro-vascular bundles consists of very narrow elongated cells. The single fibro-vascular bundle of Selaginella has many points of resemblance to that of Ferns {e.g. Pteris aquilma), as Dippel has already pointed out ; and this resemblance is only "partially obliterated in Lycopodium by the lateral coalescence of several fibro-vascular bundles'^. Only one bundle bends out into each leaf, forming, in Selaginella and Lycopodium, an axial bundle through the mid-rib, and, as has been said, uniting with the outer edge of a cauline bundle. In Selaginella the fibro-vascular bundles are surrounded by large spaces which contain air, into which transverse rows of cells pass out to the bundle like buttresses from the surrounding fundamental tissue. In Lycopodium these air-cavities are wanting. The fundamental tissue of both genera consists of elongated cells with oblique septa dove- tailed into one another in a prosenchymatous manner. In Selaginella the cell-walls are thin, the cavities wide, and there are no intercellular spaces ; in Lycopodium the walls of the fundamental tissue are usually much thickened, especially in the part that ' A similar explanation may also be given of the complicated fibro-vascular bundle of the thickish roots of L. clavatum, as described by Nageli and Leitgeb, ^ I found a stem of Pteris aqtnlina in which the two inner cauline bundles had coalesced later- ally to such an extent as to form a hollow cylinder, enclosing a part of the parenchymatous funda- mental tissue as medulla. LYCOPODIACE.E. 419 surrounds the axial cylinder; in L. Chanmcyparissus this thickening of the walls is remarkably great (Fig, 310 ^)\ . The epidermis of the stem consists, in Selaginella, of long prosenchymatous cells, and has no stomata ; these occur only in a few rows on the under side of the leaves ri<^ht and left of the mid-rib (Fig. 46, p. 47). The epidermis of the leaf consists of cells Fig. -ixz-Sela^uella inaqualifolia ; longrituclinal section through the right side of the axis of a spike S, the base of the leaf*, the ligule «, and the sporangium^/; ^ point where the cauline and foliar fibro-vascular bundles unite; /air-conducting intercellular spaces ; -I' series of cells traversing the spaces. containing chlorophyll, the lateral walls of which are beautifully serpentine. In L. Selago, on the other hand, the large and comparatively few stomata are distributed over the ^ [Hegelmaier, in an exhaustive treatise on the morphology of the genus Lycopodium (Bot. Zeitg. 1872, p. 773 et seq.), describes the stem as consisting of a fibro-vascular cylinder surrounded by a thick cortex, the first being fonned of a number of bundles penetrating a thin-walled and narrow-celled tissue. The central cylinder is composed of two parts, distinguishable from an early period and even when the tissue is mature, viz. a comparatively small external and a much more strongly developed axial portion, the latter consisting of the true fibro-vascular bundles with mter- fascicular tissue. The first of these two parts, which must not be confounded with the inner layers of the cortex, surrounds the central part of the cylinder as an enveloping sheath, and Hegelmaier proposes for it the term 'Phloem-sheath,' retaining, with previous writers, that of ' phloem ' for the interfascicular tissue. This phloem-sheath (Fig. 310 B,p) is separated from the phloem by a cylin- drical layer which unites together the outer convex surfaces of the fibro-vascular bundles, and is distinguished from it by its cells being shorter, with thinner walls and larger cell-cavities.— Ed.] £62 420 VASCULAR CRYPTOGAMS. whole of the under side of the leaf. The chlorophyll in the cells of the leaf of Sela- ginella often forms only a few — sometimes only one or two — masses variable in form ; the margin of the leaf consists, in this genus, of only a single row of cells, which, as in Mosses, develope in the form of teeth or hairs. To this brief description must be added a few further words with respect to Isoetes. The short stem of the mature plant contains an axial woody body which can scarcely be termed a bundle, consisting of short roundish vascular cells united loosely, and with spiral or reticulated thickening-bands. P'rom these the fibro-vascular bundles proceed, one into each of the very numerous leaves (Fig. 305, p. 407) and into the roots. Not- withstanding H. V. Mohl's accurate description (/. c), Hofmeister's statements, and my own researches, it is still impossible to compare this peculiar fibro-vascular body morpho- logically with the bundles of Lycopodium and Selaginella. But in opposition to the view that the layer of tissue which surrounds it is a cambium analogous to that of Dicoty- ledons and Conifers, it may be objected that this thick layer of meristem which invests the fibro-vascular body produces on the outside only parenchymatous fundamental tissue, by which the outer masses of parenchyma that annually decay away and become brown are replaced. In this respect this tissue is comparable rather to the thickening-ring of Dracaena, which also forms on the outside new cortical parenchyma and on the inside new fibro-vascular bundles. The true cambium of Dicotyledons, on the other hand, produces fibro-vascular structures in both directions, on the outside phloem, on the inside xylem. But the stem of Isoetes probably possesses no proper fibro-vascular bundle at all ; it would appear rather, from the position of the vessels, that the axial fibro-vascular body consists only of the lower (inner) commencements of the foliar bundles, which are here densely crowded. In the same manner the basal disc-like woody body may consist only of the densely crowded commencements of the radical bundles. If this view is correct, the class of Lycopodiacese presents two extremes, one in Psilotum, where the foliar development is small, and where there are, according to Nageli, no foliar bundles, but the elongated stem forms a fibro-vascular bundle belong- ing to it only ; the other in Isoetes, where the short stem possesses no cauline fibro- vascular bundle, and only the strongly developed leaves have one each. The structure of the leaves of Isoetes varies according as the species grow submerged in water, in marshes, or on dry ground. In the first case they are long and conical, penetrated by four air-cavities divided by septa into channels, with a weak fibro-vascular bundle in the axis of the organ, and the epidermis destitute of stomata : in the second case they are similar, but provided with stomata and hypodermal vascular bundles ; in the third case the epidermis is also provided with stomata, and the basal portions of the dead leaves (phyllopodes) form a firm black coat of mail round the stem. [Professor W. C. Williamson has contributed the following note on the Carboniferous Lyco- podiaceae : — ' The large and varied group of the Lycopodiaceous plants of the Coal Measures exhibits so many modifications that it is difficult to give a brief statement of their characteristic features. But so far as the Lepidodendroid and Sigillarian forms are concerned, our British forms all exhibit one type of internal organisation. In the very young state each twig has a central bundle of scalariform vessels surrounded by a 'bark,' which usually exhibits an inner parenchymatous layer surrounded by a more prosenchymatous one, which is again invested by a second but more unequal parenchyma. Bundles of vessels given off by the central vascular axis proceed to each of the leaves. As the twig enlarges the central axis invariably expands into a vascular cylinder, its interior becoming occu- pied by a cellular parenchyma of large size, and which now occupies the position and exhibits the appearance of a true medulla. The parenchyma of the leaves appears to be an extension of the outermost parenchyma of the bark. The above remarks appear to represent the common history of all the Lepidodendroid plants up to a certain stage of their growth. Beyond this stage their histories vary somewhat in the different groups. In some forms, e.g. those to which the Haloniae belong, the branches attain considerable dimensions without undergoing any great change in their internal organisation ; but in others a new development of vascular tissue invests the central cylinder at a period which seems to have varied in different species. This new growth takes place in successive PHANEROGAMS. 421 layers, which are arranged in vertical laminae disposed in radiating planes separated by tracts of muriform parenchyma ; successive additions are made to the outer margins of the woody wedges previously formed through the agency of a pseudo-cambial layer of the innermost ' bark.' These exogenous growths continued until the woody zone attained to a great thickness in the larger trunks. These exogenous layers took no part in supplying the leaves with vessels. The foliar bundles invariably pass through them on their way from their source in the inner non-radiated vas- cular cylinder to the leaves. It being now admitted that Stigmaria was the general form of root of Lepidodendroid and Sigillarian types it is necessary to correlate its tissues with those of the aerial stem. It contains a 'medulla' surrounded by a cylinder composed of radiating vascular laminae separated by cellular rays, and enclosed in a thick ' bark.' Large vascular bundles are given off from the vascular wedges to supply the rootlets. Thus the structure of the root differs from that of the aerial stem in two ^vays. (i) The inner vascular cylinder of the latter, character- ised by the non-radiating arrangement of its vessels, by the absence of ' cellular rays,' and by the numerous foliar bundles which it gives off to the leaves, is altogether wanting in the former. On the other hand, the exogenous zone of the stem is prolonged into the roots, retaining all its more important features. These however are modified in two ways — ist, in the absence of small passages for the transmission of foliar bundles of vessels ; and, 2nd, in their replacement by much larger spaces having a lenticular section, and through which large vascular bundles, directly derived by enlarging from the exogenous laminre themselves, pass outwards to the succulent rootlets. That Lepidostrobi are the fruits of Lepidodendroid plants is certain. Equally so is it that many of the former produced microspores in the upper sporangia of each cone, and macrospores in those oc- cupying its basal end. The incalculable myriads of these macrospores found in many coals renders it probable that a very large number of the Lepidostrobi possessed both kinds of spores ; indeed it is far from certain that any of them did otherwise. In the great majority of cases the spor- angia of these fniits are shrivelled and empty, the spores having been shed ; and this renders it impossible to say what their original character was*.' — Ed.] GROUP V. PHANEROGAMS. The Alternation of Generations in Phanerogams is concealed in the formation of the Seed, which, at least in its earliest stage, consists of three parts:— (i) The Testa, which is a part of the mother-plant; (2) The Endosperm'^; and (3) The E?nbfyo, the result of the development of the oospore or fertilised embryonic vesicle (oosphere). 1 [For the literature of the Carboniferous Lycopodiacese see Brongniart, Archives du Mus. d'Hist. Nat. vol. I, and Journ. Bot. vol. VII, pp. 3-8.— King, Edin. New. Phil. Journ. vol. XXXVI. —Hooker, Mem. Geol. Surv. vol. II.— Carruthers, Monthly Mic. Journ. vol.1, pp. 177-181 and 225-227 ; Quart. Journ. Geol. Soc. vol. XXV, pp. 248-254.— Williamson, Phil. Trans, vol. CLXII, pp. 197-240.— Thiselton Dyer, Quart. Journ. Mic. Sc. 1873, pp. 152-156.— Ed.] 2 The only reason why the ripe seeds of many Dicotyledons do not contain any endosperm is because it has already been absorbed and supplanted by the rapidly growing embryo before the seeds become ripe ; while in others this absorption happens only on germination after the ripenmg of the seeds, i. e. on the unfolding of the embryo ; more rarely the formation of endosperm is from the first rudimentary. 4■^ PHANEROGAMS. In Vascular Cryptogams we have already seen the sexual generation which results directly from the spore or prothallium losing more and more of its character of an independent plant. In the Ferns, Equisetacese, and Ophioglossacese it grows independently of the spore, often for a considerable period ; in the Rhizocarpese and Lycopodiace^, where male and female spores are formed, it arises in the interior of the spore, the female prothallium still protruding in the former out of the cavity of the macrospore, but remaining united with it ; while in Isoetes it fills up the interior of the macrospore as a mass of tissue which only bursts the cell-wall of the spore in order to render the archegonia accessible to the antherozoids. In the Cycadeae and Conifers) this metamorphosis is carried one step further; the prothallium \ which is now known as the Endosperm, remains during its whole existence enclosed in the macrospore or Embryo-sac ; it produces before fertilisation archegonium-like struc- tures, the ' Corpuscula,' in which the Germinal or Embryonic Vesicles arise. The processes which take place in the embryo-sac of Monocotyledons and Dicotyledons appear somewhat different, and bear a greater resemblance to what takes place in the macrospore of Selaginella. In this genus, besides the prothallium which produces the archegonia, there arises subsequently, by free cell-formation, another tissue which fills up the rest of the space of the macrospore ; to this tissue the endosperm of Monocotyledons and Dicotyledons, which is formed by free cell-formation only after fertilisation, appears to correspond ; the prothallium of Selaginella does not appear to have anything to correspond to it in Angiosperms, the embryonic cells or ' Germinal Vesicles' arising immediately from the protoplasm of the embryo- sad If, therefore, the embryo-sac is the representative of the macrospore, that part of the ovule in which the embryo-sac arises (the nucleus) must be considered the equivalent of the macrosporangium. But, as in the formation of the seeds of IMonocotyledons and Dicotyledons, certain processes of development (the formation of the archegonia or 'corpuscula'), being no longer necessary, are suppressed, and the embryonic vesicle is produced immediately from the embryo-sac as the analogue of the macrospore, so also the production of the embryo-sac immediately from the tissue of the nucleus of the ovule is more direct. Its production is due to the simple increase in size of an inner cell of the nucleus which here replaces the sporangium. But -vhile even in the most highly developed Cryptogams the macro- spore still becomes detached from the tissue of the mother-plant, and the full development of the prothallium takes place only after the dissemination of the spores, so that the embryo always arises in structures distinct from those of the mother-plant, the embryo-sac (or macrospore) of all Phanerogams remains, on the contrary, enclosed in the ovule, the endosperm in the embryo-sac, and the embryo in the endosperm. In this manner arises that structure peculiar to Phane- * The analogy of the endosperm \\-ith the prothallivun of the higher Cryptogams was first shown by Hofmeister (Vergleich. Untersuch. 1851), [Germination, Development, and Fructification of the Higher Cryptogamia, Ray Soc. 1862, p. 438.] ^ Compare Pfeffer in Hanstein's Botanical Dissertations, Heft IV, p. 24. The * Antipodal Cells ' in the embryo-sac of Angiosperms may probably be considered as the last occasional occur- rence of the rudiment of the true prothallium, and the occasional filamentary apparatus of the embiyonic vesicles as the last rudiment of the canal cell. PHANEROGAMS. 423 rogams, the Seed, the testa of which, the product of the envelopes of the ovule, closely invests both endosperm and embryo. The whole becomes separated from the mother-plant after the embryo has attained a certain very variable degree of development. Germination consists in the further development of the embryo at the expense of the endosperm. If, on the other hand, the microspores of Selaginella and Isoetes are compared with the pollen-grains of Phanerogams, a series of analogies is again seen which be- comes intelligible on comparing the intermediate phenomena presented by Gymno- sperms. Indications of the male prothallium and antheridium are indicated, as Millardet and Pfeffer have shown, by certain cell-divisions which may also be recognised in a still simpler form in the pollen-grain of Gymnosperms, but which do not occur in Angiosperms. Like the microspores, the pollen-grains contain the male fertilising principle, which, passing into the oosphere or embryonic vesicle, causes it to develope the embryo ; but a great difference is displayed in the mode in which the fertilising substance is conveyed. In Cryptogams the fertilising substance takes the form of spermatozoids or antherozoids endowed with motion and adapted to force them- selves, with the assistance of water, into the oosphere through the open neck of the archegonium. In Phanerogams, where the embryonic vesicle is enclosed in the embryo-sac and ovule, and in Angiosperms is also surrounded by the wall of the ovary, such a conveyance of the fertilising element would not serve the purpose intended; the pollen-grains are therefore themselves conveyed to the female organ by foreign agencies, such as the wind, mechanical contrivances in the flowers, and especially insects; and then germinating like spores, they emit their pollen-tubes, which, penetrating through the masses of tissue of the female organ, finally reach the embryo-sac, and transmit by diffusion the amorphous soluble fertilising substance into the embryonic vesicle. The analogy of pollen-grains to spores becomes still more evident when we examine the mode of origin of both. The mass of tissue in which the pollen is formed, the pollen-sac, shows, not only in its morphological but also in its anatomical relationships, a striking resemblance to the sporangium of. Vascular Cryptogams. As in the latter the spore-mother-cells are formed by the isolation of cells previously combined, so also are the mother-cells of the pollen; and as the former themselves produce the spores by division into four, usually after previous indication of a bipartition, the pollen-cells are produced from their mother-cells in a similar manner. Moreover, in the points here indicated Gymno- sperms again appear as a connecting link between Cryptogams and Angiosperms ; the pollen-sacs of Cycadeae and of some Coniferae resembling, in form and position, the sporangia of some Vascular Cryptogams. The general result of these observations is that the Phanerogam, with its pollen-grains and its embryo-sac, is equivalent to the spore-producing (asexual) generation of the heterosporous Vascular Cryptogams. But as in Vascular Cryp- togams the sexual differentiation first makes its appearance (in Ferns and Equise- tace«) on the prothallium only, and next (in Rhizocarpeae and Lycopodiaceae) on the spores themselves, so, in Phanerogams, this process is carried back a step further, the sexual differentiation arises still earlier, being manifested not only in the formation of embryo-sac and pollen-grains, but also in the difference between ovule and pollen-sac, and even earlier in the distinction between male and 424 PHANEROGAMS. female flowers, and last of all in the dioecious condition of the plants them- selves \ The fertilised embryonic vesicle of Phanerogams is not directly developed into the embryo ; it first of all produces a pro-embryo, the Suspensor, — growing towards the base of the embryo-sac and dividing, — which we have already met with in Selaginella, and on the apex of which arises a mass of tissue at first almost globular, and from which the embryo is developed. The development of the embryo usually proceeds, even before the maturity of the seed, to such an extent that the first leaves, the primary axis, and the first root, can be clearly distinguished. It is only in parasites and saprophytes devoid of chlorophyll that the embryo usually remains rudimentary until the dissemination of the seeds without discernible external differentiation; while in those Phanerogams which contain chlorophyll the embryo not unfrequently attains a very considerable size and external differentiation (as in Pinus, Zea, Aesculus, Quercus, Fagus, Phaseolus, &c.) Independently of any curv- ing of the embryo, the primary apex of its stem always lies originally pointing towards the bottom of the embryo-sac (the base of the ovule) ; the first root (primary root) coincides with a posterior prolongation of the primary stem ; it faces the apex (micropylar end) of the embryo-sac, and is of distinctly endogenous origin, inasmuch as its first rudiment at the posterior end of the embryo is covered by the nearest cells of the pro-embryo. The Apical Cell of the punctum vegetatiojiis, which is easily recognized in many Algae, in Characese, Muscinese, Ferns, Equisetaceae, and Rhizocarpeae, as the primary mother-cell of the tissue, has already, as we have seen, lost its significance in the Lycopodiaceae. The apical growth of the axes, leaves, and roots of Phanerogams can no longer be referred to the activity of a single apical cell from which the whole primary meristem has proceeded. Even in those cases where a single cell (not, however, of preponderating size) occupies the apex, and the arrangement of the superficial cells of the punctum vegetationis appears to point to it as the primary mother-cell, it is nevertheless by no means to be assumed that all the cells, and especially the internal mass of the primary meristem, has proceeded from it. The primary meristem of the punctum vegetationis consists of a large number of usually very small cells, more or less evidently disposed in concentric layers ; an outer simple layer, the dermatogen, may be recognized in Angiosperms as the immediate continu- ation of the epidermis of the older parts, and is continuous even over the apex of the punctum vegetatio7iis. Beneath it lies a second layer of tissue (the periblem), consisting usually of a few layers of cells, which covers the apex and passes lower down into the cortex ; this envelopes a third inner mass of tissue (the plerome) terminating beneath the apex as a single celP (Hippuris, &c.) or as a group of cells ; and out of it proceeds either an axial fibro-vascular body (in the roots and stems of water-plants), or the descending arm of the fibro-vascular bundles. In harmony with this the root-cap does not proceed, as in Cryptogams, from transverse divisions ^ Compare what is said on Dichogamy in Book III. ^ As in so many other respects, here also Isoetes shows an affinity to Phanerogams, as is evident from Nageli and Schwendener's researches on the apical growth of roots. (Compare Niigeli's Beitragen, 1867, Heft IV, p. 136.) PHA NEROGA MS. ^ 2 ^ of an apical cell, but arises, on the contrary, in Gymnosperms from a luxuriant growth of the layers of periblem of the root and from their splitting away towards the apex, and in Angiosperms from a similar process in the dermatogen \ Even the first rudiments of lateral structures, leaves, shoots, and roots, cannot be traced back in Phanerogams to a single cell in the same sense as in Cryptogams. They are first observable as protuberances consisting of a few or a larger number of small cells ; the protuberance which is to form a shoot or a leaf shows, even when it first begins to swell, an inner mass of tissue which is connected with the periblem of the generating vegetative cone, and is covered over by a continuation of the dermatogen. The normal JMode of Branchmg at the growing end of the shoot, leaves, and roots, is, with few exceptions, monopodial. The generating axis continues to grow as such, and produces lateral members (shoots, lateral leaf-branchings, lateral roots) beneath its apex ; some cymose inflorescences appear however to be the result of dichotomous branching. It is possible also that in Cycadeae the branching of the stem and leaves may be dichotomous. The monopodial branching of the axes is usually axillary ; /. e. the new rudiments of shoots appear above the median plane of very young (but not necessarily the youngest) leaves, in the angle which they form with the shoot, or somewhat above it. In Gymnosperms every axil of a leaf does not usually produce a shoot ; sometimes (in Cycadeae), the branching of the stem is reduced to a minimum. In Angiosperms, on the contrary, it is the rule that every axil of a foliage-leaf {i. c. one not belonging to the flower) produces a lateral shoot (sometimes even several side by side or one above another) ; but commonly the axillary buds, once formed, are inactive, or develope only at later periods of vege- tation. In addition to the above-mentioned cases of apparent dichotomy, there are in Angiosperms only a few cases of actual or apparent extra-axillary branching, which will be mentioned when discussing the characteristic features of this class. Phanerogams are distinguished from Cryptogams by an extraordinarily varied and complete metamorphosis of members bearing the same name ; and this is con- nected with the almost infinite variety in the mode of life, and the strict differ- entiation of the physiological functions of these plants ; and the same is the case with the differendation of tissues, which in Phanerogams greatly exceeds even that of Ferns. In these respects also Gymnosperms assume an intermediate position between Cryptogams and the rest of Phanerogams. What has now been said will serve to explain on one hand the distinction between Vascular Cryptogams and Phanerogams, on the other hand the points in which they agree, and the affinity of the two groups in their main outlines. In order, however, to facilitate the comprehension by the student of the characteristics of the separate classes of Phanerogams which are now to be described, we must in the first place keep in view a few of their peculiarities, which have at present only been briefly touched upon, and attempt to settle the nomenclature, which has become to some extent obsolete and out of harmonv with the most recent theories. The Flower, in 'the broadest sense of the term, is composed of the sexual organs and the axial structure which bears them. When the leaves which stand immediately See Hanstein, Bot. Abhandl. Heft I, and Reinke, Gottinger Nachr. 1871, p. 533. 426 PHA NER O GA MS. heneath the sexual organs on the same axis differ from the rest of the leaves of the plant in their arrangement, form, colour, or structure, and are physiologically connected with fertilisation and its results, they are considered as belonging to the flower, and are termed collectively the Floral Leaves or Perianth. The separate flowers are dis- tinguished from the Inflorescence by including, together with their sexual organs and perianth, only one axis, while the inflorescence is an axial system with more than one flower^. Roper has termed the tout ensemble of the male sexual organs of a flower the Andrcecium, that of the female organs the Gynceceum. When a flower contains sexual organs of both kinds it is called hermaphrodite or bisexual ; if it contains only male or only female sexual organs, and is therefore unisexual, it is termed diclinous ; when flowers of both sexes occur on the same individual plant, the species is moncecious, when on diff'erent individuals it is dioecious. Usually the apical growth of the floral axis ceases as soon as the sexual organs make their appearance, and frequently even earlier ; the apex of the floral axis is then concealed, and is often deeply depressed in the centre of the flower ; but in abnormal cases (and normally in Cycas) the apical gro\\i:h of the floral axis re- commences, again produces leaves, and sometimes even a new flower ; and a Proliferous Flocwer is thus produced. The sexual organs and perianth of a flower are usually crowded (arranged in rosettes either spirally or in whorls) ; the part of the floral axis which bears them remains very short, no internodes being in general distinguishable in it ; and it not unfrequently expands into the form of a club or disc, or becomes hollovv', and this part of the floral axis is called the Torus or Receptacle. In Coniferae and Gycadeae (occasionally also in Angiosperms), it is however sometimes elongated to such an extent that the sexual organs appear loosely arranged along an axis in the form of a spike. Beneath the receptacle the axis is mostly elongated and more slender, either entirely naked or bearing one or two small leaves or Bracteoles. This part of the axis is the Peduncle ; if it is very short, the flower is said to be sessile. No shoots usually arise from the axils of the floral leaves, even when they are produced in all the other leaf-axils of the plant ; there occur, however, abnormal cases (which are not very uncommon) of axillary branching or prolification even within the flower. The Male Sexual Cells {Pollen-grains), which are equivalent to the microspores of the higher Cryptogams, arise in receptacles corresponding to the sporangia in those plants, and may be termed in general Pollen-sacs. These are at first solid masses of tissue in which, as in the sporangia, an inner mass of cells becomes differentiated into the mother-cells of the pollen-grains (at first by more vigorous growth of the single cells), while the surrounding layers of tissue become developed into the wall of the pollen-sac. It has already been mentioned that the mother-cells of the pollen become separated and detached from the tissue (though this rule is subject to exceptions), and then produce the pollen-cells by division into four after actual bipartition or at least an indication of it. A special description of these processes will be given under the heading of the separate classes ; at present we must however premise a few facts relative to the morphological nature of the pollen-sac. Like the sporangia of most Vascular Crypto- gams, the pollen-sacs of Phanerogams are usually products of the leaves, which how- ever mostly undergo in this case a striking metamorphosis, remaining much smaller than all the other leaves. A leaf which bears pollen-sacs may be termed a Staminal Leaf or Stamen ; the most recent researches have, however, shown cases in which the pollen-sacs arise on the elongated floral axis itself, as Magnus has illustrated in the case of Naias, Kaufmann in Casuarina, and Rohrbach in Typha ; in these cases it is still doubtful whether the pollen-sacs may not be the only surviving portions of ^ In some cases it is however difficult to distinguish between a flower and an inflorescence; as in some Conifers, and especially in Euphorbia. (On the latter, see Warming in Flora 1870, no. 25; Schmitz, ditto 1871, nos. 27, 28; and Hieronymus, Bot. Zeitg. 1872, no. 12.) [E. Warming, El- Koppen lios Vortemcelkcn en Blomst eller en Blomsterstand, Kobenhavn 18 71.] PHANEROGAMS. .r>7 Otherwise completely abortive staminal leaves \ In the Cycadese the pollen-sacs grow singly or in groups on the under side of the relatively large stamens, often in large numbers, resembling in position the sporangia on Fern-leaves. In the Coniferse the stamens have still more lost the appearance of ordinary leaves ; they remain small, and form several or only two relatively large pollen-sacs on the under side of the lamina which is still distinctly developed. In Angiosperms the stamen is usually reduced to a slender weak and often very long stalk called the Filament, bearing two pairs of pollen-sacs at its upper end or on both sides beneath the apex, which are included as a whole under the term Anther ; the anther therefore usually consists of two longitudinal halves, united and at the same time separated by a part of the filament termed the Connecti've. The two pollen-sacs of each half of the anther are contiguous throughout their length, and frequently both halves of the anther are in close apposition. The separate pollen-sacs then appear as compartments of the anther, which is in this case quadrilocular, in contrast to those anthers (of rare occurrence) in which each half contains only a single pollen-sac, and which are therefore biiocular. The Embryo-sac, the analogue of the macrospore, is the result of a very considerable enlargement of an inner cell of the nucleus of the ovale, which itself corresponds to the macrosporangium of heterosporous Cryptogams. The nucleus is a small-celled mass of tissue of usually ovoid form, and enclosed, with a few exceptions, in one or two envelopes, each of which consists of several layers of tissue. These envelopes or Integuments grow round the young nucleus from its base, and form at its apex — where they approach and often greatly overtop it — a canal-like entrance, the Mkropyle or Foramen, through which the pollen-tube forces its way, in order to reach the apex of the embryo-sac. Very commonly the nucleus, enclosed in its integuments, is seated on a stalk, the Funiculus ; but this is sometimes wanting, and the ovule is then said to be sessile. The funiculus is, with a few exceptions (Orchideae), penetrated by an axial fibro-vascular bundle which usually ceases at the base of the nucleus. The external form of the ovule when in a state for fertilisation is very various. Independently of outgrowths of various kinds at the funiculus and the integuments, the direction of the nucleus (together with its coatings), with respect to the funiculus, is of especial importance. The ovule is orthotropous when the nucleus is in a direct line with the funiculus, and the apex of the nucleus is the apex of the entire ovule. Much more frequently the ovule is anatropous ; i.e. the apex of the nucleus, and therefore the micropyle which projects beyond it, faces the point of origin of the funiculus, which runs along the side of the nucleus, so that the ovule appears as if sharply curved at its base ; and the integuments (or at least the outer one), have united in growth with the ascending funiculus, which, so far as this union is complete, is termed the Rapf.^ ; the nucleus itself being in this case straight. Much less com- mon is the campy lotropoiis ovule, where the nucleus itself (together with its coatings) is curved ; its apical part, and therefore its micropyle, facing the base, but without any lateral cohesion with the funiculus. These are, however, only the most striking forms, which are united by transitional states. The place from which the ovules spring is called the Placenta, and belongs to the axis of the flower, or more commonly to the carpels themselves. The placentae often do not show any peculiar phenomena of growth ; but more commonly they project like cushions, and may thus assume the appearance of special organs, finally becoming detached from the surrounding tissue. While, after fertilisation, both the endosperm and the embryo are undergoing simultaneous develop- ment in the embryo-sac, the former most commonly increases considerably in size, and supplants the surrounding layers of tissue of the nucleus (sometimes even of the inner integument) ; and the tissue of the integument which is not displaced, or usually only certain definite layers of it, becomes then developed into the Testa. If a portion of » [For instances of the production of pollen-grains in abnormal positions, even in ovaries or in the ovules themselves, see Masters, Vegetable Teratology, Ray Soc. London 1869, pp. 182-188. —Ed.] 428 PHANEROGAMS. the tissue of the nucleus, tilled with food-materials, remains unchanged until the seed is ripe, it is distinguished as the Perisperm ; its food-materials, although lying outside the embryo-sac, are consumed by the embryo during germination ; and the perisperm may then act physiologically as the representative of the endosperm^ The seeds, e.g., of Cannaceae and Piperaceae, contain perisperm. Sometimes the ovule, during the period of its development into a seed, is enveloped from below by a new coating, which usually itself surrounds the tough testa as a soft mantle, and is termed the Aril. Of this* nature is the red pulp which surrounds the hard-shelled seed of the yew ; and the origin is the same of the so-called 'mace' of the nutmeg, the seed oi Myristicafragrans. If we now turn our attention to the morphological nature of those structures from w^hich the ovule immediately springs, we find a considerable variety. Only rarely does the orthotropous ovule appear as the prolongation or terminal structure of the floral axis itself, so that the nucleus forms directly the vegetative cone of the latter, as in Taxus and Polygonaceae. It is more usual for the ovule to grow laterally on the floral axis, thus corresponding in position to a leaf, as in Juniperus, Primulaceae, and Compositae. But the most common case is where the ovules spring from undoubted leaves — the carpels— and usually from their margin, like pinnae from the leaf (this is very clear, e.g. in Cycas), more rarely from their upper (or inner) side (as in Butomus, Akebia, Nymphaea, &c.). If the ordinary morphological definitions are applied to these relationships, we should have in the first-named case ovules of an axial nature, or they would be metamorphosed caulomes^; where they spring laterally from the axis, they would have to be considered as metamorphosed entire leaves ; and where they proceed laterally from the margins of carpellary leaves, as metamorphosed pinnae. For those ovules which spring from the surface of carpels there is no clear analogy with any purely vegetative structures {i. e. with any that do not subserve the purpose of fertili- sation) ; though in this case we may be reminded of the sporangia of Lycopodiaceae. It appears, how^ever, possible to regard some ovules, as, for instance, those of Orchideae, as metamorphosed trichomes (like the sporangia of Ferns and Rhizocarps). The ovules, finally, of some Cupressineae, which appear to have an axillary position on the carpels, have not yet been sufficiently investigated with respect to their true relationships. In some cases the morphological interpretation is supported by malformations which not unfrequently occur. Cramer, to whom we are indebted for an admirable investigation of this question, has shown that the ovules of Primulaceae and Compositae, which arise laterally beneath the apex of the axis of the flower, become gradually transformed into entire leaves of the ordinary form ; and that in the same manner the ovules of Delphi- nium, Melilotus, and Daucus, which spring laterally from the margins of the carpellary leaves, may become developed into ordinary parts of the lamina, as laciniae or leaflets. It appears on the other hand significant that nothing of the kind has yet been observed in those ovules which have been interpreted above as metamorphosed portions of the axis or as trichomes. The development not only of normal, but still more plainly that of ab- normal ovules, shows further that a morphological distinction exists between the nucleus on the one hand and the funiculus together with the integuments on the other hand. In those anatropous ovules which may be regarded as metamorphosed leaves or parts of leaves, the nucleus makes its appearance of a new lateral structure inserted on the body 1 [The endosperm and perisperm are generally both included in English text-books under the term * albumen,' a term vs^hich should by all means be avoided, as conveying the idea of a definite chemical composition, whereas that of the endosperm varies greatly. — Ed.] ^ Cramer, — Bildungsabweichungen bei einigen wichtigeien Pflanzen-familien, u. die morpholo- gische Bedeutung des Pflanzeneies (Zurich i864\— is inclined to consider all ovules as metamor- phosed leaves or parts of leaves. To this view I have already expressed some hesitation in the first edition of this book ; the description here given, which differs from the earlier one, is derived as much as possible from direct observation. PHANEROGAMS. ^20 of the ovule, and when this latter becomes developed in a leaf-like manner it appears as an outgrowth of the surface of the leaf. This fact, the morphological importance of which was first insisted on by Cramer, is however not universal, as is especially shown in the development of the ovules of Orchideae, the nucleus of which unquestionably cor- responds to the apex of the entire ovule, although it becomes anatropous by subsequent curvature ; still less possible does it appear to consider the nucleus of the orthotropous ovule of Taxus and Polygonaceae as a lateral formation, since it is obviously an elongation of the apex of the floral axis (see Angiosperms). The Carpellary Lea'ves are the foliar structures of the flower which stand in the closest genetic and functional relationship to the ovules. They either produce and bear the ovules, or are constructed so as to enclose them in a chamber, the O'vary, and to form the apparatus for the reception of the pollen, or Stigma. The distinct morpho- logical significance of the carpellary leaves is clearly seen by a comparison of the genera Cycas and Juniperus. In Cycas the carpels resemble the ordinary leaves of the plant, and the ovules are produced on their margins and remain entirely exposed ; in Juniperus the ovules spring from the floral axis itself, corresponding, even in their position, to a w'horl of leaves, but the preceding whorl of carpellary leaves swell up after fertilisation, and envelope the seeds in a pulpy mass, the berry-like 'fruit' of these plants. In Primulaceai the ovules spring from the elongated floral axis itself, and thus correspond in their position to entire leaves ; they are however enclosed, even at the period of their formation, by an ovary, consisting of the carpels and an elongated style bearing the stigma. In most other Dicotyledons and Monocotyledons the ovules are seated on the revolute margins of the carpels which have grown together into an ovary, and which therefore in these cases both produce and enclose the ovules. But notwithstanding these very considerable morphological diflerences, the carpellary leaves are always alike physiologically in being excited by fertilisation to further development during the maturing of the seeds, and in taking a certain share in their future history. Pollhiation and Fertilisation. The mutual action on one another of the pollen and the embryonic vesicle of Phanerogams, the latter already formed in the embryo-sac, results in two phenomena of extreme importance, to be carefully distinguished from one another: Pollination and Fertilisation. By Pollination is meant the conveyance of the pollen from the anthers to the stigma of Angiosperms or to the nucleus of Gymno- sperms. The pollen is detained there by a viscid substance, or often by hairs, and the emission is thus brought about of the pollen-tube which in Gymnosperms penetrates at once the tissue of the nucleus, but in Angiosperms grows downwards through the tissue of the stigma and the frequently very long style in order to reach the ovules ; it then forces itself into the micropyle and advances as far as the embryo-sac. It is only when it reaches the embryo-sac (in Gymnosperms however it penetrates still more deeply) that fertilisation of the embryonic vesicle results. A considerable time, occa- sionally even months, often elapses between pollination and fertilisation ; but commonly only a few days or hours. Pollination is rarely effected by the wind alone; in this case large quantities of pollen are produced in order to secure the result, as in many Coniferse ; in a few cases the pollen is thrown on to the stigma by the bursting of the anthers {e.g. in some Urti- caceae) ; but the means usually employed is that of insects. For this purpose special and often very complicated contrivances are met with to allure insects and attract them to visit the flowers; and at the same time the object is accomplished of always conveying, where possible, the pollen to the stigma of a different flower to that which produced it (even when they are hermaphrodite). In reference to this object the parts of the flower also assume definite forms and positions, which will be followed out further in Book III. Here it need only be mentioned that insects are especially attracted to visit flowers by the nectar secreted in them ; this usually sweet juice is generally produced deep down among the foliar structures of the flower, and the form of the parts is generally so contrived that the insect, while it is obtaining the nectar, must 430 PHANEROGAMS. place its body in certain definite positions by which it at one time brushes the pollen out of the anthers, at another time attaches it to the stigma of another flower. The diversity in the forms of flowers depends especially on these relationships, a com- paratively simple plan of structure lying at the base of them all. The organs which secrete the nectar, the Nectaries, are therefore of extreme importance in the life-history of most Phanerogams; they are, nevertheless, usually very inconspicuous, and, — which is very significant with respect to the relationship of morphology with physiology, — not- withstanding their enormous physiological importance, they are attached to no definite part of the flower in a morphological sense ; almost every part is able to perform the function of a nectary. This term therefore does not denote a morphological but a purely physiological idea. The nectary is usually only a small spot at the base of the carpels (as in Nicotiana), or of the stamens (as in Rheum), or of the petals (^e.g. Fritil- laria) which, without becoming more prominent, produces the nectar ; but frequently it is in the form of glandular protuberances of the floral axis between the insertion of the stamens and petals (as in Gruciferae and Fumariaceae). A particular organ, e.g. a petal, is often transformed, for the purpose of secreting and storing up the nectar, into a hollow receptacle, forming a spur-like protuberance [e. g. ViolaJ ; or all the perianth- leaves become developed into hollow^ or pitcher-like nectaries (as in Helleborus), or they assume the most w^onderful forms, like the petals of Aconitum. Even before fertilisation, pollination is usually followed by striking changes m the parts of the flow^er, particularly in the gynseceum, and especially when the parts con- cerned are delicate ; thus the stigmas, style, and corolla wdther, the ovary swells up (as in Gagea and Puschkinia), and the like. The most striking result of pollination is shown in many Orchideae, where the ovules are only formed as a consequence of this process. Those changes however which are excited by the entrance of the pollen-tube into the embryo-sac, in other words by Fertilisation, are still more energetic and varied ; the embryonic vesicle developes into the embryo ; the endosperm— formed previously in Gymnosperms— originates in Angiosperms only subsequently to fertilisation; the ovules grow along with the ovary, their layers of tissue are diiTerentiated, become lignified, pulpy, dry, &c. The increase in size of the ovary, w^hich is frequently enormous (in Cucurbita, Cocos, «&c., several thousand times in volume), shows in a striking manner that the results of fertilisation extend to the rest of the plant, in so far as it aff'ords the materials of nourishment. Striking changes in form, structure, and size take place after fertilisation, especially in the carpels, placentae, and seeds ; but very frequently similar changes result also in other parts. Thus, e.g., it is the receptacle that constitutes the fleshy swelling which is called the strawberry, on the surface of which are seated the small true fruits ; in the mulberry it is the perianth of the flowers that swells up to form the succulent coating of the fruit ; in Taxus it is a cup-shaped outgrowth of the axis beneath the ovule (the aril) that surrounds the naked seed with a red fleshy coating, &c. Popular usage includes under the term Fruit all those parts which ex- hibit a striking change as the result of fertilisation, especially when they separate as a whole from the rest of the plant ; in ordinary language the strawberry, as well as the seed of the yew surrounded by its aril, the fig, and the mulberry, are all fruits. Botanical terminology limits the idea of Fruit within narrower boundaries, which, however, are not yet sharply defined. In the most exact use of botanical terms, the whole of the gynaeceum which ripens in consequence of fertilisation may be termed the Fruit. When the gynaeceum consists of coherent carpels or of an inferior ovary, the flower produces a single entire fruit ; if the carpels do not cohere, each forms a part of the fruit, or a fruitlet. This limitation of the term is often, however, inconvenient ; and it would seem preferable to give it a definition which will vary in the diff'erent sections. The point to be most clearly borne in mind by the student is that the fruit is not a new plant-structure. All the parts of the fruit which are morphologically de- terminable originate and assume their morphological character before fertilisation ; the result of fertilisation is merely a physiological change in the parts. The only new PHA NER OGAMS. a , i parts in a morphological sense are the embryo and the endosperm, which are pro- duced in the ovule. The Inflorescence. When a shoot which has previously formed a large number of foliage-leaves terminates in a flower, the flower is said to be terminal ; if, on the other hand, a lateral shoot developes at once into a flower, with one or at most a few bracteoles beneath it, the flower is termed lateral. Sometimes the first primary axis which proceeds from the embryo terminates in a flower ; but more often the axis continues to grow, or its growth comes to an end, without forming a flower ; it is only lateral shoots of the first, second, or a higher order that terminate in flowers. In the first case the plant may be termed, in reference to the formation of its flowers, uniaxial, in the other cases bi-, tri-axial, &c. When a plant produces only terminal flowers, or when the lateral flowers spring from the axils of single large foliage-leaves, they are said to be solitary. When, on the other hand, the flowering branchlets are densely crowded, and the leaves within this region of ramification are smaller and of a different form and colour from the others, or are entirely absent, an Infloresceyice arises in the nar- rower sense of the term, usually sharply diflferentiated from the vegetative region of the plant, and not unfrequently assuming very peculiar forms which require a special terminology. This occurs however only rarely among Gymnosperms, the formation of nniltifloral inflorescences of peculiar form being characteristic of the more highly developed structure of Angiosperms ; and it will therefore be convenient to defer a more detailed classification and definition of inflorescences until we are treating of that class. With reference also to the Forms of Tissue, one point only need be mentioned here, in which Gymnosperms and Angiosperms agree. The Fibro-'vascular Bundles of Phane- rogams exhibit the characteristic peculiarity that every bundle which bends outwards to a leaf is only the upper arm of a bundle which runs downwards into the stem ; in other words, tve have here ' common ' bundles, each of which has one arm that ascends and bends out into the leaf, and another which descends and runs down into the stem ; the latter is called by Hanstein the ' inner leaf-trace' [see p. 134]. In the most simple cases {e.g. in most Conifera^) only one bundle bends out into each leaf; but when the inser- tion of the leaf is broad, or the leaf is large and strongly developed, a larger number of bundles pass from the stem into the leaf, in which they ramify when the lamina is broad. The bundles are usually thicker at the spot where they pass from the stem into the leaf than lower down in their course. Each bundle of this kind may pass downwards through only one internode or through several; in the latter case an internode with several leaves standing above it contains the lower parts of bundles which bend outwards above into leaves of diff'erent height and diff'erent age. The descending foliar bundle seldom has its lower extremity free; it is usually attached laterally to the middle or upper part of a lower (or older) bundle. This may take place by the bundle splitting below into two arms which anastomose with the lower bundles ; or the thin ends of the descending bundles may intercalate themselves between the upper parts of older foliar bundles ; or each bundle may bend right or left and become finally joined laterally to a lower bundle. In this manner the foliar bundles, originally isolated, are united laterally in the stem into a connected system ; and this, when copiously developed, gives the impression of having arisen by branching, whereas it arises in fact from the coalescence of separate portions originally distinct. Besides the descending arms of the common bundles, others may however occur in the stem of Phanerogams ; first of all net-works (as in Grasses) or girdle-like reticula- tions (as in Rubiace^ or Sambucus) are frequently formed in the nodes of the stem by horizontal bundles. Furthermore, longitudinal bundles may become diflferentiated in the stem, which have nothing to do with the leaves; and the mode of formation of these 'cauline bundles' may vary greatly. They originate either at an early period m the primary meristem of the stem, immediately after the foliar bundles and m the pith (as in Begoniaceae, Piperacese, and Cycadeae), or only at a much later period in the ;:: PHANEROGAMS. outer layers of the stem when this has continued to increase in thickness, outside the foliar bundles (as in Menispermaccce, Aloineae, and Dracaena). The further development of the foliar bundles varies in Monocotyledons on the one hand and in Gymnosperms and Dicotyledons on the other hand. In the former they are closed ; in the latter a layer of formative cambium remains, which, in stems that increase rapidly in thickness and become woody, usually prolongs itself across the medullary rays so as to form a perfect ring (the cambium-ring), and then produces regularly new layers of phloem on the outside and of xylem on the inside. In the primary roots and the stouter lateral roots of Gymnosperms and Dicotyledons, an increase of thickness also takes place by the subsequent formation of a closed cambium-ring, which, like that of the stem, is not found in Cryptogams, and commonly leads to the formation of strong persistent root-systems, which are more often replaced physiologically in Monocoty- ledons by rhizomes, tubers, and bulbs. With the persistent increase in thickness is connected, finally, the active and extensive production of cork, a process foreign both to Cryptogams and to Monocotyledons. It will be more convenient, however, to defer the special discussion of these points also until we are treating of the characteristics of the separate classes. The distinguishing characteristic of Phanerogams, as contrasted with Cryptogams, lies in the formation of the Seed. This organ is developed from the ovule, which, in its essential part the nucleus, produces the Embryo-sac, and in this the Endosperm and the Embryonic Vesicle. The latter is fertilised by the Pollen-tube, an outgrowth of the Pollen-grain, and, after first growing into a Pro-embryo, produces the Embryo. The phanerogamic plant which is differentiated into Stem, Leaves, Roots, and Hairs, corresponds to the spore-forming (asexual) generation of Vascular Cryptogams; the Embryo-sac to the Macrospore ; the Pollen-grain to the Microspore ; the Endosperm is equivalent to the female Prothallium ; and the Seed unites in itself, at least for a time, the two generations, the Prothallium (Endosperm), together with the young plant of the second (sexual) generation (the Embryo). Flowering Plants may be first of all classified as follows : — I. Phanerogams without an Ovary. The ovules are not enclosed before fertilisation in a structure (the Ovary) resulting from a cohesion of carpellary leaves. The endosperm arises before fertilisation, and forms archegonia (/. e, ' corpuscula '), in which the embryonic vesicles originate. The contents of the pollen-grains are divided before the formation of the pollen-tube, corresponding to the formation of the microspores of Selaginella. Glass XI. Gynmosperms. The first leaves produced from the embryo are arranged in whorls of two or more. A. CycadecB. Branching of the stem very rare, or entirely suppressed ; leaves large, branched. B. ConifercB. Axillary branching copious, but not from all the leaf-axils ; leaves small, not branched. C. Gnetacece. Mode of growth very various ; flowers similar in many respects to those of Angiosperms. II. Phanerogams with an Ovary. The ovules are produced in the interior of a structure (the Ovary) formed by the cohesion of carpellary leaves (often only of one carpel, the margins of which have become coherent), bearing at its summit the stigma upon which the pollen-grains germinate. The endosperm is formed after fertilisation at the same time as the GVMNOSPERMS. 433 embryo, both remaining for a time rudimentary. The contents of the pollen-grain do not divide. The branching is almost always axillary and from the axils of all the foliage-leaves ; it is rarely extra-axi41ary. Glass XII. Monocotyledons. The first leaves produced from the embryo are alternate ; endosperm usually large ; embryo small. Class XIII. Dicotyledons. The first leaves of the embryo form a whorl of two (or are opposite); endosperm very often rudimentary, often entirely absorbed by the embryo before the ripening of the seeds. CLASS XI. G Y M N O S P E R M S. This class embraces, in the orders Cycadeos, Conifera^, and Gnetacese, plants of strikingly diffcronl habil, but evidently closely allied in their morphological structure. In the peculiarities of the mode of formation of their tissue, and espe- cially of iheir sexual reproduction, they occupy an intermediate position between Vascular Cryptogams and Angiosperms, while they approach Dicotyledons among the latter especially in their anatomical structure. The PoUen-graim suggest a homology with the microspores of Selaginella, their contents undergoing before pollination one or more divisions into cells which resemble a very rudimentary male prothallium. One of these cells grows into the pollen-tube when the pollen-grain has reached the nucleus of the ovule. The pollen- sacs are always outgrowths from the under side of structures unquestionably foliar (staminal leaves), and bear a striking resemblance in many cases to the sporangia of some Vascular Cryptogams. They are produced either in larger or smaller numbers or in pairs on a staminal leaf, without cohering in their growth. The Ovule, which is almost always orthotropous, and usually provided with only one integument, appears to be either the metamorphosed end of the floral axis itself, or originates laterally beneath its apex (or is apparently axillary), or it grows from the upper surface or margins of the carpels. These never cohere so as to form a true ovary before fertilisation, although during the ripening of the seeds they often in- crease considerably in size, close together, and conceal the seeds, usually separating again when they are mature in order to allow them to fall out; the cases are, however, not rare in which the seeds remain quite naked from first to last. The embryo-sac is formed beneath the apex of the ovule which consists of small-celled tissue and remains enclosed until fertilisation by a thick layer of the tissue of the nucleus. Sometimes the formation of several embryo-sacs commences in one nucleus, but F f only one of them attains its full development. The Endosperm arises by free cell-formation long before fertilisation in the embryo-sac which is distinguished by its firm wall; but the cells soon become combfned into a tissue and increase by division. Within this mass of tissue, corresponding to the endogenous prothallium of Selaginella, arise the Archegonia (or Corpuscula^) in larger or smaller numbers. Strasburger states that each of these bodies is formed from an endosperm-cell lying at the apex of the embryo-sac, which increases considerably in size and produces the neck and central cell of the archegonium by division. According to the same authority a small upper portion of the central cell beneath the neck is even separated as the canal-cell. Whether, as Strasburger asserts, the whole contents of the central cell are to be considered as the oosphere, or whether, as Hofmeister thinks, the embryonic vesicles arise in it by free cell-formation, must for the present remain unsettled; although the first-named opinion would correspond more closely with the analogy in other respects so remarkable with the heterosporous Vascular Cryptogams-. After the pollen-tube has penetrated the tissue of the nucleus and forced itself as far as the archegonium (corpusculum), where its fertilis- ing material is conveyed to the central cell by diffusion, the Pro-embryo is formed by division of a cell that lies in the lower part of the central cell. The pro- embryonic cells are at first small, but the middle or upper ones develope into long Suspemors, which, pushing the lower ones before them, break through the central cell below, and penetrate into a softened part of the endosperm. Sometimes the suspensors which are produced side by side separate, and each produces at its apex a small-celled rudiment of an embryo. On this account, and also because several archegonia are often fertilised in one endosperm, the unripe seed contains several rudimentary embryos of which, however, only one usually increases greatly in size, the others withering away. During the development of the embryo, the endosperm becomes filled with nutrient materials and increases greatly in size ; the embryo-sac which encloses it grows at the same time, and finally entirely absorbs the surrounding tissue of the nucleus; the integument, or an inner layer of it, becomes developed into a hard shell, while frequently (in naked seeds) its outer mass of tissue becomes fleshy and pulpy and gives the seed the appearance of a drupaceous fruit {e.g. Cycas, Salisburia). The effect of fertilisation not unfrequently extends also to the carpels or other parts of the flower, which grow considerably, forming fleshy or woody coatings to the seeds, or cushions beneath them. The ripe Seed is always filled with the endosperm, in which the embryo lies and is distinctly differentiated into stem, leaves, and root. It fills up an axial cavity of the endosperm, is always straight, its radicle being turned towards ^ [The central celLs of the archegonia of Gymnosperms were discovered by Robert Brown in 1834. He called them corpuscula or embryoniferous areolae (Miscellaneous Botanical Works, vol. I. pp. 567 and 570). The structure of the neck of the archegonium was made out by Hof- meister, who applied to it the term rosette (On the Higher Cryptogamia, p. 411). Archegonium and corpusculum do not seem exactly synonymous, since the latter, properly speaking, is only equivalent to the central cell of the former. Henfrey termed the central cells ' secondary embryo- sacs ' (Elementary Course, 2nd edition, p. 608). — Ed.] ^ More will be said on this subject under Coniferce. G YMNOSPER MS. io:- the micropyle, its plumule towards the base of the seed. The first leaves which the embryonal stem produces stand in a whorl, consisting generally of two opposite, but not unfrequently of three, four, six, nine, or more members. The radicle does not project through the split testa of the seed until the period of germination ; the bud which is formed between the Cotyledons or first leaves at the apex of the stem is forced out by their elongation, the cotyledons still remaining concealed in the embryo, and remaining in it until its food-materials have been completely consumed by the embryo. Sometimes they remain concealed there as organs which have become useless; but in Coniferae they are drawn out by the elonga- tion of the embryonal stem and brought above the surface of the ground, where they unfold as the first foliage-leaves. The cotyledons of Coniferse become green even within the seed in complete darkness, the formation of chlorophyll taking place, as in Ferns, without the assistance of light. It is not known whether the same thing occurs also in Cycadeae and Gnetaceae. The young plant, freed from the seed, consists of an erect stem, passing below insensibly into the verti- cally descending tap-root, from which numerous secondary roots soon proceed in acropetal order, usually forming finally a powerful root-system. The embryonal stem grows vertically uj) wards, and is usually not only unlimited in its growth, but is much stouter than all the lateral shoots, even when these are formed in abundance, as is the case with Coniferae. In the remarkable Gnetaceous Wel- witschia the apical growth however altogether ceases at a very early period, and even the production of new leafy shoots is suppressed, as is usually the case also in Cycadeae. The Flowers are usually developed on small lateral shoots, often of a high order of ramification; terminal flowers occur on the primary stem only in the Cycadeae (and in them not exclusively). They are always diclinous; the plants themselves monoecious or dioecious. The male flower consists of a slender axis usually greatly elongated, on which the staminal leaves are arranged in large numbers usually spirally or in whorls. The female flowers are remarkably different in their external appearance, and usually very unlike those of Angiosperms. A kind of perianth of rather delicate leaves occurs only in Gnetaceae; in Coniferae and Cycadeae it is wanting or is replaced by scales. But what makes the female flowers peculiarly strange, independently of the absence of an ovary, is the elongation of the floral axis, on which the foliar structures are placed not in concentric circles as in Angio- sperms, but in a distinctly ascending spiral arrangement, or in alternating whorls when they are numerous. When only a few ovules are produced on a naked or small-leaved inflorescence, as in Podocarpus and Salisburia, the last trace of resemblance in habit to the flowers of Angiosperms ceases. But to clearly under- stand the matter it is only necessary to retain distinctly in mind the definition of a flower, viz. an axis furnished with sexual organs. On the formation of tissue in Gymnosperms see the remarks at the conclusion of the description of the whole class. F f 2 436 PHANEROGAMS, A. CYCADEy^\ The Embryo, enclosed in the large endosperm, possesses two opposite unequal Gotyledonary leaves, which lie with their inner surfaces face to face, cohering towards their apices. The tendency of the subsequent foliage-leaves to branch is sometimes displayed even in these cotyledons, a rudimentary lamina being formed on the larger one, with an indication of pinnae (as in Zamia, Fig. 313 B'). The seed germinates when laid in moist earth, but only after a considerable interval ; the testa splits at the posterior end and allows the emission of the primary root, which at first grows vigorously downwards, but sometimes assumes afterwards a tuberous form or produces a system of thicker fibrous roots. According to Fig. 313 C borrowed from Schacht, and a more recent statement by Reinke, the branching of the primary root is laterally monopodial ; Miquel, however, asserts the existence of bifurcations of the more slender roots in older plants of Cycas glaiica and Ence- phalartos, which is also confirmed by Reinke's investigations into the history of their development. By the elongadon of the cotyledons which remain in the endosperm and absorb their nourishment from it, their basal parts and the intermediate plumule are pushed out of the seed. The portion of the axis which bears the cotyledons, as well as that which developes above them, remains very short, but a consider- able lateral increase of size takes place beneath the apex due to a large develop- ment of parenchymatous tissue. The stem thus acquires the form of a roundish tuber which it retains even at a later period in some species ; but in most it lengthens in the course of years into an erect tolerably stout column which some-' times attains a height of some metres. This slow increase in height, together with the considerable increase in thickness of the growing end, is correlated with the absence of a tendency to branch as in other similar cases (Isoetes, Ophioglossum, Aspidium Filix-vias, &c.). The stem of Cycadeae usually remains perfectly simple, although old stems sometimes divide into branches of equal stoutness. But when several flowers are formed at the summit, this evidently depends on branching ; and, as far as one is able to judge from drawings, it is probable that this branching is dichotomous. In old or sickly plants small bulbous or tuberous gemmae are not ' Miquel, Monographia Cycadearum, 1842. [Ditto, On the Sexual Organs of the Cycadaceoe ; Journ. of Bot. March and April 1869.] — Karsten, Organogr. Betracht. iiber Zamia muricata, Berlin 1857. — Mohl, Bau des Cycadeen-stammes (Vermischt. Schrift. p. 195). — Mettenius, BeitrJige zur Anatomic der Cycadeen (Abhandl. der kiinigl. Sachs. Gesellsch. der Wissensch. vol. VII, i86r). — [W. C. Williamson, Contributions towards the history of Zamia gigas, Trans. Linn. Soc. vol. XXVI, 1870. — Carruthers on Fossil Cycadean Stems from the Secondary Rocks of Britain, ibid.'\ — On the structure of the pollen see Schacht, Jahrb. fiir wissensch. Bot. vol. II, p. 142 et seq. — Kraus, Ueber den Bau der Cycadeenfiedern (Jahrb. fiir wissensch. Bot. vol. IV). — Reinke in Nachrichten der konigl. Gesellsch. der Wissensch. in Gottingen, 1871, p. 532. — De Bary, Bot. Zeitg. 1870, p. 574. — Juranyi, Bau u. Entwickelung des Pollens bei Ceratozamia (Jahrb. fiir wissensch. Bot. vol. VIII, CFCADE.E. 437 unfrcquently found at the base of the stem under or above ground, the morphological nature of which is still doubtful ; in Miquel's opinion it is not impossible that they spring from old leaf-scales, and have therefore nothing to do with the branching of the stem. The whole of the surface of the stem is furnished with leaves arranged spirally; no internodes can be distinguished. The leaves are of two kinds; dry, brown, hairy, sessile, leathery scales of comparatively small size, and large, stalked, pinnate or pinnatifid foliage-leaves. The scales and the foliage-leaves alternate periodically ; a rosette of large foliage- leaves is produced annually or bien- 'nially, and among these the terminal bud of the stem is enveloped with scales, under protection of w^hich the new whorl of foliage-leaves is slowly formed. This alternation begins at once on germination in Cycas and other genera, a number of scale-leaves following the leaf-like cotyledons, and enveloping the bud of the seedling ; after these a pinnate though small foliage-leaf is then usually developed, which is again followed by scales. It is only as the strength of the plant increases after several years' growth that the foliage-leaves are produced in whorls constantly increasing in size, and forming, after the older ones have died off, the palm-like crown of leaves, the scales which stand above them enclosing at the same time the apical bud of the stem. In this bud the foliage-leaves are so far formed beforehand, that when they at length burst the bud they only have to un- fold, this process then occupying only a very short time, while one or two years elapse before the unfolding of the next rosette of leaves. The leaves which proceed from the bud are in Cycas and other genera circinate like those of Ferns ; in others the rachis of the leaf only is rolled up; in others, finally, as Dion, the growth of the leaf is straight, its lateral leaflets being also straight before expansion \ The unfolding is, as in Ferns, basifugal, and, probably in consequence of this, there is also a permanent apical growth and a basifugal development of leaflets. The leaflets are usually simple, and generally stand alternately on the rachis, which is often I to 2 metres long. The mode in which the lamina terminates above points Fig. 313. — Germination of Zayiiia spiralis (after Scliacht, reduced). B commencement of germination, ct the cotyledons coherent above their elongated base, one of them having at its apex (magnified at B') an indication of a pinnate lamina ; C seedling six months old; sa seed, w the primary root, b the first pinnate leaf, x x rudiments of the adventitious roots which afterwards grow upwards. » [This statement is not quite exact. In Zamia and Encephalartos the leaves are not circinate in vernation ; and even in Cycas it is only the leaflets and not the rachis that is so.— Ed.] 438 PHANEROGAMS. to a dichotomous branching of the leaf, the rachis of which may therefore be con- sidered as a sympodium composed of the basal portions of the successive bifur- cations, while the lateral leaflets represent the bifurcations of the lamina of the leaf, the growth of which is arrested and flattened. The whole leaf would therefore be a dichotomous cymose branch-system. Researches into the history of its develop- ment are however wanting, as in the case of the branching of the stem and root. Fig. ^\^.—Kca.xpe\oi Cycas rez'ohita (reduced about ^);y pinnae of the leaf-like carpel; j/t ovules replacing the lower pinnK ; sk' an ovule further developed. The Flowers of the Cycadeoe are always diclinous and dioecious ; both kinds of flowers appear at the summit of the stem, either singly as in Cycas as terminal flowers of the primary stem, or in pairs or larger numbers as in Zamia vitn-icata and Macrozamia spiralis, where they may perhaps be regarded as metamorphosed bifurcations of the stem\ The flower consists of a strong conical elongated axis, * The hypothesis that the male flower of Cycas Rutnphii is one, the leaf-bud by which the stem is prolonged the other bifurcation of the dichotomising apex of the stem, is not supported by De Bary's recent researches. CFCADEM. 439 sometimes supported on a naked peduncle, but densely covered in other parts by a large number of staminal and carpellary leaves arranged spirally. In Cycas the female flower is a rosette of foliage-leaves which have under- gone but slight metamorphosis (Fig. 314), the apex of the stem developing again first of all scale-leaves, and then new whorls of foliage-leaves ; the stem, therefore, grows through the female flower, or furnishes an instance of prolification. The separate carpels are, indeed, much smaller than the ordinary foliage-leaves, but are essentially of the same structure ; the lower pinnae are replaced by ovules, which attain, even before fertilisation, the magnitude of a moderate-sized ripe plum, the Fig. 31s— ^amia muricata (after Karsten). A a male flower {natural size) ; B transverse section of one; C one of its stamens with the pollen-sacs x and the peltate expansion s (seen from below) ; D the upper part of a female flower (natural size); E transverse section of one, s the peltate placenta of the ovules sk; F longitudinal section of a ripe seed ; e endosperm, c cotyledons, x the folded suspensor. fertilised seed acquiring the dimensions and the appearance of a moderate-sized ripe apple, and hanging quite naked on the carpel. Whether the male flower of Cycas also exhibits prolificalion I do not know, and it seems improbable; the very numerous staminal leaves are much smaller, 7 to 8 cm. long, and undivided ; they expand considerably from a narrow base and terminate in an apiculus. They are furnished on the under side with a number of densely-crowded pollen-sacs; the whole flower is from 30 to 40 cm. long. The male and female flowers of the remaining genera of Cycadete resemble fir-cones externally. The comparatively slender floral axis rises as a rachis on a 440 PHA NER OGAMS. short naked peduncle, and on this are seated the numerous staminal or carpellary leaves (Fig. 315). The axis terminates with a naked apex which undergoes no further development (Fig. 315 D). The stamens are, indeed, but small in comparison to the foliage-leaves of the same plant, but are, nevertheless, the largest which occur anywhere among Phanerogams. In Macrozamia, as in Cycas, they are from 6 to 8 cm. long, and as much as 3 cm. broad; they spring, with rather a narrow base, from the floral axis, and expand into a kind of lamina, terminating in an apiculus (Macrozamia) or in two curved points (Ceratozamia), or the lower part of the stamen is thinner and stalk-like and bears a peltate expansion (Zamia). They are also distinguished from the stamens of most other flowering plants by their persistence, becoming lignified and often very hard. The numerous pollen-sacs on the under side of the stamens are usually collected into small groups numbering from two to five, like the sori of Ferns, these again forming larger groups on the right and left side of the leaf. The pollen-sacs are globular or ellipsoidal, usually about i mm. in size, and are attached with a narrow base to the under side of the stamen ; Karsten states that in Zamia spiralis they are even stalked. They dehisce longitudinally, and are in all respects much more like the sporangia of Ferns than the pollen-sacs of other Phanerogams, from which they also differ in the firmness and hardness of their wall. The mode of development of the pollen-sacs and pollen-grains of Cycadeae was till lately unknown ; it has only quite recently been observed by Juranyi in Ca'atoza?nia longifolia. The pollen- sacs are formed on the under side of the stamens in the form of small papfllse, probably consisting from the first of several cells over which the epidermis of the surface of the leaf is continuous. The inner tissue is next diff"erentiated (as in the sporangia of Lycopodiaceae, Equisetaceae, and Ophioglossacese) into an outer layer of smaller cells enclosing a larger-celled tissue; since the cells of the latter continue to grow and divide in all directions, the mother-cells of the pollen are finally isolated, but densely crowded together, as in Dicotyledons. The mode of division of the mother-cells is nevertheless more like that of Monocotyledons in this respect, that they first of all split up successively into two daughter-cells, each of which again undergoes bipartition. The first division-wall is formed, as in Dico- tyledons, by the slow growth of an annular ridge of cellulose, formed in the depres- sion produced by the previous constriction of the protoplasm of the mother-cell; but in each of the two daughter-cells the second partition appears to be formed simultaneously, as in Monocotyledons. The four young pollen-cells are now freed by the rapid absorption of the cell-wall which surrounds and separates them. The pollen-grains, when free from their mother-cells, are unicellular and spherical ; but, during their further growth, the contents, enclosed by an extine and intine, divide into two cells, a smaller and a larger one, each possessing a nucleus. The smaller of these two cells, lying on one side against the intine of the pollen-grain, becomes arched on the opposite side, and projects in the form of a papilla into the larger one. This smaller cell now again undergoes a transverse division parallel to the first, and this is sometimes followed by a second; a two- or three-celled body is thus formed, attached on one side to the intine, and projecting into the cavity of the larger' cell, as in Abietineae, from which, however, Ceratozamia differs in the fact that, as in Cupressineae, the large cell, formed by the first division of the CVCADE.E. ._,j pollen-grain, developes into the pollen-tube, the mass of small cells remainino- inactive in the pollen-grain. In Cjras Riimphii, Encephalartos, and Zamia, the pollen-grain also splits up, according to De Bary, into a larger and a smaller cell, the latter also in this case again dividing once, and the larger cell developing into the pollen-tube. The spot where the intine which developes into the pollen-tube breaks through the extine, lies exactly opposite the mass of small cells (the secondary cells of the pollen-grain) ; the extine is in this place thinner, and in the dry pollen-grain deeply folded in, so that the transverse section of the dry pollen-grain is kidney-shaped. During the absorption of water however which precedes the formation of the pollen-tube, the pollen-grain again assumes a spherical form. The carpellary leaves are arranged spirally or in apparent verticils, closely crowded on the axis of the female flower. Those of Cycas have already been described; in Zamia, Encephalartos, Macrozamia, and Ceratozamia, the carpels are much smaller, and each bears only two ovules, attached right and left to a peltate expansion which terminates a slender pedicel (Fig. 315). The ovule is always orthotropous, and consists of a large nucleus and a thick integument the inner layer of which (in contrast to that of other Phanerogams) is penetrated by a number of fibro-vascular bundles. The micropyle is a slender tube, formed by the prolongation of the contracted margin of the integument beyond the summit of the nucleus. According to De Bary's recent researches a second inner inte- gument appears to exist in the case of Cycas revoluta. But little is known about the formation of the embryo-sac, or of the endosperm, which is strongly developed long before fertilisation, or of the large central cells, easily visible to the naked eye (in Cycas from 3 to 4 mm. long), or finally of the long suspensors. The main point is that in all these respects Cycadeae agree essentially with Coniferae. The central cells are formed in large numbers in the same endosperm, but not until the ovule has already attained a considerable size. The suspensors, each of which gives rise to several rudimentary embryos, but only one of which developes into a perfect embryo, may still be detected in the ripe seed as a ball of long threads, the central cells themselves being also discernible even in the ripe seed. In consequence of the form and position of the carpels, the ovules are covered and concealed before and after fertilisation, except in Cycas ; at the period of pollination, which is apparently brought about by insects, the carpels separate from one another, and the micropyle excretes a fluid to which the pollen-grains adhere. The outer layer of the testa is usually fleshy, the inner one hard, and the seed therefore resembles a plum, with its surface often brightly coloured. 4- PHANEROGAMS. B. CONIFERS ^ Gcnninatian. The endosperm surrounds the embryo in the form of a thick- walled sac open at the radicular end ; the embryo lies straight in the central cavity of P re. 316.— />/«7cj- Pi7iea ; / longitudinal section through the middle of the seed, y the micropylar end ; // commencement of gerniMiation, emergence of the root ; /// completion of germination, after the endosperm has been absorbed (the seed lay at too small a distance below the surface, and was therefore raised up by the cotyledons when the stem began to grow) ; A shows the ruptured testa s, B the endosperm e, one half of the testa having been removed, C longitudinal section of the endosperm and embryo, D transverse section at the commencement of germination ; c the cotyledons, tu the primary root, x the embryo-sac pushed out by it (ruptured ni B), he hypocotyledonary portion of the axis, w' secondary roots, r red membrane within the hard testa. the endosperm ; its axis is continuous behind with the rudiment of the primary root, and bears at its anterior end a whorl of two or more cotyledonary leaves, between ' For the structure of the flowers, see R. Brown, On the Phuah'ty and Development of the ibryos in the Seeds of Conifera^: Misc. Bot. Works, London, 1S66, vol. I, pp. 567 576.— H. von CONIFER.E. .^r. which it terminates in a roundish apex (Fig. 316 /). The Taxine^ and most Cupressinece and Araucarieae have two opposite cotyledons, although in some CupressincDe there are from three to nine, and in some Araucarie^ whorls of four cotyledons ; while among the Abietinese there are rarely so few as two, more often four or even as many as fifteen. To refer this larger number of cotyledons to the division of two opposite ones, as Duchartre proposes, is entirely opposed to the other processes of leaf-formation in these plants, especially to the common occurrence of whorls consisting of several leaves on the growing axis of seedlings. When placed in damp soil the endosperm swells up, bursts the testa at the radicular end of the embryo, which is then pushed out by the elongation of the axis, and grows into a strong descending tap-root, from which lateral roots proceed, suc- ceeding one another rapidly in acropetal succession, and subsequently branching. This is the commencement of the root-system of Conifers, which is frequently strongly developed and persistent. After the emergence of the root, the coty- ledons elongate in their turn, push out their bases from the seed and the end of the axis that lies between them ; but they themselves remain in the endosperm until it has been absorbed. In Araiicaria brasiliensis the hypocotyledonary portion of the axis remains short, and the cotyledons remain contained in the seed ; in most Conifers, on the contrary, this portion becomes greatly elongated, making a sharp bend in an upward direction, pierces the soil, and draws the cotyledons with it. As soon as these are exposed to light, the hypocotyledonary portion straightens itself, the whorl of cotyledons expand, and, having become green while still underground, act as the first foliage-leaves of the seedling, the apex of its axis having in the meantime formed a bud with new leaves (Fig. 316). I\Iodc 0/ GrocVlh and External Differentiation. The terminal bud of the stem of the seedling grows more rapidly, though frequently interrupted, than the lateral shoots which arise subsequently. The primary stem is thus a direct prolongation of the axis of the embryo ; it never ends in a flower, but grows indefinitely at the summit, becoming thickened to a corresponding extent by the activity of a cambium- ring, and thus becomes a slender cone attaining a height of 100, 200, or even more feet\ and a diameter at the base of 2 or 3 or as much as 20 feet. On this highly-developed primary axis the lateral axes of the first order are produced ; Mohl, Vermischt. Schrift. pp. 25 and 49. — Schacht. Lehib. der Anat. u. Phys. vol. II. p. 433. — Eichler in Flora 1863, p. 530 [and Nat. Hist. Rev. 1864, pp. 270-290; Fioia 1873, and Trans. Bot. Soc. Edin. 1873. pp.535 -541. — Dickson, Trans. Bot. Soc. Edin.VI, p. 420; New Phil. Journ. i86r, pp. 198, 199. — J. D. Hooker, On the Ovary of Siphonodon in Trans. Linn. Soc. XXII, pp. 137, 138. — Caspary in Ann. des Sci. Nat. 4th series, vol. XIV, p. 200, and Flora 1862, p. 377. — Brongniart, Bull. Bot. Soc. France XVIII, p. 141.— Van Tieghem, Ann. des Sci. Nat. 5th series, vol. X.] For the fertilisation, Hofmeister in Vergl. Unters. 1851 [On the Germination, Development, and Fructification of the Higher Cryptogams, Ray Soc, pp. 400-433]. — Strasburger, Die Befruchtung der Coniferen, Jena 1869. For the pollen, Schacht in Jahr. f. wiss. Bot. vol. II, p. 142.— Strasburger, Ueber die Bestiiu- bung der Gymnospermen, Jenaische Zeitschr. vol. VI. Also in addition : [Zuccarini, Morphology of the Coniferee, Ray Soc. Rep. and Pap. on Bot. 1845.]— Pfitzer, Ueber den Embryo der Coniferen, Neider- rhein. Ges. fiir Natur. u. Heilk. Aug. 7, 1871.— Reinke, Ueber das Spitzenwachsthum der Gymnosperm- Wurzeln, GGttinger Nachr. 1871, p. 530 —[Strasburger, Die Coniferen u. die Gnetaceen ; eine mor- phologische Sludie, Jena 1872.— Eichler, Sind die Coniferen gymnosperm oder nicht? Flora 1873.] ^ [The trunk oi Sequoia (Wellhigtonia) gigantea of California attains the height of 400 feet.— Ed.] 444 PHANEROGAMS. oflen periodically in terminal rosettes (pseudo- whorls) or distributed irregularly and branching again in the same manner. Each primary axis usually grows more vigorously than its secondary axes ; and hence the collective form of the system of branching, as long as the primary axis continues to grow vigorously, is that of a panicle of conical or pyramidal form. While in Cycadese the branching is almost entirely suppressed, the peculiar form and beauty of Conifers depends chiefly on the branching, the more so as the leaves are almost always small and inconspicuous, serving only, as far as the outward appearance of the plant is concerned, as a cloth- ing to the system of branching. The branching is always axillary ; but Conifers differ from Angiosperms in not producing buds in nearly all the leaf-axils ; in Araucaria and some species of Taxus, Abies, and other genera, it is chiefly or exclusively the youngest leaf-axils of a year's growth which produce branches, and these grow vigorously. \\\ Jimiperus co??imunis, indited, buds occur in most of .the leaf-axils, but only a few develope. In Piniis sylvestris and its allies shoots are formed only in the axils of the scale-like lower leaves which are borne exclusively by the primary stem and the permanent woody branches, remaining however very short, and producing two, three, or more acicular foHage-leaves, from the axils of which no lateral shoots are produced. In Larix, Cedrus, and Salisburia, buds are formed in the axils of a considerable number — but not nearly all — of the foliage-leaves, a few growing rapidly, and serving for the development of the primary branch, while others remain very short, and form annually a new rosette of leaves without lateral buds. In Thuja and Cupressus also, which are distinguished by their copious branching, the number of small leaves is still very much larger than that of the axillary shoots. IVIany Conifers exhibit a very regular arrangement of those branches of diflerent orders which arrive at their full development, the symmetry of the whole tree being at the same time increased by their difference in size. The branches of the first order on the upright primary stem are frequently formed in pseudo-whorls of several members at the conclusion of each period of vegetation, the same process being frequently repeated on the branches themselves {e.g. Pinus sylvestris^ Ai-aiicaria brasiliensis, and especially PhyUodadus trichomanoides, and many others) ; more commonly a tendency to bilateral ramification appears on the horizontal branches of the first order (as in Abies pectinata) ; and not unfrequently besides these strong branches from which the framework of the tree is constructed, smaller ones are also formed between them ( longitudinal section of the same (magnified), i' integument, kk nucleus of the ovule, X a rudimentary axillary ovule ; E longitudinal section through a more mature ovule before fertilisation, i integu- ment, kk nucleus, e endosperm, m aril, s upper scales of the envelope. Fig. 319. — Jii7iipe}-tis commiDiis ; A longitudinal section of a male flower, 5 (upper figure) a stamen seen from the front and the outside, (lower figure) seen from the back of the axis ; C longitudinal section of a female flower; a the pollen-sacs, s the peltate lamina of the stamen, b lower leaves of the floral axis, c carpels, sk ovules, kk nucleus, i the integument (A and C X about 12). a single flower. In Angiosperms the flowering shoot usually undergoes a very peculiar development at its summit, the portion of the axis which bears the flower (the receptacle) remaining very short and broad, and the floral leaves and organs of reproduction being formed in positions which diff"er greatly from those of the foliage-leaves; in Coniferae the distinction between a floral and a foliage- shoot is much less, and this is especially conspicuous in the arrangement of the leaves ; if those of the foliage-branches are arranged spirally, so also are usually those of the flowers, as, e.g., in the Abietineoe ; if, on the contrary, as in the Cupressineae, 44- PHANEROGAMS. they occur in alternating whorls, the staminal and carpellary leaves are arranged in the same way. In Jujiiperus communis even the ovules, here the representatives of whole leaves, are arranged in alternating whorls. But, occasionally, as in Taxus, greater differences are to be observed in the phyllotaxis of the flowering shoot as compared with that of the foliage-shoots. The Male Flowers always consist of a distinctly elongated axis provided with staminal leaves, and ending above in a naked apex (Fig. 319 ^). The stamens are mostly more delicate and of a different colour from the foliage-leaves, and are usually divided into a slender pedicel and a peltate lamina bearing the pollen-sacs on its under side, as in Taxus, the Cupressineae, and Abietinese (Fig. 318^, 319^,^, 320 A). The flat expansion at the end of the pedicel may, however, be entirely absent, as in Salisburia (Fig. 317 C), where it is reduced to a small knob on which the pollen-sacs hang. That the parts which bear the pollen-sacs in Coniferae are beyond doubt metamorphosed leaves, is evident not only from their form, but still more from their arrangement, which has already been spoken of. If the staminal Fig. -^fio.— Abies pectinata ; A a male flower, b the delicate bud-scales forming a perianth, a the stamens ; B a pollen-g-rain (after Schacht), e its extine, forming the two large vesicular protrusions bl. leaves of the Cycade;^ show a resemblance in more than habit to the sporangiferous leaves of Ferns, those of Coniferse may perhaps be compared to the parts that bear the sporangia of Equisetacese ; and not unfrequently, as in Taxus, Juniperus, &c., the resemblance of the male flowers to the inflorescence of Equisetum is as striking in external appearance as in the actual agreement between them from a morphological point of view. The pollen-sacs, of the structure and development of which but little is at present known, usually hang, with a narrow base, on the under side of their support, and do not cohere in their growth ; their number is usually much smaller than in Cycadeae, but much more variable than in Angio- sperms ; in the yew the peltate part of the staminal leaf bears from three to eight, in the juniper and most Cupressineae three roundish pollen-sacs (Figs. 318, 319). Those of Pinus, Abies, and their allies lie in pairs parallel or placed obliquely to one another, right and left of the pedicel, which here resembles the connective of Angiosperms; in Araucaria and Dammara, on the other hand, the long sausage- shaped pollen-sacs hang in larger numbers free beneath the peltate limb. The CONIFER.^. 449 wall of the pollen-sacs is usually delicate, and finally dehisces longitudinally to allow the escape of the pollen-grains, which are produced in extraordinarily large numbers, since they have usually to be carried by the wind to the female organs of the same or of another tree. The pollen-grains which happen to fall on the opening of the micropyle of the ovules are retained by an exuding drop of fluid, which about this time fills the canal of the mycropyle, but afterwards dries up, and thus draws the captured pollen-grains to the nucleus, where they immediately emit their pollen-tubes into its spongy tissue. In the Cupressinese, Taxineae, and Podocarpeae this contrivance is sufficient, since the mycropyles project outwardly ; in the Abietineae, where they are more concealed among the scales and bracts, these themselves form, at the time of pollination, canals and channels for this purpose, through which the pollen-grains arrive at the micropyles filled with fluid (c/. Strasburger, /. c). The large number and lightness of the pollen-grains enables them to be carried great distances by the wind ; in the true pines and the Podo- ' Fik.. :!-i — . ( pollen-praiii of Thuja orieitlalis before its escape from the pol!en-sac, / fresh, //, /// after lying in water, the extine e having been stripped off by the swelling of the intine i; B pollen-grain of Pinits Pinaster before its escape, e the extine with its vesicular protrusions bl. carpex their capacity for transport is increased by the vesicular hollow protrusions of the extine, as represented in Fig. 321, /F, F, bl. The Mode of cell- division i?i the interior of the pollen-grain of Coniferce, to which allusion has already been made, is still but imperfectly known; especially now that we know more, through Millardet's researches, of the male prothalhum of Selaginella and Isoetes, it is greatly to be desired that fresh observations should enable us to compare with them the corresponding structures in Coniferse. Schacht asserts that in Taxus, Thuja, and Cupressus, only one division-wall (Fig. 321 A) arises at right angles to the longest diameter of the pollen-grain; one of the daughter- cells is much smaller than the other, and the larger of the two developes into the pollen-tube. In Larix, Pinus, Abies, and Podocarpus, two daughter-cells of very different size are also first of all formed. But the septum between them arches into the cavity of the larger one, and the protuberance (the papilla of the smaller cell) is cut off by a partition ; a third cell is thus formed lying in the cavity of the larger of the two primary cells, grows at its apex, and again divides. A three- or four-celled body (or row of cells) is thus formed in the cavity of the pollen-grain, Gg 450 PHANER OGA MS. and is attached to the wall of the grain by a very small basal cell ; the apical cell (Fig. 320 B,y) finally enlarges and developes into the pollen-tube. The basal cells of this row appear, after they have lost their contents (in Pinus and Abies), like narrow slits in the thick wall of the pollen-grain, a phenomenon which requires further explanation (see Fig. 320, ^, 17, and 321 IV, e). A peculiarity which dis- tinguishes the pollen-grain of Conifers from that of Angiosperms lies in the rupture and final stripping off of the cuticularised extine by the swelling of its intine (Fig. 321, I, II, III). Even in this apparently insignificant fact a resemblance is again seen to the microspores of Cryptogams, and especially to those of Marsileaceae, in which the swelling endospore protrudes from the exospore. The structure of the Female Floivers is very different in the different sections of Coniferse, and in some cases the homology of the separate parts is still doubtful. The position of the ovules, as far as can be judged from advanced stages of develop- ment, is, in particular, very variable, and with this is again connected the fact that different opinions may be entertained as to the part which should be called the carpel. The following description of these structures, a full discussion of which is not permitted by our limited space, is drawn immediately from the observation of advanced stages of development; it is possible, however, that the direct obser- vations of the most rudimentary stage will cause an alteration in some points. The female flowers of Taxus spring from the axils of foliage-leaves belonging to elongated woody shoots. They have the form of short branches covered with decussate scale-like bracts (Fig. 318, C, D); the axis of the shoot ends in an apparently terminal ovule, the nucleus of which has the appearance of being the vegetative cone of the axis. In Salisburia the female flowers spring from the axils of foliage-leaves belonging to short lateral branches which annually produce new rosettes of leaves (Fig. 317 A)] the single flower consists of a stalk-like elongated axis which bears immediately beneath its apex two or more rarely three lateral ovules. Neither in this genus nor in Taxus are there any foliar structures close to the ovules which either from their position or from any other circumstance can be regarded as carpels. In the genus Podocarpus small flowering shoots are developed, springing in P. chinensis (according to Braun) from the axils of foliage- leaves, in P. chiliiia from the axils of very small scale-leaves at the end of elongated leafy shoots ; they consist of an axial structure slender and stalk-like below, club- shaped above, and bearing three pairs of very small decussate scales. The floral axis terminates between the upper pair; the ovules, in this case anatropous, with their micropyle turned downwards and towards the floral axis, spring from the axils of this pair; one ovule however is usually abortive, and the flower becomes one- seeded. In Phyllocladus the lower lateral branchlets of the leaf-like flattened shoots are transformed into female flowers which are raised upon a pedicel and are swollen above into the form of a club, the large ovules standing (according to a drawing of Decaisne's^), in the axils of small leaves. In these two genera the small scales from the axils of which the ovules spring may be regarded as carpels, if it is thought necessary to assume the existence of these organs. * [See Le Maout and Decaisne's Descriptive and Analytical Botany, edited by Dr. Hooker, London 1873, p. 747.] CONIFERM. 451 The ovules of Junipenis communis (Fig. 319, C) stand in whorls of threes beneath the naked extremity of the floral axis, the flower springing as a litde shoot from the axil of a foliage-leaf, and its axis bearing whorls of three leaves. The ovules apparently alternate with the three leaves of the upper whorl, and hence must, from their position, be themselves considered as metamorphosed leaves ; these leaves of the upper whorl swell after fertilisation, grow together and become fleshy, forming the pulp of the juniper-berry in which the ripe seeds are entirely enclosed ; they may therefore be termed carpels. In the other Cupres- sinese the flower consists of decussate whorls of two or three leaves, which grow considerably after fertilisation and attain a considerable size, enveloping the seed and forming a pericarp which may therefore correctly be said to be formed of carpels. In Sabina the pericarp is fleshy and berry-like, as in Juniperus; in the other genera, on the other hand (Thuja, Cupressus, Callitris and Taxodium), the carpels become woody artd assume the form of stalked peltate scales, or of valves separating from one another longitudinally (Frenela) ; these are closely approximate during the development of the seed, but afterwards open to allow the ripe seeds to fall out. The erect ovules of Cupressinese sometimes appear to stand in the axils of the carpels; but it is clear in other cases that they Fig. yi-i— Callitris quadrivalvis ; A female flower (mafi:nified) ; d d two pairs of decussate leaves (carpels) in tlic axils of which are six ovules (A V) ; B vertical longitudinal section of an ovule through its broader diameter; A' A' the nucleus still without an embryo-sac ; i the tubular elongated integument with the micropyle m. spring from the carpels themselves, either low down near their point of insertion or at a greater height. In Sabina and Callitris quadrivalvis (Fig. 322) only two decussate pairs of carpels separate like a star at the time of flowering; in Sabina the ovules stand in pairs in the axils of the two lower carpels, right and left of their median line, some of them being frequently abortive. In Callitris quadri- valvis a pair occurs on each of the lower carpels and a pair higher up; but this position can only be explained by further investigation of the history of their development. In Thuja and Cupressus there are three or four decussate pairs of carpels, in Taxodium a still larger number; in Thuja and Taxodium two erect ovules are situated at the base of each of the central pairs of carpels, springmg from the right and left of their median line ; in Cupressus there are a considerable number at the base of each carpel. In Juniperus drupacea and Fretiela verrucosa the fruits (in the collection at Wiirzburg) consist of alternating whorls of three carpels, opening, in the last species, after the seeds become ripe, like a six-lobed capsule. Each carpel is swollen on its inner side into a thick placenta ascending from the base to the apex, and bearing numerous winged seeds which stand in transverse rows of threes ; there are from four to six of these rows on each carpel, the whole inner side therefore bearing seeds nearly up to the apex. Gg 2 452 PHANEROGAMS. So far as the relative positions of the parts of the (lower can be explained without going back to their earliest stage, a great diversity is thus shown in the two families of Taxineae and Cupressinese ; the ovule is terminal in Taxus, lateral beneath the summit of the axis in Salisburia, carpellary leaves appearing to be entirely absent. In Podocarpus and Phyllocladus they are indicated indeed, as small scales, the ovules springing from their axils ; but they are small and do not at any time constitute a pericarp. A structure of this kind, in the form of a berry or of a chambered woody fruit, is indeed formed after fertilisation in the Cupressinese, the carpels either becoming fleshy and growing together (as in Juniperus and Sabina), or becoming woody and closing in laterally by their peltate expansions (as in Cupressus, Thuja, and Callitris), or presenting the ap- pearance of the lobes of a unilocular capsule {e.g. Frenela) ; but the carpels are in these cases at first entirely open. In Juniperus com- viunis the ovules form a whorl alternating with the carpels ; in the other genera they stand in pairs or in larger numbers at their base, or cover the whole of their inner side (as Frenela). In the Abietineae the well-known cones are the female flowers (or rather fruits). The cone is a metamorphosed shoot, its axis bearing a number of crowded woody scales arranged spirally, the ovules arising on them rarely singly, usually in pairs, occasionally in larger numbers. In the Pinese (Abies, Picea, Larix, Cedius, and Pinus) the seminiferous scales (Fig. 323, A, B, s) appear as axial structures in the axils of bracts (c) which spring from the axis of the cone ; but the examination of very young cones of Abies peclinaia shows that the seminiferous scale itself arises as a protuberance at the base of the bract (r), and is therefore not axillary. While the bract afterwards grows very little or not at all, this protuberance increases greatly, and produces on its upper surface two ovules which are attached to it by one side with the m.icropyle towards the axis of the cone. The seminiferous scale of these genera must therefore be considered as a greatly developed placenta growing out of a carpel (Fig. 323 A,B c) which is very small or even abortive'. According to Fig. -^u-i.-^ Abies pecti7iata (after Schacht). A a leaf detached from the female floral axis seen from above, with the seminiferous scale s bearing the ovules sk (magnified) ; B upper part of the female flower (or cone) in the mature state; sp floral axis or axis of the cone, cits leaves, s the largely developed senii))iferouB scales ; C a ripe seminiferous scale with the two seeds sa, / the wing of the seed (reduced), ^ Braun, Caspary, and Eichler consider the seminiferous scale in Pinus and Larix as itself a flower; i.e. as a short axis which has coalesced with its two carpels, and stands in the axil of the CONIFER.E. 453 this view the whole cone is a single flower with a number of small open carpels (hitherto considered as bracts), which are far outstripped in their growth by their seminiferous placentae (the scales). In the other Abietinese also, the female flowers of which I have had no opportunity of examining, it may be concluded from the descriptions that the cone is a single flower with numerous seminiferous scales arranged spirally, not springing from the axils of leaves, but growing immediately out of the axis of the cone, and therefore themselves leaves and of a carpellary nature. Eichler (/. c.) says, in reference to Dammara, Cunninghamia, Athrotaxis, and Sequoia :— ' The scales of a cone are in these genera all of one kind ; they consist simply of open carpels; and, in order not to introduce confusion into the definition of a flower, the whole of what is found on the axis, in other words the whole cone, must be considered a single flower; and this is also necessary in the case of the Araucarieae, the Cupressineae, and the male "catkins" of all Coniferae^' In Araucaria each scale (or carpel) bears only a single ovule, which, according to Eichler, is so enveloped by it that the only opening left is that of the micropyle which faces the axis of the cone ; in Cunninghamia there are three ovules, in Athro- taxis from three to five, in Sequoia from five to seven, in Sciadopitys as many as seven or eight on one scale, and their micropyle here also faces the axis of the cone. In Dammara the scale bears, according to Endlicher^, only one ovule which, bract (c in our figure"). Tii that cpsc the cone of these genera, in contradistinction to that of the other Conifcrar and of Cycadcx, would be an inflorescence (cf. Caspary in Ann. des Sci. Nat. 4th series, vol. XIV, p. 200, and Flora, 1862, p. 377); but this view I have already contested more in detail in my first edition, p. 427. It is impossible to consider the seminiferous scale of Pinus and Abies itself as a sinj,de carpel. In opposition also to the most recent views of Mohl (Bot. Zeitg. 187 1, p. 2 2\ I cannot bring myself to consider the seminiferous scale of the true Abietineae as a coherent structure formed of two leaves of an undeveloped branch. ^ Eichler thinks that an exception must be made in favour of Podocarpus and Cephalotaxus. 2 [Van Tieghem has been led by studying the distribution of the bundles in the different parts of the female bud of Conifercc to the opinion— different from that expressed by Sachs— that the female flower throughout this group of plants is in every case constructed after a single fundamental type which has undergone various secondary modifications. He has given in a note to his French trans- lation of the present work the following abstract of the conclusions which are worked out in greater detail in his paper already cited in the Annales des Sciences Naturelles. Neither the axis of the female bud nor its leaves or bracts of the first order ever bear ovules. It is always upon structures arising from the axils of these bracts that the ovules make their ap- pearance. This establishes a fundamental distinction between Cycadese and Conifers. In the former group it is always the leaves of the female bud of the first order that produce the ovules directly. While therefore we may regard the female bud in Cycadese as well as the male as contributing a single flower, this does not hold good in the case of Conifers. We may if we please regard the male bud of Coniferre as a single flower, but the female bud is an inflorescence. The stnicture which bears the ovule in Coniferse is always a foliar organ — the first and only leaf of an axis which undergoes no further development. This leaf, which is more or less largely developed beyond the circumscription of the ovule or ovules which it bears is an open carpel an in itself constitutes the whole female flower. It is always inverted, that is to say, it arises upon the suppressed axis which bears it with its ventral face opposite to and united with the ventral face ot the primary bract. When the ovules do not terminate the carpel, it is upon its structurally dorsal— but in respect of position upper— face that they arise, just as it is upon its structurally dorsal— but in respect of position lower— face that the pollen-sacs arise upon the stamen. This is the general type. It remains to consider the principal secondary modifications which are superinduced upon it in the different genera. Tlie axillary branch, which is reduced to its first leaf, is most frequently of the first generation 4 J 4 ^^^ ^^^ OGA MS. like those of Sequoia and Sciadopitys, are inserted near the apex and hangs down free\ The Ovules, as we have already seen, are in the Podocarpese anatropous and furnished with two integuments ; in the rest of Coniferae they are orthotropous and possess only one integument ; in the Cupressinese and Taxinese they are erect, in the Abietineae inverted, with the micropyle towards the base of the scale, to which the ovules are usually attached on one side. In these cases there is no funiculus, and the ovule consists only of the small-celled nucleus and one integument, which usually projects above it and forms a comparatively wide and long micropylar canal, through which the pollen-grains reach the apex of the nucleus, which is sometimes depressed (see Figs. 317, 318, 319, 322). Lateral outgrowths of the integument not unfrequently cause the ovule, and afterwards the seed, to appear winged on both sides, as in Calliiris quadrivalvis (Fig. 322), Frenela, &c. The wing-like appendage of the seed of Pinus and Abies, on the other hand, is the result of the detaching of a plate of tissue from the seminiferous scale, which remains attached to the ripe seed. The Embryo-sac is formed by the considerable enlargement of a cell of the nucleus lying nearly in its axis, and usually at some depth and at a considerable distance from its apex. In the Abiedneae and Juniperus it arises beneath the point at which the integument separates from the nucleus ; the embryo-sac is in these genera usually the result of the transformation of one cell only; while in Taxus, according to Hofmeister, several sacs are always formed ; several cells which lie one over another in a short axial row increase in size, and become isolated and filled with protoplasm ; only one of these, however, usually continues to grow in in respect to the axis of the female bud ; but it is also sometimes of the second (Taxus) and may even be of the third order (Torreya). The carpel itself is either entirely distinct from the parent bract (the Pinece, Taxinece) or the two leaves are united together by their ventral surfaces and are only separate towards their summit {CupressinecE, Sequoiece, Araucariece). This difference merely depends upon a different localisation of the intercalary growth of the two leaves ; it is a difference the same in kind as that which separates a dialypetalous corolla from a gamopetalous one. Whether free or united with the bract, the carpellary leaf bears its ovules sometimes towards its base {CupressinecE), sometimes towards its middle (Piyiece), sometimes towards its summit {Arau- cariece) ; each represents a lobe, more or less developed, of the dorsal face of the carpel. In the TaxinecB the ovules terminate the carpellary leaf; they result in this case from the trans- formation of its whole entire limb, whether each half of the limb forms an ovule (Salisburia, Cepha- lotaxus), or whether the entire limb has only produced a single one (Podocarpus, Phyllocladus, Taxus, Torreya, &c.). In this case it is evidently only the petiole of the ovuliferous leaf which represents the carpel ; if the petiole is long (Salisburia) the carpel is obviously developed ; but if it remains very short (Cephalotaxus, Podocarpus, Phyllocladus, Taxus, Torreya, &c.) the carpel is almost absent — in other words, the carpellary leaf is reduced to a sessile limb completely converted into a single ovule (Podocarpus, Taxus, &c.) or into two ovules (Cephalotaxus). The number of the ovules which each carpellary leaf bears, as well as the number of carpellary leaves themselves, that is to say, of the female flowers which enter into the composition of the inflorescence, both vary, and may even be simultaneously reduced to unity, which is the ordinary case in Taxus. — Ed.] * [For a review of the literature of the question whether the ovules of Coniferce are really naked or whether there is a true ovary, see Eichler, ' Sind die Coniferen gymnosperm oder nicht, in ' Flora' for 1873, translated in Trans. Bot. Soc. Edin. 1873, pp- 535-541. Dr. Eichler here, in opposition to the contrary view of Strasburger, sums up the whole argument strongly in favour of the opinion that the Conifers are really gymnospermous. — Ed.] CONIFERS. 455 order to form the permanent embryo-sac. The nucleus of the embryo-sac is soon absorbed, fresh nuclei being then formed in the parietal protoplasm, and free cell- formation takes place round them. These cells soon unite laterally, grow in the radial direction, and divide in such a manner that the embryo-sac is filled with parenchymatous tissue. In those Coniferae in which the seeds take two years to ripen, as Piniis sylvestris 2>x^A Juniper us communis, the endosperm formed in the first summer is again absorbed in the spring, the protoplasm of the primary endosperm- FIG 324.— Taxus canadensis (after Hofmeister). A longitudinal section through the upper end of the endosperm ee and the lower end of the pollen-tube/, cc the archegonia, ci their stigmatic cells, the left archegonium is fertilised, X rudiment of the pro-embryo (June 5), (X300). B part of the endosperm with an archegonium, the pro-embryo of which V is already further developed, /> the pollen-tube (June 10) (X200); C longitudinal section of a nucleus (June 15), /fe /t nucleus, ee endosperm, p pollen-tube, vv two pro-embryos proceeding from two archegonia (X50). cells is set free by the deliquescence of their cell-walls, and forms by division a number of new cells which, in May of the second year, again fill with parenchy- matous tissue the embryo-sac now considerably increased in size. According to Strasburger's recent researches, the mother-cells of the 'corpuscula' (archegonia' ) arise in the embryo-sac by free cell-formation in the same manner as the first endosperm-cells ; but the septa by which the latter are transformed into a multicellular tissue are not produced. The cells grow, on the other hand, more vigorously, and divide near their apex where they touch the embryo- sac ; a large inner [See foot-note to p. 434.] 45^ PHANEROGAMS. (lower) cell is thus formed, the central cell of the archegonium, and an upper small one, lying next the embryo-sac from which the neck of the archegonium is formed \ In Abies ca?iadensts this neck remains simple and unicellular, and elongates considerably with the increase in size of the surrounding endosperm; but usually the original cell which constitutes the neck divides into several cells which either lie only in one plane (Figs. ^24 A, d, 325 /, ^), the ' stigmatic cells,' or form several layers lying one over another (as in Abies excelsa and Pmiis Pinaster). Seen from Fig. 325. — Juniperus coMityiunis {after Hofmeister). / three archegonia cp close beside one another, in two of them the fertilised embryonic vesicles is imbedded in the upper end, (Astigmatic cells,/ pollen-tube (July 28) (X300). // a similar section, e e the endosperm, v v the pro-embryos ; /// lower end of one of the longitudmal rows of cells of a pro-embryo with the rudmient of the embryo eb ; IV longitudinal section of the nucleus kk, e the endosperm, e' por- tion of the endosperm that is broken up, p pollen-tube, cp the archegonia, v the pro-embryos (beginning of August) (X80). above the neck appears to form a four-celled, or, in Abies excelsa^ even an eight- celled rosette. The homology of the archegonium ' corpuscula ' with the arche- gonium of Vascular Cryptogams, already established by the earlier investigations of Hofmeister, is carried a step further by Strasburger, who discovered the formation also of a canal-cell. He considers that the part of the protoplasmic contents of the large central cell which lies immediately beneath the neck are separated from the rest by division, and a small cell is thus formed shortly before fertilisation {i. e. ^ Hofmeister (Vergleichende Untersuchungen, p. 129) gives a somewhat different account of the origin of the archegonium [Germination, &.C., p. 410]. CONIFERJE. 457 before the access of the pollen-tube to the endosperm) ; this cell being clearly equivalent to the canal- cell so often mentioned in Vascular Cryptogams which is afterwards converted into mucilage \ In Abies canadensis and excclsa and Finns Larix this canal-cell is, according to Strasburger, very evident ; while in the Cupressineae (Thuja, Juniperus, and Callitris), its demarcation from the rest of the contents of the central cell is only slight. As in those Vascular Cryptogams where the ventral part of the archegonium is plunged in the tissue of the prothallium, the neighbouring cells become transformed by further divisions into a parietal layer surrounding the central cell, so the same thing takes place also in the endosperm of Coniferce. In the Abietineag each archegonium is separated from an adjacent one by at least one, often by a large number of layers of cells : those of the Cupressincce, on the other hand (Fig. 325, cf), are in lateral contact. The arche- gonia of Taxus are short ; in those of the Abietineoe the central cell is elongated ; in the Cuprcssinese it becomes angular from the pressure of the adjacent cells. The number of the archegonia which are formed in the endosperm beneath the apex of the embryo- sac is very various; Hofmeister and Strasburger state that in the Abietineoe it is from three to five, in the Cupressineae from five to fifteen (according to Schacht it may even be thirty) ; in Taxus baccata from five to eight. The continuous growth of the surrounding endosperm causes the formation of funnel-shaped depressions above the archegonia, which in some Abietinese are but shallow, in Finns Finaster^ F. Slrobus, &c., deep and narrow. In these species each of the funnel-shaped depressions leads down only to the neck of one archegonium ; in the Cupressineae (Callitris, Thuja, and Juniperus), where they lie closely crowded together, the cluster of them is walled round by the endosperm, and a funnel is formed common to them all, which still remains closed by the cell-wall of the embryo-sac. Fertilisation. The pollination of the ovules takes place before the archegonia are formed in the endosperm ; the pollen-grains, having reached the apex of the nucleus, put out their tubes at first only for a short distance into its tissue ; their growth is then for a time suspended. After the archegonia are completely de- veloped, the pollen-tubes begin to grow again into the endosperm in order to reach them. This interruption of their growth lasts, in those Coniferae whose seeds ripen in a single year, for only a few weeks or a month ; when the seeds take two years to ripen, as in Juniperus sibirica and communis, and Finus sylvestris and F. Strobus, until June of the next year. Whilst the pollen-tubes penetrate through a loose portion of the tissue of the nucleus, their width gradually increases at their lower end, their wall becoming at the same time thicker ; until at length they meet the wall of the embryo-sac which has now become soft, break through it, penetrate into the funnel of the endosperm mentioned above, and attach themselves firmly to the cells of the neck of the archegonia. In the Abietineae and Taxineae each pollen- tube fertilises only one archegonium; and several tubes therefore penetrate into the funnel at the same time ; in the Cupressineae on the contrary one pollen-tube suffices for the fertilisation of the whole group of archegonia beneath the broad ' In Figs. 324 and 325, which are transferred from the first edition, the canal-cell is not indicated. 4 , ') '^ PHA NER OGA MS . lunnel of the endosperm. The tube entirely fills up the funnel and applies itself to the necks of the whole group of archegonia ; short narrow protuberances from the wide pollen-tube now grow into the separate necks of the archegonia, forcing the stigmatic cells from one another and destroying them, and at length reaching the central cell. The same process takes place in the Abietineae and Taxinese ; the pollen-tube, after widening again, becoming narrower and entering the neck of only one archegonium, whence it penetrates finally as far as the central cell. A thin spot may be observed at the extremity of this protuberance of the thick- walled pollen-tube, which obviously facilitates the escape of the fertilising substance by diffusion ; and this is probably assisted by the pressure exerted by the tissue which lies above on the part of the pollen-tube outside the archegonium. Hof- meister states that a few free primordial cells (Fig. 325, /) are sometimes formed in the end of the pollen-tube, which he was inclined to consider as rudimentary indications of mother-cells of antherozoids (corresponding somewhat to those in Salvinia) ; but Strasburger denies the existence of bodies of this kind, and admits only the presence of a number of grains of starch in the protoplasm at the end of the pollen-tube. In reference also to the processes in the central cell of the archegonium, the statements of these two observers differ. According to Hof- meister a number of primordial cells arise in the protoplasm of the central cell, all of which he considers to be embryonic vesicles (oospheres) ; one of- these, however, is distinguished from the rest even before fertilisation by its size and contents; it lies in the upper or middle part of the central cell, but after fertilisation sinks to its base and adheres closely to it, filling up the lower part of the central cell as the rudiment of the embryo, while the remaining embryonic vesicles perish. Stras- burger, on the contrary, considers the whole protoplasmic contents of the central cell as the ' oosphere,' and regards Hofmeister's numerous embryonic vesicles only as vacuoli (vesicles of protoplasm). The effect of fertilisation is manifested first of all in the central cell by the turbidity of the protoplasm and by the formation of granular bodies in it ; these collect in the lowTr part of the central cell, which then becomes separated by a septum from the larger remaining part, and forms the rudiment of the pro-embryo. Our figures, which are borrowed from Hofmeister (Fig. 324,^, X, and 325, /, ei, the rudiment of the pro-embryo mentioned above), may point to both explanations ; that of Strasburger, however, is most in accordance with the processes that take place in the archegonium of the highest Cryptogams, as well as with those in the embryo-sac of Angiosperms, and connects the two. My own observations, however, are not sufficient to decide definitely in favour of one or the other view. The further development of the rudiment of the pro-embryo (Fig. ^24, A, x, and Fig. 325, /, ei), is brought about by longitudinal divisions at right angles to each other, which are soon followed by transverse divisions ; a mass composed usually of three layers of cells is thus formed at the base of the central cell ; the bottom of the cell is broken through by a considerable extension of the uppermost (in Taxus and Juniperus) or middle cells (Abietineae) of the pro-ernbryo (Fig. 324, B, v) ; these cells elongate, continue to grow, and transverse divisions are formed in them (Fig. 325, IV, v), and penetrate into the softened part of the endosperm, bending in different directions. In Taxus the elongated cells of the pro-embryo CONIFERM. .-Q remain for a long time adjacent and united, the pro-embryo producing only a small- celled rudiment of an embryo at its apex (Fig. 324, B, C) ; while in the Abietinecs (Abies, Pinus) and Cupressineae (Thuja, Juniperus) the elongated cells of the pro- embryo separate from one another, continue to grow in this condition, and each forms the rudiment of an embryo at its apex^ (Fig. 325, IV, v, III). By this means several embryos can be produced from one embryonic vesicle ; the number within a single endosperm being increased by the simultaneous fertilisation of several archegonia. Polyembryony, which is rare among Angiosperms, is thus the typical condition among Conifers and generally among Gymnosperms, but only in the very earliest stage; for usually only one of the rudiments developes into a vigorous embryo, such as has already been described. During its development the endo- sperm also continues to grow vigorously; its cells become filled with reserves of food-material (fat and albuminoids) ; the embryo-sac which surrounds it grows at the same time, and finally supplants the tissue of the nucleus, the tissue of the integu- ment hardening at the same time into the testa. In Salisburia, how^ever, an outer strong layer of tissue forms the pulpy envelope which causes the seed to resemble a drupe. The elongated cells of the pro-embryo usually disappear during these processes, but according to Schacht are permanent in Larix. During the period that the seeds are ripening, the carpels and the placentae also continue to grow and to undergo changes in texture. In Taxus a red aril which afterwards becomes pulpy grows round the ripening seed (Fig. 318 w); in Podocarpus the part of the floral axis that bears the scales and the seeds, and which was already considerably swollen, becomes fleshy ; in Juniperus and Sabina the carpels themselves form the blue 'berry' which envelopes the seeds: in most other Cupressincoe the carpels grow, close up laterally and become woody ; and the same occurs in those Abietineae which are without bracts (in respect to Cunninghamia, vide supra) ; while in Pinus, Abies, Cedrus, and Larix, it is the placental scales which after fertilisation grow vigorously, outstripping in their growth the true carpels (bracts), become woody, and form the mature cone. In all these cases (except Podocarpus, Salisburia, and Taxus), the seed is closely and firmly enclosed during ripening by the carpels or placental scales ; it ripens within the fruit, the parts of which do not again separate or become detached in order to allow of the escape of the seeds until they are completely ripe (as in Ahies pedinata). • So long as we are still in doubt as to the nature of the female flowers of various genera, the systematic arrangement of the Coniferae can only be considered as provi- sional ; Endlicher (Synopsis Coniferarum, 1847) distinguishes the following families:— First Family. Cupressineae. Leaves, including those of the flowers, opposite or verticillate (in Division e single) ; flowers moncecious or dioecious ; stamens terminating in a shield-like expansion bearing pollen-sacs in twos or threes or larger numbers ; female flower consisting of alternate \vhorls of carpels, bearing at their base or on their inner surface two or a larger number of erect ovules (in Juniperus comtnunis the ovules ^ See in addition Schacht, Lehrhuch der Anat. u. Phys. vol. II, p. 462. According to Pfitzer (/. c.) the young rudiment of the embryo has at first an apical cell, which however soon disappears ; in the Abietine^ the mode of formation of the embryo is from the first like that in Angiosperms. O PHANEROGAMS. are alternate with the three carpels on the floral axis) ; embryo with two, rarely three or nine cotyledons. (a) Jimiperinea. Fruit berry-like (Juniperus, Sabina). (b) Act'mostrobecB. Carpels united into valves; afterwards separating as a four- or six-rayed star (Widdringtonia, Frenela, Actinostrobus, Gallitris, Libocedrus). (c) ThujcpjidecB. Carpels partially overlapping one another (Biota, Thuja, Thujopsis). (d) Cupress'mece -vertB. Carpels peltate and polygonal in front (Gupressus, Cha- maecyparis). (e) Taxodineoe. Carpels peltate or overlapping ; leaves alternate (Taxodium, Gly- ptostrobus, Cryptomeria). Second Family. AbietinesB. Leaves usually acicular and arranged spirally, singly, or in twos, threes, or rosettes on special short shoots; flowers monoecious, rarely dioecious; stamens numerous, with two or more long pollen-sacs; female flower con- sisting of a number of scale-like placentae arranged spirally, which are either themselves carpels or are the result of the coalescence and lignifying of small carpels ; micropyle of the ovule turned towards the base of the placenta ; embryo with from two to fifteen cotyledons. (a) Pineae, Seeds in pairs on a scale-like placenta which springs from a small open carpellary leaf (Pinus, Abies, Tsuga, Larix, Cedrus). (b) Araucariece. Seed single on the carpel, and enveloped by it (Araucaria). (c) Ctmmnghamiece. Seeds single or numerous on a carpel (Dammara, Cunning- hamia, Athrotaxis, Sequoia, Sciadopitys). Third Family. Podocarpese. Leaves acicular or broader, and arranged spirally ; flowers monoecious or dioecious; stamens short, wMth two roundish pollen-sacs; female flow^er consisting of an axis swollen above with small scale-leaves, from the axils of which (?) the ovules spring ; embryo dicotyledonous. Podocarpus (Dacrydium, Microcachrys). Fourth Family. TaxinesB. Leaves arranged spirally, acicular or often of con- siderable breadth ; in Phyllocladus there are no foliage-leaves, these being replaced by leaf-like branches ; flowers always dioecious ; stamens of various forms, bearing two, three, four, or eight pendent pollen-sacs ; female flowers always consisting of a naked axis or of one furnished with small leaves, bearing the erect ovules terminally or laterally ; ripe seed enclosed in a fleshy aril or with the outer layer of the testa fleshy ; embryo dicotyledonous, Phyllocladus, Salisburia, Cephalotaxus, Torreya, Tax us. C. GNETACE/E. This order includes three genera which differ strikingly in habit. The Ephedra are shrubs with no foliage-leaves and with long, slender, cylindrical green-barked branches; at the joints of the stem are two opposite minute leaves which grow together into a bidentate sheath, and from their axils the lateral branches spring. In Gnetum the leaves are also opposite on the jointed axes, but large and stalked, with a broad lanceolate lamina and feather-veined venation. Thirdly, Wehvitschia mirahilis, so remarkable a plant in many other ways, possesses only two GNETACEJE. .5j foliage-leaves (probably the cotyledons) of immense size. They are extended on the ground and become divided into strips as they become old ; the stem remains short, rising only slightly above the ground, and is broad above with a furrow across the top, while it is tuberous below, and passes into the tap-root \ The Flowers of Gnetaceae are unisexual, and are arranged in dioecious (Ephedra) or monoecious inflorescences ; the inflorescence has a well-defined form, and in Ephedra and Gnetum springs from the axils of the opposite leaves. The male flower of these genera consists of a small bifid perianth, in the middle of which rises a staminal column, which in Gnetum is bifurcate above and bears two bilocular anthers, in Ephedra a larger number crowded into a head. The female flower of Gnetum (Eichler, in Flora 1863, p. 463), like that of Ephedra, also possesses a perianth, flask-shaped in the former, obscurely trigonous in the latter genus ; it envelopes a central ovule possessing in the case of Ephedra one integument, in that of Gnetum two, the inner of which is elongated like a style. The more exact morphology of these flowers is still doubtful. The endosperm of Ephedra is said by Schacht to produce only one archegonium, and the contents of the longish pollen-grain to divide like those of the-Abietineae. In Gnetum the inflorescence, which springs from the axil of the foliage-leaves, consists of a jointed axis with verticillate leaves, in the axils of which the flowers, male and female, are agglo- merated. The inflorescence of Welwitschia^ is a dichotomously branched cyme ' For a full description of this remarkable plant see J. D. Hooker in Trans. Linn. Soc. vol. XXIV. ^ [According to Professor W. R. M^Nab, ' The cones of Welwitschia consist of numerous opposite and decussate bracts, with a sessile flower in the axil of each of the bracts. The per- fect flowers in the male cone consist of two outer perianth leaves (calyx) placed right and left, two inner ones (corolla) placed anteriorly and posteriorly, six stamens united below, and two carpels anterior and posterior, the conical end of the axis projecting as a rudimentary axile ovule surrounded by the two carpels. The outer parts of the perianth are first developed, appearing as two shoulders at the very base of the young floral branch. The flower next in age has the floral axis more elongated, the outer parts of the perianth larger, and a distinct swelling is visible above the outer parts. These swellings are anterior and posterior, and much larger than the outer parts. Above the inner parts of the perianth the axis is expanded, and contracts near the rounded apex. The expanded portions are superposed on the outer lateral parts of the perianth, and are the two primordial staminal cushions. These cushions are semilunar, and in the earlier stages show no trace of division into three. At this stage the parts of the perianth rapidly enlarge and cover in the central parts of the flower. A projection now forms anteriorly and posteriorly, the first indication of the two carpels. The next' stage shows the two staminal cushions each forming three elevations, the central one larger than the two lateral ones. The six stamens are thus produced by the branching of two primordial stamens. In the next stage the carpels elongate and cover in the punctum vegetationis, ultimately developing the peculiar style and stigma-like process. The axis elongates slowly and forms a conical projection which is undoubtedly a rudimentary axile ovule, but it never shows any appearance of an embryo-sac' In the female flowers, which are produced in different cones from the male flowers, the develop- ment is very different. A very short stalk is developed in the female, which is wanting in the male ; then two shoulders are developed exactly like the two outer parts of the perianth in the male flower, to which Dr. Hooker considered them to be equivalent. Judging from the construction of the male flower. Professor McNab was disposed to accept this view ; but with hesitation, as he could not account for the stalk-like process. Strasburger however concludes that they are carpels, and in that M<=Nab quite concurs. Above the carpels the axis elongates slightly, and a ring is formed surrounding the punctum vegetationis. This ring is the ovular integument. Comparing the two flowers, it will be seen that in the male there are four series ofpaits, in the female the three outer 46 a PHA NER OGA MS. nearly a foot in height, rising above the insertion of the two enormous leaves on the periphery of the broad apex of the stem. The branches of the inflor- escence are terete and jointed, spring from the axils of the bracts, and bear upright longish cylindrical cones ; these are furnished with from seventy to ninety broadly ovate scale-leaves standing closely one above another in four rows, a single flower being seated in each axil, male and female in different cones. The male flowers are pseudo-hermaphrodite, and possess a perianth consisting of two pairs of decussate leaves; the lower ones are entirely free, sickle-shaped and pointed, the upper ones broadly spathulate and coherent at their base into a com- pressed tube. Within this tube are six stamens monadelphous at the base, with cylindrical filaments and terminal spherical bilocular anthers, which dehisce above the apex with a three-armed fissure; the pollen-grains are simple (?) and elliptic. The centre of the flower encloses a single erect orthotropous sessile ovule with broad base, and with no other investment than a simple integument, w^hich is drawn out into a style-like tube with a margin expanded in a discoid manner ; the nucleus however has no embryo-sac, or is sterile. In the female flowers the perianth is tubular, greatly compressed, somewhat winged, and altogether undivided; there is no indication of any male organ ; the ovule (in this case of course possessing an embryo-sac) is entirely enclosed in the perianth, and is similar in its external form to that of the male flower, but with this difference, that the elongated point of the integument is only simply slit, not expanded into the form of a plate. When ripe the cone is about two inches long and of a scarlet colour; the scales are persistent ; the perianth enlarges considerably and becomes broadly winged ; its cavity is narrowed above into a narrow canal, through which the apex of the in- tegument, passes. The seed is of the same form as the unfertilised ovule, and con- tains abundant endosperm, in the axis of which lies the dicotyledonous embryo ; the embryo is thick at the radicular end, and is there attached to the very long spirally coiled suspensor. The formation of endosperm commences in the embryo- sac even before fertilisation ; archegonia are formed which grow out of the embryo-sac to the number of from twenty to sixty, and penetrate into the canal-like cavity of the nucleus; there they are fertilised by the pollen-tubes which have grown to meet them, the pro-embryos being then formed in the lower part of the central cells, the (coiled) suspensors attaining a length of three inches. Although from two to eight archegonia are fertilised, only one embryo is developed. 7/je Formation of Tissue in Gymnosperms. From the abundant though still unsifted material I will only adduce a few particulars as a contribution to the special character- istics of this section. The Fibro-'vascidar Bundles^ are similar in their structure to those of Dicotyledons. series are wanting and only the carpels remain. But in the male flower the carpels are anterior and posterior, while in the female they are lateral. This is to be explained by the fact that the carpels are here the first leaves of the branch, and that it is very rare (except in Grasses) that the first leaves of a shoot are anterior or posterior, and not lateral. The ovular integument of the female flower is wanting in the male. While therefore the male flower is complex, the female is remarkably simple. For further details see Transactions of the Linnean Society 1873, vol. XXVIII, pp. 507-512. — Ed.] Mohl, Bau des Cycadeenstammes (Verm. Schr. p. 195).— Kraus, Bau der Cycadeen-Fiedern GNETACE.E. ^6^ There is a system of bundles cominon to the stem and leaves ; the portions whicli descend into the stem forming a circle, ^vhere a closed cambium-ring is produced b\ the formation of interfascicular cambium. This ring causes the permanent growth in thickness of the stem. The ascending portion, which curves out into the leaf itself, assumes in Cycadeae more or less the character of a closed bundle, while in the leaves of many Coniferse it at least retains the appearance of an open bundle. No exclusively cauline bundles are produced in the stem of Coniferae or of Ephedra ; but in Cycadege and Wehvitschia bundles arise in the older stem which are nothing but ramifications of the common bundles, although in their further development, to a great extent, independent of them. Thus in the tissue of the pith of some Cycadese slender isolated bundles occur ; while in some a system of thicker branches of bundles is developed in the bark which may form there in old age one or more apparent rings of wood. As far as we can judge from Hooker's description, bundles occur in the bark of Wel- witschia which owe their origin to a layer of meristem enveloping the w^hole stem. The Coniferae, as has been mentioned, possess only common bundles, the descending portions passing through a number of internodes, and then joining others lower down either unilaterally or on both sides by splitting into two arms and turning to both sides. The leaves of Coniferae, when narrow, contain only one fibro-vascular bundle from the stem, which then usually splits into two halves running parallel to one another; when the leaves are broader, two (Salisburia, Ephedra) or even three bundles occur ; when the leaf forms a flat broad lamina, as in Salisburia and Dammara, the bundles ramify in it, but without forming a net-work ; in Salisburia they repeatedly branch dichotomously. In Conifera: these bundles seldom form prominent veins, but nm through the middle of the tissues of the leaf. In the two gigantic leaves of Wehvitschia there are a number of bundles, the parallel ramifications of which run into the middle layer of tissue. In the large pinnate leaves of Cycadeae there are also several bundles which curve nearly horizontally within the cortical parenchyma, and split into a number of stout bundles in the leaf-stalk when it is thick ; these bundles exhibit a beautiful arrangement when seen in transverse section (in Cycas revoluta, e.g. in the form of an inverted 12). They run parallel in the rachis of the pinnate leaf, and give off branches into the pinnae, where they either run parallel in the middle layer of tissue (as in Dion) or dichoto- mise {e. g. Encephalartos) ; while in Cycas they form a mid-rib projecting beneath. The course of the bundles in the leaf therefore shows a decided resemblance to that of many Ferns. The substance of the wood of the stem is formed from the descending bundles, which are at first completely isolated, but soon coalesce into a closed ring by portions of cambium which cross the medullary rays. The primary wood or xylem, termed the Medullary Sheath, which consists of the xylem-portions of the descending arms of the common bundles, contains, in all Gymnosperms, as in Dicotyledons, long narrow vessels with annular or spiral thickening-bands, while further outwards occur scalariform or reticulately thickened vessels. The secondary wood produced from the cambium-ring after the cessation of growth in length consists, in Cycadeae and Coniferse, of long tracheides grown one into another in a prosenchymatous manner (cf. p. 25) with a few large bordered pits, which are usually circular, at least when the wood is mature. Every possible stage of transition occurs between these tracheides (p. 99) and the spiral vessels of the medullary sheath. The secondary wood of Cycadeae and Coniferae is distin- guished from that of Dicotyledons by the striking peculiarity that it is composed only (Jahrb. f. wiss. Bot. vol. IV. p. 329).-Geyler, Ueber Geflssbundelveilauf bei Coniferen (ditto, vol. VI. p. 68).— Thomas, Vergl. Anat. des Conifer-Blattes (ditto, vol. IV. p. 43).— Mohl, Ueber die grossen get.ipfelten R5hren von Ephedra (Verm. Schr. p. 269).-!. D. Hooker, On Wehvitschia (Trans. Linn Soc. vol. XXIV.).— Dippel, Histologie der Coniferen (Bot. Zeit. 1862 and 1S63).- Rossmann, Bau des Holzes (Frankfurt-a-M. 1863).— Mohl, Bot. Zeit. 1871. 464 PHANEROGAMS. of this a prosenchymatous form of cells^); and that the wide dotted vessels composed of short cells are wanting which penetrate the dense narrow-celled masses of the wood of Dicotyledons. In the younger stems of Cycadeae the tracheYdes with broad bordered pits and hence with a more or less scalariform wall, are very much like the long prosen- chymatous vessels of Vascular Cryptogams; and this resemblance extends even to the tracheides of Coniferae, so far as they are distinctly prosenchymatous, although the smaller number and round form of the bordered pits shows a more marked difference (pp. 25-27). The bordered pits of Coniferae are usually developed only on the wall M'hich faces the medullary rays, in one or two rows, but in Araucaria in larger numbers and densely crowded. In the structure of the secondary wood, as in that of their flowers and in their habit, Gnetaceae approach Dicotyledons ; in Ephedra broad vessels occur in it together with the usual tracheides in the inner part of the ring of wood, but their component cells are separated by oblique septa, and are therefore still prosenchymatous, and are penetrated by several roundish holes ; their lateral walls show bordered pits like the tracheides, and furnish a striking evidence that the true vessels in the secondary wood of Dicotyledons are connected by intermediate forms with the vessels of Vascular Cryptogams formed from prosenchymatous cells. It is stated that in the wood of Wehvitschia tracheides with doubly bordered pits are entirely wanting, and that it contains in their place thick-walled ' porous vessels.' The rays of the secondary wood of Coniferae are very narrow, often only one cell in breadth ; the cells are strongly lignified, and their lateral faces in contact with the adjoining tracheides are provided with closed dots. In Cycadeae the rays are broader, and their tissue bears a closer resemblance to the parenchyma of the pith and cortex ; their number and width cause the whole substance of the wood to appear spongy, and its parenchymatous cells to be strongly curved in different directions when cut across. The phloem-portion of the fibro-vascular bundles of Gymnosperms resembles that of Dicotyledons ; it is mostly composed of true strongly-thickened bast-fibres, cambiform cells, latticed-cells, and parenchymatous cells ; while in Conifera3 they are formed in alternate layers. Usually the soft bast predominates. The Fimdamental tissue of the stem of Gymnosperms is separated by the ring of wood into pith and primary cortex. Both are very strongly developed in Cycadeae, especially the pith, and consist of true parenchyma, while the woody portion is considerably smaller. In Wehvitschia the parenchymatous tissues appear also to prevail ; but the greater part of their substance can only originate from the meristem-layer of the stem already mentioned. A large number of so-called spicular cells occur dispersed in all the organs of this remarkable plant, they are fusiform or branched and greatly thickened ; and a number of beautifully developed crystals are found imbedded close to one another in their cell-wall. Similar structures also occur in Coniferae (p. 66). The parenchymatous fundamental tissue of Coniferae decreases greatly with the increase in age of the stem (and of the root). With the exception of the pith, which is here small, the stem consists exclusively of the products of the cambium-ring, since the primary cortex, and afterwards also the outer layers of the secondary cortex which always have a subsequent growth, are used up in the formation of cork. In the stem of Cycadeae, the increase of which in thickness is inconsiderable, the formation of cork is also very small ; in Wehvitschia it appears to be entirely wanting (?). Sap-conducting Intercellular Passages are widely distributed in Gymnosperms; their structure is that which has been explained generally at pp.73 and 115. In Cycadeae they are found in all the organs in large numbers, and contain gum, which exudes from incisions in thick viscid drops ; in Coniferae they contain oil of turpentine and resin. In this latter order they occur in the pith of the stem, in the whole substance * Wood-^ arenchyma is not formed, or only in small quantity. GNETACE.E. ,5^ of the wood, and in the primary and secondary cortex, as well as distributed through the leaves (p. 105) ; always following the direction in length of the organs, like the gum- passages of Cycadea^. In many Conifers with short leaves roundish resin-glands also occur in them (as in Callitris, Thuja, and Gupressus, according to Thomas); in Taxus the resin-canals are entirely wanting ^ The Lea'ves of Cycadeae and Goniferae are covered by a firm epidermis, usually strongly cuticularised, and furnished with numerous stomata, each with two guard-cells. In the Gycadeae the guard-cells are more or less deeply depressed, and the stomata occur only on the under side of the lamina, and are either irregularly scattered, or arranged in rows between the veins (Kraus). In the leaves of Conifers the guard-cells are also, according to Hildebrand (Bot. Zeit. 1869, p. 149), always depressed in the epidermis; and the stoma has hence always a border (r/. p. 86). In Goniferae the stomata are developed either on both or only on one side of the leaf; when the leaf is broad, as in Dammara and Salisburia, they are irregularly scattered; when the leaves are acicular they mostly lie in longitudinal rows ; and in the large leaves of Welwitschia they are also arranged in rows. The firm texture of the leaves of Cycadeae and Goni- ferie is due to a hypodermal layer (p. 105), often strongly developed, consisting of Fir.. 336. — Pinus Ptiiaslry : two cells of the colourless parenchyma surroiincling the fihro-vascular bundle of the leaf; r/ the ilot-like structures. cut across, t' the same seen from the surface. strongly-thickened, generally long, fibre-like cells lying parallel to the surface; in the leaf of Welwitschia this hypoderma consists of spongy succulent tissue penetrated by bundles of fibres, which acquires its hardness from a mass of spicular cells. The chlorophyll-tissue of the leaves lies beneath this layer, and is developed on the upper side of the leaves of Cycadeae and of the broader leaves of Goniferae as the so-called Pallisade-tissue ; /. e. its cells are elongated in a direction vertical to the surface of the leaf and are densely packed together. In Pinus, Larix, and Gedrus the cells which contain chlorophyll exhibit the infoldings of the cell-wall which have been already * [Van Tieghem (Ann. des Sci. Nat. 1872) distinguishes the six following modifications of the distribution of the secretory organs in Coniferce :— i. No canals in the root nor stem : Taxus. 2. No canals in the root ; canals in the cortical parenchyma of the stem : Cryptomeria, Taxodium, Podo- carpus, Dacrydium, Torreya, Tsuga, Cunninghamia. 3. No canals in the root; canals in the cortical parenchyma and in the pith of the stem : Salisburia. 4. A secretory canal in the root ; canals in the cortical parenchyma of the stem : Gedrus, Abies, Pseudolarix. 5- Canals in the wood of the fibro-vascular bundles of the root and stem ; canals in the cortical parenchyma of the stem : Pinus, Larix, Picea, Pseudotsuga. 6. Canals in the liber of the fibro-vascular bundles of the root and of the stem; canals in the cortical parenchyma of the stem: Araucaria, Widdrmgtonia, Thuja, Gupressus, Biota. In Cycade^ the canals are found disseminated through the cortical parenchyma of the stem ; the pith of Cycas appears destitute of them. In their distribution they resemble therefore that which occurs in the second class of Conifer^.— Ed.] Hh 466 PHA NER GA MS. mentioned at p. 72 (Fig. 60). The middle layer of the tissue of the leaf, in which also the fibro-vascular bundles run, has usually a peculiar development in Gymnosperms ; in Cycadeae and Podocarpeae it consists of cells elongated in a direction transverse to the axis of the leaf and to the bundles, but parallel to the surface of the leaf, leaving large intercellular spaces (Transfusion-Tissue of Mohl), In the acicular leaves of the Abietineae the fibro-vascular bundle, split into two, is enveloped by a colourless tissue, which is sharply differentiated from the surrounding chlorophyll-tissue (Fig. 89, gby p. 105). It is parenchymatous, and is distinguished by the large number of peculiar pit-like markings (Fig. 326) ^ A NG I OS PERM SI Monocotyledons and Dicotyledons are distinguished from Gymnosperms by the following characters : — their ovules are form.ed within a receptacle, the Ovary ; the endosperm originates in the embryo-sac only after fertilisation, the pollen-grain emits its pollen-tube as an outgrowth of its inner cell-wall (intine) without any previous internal cell-formation ; — characteristics, the immense importance of which has already been shown in the general introduction to Phanerogams. Concurrent with these distinctions there are however a number of other peculiarities in these plants taken as a whole which distinguish them from all other vascular plants ; and this is especially the case with the structure of the flowers and the fruit, the normal morphological characters undergoing such peculiar combinations and changes that a more detailed description of them must precede the special description of the two classes which they include. The Floiver as a ivhole^. The flower of Angiosperms is rarely terminal, /. e. the primary stem, which is a prolongation of the axis of the embryo, rarely termi- nates in a flower, making the plant uniaxial. When this is the case a sympodial or cymose inflorescence is usually developed, new axes with terminal flowers arising beneath the first flower ; but it is more common for only axes of the second, third, or a higher order to terminate in a flower, so that the plant may in this respect be termed bi-, tri-, or multi-axial. While in Gymnosperms the flowers are typically unisexual or diclinous, herma- phroditism largely prevails among Angiosperms, although monoecious and dioecious species, genera, and famihes are not uncommon. The male flowers are sometimes essentially different in structure from the female flowers (as in Cupuliferae and Cannabinese), but in most cases the unisexuaHty arises merely from the partial or ' For further details, see Mohl, Bot. Zeit. 1871, Nos. 1,2. ^ From ar^^uov^ a receptacle, capsule, ovaiy, and crirtpfia, seed. ^ The most important and comprehensive work on the flowers of Angiosperms is Payer's Traite d'Organog('nic de la Fleur (Paris 1857) with 154 plates. ANGIOSPERMS. 467 entire abortion either of the androecium or the gynajceum, the flower being in other respects constructed on the same type (Fig. 327, A); and in such cases it also frequently happens that hermaphrodite flowers are developed in addition to the male and female (polygamous species, as the ash, Acer, Saponaria ocymoides, &c.). But even in the greater number of cases where the male and female organs e completely developed in hermaphrodite flowers and functionally perfect, fertilisation takes place by the conveyance of the pollen of one flower to the gynseceum of other flowers or even of other individuals of the same species, because either polli- nation within tHe same flower is impossible in consequence of special contrivances (such as dichogamy), or because the pollen is potent only in the fertilisation of Fig. ■^^■;.—Akebia qttiiiata; A part of an inflorescence, i female, 6 male flowers; /> a male flower cut through leng^th- wise, fits sterile carpels; C horizontal section^ of a female flower (magnified); D horizontal section of a male flower; K gynaceum of the female flower with the sterile stamens a; Fan ovary cut through horizontally ; G an ovule ; // horizontal section of an anther; a (in /> and C) the outer, a' the inner stamens, c (in £) the carpels ; p {in B and C) the perianth. ovules of another flower (as in Orchideae, Corydalis, &c.). To these phenomena we shall recur more in detail in the Third Book, when speaking of the physiology of sexual reproduction. While in Gymnosperms the floral axis is usually elongated to such an extent that the sexual organs, especially if numerous, are evidently arranged one above another in alternate whorls or in spirals, — in Angiosperms, on the contrary, the floral axis which bears the floral envelopes and sexual organs is so abbreviated that space can only be found for the various foliar structures by a corresponding expansion or increase in size of the receptacle or torus ; this receptacle swells even before and during the formation of the floral leaves in a club-shaped manner, and is not unfrequently expanded flat like a plate or even hollowed out like a cup in such a H h 2 468 PHANEROGAMS. manner that the apex of the axis is placed at the bottom of the hollow {cf. Fig. 152, p. 200), while the cup thus formed encloses the carpels (as in perigynous flowers), or even takes part in the formation of the ovary, which is then inferior (Fig. 328). But in every case, owing to the abbreviation of the axis, the separate parts do not usually stand one above another, but rather in concentric whorls, or in scarcely ascending spirals, for which reason the explanation of the relative positions expressed by a diagram in the sense explained on p. 167 appears the most obvious. This abbreviation of the axis is also obviously the immediate cause of the numerous cohesions and displacements which are nowhere met with so frequently as in the flowers of Angiosperms. The small development of the floral axis in length depends on the early cessation of its apical growth ; the acropetal or centripetal order of suc- cession of the floral leaves may therefore be disturbed^ by the production of inter- calary zones of growth, although even in these cases the disturbance of the ordinary regularity remains inconsiderable. The acropetal order of succession is however even here in most cases strictly carried out, and the apical growth of the floral FlG.328.—^sarit7/! ctnadense; .-/ tlie flower cut through lengthwise, / the perianth; ^ horizontal section of the flower above the ovary; C horizontal section of the sex-locular ovary ; D a stamen with its lateral anther-lobes a. axis not unfrequently continues long enough to allow the foliar structures to arrange themselves in evident whorls placed one over another or in spirals (e.g. Magnolia, Ranunculaceae, Nymphseaceae). Occasionally also particular portions of the axis are greatly elongated within the flower, as the portion between calyx and corolla in Lychnis (Fig. 330 dis, p. 472), in Passiflora that between corolla and stamens, in Labiatse that between stamens and ovary. The flower of Angiosperms, like that of Gymnosperms, is a metamorphosed shoot, a leaf-bearing axis; but this section of the vegetable kingdom is especially characterised by the high degree of metamorphosis which the floral shoot has undergone, and by the very peculiar characteristics and the diff'erent arrangement of the foliar structures as contrasted with those of the purely vegetative shoots- As far as external appearance goes, the flower of Angiosperms is an altogether peculiar structure, sharply differentiated as a whole from the rest of the organism. This peculiar appearance is due not only to the special properties of its axis, ' The cases adduced by Hofmeister (Allgemeine Morphologic, § 10) of the absence of strict acropetal succession in the foliar structures all belong to this category. ANGIOSPERMS 469 but especially to the presence of the floral envelopes, and most of all to the circumstance that the foliar structures of the flower are arranged, with rare exceptions, in the form of whorls, even when the leaves of the vegetative shoots arc alternate or distichous, or disposed in other similar arrangements. Each of the distinct appendicular organs of the flower, viz. the perianth, androecium, and gynaeceum, is usually represented by several members arranged in concentric circles or a spiral ; so that one or more perianth-whorls are immediately succeeded within by one or more whorls of stamens, and these by the gynseceum in the centre of the flower. One or other of these whorls may however be absent, or each of the separate whorls may be represented by only a single member, as in Ilippuris (Fig. 330), where only one stamen and one carpel are contained within a scantily developed perianth. It is only rarely that the whole flower is reduced ri(.. ■>,-r),—Clu>tofi's), Saponaria, Nerium, Hydrophyllese, &c. When the corolla itself is gamopetalous, the parts of the corona also coalesce, as in Narcissus, where it is very large. The complete form of the perianth, especially when its structure is decidedly petaloid and its dimensions considerable, always stands in a definite relation to pol- lination by the aid of insects [or birds] ; and large, brilliantly coloured, odoriferous flowers only occur where the fertilisation is brought about by this means. The purpose of these properties is to attract insects to visit the flowers ; and the in- finitely varied and often wonderful form of the perianth is especially adapted to compel certain positions of the body and certain movements on the part of insects of a definite size and species when searching for the nectar, by which the conveyance of pollen from flower to flower is unintendonally accomplished by them. We shall recur in detail to these physiological questions in the Third Book. Fig. 330 I'jzj.— Longitudinal section throu,t,'h the flower of Lychnis /los-'Jcn'ts ; y the elongated portion of the axis between calyx and corolla ; at ligiile of the petals or corona. The radial or bilateral symmetry of the perianth is usually associated with that of the other parts of the flower, and will therefore be discussed in connection with it. Besides the perianth in the narrower sense which we have hitherto considered, there are often additional envelopes to the separate flowers. In the Malvaceae and some other plants the true calyx appears to be surrounded by a second calyx {Epicalyx or Calyculus), the morphological homology of which, however, varies. In Malope trifida, for example, the three parts of the epicalyx represent a sub-floral bract with its two stipules ; in Kitaibelia vitifolia, the six-parted epicalyx consists (according to Payer) of two such sub-floral leaves with their four stipules. But the epicalyx may be purely illusory from the production of stipular structures by the true sepals, as in Rosa and Potentilla. In Dimithiis Caryophyllus and some other species a kind of epicalyx results from two decussate pairs of small bracts which are found immediately beneath the calyx ; in the terminal flowers of Anemone a whorl of bracts stands at a short distance below the flower, which takes the form in the nearly allied Eranihis hyemalis of a kind of epicalyx \ The epicalyx of ^ [The garden Clematis known as ' Lucie Lemoine ' possesses a well-marked seven-leaved involucre which has evidently originated from the growth of the axis above the outermost whorl of the multiplied petaloid sepals. — Ed.] ANGIOSPERMS. ^y a^ the small flowers of Dipsacacece is of special interest, each being surrounded, within the crowded inflorescence, by a membranous tube, which here forms the epicalyx. Sometimes, after the perianth and sexual organs have begun to be formed, an ele- vation of the flower-stalk, at first annular, is formed below the flower, growing up afterwards in the form of a cup or saucer, and bearing scaly or spiny protuberances. A structure of this kind is called a Cupule ; and the cup in which the acorn of the various species of oak is seated is of this nature ^ In this case the cupule surrounds only one flower, in the sweet-chestnut and beech on the other hand it encloses a small inflorescence. This spiny cupule afterwards splits from above, separating into lobes, to allow the escape of the fruit which has ripened within it. When an inflorescence is surrounded by a peculiarly developed whorl or rosette of leaves, as in Umbelliferce and Compositce, this is called an Involucre ; when a single sheathing leaf envelopes an inflorescence springing from its axis, it is a Spathe. Both involucre and spathe may assume a petaloid structure, the former, for example, in Cornus florida, the latter in Aroidece. The Andrcccium is composed of the assemblage of the male sexual organs of a flower. Each separate organ is called a S/ainen, and consists of the Anther and its stalk the Fihvmnt, which is usually filiform, but sometimes expanded like a leaf. The anther consists of two longitudinal halves (anther-lobes) placed on the upper part of the filament right and left of its median line; and the portion of the filament which bears the lobes of the anthers is distinguished as the Connective. The lateral position of the stamens on the floral axis (the receptacle) is quite unmislakcable in all hermaphrodite and in most exclusively male flowers. Their lateral position, their exogenous origin from the primary meristem next the ptinctu?n vegetalionis of the floral axis, their acropetal order of development, and the frequent monstrosities in which the stamens assume more or less the nature of petals, or even of foliage-leaves'-, place it beyond doubt that they must be considered morpho- logically as foliar structures, and make it convenient to term them Staminal Leaves ; the filament, together with the connective, being considered as the leaf, of which the two anther-lobes are appendages. From a morphological point of view it is therefore indiff^erent whether the filament (or true leaf) greatly preponderates in size, or is inconsiderable as compared to that of the anther. Only very recently three cases have become known in which the anther appears itself to be a product of the floral axis, and the stalk which corresponds to the filament is the floral axis itself. According to Magnus^, the vegetative cone of the male floral axis of Naias becomes transformed into quadrilocular anthers by the formadon of pollen-mother-cells in four peripheral longitudinal strips of its tissue. Kaufmann had previously described a somewhat similar process in the case of the anther of Casuarina ; and, according to RohrbachS the apex of the floral axis of Typha either itself developes into the anther, or it first of all branches and then forms an anther on each branch. It would carry us too far to give reasons for the doubt already expressed (p. 426), ' On the development of the acorn-cup see Hofmeister, Allgemeine Morphologie, p. 465. 2 [On ' phyllody ' and ' petalody ' of stamens see Masters, Vegetable Teratology, Ray Soc. 1869, pp. 253-256, and 285-296. — Ed.] 3 Magnus, Bot. Zeitg. 1869, p. 771. * Rohrbach, in Sitzungsber. der Gesellsch. naturf. Freunde m Berlin, Nov. 16, 1869. I "4 PHANEROGAMS. whether these facts are sufficient to estabHsh the axial character of these anthers; and these cases may, therefore, be considered for the present as exceptions to the foliar nature of stamens. But, besides, the morphological homology of the separate parts of the ordinary stamens is not yet altogether determined, more precise investigations into the history of development being still wanting in this direction. Cassini and Roper consider the two anther-lobes as the swollen lateral halves of the lamina of the stamen ; their loculi would therefore in that case be mere excavations in the tissue of the leaf; the pollen-mother-cells become differ- entiated inside the young tissue of the leaf, like the spore-mother-cells in the fertile segment of the leaf of Ophioglossaceae. According to this view the furrow between the two pollen-sacs of an anther-lobe (see Fig. 327, H) would correspond to the margin of the staminal leaf; but this cannot be the case^, at least not always, according to Mohl's observations. When the stamens become transformed into petals (by the so-called ' doubling ' of the flower) as in the rose, poppy, Nigclla damasceiia, &c., it may be observed with certainty that the anterior and posterior loculi do not stand opposite one another, which would be the case if one belonged to the upper, the other to the under side of the staminal leaf; but that both are formed on the upper surface, the anterior loculus nearer the median line of the leaf, the posterior one nearer its margin. It is further observable that in such cases the two pollen-sacs of an anther-lobe do not always stand close to one another, but that they are frequently separated by a tolerably broad piece of the leaf, and that this inter- mediate piece contracts in the normal state into the partition-wall between the two pollen-sacs. The greater stress must be laid on these observations of ]Mohl, because in them the abnormal development only shows more plainly what can often enough be seen in a horizontal section of the anther and connective of normal stamens, viz. that the pollen-sacs of an anther-lobe evidendy belong to 07ic side of the stamen ; it appears, however, that they must in some cases be referred to the under (Fig. 327, C, H), in others to the upper side (Fig. 330 C). The origin of the pollen-mother-cells and the development of the wall of the separate pollen-sacs calls to mind so vividly in all essential features the corresponding phenomena in the sporangium of Lycopodiaceae and even of Equisetaceae, that it may be assumed, until more exact observations bring something different to light, that each pollen-sac {i. e. each loculus with its wall) corresponds to a sporangium, and hence also to a single pollen-sac of Cycadeae and Cupressineae ; and that therefore the anther usually consists of four pollen-sacs springing side by side from the anterior or posterior side of a staminal leaf, the sacs lying in pairs so close to one another right and left of the connective, that they coalesce more or less laterally to form one anther-lobe. But before we pass on to the consideration of the pollen-sacs and their contents, we must again recur to the discussion of the entire stamen and androecium. The stalk of the anther (the filament with its connective) is either simple or segmented. The simple filament may be filiform (Fig. 329) or expanded into the form of a leaf (Fig. 328), sometimes even very broad, as in Asclepiadeae and Apocynacese ; or it may be broad below (Fig. 332/") or above; it generally termi- nates between the two anther-lobes, but is not unfrequently prolonged above 11. V. Molil, Vermischte Schriflen, p. 42. ANGIOSPERMS. 47, them (Fig. 328 Z)) as a point, or in the form of a long appendage as in the ole- ander. If the upper part of the stalk, the connective, is broad, the two anther- lobes are distinctly separated (Figs. 328, 331) ; if it is narrow, they lie close to one another. The articulation of the stalk is very commonly the result of the con- nective being sharply separated from the filament by a deep constriction; the connection of the two is then maintained by so thin a piece that the anther, together with the connective which unites the anther-lobes, swings very lightly as a whole on the filament (versatile anther). The point of connection may be at the lower end, at the centre (Fig. 332), or at the upper part of the connective; sometimes the detached connective attains a considerable size, and forms appendages beyond the anther (Fig. 333, /i, x), or it is developed between the two lobes like a cross-bar, so that the filament and connective form a T, as in the lime, and to a much greater extent in Salvia, where the transversely extended connective bears an anther-lobe on one arm only, while the other is sterile and is adapted for a different' purpose. Whether the anther-lobes are parallel depends on the mode of their connection with the Fig. 331. — Stamen of .]/a/ioi!i\i .li/tn- folium ; B with the anther open (by re- curved valves). Fic;. 332.— Stamen ol Arbutus hybrida, anllier open (by pores) ; x appendajje. Fig. 333. — Stamens of Centradettia rosea ; A a larijer fertile one. B a smaller sterile one of the same flower. connective ; if they are so, they are usually attached to the connective for their whole length ; or in other cases they are separated above, or free below and coherent above, in which case they may become placed at such a distance from one another that the two lobes lie in one line above the apex of the filament, as in many Labiates. Not unfrequently the filament also has appendages ; as, for example, the membranous expansions or appendages right and left below in Allium which resemble stipules, or a hood-shaped outgrowth behind as in Asclepiadeae, or ligular structures in front as in Aljsstnn viontaniun, or conical prolongations beneath on one side as in Crambe, or on both as in Mahonia (Fig. 331 x). A phenomenon of great importance from a morphological point of view is the branching of stamens which occurs in many Dicotyledons, a peculiarity of structure which was erroneously confounded by the older botanists with their cohesion, although the two are fundamentally distinct. Sometimes the branching of stamens takes place, like that of foliage-leaves, bilaterally in one plane, right and left of the median line, so that the branched stamen has a pinnate appearance, as in Calo- thamnus (Fig. 334 st\ where each division bears an anther. In other cases the branching takes place in a kind of polytomy, as in Ricinus (Fig. 335), where the separate stamens arise in the form of simple protuberances from the receptacle, each 47 PHANEROGAMS. one repeatedly producing new protuberances, which at length develop 3 by inter- calary growth into a compoundly and repeatedly branched filament; the ends of Fig. 3-!4. — Longitudinal section of tlic flower of Calothaninus ; / the ovary, s calyx, / petals, ^ style, st branched stamens. Fig. 333. — Part of a male flower of Rtciiiits co)innunis cut through lengthways ; ff the basal portions of the coinpoundly-branched stamens ; a the anthers. the branches all bearing anthers. In the Hypericineae, three or five large broad protuberances (Fig. 336, II-V, a) spring from the periphery of the floral axis after the formation of the corolla, on each of which smaller roundish knobs are produced Fig. 336. — Development of the flower oi Hypericii7n perforatinn ; /young flower-bud in the axil of the bract B, with its two bracteoles 6 i, s the sepals, / first indication of the petals ; //middle part of a somewhat older bud, /rudiment of the ovary, a, a, a the three stamens with the rudiments of their branches arising as protuberances ; ///a flower-bud of nearly the same age as in //, but seen from the side, s a sepal, a a the stamens.y the ovar>' ; /y and l^ flower-buds in further stages of development, the letters indicating the same as in /, //, and ///,- i, 2, 3 ovary in various stages of development cut through horizontallj-. in basipetal succession from its apex ; these latter become the filaments, each of which eventually bears an anther, and are connected at their base with the primordial protuberance of which they are branches. A horizontal section through the flower- ANGIOSPERMS. All bud before tlie opening of the flower shows, especially in Hypericum calydnu?n, the numerous filaments which spring from one original protuberance densely crowded into one bundle. In this and many similar cases the common primordial basal portion of the stamen remains very short, while the secondary filaments lengthen considerably and subsequently present the appearance of a tuft springing from the receptacle, the true nature of which can only be ascertained by the history of its development. If, on the contrary, the primordial basal portion lengthens, as in Calothamnus, the whole stamen is easily recognised as branched even in the mature condition. Of no less importance for understanding the entire plan of structure of a flower, and especially the relations of number and position which actually occur, is the cohesion of stamens which grow side by side in a whorl. In Cucurbita, for example, there are, in the earliest stage, five stamens, but at a later period only three are found, two of which are, however, broader than the third ; these are each the result of the lateral coalescence of two stamens. In this case the filaments Fig. 337.— Dcvelnpinent of the andrceciuin of Cucurbita Pepo (after Payer); in all the fiiriires the simple stamen is to the riiflit, behind and to the left two double ones. The anthers grow vigorously in length and form verniiforni coils. become combined into a central column, on which (as is shown in Fig. 337, ///) the pollen-sacs grow more rapidly in length than the filaments, forming vermiform coils. The relationships are much more complicated and more difficult to understand when cohesion and branching of the stamens occur simultaneously, as in Mal- vaceae. In Althcca rosea, for instance, the filaments form a membranous closed tube which completely envelopes the gynaeceum ; springing from this tube are five verdcal and parallel double rows of long filaments, each of which (Fig. 338, B) again splits into two arms (/), and each of these arms bears a single anther-lobe. The history of development and a comparison with allied forms shows that the tube is formed by the lateral coalescence of five stamens ; but the coherent margins produce double rows of lateral ramifications, in other words, of filaments, which then again split into two arms. A horizontal section of the young staminal tube (Fig. 338, A) shows plainly these double rows of split filaments ; the part [v) which lies between two of these must be considered as the substance of a stamen, the margins of which each bear right and left a simple row of filaments as lacinioe or branches \ In ^ The strangeness of this conception will disappear if the structure is recalled of a unilocular ovary with numerous carpels coherent at the margins, e.g. Viola, where the ovules arise in double rows on the lines of junction (the placenta). What takes place in one case in the mside m refer- ence to the ovules takes place in the other case on the outside in the formation of the filaments. 47 ^' PHANEROGAMS. the lime, where the five primordial stamens also branch at the margins, and form anthers on their branches, the stamens remain free, but in other respects the phenomena are altogether similar [cf. Payer I. c.) Fig. ■i'^.—Althaa rosea ; A horizontal section through the young andrcecium ; B a piece of the tube of a mature andrcecium with several stamens ; h cavity of the tube, -o substance of the tube, a anthers, / the spot where the filament divides, /"the spot v/here two filaments spring from the tube {A much more strongly magnified than B). The stamens not unfrequently suffer conspicuous displacements by the inter- calary growth of the tissue of the receptacle in the region of their insertion ; and such displacements are also ordinarily included under the term cohesion (or adhe- sion) \ Thus the stamens often adhere to the calyx or corolla; and then, when Y\C..->,y^.—V\ov&x oi Mauglesia glabraia; 4 before open- ing ; B open ; C the gynaeceum, ^p the gynophore ; D hori- zontal section of the ovary ; E fruit ripening on its pedicel. Fig. 340. — Flower oi Sterciilia Ealaiis^has ; A, ss the gynophore, y ovary, n stigma ; B hori- zontal section of the ovary. mature, the filaments appear as if they sprang from the inside of the perianth ; the earliest stages of development show, however, that the perianth-leaves and the stamens spring in succession and separately from the receptacle ; it is not till a later period that intercalary growth begins at the part of the receptacle from which both spring ; in this manner a lamella grows up which structurally forms the basal portion of the perianth-leaf, and which at the same time bears the stamen, so that the * [It has come to be the usage in English works on descriptive botany to apply the term ' cohesion ' to the apparent union of organs of the same kind, ' adhesion ' to the apparent union of organs of a different kind.— Ed.] ANGIOSPERMS. 479 appearance is presented as if the stamen sprang from the centre of its inner surface. This is shown in Fig. 339, B, where /• is a perianth-leaf and a an anther sessile upon it ; the two stand at first distinct on the young receptacle one over the other ; the portion of leaf lying beneath a and p is not formed till a much later period by intercalary growth, and pushes up at the same time the true perianth-leaf p, and the stamen a. This kind of adhesion is especially frequent in those flowers whose petals have also become coherent laterally into a tube, such as Compositae, Labiatae, Valerianaceae, &c. On the other hand, the stamen may also become 'adherent' in various ways to the gynaeceum. In Sterctilia Balanghas (Fig. 340) this structure is only apparent, depending simply on the small stamens, which are placed close beneath the ovary, becoming raised up together with it by the elongation of a part of the receptacle ; from their small size they appear like a mere appendage of the large ovary ; the part which bears both the organs, the Gynophore, is therefore in this case an internode of the floral axis. Much more complicated is the history Fig. 341.— Flower of Cypripedium Calceolus after removal of the perianth. of the formation of the true Gynosiemium (column) which is formed above an inferior ovary, as in the Aristolochiaceae, and especially in the Orchide^e, where these adhesions and displacements of the parts of the flower are also combined with abortion of certain members. Since these relationships will be explained in the sequel, the examination of Fig. 341 will suffice for the present, where the flower of Cypripedium is represented from the side (yl), from behind {B), and from front (C), after removal of the perianth (//). / is the inferior ovary, gs the gynostemium, resulting from the adhesion of three stamens— two of which {a a) are fertile, while the third (j) forms a sterile staminode— with the carpel, the anterior part of which bears the stigma {n). In this case the gynostemium consists entirely of coherent foliar structures, or of the basal portions of the staminal and carpellary leaves, both of which spring from the upper margin of the hollowed-out receptacle which constitutes the inferior ovary ^ ' Compare the account of the development and significance of the flowers of Orchideoe in the sequel. 4.>u PHANEROGAMS. The size and form of the stamens frequently varies within one and th'e same flower ; thus, for instance, in the Cruciferai there are two shorter and four longer (tetradynamous), in the Labiatae two larger and two shorter (didynamous) stamens ; in Centradenia, as was shown in Fig. 333, A, B, they are not only of different size, but are also differently segmented. A correct conception of the history of develop- ment and a comparison of the relationships of number and position in nearly allied plants enable one to apply the term stamen even to structures which have no anther and therefore want the ordinary physiological character of stamens. Thus, for example, in Geranium there are two whorls of fertile stamens, while in the nearly related genus Erodium those of one whorl are without anthers. Such sterile stamens or Staminodes generally undergo further metamorphosis, by which they become unlike the fertile ones and not unfrequently petaloid, as the innermost staminal leaves of n •<■.*-=-. r Fig. 342.— Various stages of development of tlie flower of I.amiiim album; /, //, /// very young budb seen from above ; in / the rudiments of the sepals s are formed, in // those of the petals/, in /// those of the stamens st and of the carpels c; /A' horizontal section of an older bud, i- tube of the gamosepalous calyx, / that of the gamopetalous corolla; a anthers, ii stigmas ; V upper lip of the corolla with the epipetalous stamens ; l^I entire mature flower seen from the side. Aquilegia ; or assume very peculiar forms, as in Cypripedium (Fig. 341 s). In some Gesneracese a glandular structure or nectary is found in place of the poste- rior stamen (compare the drawing of Columnea, Fig. 385). Metamorphoses of this kind may be considered as the first steps to a condition of abortion, the final stage of which is the production of a vacancy at the spot where the stamen should be, as in the Labiatae, an order closely allied to the Gesneraceae, where, in the place of this staminode there is no structure whatever ; instead of the five stamens to which the plan of construction of the flower points, there are only four, even the rudiment of the fifth, the posterior one, being suppressed, as is seen in Fig. 342 \ Phenomena of this kind altogether justify the hypothesis of abortion in those cases also where * [Peyritsch however (Sitzungsb. der k. Akad. der Wissen. zu Wien, 1873) infers, from the constant reversion to fours in the peloric flowers of Labiatoe, and from other considerations, that the original type of the flower is letramcrous. — Ed.] ANGIOSPERMS. ^^8 j the absent organ does not disappear in the course of development, but never comes into existence at all, if the hypothesis of the suppression of the part is confirmed by a comparison of the relationships of number and position in nearly-allied plants The hypothesis of an abortion of this kind was, however, for the first time placed on a firm basis by the theory of descent. The number of stamens in a flower is only rarely so few as one or two ; it is usually larger, and equal in number to that of the perianth-leaves, and they are then arranged in the form of rosettes, either spirally or in whorls. If the arrange- ment of the perianth-leaves is spiral, that of the stamens is usually the same, and the number of the latter is then very commonly large and indefinite, as in Nymphaea, MagnoHa, Ranunculus, Helleborus, &c. ; but in this case they are sometimes also few in number and definite. IMuch more often, however, the stamens are arranged in one or more whorls, those in one whorl being then usually equal in number and alternate with those in the other whorls, and with the perianth-leaves [symmetrical flowers of English text- books]. There are, however, numerous deviations from this rule [unsymmetrical flowers of English text-books] occasioned frequently by the abortion of particular members or of whole whorls, or by their multiplcation, or by the superposition of consecutive whorls ; and not unfrequently in the place of a single stamen two or even more will arise side by side [dedoublemcnt). These phenomena, which are often difficult to make out, are nevertheless of great value in the determination of natural affinities, and will be still further examined in the sequel. Dei'clopmenl of the Pollen a?id of the Anther-walP. The description given in this place will apply only to the ordinary cases in which the pollen is formed in separate grains in the four loculi of the anther, and falls out of the anther after it has opened ; some of the more important exceptions will be mentioned hereafter. Immediately after the perianth-leaves, or their innermost whorl, first become visible on the receptacle as roundish protuberances, the rudiments of the stamens make their appearance in a similar manner, but usually obtain a considerable start in growth of the corolla, which not unfrequently remains for a considerable time in a very rudimentary condition. The form of the stamen, which consists of homogeneous primary meristem, very soon shows the outlines of the two anther- lobes united by the connective; the filament is still very short, subsequently it also grows slowly, and it is only just before the expansion of the flower that it elongates very rapidly by vigorous intercalary growth. When the four pollen-sacs make their appearance externally on the young anthers as longitudinal protuberances, a layer of cells becomes difl"erentiated in the direction of their length^, through * Nageli, Zur Entwickelungsgeschichte des Pollens ; Zurich 1842.— Hofmeister, Neue Beitnlge zur Kenntniss der Embryobildung der Phanerogamen, II. Monocotyledoiien. M am indebted to a letter from Dr. Warming for the following account of the first origin of the mother-cells of the pollen :— ' The mother-cells of the pollen originate by the division of the cells of the outermost or sub-epidermal layer of the periblem from one to three times by tangential walls, the outermost of the cells which are thus formed being also divided by radial walls. In those plants which have been more minutely examined (Hyoscyamus, Datura, Cyclanthera, Euphorbia) the innermost layer of these cells becomes immediately converted into the primary mother-cells of the pollen, the inner of the layers which lie between them and the epidermis becoming absorbed, I i 4.S2 PHANEROGAMS. ti Stronger growth and slower production of divisions, which are formed more rapidly in the surromiding meristem. This layer consists of the primary mother- FlG. 343. — Fjotkia cordata ; A transverse section through a young pollen-sac before the isolation of the mother-cells S7n, ep the epithelium which clothes the anther-lobe, iv wall of the pollen-sac ; B the anther-lobe after isolation of the mother-cells sm ; ep indication of the epithelium ( X 500). Fig. 344. — Mode of formation of the pollen ol Funkia ' ovata (X550). In VII the wall of the daughter-cell has absorbed water till it has burst ; its protoplasm is forcing itself out through the fissure, and is lying before it rounded off into a spherical form. Fig. 345.— .5 a young pollen-cell oi Funkia ovata ; the thickenings which project outwardly are still small, in the older pollen-cell C they are larger ; they are arranged in lines connected into a net-work. cells of the pollen, which produce, by a few further divisions, a longish assemblage of mother-cells united into a tissue (Fig. 343, A, sm; Fig. 346, ?n) ; the whole large- celled mass being surrounded on the outside by a small-celled tissue consisting of so that usually only one layer remains ; and this, together with the epidermis, forms the wall of the anther.' ANGIOSPERMS. 483 several layers, the future wall of the pollen-sac (Fig. 343, A). The innermost layer, which is continued round the whole mass of mother-cells, is transformed at an early Fig 346.— -4 pollen-sa: of Althaa rosea seen from the side ; B transverse section of an anther-lobe showing the two pollen-sacs, m the mother-cells of the pollen, in A still united into a tissue, in 5 already divided each Into four -'; pollen-cells, n the epithelium of the pollen-sac. Each anther-lobe, consisting of two pollen-sacs, is here borne on a long branch of the filament. period into a delicate thin-walled epithelium {ep) filled with coarse-grained protoplasm, the cells of which usually divide radially and elongate, but are afterwards destroyed Fig 347 —AltJuEa rosea ■ A^E division of the mother-cells of the pollen into four ; /^ and G a tetrahedron, the walls of whose special mother-cells have burst under the influence of water, and have allowed the protoplasmic body of the young pollen-cells to escape ; H a mature pollen-grain seen from without magnified to the same extent (C/t Fig. II, p. 14). like the inner layer of cells in the sporangium of Vascular Cryptogams. The de- velopment of the outer cell-layers which subsequently cause the rupture of the wall I i 2 4^4 PHANEROGAMS. does not take place till a much later period. The mother-cells of the pollen are at first large and their walls thin (Fig. 343, A, sm) ; but these increase considerably in thickness, though generally not uniformly (Figs. 344, 347, A), the thickening matter being usually distinctly stratified. In many Monocotyledons the mother-cells now become completely separated, the pollen-sac becomes broader, and the cells float singly or in connected groups in a granular fluid which fills up its cavity, as is shown in Fig. 343, ^, a phenomenon which calls strongly to mind the formation of the spores of Vascular Cryptogams. In other cases, however, as for instance in many Dicotyledons (Tropaeolum, Althaea, &c.), the very thick-walled mother-cells do not become isolated ; they completely fill up the pollen- sac, but are usually found separated after the rupture of the anther-wall in water. With the thickening of the cell-wall is connected a rounding-off of the protoplasm, the large central nucleus of which is absorbed when the preparation is commencing for the formation of the pollen-cells. Instead of the nucleus which has disappeared by absorption, either two Fig. 348.— Mother-cell of the pollen ofCucitrbiia Pepo ; S2 the outer common layers of the mother-cell in the ac of being absorbed ; sp the so-called ' special mother-cells,' consisting of masses of layers of the mother-cell which surround the young pollen-cells ; they also are afterwards absorbed ; ph the wall of the pollen-cell ; its spines grow outwards and penetrate the special mother-cell ; v hemispherical deposition of cellulose on the inside cf the pollen cell-wall, from which the pollen-tube is afterwards formed ; / the protoplasm contracted (X550). (The preparation was obtained by making a section of an anther which had lain for some months in absolute alcohol.) fresh nuclei first of all make their appearance and undergo an immediate simulta- neous bipartition (as represented in Fig. 344, /, //), or these two are again absorbed and four nuclei are formed in their place, followed by the simultaneous division of the cell into four. These cases have been observed especially in Liliacese among Monocotyledons ; but a third process is especially characteristic of Dicoty- ledons, in which, immediately after the absorption of the nuclei of the mother-cell, four fresh nuclei are formed simultaneously, which arrange themselves at diff'erent points of a plane or in the corners of a tetrahedron, the protoplasm becoming then constricted into four lobes, each nucleus forming the centre of one of the lobes. During this constriction the thick wall of the mother-cell grows inwards, following the constriction of the protoplasm, until at length the four lumps of protoplasm which have become rounded off" during the division lie quite distinct in four cavities of the mother-cell (Fig. 347, A-E). The mass of cellulose now becomes diff'er- entiated round each of the daughter-cells of the tetrahedron into concentric systems ANGIOSPERMS. 485 of layers -(the so-called 'special mother-cells'), and these are again enveloped by layers which are common to the whole tetrahedron (Figs. 347 £, 348). If the tetra- hedra have lain for some time in water, the masses of layers usually burst, and the protoplasmic contents of the young pollen-cells are forced out through the fissure, and become rounded off into a sphere (Figs. 344 VII; 347 F, G). Soon after the conversion of the mother-cells of the pollen into a tetrahedron, each protoplasmic mass becomes clothed with a new cell-wall, at first very thin and not continuous with the inner layers of the wall of the mother-cell, as is shown by its becoming detached from them when caused to contract by alcohol. This is the true cell-wall of the pollen, which now increases greatly in thickness, and becomes differentiated into an outer cuticularised layer and an inner one of pure cellulose, the Extine and the Intine. The former becomes covered on the outside with spines (Fig. 348,//^), warts (Fig. 345), ridges, combs, &c. ; w^hile the latter frequently forms considerable thickenings which project inwards at particular spots (Fig. 348, v), and at a later period are employed to form the pollen-tube. During these processes the masses of layers forming the envelope of the tetrahedron become slowly absorbed, their substance is converted into mucilage, and they at length entirely lose their form; their disorganisation may commence either on the inner (as in Fig. 344, VII, x) or outer side (Fig. 348, sg) of the wall of the mother-cell. By the absorption of the chamber in which the young pollen-cells have hitherto been enclosed, they now become free, separate, and fioat in the granular fluid which fills up the cavity of the anther ; and within this they now attain their definite development and size. The fluid being thus used up, the mature pollen-grains finally fill up the cavity of the anther in the form of a powdery mass. The ripe pollen-grain of Angiosperms ^ does not undergo any further divisions, like that of Gymnosperms; it remains unicellular; the pollen-tube is developed immediately on the stigma as a protuberance of the intine, which perforates the extine at certain definite spots that have usually been prepared beforehand. The spots where this perforation takes place are often more than one, or even very nume- rous (Fig. 349 a, 350 0) ; yet, notwithstanding the possibility of the formation of this number of pollen-tubes from one grain, only one usually grows to an extent sufficient to effect impregnation. Independently of the sculpture of the extine itself which has already been mentioned, the external form and structure of the outer coat of pollen-grains depends chiefly on the number of the spots at which the perforation takes place, on the mode in which these are arranged, and on the circumstance whether the extine is at these spots merely thinner and the intine projects in the form of a wart (Fig. 349), or whether roundish pieces of the extine become detached in the form of a lid, as in Cucurbitacese and Passiflora (Fig. 37, p. 33), or whether it splits into bands by spiral fissures, as in Thunbergia (Fig. 38, p. 34), &c. At the points of perforation the intine is generally thicker, often forming hemispherical protuberances which furnish the first material for the formation of the pollen-tube (Fig. 350, 0, or the extine only forms thinner longitudinal striae which fold inwards » For more minute details see Schacht, Jahrb. ftir wissensch. Bot. II, p. 109, and Luerssen, ibid. VII, p. 34.— [Fritzsche. Beitriige, zur Kenntniss des Pollen, Berlin, 1832.— Mohl, Beitrage zur Anatomic u. Physiologic der Gewiichse, ist Heft, Bern, 1834.] 486 PHANEROGAMS. when the pollen-grain becomes dry (as in Gladiolus, Yucca, Helleborus, &c.). Very commonly however the intine is uniformly and continuously thickened, as in Canna, Strelitzia, Musa, Persea, &c. ; and in this case, according to Schacht, no definite spots are prepared beforehand where the perforation is to take place. The number of these peculiarly organised points of perforation is definite in each species, often in whole genera and families ; there is only one in most Monocotyledons and a few Dicotyledons, two in Ficus, Justicia, &c., three in the Onagrarieae, Proteaceae, Cupu- liferae, Geraniacese, Compositae, and Borraginese; four to six in Impatiens, Astra- pasa, Alnus, and Carpinus, while the number is large in Convolvulacese, Malvaceae, Alsineae, &c. (see Schacht, I.e.). The extine is rarely smooth, more often marked JW;> Fig. 349.— Transverse section of a pollen-grain of Epilo- binm angtisti/oliutn: a the points where the intine i pro- trudes, the intine being there thicker and the extine e thinner Fig. 350.— Pollen-grain of Althaa rosea: A a piece of the extine seen from without ; B the half of a very thin section through the middle of the pollen-grain, st large spines, A j small spine of the extine, o perforations through the extine e, t'the intine,/ the protoplasm of the pollen-grain contracted (X800). on the outside by the sculpture to which reference has already been made. When it is very thick, layers of different structure and texture may frequently be detected, and differentiations sometimes occur in a radial direction, penetrating the thickness of the extine (Fig. 350), and giving it in some cases the appearance of consisting of rod-shaped prismatic pieces or of honeycomb-like lamellae, &c. These peculiarities of structure recall those of the exospore of Marsileaceae, and probably only de- pend, as in that case, on a further development of the radial striation, accompanied possibly by subsequent absorption of the soft areolae and hardening of the denser parts (see p. 30). The contents of the ripe pollen-grain, the Fovilla^ of the older ^ [On the constitution of the ' amyloid corpuscles ' in the fovilla of pollen see Saccardo, Nuovo Giornalc Botanico Italiauo, 1872, p. 241. — Ed.] ANGIOSPERMS. 487- botanists, usually consists of a dense coarse-grained protoplasm in which o-rains of starch and drops of oil may be recognized. When the grain bursts in water, the fovilla escapes in masses connected by mucilage and often in long vermiform threads. The surface of the extine is commonly found coated with a yellow oil, or of some other colour, often in evident drops, which renders the pollen viscid and adapted to be carried by insects from flower to flower; in only a comparatively few cases is it quite dry and powdery, as in Urticaceae and many Grasses, where it is projected with violence from the anthers or simply falls out. At the time when the pollen-grains are nearly mature, and the flower-bud is preparing to open, the wall of the pollen-sacs undergoes a further development \ The outer layer of cells or epidermis always remains smooth-walled (see Fig. 351, p. 489) ; the inner layers or endothecium are also smooth if the anther does not dehisce. If on the other hand it opens by recurved valves (Fig. 331 k, p. 475); the cells of the innermost layers only of these valves are provided with thicken- ing-bands (or are fibrous) ; while, when the pollen-sacs dehisce longitudinally, the whole of their endothecium contains fibrous cells. There is usually only one such layer, sometimes several ; in Agave ame7-icana as many as from eight to twelve; The thickening-bands of the fibrous cells which project inwards are usually wanting on their outer wall ; on the side-walls they are generally vertical to the surface of the pollen-sac ; on the inner wall they run transversely and are united in a reticulate or stellate manner. Since the epidermal cells contract more strongly when the ripe anther-walls dry up than those of the endothecium which are provided with thickening-bands, they exert a force which has a tendency to make the anther-wall bulge outwards and give way at its weakest point. The modes in which the anthers open are very various, and are always intimately connected with the other contrivances which are met with in the flower for the purpose of pollination with or without the agency of insects. Sometimes only a short fissure (pore) is formed at the apex of each anther-lobe, as in Solanum, Ericaceae (Fig. 33-2, p. 4'75), &c., through which the pollen of both the contiguous pollen-sacs escapes ; but more com- monly the wall gives way in the furrow between the two sacs (the suture) along its whole length, the tissue which separates them becoming at the same time more or less destroyed, and thus both pollen-sacs dehiscing at the same time by the longitu- dinal fissure (Fig. 351). It is this phenomenon that has given rise to the erroneous description of these anthers as being bilocular; but if nomenclature is to have a scientific basis, they must be termed quadrilocular, in contrast to the really bilo-' cular anthers of Asclepiadeae and the octilocular ones of many Mimosese. Some- times again the anther-lobes open at the apex by a pore which results simply from the destruction of a smafl portion of tissue at this spot (Hofmeister). In other respects \ve still want a detailed and comparative investigation of these processes, which are very various and of great physiological importance; only the addi- tional remark need be made here, that it is very important from a systematic point of view whether the anthers open inwards towards the gyn^^ceum (introrse), or outwards (extrorse), the diff'erence depending on the position of the suture and hence on that of the pollen-sacs on the inner or outer side of the filament. , , » Compare li. v. Mohl, Vermischte Schiiften, p. 6:. 488 PHANEROGAMS. In several families of Monocotyledons and Dicotyledons more or less con- siderable deviations^ occur from the course of development of the pollen and from its final structure which has been here described. Naias and Zostera deviate only to this extent, that no thickening of the wall of the mother-cells takes place, and that the pollen-cells themselves are very thin-walled, acquiring in Zostera a very strange appearance from assuming, instead of the ordinary rounded form, that of long thin tubes lying parallel to one another in the anther. The deviations are more considerable in the formation of compound pollen-grains. The origin of these is either that only the four daughter-cells (pollen-cells) of one mother-cell remain more or less closely united, like the pollen-tetrahedra (four-fold grains) of some Orchideae, Fourcroya, Typha, Anona, Rhododendron, &c. ; or the whole contents of one primary mother-cell remains unseparated and forms a mass of pollen con- sisting of eight, twelve, sixteen, thirty-two, or sixty-four connected pollen-cells, as in many Mimoseae and Acacieae^. In these cases the cuticle or extine is more strongly developed on the outer surface of the daughter-cells lying at the circumference of the mass, and covers the whole as a continuous skin ; while only thin ridges of the cuticle project from this skin inwards between the separate cells. In the various sections of Orchideae every gradation occurs from the ordinary separate pollen-grains of Cypripedium, through the four-fold grains of Neottia, to the Ophrydese, where all the pollen -grains which are formed from each primary mother-cell remain united, and thus a number of pollen-masses lie in one pollen-sac ; and finally to the Pollinia of the Cerorchideae, where all the pollen-grains of a pollen-sac remain united into a cellular mass. In this case, as in the Asclepiadeae with only bilocular anthers, where the grains of each pollen-sac are firmly united by a waxy substance, it is obvious that the pollen cannot be dispersed, nor can the pollen-masses fall out spon- taneously from the anthers ; but the flower is provided with very peculiar con- trivances by means of which insects in search of honey extract from the pollen-sac the pollinia or the masses of pollen which are glued together, and again get rid of them on to the stigmas of other flowers of the same species (see Book III on Sexual Reproduction). The Female Sexual Apparatus or GyncEceum^ of the flowers of Angiosperms consists of one or more closed chambers in which the ovules are formed ; the lower, hollow, swollen part of each separate seed-chamber which encloses the ovules is called the Ovary ; the place or the mass of tissue from which the ovules spring directly into the ovary is a Placenta. Above the ovary the seed-vessel narrows into one or more thin stalk-like structures or Styles, which bear the Stigmas ; these are glandular swellings or expansions of various forms which retain the pollen that is carried to them, and by means of the moisture which is excreted from them induce the emission of the pollen-tubes. ^ In reference to what follows compare Hofmeister, Neue Beitrage, pt. II. (Abhand. der kcinig. Sachs. Gesellsch. VII); also Reichenbach, De poUinis orchidearum genesi, Leipzig 1852; and Rosanoff, Ueber den Pollen der Mimosen (Jahrb. fiir wissensch. Bot. VI, p. 441). 2 In many Mimosege the anther is, according to Rosanoff, octilocular, two pairs of small locuU being formed in each anther-lobe ; the pollen-cells of each pollen-sac remain united into a mass. ^ Compare with this Payer's view (Organogenic de la fleur, p. 725), which differs in some essential points. ANGIOSPERMS, a^q The Gynoeceum is always the final structure of the flower. When the floral axis has attained a suflicient length, the gynaeceum is formed at its apex ; if the axis is flat, disc-like, or expanded, it stands in the centre of the flower ; if it is hollowed out or cup-shaped, the gynaeceum is placed at the bottom of the hollow, in the centre of which lies the apical point of the floral axis. In the diagram of the flower, Figs. 351 /, and 353 B, where each outer circle represents a lower transverse section, and each inner circle a higher one, the gynaeceum necessarily appears always as the innermost central structure of the flower, the longitudinal displacements on the floral axis being neglected in the construction of the diagram. When the axial part of the flower, the Receptacle or Torus, is so elevated in the Fig. ^sx.—Fiitomus umbellatus: A flower (natural size); B the gynaeceum (magnified), the perianth and stamens removed, n the stigmas ; C horizontal section through three of the monocarpellary ovaries, each carpel bearing on its inside a number of ovules ; D a young ovule ; E an ovule immediately before fertilisation, ii the integuments, K the nucleus, KS the raphe, em the embryo-sac ; F horizontal section through the stigmatic portion of a carpel (strongly magnified), pollen-grains attached to the stigmatic hairs ; G horizontal section of a quadrilocular anther, but the valves z are so separated at ^ that it then appears bilocular ; // part of an anther-lobe (corresponding to /3 in G), y the point where it has become detached from the connective, e the epidermis, x the fibrous layer of cells (endothecium) ; / diagram of the entire flower ; the perianth // consists of two alternate whorls of three leaves, as also does the androecium, but the stamens of the outer whoriy are double, those of the inner whorl y simple and thicker ; the gynaeceum also consists of two whorls of three carpels, an outer c and an inner whorl c' ; there are therefore six alternate whorls of three, the members of the first staminal whorl being doubled. centre that the base of the gynaeceum lies evidently above the stamens, or at least in the middle of the androecium, the perianth and the androecium, or even the whole flower, is said to be hypogynous (Fig. 351). When, on the contrary, the receptacle is hoflowed out like a cup or saucer, bearing the perianth and stamens on its annular margin, while the gynaeceum springs from the bottom (Fig. 353 A), the flower is said to be perigynous. It is obvious that intermediate forms are possible between extreme cases of hypogynous and perigynous flowers; and these are in fact 4^0 PHANEROGAMS. common, especially among Rosiflorae. In both these forms of flower the gynaiceum is free, the receptacle taking no part in the formation of the wall of the ovary, although this appears to be the case externally in some perigynous flowers, as Pyrus and Rosa, The flower finally is epigynous when it possesses an actually inferior ovary. This latter is distinguished from the ovary which is buried in the receptacle of perigynous flowers by its wall being formed of the receptacle itself hollowed out into the form of a cup or even of a long tube. The carpels, which in the case of the free superior ovary form its whole wall, spring in the inferior ovary F'^- 3S3-— I'lower oi Elaaguusfusca A longitudinal section, d disc ; B dia- gram. Fig. 352. — Longitudinal section through the inferior avsxy oi Erynguan campestre; I sepals, c petals, y filament, gy style, h disc, A' A" nucleus of the ovule, z' integument. (like the perianth and the androecium) from the margin of the hollow receptacle, and only close up the cavity above, where they are prolonged into the style and bear the stigmas (Fig. 352). Intermediate forms are also not uncommon between the superior ovary of hypogynous and the inferior ovary of epigynous flowers ; the ovary may, for example, be composed in its lower half of the receptacle, in its upper part of the coherent carpels; transitional forms of this kind are found especially among Saxifragaceae. It will be easier to understand the diff'erent forms of the gynaeceum if the more important ones are considered separately ; and for this purpose the following classifi- cation mav be made : — ANGIOSPERMS. ^n I I. Gynseceum Superior ; flower hypogynous or perigynous. A. Ovules attached to the carpels. a. Ovary monocarpellary ; (a) flower with one ovary, (/3) flower with two or more ovaries. 6. Ovary polycarpellary ; • (y) ovary unilocular, (8) ovary multilocular. B. Ovules attached to the floral axis ; (e) ovule solitary, terminal, (C) ovules one or more, lateral. II. Gynseceum Inferior ; flower epigynous. C. Ovules parietal ; {r}) ovary unilocular, (6) ovary multilocular. D. Ovules axile ; (t) ovule solitar}-, terminal, (k) ovules one or more, lateral. The Superior Gymrccum is constructed essentially from a peculiar foliar structure, the carpellary leaves or carpels. These usually produce the ovules, which generally spring from the margins of the carpels, as in Fig. 354, but fre- quently also from the whole inner surface, as in Fig. 327 -^ (p. 467), and Fig. 351 C. The ovary is monocarpcUary when it consists of only a single carpel, the margins of which are coherent, so that the mid-rib runs along its back, and the ovules, when they are marginal, form a double row opposite to it. The inflexed margins of the carpellary leaf ma}-, however, swell up into thick placentae (as in Fig. 355) and produce a larger number of rows of ovules. The number of ovules is, on the other hand, not unfrequently reduced to two (as in Amygdalus), or only one {e.g. Ranunculus). In monocarpellary flowers there is only one such carpellary leaf, as in Figs. 353, 354 ; in polycarpellary flowers there may be two, three, or more, or even a very large number; if the number is two, three, or five, they usually stand in a whorl ; if four, six, or ten, they are generally arranged in two alternating whorls (see Fig. 351, B, I). When the number of monocarpellary ovaries in a flower is considerable, as in Ranunculaceoe, Magnolia, &c., the part of the axis which bears them is commonly elongated (to a very considerable ex- tent for example in Myosurus), and their arrangement is then spiral. The mono- carpellary ovary is originally always unilocular, though it may subsequently become multilocular from the production of ridges by the luxuriant growth of the inside of the carpel, which divide the cavity longitudinally into compartments, as in Astragalus, or transversely, as in Cassia fistula. Ovaries of this kind may be distinguished as monocarpellary with spurious loculi, but ought not to be called polycarpellary. 49^ PHANEROGAMS. A poly car pellary ovary is always the result of the union of all the carpels of a flower, the number being usually two, three, four, or five, arranged in one whorl, and the floral axis terminating in the midst of them. If the separate carpels remain open, and cohere in such a manner that the right margin of one unites with the left margin of another, the result is a unilocular polycarpellary ovary. The placentation FIG. ■i'^^.—Phaseohis vulgaris; A horizontal section through the flower-bud, / calyx-tube, c corolla, y^ filaments of the outer, a anthers of the inner starainal whorl, K carpel ; B longitudinal section of the carpel, with the ovules SK and stigma n ; C, D, E horizontal sections of carpels of different ages, SK the parietal ovules, g- mid-rib of the carpel. is in this case parietal when the coherent margins project only slightly inwards, as in Reseda, Viola, &c. But if the coherent margins of the carpels project further inwards, the cavity of the ovary becomes imperfectly multilocular, the chambers being connected with one another in the centre, as in Papaver, where the imperfect dissepiments are covered on both sides by a number of ovules. A bi- or multilocular Fig. 3SS.— Gynxceum of Saxifyaga cordi/olia ; A longitudinal section, g style, n stigma ; B horizontal section " at different heights, / placenta. polycarpellary ovary results when the margins of the carpels project inwardly so far that they meet or cohere either in the axis or periphery of the ovary, the elongation of the floral axis in the centre frequently contributing to this result. The mode of cohesion of the carpels in multilocular ovaries may vary greatly in other respects, according as it takes place along the whole length of their inflexed margins, or only below, while the upper parts resemble a whorl of monocarpellary ANGIOSPERMS. 493 ovaries (Figs. 355—358). Since the margins of the carpels which meet in the centre become developed into the placentae, the ovules make their appearance in the central angles of the loculi, as is seen in Fig. 357 ; but very commonly the margins of the carpels which turn in as far as the centre then split into two Z? Fig. jsC.—GynTcceum of Pyro/a umM/a/a ; y^ longitudinal section, J sepals, / petals, j;* filaments, /ovary, « stigma, (/ nectar-glands ; B horizontal section through the ovary, y the wall, // placentre. lamellae which are bent back and swell out into placentae in the middle of the loculi, as is shown in Fig. 356. It is clear that in this case the two placentae within FIG. riS7.-D^ctam„us Fraxinella; A young flower-bud, with rudiments of sepals s; B older flower-bud, ^^'^ ;;f '■"^"'^J' petals/; ' C still older state, with rudiments of the five stamens a, five more stamens a> arise between them, of which three are already visible ; b the b«act, b' a bracteole ; D-H development of the ovary //6, sk ovules, gp gynophore, g style. each loculus correspond to the margins of the same carpel which forms the outer wall of the loculus. Spurious dissepiments may arise in polycarpellary as in monocarpellary ovaries; if the polycarpellary ovary consists of two loculi, it may thus become quadrilocular, or five original loculi may become divided into ten. The first case is universal in 4^;4 PHANEROGAMS. I^abiatae and Borragineae. Fig. 359 shows that the ovary is formed of two coherent carpels, the margins of which (I-IV) projecting inwards form a right and a left placenta {pi)', on each of these placentae which correspond to the margins of the carpels a posterior and an anterior ovule are produced, but an outgrowth from Fir,. 35?.— Ripe fruit oi Dictamnns Fraxinella : the anterior carpel has been removed and tlie tivn lateral ones opened ; g gynobasic style (natural size). the mid-rib of the carpel (/F, VI, x) inserts itself between the two ovules belong- ing to each loculus, dividing it into two one-seeded lobes. Since at a sub- sequent period the outer part of the wall of each of the four lobes bulges strongly outwards and upwards {B), the separation of the bicarpellary ovary into four separate parts becomes still more distinct ; and finally they completely separate as Fig. 359. — / — W/ stages of development of the ovary oi Phlornis pKngens, Vm longitudinal, the rest in horizontal section ; A a gj'njeceum seen from without ready for fertilisation ; B the same in longitudinal section, the lines it n, oo correspond to the horizontal sections I'l and V'll ; pi the placenta, x the spurious dissepiment, /"loculi, sk ovules, <: wall of the carpel, t disc, g stj'le, n stigma. one-seeded lobes of the fruit ; while in Borragineae the separation is still more complete. The division of the five loculi of the ovary of Linum into ten by spurious dissepiments is not so perfect, the projections from the centres of the carpels not reaching the central axis of the ovary. Before passing to the consideration of ovaries with axile placentation, it should ANGTOSPERMS. and C tlie apical swelling of two embryo-sacs e with the embryo eb attached to it ; the pro-embryo in A' is two-celled. later, and the centre of the sac is filled in the unripe seed with a clear vacuole-fluid. In the embryo-sac of the cocoa-nut, which grows to an enormous size, this fluid — the cocoa-nut-milk — remains until the seed is fully ripe, the tissue of the endosperm forming a layer only some millimetres in thickness, which lines the inside of the testa. The very narrow elongated embryo-sacs of plants with small seeds, as Pislia and Arum, arc filled up by a single longitudinal row of cells formed by free cell- formation. In a large number of dicotyledonous plants (as Loranthaceas, Oro- FlG. 370. — P'iola tricolor, posterior part of the embryo-sac, £• its cell-wall, 3" the cavity of the cell, A', A' young endosperm-cells wliich have been produced in the protoplasm pr. banchcoe, Labiata?, Campanulacea?, &c.), with long narrow tubular embryo-sacs, the space of the embryo-sac is first of all divided by two septa, further divisions succeeding in all or some of the cells thus formed ; the tissue of the endosperm is produced from these last cells, and in this case often fills up only certain parts of the embryo-sac; or the sac is divided by a septum into two daughter- cells, the upper of which contains the rudimentary embryo, and produces endosperm in small quantities by free cell-formation {e.g. Nymphaea, Nuphar, Ceratophyllum, Anthurium^). In a few families only the formation of endosperm is rudimentary, and limited to the temporary appearance of a few free cells or nuclei, as in Tropie- * For further details of these processes desciibcd by Hofmeister, vide infra, under the character- istics of Dicot\ledon3. ,IZ PHANEROGAMS. olum, Trapa, Naiadeoe, Alismacese, Potamogetoneae, Orchideae ; in Canna even this rudimentary production of endosperm appears to be suppressed. During the formation of the endosperm, the embryo-sac usually increases in size, and thus displaces the tissue of the nucleus which still to a certain extent surrounds it ; only in a few cases is the nucleus still partially or entirely preserved ; it becomes filled with food-materials, like the endosperm, and replaces this latter as a reservoir of reserve-materials for the embryo. In Scitamineae {e. g. Canna), this tissue, the Perispcr??i, is very strongly developed, while the endosperm is alto- gether wanting ; in Piperaceae and Nymphaeaceae there is a small endosperm in the ripe seed, lying in a protuberance of the much larger perisperm. While the endosperm surrounded by the embryo-sac increases in size, the Tes/a is formed from the development of the integuments which accompanies that of the endosperm ; but in Criiium capense and some other Amaryllideae the growing endosperm is stated by Hofmeister to burst the testa and even the wall of the ovary; its cells produce chlorophyll, and the tissue remains succulent and forms intercellular spaces (which does not occur in other cases). In Ricinus a similar growth takes place when the ripe seed germinates in moist earth, bursting the testa (according to von Mohl) ; and the endosperm, previously ovoid and from 8 to lomm. long, is transformed into a flat broad sac 20 to 25 mm. in length, which surrounds the growing cotyledons until they have absorbed all the food-materials from it. In Monocotyledons and many Dicotyledons the embryo remains small and either enveloped by the endosperm or lying by its side (as in Grasses) ; its cells, which are in close contact without intercellular spaces, become filled, until the seed is ripe, with a protoplasmic substance and fatty matter or starch or both, in which case they remain thin-walled; the endosperm then appears as the mealy (full of starch) or fatty nucleus of the ripe seed, the embryo being found by its side or within it ; but it is often horny in consequence of a considerable thickening of its cell-walls which have the power of swelling {e.g. the date and other Palms, Umbelliferse, Coffea, &c.) If this thickening has taken place to a very great extent, the endosperm may fill up the testa as a hard mass, forming, for instance, the ' vegetable ivory ' in the Phytelephas. In these cases the thickening-masses of the endosperm-cells, which are absorbed during germination together with their proto- plasmic and fatty contents, serve for the first nourishment of the embryo. The ripe endosperm, when copiously developed, has usually the form of the entire ripe seed, being uniformly covered by its testa ; its external form is therefore generally simple, often round ; although considerable deviations from this frequently occur, especially among Dicotyledons. Thus, for instance, the substance known as the 'coffee-berry' consists, with the exception of the minute embryo w^hich is concealed in it, entirely of the horny endosperm ; but this, as a transverse section shows, is a plate folded inwards at its margins. The marbled (ruminated) endosperm which forms the nucleus of the nutmeg (the seed of Myristica /ragrans), and the areca-nut (the seed of the areca-palm), owes its appearance to the circumstance that an inner ^dark layer of the testa grows in the form of radiating lamellae between narrow fold-like protuberances of the light-coloured endosperm. The ripe endosperm is either a perfectly solid mass of tissue, or it possesses an inner cavity, as in Sirychnos ANGIOSI ERMS. 5-^3 Niix-vomica, where, like the seed itself, it is broad and flat. This is clearly the result of the endosperm which grows inwards from the periphery of the embryo-sac leaving a free central space, which, as has already been mentioned, is very large and filled with fluid in the case of the cocoa-nut. In these cases the endosperm is therefore a hollow thick-walled sac, enclosing a roundish or flattened cavity. In a large number of families of Dicotyledons, the first leaves of the embryo, the Cotyledons, grow, before the seeds are ripe, to so considerable a size that they displace the endosperm which was previously present, and finally fill up the whole space enclosed by the embryo-sac and the testa ; while the axial part of the embryo, and the bud (plumule) that lies between the bases of the cotyledons, attain even in these cases only inconsiderable dimensions. In these thick fleshy or foUaceous cotyledons (which are then usually folded), the reserve of food-material accumulates, consisting of protoplasmic substance or starch and fatty matter, which is in other cases stored up in the endosperm, and is made use of during the development of the seedling. This storing of the cotyledons with so large a quandty of food-materials appears to take place by its transference from the endosperm; and hence the diff"erence between those seeds which in the ripe condition contain no endosperm ['exalbuminous'], and those which do contain it ['albuminous'] con- sists essentially only in the fact that the food-material of the endosperm has passed over in the former case before germination into the embryo ; while in the latter case this only takes place during the process of germination. The presence or absence of the endosperm in ripe seeds is more or less constant within large groups of forms, and is therefore of value in classificadon. Of the better-known families, for example, the Compositoe, Cucurbitaceae, Papilionaceae, Cupuliferae (the oak and beech), &c., are destitute of endosperm. Sometimes also the embryo increases so greatly in size that the endosperm appears as a thin skin surrounding it. We must now recur to the fertilised ovule in order to follow the formation of the Embryo. In Angiosperms, as in Gymnosperms, the embryonic vesicle is not immediately transformed into the embryo; the end which faces the micropyle coalesces in its growth with the wall of the embryo-sac at its apical swelling; its free end turned towards the base of the ovule then lengthens, and under- goes one or more transverse divisions. The Pro-embryo, or Suspensor as it is more frequently called, thus formed, usually remains short (Fig. 369, p. 511); somedmes, as in Funkia, its basal cell swells up into a globular form (Fig. 368, p. 508) ; in other cases (as, according to Hofmeister, in Loranthus) the embryonic vesicle lengthens, and penetrates to the considerably enlarged base of the long tubular embryo-sac, and there forms the embryonic vesicle within the endosperm. In those Dicotyledons where the endosperm is formed only at certain lower parts of the embryo-sac by division, a similar elongation of the embryonic vesicle is usual, although not to so great an extent {e.g. Pedicularis, Catalpa, Labiatse). The apical cell of the two- or more-celled pro-embryo which is turned towards the base of the embryo- sac, and therefore also towards that of the ovule, is rounded off" into a spherical form, a longitudinal or only slightly oblique division-wall first of all makes its appearance in it, indicadng the commencement of the formation of the embryo (see also Fig. 14, p. 17). As this grows by rapidly repeated divisions, a spherical or ovoid mass of small-celled tissue is produced, on which the first foliar l1 5H PHANEROGAMS. Structures, the cotyledons, subsequently arise, while the rudiment of the first root may be observed in the differentiation of the tissue at the boundary-line of the pro- embryo and embryo. The first cells of the embryo are not unfrequently disposed as if they had resulted from oblique divisions of an apical cell in two or three directions (Fig. 369 C), a supposition which is completely supported by the oblique position of the first septum of the apical cell in the pro-embryo ; in Rheum I also found the apex of young embryos to present an appearance which suggested the existence of a three-faced apical cell. According to Hanstein's new and prolonged researches, the process is, nevertheless, different ; he asserts that the first longitudinal wall, even Fig. 371.— Formation of the embryo of Monocotyledons (Alisma) (after drawings by Hanstem) ; I—VIII various stapes of development; v the pro-embryo, h the liypopliysis. iv the region in which the radicle is formed, / the region in which the plumule is formed, c cotyledon, b first leaf (VII and VlII much less magnified than the rest ; the dermatogen is shaded). when it stands obliquely to the last transverse wall, is still in the median plane of the body of the embryo which is being formed, and is frequently at right angles to the last transverse wall, and therefore in the axis of growth of the pro-embryo \ The formation of this median longitudinal wall in the primary embryonic vesicle com- pletely excludes the possibility of a bi- or pluri-seriate segmentation of the apical cell. We learn from Hanstein that the mode of formation of the embryo of Monocotyledons may be seen remarkably clearly in Alisma. In Fig. 371, //, are shown, above the pro-embryonic cell v, two other cells a and c lying one over the other, the last of ^ The description in the text is taken from Hanstein's preliminary publications (Monatsberichte der niederrhein. Gesellsch. fur Natur- und Heilkunde, July 15 and August 2, 1869), as well as from more detailed communications in letters. Professor Hanstein has also had the kindness to allow me the sight of a number of drawings; and, with his permission, the figs. 371-374 are copied from them. I have also had the opportunity, in the summer of 1869, of seeing preparations of Hanstein's ^similar to Fig. 372. Compare also Hanstein, Botanische Abhandlungen, Heft I, for a more detailed description of the development of the embryo in Monocotyledons and Dicotyledons. [See also Quart Joum. Micr. Soc. 1873, p. 51.] ANGIOSPERMS. cir which is already divided by a longitudinal and a transverse wall into four cells arranged like quarters of a sphere. A comparison of the states //— V shows that the further development advances first of all in a basipetal direction. A cell w or h, the result of intercalary division, which arises between the end of the pro-embryo and the body of the embryo a c already formed, is especially to be noted. It is from this that the root is subsequently developed. Hanstein calls it and the tissue which proceeds from it the Hypophysis. Before the body of the embryo undergoes any external differentiation, its primary meristem separates into a single peripheral layer (shaded in the drawing), and a tissue internal to this ; the former is the primary epidermis or dermatogen, which continues to grow only at the surface and divides only in a radial direction ; the figures IV— VI show that the dermatogen is marked off from the primary cells of the embryo by tangential divisions proceeding towards the base. The inner mass of tissue soon undergoes further differentiation ; an axial string of tissue is produced by divisions, especially longitudinal, forming the plerome or tissue which subsequently produces the fibro-vascular bundles ; the primary meristem lying between the plerome and the dermatogen, and which undergoes copious transverse divisions, is the periblem, /. e. the primary cortical tissue. At the same time that this differentiation of tissue is first indicated in the upper part a c oi the embryo, it begins also in the hypophysis h. The lower layer of this hypophysis takes no part in the formation of the dermatogen, while from its upper layer (in VF) is formed a prolongation of the dermatogen and of the periblem of the body of the embryo, from which, as will be explained further on, the root is developed as a posterior appendage of the embryo. Hanstein designates the apical part c of the embryo the first cotyledon, at the base of which h the apex of the stem is afterwards formed laterally. But if the cotyledon is really the apical structure of the embryo, which seems to me to be not yet sufficiently established, it cannot possibly be a foliar structure, even if (as in Allium) it subsequently assumes altogether the appearance of a foliage-leaf. The different stages in the development of the embryo from the embryonic vesicle are much more clearly seen in Dicotyledons than in Monocotyledons, the Grasses in particular among the latter presenting difficulties. Hanstein has singled out Capsella Bursa-pas ton's for detailed description. Fig. 372 shows first of all that the mass of the embryo is developed from the spherical apical cell of the pro- embryo V, and in what manner this takes place; here also a basal cell k of the body of the embryo forms the hypophysis, from which the radicle is developed. The spherical primary cell of the embryo divides first by a longitudinal wall i, i (in / — IV), followed in each of the two halves by a transverse division 2, 2, so that the body of the embryo consists at first of four quarters of a sphere, each of which next undergoes another tangential division, by which four outer cells are formed as the rudiment of the dermatogen, and four inner central cells (//). While the first only multiply by radial divisions, the inner mass of tissue grows in all directions, resulting at an early .period in its differentiation into plerome (///, IV, V, shaded in the drawing), and periblem. The mass of tissue which is produced from the primary cell of the embryo thus increases rapidly by the multi- plication of its cells, and two large protuberances (F, cc), the first leaves or cotyledons, soon make their appearance one on each side of the apex {s) ; the L 1 2 5i6 PHANEROGAMS. apex of the stem exists for the present only as the end of the longitudinal axis of the embryo; an elevated piece of tissue, the vegetative cone of the stem, is not formed till later deeply enclosed between the cotyledons. The posterior or basal end of the axis of the embryo after the differentiation of its primary meristem into dermatogen, periblem, and plerome (//, ///, IV), is, so to speak, open as long as this differentiation has not also taken place in the hypophysis (/z) ; but finally it takes place in it also and in such a way (as is shown in Fig. 372, V), that ff 1 Fig 372.— Formation of the embryo of Cap^elld Bursa-pastOyis (after Hanstein) ; I— VI various stages of development, Vb apex of the root seen from below; i, i, z, 2, the first divisions of the apical cell of the pro-embryo (suspensor), h h the hypophysis, u the pro-embryo, c the cotyledons, s apex of the axis, w root (the dermatogen and plerome are shaded dark). the upper of its two cells breaks up into two layers [K), the outer of which becomes continuous with the dermatogen of the axis, while the inner layer forms a pro- longation of the internal axial tissue. The low^r cell of the hypophysis {h) divides cross-wise {V h, seen from below) and may be regarded as a transitional structure between pro-embryo and root (appendage of the root) or as the first layer of the root-cap. Hanstein's description of the growth of the root-cap of Phanerogams, confirmed by Reinke\ is of very great value, showing, as may be seen from » Compare also Reinke, Wachsthumsgeschichte und Morphologic der Phanerogamenwurzek in Hanstein's Bolanische Abhandlungen, Bonn 1871, Heft III. ANGIOSPERMS. 517 Figs. 373 and 374, that it is simply a luxuriant growth of the dermatogen. This peripheral layer of tissue, which elsewhere remains simple, and passes over into permanent tissue in forming the epidermis, increases in thickness, on the contrary, where it covers the puncium vegetaiionis of the root, and undergoes repeated tangential divisions (parallel to the surface). Of the two layers which are succes- sively formed on each of these occasions, the outer becomes a layer of the root-cap (Fig. 373 wh, and Fig. 374, 2); the inner remains as dermatogen and again undergoes the same process. This dermatogen which covers the vegetative cone of the root behaves therefore like a layer of phellogen, with this difference, that the cells produced from cork-cambium become at once permanent cells, while those of the root-cap remain still capable of division; so that each layer split off as it were from the dermatogen forms a cap consisting of several layers of cells ; its growth being most active in the centre, and diminishing towards the periphery. The splitting of the dermatogen into two lamellae usually progresses from the V\C,. -573. — Diagrammatic rcpresentatif>n of tlie formation of the primary root in Monototylcilons and its connection with the stem (after Hansteiii) ; V pro-embryo, h liypophysis, iv iu line of separation of the root and stem, ivh layer of the root-cap, d derma- togen, pb pcriblem, // pleronie. Fig. 374. — Diagi-ammatic representation of the formation of the embryo of Dicotyledons (after Hanstein) ; i, 2, the first layer Of the root-cap,/ peri- blem, d dermatogen, pi plerome. apex towards the periphery of the apex of the root ; in the secondary roots of Trapa, Hanstein and Reinke state that the reverse is the case. Lateral roots not unfrequently arise in the embryo even before the ripening of the seed, in addition to the primary root which we have hitherto alone considered ; as, for instance, in many Grasses and some Dicotyledons {e.g. Impatiens, according to Hanstein and Reinke, Cucurbita from my own observations). In Trapa natans the primary root soon becomes abortive, laferal roots arising at an early period from the hypocotyledonary portion of the axis. Hanstein and Reinke state that the lateral roots of Angiosperms have their origin in the pericambium, in Nageli's sense of the term\ Their development was found in several plants to harmonise with this. In Trapa naians, for example, it is as follows : — A group of cells of the mantle of pericambium which consists of only one layer divides radially; the newly formed cells elongate in the same direction, and then divide tangentially ; the outer of the two layers produces the dermatogen, * Compare what was said on Fig. 1 15, p. 145. 5l8 PHANEROGAMS. the inner the body of the root. The dermatogen, pushed outwards by the development of the body of the root, produces the root-cap in the way already mentioned ; the tissue of the body of the root itself which is covered by it becomes differentiated into plerome and periblem. The same process takes place in Pistia, and probably also in Grasses. Hanstein and Reinke do not find 'anywhere an apical cell which originates the growth, as in Cryptogams ; a group of cells always obeys the common direction of growth.' The variation in the size of the embryo in the ripe seed of Angiosperms has already been mentioned w^hen speaking of the endosperm. The external differ- entiation sometimes goes no further than the rudiment of the root (radicle) at the posterior end of the stem of the embryo and the cotyledons {e.g. in Cucurbita, Helianthus, Allium Cepa, &c.), between which lies the naked punctum vegeiaiionis. But frequently this latter undergoes further growth before the seed is ripe, and produces additional foliar structures (as in Grasses, Phaseolus, Faba, Quercus, Amyg- dalus, &c.), which are then included, in the ordinary nomenclature, under the term Plumule, but do not unfold until the germination of the seed. The systems of tissue are usually sufficiently clearly differentiated as such at the period of maturity of the seed ; but the different forms of permanent tissue do not become developed till later, during germination. A striking exception to this advanced development of the young plant within the ripening seed is afforded by parasites and saprophytes destitute of chlorophyll, but especially by Orchidese. In them the embryo remains until the seed is ripe as a roundish corpuscle consisting sometimes of only a few cells, without any external differentiation into stem, leaves, and root; this takes place only after germination, and even then sometimes quite imperfectly. Development of the Seed and Fruit. While the endosperm and embryo are becoming perfectly formed in the embryo-sac, growth proceeds not only in the ovule but also in the wall of the ovary that encloses it. Since the testa is formed at the expense of the whole or part of the cellular layers of the ovular integuments, and presents extreme diversities in its structure, the ovule, together with its contents which have resulted from fertilisation, becomes the Seed. The wall of the ovary, the placentae, and the dissepiments, not only increase in dimensions, but undergo the most various changes of external form and still more of internal structuie. Together with the seeds they constitute the Fruit. The transformed wall of the ovary now takes the name of Pericarp ; if an outer epidermal layer is specially differentiated it is called the Epicarp, and an inner one the Endocarp ; while a third layer, the Mesocarp, frequendy lies between these two. A number of typical kinds of fruit are distinguished according 'to the original form of the ovary and the structure of its tissue when ripe, the nomenclature of which will be given in the sequel. But sometimes the long series of deep-seated changes induced by fertili- sation extends also to parts which do not belong to the ovary, and even to some which have never belonged to the flower. But as they are part of the fruit from a physiological point of view, and are usually associated with it as a whole, while sharply differentiated from the rest of the plant, a structure of this kind (such as the fig, strawberry, and mulberry) may be termed a Pseudocarp. At a certain period either the fruit together with its seeds becomes detached from the rest of the plant, or the seeds alone separate from the dehiscent fruit ; and ANGIOSPERMS. -, this is the period of maturity. In many species the whole plant dies down when the fruit IS ripe, and a plant of this description is termed mo?wcar/>/c (bearing fruit only once). Monocarpic plants may be distinguished into those which fructify in the first period of vegetation (amma/ plants), those which do not till the second year (dienma/ plants), and finally not till several or a large number of periods of vege- tation (monocarpic perejmial plants, as Agave aviericana). Most Angiosperms are however poly car pi c ; i e. the vital power of the individual is not exhausted by the ripening of the fruit ; the plant continues to grow and periodically fructifies afresh, or is polycarpic and perennial. anse The Inflorescence. It is comparatively rare for the flowers of Angiosperms to singly at the summit of the primary shoot or in the axils of the leaves; peculiarly developed systems of branching are much more commonly produced at the' end of the primary shoot or in the axils of its foliage-leaves, which usually bear a considerable number of flowers and are distinguished by their collective form from the rest of the vegetative system ; in polycarpic plants these may even be thrown off after the ripening of the fruit. Such a system of branching is termed an hflorescence. The habit of the inflorescence docs not depend merely on the number, form, and size of the flowers which it bears, but also on the length and thickness of the branches of different orders, as well as on the degree of development of the leaves from the axils of which the branches spring. These leaves are generally much simpler in form and smaller than the foliage- leaves ; frequently coloured (/. e. not green) or altogether colourless. They are dis- tinguished as Hypsophyllary Leaves or Bracts ; and in this term are frequently included the small leaves which spring from the pedicels and which often have no axillary shoots {Bracteoles). Leaves of this kind are sometimes entirely absent from the inflor- escence or from certain parts of it ; the ultimate floral axes or pedicels of the flowers are then not axillary, as in Aroideap, Cruciferae, . PHANEROGAMS. be placed after the niiniber ol" the first whorl ; thus 5,-^ P^ | St^' Q might represent the formula for Hypericum calycinum (Fig. 377, p. 524), Str^ indicating that the androecium consists of five branched stamens which are superposed on the petals. If, finally, it is desired to signify that members of a second whorl are interposed at the same level between those of one already in existence, the number of the new members may be placed simply beside those of the original whorl ; thus the formula 5^ Fr^ 5/5.-, C5 would correspond to the diagram, Fig. 383, p. 528. In the formulae already given no cohesions of any kind have been indicated ; they can however under certain circumstances easily be expressed by special symbols. Thus, in the formula for Convolvulus, S^P^Str^C^, the sign P^ indicates a gamopetalous corolla of five petals, Q a syncarpous ovary of two carpels. In the formula for the flowers of Papilionaceae again S-^Pr^Sff+^^^C^, the expression Stf^^^^ signifies that the five stamens of the outer and four of those of the inner whorl have united into a tube, while the posterior stamen of the inner whorl remains free^. The mode of writing the formulae must vary according to the object which one has in view ; the greater the number of relationships it is intended to express, the more complicated will they become ; and care must be taken that they do not lose their ' clearness by being overladen by too many signs. The examples of formulce which have hitherto been adduced all illustrate cyclic flowers; those parts of flowers which are arranged spirally may be denoted by the symbol '^ placed before them, and the angle of divergence may also be affixed to their number. Thus, for example, the relative numbers and positions of the parts of the flower of Aconitum, according to Braun's investigations, m.ay be expressed by the formula 8,^2 5 Pr^.-. s ^^^^k/. 00 C^.^, which indicates that all the foliar structures of this flower are arranged spirally, and that the calyx consists of five sepals with the divergence ^/g, the corolla of eight petals with the divergence ^ g, and the androecium of an indefinite number of stamens with the divergence V^i- It would however be sufficient in this case, since the spiral arrangement runs through the whole flov.-er, to place the symbol only once before the whole formula, thus — So p, o 6*/^ Co In flo\\ers with a cyclic arrangement of their parts a statement of the angle of divergence is generally unnecessary, since the members of each whorl usually arise simultaneously, and are arranged so as to divide the circle into equal parts. When they do not arise simultaneously but successively in the circle with a definite angle of divergence, as in most trimerous or pentamerous calyces, this can be indicated by placing the angle of divergence after the number of the members; thus the formula for Linaceae would be Sr^i . Pr,St^C^. If, on the other hand, the members of a whorl are formed in succession from front to back, this may be shown by an arrow pointing upwards ^, as in the formula for Papilionaceae S-] P^T 5/-t45t Q. If they are formed in succession from back to front, the arrow may be made to point downwards \, as in the formula for Reseda S^i Pni Stpi^gi Q., where the number of the parts is expressed by letters instead of figures in consequence of its variability''^. Order of Succession of the Parts of the Flo-iver. The foliar structures arise on the axis of the floral shoot, as on other axes, in acropetal order below the growing apex. It is however not uncommon in the formation of flowers for the apical growth of the axis (o cease altogether or to become extremely slow, while the receptacle continues to increase in breadth, and to develope transverse zones of intercalary growth. When this is the case the acropetal order of development is disturbed, and new whorls may become interposed between those already in existence. But even within the same floral whorl the individual members may be formed in a very different order of succession, according as the zone of the receptacle which bears the floral leaves is developed in a uniform * See also Rohrbach, Bot. Zeitg. 1870, pp. S16 et seq. ^ See Payer, Organogonie tie la fleur ; also our Fig. 137. p. \(X). ANGIOSPERMS. 53' manner all round (as in polysymmctrical flowers) or more rapidly on the anterior or the posterior side (which is especially the case in monosymmetrical or zygomorphic flowers). In flowers with a spiral arrangement of their parts i, disturbances of the acropetal order of development are of less importance the more numerous the parts w ith a spiral arrangement, and the longer the apical growth of the floral axis continues. Those mem- bers which have a spiral arrangement arise one after the other in ascending order; the angle of divergence may either be constant or may change. Thus, according to Payer, in Ranunculaceae and ^Nlagnoliaceae the perianth-leaves and stamens arise in a continuous spiral, but each whorl of stamens consists of a larger number of members than the whorls of perianth-leaves ; thus, e. g., in Hellebona odorus, where all the organs of the flower are arranged spirally, each whorl of the corolla consists of only thirteen petals, while each whorl of stamens numbers twenty-one. According to Braun the whorls of the calyx of Delph'mium Consolida have a % arrangement - ; the divergence then under- goes a small change, but without materially deviating from %; the first whorl with this altered arrangement is the corolla ; the three following ones are the stamens, and the spiral terminates with a single carpel. In the section Garidella of Nigella the first of the w^horls with a- 5 angle of divergence is the calyx and the second the corolla; then follows a slight change in the angle to "^ ,-, the stamens forming one or two whorls with this arrangement ; and the spiral closes with three or four carpels. In the section Delphi- nellum of Delphinium the calyx constitutes a whorl with '^/g, the corolla one with % angle of divergence ; then follow two or three whorls of stamens with the angle very near %, the spiral closing with three carpels. In the section Staphisagria of the same genus, and in Aconitum, the calyx forms a whorl with %» the corolla one with ^, g angle ; the stamens stand in one or two whorls with the divergence Vsi 01* ^'Ai 5 concluding with three, five, or rarely a larger number of carpels. It must be noted in reference to these arrangements that the members of successive whorls stand in orthostichies when the angle of divergence remains constant ; but that the orthostichies pass into oblique rows when the divergence undergoes a small change. The first thing to observe in cyclic flowers [i.e. those in which the parts are arranged in whorls) is the order of formation of the whorls with respect to one another, and then the order in which the members of each whorl are themselves formed ; although the two are in fact closely connected. A disturbance of the acropetal order of succession in the formation of the whorls occurs when the carpels have begun to be formed before all the stamens which stand below them have been produced, as in Rubus, Potentilla, and Rosa\ or when the calyx is not formed until after the androecium (as in Hypericum cnlycimim according to Hofmeister), or when the calyx is not observable until after the corolla has become considerably developed or even after the formation of the stamens and carpels, as in Compositgp, Dipsacaceae, Valerianaceap, and Rubiacese. One of the most remarkable deviations from the general rule of the order of develop- ment of the floral whorls occurs in Primulaceae, where five protuberances (primordia) appear on the receptacle above the calyx, each of which grows up into a stamen, while on the posterior or lower side of the base of each primordial stamen a lobe of the corolla subsequently appears. Pfefl'er, who has observed this order of development (Jahrb. flir wissensch. Bot. vol. VII, p. 194), considers that the same probably also happens in the pentandrous Hypericineae and in Plumbagineae ; he therefore explains the corolla- lobes as posterior outgroAvths of the stamens (a posterior ligular structure), such as, for ' Compare Payer, Organogenie de la fleur, p. 70; e/ seq. ; and Braun, Jahrb. fiir wis-ensch. Bot., Ueber den Bltithenbau der Gattung Delphinium. 2 Compare with this what is said below respecting sepals and petals which are formed with the angle of divergence Y3 and Y5. 2 Compare Plofmeister, AUgemeine Morphologic,, pp. 436 ei seq., where Payer's ol servations on this point will also be found. >I m 2 532 PHANEROGAMS, instance, occur on the stamens of Asclepiadeae in the form of hood-shaped nectaries, where a true corolla is also present. The flowers of Primulaceae would therefore be strictly apetalous in the morphological sense of the word, since their corolla is not a true floral whorl, but only an outgrowth of the staminal whorl. In other families of Dicoty- ledons, on the other hand, superposed corollas and androecia arise separately and in acropetal order ; as, for instance, in Ampelidese, probably also in Rhamnacese, Santalaceae, and Chenopodiaceae. The individual members of a floral whorl may arise in succession from front to back or the reverse, especially when the flowers themselves are subsequently developed zygomorphically. Thus, for instance, in Papilionaceae the ' anterior median sepal is formed first, then simultaneously one to the right and one to the left, and finally the two posterior ones ; but before these last arise the two anterior petals appear, followed by the two lateral and finally the posterior one ; and the androecium, consisting of two alternating whorls of five stamens each, is formed in the same manner from front to backi. In the Resedaceae on the contrary (Reseda and Astrocarpus), Payer states that the petals, stamens, and carpels are developed from behind forwards on both sides {cf. Fig- 137, p. t66). When the calyx consists of pairs of sepals, those of each pair are formed, as Payer has shown, simultaneously ; but if the calyx consists of three or five sepals, they are usually formed one after another, and with the angle of divergence in one case Vs in the other %; but the succeeding whorls, the petals stamens and carpels, usually arise as simultaneous whorls, with the exceptions already named and others still to be spoken of. It is well to draw attention here to the circumstance that it does not follow from the order of succession advancing from one point, with a definite angle of divergence, say Vs or 75, that the arrangement is a spiral one^; it may just as well in such cases be a whorl. The nature of the arrangement depends on the circumstance whether the foliar structures in question are formed at the same height or not, /. e. at an equal distance from the centre of the flower ; if this is the case, we have a whorl ; but if the members arise in acropetal order at different heights, i.e. approaching the centre of the flower with each step in the divergence, the arrangement is a spiral one. I'he last appears to be actually the case in many calyces ; but it is doubtful whether it ever occurs where the angle of divergence of the sepals is V3 oi" " 5- We must now refer again to the cases already mentioned, where new members of a whorl are formed between those already in existence and at the same heights In the Oxalidcce, Geraniaceae, Rutaceae, and Zygophyllaceae, an entire w-horl of five stamens is thus interposed between those already present ; according to Payer, in Peganum Harmala^ a whorl of ten stamens is even formed in this manner, arising, not in pairs between the first five, but lower down at the bases of the petals ; whether the later formed stamens arise on the same level with the first or lower down is obviously regulated according to the space aff'orded by the changes of form of the growing receptacle. A still further departure from the ordinary process occurs in the Acerineae, Hippocastaneae, and Sapin- daceae, where Payer asserts that a whorl of five stamens is first of all formed alternating with the corolla, in which an imperfect whorl of tw'o or four stamens is subsequently interposed at the same height, as is shown by his illustrations. In Tropaeolum, on the other hand, according to Payer and Rohrbach*, three stamens first of all appear after ' On the nearly related Ccesalpineae see Rohrbach, Bot. Zeitg. 1870, p. 826. ^ Compare the successive true whorls of Chara and Salvinia, pp. 279, 389. ^ Compare also on this point Pfeffer, Jahrb, fiir wiss. Bot. vol. VIII, p. 205. * Rohrbach (Bot. Zeit. 1869, Nos. 50, sO however gives a different explanation to these observa- tions from that mentioned here. The equal or greater distance at which the later stamens arise from the centre of the flower is a distinct proof that one cannot in this case suppose that the parts are produced in a spiral arrangement advancing from without inwards. ANGIOSPERMS. 133 the formation of the petals, and then between them five others, the distance of which from the centre of the flower is however rather greater than that of the three earlier ones. Symmetry of the Floiver. If the observations which will be found on p. i66 et seq., under the head of General Morphology are now applied to the floral shoot, it is seen that true symmetry and distinctly bilateral structure occur here far more commonly than on the vegetative shoots. In contrast to the lax mode of expression used by many botanists, I understand by Symmetrical Structures those which may be divided into two halves, each of which is an exact reflex image of the other. If a flower can be divided in this manner by only one plane, I call it simply symmetrical or monosymtnetrical ; if, on the contrary, it can be symmetrically divided by two or more planes, it is, as the case may be, doubly or poly-symmetrical. The happy expression zygomorphic already used by Braun may be applied equally to monosymmetrical flowers and to those polysymmetrical ones in which the median section produces halves of quite a different shape from those caused by lateral section {e.g. Dicentra). I apply the term regular to a poly- symmetrical flower only when the symmetrical halves produced by any one section are Fig. 384.— Flower of HeracUum pubescens \svCs\ zygoraorphic corolla. exactly like or very similar to those produced by any other section; or— which comes to the same thing— when two, three, or more longitudinal sections divide a flower into four, six, or more equal or similar portions. In exactly defining the symmetrical relations of a flower, the relative positions of the parts, as represented by the diagram, must first of all be distinguished from the entire form of the flower, such as is realised in the development of the organs. If attention is paid first of all only to the relative positions of the parts, it is clear that they can never be distributed symmetrically in flowers with a truly spiral structure ; while in hemicvclic flowers those members at least which are arranged in whorls may possibly be distributed symmetrically. If, on the contrary, the parts are all arranged m whorls, they are usually distributed monosymmetrically or polysymmetrically on the receptacle. Thus, for example, the diagram Fig. 375 (P- 524) can be divided symmet- rically and irregularly by three planes, Fig. 376 by four, and Fig. 377 by five planes. The diagrams Fig. 378 B and C, as well as Fig. 379, can, on the contrary, be symmetri- cally halved by only one plane, which is at the same time the median plane. The diagram Fig. 380 can be divided by the median plane into two symmetrical halves which are 534 PHANEROGAMS. unlike those produced by the hiteral section; this diagram is, hke those in Figs. 378 ii, Cand 379, zygomorphic, but is doubly while these are only singly symmetrical. The symmetry of mature unfolded flowers is indeed usually connected genetically with the relations of symmetry of the diagram (which represents only the position and number of the parts); as will be made clear by a comparison of Pigs. 385 and 387 with Fig. 379 ^. But inasmuch as the entire form of the mature flower is essentially deter- mined by the shape, size, torsion, and curvature of the separate parts, these circum- stances also exert a preponderating influence on the relations of symmetry of the open Fig. 385. — Zygomorphic flower of Cohantica Schiedeana : A entire flower after removal of two sepals ; E andruecium ; C gynasceum ; D the coherent anthers magnified and seen from behind ; E horizontal section of the ovary ; F diagram ; a anthers, n stigma, g style, fk ovary, d the stamiiiode developed into a nectary, // the lateral oblique placentae. f ower, and to such a degree that even flowers that have their parts arranged spirally may become monosymmetrically zygomorphic in reference to their entire form, as is the case to a high degree, for example, in Aconitum and Delphinium. It must however be observed that the zygomorphism of the flower is here brought about principally or entirely by the calyx and corolla, the spiral arrangement of which may perhaps still be doubtful, but which always occupy so narrow a zone on the receptacle that their position may be considered practically to be verticillate. If, on the other hand, the floral axis is sufliciently elongated to show that the arrangement is a distinctly ascending spiral one, as in the perianth and androecium of Nymphaea and the androecium and gynaeceum of ANGIOSPERMS. 535 Magnolia, the subsequent development of the organs appears also not to show any zvgo- morphic nor indeed generally any kind of actually symmetrical arrangement The zygomorphic and monosymmetrical form occurs, on the contrary, very com- monly m those flowers the parts of which are arranged in whorls. A very distinctly zygomorphic arrangement is not unfrequently united with a partial or entire abortion of particular members, as, e.g., in Columnea, Fig. 385, and other genera of Gesnerace^, where the posterior stamen is transformed into a small nectary ; while in LabiatcT it is entirely wanting. This abortion is carried still further in Orchidcce, where, of the six typical stamens, only the median anterior one of the outer whorl or the two lateral anterior ones of the inner whorl are developed (see Fig. 379, p. 526). The final mono- symmetrical arrangement is sometimes to a certain extent indicated by the order of their formation, even in the rudimentary condition of the parts of the flower, when their origin is not simultaneous in the whorl, and does not progress wich a definite angle of divergence, but is so arranged that the development commences with one anterior or one posterior member, and then advances simultaneously right and left from Fig. 386.— Zyi,'oinnrphic flower of ro/j'^<7/a s>(i>'difIora : A entire flower seen from the side after removal of one sepal /: ; A flower divided syniiuetrically without the frynjeceum ; C the gyn:eceum magnified ; D horizontal section of the ovary ; /:" median longitudinal section of the ovary ; /•" horizontal section of the flower ; k calyx, c corolla, St staiiiinal tube. ' and C its folded lamina is seen cut across. Fig 3S9. — VX-aM. o{ Po!ygotiaf!i7n mnlliJJoruyn in its second year ; B its stem magnified, lu the unbranched primary root, iv' lateral roots spring- ing from the stem st, I foliage-leaf of the second year, k bud, c the scar where the cotyledon was attached, i and 2 scars of the first sheath-leaves which precede the foliage-leaf, /. /, // the succeeding sheath or cataphyllary leaves of the bud in B. (Cf. Fig. 135. p. 165.) developed from the primary root of jNTonocotyledons as is found in Gymnosperms and in many Dicotyledons ; sometimes no roots at all are produced, as in some Orchidaceous saprophytes destitute of chlorophyll (as Epipogium and Corallorhiza), which never possess any roots. The plumule of the embryo is usually completely enclosed in a single MONOCOTVLEDONS. r.^ sheath-like structure, the first leaf or cotyledon, which developes either into a sheath-like cataphyllary leaf or at once into the first green foliage-leaf of the young plant (as in Allium). Within the cotyledon there is generally a second and sometimes (in Grasses) a third and fourth leaf, which protrude on germination out of the sheath of the cotyledon, increasing by intercalary growth at their base ; these and the leaves which are formed subsequently are larger the later they are formed on the growing axis. The axis usually remains very short during germi- nation without forming any distmct internodes (Allium, Palms, &c.), or it lengthens more rapidly and becomes segmented into evident internodes (Zea and other Grasses). The increase in strength of the plant may take place by the powerful growth of the axis of the embryo itself, so that this at length forms the primary stem of the mature plant bearing the organs of reproduction, as for instance in most Palms, Aloes, Zea, &c. If the axis of the embryo remains short while it increases in strength, it may grow considerably in thickness and form a tuber (Fig. 389), or, if the bases of the leaves become thick (as in Allium Cepa), a bulb. If the axis of the embryo itself developes into the primary stem, whether into an upright one or a creeping rhizome, it first of all takes the form of an inverted cone, which is elongated or abbreviated according to the length of the internodes. lliis peculiarity, which belongs to Monocotyledons in common with Ferns, depends on the absence of any subsequent increase in thickness ; the portions of the stem first formed retain their size, while each successive portion is larger ; the diameter of the stem is therefore so much larger the nearer it is taken to the apex. As long as this growth proceeds, the stem continues to grow stronger; but sooner or later there comes a time when ever}^ portion of the stem acquires the same thickness as the previous one ; the stem then becomes cylindrical, or, if it is compressed like some rhizomes, still with a uniform breadth. The lateral shoots exhibit the same peculiarity when they spring low down from the primary stem (as in Aloe, &c.). But the primary shoot which springs from the embryo not unfrequently disappears after producing lateral shoots which grow more vigorously than it and then again transfer the further grow^th to new shoots, which now produce from generation to generation thicker axes, larger leaves, and stouter roots, until at length a condition again results in which each successive generation of shoots produces others of equal strength. If the portions of the axes of the shoots beneath the points where the shoots of the next order arise are persistent, synjpodia arise (as represented in Fig. 135, p. 165); but frequently each shoot entirely disappears after producing one of the next order, as for instance in our native tuberous Orchids (Fig. 150, p. 198), or in the crown-imperial (Fig. 390) or autumnal crocus (Fig. 391 )^ The normal Mode of Branching of Monocotyledons is always monopodia! and usually axillary 2; a bud is generally formed in the axil of each leaf, but often ^ Further details of the great vaiiety of modifications of these processes of growth will be found in Irmisch, Knollen und ZwiebelgewSchse (Berlhi 1850), and Eiologie und Morphologie der Orchidcen (Leipzig 18^3^. - According to Magnus (Bot. Zeit. i' 69, p. 770) the flower of Naias occupies exactly the place of 44 PHANEROGAMS. does not unfold, so that the number of branches visible is often less than that of the leaves (as in Agave, Aloe, Dracaena, Palms, many Grasses &c.). But some- times several buds are formed in the axil of a leaf, and if the insertion of the leaf is broad these are placed side by side, as occurs in many bulbs (Fig. 122, p. 154). In Musa a number of flowers even stand side by side in the axil of a bract, and in Musa Etisefe two rows one over the other. In the Spadiciflorse the bracts are often absent \ and the ebracteate flowers stand on the rachis of the inflorescence, but are distinctly lateral in their origin. This is also the explanation of the branching of Lemna, w'hich does not in general form any foliage-leaves, but the vegetative portion of the plant consists of disc-like or swollen pordons of the axis containing chlorophyll which braii-fh laterally out of one another, and are connected together only by slender stalks, or soon separate. The plane of Fig. 390.— Bii'.b oi Fri/iliaria iinperiali^ in November: A longitudinal section of the whole bulb reduced, z z the coalescent lower portions of the bulb-scales, bb their free upper portions; the scales enclose a cavity / which contains the decayed flower- stem ; next year's bud is formed in the axil of the innermost scale ; its first leaves Vill form the new bulb, while its axis will develope into the flower-stem; the root lu springs from the axis of this bud. B longitudinal section of the apical region of next year's bud, j- apex of the stem, bb' b'' youngest leaves. ramification coincides with the surface of the water on which they float ; each shoot produces only one or a pair of opposite lateral shoots, and the branching is therefore distinctly cymose, sympodial, or, as in Lenma trisuka, dichasial. Besides the formation of shoots by the branching of the axis, adventitious shoots also sometimes occur on leaves which perform the function of gemmae; as for instance on the margins of the leaves of Hyacinthus Pouzohii and some Orchids (Diill, Flora p. 348)-. The large gemmae which appear very regularly at \h^ point of junction of the leaf-stalk and lamina, and at the base of the lamina of Athenirus tenia/us, are especially striking. The small bulbs on the stem of the first leaf of a branch; but it appears from p. 771 as though the flower and the shoot that bears it were the bifurcations of a dichotomy. ^ Compare under Dicotyledons p. 554 ^ [On the buds developed on the leaves of Malaxis which exhibit a striking resemblance to the ovules of Orchideae, see Dickie, Journ. Linn. Soc. vol. xiv, pp. i and 180. Dr. Dickie considers the structure of these buds to favour the theory that the ovule is homologiDUS to a bud, the nucleus-like body of the bud corresponding to an axis. See also Henslow on Malaxis, Mag. Nat. Hist, vol.1. 1829, pp. 441. 442. — Ed.] MONOCOTYL EDONS. 5-15 Lilium hulbiferum are, on the other hand, normal axillary shoots, and probably the same is the case with those on the inflorescence of some species of Allium. Adventitious buds are stated by Hofmeister to occur on the roots of Epipactis inicrophylla. The Leaves of Monocotyledons are seldom verticillate, though this occurs in the foliage-leaves of Elodea and the bracts of Aiisma; they are very commonly arranged alternately in two rows, as in Gramineae, Irideoe, Phormium, Clivia, Typha' Fig. 391.— The underground part of a floweringr plant of Colchictim autiottnale : A seen in fiout and from without, k the corni, s' , x"^ataphyllary leaves embracing the flower-stalk, luh its base from which proceed the roots w ; B longitudinal section, A A a brown skin which envelopes all the underground parts of the plant, st the flower- and leaf-stalk of the previous year which has died down, its swollen basal portion k only remaining as a reservoir of food-materials for the new plant now m flower. The new plant is a lateral shoot from the base of the corm k, consisting of the axis from the base of which proceed the roots -wt, and the middle part of which (/&') swells up in the next year into a corm, the old conn k disappearing ; the axis bears the sheath-leaves s, s', s'' and the foliage-leaves /', /" ; the flowers l>,b' are placed in the axils of the uppermost foliage-leaves, the axis itself terminating amongst the flowers. The foliage-kaves are still small at the time of flowering ; in the next spring they emerge from the ground together with the fruits ; the portion of the axis k then swells up into the new corm, on which the axillary bud k" developes mto the new flowering plant, sheath of the lowermost foliage-leaf is changed into the brown enveloping skin. v\n\& the &c. This arrangement either prevails over the whole shoot together with its secondary shoots, or occurs only at first, and then passes into spiral arrangements, which very commonly lead to the formation of rosettes radiating on all sides, as in Aloe (see Fig. 144, p. 172), Agave, Palms, &c. The arrangement with the angle of divergence V3 is much rarer, but occurs in some species of Aloe, Carex, Pan- danus, &c. Spiral arrangements Vv'ith a smaller divergence than \ ., also occur N n 54'^ PHANEROGAMS. sometimes ; as e.g. in IMusa (in 31. rubra the angle is, according to Braun, V? i^^ the foliage-leaves, Vn i^^ the bracts), and Costus (where the angle of the foliage- leaves is from V'4 to ^4) &c. The axillary shoots of Monocotyledons mostly begin with a leaf in close contact with the primary axis and with its back turned towards it, and usually bicarinate. Of this character must be considered, for instance, the upper pale of the flower of Grasses, which is itself an axillary shoot of the lower Fig. 392. — Crocus ■vermes: A the bulbous stem seen from above, B seen from below, C from the side and cut throujjh lengthwise ; fff the circular line of scars of the cataphyllary leaves, k k the corms which t,frow in their axils ; b the base of the decayed flower- and leaf-stem, by its side (lik in Q next year's bud, from which a new corm and flower-stem will be produced; D longitudinal section through this bud, n « its cataphyllary leaves, // foliage-leaves, h bract, / perianth, a anthers, k a bud in the axil of a foliage-leaf. pale. When the phyllotaxis of successive orders of shoots is alternate in two rows, the result of this arrangement is that a whole system of shoots is bilateral, or may be divided by a plane which bisects the leaves (as in Potamogeton, Typha, &c.). The mode of insertion of the cataphyllary and foliage-leaves, and very often that of the hypsophyllary leaves (as for instance that of the spathe which is of common occurrence) is generally entirely or for the greater part amplexicaul, Fig. 393.— Bud in the inside of a bulb of Allium Cepa, the scales having been removed, st the short flat base of the stem on which the bulb-scales are inserted ; I'm A B lamina, sh the sheath of the foliage-leaves still short ; in B the outer leaves have been removed, and an axillary bud k" has made its appearance in addition to the terminal bud k' . and the lower part of the leaf is in consequence sheathing ; and this is evidently connected with the want of stipules, which are so frequent among Dicotyledons. The cataphyllary and many of the h}-psoph3'llary leaves are usually reduced to this sheathing part, which generally passes immediately into the green lamina in the case of the foliage- leaves ; but in Scitaminece, Palmacece, Aroideae, and MONOCOTYLEDONS. 547 some others, a long and comparatively slender stalk developes between the sheath and the lamina. When the leaf-stalk is absent, and the lamina sharply marked off from the sheath, a Ligule is not unfrequently present at the point where the two meet, as in Grasses and Allium (Fig. 394). The lamina is generally entire and of a very simple form, commonly long and narrow (ligulate), rarely roundish and disc-shaped {e. g. Hydrocharis), or cordate or sagittate (as in Sagittaria and some Aroidese). Branching of the lamina is a rather rare exception among IMonocotyledons ; and then takes the form either of lobes from a broad common base or less often of deep divisions, as in some Aroidese {e.g. Amorphophallus, Fig. 133, p. 162, Atherurus, and Sauromatum). The division of the com- pound and pinnate leaves of Palms is not due to a branching occurring at an early stage, but to a splitting which takes place on unfolding, and is caused by the drying up of certain strips of tissue with- in the lamina, which is at first sharply folded up. The formation of the tendrils of Smilax appears, on the other hand, to depend on actual branching of the leaf- stalk. The Venation of the foliage-leaves differs from that of most Dicotyledons, in the weaker veins not generally pro- jecting on the under side of the leaf, but running through the mesophyll ; in the smaller leaves there is even no projecting mid-rib. The mid-rib is, on the other hand, strongly developed in the large stalked leaves of the Spadiciflorae and Scitamineae, and is permeated by a num- ber of fibro-vascular bundles. When the leaf is ligulate and its insertion broad, the fibro-vascular bundles run nearly parallel to one another; in broader leaves with- out a conspicuous mid-rib they describe curves from the mid -rib to the margins (as in Convallaria). But when a strong mid-rib occurs in a broad lamina, as in Musa &c., the fibro-vascular bundles which run through it give off laterally smaller thin bundles, running parallel to one another in large numbers to the margin of the leaf. These parallel transverse nerves are sometimes united into a lattice-like network by short straight anastomosings (as in Alisma, Costiis, and Ouvirandra, the mesophyll being absent within the meshes of the latter). It is only rarely (as in some Aroidese), that projecting lateral veins are given off from the mid-rib, a finer reticulated venation springing from them. The Floiver of jMonocotyledons usually consists of five alternating whorls each N n 2 / Fig. 394.— a leaf of AUnim Cepa divided length- wise ; z the thickened base of the sheatli, which persists as a bulb-scale after the upper part of the leaf has died down, s the membranous part of the sheath, / the hollow lamina, h hcUow of the leaf, i' inner side of the lamina, X ligule. 548 PHANEROGAMS. with an equal number of members; viz. an outer and an inner perianth-whorl, an outer and an inner whorl of stamens, and a carpellary whorl, which is succeeded by a second carpellary whorl only in Alismaceae and Juncagineae. The most common typical flora formula is therefore S^F^S/,,^nCn(+u)- It is only in the Hydrocharideee and a few other isolated cases that the number of whorls of stamens is larger. Where in other cases, as Butomus, an increase of the typical number of stamens occurs, this takes place by de'donhlement without any increase of the number of whorls (Fig. 400 A). The number of members in each whorl is two {S.^ P^ ^4+2 Q)' i^^ only a very few cases scattered through the most different families {e. g. in P>Iaianthemum and some Enantioblastce ; it is sometimes four or five (occasionally in Paris Fig. 395. — Diagram of Scirpus (Cyperacere). Fig. 396. — Diacjraui of Iridece. Fig. 397. — Diagram of Musaceie. quadrifolia and in some Orontiaceae) ; but the usual number of members in each whorl is three, and the typical formula therefore ^3 Pg ^4^3 (73(^3) . In the large section of Liliiflorse, in some Spadiciflorse, and in many Enantioblastae, Juncagineae, and Alismaceae^ this typical floral formula is at once obtained empirically; but in most others particular members or whorls are wanting; but the abortion of these is generally at once evident from the position of those that are present. In the Scitamineae with only one or even with only half an anther (Fig. 398, 399) Fig. 398.— Diagram of Zingiberaceas ; A Hedychium (after Le Maout and Decaisne), B Alpinia (after Payer). Fig 399.— Diagram of Caima (Musacea-), after Payer. the rest of the members of the androecium are present or only partially deficient, but are transformed into petaloid staminodes. It has already been pointed out how the flowers of Gramineae and Orchideae can be traced back to the trimerous pentacyclic type; the theoretical diagrams here given (Figs. 395-402) will answer the same purpose for some of the other more important families. If the pentacyclic flower wdth the formula »S'„P^i67„^„ C„(^,j) is considered as ^ The dimerous flower of Potamogelon {S.^P.,^St.^^C^) (see Hegelmaier, Bot. Zeit. 1870, p. 287) differs from the typical formula only lo this extent, lliat the four carpels arise simultaneously, and are placed diagonally to the preceding pairs. MONOCOTYLEDONS. 549 the typical one for Monocotyledons, it will be seen that the great majority of families the number of whose parts deviates from this type, do this only by the suppression of single members or of whole whorls, the typical position of those that still remain with respect to one another not being disturbed. The variety in the forms of flowers in this class is therefore brought about almost entirely by abortion^; and it is not uncommon for abortion to be carried to such an extent in Monocotyledons that nothing is left at last of the whole flower but a single naked ovary or a single stamen, as happens frequently in Aroideae. In these cases a similar explanation of the relationships of the parts of the flower is approached and elucidated by the occurrence of flowers with the actual typical structure, and by a complete series of transitions caused by partial abortion. It is Fic;. 400.— Diagram of Alisniacerc ; .-i Butomus, B Alisina. Fig. 401.— Diagram of Triglochin (Juncaginea'). especially in small closely crowded flowers, as those of Spadiciflora^, Glumiflorse, &c., that so great a reduction of the typical number of members is observed ; while in larger and more isolated flowers the number of members in each whorl is usually complete or even excessive (as Butomus and Hydrocharis), and deviations usually result from petals (or petaloid staminodes) being formed in the place of fertile stamens {e. g. Scitamineae). With reference to the abortion which is often carried to so great an extent in small flowers, it may in certain cases even be doubtful whether Fir.. 402.— Diagram of Gymnostacliys (Aroidew), after Payer. in an assemblage of stamens and carpels we have a single flower or an inflorescence consisting of several flowers reduced to a very simple state by abortion, as for example in Lemna. When both the perianth-whorls are well developed, they are usually similar in structure; in large flowers they are generally delicate and petaloid and either brightly coloured or not (Liliacese, Orchide^, &c.); in small flowers on the con- trary they are firm, dry, and membranous, as in Juncaceae, Eriocauloneoe, &c. Compare what was said on Abortion at p. 201 and in the Introduction to Angiosperms. 5^0 PHA NER GA MS. Sometimes however the outer perianth-whorl is green and sepaloid, the inner whorl larger, delicate, and petaloid (Canna, Alisma, Tradescantia) ; in the very small and closely crowded flowers of the Glumiflorse, the perianth-leaves, when present, take the form of hairs (the setae of Cyperaceas) (Fig. 395), or of small membranous scales (the pales and lodicules of Grasses). The Siameiis generally consist of a filiform filament and a quadrilocular anther ; though variations frequently occur, especially in the form of the filament and connective. Among the most striking deviations from the ordinary type are the petaloid staminodes of Cannacese and Zingiberacese. It has already been pointed out (pp. 426, 473), that the foliar nature of the stamens is subject to an exception in the Naiadese (at least in Naias) according to the researches of Magnus, and in Typha according to those of Rohrbach. The stamens of Monocotyledons scarcely ever branch, as is often the case in Dicotyledons ; and this corresponds to the customary absence of branching in the other foliar structures also. If the diagram of the flower of Canna (Fig. 399), drawn according to Payer's description, is correct, the petaloid staminodes are branched; according to Rohrbach the (axial) stamen of Typha is also branched. The GyncBceum has usually a trilocular ovary; less often it is tricarpellary but unilocular; in both cases it may be either superior or inferior, but the latter occurs only in plants with large flowers (Hydrocharis, Irideae, Amaryllideae, Scitaminese, Orchidere, &c.). The formation of three or more monocarpellary ovaries is limited to the alliance of the Juncagineae and Alismaceae, in which the ordinary number of members and of whorls of the gynseceum is also exceeded, reminding one of the Polycarpas among Dicotyledons. Adhesion and displacement are not so common in the flower of Monocoty- ledons, and usually not so complicated as among Dicotyledons ; among the most striking phenomena of this nature are the formation of the gynostemium of Orchids ; the cohesion of the six similar perianth-leaves into a tube in Hyacinthus, Con- vallaria, Colchicum, &c. ; and the epipetalous and episepalous position of the stamens in the same plants and in some others. Adhesion of the stamens to the calyx or corolla occurs much less constantly in particular families among IMono- cotyledons than among Dicotyledons. Terminal flowers to a leafy primary shoot occur very rarely among jMono- cotyledons {e.g. in Tulipa) ; terminal inflorescences are more common. The flower acquires a tendency to zygomorphism, especially as it increases in size ; but this is often only feebly indicated, and attains its highest development in Scitamineae and Orchideae. The Ovules of Monocotyledons usually spring from the margins of the carpels, rarely from their inner surface (as in Butomus) ; the single orthotropous ovules of Naias (according to Magnus) and Typha (Rohrbach) arise by the transformation of the end of the floral axis itself (see p. 496) ; in Lemna and in some Aroideas one or more ovules stand at the bottom of the cavity of the unilocular ovary. The prevailing form of the ovule is anatropous ; but in Scitamineae, Gramineae, and some other orders, campylotropous ovules occur; in the Enantioblastse and a few Aroideas they are orthotropous, either erect or pendulous. The nucleus is almost without exception enclosed in two envelopes (Crinum however forms an exception). MONOCOrVLEDONS. crj The Embryo-Sac^ generally remain^ surrounded by one layer of the tissue of the nucleus till the time of impregnation ; the apex is sometimes destroyed so that the embr\-o-sac projects (as in Hemerocallis, Crocus, Gladiolus, &c.) ; but, on the other hand, the apex not unfrequently remains as a cap of tissue covering the apex of the embryo-sac (as in some Aroideaj and Lihacese). In Orchidese the growing- embryo-sac completely destroys the layer of tissue that envelopes it together with the apex of the nucleus; and this happens after impregnation in all the other Monocotyledons that possess an endosperm, and in this case the embryo- sac some- times advances even to the inner integument and destroys it {Allium odor am, Ophrydeae). In the greater number of IMonocotyledons a copious development of endo- sperm-cells follows quickly after impregnation ; these are all formed simultaneously, and remain free in the parietal protoplasm. When they lie near together they soon unite into a layer of tissue and divide tangentially, new cells being formed at the same time by free cell-formation on the inner side of the first layer which behave in the same manner, until at length the embryo- sac is filled with radial rows of cells the result of division. Narrow embryo-sacs are filled up by the growth of the first endosperm-cells; but sometimes the cells formed by free cell-formation in the parietal layer of protoplasm constitute at first a loose mass which fills up the embryo-sac and only closes up into a tissue at a later period {e.g. Leucojum, Gagea). The narrow embryo-sac of Pistia is filled up by a row of broad disc- shaped cells which lie in it like transverse compartments and are perhaps the result of division of the sac itself In some Aroideae only a part of the embryo-sac is filled with endosperm, the rest remaining empty. The endosperm still continues to grow after it has filled up the embryo-sac, the seed which it fills increasing also in size. It has already been mentioned how considerable this growth is in Crinum (p. 512). In all those Monocotyledons which form an endosperm (albuminous), it becomes closed up into a continuous tissue enveloping the embryo before this has completed its growth. By the growth of the embryo a part of the endosperm which surrounds it is again forced aside ; and on this displacement depends the lateral position of the embryo in Grasses by the side of the endosperm, and the absence of this latter in some Aroidese. But in all the other Monocotyledons which have no endosperm (exalbuminous), Naiadese, Potamogetonese, Juncaginex, Alismaceae, Cannaceae, and Orchidese, its formation is altogether suppressed, or transitory preparations for it only take place. On the first origin of the embryo reference must be made to what was said in the Introduction to Angiosperms (p. 510) ; there are many points which are still doubtful in the formation of the plumule, scutellum (in Grasses), and root, from the orio^inal small-celled mass of tissue of the embryo. ^ See Ilofmeister, Neue Beitrage (Abhandl. der konigl. Sllchs. GeselLsch. der Wissensch. vol. VII). 55- PHANEROGAMS. With respect to the Formation of Tissue'^, Monocotyledons differ from Dicotyledons and Gymnosperms chiefly in the course of the fibro-vascular bundles in the stem, and in the want of a true cambium-layer. A number of the common bundles {i.e. those common to the stem and leaves) enter the stem side by side from the broad insertions of the leaves, pass obliquely downwards into it, and then again bend outwards as they descend, approaching gradually the surface of the stem. The common bundle is usually thickest and most perfectly developed at the curved portion which lies deepest in the stem, while the arm which bends upwards into the leaf becomes thinner and simpler upwards, and the descending arm of the bundle behaves similarly down- wards. Hence a transverse section of the stem which cuts through the different descending arms at different heights in their course, shows bundles of different structure and of various sizes. A radial longitudinal section through the bud or through mature stems with short internodes (as Palm-stems, thick rhizomes, bulbs, &c.), shows how the bundles which descend from different leaves, the curves of which lie at different heights, cross one another radially, some of them bending inwards where others are already turning outwards. In elongated internodes, as for instance those of the stalks of Grasses and of some Palm-stems (like Calamus), the long scapes of Allium, &c., the bundles run nearly parallel to one another and to the surface ; the curves and inter- sections of the bundles may be easily distinguished at the apex of such stems, and localise themselves in the transverse plates or nodes which do not elongate between each pair of internodes. The nodes are not unfrequently traversed by a network of horizontal bundles ; and this is very conspicuous in the maize. The course of the fibro-vascular bundles which has now been described renders impossible the separation of the fundamental tissue of the stem into pith and cortex in the sense in which this occurs in Conifers and Dicotyledons. The parenchymatous fundamental tissue fills up homogeneously the spaces between the bundles which are generally numerous ; but a separation takes place not unfrequently into an outer peri- pheral layer and an inner region, a layer of tissue being formed between the two the cells of which are thickened and lignified in a peculiar way (as for instance in most thickish rhizomes, in the hollow scape of Allium, &c.). In consequence of their not being parallel, and of their scattered distribution in the transverse section of the stem, the descending bundles of JMonocotyledons have not the power of coalescing into a closed sheath by connecting bands of cambium (interfasci- cular cambium), as is the case in other Phanerogams. In consequence of this the layer of cambium between the pfiloem and xylem is also absent ; the fibro-vascular bundles are closed. When a portion of the stem ceases to grow in length, the whole of the tissue of the bundles becomes transformed into permanent tissue (see e.g. Fig. 91, p. 107); and there is in consequence usually no subsequent increase in thickness; each portion of the stem, when once formed, maintains the thickness which it had already attained within the bud near the apex of the stem. But in Dracaena, Aloe, and Yucca, a renewed increase of thickness begins afterwards at a considerable distance from the apex of the stem, which may even continue for centuries and may cause a considerable though slow increase in its circumference. But this subsequent growth in thickness takes place in a way quite different from that w^hich occurs in Gymnosperms and Dico- tyledons ; — a layer of the fundamental tissue parallel to the surface of the stem becomes transformed into meristem which continually produces new closed fibro-vascular bundles, and between them parenchymatous fundamental tissue (Fig. 91). A more or less evidently stratified network of slender anastomosing bundles is thus formed, the posi- tion and connection of which is easily recognised on stems which have been exposed ^ Von Mohl, Bail des Palmenstammes, in his Vermischte Schriflen, p. 129. — Niigeli, Beitnige zur wissensch. Bot. Heft 1. — Millaidet, INIemoires de la Soc. Imp. des Sci. Nat. de Cherbourg, vol. XI, i86^. MONOCOTFLEDONS. r-^ to the weather, and in which the parenchyma which fills up the interstices has decayed. This network of closely-placed closed fibro-vascular bundles now forms a kind of secondary wood which surrounds like a hollow cylinder the space in which the original fibro-vascular bundles of the stem run isolated and loose in the form of long threads. This thickening ring of the arborescent Monocotyledons resembles the secondary woody mass of Conifers and Dicotyledons in the fact that it belongs altogether to the stem and has no genetic connection with the leaves, differing in this from the original common bundles. An exception to the ordinary structure of Mono- cotyledons occurs in submerged water-plants (Hydrilla and Potamogeton), in which, according to Sanio (Bot. Zeitg. 1864, p. 223, and 1865, p. 184), an axial cauhne bundle in the stem lengthens continuously, while the foliar bundles do not unite with it till a later period, a peculiarity which recurs in some dicotyledonous water-plants, and re- minds one of the corresponding processes in Selaginella. The Systematic Classification^ of the sub-sections of IMonocotyledons here adopted is that of A.Braun (in Ascherson's Flora of the province Brandenburg, Berlin 1864); but with the variation that the order Helobiae there given is broken up into a series of orders. In short diagnoses of the orders only a few of the characters are specified which are most important from a systematic point of view; the figures placed within brackets refer to those attached to the families belonging to the order in which the characters named are present or absent. A complete account might have been given of the characters of the separate families of Monocotyledons ; but since a similar treat- ment of the class of Dicotyledons would have far exceeded our limits, the mere enumer- ation of the families must, for the sake of uniformity, suflice. SERIES I.— Heloble. \\'atcr-plants ; seed with little or no endosperm, but a strongly developed hypo- cotyledonary axis to the embryo. The number of parts of the flower usually vary from the ordinary type of Monocotyledons. Order i. Centrospermae (so named from the central position of the seed in (i) and in Xaiasi. Flowers imperfect, very simple, usually without a perianth; in (i) consisting of two stamens and a unilocular ovary (containing from i to 6 basilar ovules) surrounded by a sheath (perianth or spathe) ; ovary in (2) unilocular, usually one-seeded ; seed with but little endosperm. The Lemnaceae consist of small branched leafless floating vegetating bodies, generally with true pendent roots ; the Naiadeae are slender branched long-leaved submerged plants ; this family is not definable systematically, and should be split up into several. (The Lemnacea: should perhaps be united to the Aroidesp.) Families: i. Lemnaceae. 2. Naiadeae. ' [The systematic classification adopted in this book is not one which the reader will find followed in any standard English work, either as respects Monocotyledons or Dicotyledons. The M'ork now generally adopted as containing the most satisfactory system of distribution of the vege- table kingdom into classes, orders, and genera, is Bentham and Hooker's Genera Plantarum (London 1 862 -1873), which is however at present only completed so far as to include the Gamopetaloe with inferior ovary. In Dr. Hooker's edition of Le Maout and Decaisne's Traits Generale de Botanique (London 1873), will be found the outlines of this classification completed as far as relates to the classes and orders. De Candolle's Prodromus Systematis Naturalis Vegetabilium in 17 vols. (Paris 18 18-1873), contains a description of every known species of Dicotyledons ; Walpers' ' Repertorium ' and ' Annales,' serving as supplements to the earlier volum.es, which are far less complete than the later ones. For an admirable epitome and illustrations of the character of each of the natural orders see also Oliver, Illustrations of the Principal Natural Orders of the Vegetable Kingdom ; London, 1S74. — ^^-1 554 PHANEROGAMS. Order 2. Polyearp£e. Flowers pentacyclic or hexacyclic (2, 3); whorls in (i) dimerous and decussate, with four monocarpellary ovaries placed diagonally; in (2, 3) trimerous, or with a larger number of stamens and carpels (see p. 549) ; the gynjeceum consists of three or more monocarpellary ovaries, which are one- or more-seeded ; endosperm absent. Perennial floating water- or upright bog-plants, with large lattice-veined or long narrow (2) leaves Families: i. Potamogetoneae. V^ 2. Juncagineae. 3. Alismacese. Order 3. HydrocharideaB. Flowers dioecious or polygamous, with trimerous whorls, and perianth consisting of both calyx and corolla ; male flowers of from one to four whorls of fertile stamens and within these several whorls of staminodes ; female flowers with an inferior tripartite or six-chambered (3) many-seeded ovary; endosperm absent. Perennial submerged or floating water-plants with spiral or verticillate (i) leaves. Family i, Hydrocharidere; with the subsections — 1. Hydrilleas. t-^; 2. Vallisnerieae. 3. Stratioteae. SERIES II.— MlCRANTH^. Land- or bog-plants; the individual flowers usuafly very small and inconspicuous, but collected in large numbers in the inflorescence, and almost always referable to the dimerous or trimerous pentacyclic type. Order 4. SpadiciflorsB. Inflorescence a spadix or panicle with thick branches (4), generally enveloped in a large sometimes petaloid (i) spathe ; bracts small or altogether absent ; perianth never petaloid, usually inconspicuous or altogether abortive (1-3); sexual organs generally diclinous by abortion; fruit always superior and often very large (2, 4); the seed mostly large or of an immense size and with a very large endosperm ; embryo small, straight. Mostly large strong plants with the stem strongly developed, chiefly above ground, and a great number of large foliage-leaves; in (i, 3, 4) they have a broad branched or apparently pinnate or compound lamina, a leaf-stalk and sheath, in (2) they are sessile, very long and narrow. Families: i. Aroideae. 2. Pandanaceae. 3. Cyclantheae. 4. PalmaceES. Orders. Glumifilorse. Inflorescence spicate or panicled, without a spathe; flowers very small and inconspicuous, usually concealed among thickly placed dry hypsophyllary leaves (glumes or pales) (2, 3); perianth absent, or replaced by hair- like structures or scales ; fruit superior, small, one-seeded, dry and indehiscent (a caryopsis); embryo in (i) long and in the axis of the endosperm, in (2) by its side and very small, in (3) also by the side of the endosperm, but considerably developed and provided with a scutellum. Plants with persistent underground elongated rhizomes, and upright foliage-leaves in two or three (2) rows; (i) should perhaps rather be included in the fourth order. MONOCOTYLEDONS. ' j-j-c; Families: i. Typhacese. ^ 2. Cyperaceae. 3. Gramineap. Order 6. Enantioblastae. Flowers in crowded (4) cymose inflorescences inconspicuous (i, 2), or conspicuous (3, 4), pentacyclic, and usually trimerous (in (i, 2) often dimerous); perianth-whorls glumaceous in (i, 2), developed into calyx and corolla in (3,4); fruit a superior bi- or trilocular capsule with loculicidal dehiscence ; ovule orthotropous, and the embryo (^Xc'kttt]) therefore opposite (iuau- Ti'of) the base of the seed. Plants with grass-like (1-3), or succulent habit (4). Families: i. Restiaceae. 2. Eriocauloneae. ^ 3. XyrideiF. 4. Commclynacea?. SERIES III.— COROLLIFLOR^. Both the perianth-whorls conspicuous, usually large and petaloid; the two stamina! whorls completely developed or partially wanting by abortion, and then replaced by staminodes ; one carpellary whorl ; the five whorls, with few exceptions, trimerous. Order 7. LiliiflorfB. Inflorescence very various, racemose or cymose ; the large flowers sometimes single. Flowers pentacyclic and trimerous, except a few cases where they are dimerous, tetramerous, or even pentamerous ; in (3) the inner staminal whorl is wanting ; perianth -whorls similar, in (i) inconspicuous and membranous, but usually petaloid (2, 3, 5-8) and often large; sometimes all the six leaves are coherent into a tube (6 and elsewhere), often with epipetalous and episepalous stamens; ovary superior in (i, 2), inferior in the other families, usually forming a trilocular capsule or berry ; embryo surrounded by endosperm. Plants of very various habit ; with strong woody stems increasing in thickness in Alo-', Yucca, and Dracaena (2) ; more often with underground rhizomes, corms, or bulbs, from which spring leafy annual shoots; leaves mostly long and narrow, in (4) with a broad lamina and slender stalk. ^\imilies: r. Juncaceap. 2. Liliaceae. 3* Irideae. 4- Dioscoreap. 5- Taccaceap. 6. Haemodoracea?. 7- Pontaderiaceae. Order 8. Ananasinese. Flowers consisting of the typical five trimerous whorls; outer perianth-whorl developed into calyx, inner one into corolla ; ovary trilocular and many-seeded, superior or inferior; embryo by the side of the endosperm; leaves long, often very narrow. Family: i. Bromeliaceai. Order 9. Scitaminese. Floral whorls trimerous and zygomorphic ; both perianth-whorls or onlv the inner one (2, 3) petaloid; of the stamens the pos- terior one of the inner whorl is abortive in (i), this alone being fertile m (2, 3) (in 3 with only half an anther), while the rest are changed into petaloid staminodes (see Figs. 397-399, p. 548); fruit inferior, trilocular, a berry or capsule; endosperm PHANEROGAMS. absent, but replaced by a copious perisperm. Usually handsome, often very large (i) leafy shrubby plants springing from a persistent rhizome, with large leaves, generally divided into a broad lamina, leaf-stalk, and sheath. Families: i. Musaceae. ^ 2. Zingiberaceae. 3. Cannacese. Order 10. Gynandrse. The entire flower zygomorphic in origin and de- velopment; by the torsion of the long inferior ovary (i) the anterior side of the mature flower usually becomes posterior ; both of the trimerous perianth- whorls petaloid, the posterior leaf of the inner one (the labellum) generally provided with a spur ; of the six typical stamens of the two w^horls only the anterior ones are eventually developed, and in (i) (with the exception of Cypripedium) the anterior one of the outer w^horl is alone fertile and has large anthers, the two anterior ones of the inner whorl forming small staminodes ; but in Cypripedium it is these latter that are fertile, the anterior one of the outer whorl forming a large staminode; in fa) the same occurs, or the three anterior ones are fertile; filaments of the fertile and sterile stamens coherent with the three styles into a gynostemium ; pollen in single grains, tetrahedra, masses, or poUinia ; ovary inferior and unilocular with parietal placentation (i) or trilocular wnth axile placentation (2); ovules anatropous; seeds very numerous, very small, without endosperm and with the embryo undifferentiated. Small herbs or larger shrubby plants; the tropical Orchide£e often epiphytal and furnished wnth peculiar aerial roots; our native species perennial with underground rhizomes or tubers ; some Orchidese are sapro- phytes destitute of chlorophyll, and a few have even no roots (Epipogium, Coral- lorhiza). Families: i. Orchideac. 2. ApostasiacesE. The Burmanniacess with cymose inflorescence, three or six fertile epipetalous stamens, free tripartite style, and uni- or tri-locular inferior ovary, are allied to the Gynandrae by their small seeds without endosperm and their undifferentiated embryo ; and in this order, which consists for the most part of small plants, there are some saprophytes destitute of chlorophyll. CLASS xni. DICOTYLEDONS. The ripe Seed of Dicotyledons contains either a large endosperm and a small embryo (as in Euphorbiaceae, Coffea, Myristica, Umbelliferae, Ampelideae, Polygon- aceae, Caesalpineae, &c.) ; or the embryo is comparatively large, and the endosperm occupies but a small space {e. g. Plumbaginese, Labiatae, Asclepiadeae, &c.) ; or, thirdly, the endosperm is entirely wanting, and the embryo fills up the whole of the DICOTYLEDONS. 557 space enclosed by the testa, and thus, when ripe, often attains a very considerable size {e.g. Aesculus, Juglans, Cucurbita, Tropseolum, Cupuliferse, Leguminosse, &c.) ; though in small seeds it still remains of moderate dimensions (as in Cruciferse, Com- posit;^, Rosiflorae, &c.). The absence of endosperm generally results from its absorption by the rapid growth of the embryo before the ripening of the seed; only in a very few cases is it rudimentary from the first (Tropieolum, Trapa). In Nymphaeaceae and Piperaceae the embryo and the endosperm which surrounds it both remain small, the rest of the space within the testa being occupied by perisperm. The Embryo generally attains but very small dimensions in the small-seeded parasites and saprophytes destitute of chlorophyll, and remains without differentiation until the time of ripening of the seed ; in Monotropa it never consists of more than two cells, and even in Pyrola secunda, which possesses chlorophyll, only of from eight to sixteen (Hofmeister). The ripe seeds of Orobanche, Balanophora, Rafflesiacea^, &c., contain a very small undifferentiated embryo in the form of a roundish mass of tissue ; the embryo of Cuscuta is of moderate size and length, but the formation of leaves and roots on the filiform stem^ is suppressed. The mistletoe (Loranthaceee), I-ir,. \o\—Chivio}ianthus/ra):ians: A liorizontal section of the nearly ripe fruit: B longitudinal section of t/ie saine.ytlie tliin pericaqi. e reni.-iins of the endosperm, c cotyledons; C the embryo removed from the seed, showing the cotyledons rolled round one .nnother, the radicular end below. on the other hand, parasitic but containing chlorophyll, produces an embryo which is not only large but well-developed. If the embryo of the ripe seed is differentiated, as is generally the case, it consists of an axis and two primary opposite leaves (cotyledons) betvv^een which the axis terminates as a naked vegetative cone (Cucurbita), or bears a bud which sometimes consists of several leaves ( Vicia Faba, Fig. 405, Phaseolus, Quercus, &c.). Instead of the two opposite cotyledons, a whorl of three is not unfrequently formed in those plants which normally possess only two" (Phaseolus, Amygdalus, Quercus, &c.). The opposite cotyledons are usually alike in form and vigour; in Trapa however one remains much smaller than the other ; and cases even occur in which only one has been formed, as in Ranunculus Fican'a^, where it remains below in the form of a sheath, and in Bulbocapnos, a section of Corydalis. The two cotyledons generally form by far the larger part of the ripe embryo, so that the axis has the appearance » According to Uloth (Flora i860, p. 265) the root-cap is also absent. On parasites see especially Solms-Laubach in Jahrb. fiir wissensch. Bot. vol. VI, p. 599 et ^eq. ■' Numerous additional instances are given in the Bot. Zeitg. 1869, p. 875. [Masters, Vegetable Teratology, Ray Soc 1869, p. 370.] 2 Irmisch, Beitr"ge zur vergleichendcn Morphologic der Pfianzen, Halle 1S54, p. 12. vS PHANEROGAMS, only of a small fusiform appendage between them ; and this structure is especially striking when the embryo attains a very considerable absolute size in those seeds which possess no endosperm, and the cotyledons swell up into two thick fleshy bodies (as in Aesculus, Castanea, Quercus^ Fig. 407, Amygdalus, Victa Faba, Phase- olus, the Brazil-nut, &;c.); but more often the cotyledons remain thin like shortly stalked foliage-leaves of simple form (as in Cruciferae, Euphorbiacese, and Tilia, the last with a three- to five-lobed lamina). Most often they lie with their inner faces flat against one another (Figs. 404, 405) ; but are not unfrequently folded or wrinkled and curved backwards and forwards (as in Theobroma with thick, Acer and Convol- FlG. 442. — Ricin7ts co7nntJ!nis ; I longitudinal section of the ripe seed ; // germinating seed with the cotyledons still in the endosperm (shown more distinctly in A and B), s testa, e endo- sperm, c cotyledon, he hypocotyledonary portion of the stem, 7v primary root, iu' secondary root, x the caruncle, an appendage of the seed characteristic of Euphorbiaceae. Fig. 405. — Vicia Faba : A seed with one of the coty- ledons removed, c the remaining cotyledon, iu radicle, kn plumule, j testa ; B germinating seed, s testa, / a por- tion of the testa torn away, ;/ hiluni, st petiole of one of the cotyledons, k curved portion of the axis above the cotyledons, he the very short hypocotyledonary portion of the axis, h the primary root, ivs its apex, kn bud in the axil of one of the cotyledons. vulaceae, &c., with thin cotyledons) ; less often they are rolled spirally round one another. The axis of the embryo beneath the cotyledons is generally elongated and fusiform, and when of this shape is described in works on descriptive botany as the Radicle. This fusiform body consists however in its upper and usually larger part of the hypocotyledonary portion of the stem, and only the lower posterior terminal piece, which is often very short, is the rudiment of the primary root (Fig. 406). The rudiments of the secondary roots can sometimes be distinguished in the tissue of the primary root (in Cucurbita, and according to Reinke in Impatiens). Germmalw7t generally takes place — after the testa, or in dry indehiscent fruits the pericarp, has burst from the swelling of the endosperm or of the cotyledons them- DICOTYLEDONS. 559 selves — by the elongalion of the hypocotyledonary portion of the axis to such an extent as to push the radicle out of the seed, the root then beginning to grow rapidly and generally attaining a considerable length and forming secondary roots in acropetal succession, while the cotyledons and plumule still remain in the seed (Figs. 404, 405, 406). Thick fleshy cotyledons usually remain in the seed during germination, finally perishing after their food-material has been consumed (as in Fig. 406.— Longitudinal section of the axis of the embryo m the ripe seed of Phaseolus miiltiflorus, parallel to the cotj'lcdons (X about 30), jj apex of the stem. 7US of the root, ct cushion at the insertion of the cotyledons, zthe first internode,/A the petioles of the first foliage-leaves, -v, v, f the procambium of the fibro-vascular bundles. Fig. 407. — Querciis Rohur : /longitudinal section of the embrj-o (mag- nified) after removal of the anterior half of both cotyledons c, c; the hypocotyledonary portion of the axis he the primary root iv and plumule b are concealed between the lower portion of the thick cotyledons ; st petiole of the cotyledons ; // seed at the time when germination is com- mencing (natural size), the pericarp and one cotyledon have been removed, the hypocotyledonary portion of the axis and the radicle have elongated ; /// further stage of germination, the plumule having emerged from the testa sk and pericarp s by the elongation of the petiole of the cotyledons st, IV primary root, 2v' secondary roots. Phaseolus mul/iflorus, Vkia Faba, Fig. 405, Quercus, Fig. 407). In this case the petioles of the cotyledons lengthen so much that the plumule which is concealed between them is pushed out (Fig. 407), and now grows upright so that the seed and cotyledons together have the appearance of being a lateral appendage of the axis of the embryo. But usually the cotyledons are destined for further develop- ment, especially when they are thin, and form the first foliage-leaves of the plant. PHANEROGAMS. In order to liberate them and the phimule which lies between them from the seed, the hypocotyledonary portion of the axis increases considerably in length, making first of all a curve which is convex on the upper side (Fig. 404), because the coty- ledons still remain in the seed while the lower end of the stem is attached by the root to the ground. Ultimately, by a final lengthening of the hypocotyledonary portion, the upper part of the axis together with the cotyledons is drawn out of the seed in a pendent position. The axis now straightens as it continues to grow, and the cotyledons expand in the air, the plumule developing more completely and pushing up between them. The cotyledons which thus become exposed to the light usually increase rapidly in size, and constitute the first green leaves of the plant, which are of simple form {e.g. Cruciferse, Acer, Cucurbita, Convol.vulaceas, Euphorbiaceae, &c). If the seed contains an endosperm, the cotyledons do not emerge till after it has been absorbed (Fig. 404). ]\Iany transitional forms occur between the different modes of germination now described ; peculiar phenomena sometimes appearing which are caused by special vital conditions. In Trapa, for example, the primary root is from the first rudimentary, and remains altogether unde- veloped ; the hypocotyledonary portion lengthens considerably, curves upwards, and protrudes a great number of lateral roots in rows which fix the plant into the ground \ The further development of the young plant may take place by the rapid enlargement of the primary axis of the embryo. While the axis is growing, generally in an upright direction, the shoot which developes from the plumule be- comes the primary stem of the plant, lengthen- ing at the summit, and usually producing weaker lateral shoots [e. g. Helianthus, Vicia, Populus, Impatiens, &c.). When the main stem is perennial, it sooner or later ceases to develope further at the apex, or the lateral shoots nearest to the apex become equally strong. An arborescent head is thus formed, the main stem or trunk becoming denuded by the dying off of the lo.wer branches, or the main stem continues to grow erect as a sym- podium (as in Ricinus, the lime, &c.) ; or lateral shoots are formed at an early Fig. 408. — Almond-seed germinating, one of the cotyledons c' c" being split ; the letters as in Fig. 407, i the first interiiode strongly developed. [See De Candolle, Organographie Vegetale, PI. 55. — Ed.] DICOTYLEDONS. 561 period at the base of the primary stem which grow as strongly, and thus give rise to a shrubby plant. When the axis of the embryo grows vigorously, the primary root generally also grows vigorously in a downward direction^; and a Tap-root is thus formed, from which, as long as it increases in length, the lateral roots spring in great numbers in acropetal succession. When the growth in length of the tap-root ceases, adventitious roots become intercalated among the lateral roots already formed, and like them, grow vigorously, and may themselves produce lateral roots of higher orders. A strong root-system is thus produced with the primary root of the embryo for its centre, which endures as long as the stem itself. By the subse- quent increase in thickness the primary stem /as well as its branches) assumes the form of a slender upright cone, the base of which rests on the base of the inverted cone formed by the primary root which has also increased in thickness. While these processes, which are here described in their main outlines, take place almost invariably among Conifers, a number of deviations occur, on the other hand, among Dicotyledons similar to those which have been spoken of under the head of Monocotyledons. The primary axis may die soon after germination or at the end of the first period of vegetation, the primary root often perishing as well, while the axillary shoots of the cotyledons or of subsequent leaves continue the life of the individual. Thus, for example, in the dahlia, a strong adventitious root is given out laterally from the hy]iocotyledonary portion of the axis at the close of the first period of vegetation of the young plant, and swells into a tuber ; the primary root-system and the portion of the axis above the cotyledons disappear, and there remain only for the continuance of the life of the plant the new tuberous root, the hypocoty- ledonary portion of the axis, and the axillary buds of the cotyledons. The process is still more striking in Ramuiculus Ficaria, where, after the development of the primary root, a tuberous lateral root is produced below the primary axis of the embryo, sheathed by a coleorhiza, and maintains its existence together with the axis, while the primary root and the first leaves perish. Among the numerous cases belonging to this category may be mentioned also Physalis Alkekengi, Mc7itha arvensis, Bryonia alba, Polygonum amphibium, and Lysimachia vulgaris"-. The pro- duction of bulbs also occurs among Dicotyledons (as in species of Oxalis), though not so commonly as among IMonocotyledons ; of more common occurrence are tubers or swellings of underground branches, stolons, or rhizomes of greater or less thickness. The greater number of Dicotyledons have perennial underground roots or stems which send up periodically leafy and flowering shoots that die at the end of each period of vegetation. In all such cases, where the primary root-system of the seedling perishes, new roots are repeatedly developed from the stem; and the power possessed by most Dicotyledons of producing adventitious roots from the ' One of the most remarkable exceptions is afforded by the genus Cuscuta, which has no primary root, the posterior end of the axis penetrating into the ground on germination, but soon dying off when the upper filiform portion of the stem has embraced the plant on which it becomes parasitic, and has fixed itself on to it by its short suckers; the plant afterwards grows vigorously and branches. 2 The above is taken from Irmisch's detailed descriptions in his Beitrage zur vergleichenden Morphologic der Pflanzen, Halle, 1854, 1S56 ; Bot. Zcltg. i85i ; and elsewhere. o 562 PHANEROGAMS. Stem, especially when kept moist and dark, enables them to be reproduced to almost any extent from branches and portions of branches. Some species climb, like the ivy, by roots put out regularly from the weak stem which requires a support ; others send out runners to a distance, on which the bud forms a new plant, as in the straw- berry, the stem which is thus formed putting out roots. The order of succession of new roots from the stem is in general acropetal^ but they do not usually make their appearance except at a considerable distance behind the growing bud ; many Cac- taceae however not un frequently produce them close below it. The norm^al Mode 0/ Branchmg at the end of growing shoots is monopodial ; the branches are produced laterally beneath the apex of the punciiim vegetatiojiis. Up to the present time only one instance is known of dichotomous branching, and in that the bifurcations are developed sympodially ; according to Kaufmann, the formation of the circinate inflorescence of Borragineae depends, as has already been mentioned, on this mode of development. The normal monopodial branching is axillary; the lateral shoots are produced in the angle which the median line of the leaf forms with the internode. On a vegetative shoot at least one lateral shoot is produced in the axil of each leaf, although only a few of the axillary buds unfold. Sometimes other axillary buds are produced in rows above the original one; as, for instance, above the axils of the foliage-leaves in Aristolochia Sipho, Gleditschia, Lonicera, &c.\ above the axils of the cotyledons in Juglans regia, and that of the larger cotyledon in Trapa. In woody plants the axillary buds destined to live through the winter are not unfrequenlly so completely surrounded by the base of the leaf-stalk that they are not visible until the leaf has fallen off, as in Rhus iyphinum, Virgilia lu/ca, Platanus, &c., and are then called Intrapetiolar Buds. Besides the ordinary axillary branching, some cases are known among Dicotyledons of lateral and monopodial but extra-axillary branching. To this description belong the tendrils of Vitis and Ampelopsis which are produced (accord- ing to Niigeli and Schwendener) beneath the punctum vegetatmiis of the mother- shoot, opposite to the youngest leaf and somewhat Inter than it. In Asclepias syriaca and some other plants a lateral vegetative branch stands beneath the terminal inflorescence between the insertions of the foliage-leaves, which themselves also produce shoots in their axils. According to Pringsheim^ lateral shoots arise on the concave side of the long spirally-curved vegetative cone of Utricularia vulgaris which he considers to be extra-axillary branches, while 'normal' shoots are formed in the axils of the leaves which stand in two rows on the convex side of the shoot or by their side. It appears to me however certain that these extra- axillary structures on the concave side of the mother- shoot are leaves of peculiar form^, since inflorescences are produced in their axils. The suppression of the bracts of the inflorescence, which is not uncommon, » See Guillard, Cull. Soc. Cot. de Frar.ce, vol. IV, 1857, p. 239 (quoted Ly Duchartre, Elements do Cotanique, p. 408). 2 Zur Morphologie der Utriculai ien, Monalsber. der kr-nigl. Akad. der Wissensch. Feb. 1 869. ^ This of course depends on what is considered leaf and what shoot; this is not however a matter of simple observation, but rather of conventional conceptions convenient for a special purpose. DICOTYLEDONS. 5^3 cannot be placed in the same category as the cases just mentioned of extra-axillary branching, where large leaves in the axils of which buds are also formed exist near the extra-axillary lateral branches. Here, on the contrary, as for instance in Crucifer?e and the capitulum of many Compositae, the formation of leaves on the axis of the inflorescence is itself entirely suppressed ; there are no leaves in the axils of which the branches could stand. The branches are hov/ever produced as if the leaves were actually there ; and there are reasons for supposing that we have here a case of abortion of the bracts in the same sense as the abortion of the posterior stamen of Labiatae (p. 480), Musaceae (Fig. 397, p. 548), &c. Since it is common for the hypsophyllary leaves on the inflorescence to remain very small and to disappear early, it would not be surprising, according to the theory of descent, that functionless organs of this kind should at length entirely disappear, their deve- lopment being in such cases altogether suppressed, while the lateral branches which belong to them (according to the theory of descent typically axillary) should be strongly developed. Adventitious buds are rare in Dicotyledons, as they are in Phanerogams generally. Those which are commonly formed with an exogenous origin in the indentations of the margins of the leaves of Bryophyllum calycinmn are v/ell known, and serve to propagate the plant. They sometimes occur (according to Peter- hausen') in Begoiiia coriacea in the form of small bulbs on the peltate surface of the leaf where the i)rincipal veins radiate'-. On the adventitious buds on the leaves of Utricularia, Pringsheim's treatise already quoted may be consulted. Adventitious buds more often spring from roots, e. g. in Anemo7ie japonica, Linaria vnlgaris, Cirsiiim arvcnsc, and Popiiliis ircmula, according to Irmisch^ The shoots which spring from the bark of the older stems of trees must not at once be set down as the development of adventitious buds ; since the numerous dormant buds of woody plants may long remain buried and yet retain their vitality. The Leaves of Dicotyledons exhibit a greater variety both in their position and their form than those of all other classes of plants put together. The ordinary phyllotaxis of seedlings begins with a whorl of two cotyledons, and continues either in decussate pairs or passes into a distichous arrangement or into whorls consisting of larger numbers or spiral arrangements with the most various angles of divergence. More simple arrangements, especially that of decussate pairs, are generally constant in whole families, the more complicated arrangements usually less constant. Axillary branches' usually begin with a pair of leaves which are either opposite or alternate, and stand right and left of the median line of the mother-leaf. It is quite impossible to give in a short space even a general account of the forms of leaves, even apart from cataphyllary leaves (scales on underground stems and those which envelope persistent buds), hypsophyllary leaves or bracts, ^ Beitr:;ge zur Entwickelung der Brutknospen (Hameln 1S69). where various examples are also given of axillary buds of Dicotyledons which form deciduous gemmx ; :,s\n Polygonnvi viviparrmi, Saxifraga granulata, Dentaria bulbifera. Ranunculus Ficaria, &c. , r i -r i ■' [The common method of propagating Begonias is by culling or tearing tne leaf, which, if then placed on moist soil, produces buds on the edges.— Ed.] 3 [Irmisch, Bot. Gaz. Ill, pp. 146 and 160.] 002 5^4 PHANEROGAMS. and floral leaves; only a few of those forms of foliage-leaves can be mentioned here which are peculiar to or characteristic of Dicotyledons. The foliage-leaves are usually divided into a slender leaf-stalk {petiole) and a flat blade {lamina) ; the lamina is very commonly branched, i. e. lobed, pinnate, compound, or incised ; and even where it forms a single plate (simple leaf) the tendency to branching is gene- rally indicated by indentations, teeth, or incisions in the margin. The branching of the lamina has usually a distinctly monopodial origin, but its development may continue in a cymose manner, a helicoid succession of lateral lobes being formed on each side right and left of the centre of the leaf (as in Rubus, Helleborus, &c., see Fig. 133, p. 162). The sheathing amplexicaul base is not common in Dicotyledons (but occurs in Umbelliferse) ; and the occurrence of Stipules in its place is more common. The cohesion of opposite leaves into a single plate pierced by the stem is not uncommon (' perfoliate ' leaves, as in Lamiiim amplex- icaule, Dipsacus FuUomim, Lojiicera Capri/olium, species of Silphium, Eucalyptus, &c.) ; as well as the downward prolongation of the lamina of the leaves (' decurrent leaves '), which distinguishes the * winged ' stem of Verbascum, Onopordon, &c. The not uncommon ' peltate ' leaf also scarcely occurs in so marked a manner in any other class (Tropaeolum, Victoria regia, &c.). The power of Dicotyledons to develop from their foliage-leaves organs of the most diverse functions adapted to the most various conditions of life is seen in a very striking manner in the common occurrence of leaf-tendrils and leaf-thorns, and still more in the formation of the ascidia or ' pitchers ' of Nepenthes, Cephalotus, Sarracenia. &c. The Venation of the foliage-leaves (with the exception of the thick leaves of succulent plants) is distinguished by the numerous veins which project on the under side, and by their curvilinear anastomoses by means of fibro-vascular bundles running through the mesophyll itself. The mid-rib, which usually divides the leaf into two symmetrical but sometimes into very unsymmetrical halves, gives off lateral veins right and left ; one, two, or three strong nerves, similar to the mid-rib, often springing in addition from the base of the lamina right and left of the median line. The whole system of the projecting veins of a foliage-leaf behaves like a monopodial branch-system developed in one plane, the interstices being filled up by the green mesophyll in which lie the anastomoses combined into a small-meshed network. Within the meshes still finer bundles are usually formed which disappear in the mesophyll. In membranous cataphyllary and hypsophyllary leaves and the perianth-leaves of the flowers the projecting veins do not usually occur; the venation is more simple and more like that of Mono- cotyledons\ The Flower'^. In the great majority of Dicotyledons the parts of the flower are ^ [The structure of the leaf compared with that of the stem has been worked out by Casimir De Candolle, Archives des Sciences, 1868; the 'Student' for the same year contains an abridged translation of his paper.— Ed.] ^ The floral diagrams given here are drawn partly from my own investigations, but chiefly from the researches of Payer into the history of development, assisted by DjITs Flora of Baden. The figures placed beneath the diagrams are intended to indicate the number and cohesion of the carpels as well as the placentation in those plants the diagram of which is otherwise the same. DICOTYLEDONS, c5^ arranged in whorls, or the flowers are cyclic ; only in a comparatively small number of families (Ranunculacese, Magnoliaceae, Calycanthaceae, Nymph^acese, and Nelum^ biaceae) arc all or some of them arranged spirally {acyclic or hemicyclic^). In Cyclic Floiuers the whorls are usually pentamerous, less often tetramerous, both numbers occurring in nearly-related plants. Dimerous or trimerous, or combi- nations of dimerous and tetramerous whorls are much less common than penta- merous, and are usually characteristic of smaller groups in the natural system. When the floral whorls are tetramerous or pentamerous, they are generally four in number, and are developed as Calyx, Corolla, Androecium and Gynceceum. In dimerous or trimerous flowers the number of the whorls is much more variable, and then it is not uncommon for each organ to be made up of two or three whorls ; while in the previous case the multiplication of the whorls is almost entirely confined to the androecium. The corolla is frequently absent, and the flow^ers are then said to be apelaloiis. When the calyx and corolla are both present the number of their parts (sepals and petals) is almost always the same (Papaver is an exception) ; but this is not the case with the number of the whorls. In Cruciferae, for example, the calyx consists of two decussate whorls of two sepals each, the corolla of one whorl of four petals. When the perianth and androecium are both present (whether the former consist of calyx only or of both calyx and corolla), the number of their parts is usually the same, or the flower is isosle7?iofwus, but the stamens are often more, rarely fewer in number than the parts of the perianth, and the flower is then anisosiemonoiis. When the flower is tetramerous or pentamerous the number of carpels is usually less ; when the flower is dimerous or trimerous, or when the parts are arranged spirally, the number of carpels is not unfrequently larger. It will be seen from this brief outline that the relations of number and position in the parts of the flowers of Dicotyledons are very various, and cannot be referred, as is the case with ^lonocotyledons with but few exceptions, to a single type. Even the establishment of difl'erent types for the larger groups is attended with great uncertainty, since the knowledge of development necessary in order to refer par- ticular forms of flowers to general formulae is often wanting. The too universal application of the spiral theory to phyllotaxis even in the case of cyclic flowers has often increased the difficulty, and has even occasioned doubts which would not have arisen without the theory. For the great majority of Dicotyledons the floral formula may be given 6'»/'„6'/,,(^„^...) C„(_„i). This formula holds good for most pentamerous flowers and for those which are truly tetramerous (or octamerous as Michauxia) ; so that n is in these cases 5 or 4 (or 8 as the case may be). In the androecium an indefinite number of (alternating) whorls ^/,(+„4....) must be assumed in order to include the large number of flowers in which the androecium consists of more than one whorl (as^.^. Fig. 420). The mode expressing the gynaeceum C„(_^) is intended to show that very commonly the number of carpels is fewer than 5 or 4 (or 8 as the case may be) ; m may be of any value from o to «. In the majority of gamopetalous Compare pp. 523 and 531. i66 PHANEROGAMS. orders and elsewhere there are very commonly only two carpels ; and in this case they stand in a median line posterior and anterior ; but on the hypothesis that the typical gynDeceum consists of five alternating carpels and has been reduced to two by abortion, one must stand in the median position in front, the other obliquely behind. A similar difficulty is also presented when the gynseceum consists of three or of G. 409.— Diagram of Caprifoliace2e ; A Ley- cesteria, a Lonicera, b Syinphoricarpus. I'IG. 410. — Diagram of Par- nassia (Saxifragacea). FIG. 411. — Diagram of Campanulacece ; A Campanula, a Lobelia. Fig. 412. — Diagram of Valerianacece B Centranthus. FIG. 413.— Diagram of Ci curbitacciie. Fig. 414.— Diagram of Com- positae. Fig. 415.— Diagram of some Rubiacete. Fig. 416. — Diagram of Plantagine.x. Fig. 417.— Diagram of Oleaccse. Fig. 418.— Diagram of Menispermaceae. Fig. 419.— Diagram of Ciimamomum (LauraceK). only one carpel. , It would carry us too far to detail the reasons which nevertheless determine me to retain the formula above given for the gynseceum of flowers of this description ; it need only be mentioned that species or genera with the typical five carpels occur in the most diverse families and orders where a smaller number is the normal one. The diagrams Figs. 409-419 represent a selection of cases which can be DICOTYLEDONS. ^^^ reduced (if no further reference is made to the considerations mentioned above) to the general formula which here assumes the simpler expression S,,P,^St,,C,^,^^. A comparison with nearly-allied forms leaves little room for doubt that the vacant spaces indicated by dots in the three outer whorls correspond to abortive members Fig. 4-'o.— Diagram of Aquilegia (Raiiunculaceae). in the sense already frequently indicated, even when the absence of these members is so complete that even the earliest stages of development of the flower give no indication of them. The same is the case also when the number of carpels' is less than the typical one. Other cases however occur, as in the case of Rhus (Fig. 421), Fig. 421.— Diagram of Rhus (Anacardiacea;). Fig. 422.— Diagram of Cio2oi)Iiora (Euphorbiacere), a female, b male flower. where certain members, in this case two out of the three carpels, disappear in the course of development. Crozophora tmctoria (Fig. 422) is especially instructive in regard to the relationships here suggested, the flowers becoming diclinous from the stamens in the female flowers developing as sterile staminodes, which may be Fig. 423.— Diagram of pentainerous Ericacea; and Epacridere, Fig. 424.— Diagram of .-Esculus (Hippocastanea;). considered as the first step towards abortion, while in the male flowers the three carpels are replaced by as many fertile stamens (Payer). Reference was made in the Introduction to Angiosperms (p. 481) to the inter- position of a whorl of stamens between the members of a previously formed staminal ,',S PHANEROGAMS. whorl ; and it was mentioned that the interposed whorl has sometimes not the full number of members. These phenomena occur in various large groups of Dicoty- ledons \ In Fig. 423 the five stamens of the decandrous flower of the group of Bicornes which are interposed as a whorl of full number within the first whorl are indicated by the lighter colour. The same is the case with the larger number of Gruinales, among which however the Balsamineae possess only the typical five stamens; the Linese and the genus Erodium have five additional rudimentary stamens interposed between them ; while in Peganum Hannala and INIonsonia the number of stamens in the interposed and outer whorl is doubled. The order ^sculinea^ is of special interest in this connection, since in some of its families (Acerineae and Hippocastanese, Fig. 424) the interposed staminal whorl remains incomplete, so that the total number of stamens is not a multiple of the typical fundamental number (five). Among pentamerous flowers Lythrarieae, Crassulacese, and Papilionaceae may be mentioned in addition, and among tetramerous ones Q^nothereae, in which a complete staminal whorl is interposed. One of the most remarkable deviations from the ordinary structure takes the form in not a few families of Dicotyledons of the simple staminal whorl being ric;. 425.— Diagram of Primulaceae. I'IG. 426.— Diagram of Vitis (Ampelidea-). superposed on the corolline whorl, as shown in Figs. 425, 426, and as occurs also in the Rhamnaceae, Celastrinece, the pentandrous Hypericinege, and Tilia. Pfeffer^ has shown that the two superposed whorls of Ampelidece arise independently of one another and in acropetal order, while on the other hand in Primulacese they first appear in the form of five projections each of which forms a stamen, and from each of which a petal subsequently grows outwards ^ In these cases we have no sufficient ground for the hypothesis that an alternating whorl has been suppressed between the two superposed ones : although in other cases this supposition is justified, or at least is very probable. Thus in the order Caryophyllineae, families, genera and species occur in which the corolla is absent and the stamens are superposed on the sepals ; and since in the same natural group species also occur with a corolla, it may be assumed that where the corolla is absent this is the result of abortion. The diagram of these plants (Figs. 427, 428) is complicated still further by the tendency which they exhibit to a de'doublement of the stamens and even of the carpels. . ' Payer's figures show that the interposed whorl, although of later origin, is sometimes exterior to the typical whorl. The main point is that the position and number of die other parts of the flower are exactly as if there were no interposed whorl. ^ Pfeffer, Bot. Zeitg. 1870, p. 143; and Jahrb. fiir wissensch. Bot. vol. VIII, p. 194. ^ Compare on this point what was said on p. 531. If the theory that is here objected to of the flowers of Primulaceae is maintained, it is clear that the mode of expressing the floral formula must then be altered, and the diagram be somewhat diffcrcnUy drawn. DICOTYLEDONS. 569 When a flower has more stamens than sepals or petals, this may be the result, as has already been mentioned, on the one hand of an increase in the number of staminal whorls (as in Fig. 420), or on the other hand, of the interposition of a perfect or imperfect whorl among the typical ones, or of dedoublement of the stamens (as in Fig. 427). These cases must be clearly distinguished from those in which a larger number of stamens results from the branching of the original ones, a phenomenon which is found in different sections of Dicotyledons, and is some- times constant in whole families (see p. 475). Thus, for instance, in DiUeniaceas I'"IG. 427. — niat;r.-im of scl (Paronychiace.-c). Fig. 428. — Diagram of Phytolacca (Phytolaceacepe). I'lG. 429. — Diagram of Celosia {Amaranthacese). (Fig. 430), Aurantiaccx (P'ig. 431), and Tiliacene (Fig. 432), each symbol which indicates a group of anthers corresponds to a single original stamen. In this case the number of original stamens is the same as that of the petals and sepals ; but sometimes it is less (as in Hypericum perforatum with three staminal bundles in the pentamerous flower) ; so that an increase in the number of stamens is united with a decrease of the typical number of staminal leaves. The branching of carpels is much less common than that of stamens. It occurs very clearl}' in Malvaceae, where the typical number of carpels is five, Fig. 430. — niigram of Candollea (Dilleniacca-). FIG. 43t —Diagram of Citrus (Aurantiacese). FIG. 432.— Diagram of Tiliaceffi. and they are often developed as such (as in Hibiscus). In some genera however (as Malva, IMalope, and Althaea) five original rudiments of carpels first of all make their appearance in the form of a low cushion. Each of these forms very early a larger number of outgrowths lying side by side, and each of these produces a style and a one-seeded compartment of the peculiarly-shaped gynaeceum^ This short sketch will be sufficient to show what variations are possible in the numbers and positions of the parts that may be included under the expression Cn (_m)5 which, as has already been said, is especially characteristic '^n Pn St. n "^ni {+n+. . .) ^n ( of flowers with pentamerous or truly tetramerous whorls. True tetramerous flowers ^ See Payer, Organogenic dc la fleur, Tl. 6 8. J JO PHANEROGAMS. are allied not only to those that are octamerous (like INIichauxia) but also to those with dimerous whorls, among which CEnothereae may be especially men- tioned. Of genera belonging to this family, Epilobium, for example, is constructed on the formula S^^^ P^^ St^_^ C^, Circaea on that of S^ P^ S\ C^ ; and Trapa, with the formula ^2+2 ^X4 '^''^4 ^2? ^^^^t also be included here. Although in Epilobium and Trapa the calyx really consists of two whorls, this pseudo-whorl formed of two decussate pairs is followed by the other whorls exactly as if it were a true tetramerous whorl. But other dimerous and tetramerous flowers exhibit a more considerable deviation from the type, inasmuch as the two dimerous perianth-whorls which develope as if they were a tetramerous calyx or corolla are followed by a staminal whorl which is superposed on the pseudo-whorl consisting of two decussate pairs, as in Urtica and other genera of the order, and in Proteaceae with the formula ^',^.2 Si^ C, (Fig. 339, p. 478). Among the dimerous and trimerous flowers of the orders Polycarpie and Cruciflorae, where they are the most perfectly developed, a tendency prevails for more than one whorl to go to the formation of the calyx, the corolla, the androecium, and even the gynaeceum, a tendency which may be expressed by the formula ^p(+p+...) ^pi+p+...) 'Sy^.(+p+...) C^,(+p4....); for example Fumariaceae, S^ P^j^^ -^^2+... ^r Berberideae, Epimedium, S^^.-^ P^^.> Si^^^ C^ Berberis, ^'3+3 ^3^.3 Sl^^^ C^ Podophyllum, S^ P^^^^ Si.,\^ C, Cruciferai, S^^^ P^, St,^^ C^^^,^). A large number of examples of this general formula are aftbrded by the family IMenispermaceae, in which the whorls are sometimes dimerous, sometimes trimerous, while sometimes whorls of each description occur in one flower; and where almost every one of the organs may disappear by abortion \ In addition to the trimerous flowers already mentioned, there are also some which come under the first-mentioned general formula S^ Pu •S^ni+u} ^« (-m) ; as, for example. Rheum \\ith the formula S.^ P^ ^'4"+3 Q- Other trimerous flowers again appear to belong to a third type, as Asarum with the formula -5'3 S/^a.^ C^. When the number of staminal whorls is considerably increased, it not unfre- quently happens that the number of stamens in each whorl also undergoes change, and complicated alternations arise. Flowers the structure of which is otherwise altogether difl"erent resemble one another in this respect, as is shown by the Papaveraceae on the one hand (Fig. 433), and by Cistineae and some Rosaceas on the other hand. The reduction of the flower to a simpler condition is often carried so far in many Dicotyledons (as in Monocotyledons) that each individual flower consists only either of an ovary with one or several stamens, or, when the arrangement ^ Eichler, Ueber die Menispermaceen, Denkschrift der k. bayer. Ges., Regensburg 1S64. — Payer, Organogenie dela fleur, PL 45-49. — Eichler, Flora 1S65, Nos. 2-8 et seq. DICOTYLEDONS. is diclinous, even only of a single ovary or of a single or several stamens; the perianth being either entirely absent (as in Salix and Piperacecfi) or reduced to a cup-like structure (Populus, the female flower of Cannabinece &c.) or to hair-like FIG. 433.-Diag:raiu of PapaveraceK ; A Chdidonium, a Papaver. scales among the sexual organs which represent the flower {e.g. Platanus). Flowers of this kind are generally very small and densely crowded in large numbers in the inflorescence (such as capitula, spikes, or catkins). In some cases it may even be doubtful whether we have an inflorescence or a single flower, as in the genus Euphorbia \ The development of the separate parts and the entire form of the flower in the mature state is so various that it is scarcely possible to state any general facts concerning them. The perigynous structure of the flower is peculiar to Dicotyledons, as is also the occurrence of hollowed axes of the inflorescence, like the fig and similar structures, and the cupule, which occur in some families, and are dependent on similar processes of growth. The Ovules exhibit, in the difl"erent divisions of Dicotyledons, all those varieties of structure which have already been mentioned in the introduction. Very commonly, especially among the Gamopetaloe, the nucleus is covered by only one integument, which is then often very thick before impregnation. But on the other hand the third integument or aril is much more common than among Monocotyledons. When there are two integuments, the outer one, — diff"ering again in this respect from most Monocotyledons — takes part in the formation of the micropyle, enveloping the exostome or entrance to it. In some parasites the ovules are rudimentary, and in many Balanophoraceae are reduced to a naked few-celled nucleus; while in Loranthaceae they are coherent with the tissue of the floral axis in the inferior ovary. The behaviour of the Embryo-sac'^ before and after impregnation is similar in most Dicotyledons to that which occurs in Monocotyledons. The endosperm usually originates by free cell-formation, and is transformed by repeated divisions of the first cells which are formed in this manner into a more or less dense tissue, which fills up the embryo-sac either before or after the formation of the multi- cellular rudiment of the embryo. But in a very considerable number of families belonging to altogether different groups the embryo-sac exhibits on the one hand striking phenomena of growth, elongating considerably before impregnation into ^ See Payer, /. c. p. 529 ; [also foot-note to p. 426]. 2 Hofmeister, Jahrb. fur wiss. Bot. vol. I, p. 1S5 ; and Abhandl. der kon. Sachs. Ges. der Wiss. vol. VI, p. 536. -;2 PHANEROGAMS. Li long tube, and emitting after impregnation one or more vermiform protrusions which penetrate into and destroy the tissue of the nucleus and of the integu- ments, or even protrude altogether out of the ovule (as in Pedicuiaris, Lathrgea, -ind Thesium). On the other hand in those plants in which the endosperm originates by cell-division we learn from Hofmeister that the following variations occur : — ' The whole of the cavity of the embryo-sac behaves like the first cell of the endosperm in Asarineae, Aristolochiace^, Balanophoraceas, Pyrolese, and Monotropeae ; the first division of the sac is the result of a partition-wall which divides it into two nearly equal halves, each of which encloses a cell-nucleus and again divides at least once into daughter-cells. In other cases the first cell of the endosperm includes the upper end of the embryo-sac ; the embryo-sac which has just been fertiHsed appears to be divided by a transverse septum into two halves, the upper one of which developes into the endosperm by a series of bipartitions ; while no such bipartition of the lower one occurs in Viscum, Thesium, Lathraea, Rhinanthus, Mazus, Melampyrum, or Globularia. The first cell of the endosperm fills up the middle part of the embryo-sac in Veronica, Nemophila, Pedicuiaris, Plantago, Campanula, Loasa, and Labiatse ; its lower end in Loranthus, Acanthus, Catalpa, Hebenstreitia, Verbena, and Vaccinium.' In Nymphsea, Nuphar, and Ceratophyllum, the upper end of the embryo-sac is cut off from the rest of the space by a septum soon after impregnation, and the further development of the daughter-cells or endosperm takes place only in the upper part which also includes the ' embryonic vesicles.' This mode of formation of the endosperm differs however from that which occurs in the plants mentioned above, in taking place in the upper half of the embryo-sac by free cell-formation. In the very large majority of true parasites (except Cuscuta) and saprophytes, the endosperm is formed by cell-division ; in Cuscuta however by free cell-formation. Hofmeister states that only slight indications of the formation of endosperm are to be found in Tropaeolum and Trapa. The mode of formation of the Emb}yn of Dicotyledons, as it has now been elucidated by Hanstein's recent researches, has already been explained in the introduction to Angiosperms (see Fig. 372, p. 516). It need now only be stated in addition that in parasites destitute of chlorophyll and in some saprophytes the seeds become ripe before the embryo has emerged from the condition of a roundish mass of tissue still without external differentiation of parts {e. g. in Monotropa, Pyrola, Orobanche, Balanophoraceae, and Rafiflesiaceae). With reference to the Formation of Tissue^, I will confine my remarks here to a description of the behaviour of the fibro-vascular bundles and of the mode in which the stem increases in thickness. With the exception of a few water-plants of simple structure, in which a purely cauline fibro-vascular cylinder runs through the stem and increases in length at its ^ Hanstein, Jahrb, fiir wiss. Bot. vol. I, p 23^ et seq., and for the girdle-shaped combinations of vascular bundles Abh. der Berl. Akad. 1857, 8, — Nageli, Beitrage zur wiss. Bot, Leipzig, Heft I, 1858 ; and Dickenwachsthum und Anordnung der Gefassstrange bei den Sapindaceen, Miinchen 1864. — Sanio, Bot., Zeit. -1864, p. 195 et seq. and 1865, p. 165 et seq. — Eichlcr, Denkschiift der kon. bayer. bot. Gesells. vol. V, Heft I, p. 20, Rcgensburg 1864. DICOTYLEDONS 573 summit, the foliar bundles originating from it later (in Hippuris, Aldrovanda, Cerato- phyllum, and to a certain extent also Trapa, according to Sanio), it is the general rule that 'common' bundles are first formed, the ascending branches of which enter the stronger foliage-leaves generally in large numbers, and then pursue their course as isolated bundles in the leaf-stalk and mid-rib, giving off the secondary bundles which constitute the venation of the lamina^ The branches which descend into the stem mostly run downwards through several internodes, become first interposed between the upper parts of the older bundles, and sometimes (Fig. 434) first split and then coalesce laterally with the older bundles lower down. Sometimes (as in Iberis) every bundle is twisted in the stem and in the same direction, so that the bundles which have coalesced sympodially, belonging to leaves of different heights on the stem, ascend spirally within the bark. But most commonly they run parallel to the axis of the stem, until they anastomose with older bundles lower down. The bundles do not bend deeply into the inner tissue of the stem, but turn downwards and run parallel to one another at the same distance below the surface, so that they lie in one layer, which presents the appearance of a ring on transverse section separating the fundamental tissue into pith and primary cortex. The portions of the fundamental tissue which lie between the fibro-vascular buntilcs connect the pith with the primary cortex, and form the primary Fir,. 434. — The course of the bundles in two internodes of Satnbucus P.biihis: they lie in a cylinder wliicli is here flattened out ; eaci) internode be.irs two opposite leaves, and each leaf receives from the stem a middle bundle h h and two strong later.il bundles s' s' ; tlie descendint? arms of the bundles split and interpose between the lower bundles; there arc in .i:62, pp. 298-329, and i86,^ pp. 251-258.— Ed.] - ; C ) PHA NER GA MS. cambium-ring is then formed outside the one which has disappeared, and another one again outside this one when it. has in turn disappeared. Several circles of fibro-vascular bundles are thus formed, continually increasing in number. In many Menispermacex (e-g. Gocculus), the new outer circle of vascular bundles together with its cambium-ring is developed from a ring of meristem which lies in the primary cortex and therefore outside the primary bast,— a phenomenon which is repeated in the primary cortex as its growth proceeds (Nageli). In Phytolacca, on the other hand, and, according to Eichler, also in Dilleniaceae, Bauhinia, Polygaleas (Securidaca and Comesperma), Cissus, and Phytocrene, the successive circles of bundles originate in the secondary cortex. Phytolacca agrees moreover with the cases mentioned under a in the primary bundles lying also in the pith, and in the first closed ring which surrounds them being a secondary production due to increase in thickness. Second Group. The secondary bundles arise early after the primary bundles further inwards or nearer the axis of the stem (endogenous). a. Both the primary and the secondary endogenous bundles remain isolated ; they are net united by a closed cambium-ring, but anastomose with one another, as in Gucurbita, Nymphaeaceae and Papayer(?). The transverse section of the stem bears a greater or less resemblance to that of a Monocotyledon, especially in Nymphaeaceee. 6. The primary bundles lie in a ring on the transverse section, and are united by a cambium-ring ; the secondary bundles arise at an early period in the pith and remain isolated and scattered on the transverse section ; they anastomose with one another and with the primary bundles in the nodes of the stem. Examples are furnished, according to Sanio, by Piperaceae, Begoniaceac, and Aralia. The cell-forms of the phloem and xylem of Dicotyledons have already been described in general terms {see p.gS et seq.). Only two peculiar phenomena need be mentioned here. In Cucurbitaceae, some Solanacese, and Nerium (and in a certain sense also in Tecoma radicam^, a phloem-tissue is found not only on the outside but also on the inside of the fibro-vascular bundles, which is developed with especial strength in Cucurbitacex. The isolated fibro-vascular bundles of the pith which are enclosed by the ring of wood sometimes show an abnormal arrangement of their phloem and xylem. Thus, according to Sanio, Jlralia racemosa has an endogenous circle of closed fibro-vascular bundles in which the xylem is outside and the phloem inside. The isolated bundles in the pith of Phytolacca dioica on the other hand consist, according to Nageli, on a transverse section, of a hollow wocdy cylinder which surrounds the phloem on all sides and is itself penetrated by xylem-rays. The isolated fibro-vascular bundles of the pith in the rachis of the inflorescence of Ricinus commuriis also consist of a thin axial bundle of phloem (?), surrounded by a sheath of cells (xylem ?) arranged in rays. A layer of collenchyma is very common in Dicotyledons beneath the epidermis of the internodes and leaf-stalk. The Clasjification of Dicotyledo?is^ has now been carried out so completely that the smaller groups which are called Families^, and w^hich usually comprise genera very nearly related to one another, have been united into larger groups or orders ; so that at present only a few families remain unplaced. The greater number of the orders can also be again arranged into larger groups which are clearly connected by actual relation- ship. Systematists have not however up to the present time agreed as to how many of these cycles of aflinity should be established, so as to make the primary division of the whole class of Dicotyledons in accord with the requirements of scientific classification. The grouping of all Dicotyledons into three sections, Apetalae, Gamopetalai, and Eleu- ' [See note to p. 553.] "^ Le Maout and Decaisne's Traits gs^n^ral de Botanique, descriptive et analytique, is strongly to Le recommended for a study of the diagnosis of the families [translated by Mrs. Hooker; London J873I. DICOTYLEDONS. r-^ 0/ / thcropetalae, proposed by De Candolle and Endlicher^, is now abandoned by most, although still much in use for practical purposes. A. Braun" placed among the Eleu- theropetalae the greater number of plants previously classed among Apetala^;* and Hanstein^ has now distributed among them the remainder, so that the whole class consists of only two sub-classes, Gamopetalae and Eleutheropetalse. This classifica- tion however assigns far too great an importance to this particular point of structure, considering that on the one hand flowers occur among the Eleutheropetalse which difl'er greatly from one another not only in this but also in every other respect; while on the other hand the most intimate relationship exists between particular sections of Eleutheropetalce and of Gamopetalsp. I therefore think it convenient, while retaining the largest sub-divisions of the class, to employ also other characters in the classifi- cation ; and to make use of the character drawn from the cohesion or non-cohesion of the petals in the subdivision of the largest group, that provided with two perianth- whorls. In the following classification Dicotyledons are split up into five divisions of equal systematic and morphological value, which should rather be arranged parallel to one another than in a single linear series. This classification has also, I think, a practical advantage; since the extraordinarily large number of families and orders can be more easily kept in the memory when they are at once arranged in several comprehensive groups of equal value. DICOTYLEDONS. Juliflorse : A. Piperineap, B. Urticinea^, ('. Amentifera;, l^ MonochlamydeaB: ,^ A. Serpentariea-, x/ B. Rhi'/anthea-. ill. Aphanocycias : A. HydropeltidincK, B. Polycarpae, C. Cruciflorae. IV. Tetracyelse: ( r J ) Ganiopetalo' : A. Anisocarpae, B. Isocarpa:. (/3) Eleutheropetalce : C. Eucycla^, D. Centrospermae, E. Discophorae. V. Perigynee: A. Calyciflorae, B. Corolliflorae, The sections designated by capital letters correspond partly to single orders, partly to whole series of orders in the system referred to above. ^ ' Endlicher, Genera plantarum secundum ordines naturales disposila, Vindobonoe, iS.^6-1840; and Enchiridion botanicum, Lipsioe — Vienna, 1841. •-' A. Braun, Uebeisicht des naturlichen Systems, in Ascherson's Flora der Provinz Brandenbur^S 1864. ■-> Hanstein, Uebersicht des natiirlichen Pflanzensystems, Bonn 1867. In the first edition oflhis book I followed this work with but little deviation. Compare also Giisebach, Grundnss der systematischeu Bolanik. P p -S PHANEROGAMS. I. JULIFLORvE. Flowers very small or inconspicuous, crowded in dense inflorescences— spikes, capi- tula, or less often panicles— which are often of very peculiar form ; naked or with a simple sepaloid perianth, and usually diclinous; the male and female flowers often different. Leaves simple. A. Piper ineop. Flowers very small, in dense spikes subtended by bracts, without a perianth. The small embryo lies, surrounded by the endosperm, in a hollow of the copious perisperm. Herbs or shrubs, often with verticillate leaves. Families: i. Piperaceae, 2. Saurureae, 3. Chlorantheae. B. UrticinefP. Perianth simple, sepaloid, three- to five-partite, sometimes absent; stamens superposed on the segments of the perianth ; flowers hermaphrodite or diclinous, and then the male and female flowers different (3), usually in densely crowded inflorescences, the female flowers in spikes, umbels, capitula (2) or some- times panicles (3), not unfrequently developing into peculiar pseudocarps (as the mulberry, fig, bread-fruit, and Dorstenia). Fruit usually unilocular, rarely bilocular; ovules one or rarely two in each loculus ; seed usually with endosperm. Large shrubs or trees ^ ; leaves stalked, usually stipulate. Families: i. Urticacege, Urticese, Moreae, Artocarpeae, 2. Platanaceae, 3. CannabinecP, 4. Ulmaceae (including Ccltideae). C. Amentiferce. Flowers diclinous, epigynous, in compact panicles (false spikes) ; the female few-flowered inflorescence in (2) surrounded by a cupule. Fruit dry, indehiscent, one-seeded ; seed without endosperm. Trees with deciduous stipules. Families: i. Betulaceae, 2. Cupuliferae. H. IMOXOCHLAMYDE/E. Flowers large and conspicuous and consisting of a simple more or less petaloid, usually gamophyllous perianth, one or more staminal whorls, and a polycarpellary ovary ; carpels equal in number or double the segments of the perianth. The number of mem- bers of the whorls is derived from the typical numbers two, three, four, or five, and generally increases inwards. Ovary generally inferior and surmounted by a short thick columnar style, to which in the hermaphrodite flowers the stamens are usually partially or entirely adherent. Flowers often diclinous. Seeds numerous. A. SerpentariecE. Creeping or climbing plants with slender stems and large simple leaves; floral whorls dimerous and tetramerous(i) or trimerous and hex- amerous; perianth-leaves free (i) or coherent into a tube; ovary of four or six loculi ; embryo small but segmented. Families: i. Nepentheae, 2. Aristolochiaceae, 3. Asarineae. [The Urticece include a number of herbaceous genera. — En.] DICOTYLEDONS. r-^., 5/9 B. Rhizanthece. Root-parasites without chlorophyll or foliage-leaves, generally with stunted vegetative organs and very large solitary flowers or small flowers on a dense inflorescence (i) ; whorls dimerous to octamerous (i), trimerous (2), or pentamerous and decamerous (3); ovary with one or eight (i) loculi ; the placentae and anthers of very peculiar form ; a very great number of small seeds with rudi- mentary embryo. Families: i. Cytinese, 2. Hydnoreae, 3. Rafflesiaceae, III. Aphanocycl^. Flowers hemicyclic or cyclic, or the parts arranged spirally ; the members of each whorl usually free, not coherent with one another, or only in the gynapceum ; perianth generally distinctly separated into calyx and corolla ; the numbers of the parts in the four whorls very variable ; stamens usually more in number than perianth-leaves ; carpels forming generally one, several, or a large number of monocarpellary ovaries ; in G the ovary is superior and bi- or quadri-locular. Ovules springing occasionally in all the sections from the inner surface of the carpels. A. Hydropeltiditiecp. Water-plants with solitary lateral and usually large flowers, the perianth-leaves and stamens variable in number and arranged spirally ; ovaries several and monocarpellary (i, 2), or one only polycarpellary and multilocular ; embryo small, surrounded by a small endosperm in a hollow of the perisperm. Families; i. Nelumbiaceae, 2. Cabombeae, 3. Nymphaeaceae. B. Polycarpee. Parts of the flowers arranged spirally or in whorls, when in whorls usually dimerous or trimerous, each organ generally consisting of more than one whorl, rarely in four pentamerous whorls (2); gynasceum consisting of one, several, or a larger number of monocarpellary ovaries, which are one- or many- seeded ; embryo small ; endosperm none (8), abundant, or very large (9). Families: i. Ranunculaceas, 2. Dilleniaceae, 3. Schizandreae, 4. Annonaceae, 5. Magnoliaceae, 6. Berberideae., 7. Menispermaceae, 8. Laurineoe, 9. IMyristicaceae. C. Crucljiorcp. Perianth-whorls dimerous; in (3) and (4) corolla of four petals placed diagonally ; staminal whorls two or more, each consisting of two stamens or divisible into two: ovarv single, bi- quadri- or multi-locular ; seed with (i, 2) or without endosperm. Families: i. PapaveraceEP, 2. Fumariaceae, 3. Cruciferae, 4. Capparidenp. p p 2 ;So PHANEROGAMS. IV. TETRACYCLiE. Parts of the flower always arranged strictly in whorls ; the typical number of whorls is four, the calyx, corolla, androecium, and gynaeceum each consisting of a single whorl ; whorls generally pentamerous, rarely tetramerous (very rarely dimerous or octamerous) ; any one of the whorls may be entirely wanting, or individual members may be abortive ; this occurs most often with the stamens and carpels. Increase in number of the stamens usually takes place by the interposition of one perfect or imperfect whorl between the members of the typical whorl or a little outside it, or by doubling of the members, or by branching of the original staminal leaves ; increase in number of the staminal whorls themselves is rare. All the whorls usually alternate, but the stamens are not unfrequently superposed on the petals. A tendency prevails in all the sections to a diminution of the number of carpels below that of the members of the perianth-whorls ; very commonly there are only two, one anterior and one posterior. Ovary almost always single and polycarpellary, inferior or superior, unilocular or multilocular. I. Gamopetalse or Sympetalse. The petals united at the base into a tube or cup ; corolla never wanting. A. AnisocarpcE. The whorls or members of the whorls never larger than the typical number ; calyx or some of the stamens sometimes abortive ; carpels usually only two, one anterior and one posterior, or three and united into a single ovary •'. a. Hypogynse. Order 1. Tubiflorse. Families: i. Convolvulacea' (including Guscuteue), 2. Polcmoniacese, 3. Hydrophyllaceac, 4. Borragineae, 5. Solanaceae. Order 2. Labiatiflorse. Families: i. Scrophulariaceai, 2. Bignoniaceae, 3. Acanthaceae, 4. Gesneraceae, 5. Orobanchese, 6. Ramondiea?, 7. Selagineae, 8. Globulariaceae, 9. Plantagineae, 10. Verbenaceae, 1 1. Labiatae. Order 3. Diandrse. Families: i. Oleaceae, 2. Jasminiaceae. Order 4. Contortae. Families: r. Gentianacea?, 2. Loganiaceae, 3. Strychnaceap, 4. Apocynaceae, 5. Asclepiadeae. ^ The orders are arranged mainly after Braiin and Uanstein. DICOTVLEDONS. „ b. EpigynaB. Order 5. Aggregatse. Families: i. Rubiacese, 2. Caprifoliaceae, 3. Valerianacea?, 4. Dipsacacese. Order 6. Synandrse. Families: i. Cucurbitacecc, 2. Campanulacea:, 3. Lobeliacesp, 4. Goodeniaces, 5- Stylidieae, 6. Calycereae, 7. Gompositae. B. Isocarpa^. Carpels equal in number to the sepals and petals, usually five rarely four, and coherent into a generally superior ovary (except Orde ,, Family" where there are only two median carpels); diminution of the number o( sUmen; does not occur (except in Order ,, Family ,); in Orders . and 3, on the o"he hand, a perfect stan.inal whorl is usually interposed; in Order i the stamens are superposed on the petals, and a number of seeds spring from an elevated axial placenta m the unilocular ovary ; in Orders 2 and 3 the ovary is multilocular and many-seeded. Order 1. PrimulinesB. Families: i. LentibulariacecP, 2. Plumbagineac, 3. Primulaceae, 4. Myrsinacea', Order 2. DiosporinesB. Families: i. Sapotaces, 2. Ebenaceae (including Styracaces). Order 3. Bicornes. Families: i. Epacrideas, 2. Pyrolaceae, 3. IMonotropeae, 4. RhodoraceiE, 5. Ericaceae, 6. Vaccinieae. II. Eleutheropetalae or Dialypetalse. Petals free, sometimes wanting. G. Eucyclcp.. Gorolla very rarely wanting; stamens very commonly twice or three times as many as petals by the interposition of a perfect or even double (Orders 6, 7) whorl, or by the interposition of an imperfect whorl differing in number from the corolla (Order 5); the isostemonous stamens sometimes superposed on the petals (Order 4), or the original stamens branch (especially in Orders 2, 3, and 8) ; the number of carpels often the same as that of the sepals and petals (Orders 7, 8), but commonly less — two, three, or four ; ovary unilocular with parietal placentse in Order i, in the others multilocular; seed generally without endosperm. c\^2 P^^^ ^^^ OGA MS. Order 1. Parietales. Families: i. Resedaceae, 2. Violacese, 3* Frankeniaceae, 4- Loasaceae, 5- Turneraceae, 6. Papayaceae, 7- Passifloracese, 8. Bixacese, 9- Samydaceae, lO. Cistineae. Order 2. Guttiferae. Families: i. Salicineae, 2. Tamariscineae, 3- Reaumuriacese, 4. Hypericineae, 5- Clusiaceae, 6. Marcgraviaceae, 7- Ternstroemiaceae, 8. Chlaenaceae, 9- Dipterocarpeae. Order 3. HesperideaB. Families: i. Aurantiacea;, 2. Meliaceae (including Cedreleae), 3- Hiimiriaceae, 4- Erythroxylaceae. Order 4. Frangulineas. Families: i. Ampelideae, 2. Rhamnaceap, 3. Gelastrineae, 4. . Staphyleaceae, 5 . Aquifoliaceae, 6 . Hippocrateaceae, 7 . Pittosporeae. Order 5. ^sculineaB. Families: i. , Malpighiaceae, 2, , Sapindaceae, a. Acerineae, b. Sapindaceae, c. Hippocastaneae, 3 . Tropaeolaceae, 4 . Polygalaceae. Order 6. TerebinthineaB. Families: i. Terebinthaceae, a. Anacardiaceae, b. Burseracese, c. Amyrideae, DICOTVLEDOXS. r^j 2. Rutaceae, a. Rutea', b. Diosineae, c. Xanthoxylacex, d. Simarubeae, 3. Ochnaceae. Order 7. Gruinales. Families; i. Balsaminea;, 2. Limnanthacese, 3. Linacea^, 4. Oxalideae, 5. Geraniaceee, 6. Zygophyllaceap. Order 8. Columnifera©- Families: i. Sterculiacea?. 2. Biittneriaceae, 3. Tiliaceae, 4. Malvaceae. Order 9. Tricoccse^ Families: i, Euphorbiaceae, a. Euphorbieae, b. Acalypheae, 2. Phyllanthaceae ; a. Phyllantheap, b. Buxineae. D, Cetitrosperma. Corolla usually wanting [except in Fam. 6] ; stamens fewer or more often more than tlie sepals, in the last case generally double as many (4 or 6) ; ovary usually superior and unilocular, with one or more basal often cam- pylotropous ovules, less often multilocular with central placentation. Order 1. CaryophyllinesB, Families: i. Nyctagineae, 2. Chenopodiaceae, 3. Amaranthaceae, 4. Phytolaccaceae, 5. Portulacaces, 6. Caryophylleae : a. Paronychieae, b. Sclerantheae, c. Alsineae, d. Sileneae. E. Discophorce. Ovary inferior (Order i) or half inferior or even superior, and then (Order 2, Family 5) carpels distinct; carpels as many as or fewer than sepals and petals (often two) ; when the ovary is inferior or half inferior a necta- riferous disc usually occurs between the styles and the stamens; stamens equal in number to sepals and petals (Order i) or twice as many, or even a still larger number ; calyx-limb usually obsolete in Order i ; seed generally with copious endosperm. _ 1 The position of this order is doubtful. 5^4 PHANEROGAMS. Order 1. Umbellifloras. Families: i. Umbelliferae, 2. Araliaceae, 3. Cornaceae. Order 2. SaxifraginesB. Families: i. Saxifragaceae (including Hydrangeae, Escallonieae, and Cunoniacese), 2. (?) Grossulariaceae, 3. (?) Philadelphese, 4. (?) Francoaceae, 5. (?) Crassulaceae. V. Perigyn^. Flower displaying a tendency towards the perigynous structure. An annular body is elevated from the floral axis bearing the perianth and the stamens, and enveloping the gynaeceum as a cup-, saucer-, or urn-like receptacle ; or it becomes adherent in its growth to the carpels (B, Order 2, Family 2). In a few families which are placed here provisionally (B, Order 3, Families 4-6) the ovary is truly inferior. A. Calyciflora. Perianth simple, either sepaloid or petaloid and usually tetra- merous ; the tubular receptacle is generally of the same nature, and in Family 3 is even quadripartite, corresponding to the four perianth-leaves and to the four stamens superposed on them (see Fig. 339, p. 478); stamens fewer than, as many as, or twice as many as the perianth-leaves; ovary monocarpcllary, rarely bilocular, with one or a few seeds ; seed with little or no endosperm. Order 1. ThymelsBinesB. Families: i. Thymelaeaceae, 2. Elaeagnaceae, 3. Proteaceae. B. CoroUiJlorcB. Calyx, corolla, and androecium placed on a flat (Order i) or cup-shaped receptacle, or on one hollowed out into a deep urn-shape (Order 2 and in part 3), which is often (Order 2) thick and succulent (as in the apple, rose-hip, &c.); sepals distinct or coherent (Order i;); petals always distinct (corolla dialy- petalous) ; the two perianth-whorls usually pentamerous, sometimes tetramerous ; stamens as many as or twice as many as (Order i) sepals and petals, or a much larger number (Order 2), in Order 3, Family 3, commonly branched; gynaeceum composed of one (Order i, and in part 3) or several or a large number of mono- carpellary ovaries; or (in Order 3) ovary polycarpellary, and sometimes inferior (Families 4-6). Order 1. Leguminosae. E^^amilies : i . Mimoseae, 2. Swartzieae, 3. Caesalpineae, 4. Papilionaceae. Order 2. Rosiflorfie. Families: i. Calycanthaceae, 2. Pomeae, 3. Rosacea*, DICOTYLEDONS. 5«5 4. Sanguisorbese, 5. Dryadeae, 6. Spiraeeae, 7. Amygdaleae, 8. Chrysobalaneae. Order 3. Myrtiflorse. Families: i. Lythrarieat-, 2. Melastomaceae, 3. Myrtaceae, 4. Combretaceae, 5. CEnothereae, ^ ' 6. Haloras'ideae. BalanophoriE. Santalacea:. Loranthaceae. Podostcmoncic. Funiilies of unknown or "very doubtful offinity. Hippurideae. Polygonaceae. Callitrichaceae. Begoniaceae. CeratophyllaccaL". Mesembryanthemeae. Tetragonieac. Empctraccac. Cactaceae. Elatincac. Casuarineae. Myricaceae, Jugiandeac. ' The position of lliese families here is very doubtful. BOOK III. PHYSIOLOGY. CHAPTER I. MOLECULAR FORCES IN THE PLANT. Sect. i. — The Condition of Aggregation of organised structures \ Cell-walls, starch-grains, and protoplasmic structures consist, in their natural con- dition, at every point that can be seen even under the microscope, of a combination of solid material with water. If these organised structures are placed in a sub- stance capable of removing water, a part of their aqueous contents is withdrawn; while, on the other hand, if they are in contact with aqueous solutions possessing certain chemical properties and of a proper temperature, they absorb more w^ater. The volume alters with the change in the proportion of water ; loss of water causes contraction, absorption of water a corresponding augmentation of volume. Since the absorption of water occasions a considerable elevation of temperature (air-dry starch rises 2° or 3° C. when mixed with water of the same temperature), it must be supposed that the water contracts as it is absorbed^. Within certain limits these variations in the proportion of water may occur without occasioning any per- manent change in the molecular structure ; but if, with a higher temperature and in the presence of chemical reagents, the proportion falls below a certain minimum or exceeds a certain maximum, permanent changes of the internal structure take place which can no longer be reversed ; and the internal organisation of the body becomes partially or entirely destroyed. ' See Sachs, Handbuch der Experimental-Physiologie, p. 398 '^ seq.^mgdi u. Schwendener, Das Mikroscop, vol. II, p. 402 e( seg.; compare also Book I of this work, p. 31 et sey.— Cramer, Naturforsch. Gesells. in Zurich. Nov. 8, 1869. 2 Jungk, in Pogg. Ann. 1865. vol. 125, p. 292 et seq. J (So MOLECULAR FORCES IN THE PLANT. These facts, in connection witli a number of other phenomena, first led Niigeli to the hypothesis that organised bodies consist of isolated particles or Molecules between which the water penetrates, and which are solid and relatively unchange- able, and invisible even with the most powerful microscopes. Every molecule of a saturated organised body is, on this hypothesis, surrounded by layers of water by which the adjacent molecules are completely separated from one another. These molecules may be supposed to be of various sizes, and it is evident a priori that, if the thickness of the aqueous envelope is the same, larger molecules will form a denser, smaller molecules a less dense substance ; and it may therefore be concluded conversely that the layers and lamellae of organised bodies of different thickness, especially those of the cell- wall and of starch-grains, are composed of molecules of different sizes ; and the difference in the proportion of water in such cases leads to the hypothesis that the densest substance consists of molecules which are several thousand times larger than those of the more watery substance. As the molecules increase in size, the density of the whole substance is moreover increased by the smaller distance that intervenes between them, so that larger molecules are separated from one another by thinner layers of water. The changes in volume of organised bodies due to the removal of water or its absorption, depend, according to this view, on the fact that when swelling takes place the molecules are forced further apart by the water which penetrates between them ; while, on the other hand, when water is removed they approach one another in proportion as the water is withdrawn from their interstices. The forces which are concerned in these processes in the interior of an organ- ised body may be divided into three kinds :— (i) the Cohesion within each separate molecule impermeable to water, which is itself an aggregate of smaller molecules and atoms; (2) the Attraction of the adjoining molecules for one another, in consequence of which they tend mutually to approach; and (3) the Attraction of the surfaces of the molecule for the absorbed water, which counteracts the mutual attraction of the neighbouring particles. In starch-grains, cell-walls, and to a certain extent in crystalloids \ the absorbed water is not deposited uniformly in all directions ; the molecules are, on the con- trary, forced further from one another in certain directions, as is clearly seen from the change of form of the whole, from the formation of fissures, &c. One of the most remarkable effects of the tensions thus caused in the interior of the body is the fact that when swelling takes place particular dimensions may even decrease ; thus, for example, the layers of bast-fibres become very considerably shorter when they sv,^ell up under the influence of dilute sulphuric acid, the coils of the spiral striation be- coming closer and larger in circumference. Crystalloids change their angles several degrees when they swell. These phenomena are explicable only on the supposidon that the molecular forces in the interior of organised substances vary in intensity in different directions ; and this again is conceivable only on the hypothesis that the form of the molecules is not spherical. Nageli and Schwendener obtained a deeper insight into these laws by a very careful observation of the phenomena produced by ^ [See Ijook I, pp. 40-57, on Crystalloids.] CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. ySt; polarised Vv^hi in cell- walls, starch-grains, and crystalloids \ They inferred from these facts a crystalline structure of the individual molecules, and that the crystals are doubly refractive, and have two optic axes which are so arranged, at least in the greater number, that one axis of elasticity of the ether within each molecule of the starch-grains and cell-walls is placed radially, but the two other axes of elasticity tangentially. In crystalloids the molecules are probably arranged as in true crystals, but separated also by layers of water parallel to the faces or lines of cleavage. The behaviour of grains of chlorophyll and of colourless protoplasm towards polarised light, as well as under the addition and removal of water, is at present but little known ; and a more definite idea of the form of their molecules is therefore not yet possible. The solid molecules of one and the same organised body which are separated by aqueous envelopes always vary in their chemical nature ; so that at every visible point molecules which possess chemically different properties lie by the side of and among one another separated by layers of water. In starch-grains, cell-walls, and crystalloids this fact is inferred from the circumstance that certain substances are extracted by the application of certain solvents, while other substances remain behind, constituting what is called the skeleton. This skeleton is of course less dense than the original substance ; and it is evident that the extraction has taken place at all visible points, without the external form or internal structure having undergone any essential change. Thus, for example, a skeleton of cellulose remains behind when the lignin has been extracted from wood-fibres by Schultz's maceration ; and again, a skeleton of silica remains behind with all the optical properties of the cell-wall when the organic substance has been burnt away. In the same manner a grain of starch leaves behind a skeleton containing very little solid material when the granulose has been extracted by saliva or some other substance. From crystal- loids also a skele in this sense of the term containing very httle solid matter can be obtained by the solution of a part of their substance, especially of the colour- ing material contained in them. The properties of these skeletons show that the molecules which remain behind after solution of the rest still occupy essentially the same position and are endowed with the same forces as before ; but it is probable that the extracted substance lay previously between these molecules without being contained in them. This view is also more or less probable in the case of chloro- phyll-grains and protoplasm ; in the former the fundamental protoplasmic substance remains behind as a very solid skeleton when the green colouring material is ex- tracted by ether, alcohol, oil, &c. Very different substances are certainly combined in the protoplasm ; and when a naked primordial cell secretes a cell-wall, it may be assumed that the molecules which form the cell-wall were previously di4ributed between those of the protoplasm, and only change their position and their chemical nature when they are secreted m the formation of the cell-wall ; the protoplasm which remains behind retaining essentially its original properties. The same is the case when grains of starch or chlorophyll are formed in the protoplasm. A funda- mental substance is clearly present in the protoplasm which always retains the » Hofmeister (Handbuch der phys. Bot. vol. I, p. 348) h^^ arrived at altogether different con- cUisions, with which I cannot agree. >jO MOLECULAR FORCES IN THE PLANT. essential properlies of protoplasm ; but various other substances penetrate between its molecules and are afterwards again separated. This is especially observable in the formation of zygospores and swarmspores. The nutriment and growth of organised structures takes place, as has already been shown in Book I, by intussusception ; the nutrient solution penetrates between the molecules already in existence, and either occasions by apposition an enlarge- ment of the- individual molecules ; or new molecules of small size are produced in the spaces filled with water, which then increase by the apposition of new matter, or the increase takes place in both ways at different points. The increase in mass of the cell-wall, starch-grain, &c., is therefore brought about by the molecules being forced apart from within. Connected with the growth of the molecules already in existence and with the formation of new ones, is a continual disturbance of the osmotic equilibrium between the surrounding fluid (the cell-sap in the widest sense of the term, see p. 62), and that within the body, which has the effect of constantly drawing fresh particles from the surrounding fluid to the interior of the body which is undergoing augmentation. Chemical processes in the interior of the growing body are also always con- nected wnth these processes of growth. The nutrient fluid which penetrates from without contains in fact the material for the formation of molecules of a definite chemical nature ; but this material is chemically different from the molecules which it produces. Thus starch-grains are nourished by a fluid which clearly does not contain any starch in solution ; and again the cell-wall grows by the absorption of substances out of the protoplasm which are not dissolved cellulose. The colouring matter of the chlorophvH arises in the interior of the chlorophyll-grain ; and the substances by which the protoplasm is nourished by intussusception are clearly only produced in the interior of the protoplasm, as is shown in particular by naked Plasmodia and by unicellular Algae and Fungi. Growth by intussusception is therefore connected not only with a continual disturbance of the molecular equi- librium, but also with chemical processes in the interior of the growing structure. Chemical combinations of the most various kinds take place between the molecules of an organised body, so tlvat they act upon and decompose one another. It is certain that all growth continues only so long as the growing parts of the cell are exposed to atmospheric air ; the oxygen of the air has an oxidising effect on the chemical comipounds contained in the organised structure ; with every act of growth carbon dioxide is produced and evolved. The equiHbrium of the chemical forces is also continually disturbed by the necessary production of heat ; and this may also be accompanied by electrical actions. The movements of the atoms and molecules w'ithin a growing organised body represent a definite amount of work, and the equi- valent forces are set free by chemical changes. The essence of organisation and of life lies in this : — that organised structures are capable of a constant internal change ; and that, as long as they are in contact with water and with oxygenated air, only a portion of their forces remains in equilibrium even in their interior, and determines the form or framework of the whole ; while new forces are constantly being set free by chemical changes between and in the molecules, which forces in their turn occasion further changes. This depends essentially on the peculiarity of molecular structure, which permits dissolved and gaseous (absorbed) substances to CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 591 penetrate from without into every point of the interior, and to be ao-ain conveyed outwards. This internal instability attains its highest degree in chlorophyll-grains and pro- toplasm. In the former chemical processes take place wiih great energy and activity under the influence of light, such as the formation of the green colouring matter and of starch ; and when deprived of light other chemical changes at once ensue, which terminate only with the complete destruction of the entire chlorophyll-grain. The remarkable properties of protoplasm, w^hich we have already examined from different sides in discussing the structure of the cell, attain their climax in its spontaneous automatic power of motion, and in its capacity of assuming different forms and changing both its shape and its internal state, and therefore of bringing into action internal forces, even when corresponding impulses from without cannot be observed. It is impossible to enter here in detail into the explanation of these remarkable facts; but they will be understood, at least generally and to a certain extent, if it is borne in mind that neither the chemical nor the molecular forces are ever in equilibrium in the protoplasm ; that the most various elementary substances are present in it in the most various comiMnations ; that fresh impulses to the disturbance of the internal equilibrium are constantly given by the chemical action of the oxygen of the air ; and that forces are continually being set free at the expense of the substance of the protoplasm itself, which must lead to the most complex actions in a substance of so complicated structure. Every impulse fiom without, even when in-sperceptible, must call forth a complicated play of internal movements, of which we are able to perceive onl)- the ultimate effect in an external change of form. The destruction of the molecular structure of organised bodies may take place in many different ways, and affords an insight into many physiological processes. The most important forces by which the molecular condition of organic substances is permanently altered are changes, in temperature, chemical reagents, and substances which have a powerful attraction for water. But these agencies do not in general cause destruction until they have exceeded a definite degree of intensity; while dif- ferent changes of temperature and different states of concentration of the reagents not unfrequently give rise to phenomena differing not only in degree but even in kind. The effect of most external influences depends moreover to a great extent on the chemical nature of the substance which forms the material and molecular framework of an organised body. Cellulose ^ and starch may therefore be distinguished from crystal- loids, chlorophyll-grains, and protoplasm, the former consisting mainly of carbo-hydrates insoluble in water, the latter chiefly of albuminoids. (a) Temperature does not usually cause any striking or permanent change or destruc- tion of organisation till it exceeds 50^, or sometimes even 60° C, and the substance affected is completely saturated with water. Air-dry organised bodies can generally bear much higher temperatures without injury. Thus, for example, dense starch-grains con- taining but little water are not converted into paste below 65° C, while the more watery grains undergo this change at 55° G. (Nageli), the capacity for absorbing water and in consequence the volume then increasing enormously. Payen gives the increase m volume of starch in water of 60° C. as 142 p. c, at 70° to 72^ C. as 1255 P- c, the starch originally containing, according to Nageli, only from 40 to 70 p. c. water. Air-dry 1 The cell-wall I suppo^.e here and in the sequel to be neither culiculariscd, lignified. nor converted into mucilajje. />- MOLECULAR FORCES IN THE I'LANT. starch must be heated to nearly 200' G. before its power of absorbing water materially increases ; but it is then changed chemically and converted into dextrine. The cor- responding action of temperature on cellulose is not yet accurately known, but it is certainly different from that on starch. Like albuminoids, protoplasmic structures consisting for the most part of these substances are, when saturated, coagulated. by a temperature of from 50° to 60° C, while when air-dry they can stand much higher temperatures without their molecular structure being destroyed \ The remarkable difference in the action of temperature on saturated starch on the one hand and on saturated protoplasm on the other hand must not be overlooked. In the former case the power of absorbing water is enormously increased ; its structure becomes looser and more easily susceptible to chemical action ; while the coagulation of protoplasm di- minishes its power of absorbing water and the diffusibility of its molecules, and in- creases its power of resisting chemical action. This difference is also manifest when the change of molecular structure is caused by acids ; and in this respect normal cellulose behaves in a similar man- ner to starch. (/6) jlcids (especially sulphuric acid) when greatly diluted cause starch-grains and cellulose at the ordinary temperature to swell up much more violently than pure water, without however destroying their organisation ; and the previous condition returns when the acid is washed out. If, on the other hand, the acids are more highly concentrated, a violent absorption takes place in cellulose and starch-grains, and they pass into a pasty state. Proto- plasmic substances, on the contrary, co- agulate, as they do under the influence of higher temperatures. Concentrated sul- phuric acid finally completely destroys the molecular structure of both with a smaller or larger amount of chemical change, and they deliquesce. (f) Solutio}! of Potash acts on starch- grains like sulphuric acid, especially in caus- ing them to swell up. Its action on pro- toplasmic substances is on the other hand very different from that of acids ; if the solution is dilute they swell up strongly or deliquesce, and this is especially the case with protoplasm and the nucleus of very young cells (the nuclei of older cells often resist the action strongly). But in a highly concen- trated solution of potash protoplasmic structures often retain their form and apparently their structure ; they neither coagulate nor deliquesce. The fundamental destruction of their molecular structure which has nevertheless taken place is evident from the fact that they immediately deliquesce if water is added copiously. Fig. 435. — Bast-cells from a leaf of Hoyn ca7->icsa (see Fig. 32, p. 29) ; a and d after the coniiuencenient of the action of iodine and dilute sulphuric acid ; c, wlien the swelling in dilute sulphuric acid has proceded further. a and ^ in a are the outermost layer not capable of swell- ing, and coloured blue, which breaks up somewhat irre- gularly in these cases, but in c more regularly, into a spiral band, while the mner layers swell between them, and are coloured blue by iodine ; 7 in r is the cavity of the bast- fibfe ; 6 and i\ are constrictions at points where the outer layer is especially firm ; at 6 the greatly swollen substance is beginning to become disorganised (x8oo). "^ See Sachs, Handbuch der Experimcnlal-PhysloloL,ne. p. 6_', et seq. CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 593 (ri) Mechanical InJJuences. Organised structures bear without injury small mechanical disturbances ; they are either sufficiently elastic, like starch-grains and cell-walls, again to bring to equilibrium the changes which are thus caused in their internal tension and external form ; or they are inelastic like protoplasm and chlorophyll-grains, and can then equalise small passive changes of form in another way. But stronger disturbances cause disruptions which cannot be again effaced. The molecular structure of the separated portions may however still be perfectly retained, as is shown by fragments of starch- grains and cell-walls. This is still more evident in motile protoplasm, where the sepa- rated portions of the previously continuous substance behave like so many individuals, and have the power of independent motion ; as, for example, separated portions of Plasmodia, the detached halves of the rotating protoplasm in the root-hairs of Hydro- charis when contracted by a solution of sugar, &c. In the same manner two or more separated portions of protoplasm may unite into a whole, as in the formation of large Plasmodia and of zygospores, the impregnation of oogonia, &c. The only purely mechanical mode in which complete destruction of an organic structure can be accom- plished is by crushing ; /. e. by complete disseverance of its molecules and their subse- quent promiscuous intermixture. In this case a chemical change usually directly follows the mechanical destruction of the molecular structure of the protoplasmic substance. In some cell- walls the mere interruption of continuity by a cut causes striking changes in the adjoining and the more distant parts ; thus, according to Nageli, cell-walls of Schizomeris that have been cut through become shorter and thicker to a remarkable extent. {e) Changes in the molecular arrangement of organised structures caused by injurious influences determining their death are often accompanied by striking changes in their power of diffusion. With respect to starch and cellulose but little is known in this respect ; but the phenomena connected with protoplasm, including the nucleus, are very remarkable'. Normal living protoplasm does not, for example, absorb any colouring material from the surrounding solution ; but as soon as it has been killed by heat or by a chemical re-agent, the dissolved colouring material not merely penetrates into it, but accumulates in it to such an extent that the dead protoplasm appears of a much deeper colour than the surrounding solution of the colouring substance. Starch and cellulose, on the contrary, even in a fresh unchanged condition, absorb from a solution of iodine a comparatively much larger quantity of iodine than of the solvent, and become of a much deeper colour than the surrounding solution ; the colour is also different, usually blue, while the surrounding solution is yellowish brown. The protoplasm which fills the cells and has been killed in any manner, by frost, heat, or chemical agents, is more permeable (whether cellulose is so also is not knowm) ; it allows the cell-sap, which in living and growing cells is always subject to high pressure, to filter out as if it had become porous. This is well seen when coloured cells or tissue are frozen or heated above 50° C; they then allow their coloured contents to diffuse out, which they do not do when living. (/) The true nature of the change which the molecular structure of moist organ- ised bodies undergoes by heating above 50° or 60° C, or when they are made to swell up strongly by treatment with acids or alkalies, is considered by Nageli to lie in the destruction of the crystalline molecules. In the case of starch-grains and cell- walls this view is supported by a few facts which have hitherto not been explained in any other manner. The increase of the power of absorbing water under such conditions is then explained on the hypothesis that the number of particles which attract water is increased and their size diminished by the destruction of the molecules ; and this must necessarily be connected with an increase in the proportion of water and a correspondmg mcrease 1 Niigeli, Pflanzenphysiologische Untersuchungen, vol. I, p. 3 et seq.-Wxxgo de Vries, Sur la permt'abilite du protoplasm des betteraves,_Arch. Neeiland, vol. VI, iSyr. Q q ^y4 MOLECULAR FORCES TN THE PLANT. in volume. It is especially noteworthy that the denser layers of starch-grains and cell- walls become under these circumstances homogeneous with the least dense and most watery layers. But since the denser layers probably consist of large, the less dense layers of small molecules, the explanation may lie in the fact that the large molecules of the dense substance are broken up into a number of small molecules, and thus become similar to those of the less dense substance. The same explanation may be given of the fact that when the organised structure is changed by undergoing strong swelling, the optical properties of starch and cellulose also undergo change ; their pre- vious action on polarised light disappearing altogether. This is also explained if we suppose that under the action of these agents the molecules which produce the optical effect lose their form, and that their fragments are irregularly intermixed. How far these views can be applied also to protoplasmic structures and their coagu- lation remains at present uncertain. (g) The disorganisation of the molecular structure of organised bodies may take place gradually ; and when it has exceeded a certain limit, a new substance is produced from the originally organised material, the molecular condition of which has, since the time of Graham, been termed colloidal. From the similarity which, according to Nageli and Schwendener, exists between organised and crystalline bodies, it is not surprising that there are also mineral substances, which, like silica, are usually crystalline, but become under certain circumstances colloidal ^ Organised bodies absorb water and other fluids, increasing at the same time in volume up to a certain maximum at which they are saturated ; crystalline bodies dissolve in a definite minimum of water and produce a saturated solution which can be diluted ad libitum. Colloidal bodies show in this respect intermediate properties ; they can be mixed with water in all proportions without any minimum or maximum. Solvents cause in organised and crystalline bodies a sudden passage from the solid to the fluid condition. Colloidal bodies pass from the solid to the fluid condition, when they are soluble, through all stages of softening ; in a certain state when they contain but little w^ater they are hard, then tenacious, then tough and scarcely fluid, finally when mixed with abundance of water perfectly fluid. Even in the fluid state they may be mucilaginous, cohering strongly to organic, less strongly to crystalline substances ; and even when greatly diluted they diflTuse very slowly, and some of them appear unable to penetrate organic membranes such as cell-walls. On drying they afl^'ord a homogeneous substance which diff'ers greatly in its capacity for swelling and in its optical properties from the molecular structure of crystals and of organised bodies. In contradistinction to these latter, colloidal bodies may be considered amorphous in- ternally as wefl as externally. Colloidal bodies occur abundantly in plants as products of the decomposition of organised bodies, and under certain circumstances they supply material for the production of new organised bodies. Thus gum-bassorin and perhaps also gum-arabic, as well as the mucilage of quince and linseed, result from the decom- position of cell-walls ; perhaps also the formation of the substance of the cuticle must be included in this category. Viscin is the product of decomposed cellulose ; the origin of colloidal pectin and caoutchouc is still unknown ; but none of these substances are of any further use to the plant. (/j) Traubes Artificial Cells-. Among the most important of the phenomena belonging to the growth of the plant are those connected with the cell-wall ; and everything which contributes to a more exact knowledge of its development must always be welcome. The researches of Traube, of which an abstract is here given, are of great interest from this point of view ; even though it may not always be possible to transfer all the properties of his artificial cells to the real plant. ^ See, among other authorities, Graham, Phil. Trans. 1862; Journ. Chem. Soc. 1862. 2 Traube, Experimente zur Theorie der Zellbildung u. Endosmose, in Arch, fiir Anat., Phys., u. wissenscli. Medecin, von Reichert u. Dii Bois 1S67, p. 87 et sej. CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 595 Starting from Graham's observation that dissolved colloids cannot ditiuse through colloidal membranes, and from the empirical fact that precipitates of colloidal substances are usually themselves colloidal, Traube found that a drop of a colloid A placed in a solution of a colloid B must become surrounded by a pellicle. If A is also more con- centrated (or rather if its attraction for water is greater) the cell must become turgid, /. e. the precipitated pellicle must become stretched by the additional water that is absorbed ; and the molecules of the pellicle thus become separated to such an extent that a fresh precipitate takes place between them which occasions increase in the superficies of the pellicle. For a more exact study Traube chiefly employed cells the pellicle of which consisted of a precipitate of gelatine tannate. For this purpose the tendency of the gelatine to coagulate was destroyed by boiling for thirty-six hours. A thick drop of this gelatine (called /S) of the consistency of syrup was taken up by a glass rod, allowed to dry for some hours in the air, and then plunged into a flask half filled with a solution of tannic acid, into the cork of which the rod was fixed. The portion of gelatine which undergoes solution on the outside of the drop immediately forms a completely closed pellicle with the surrounding solution of tannin ; and the water which penetrates through it constantly dissolves the gelatine within. In a dilute solution of tannin of o'8 to i*8 p. c. a tense pellicle which is not iridescent and is there- fore thick is formed ; in a concentrated solution of from 3*5 to 6 p.c. (in which therefore there is a smaller difference between the concentration of the two fluids) a thin flaccid iridescent pellicle is formed ^ Traube found that the cells which are at first thick- walled go through various stages of development ; they remain spherical so long as the nucleus of gelatine is not completely dissolved ; a turbidity then sets in from above downwards owing to the solution of a part of the pellicle in the solution of gelatine which is more dilute in its upper part ; the pellicle at the same time begins to collapse and to become iridescent ; and finally the contents become clear and tension again takes place. After the lapse of some weeks the cell still allows gelatine to escape when torn. The greater the difference in the concentration of the two fluids, the firmer and more tense is the pellicle; i.e. the greater the intensity of the endosmotic attraction the greater is the number of layers of atoms which coagulate so as to produce the pellicle, and therefore the thicker it is. With reference to the properties of the pellicle, Traube shows that all pellicles hitherto employed in experiments on diffusion have perforations-, while the precipitated pellicles have only molecular interstices; and indeed these latter are, according to him, smaller than the molecules of the precipitate of which the pellicle is composed. But in spite of the greater density, the endosmose is quicker than with all other membranes, because they are thinner. The pellicle becomes firmer (stiffer?) when lead acetate or copper sulphate is added to the (3 gelatine. As soon as the molecules of the stretched pellicle have become so far separated by the pressure of the cell-contents which have increased in quantity by the action of endosmose that their interstices allow the passage of the two substances from which the pellicle is formed, these substances must obviously again at once mutually react upon one another at those points, and must * Only pellicles of gelatine behave in this way ; all others are iridescent when tense. 2 It is easy to convince oneself of the presence of actual perforations in pig-bladder, ox-bladdcr, the pericarduim, amnios, collodion-membrane, or parchment, with which experiments on diflusion have hitherto usually been made, by stretching them over a wide glass tube, pouring in a column of water from 20 to 40 cm. high, and repeatedly drying the free surface of the membrane with filtcinig paper. Water is then almost always seen to ooze out at particular spots; a piece of membrane 2 or 3 cm. square is seldom watertight. The perforations are still more evident if the tube is filled with a concentrated solution of common salt and the membrane dipped in water. Instead of a diffusion-current equal over the whole surface of the membrane, separate threads of the solution of salt are seen to sink down into the water. These experiments show how little dependence is to be placed on the researches hitherto made on memljranes. Q q 2 596 MOLECULAR FORCES IN THE PLANT. cause the production of new molecules of pellicle, which are deposited between those already in existence. Growth therefore takes place by intussusception, and is caused by the stretching of the pellicle, which stretching is on its part occasioned by endosmose. That the growth takes place not only by stretching but also by deposition Traube proved by replacing the tannic acid by water. As soon as this was done (i. e. as soon as the formation of new molecules of the precipitate in the pellicle was prevented, the endos- mose still continuing) the growth ceased. As long as the concentration of the contents of the artificial cell is everywhere the same, the pellicle remains everywhere equally thick, and the cell retains its spherical form. But when the contents are diluted, a denser solution is formed in the lower part of the cell, a more watery solution in the upper part. The pellicle becomes in consequence thinner above and therefore more extensible, because the difference of concentration is smaller there ; hence the pellicle becomes more strongly stretched above and increases more rapidly in superficies, and protuberances directed outwards are not unfrequently formed. This may be expressed shortly by saying that endosmose takes place principally in the lower part of the cell, growth in the upper part. The difference however in the concentration in the interior of the cell which causes this is the con- sequence of the water which penetrates by endosmose not mixing at once uniformly with all parts of the interior solution, so that layers of different specific gravity lie one over another. Further experiments showed that growing pellicle-precipitates having the form of cell-walls are produced also by mixing colloids with crystalloids^; e.g. tannic acid with copper and lead acetates, soluble glass with the same substances or with copper chloride, or finally crystalloids with one another, as potassium ferro-cyanide with copper acetate or chloride. Traube came to the conclusion that every precipitate the interstices of which are smaller than the molecules of its components must assume the form of a pel- licle when the solutions of its components come into contact with one another. Since the pellicle-precipitates, as has already been mentioned, contain molecular interstices but no perforations, they are peculiarly well adapted for the study of endosmotic pro- cesses. They behave in this respect very diff'erently from other membranes, being themselves often perfectly impermeable to the most diflfusible substances, but allowing other chemical compounds to pass through them ; and every kind of pellicle has in this respect its own peculiarities. Independently of the fact that every pellicle-precipitate is impermeable to the fluids from which it is itself produced, the /3 gelatine tannate is, moreover, impermeable for example also to potassium ferro-cyanide, but permeable to ammonium chloride, barium nitrate, or water. The pellicle of copper ferro-cyanide which is formed round a drop of copper chloride in potassium ferro-cyanide is imper- meable to barium chloride, calcium chloride, potassium sulphate, ammonium sulphate, or barium nitrate, but permeable to potassium chloride or water. Traube considers that in the permeability of the pellicle-precipitates we have a means of determining the relative size of the molecules of different solutions, since only those molecules can pass through the pellicle which are smaller than its molecular interstices and therefore smaller than the molecules of the solutions which produce it. If a small quantity of ammonium sulphate is added to a solution of /3 gelatine, and a small quantity of barium chloride to one of tannic acid, and the two mixtures thus obtained are themselves mixed, a pellicle is formed of calcium tannate, and on it a precipitate of barium sulphate which diminishes the size of the interstices; the two solutions which cause the deposit can no longer diifuse ; but the incrusted pellicle is still permeable to the smaller molecules of ammonium chloride and water. ^ [The term 'crystalloid' is here used in the sense in which it was first employed by Graham, to indicate those substances— as opposed to ' colloids' — which may be susceptible of crystallisation, and which are endowed with the power of diffusion through a porous septum. — Ed.] CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 597 Traube maintains that there is no such thing as an endosmotic equivalent in the sense of the older theory. Endosmose is independent of any interchange, since it results entirely from the attraction of the dissolving substance for the solvent; and this attraction is invariable at the same temperature and may be termed Endosmotic Force. The endosmotic force of grape-sugar, for instance, is very great, that of gela- tinous substances very small. To these researches, which are of extreme importance in reference to vegetable physiology, and of which we shall make much use in t)ie sequel, though with a cautious selection, Traube has added observations on the growth of the pellicle-precipitates of copper fcrro-cyanide, the main results of which however I have been unable to confirm after a number of experiments. If a drop of a very concentrated solution of copper chloride is dropped into a dilute solution of potassium ferro-cyanide, it immediately becomes coated with a thin brownish pellicle of copper ferro-cyanide which exhibits peculiar phenomena. It is more convenient to place small pieces of copper chloride in the ferro-cyanide solution, where a green drop is immediately formed at the expense of the water of the solu- tion, producing the pellicle on its surface, and still enclosing the solid copper chloride which dissolves gradually from the permeation of the water. These cells manifest active growth and a variety of differences not easy to explain and dependent on secondary circumstances; some have very thin pellicles, are roundish, and exhibit a slight tendency to grow upwards ; they usually form a number of small wart-like outgrowths and attain very considerable dimensions (from i to 2 cm. in diameter). They appear to be formed chiefly by the solution of large pieces of the copper chloride. Others have thick reddish brown pellicles, grow quickly upwards in the form of irregular cylinders, rarely branch, and attain a diameter of from 2 to 4 mm. and often a height of several cen- timetres. Combinations of the two forms also occur which sometimes form a kind of horizontal tuberous rhizome-like structure from which long stalk-like outgrowths arise upwards, and root-like protuberances downwards. It is impossible, in the space at our disposal here, to give a detailed description of these phenomena ; one only may be specially mentioned :— that these pellicles of copper ferrocyanide do not grow, as Traube supposes, entirely by intussusception, but also in quite a different way (by eruption). When a brown pellicle has been formed round the green drops, water penetrates quickly from without through the pellicle to the copper chloride ; this becomes rapidly stretched, and, as may be clearly seen, at length ruptured. The green solution immediately escapes through the fissure, but becomes at once coated with a pellicular precipitate which appears either as an intercalated piece of the previous one, or as an excrescence or branch of it, a process which is repeated as long as any copper chloride remains inside the cell. We cannot therefore in this case conclude that deposition of fresh molecules of the pellicle takes place between those already in existence. These cells cannot, so to speak, be injured ; if they are pricked, then at the moment when the point which pricks them is with- drawn an outgrowth follows immediately, which is easily to be explained from what has been said. In consequence of the rapid flowing in of water through the perforation, the dissolved or the still solid copper chloride has no time to form a homogeneous solution ; a stratification arises which begins in the lower part of the cell with a very concentrated solution, and passes in the upper part into almost pure water when the cell has already grown to some height. Since the dilute upper fluid is lighter than the surrounding solution, it exerts an upward pressure upon the membrane-just as a cork held down under water attempts to rise— till it is ruptured below or at the apex (in the second form of cell). But the lighter fluid, when on the point of ascending, becomes at once surrounded by a pellicle which remains attached to the walls of the fissure of the old one; and thus apical growth takes place in cells of this description m the form of eruptions, just like the formation of branches and excrescences in the round ones. If the fluid in the upper part of the cell is pure water, large pieces of the pellicle break off" -ycS MOLECULAR FORCES IN THE PLANT. and rise up into the surrounding solution like air-balloons open below. If the copper chloride is entirely consumed in the formation of the pellicle, the opening caused by the tearing off of the upper cap does not close, or the whole cell ascends like an air- balloon. If rapidly growing cells of the second form are placed in a horizontal posi- tion, an outgrowth takes place at the extreme apex as the least solid point, which is directed vertically upwards, and then grows in this direction like the earlier apex of the cell. This process, even though it calls to mind distantly the bending upwards of grow- ing stems which are placed horizontally (geotropism), bears in fact not the least actual resemblance to this phenomenon, as will be shown in Chap. IV; and this is at once evident if it is remembered that in these cells there is no such thing as growth by intussusception. Sect. 2. Movement of Water in Plants^ The growth of the cells of plants is always connected with the absorption of water, and not only as regards the increase of size of the cell-cavity ; the growth of the cell-wall and of other organ- ised structures is also accompanied by the intercalation of particles of water be- tween the solid molecules. Water must therefore be conducted to the growing cells and tissues ; and when the organs which absorb the water lie at a distance from those which require it for their growth, the movement which results is necessarily considerable. Water is in the same manner required by the organs of assimilation, since it furnishes the hydrogen required for organic compounds. The reservoirs of food-material in which the assimilated compounds are for a time accumulated also require water for the purpose of again dissolving these substances, in order that they may be carried as formative materials to the leaves and the growing apices of roots and stems. All these movements of water, which are necessarily connected with nutrition and growth, proceed slowly like growth itself; their direction is in general determined by the relative positions of the organs which absorb the water from without and of those which make use of it. In plants which grow under water or beneath the ground where no loss of water takes place or only to a very inconsiderable extent, there is no need for these processes. The case is nearly the same also with some land-plants which are almost completely protected by a peculiar organisation from loss of water by evaporation when it has once been absorbed, as the Cactus-like Euphorbias, Stapelias, &c., which are by this means enabled to live in the most arid localities. But the great majority of plants have foliage with a very large superficial development ; when the leaves are also delicate, as in most plants with a rapid growth, a very considerable portion of the water of their cell-sap is removed by evaporation within a short time, so that in the course of a single period of vegetation the quantity of water which has been withdrawn by evaporation may exceed many times the weight and volume of the plant itself. It is easy to understand that this is possible only when the loss is compensated by the absorption of corresponding quantities of water through the roots, and that the water withdrawn from the leaves is replaced in this way. As long as the tissue of plants in which transpiration ' See Sachs, Handbuch der Experimental-Physiologie, the section on the movement of water, p. 196, where the literature up to 1865 is mentioned; the most important of the more recent publi- cations are quoted in the sequel. MOVEMENT OF WATER IN PLANTS. /^ng takes place remains turgid, the addition must nearly equal the loss by evaporation ; so long therefore as evaporation proceeds continuously from the leaves or other surfaces, a constant current of water exists from the roots to the leaves. When evaporation ceases, as in very moist air when the leaves are wetted by dew or rain or after the falHng of the leaves, the current of water also ceases as soon as the tissues which have become somewhat flaccid are again turgescent. Since evapor- ation is accelerated by a high temperature of the air, by its dryness, and above all by sunshine, and as these conditions are constantly changing, the rapidity of the current of water is also subject to continual change. The current of water occasioned by evaporation has, as will be seen, no immediate connection with the processes of growth and nutriment ; the horse- chestnut and other trees and shrubs which put out in spring only a definite number of leaves, and during the summer do not any further increase their foliage, transpire the most rapidly during this time ; and at this time also the current of water is most considerable in them. In winter both growth and evaporation, and with this last the amount of water also in the tissues, remain stationary ; when the buds are put out, the water is first of all only set in motion to the extent required by the increase of the growing organs ; but as the development of the organs increases their surface, the amount of evaporation again rises, and the current begins afresh. While the movement of water required for purposes of growth and nutrition must take place in the most different forms of tissue — in the parenchyma and even in the primary meristem of buds and of the apices of roots — it is nevertheless certain that the current of water caused by evaporation passes exclusively through the woody portion of the fibro-vascular bundles ; all the rest of the tissue may be destroyed at any place without the current of water ceasing, if only the wood remains entire. In Conifers and Dicotyledons a strong current passes through the root and stem, dividing in die branches and leaves into constantly narrower channels ; while in Ferns and Monocotyledons the current of water passes, even in the primary stem, through isolated narrower channels corresponding to the course of the isolated woody bundles. That the lignified elements of the xylem of the fibro-vascular bundles determine the channel of the current, is seen not only from direct observa- tion, but also from the fact that the formation of wood proceeds the more rapidly the more considerable is the evaporation and the stronger the current of water in a plant. In submerged and underground parts of plants from which no evaporation takes place the xylem remains entirely or nearly unlignified ; in Dicotyledons and Conifers, where the evaporating surface increases with age, the channel taken by the current is also annually widened by the increase of the wood. The crown of leaves of palm-trees remains after a certain time of nearly the same size, and the stem and the channels of the current (woody bundles) which traverse it consequently retain their diameter unchanged. The movements of water caused by growth as well as those induced by evaporation have this in common, that their direction is towards the places wliere they are required. If the growth or the evaporation begins at a certain time at a definite spot, the nearest portions of the tissue give up their water first of all, then the more distant ones, until at length the organs at the greatest distance, generally the roots, are compelled to absorb water from without. The movement 6oo MOLECULAR FORCES IN THE PLANT. therefore propagates itself continually further and further from the point to which it tends, and finally over the whole plant to the medium which surrounds the root. The kind of motion may therefore — without considering for the moment its actual causes — be described as a process of suction. Tliis is especially evident in leafy stems and branches which, having been cut off and placed with their cut surface in waier, suck up as much water through their woody bundles as is required for transpir- ation and for the unfolding of fresh leaves, unassisted in this case by any pressure from below. Another kind of motion of water in the plant, depending not on suction but on pressure from below, is caused by the roots, and is altogether independent of the use of the water for the purpose of growth or of evaporation. If the woody stem of a land-plant is cut through above the root, the root being attached to the ground in the ordinary manner, and if the ground is damp and suffici- ently warm, water exudes from the trans- verse section of the stem either at once or after some time, the current continuing for days, and the quantity of water which flows out amounting sometimes to many times the volume of the root. This cur- rent of water, which rises in wood as well as in glass tubes, can only be in- duced by a pressure exerted on the lower parts of the root. If a manometer of a proper form is fixed in the section (Fig. 438), it, shows that even in smaller plants with but litde wood (as tobacco, maize, the stinging nettle, &c.) the water which exudes stands at a pressure which holds in equilibrium a column of mercury several centimetres in height ; while in some woody plants, as for instance the vine, this pressure may amount to 76 cm. (or one atmospheric pressure). In many plants of small height this root-pressure is observable from the fact that water exudes at particular points of the leaves in the form of drops, pro- vided that the internal supply of water is nowhere diminished by powerful evapor- ation, and the pressure thus removed. Thus drops of water appear abundantly and repeatedly on the margins and apices of the leaves of many Grasses (especially striking in the maize), Aroideae, Alchemilla^ &c., when transpiration is diminished KiG. 438. — Apparatus for observing the force with whicli water escapes under root-pressure from tlie trans- verse section of a stem r. The glass tube H is first of all firmly fastened to the stem, and the tube r then fixed into it by the cork Jt. Ji is completely filled with water, the upper cork A then fixed in it, and mercury poured into the tube r so as to stand from the first higher at g' than at \.?L°-^y'";- leaves, especially those of water-plants, or altogether imperceptible as m roots, these parts dry up very quickly in ordinary air; while an intermediate condition is presented by the cuticularised outer layer of the epidermis of leaves and young internodes. Ma contradistinction to this the evaporation is very small from hard evergreen leaves, Cactus-stems &c., \Yhich are covered by a thick cuticular coating. yinay be ass umed-that., in, j)lants„ pxQvided with a thick cuticle transpiration takes £lace_ prmdpal]y. through the stomata, and is therefore dependent on their smaller or Jiarger number ..and size. The evaporation does not in this case proceed from the surface of the organ (or only to an imperceptible extent) but in its interior, 'viz. at the places where the cells of the parenchyma bound the intercellular spaces. These spaces may be supposed to be always at least nearly saturated with aqueous vapour ; but the vapour will escape through the stomata with every increase of its tension or decrease of the tension of the vapour without, and will thus give rise to the production of more vapour in the inside. The_£rodiictiaiL of. vap.our in the intercellular spaces is moreover the more abundant theJargELcthey are themselves, or in other words the larger the superficies of cell- wall ..which bounds. 4,hem. This circumstance, and the much larger number of stomata on the under side of the leaves, are clearly the reason why evaporation is gener- ally so much more copious from it than from the upper side. Since water containing any substance in solution evaporates more slowly than pure water, and the more slowly the more concentrated and denser the solution, this force must also be considered among the conditions which limit the transpiration of water from the sap of plants. It must not however be forgotten that evaporation takes place only on the external surfaces ^of the cell-walls of tissues, which on their part remove the water by imbibition from _the._oeli-sap. The conditions now named which regulate transpiration are combined in the most various ways, and not only cause different plants to show different amounts of transpir- ation, but also the amount to be very different in the same plant at different times. A definite statement cannot however be made of the total amount of transpiration, /. e. of the quantity of water required by a plant during its period of vegetation, although certain very variable limits can always be assigned to each species in this respect. Two plants of the same species may, as any one may see, thrive equally well if one grows in damp soil and dry air, the other in dry soil and damp air, the former thus using up a large, the latter a small amount of water. In general the conditions of transpiration which have been mentioned exhibit periodic variations related to the meteorological distinction of day and night ; the temperature, the moisture of the air, and light, are usually favourable to evaporation by day, unfavourable by night; but under certain circumstances this condition may even be reversed. (f) Currents of IVater in the H^ood. Superficial cells or those which bound intercel- lular spaces and lose water directly by evaporation, would very soon collapse and dry up if they were not able again to replace that which they have lost. This can only take place by the flow of water from the adjoining cellular tissue from which no evaporation occurs ; but when this tissue is placed in the same condition as the former, it must also compensate its loss from more distant layers of tissue, and these again from those which are connected with the conducting organs or woody bundles which convey the water from the roots. The question here presents itself whether this movement of water within the succulent tissue (especially in the parenchyma of the leaves) is caused by endosmose from cell to cell, or whether it does not occur at least principally along the cell-walls, these latter forming the channels of communication between the woody bundles and the surfaces where the evaporation takes place, the contents of the cells being only incidentallv carried along with the transmitted fluid. The chief evidence of the fact that the currents of water m the roots, stem, and branches caused by transpiration take place only in the wood, /'. e. in the lignified xylem, has already been stated. It can be demonstrated in a more conspicuous manner 6o4 MOLECULAR FORCES IN THE PLANT. by placing a cut stem or branch with its cut surface in a coloured solution^ while the leaves are transpiring. If the stem or branch is cut through at various heights after a few hours, or according to circumstances after a longer period^ the colouring of the wood will show^ how high the solution has been sucked up in it, and will be seen only in the woody bundles and not in the cortex or pith. If branches with pure white flowers are employed in this experiment (according to Hanstein's process), such as a w^hite- flowered Iris or Deutzia, and if they are placed in a dark aqueous solution of aniline, the white petals are found, after from ten to fifteen hours, to be permeated by dark blue veins corresponding to the fine woody bundles of the venation. This beautiful appear- ance however soon vanishes, the poisonous colouring material subsequently killing the adjoining layers of parenchyma, and colouring the spaces between the veins blue by diffusion, and the corolla thus becomes flaccid^. The difference in the amount of transpiration under different external conditions must also correspond to a difference in the rapidity of the current of the water in the wood. In rainy weather, when there is no evaporation or but very little from the leaves, the movement of the water in the stem will be very slow ; but when the transpiration increases with sunshine and wind, the current of water in the w^oody bundles is also accelerated. Under the hypothesis that the water moves only in the woody substance of the walls of the wood-cells themselves and not in their cavities, I have calculated the rapidity of the ascending current of water in a branch of the silver poplar in w^hich there w^as strong evaporation, and obtained a rate of 23 cm. per hour. M^Nab placed branches of Primus Laurocerasus ^ from which evaporation was taking ^ I must take this opportunity of making the remark that I still entertain, and in a high degree, the doubt previously expressed, whether it is not a purely pathological phenomenon that is produced in this manner. "^ [This is a method of experimentation which has been practised by numerous observers since the commencement of the last century, when it was apparently first tried by Magnol. Sarrabat (otherwise Delabaisse) coloured the veins of the flovvei-s of the Tuberose {Polyanthes tuberosa) and Snapdragon {Antirrhinum inajus) by watering the plants with the juice of the berries of Phytolacca. (Dissert, sur la circul. de la ^h\e, Bordeaux, 1733.) Van Tieghem (in the French edition of this work, p. 791) quotes Reichel as having plunged the roots of a flowering plant of Datura Straniofiium into a decoction of the wood of Fernambouc ; the liquid followed the course of the vessels, and after eight days veined the corolla with red, and made its appearance also in the stamens, the walls of the fruit, and even in the style. (De vasis plantarum spiralibus, Leipzig, 1748.) For other old authorities see De CandoUe, Phys. Veg. i. 82. De Saussure found that the stem of a bean became coloured by a decoction of Brazil-wood ; and this was one of the facts upon which he based the conclusion that organic matters were capable of being taken up by the roots of plants (Ann. des Chem. u. Phys. xlii. p. 275). Biot noticed that the red colouring matter of Phytolacca was absorbed by white hyacinths when poured upon the soil in which they were grown ; after two or three days, however, the red colour disappeared from the flowers. (Comptes Rendus, 1837, i. 12.) linger also made the same experiment (Botanical Letters, p. 38). Hallier immersed the ends of cuttings of plants in solution of indigo or black cherry juice. (Phytopathologie, 1868, p. 67). Persoz states (Introd. a I'c'tude de la Chimie moleculaire, p. 553) that plants oi Impatie7is parviflora, the roots of which are immersed in a solution of sulphindigotic acid, absorb that fluid in a reduced or colourless state due to the action of the roots upon it ; in the petals it again undergoes oxidation and becomes blue. The experiments of Herbert Spencer (Prin- ciples of Biology, i. p. 53S) may also be referred to. — Ed.] ^ M^Nab, Transactions of the Botanical Society of Edinburgh, 1871. [Dr. Pfitzer has suggested that the result may be anived at by the much simpler mode of allowing the plant grown in a pot to become so flaccid from want of water that the leaves droop perceptibly, and then, after supplying the root with water, to observe the length of time that elapses before the leaves at various heights from the ground recover their normal position. Pfitzer found by this means a much more rapid rate of ascent indicated than that stated by M<^Nab ; and believes that there is a serious source of error in M^Nab's experiments, from the saline solution not rising so fast as pure water. — Ed.] MOVEMENT OF WATER IN PLANTS, 605 place in a solution of lithium citrate, and then examined the ashes of successive inter- nodes by the spectroscope. The solution was found to rise from 42 to 46 cm. in one hour. But neither method of calculation is exact or probably of much value. The current of water in the woody substance which replaces the loss occasioned in the leaves by transpiration is not caused by osmose, since at the time when the evaporation is strongest and therefore the current in the wood quickest, the cavities of the conducting wood-cells do not contain sap but air, or at the most are only partially filled with fluid. If the rising of the water took place by endosmose from cell to cell, the cells would themselves possess closed cell-walls and be full of sap, the concentration of which would constantly increase from below upwards in the wood. But the conducting cells are at this time not closed, but partially or altogether (as in Goniferie) connected with one another by open bordered pits. In the spring, before strong transpiration sets in, and therefore at a time when the water in the wood is com- paratively at rest, the wood-cells also, it is true, contain sap, flowing in quantities out of their communicating cell-cavities when holes are bored in the trunks (as in the birch, maple, &c.) '. But this sap does not, as is proved by analysis'^, show a concentration increasing from below upwards. The fact also that water rises in cut leafy branches placed with their upper end in water or planted and taking root, and flows there- fore in a direction opposite to the ordinary one in the branch, shows that endosmose depending on a definite distribution of the concentration of the sap cannot be the cause of the current of water. Since vessels and wood-cells communicating with one another through their open pores form narrow cavities which sometimes become wider as they proceed, sometimes narrower, the woody substance may be represented by a bundle of narrow glass tubes alternately bulging and contracting, in which the water which fills them rises by capillary attraction. But how little efficacious a contrivance of this kind would be is seen at once from the width of the capillary tubes, which is much too great to raise water to a height of 100 feet or more. It must also be pointed out that in the summer, when the current of water is strongest, it is principally air and not fluid that is conveyed through the cavities of the cells. Since it is evident from what has been said that the movement of the water takes place in the woody substance and not in the cell-cavities filled with water, there remain only two hypotheses; viz. (i) that the movement takes place in the water contained in the lignified cell-walls (or in other words imbibed by them) ; and (2) that it is caused by a very thin stratum of water which overspreads the inner surface of the wood-cells and vessels^. In both cases it must be assumed that the transpiration in the tissue of the leaves causes the upper parts of the wood to contain less water, and therefore to draw up th^ water from the parts which lie lower. The woody bundles of the roots are surrounded by succulent parenchyma, from which they remove the water ; and these again absorb it from the soil by endosmose. It may however be imagined that both the kinds of motion mentioned proceed along the surface as well as in the substance of the cell-walls (the contents not participating in it) to the surface of the root, where the water contained in the soil is sucked up. The question whether the attraction of the cell-walls for water,— putting aside the question whether it moves in their substance or only on their surface,— is sufficiently powerful to sustain the weight of a column of water of the height of 100 or even 300 feet or more attained by some trees. • The older statements of Unger are referred to in my ' Experimental-Physiologie '; others will be found in Schroder, Jahrb. flir wiss. Bot. vol. VII, p. 266 et seq. 2 The conduction is however by no means so considerable in the reversed as in the ordinary direction, as Baranetzky found in the laboratory at Wiirzburg; but this may be connected with other peculiarities of the organisation. 3 This hypothesis follows from the discoveries of Quincke on capillarity and has been commu- nicated to me by him. )o6 MOLECULAR FORCES IN THE PLANT. may be answered without hesitation in the affirmative, since we have to do here with molecuhir forces in opposition to which the action of gravity altogether disappears. But it is another question whether the rapidity of the molecular movements of water of this nature is sufficient to cover the requirements of the foliage of a tree which amounts on a hot day to hundreds of pounds \ The hypothesis finally that the w^ater is forced up into the stem and even into the leaves by root-pressure must be abandoned, since this could only operate in the cavities of the wood ; and these are always empty in energetically transpiring plants. In the case of tall trees the pressure would also not be sufficient ; and if 1 at one time assumed that this might be a cooperative cause at least in shrubs and annual plants, I must retract this after my observations made in the year 1870; since these show that the root-stock of such plants as the sun-flower, gourd, &c., is even subject to a negative pressure when they are transpiring strongly ; i. e. does not press water up, but greedily sucks it in at a cut surface above the ground {'vide infra). The insufficiency of all attempts hitherto made to explain the movement of water in the wood due to transpiration is especally noticeable from the fact that it is only under certain internal conditions which cannot be more accurately ascertained that wood is capable of conducting w^ater with the force and rapidity required by the eva- poration from the leaves. \¥oody but air-dry branches wath a lower cut surface placed in water are never able to raise up as much water as is necessary to' replace the evaporation even from an upper cut surface ; while the same branch in a fresh state conducts water fast enough to replace the much greater amount of evaporation from the numerous leaves. A change is thus caused in wood simply by drying up which deprives it of the power of conducting water rapidly. The natural alteration which takes place in w^ood, by which it is transformed as it increases in age into ' duramen ' — the cell-walls becoming harder and of a deeper colour — also deprives it of this power. If a tree is deprived not only of the bark but also of the ' alburnum ' (the light-coloured younger wood on the outside), in an annular zone, the foliage of the tree, according to the statement of different writers, dries up, because the water is not conducted suffi- ciently rapidly through the duramen. Among the most remarkable of the phenomena related to this is the fact that the younger terminal portions of the stems of large-leaved plants partially lose the power of conducting water w^hen cut off in air. If the cut leafy end of the stem of He I i ant bus anniuis, H. tuberosus, Aristolochia Sipho, &c., be placed with the cut section in water, the suction is not sufficient to compensate the evaporation from the leaves, which therefore wither after a shorter or longer time. As I have already shown in the second edition of this book, the withered shoot may in a short time be revived by forcing in water by means of the contrivance represented in Fig. 439. I did not discover till afterwards that the shoot remains turgid even when the pressure is reduced to zero, and even when the mercury is raised up by the suction of the shoot in the same arm of the tube {a), when therefore a force acts on the section of the shoot in the opposite direction. This show^s that the forcing in of w^ater is only necessary at first, but that the revived shoot has itself sufficient power of suction even to raise up a column of mercury several centimetres in height, and thus to replace the loss by transpiration from the leaves. Thus much \vas known about the phenomenon of the withering of cut shoots placed in water, when Dr. Hugo de Vries took up the further investigation of it in the laboratory of the Wiirzburg Institute. The results obtained by him I will now quote : — ' If rapidly-growing shoots of large-leaved plants are cut off at their lower part which has become completely lignified, and are placed with the cut surface in water, they remain for some time perfectly fresh. But if they are cut through at the younger parts of their stem and are then placed in water, they soon begin to wither, and the ' See Nilgeli u. Schwendener. Das Mikroskop. vol. IT. p. 364 ef seq. MOVEMENT OF WATER IN PLANTS. 607 more rapidly and completely the younger and less lignified the part where the section is made. This withering can be easily prevented by making the section under water, and taking care that the cut surface does not come into contact with the air, the con- duction of water through the stem thus suffering no interruption. If care is taken that while the section is being made in the air the leaves and upper parts of the stem lose only a very small quantity of water by evaporation, withering does not begin till later and increases only slowly after the cut surface is placed in water and the leaves again transpire.' It results from these experiments that the cause of withering is the interruption in the conduction of water from below; and this interi'uption produces withering not only from the conduction of the water ceasing for a short time, but chiefly also from the power of conducting water in the stem being diminished by the loss of water above the cut surface, which loss cannot be restored simply by placing the cut surface in contact with water. If the cut surface does not remain too long in contact with the air, the diminution FIG. 439.— Apparatus for showing the revival of withered shoots by forcing water into them. The U-shaped glass tube is first filled with water, and the perforated stopper of caoutchouc i in which the stalk of the plant is niserted. is then fixed in. When the shoot is withered, as represented by a, mercury is poured into the other arm of tlie tube, so as to stand at / some 8 or 10 cm. above ?, and the shoot then revives, as represented by d, even when the level ? becomes subsequently higher than g'. of the capacity for conduction takes place in only a short piece of the stem above the cut. When placing in water ends of shoots which have begun to wither after being cut off, it is only necessary to remove by a new cut a sufficiently long piece above the first cut, but this time beneath the water, for the shoot to revive. In the case of shoots 20 centimetres or more in length which at this distance from the apex are not ligni- fied, the removal of a piece 6 cm. long is usually sufficient to revive the withered shoot {e.g. in Reliant bus tuberosus, Sambucus nigra, Xanthium echinatum, &c. This experiment proves beyond question that the change, whatever its nature may be, takes place only in this relatively short piece above the cut. That it consists in a diminution of the power of conducting water is shown by the following experiment :— When a sufficient number of the lowest and largest leaves have been removed from a stem oi Hehanthus tuberosus cut off in the air and placed in water, and which has begun to wither, the leaves that are left and the terminal bud will after some time begin to revive even without again cutting the stem. The water which is required for the transpiration of a great number of leaves can therefore no longer be conducted through the stem after oS MOLECULAR FORCES IN THE PLANT. it has been cut off in air, although that which is wanted for the transpiration of a few leaves can be. The cause of this phenomenon is therefore a diminution in the power of conducting water in a short piece above the cut surface of the stem. This is evidently occasioned by the loss of water from the cells caused by the suction of the higher parts not being compensated by absorption from below. All circumstances which favour this loss of water increase also the loss of power of conducting it, and cause the shoot w^hich is placed in water to wither more rapidly and completely. It must therefore be assumed that the conducting power of the cells depends on the quantity of water they contain. The probability of this hypothesis is increased by the fact that by artificially increasing the amount of water in the cells of this piece, its conducting capacity can also be in- creased, as is proved by forcing in water from below. If the modified portion is dipped in water of from 35° to 40° C, the withered shoots soon revive, and if then placed in water of 20 G°. remain fresh for days (as in the case of the elder), or at least wither more slowly [e. g. the artichoke). {d) Water retained in the ^wood hy Capillary Attraction. If the capillarity of the cavities in the wood must be considered as w^ithout any immediate action on the currents of water, this force must nevertheless be taken into account with respect to other processes connected indirectly with the movement of water in the plant. In winter and after long-continued rain in summer a large quantity of water is found in the cavities of the wood together with bubbles of air which occupy the wider spaces. It is not known how this water has reached the higher parts of the trees, though it is possibly by the forma- tion of dew as the temperature varies ; it is however to a great extent retained by capillarity. A part of the w-ater flows out in many cases through holes bored in the stem if they are not placed too high, as in the birch, maple, vine, &c. It may be sup- posed that the water which flows out has been forced up by the root-pressure which must also be taken into account ; though how far up this pressure extends is not yet ascertained. The water which does not flow out of the cavities when there is less transpiration is clearly retained by capillarity, assisted by the air in the cell-cavities ; for Montgolfier and Jamin have shown that in capillary spaces which contain water and air the water is not easily set in motion. This explains also the phenomenon already mentioned, that w^ater escapes when pieces of wood which have been cut off in cold weather are warmed, because the air expands and forces out the water. Sub- sequent cooling causes on the contrary water to be sucked in at the cut surface, because the air contracts, and the pressure of the external air forces in water from without. (^) The ascent of water from the root into the 3tem^. The most important features of this phenomenon have already been briefly mentioned. It is to be observed in the open air in plants of the most diff'erent kind, if they possess vigorous root-systems and well-developed wood ; as, for instance, in the birch, maple, and vine, and among annual plants, in the sunflower. Dahlia, Ricinus, tobacco, gourd, maize, stinging nettle, &c. In order to study the phenomenon accurately, it is best to grow the plants for some time previously in large flower-pots until they have developed a strong root-system. Land- plants such as maize grown in water and artificially fed by nutrient substances, are also well adapted for the investigation. If the stem of such a plant is cut across smoothly 5 or 6 cm. from the ground, and a glass tube fixed to the stump by means of an india-rubber tube, the result will be seen as follows. If the plant was in a condition to transpire freely before it was cut, the cut surface of the root-stump remains at first quite dry, and if water is poured into the glass tube it is at once sucked up^. The ^ See in particular Hofmeister, On the tension and the quantity and rapidity of the flow of the juices of living plants; Flora 1862, p. 97. ^ This fact is sufficient to prove that the root-pressure has no share in the ascent of the water at the time when transpiration is active. MOVEMENT OF WATER IN PLANTS. 6oQ woody substance of the root-stump has evidently been exhausted by transpiration before the operation, and contains but very Httle water; not only are its cavities empty, but even the cell-walls of the wood may not be saturated. After a shorter or longer time however the exudation of water at the cut surface begins— rising higher and higher in the tube — and continues from six to ten days if the plant is properly treated, be- coming during the earlier part of the time continually more copious, attaining a maxi- mum, and finally diminishing until it ceases with the death of the root-stock. If the cut section is repeatedly dried with blotting paper during the time that the water is flowing, it is clearly seen that the water exudes from the woody tissue— in Monocoty- ledons from the xylem of the separate bundles — and that it comes principally from the openings of the larger vessels. That the water which flows out had previously been absorbed by the roots out of the ground, and not merely from the store in the root- stock, is at once evident from the fact that the quantity which exudes at the cut section is after a few days greater in volume than the whole of the stock. Under the conditions here described, the water which flows out contains only traces of organic substances in solution ; but the presence of mineral constituents can be easily proved, especially lime, sulphuric acid, phosphoric acid, and chlorine, which the plant has absorbed out of the ground. The water which flows in the spring from holes bored in trees such as the birch and maple, contains however considerable quantities of sugar and albuminous sub- stances ; since the longer stagnation in the cavities of the wood gives it the opportunity of absorbing these substances out of the closed living cells of the wood and out of the surrounding parenchyma, a result which cannot be expected, or only in a smaller degree, in the case of the rapid flow from the smaller root-stocks of quickly-growing plants. In order to determine the quantity of the outflow, a narrow burette may be used instead of the tube, in which the amount can be read off hourly in cubic centimetres when the outflow is at all considerable. The root-pressure which acts upon the cut surface is however then considerably altered. In order to avoid this, a tube of the form shown in Fig. 438 i? (p. 600) is fixed to the stump, and to it is attached a narrow tube instead of the manometer ; the free end of this tube is bent downwards into a graduated burette. If the reservoir is from the first filled with water, as much runs into the burette as flows out from the cut section, and the pressure therefore remains constant. This experiment shows that the flow of water varies from day to day, from one time of the day to another, and even from hour to hour ; but the causes of these variations in the outflow, which must depend on the activity of the roots, are not yet known ; it would even seem as if a periodicity were established independent of the temperature and of the moisture of the ground ^ The measurement of the lowest pressure at which the outflow can take place at the cut surface can be effected by the apparatus figured in Fig. 438, where it is expressed by the difference of level of the mercury in the two arms of the tube, or by q—q. This will however only afford a measurement of the pressure which the outflowing water may still have to overcome at the cut surface ; but it has obviously had also to overcome other resistances of unknown magnitude in the interior of the root-stock. With respect to this point I was interested in ascertaining how great is the difference in the outflow if one of two equal root-stocks has no pressure to overcome at the cut surface, the other a considerable but constant pressure. If, in Fig. 440, a indicates the cut stem of a sunflower or similar plant grown in a pot, c, d, e the tube which is attached to it by the india-rubber tube b, and / a glass tube bent downwards, which (not as in the figure) reaches beyond the rim of the pot and terminates in a burette, while the opening of/ lies exactly on the level of the cut surface of the stem; then, when the tube r, d, e,f has been filled with water, we have an apparatus for observing the outflow when the ^ Very detailed observations on this point have just been made by Baranetzky in the WUrzburg laboratory, in the summer of 1872, R r 6io MOLECULAR FORCES IN THE PLANT. pressure at the cut surface is at zero. A second root-stock from a plant of exactly the same age and vigour and grown in a pot of the same size is provided with the apparatus figured in Fig. 440, where the tube / through which the outflow takes place reaches the vessel h through the cork g. This vessel contains water above, mercury below. A tube k rises from the cork i to a certain height and is bent round at the free end where it dips into a graduated tube. If the apparatus is so contrived that, for example, the opening for the outflow o stands about 1 5 cm. above the level «, then the column of mercury o?i exercises a pressure of 15 cms. on the water h, and through it on the cut surface at b. When the water begins to flow out from the cut surface at b, the quantity of water in h will be increased, and an equal volume of mer- cury will flow out at 0. The mercury collects in the burette, and its level enables Fig. 440. — Apparatus for measuring; the root-pressure when considerable and constant. The cork I has a lateral incision in order to allow of the escape of the air when the mercury is dropped in. the quantity of water which has flowed from the cut surface to be read off" from hour to hour, and to be compared in the other apparatus where there is no pressure. After a longer period of observation, the level n falls sensibly and the pressure on augments a little. But it is easy to bring it again to the original amount if a fresh quantity of mercury is poured in every twelve hours. I observed in this manner in the summer of 1870 for five days two equally strong root-stocks of the sunflower^; and the result was that the diff"erence of the outflow was but small, although the amount of pressure in one case was zero, in the other case 17 cm. of mercury. In the first thirty-three hours the outflow where there was no pressure at the cut surface amounted to 26-45 cubic cm.; when the pressure was 17 cm. ^ I cannot here describe the whole series of minute observations. I MOVEMENT OF WATER IN PLANTS. 6ll of mercury it was 20-9 cubic cm. A sudden change in the pressure of the mercury of I or 2 cm. also caused no considerable alteration in the rapidity of the outflow. It is necessary then to make some conjecture as to the cause of this powerful ascent of water in the wood of the root-stock ; how it happens that the water sucked up at the surfaces of the roots not only passes into the cavities of the wood, but is pressed upwards with so great a force as to be able to overcome a considerable resistance at the cut surface ; for it is obvious that the water which flows out above must have been sucked in below at the surfaces of the roots. This suction can only be induced by the en- dosmotic action of the parenchymatous cells of the cortex of the root. If we suppose that this endosmotic force is very considerable, these cells will swell greatly; and as much water will filter through the cell-walls to the cavities of the wood as is sucked up from without by endosmose. The parenchymatous cells which are gorged by endos- mose drive into the vessels the water which presses into them in consequence of the endosmose, and with such force that in flowing out above from the vessels it is still able to overcome a considerable pressure. It follows from this explanation that the pressure which acts at the cut surface must, in accordance with the laws of hydrostatics, be exerted also against the inside of the vessels which receive the water from the turgid parenchymatous cells. But the water which enters them has also to overcome the re- sistance to filtration exercised by the cell-walls. The endosmose of the cortical cells of the root must overcome these resistances. Although we do not know the magnitude of the endosmotic force, yet we have ground for supposing that it is much greater than that given by Dutrochet's experiments on animal membranes; and this explanation would therefore be very probable. But a difficulty occurs in answering the question why the turgescent cortical cells of the root expel their water only inwards into the woody tissues and not also through their outer walls. We may however here be helped by the supposition that the molecular structure of the cells is different on their outer and inner sides, and that those facing the exterior of the root are best adapted to allow endosmose, while those facing the interior of the root are best adapted for permitting filtration under high endosmotic pressure. It must however be observed that this supposition is at present only a hypothesis for the purpose of explaining to a certain extent the processes which take place in the root. The exudation of drops of water from the upper cell of the Fungus Pilobolus crystallinus, from the root-hairs of a Mar- chant ia grown in damp air, &c., shows moreover that cells distended by endosmotic tension can in fact exude water at certain spots. It is difficult to give any other ex- planation of the exudation of nectar in flowers ; the excreting cells must evidently absorb the water or the sap with great force on one side, and then exude it on the other side. That in this case pressure from the root does not directly cooperate is shown by the fact that this exudation of nectar, which is often very copious, as in the flowers of Fritil- laria imperkdis, takes place even when cut flowers are simply placed in water. In this respect these exudations of fluid diflfer from the exudation of drops on the leaves of many plants, which only takes place when they are still in connection with the root, and which is clearly caused by the forcing power of the root (as in Aroidece, &c.). It also happens however sometimes that drops of water are exuded from cut surfoces of the tissue, while another cut surface of the organ sucks up water. This I found, for instance, to be the case with pieces of the young stems of diff"erent Grasses, cut off" from 6 to 10 cm. in length, which were placed with the lower end in damp soil; the free upper end then repeatedly and continuously exuded drops of water in darkness and in an atmosphere saturated with moisture. Here the parenchymatous cells of the lower cut surface clearly acted as the cortical cells of the root, sucked up by endosmotic action, and probably pressed the water thus sucked up into the vessels, from which it then escaped to the upper cut surface. (/) The combined action of transpiration, conduction, and absorption of nvater through the roots takes place under ordinary and favourable conditions in such a manner that nearly as much water is absorbed through the roots and conducted upwards through the R r 2 6l2 MOLECULAR FORCES IN THE PLANT. wood as is transpired from the leaves. As long as this equilibrium lasts, the plant is turgid and tense in all its parts ; and conversely it may be concluded from the unaltered turgidity and tenseness of the leaves and internodes, that the conduction of water is compensated by the evaporation from the leaves. Hence, under these conditions, the quantity of water evaporated may be taken as the measure of the suction of the root (or of a cut surface), and conversely the suction observed as the measure of the evapor- ation from the leaves. Since however the tissues can be more or less turgid without its being immediately perceptible, evaporation and suction are not usually exactly equal. But for most observations the small occasional difference may be neglected so long as no actually perceptible amount of flaccidity, i. e. of withering, caused by the collapse of the cells, takes place when the evaporation is stronger and the suction weaker; or so long as, in the opposite case, no exudation of drops of water results on the leaves of rooted plants. It is only when longer observations are made on growing plants that the comparatively small quantities of water have to be taken into account which are needed for the increase in size of growing organs. Without going more minutely into the various cases which present themselves^, it need only be pointed out in addition that withering is the consequence of the quantity of water evaporated being greater than that absorbed through the roots or through a cut surface of the stem. This only occurs in general when the amount of transpiration is very considerable, or when the ground is very dry, or when in cut shoots the power of the stem to conduct water has ceased. The exudation of drops of water already mentioned is, on the other hand, the consequence of a smaller quantity of water evaporating from the leaves than is absorbed by the roots and forced up into the upper organs. If a branch of a potato-plant, a leaf of an Aroid, a cut stem of maize, or the like, is fixed in the cork k in Fig. 439, and if, when the evaporation is weak a pressure of mercury of 10 or 12 cm. is allowed to act for some time, drops of water appear at the same spots on the apices or margins of the leaves, where they would appear in plants with roots in the evening or night or in damp weather. In the same manner the exudation of drops from plants with roots can be produced or increased by warming the ground and covering the leaves with a bell-glass in order to hinder evaporation^. The pressure due to the root which is so conspicuous in stems when cut across and when the amount of evaporation is very small, can scarcely be of any considerable use in promoting the current of water in the wood caused by strong transpiration. The fact already mentioned that strongly transpiring plants suck up water at the cut surface of their stems immediately after the upper part has been cut off, shows that the pro- pelling force of the root does not act sufficiently quickly to protect even the vessels of the root-stock of strongly transpiring plants from complete exhaustion ; that is, although the force which drives the water into the root-stock is great, as we have seen, it acts too slowly to be taken into account when the evaporation is rapid. The same conclusion is reached if the quantity of water which exudes in the same time from the cut stem of a plant above the root is compared with that which is absorbed at the lower cut surface by the upper part of the same plant. The absorption of the upper part is always much more considerable in amount than the outflow from the root-stock, even when the withering of the upper part indicates that the capacity of its wood for conduction has diminished, and that it absorbs less than it would absorb in the normal condition. Thus, for example, the water absorbed by the cut leafy top of * See Rauwenhoff, Phytophysiologische Bijdraden in Versagen en Mededeelingen der kon. Akad. van Wetens, Afdeeling Natuurkunde, 2^0 Reeks, Deel III, 1868, where however the indis- pensable thermometric observations are wanting. 2 The exudation of drops on the margins of the leaves of plants, the roots of which are sur- rounded by damp wann earth, their foliage rising into moist air, is an altogether different pheno- menon, as I know from the experience of many years. MOVEMENT OF WATER IN PLANTS. 613 a tobacco-plant amounted in five days to 200 cubic cm., while the root-stock exuded only 15-7 cubic cm. In the same manner in Cucurbita Pepo (when much withered) the amount absorbed was 14 cubic cm., the exudation from the root-stock only 11-4 cubic cm. The withered upper part of a sunflower absorbed in a few days 95 cubic cm., while the root-stock exuded only 52*9 cubic cm. The result is also the same when the relative amounts which extend over a shorter time are compared. It follows from these facts that, with the exception of times when the amount of transpiration is small or when drops of water exude from the leaves, no root-pressure at all exists when the plant is uninjured; and that this pressure is exerted only after evaporation and absorption have ceased or when they are very small. The exhaustion of the root-stock of a strongly transpiring plant (as after it has been cut off) proves rather that a plant with roots behaves in exactly the same way as a cut shoot. Just as the latter absorbs water from a receiver, so the wood of the root-stock which has lost water in consequence of evaporation above absorbs water from the cortical cells of the root which obtain it by endosmose. From all this it still remains in doubt whether in such cases the contents of the cortical cells of the root must not be left altogether out of consideration, since it is possible that the suction of the cell-walls merely, due to imbibition or surface-action, reaches as far as the surface of the roots. {g) The parts of land-plants which are covered with a cuticle and which serve for transpiration appear to have no power of absorbing in any considerable quantity the water by which they are moistened, such as the rain and dew which is deposited on the leaves. As long as the tissues and leaves of uninjured plants with roots become turgid and are supplied with water from below, any considerable absorption through the sur- faces of the leaves themselves, if they are already quite moist, is not to be expected, since it is not easy to see where the water can go in cells that are already gorged ^ But even when a plant has withered, it is still doubtful whether its revival depends on the ab- sorption of water by the leaves, since it is not impossible for an upward pressure to take place subsequently. Greatly withered shoots do not under such circumstances become turgid or do so only very slowly unless the cut surface is placed in water, and even in this case there is doubt as to the absorption of water through the surfaces of the leaves. In harmony with this Duchartre found also^ that rooting plants (Hortensia, Helian- thus amiuus), which wither in the evening in consequence of the dryness of the earth in the pot, did not recover or become turgid if copiously moistened by dew during a whole night, the pots in which the roots spread being provided with a closed cover. Epidendral Orchids, Tillandsias, &c., behave in the same way in this respect ; they also absorb neither water nor aqueous vapour through their leaves, nor even in any con- siderable quantity through the roots. The water which they require for their trans- piration and growth must be conveyed to them in the form of rain^or dew which moistens the root-envelopes or wounded surfaces^. When land-plants wither on a hot day and revive again in the evening, this is the result of diminished transpiration with the decrease of heat and increase of the moisture in the air in the evening, the activity of the roots continuing— not of any absorption of aqueous vapour or dew through the leaves. Rain again revives withered plants not by penetrating the leaves, but by moistening them and thus hindering further transpiration, and conveying water to the roots, which they then conduct to the leaves. A simple experiment will afford much instruction to the student in these matters. The pot in which a leafy plant is growing is enclosed in a glass or metal vessel provided ' Duchartre has neglected this obvious reflection in his researches (Bulletin de la Soc Bot. de France, Feb. 24, i860) ; in other respects also these experiments are very defective. 2 Duchartre, I.e. 1857, pp. 940-946. ■' Duchartre, Experiences sur la vegetation des plantes epiphytes (Soc. Imp. et ccntrale d horH^ cultme, Jan. 1856, p. 67; and Comptes Rendus, 1868, vol. LXVII, p. 775). /^I_|. MOLECULAR FORCES IN THE PLANT. above with a lid in two portions, and surrounding the stem so as completely to cover the earth in the pot. If the soil is dry the plant withers. If a bell-glass is placed over it the plant revives, and again withers if it is removed. This shows that the withering is the result of increased, the revival the result of diminished evaporation from the leaves when the roots convey but very little water to the plant. If cut shoots are allowed to wither and are then suspended in air nearly saturated with aqueous vapour, the leaves and younger internodes again revive, although the whole shoot continues to lose weight from evaporation. This phenomenon results from the water passing from the older parts of the stem to the younger withered parts, as must be concluded from Prillieux's experiments ^ Sect. 3. — Movements of Gases in Plants^. All growing cells of a plant, or all that are otherwise in a condition of vital activity, are continually absorbing atmo- spheric oxygen and giving back in its place a nearly equal volume of carbon dioxide. The cells which contain chlorophyll have in addition the property, under the influ- ence of sunlight, of absorbing carbon dioxide from without, exhaling at the same time a nearly equal volume of oxygen mixed with nitrogen. In proportion to the activity of the chemical processes which take place within the cells, the movements of gases occasioned by them vary greatly in rapidity. The formation of carbon dioxide at the expense of the atmospheric oxygen takes place continuously and in all the cells; but the quantities concerned are small in proportion to the large amount of carbon dioxide which is decomposed in the green tissues, and in ex- change for which equal volumes of oxygen are exhaled. Some idea of the activity of this last-named process is obtained by reflecting that about one-half the (dry) weight of the plants consists of carbon which has been obtained by the decompo- sition of atmospheric carbon dioxide in tissues containing chlorophyll under the assistance of light. Oxygen and nitrogen are permanent gases, as also is carbon dioxide within the limits of the temperature of vegetation, and indeed far below it. Aqueous vapour, on the contrary, is only produced from water within these limits, and under certain conditions even returns to the liquid state. In other respects aqueous vapour be- haves just like oxygen and nitrogen in reference to the processes to be considered here. When the gases with which we have to do are traversing closed cell-walls, expanding w^hen diffusing^ themselves through the cell-sap, or permeating or escaping from the protoplasm, chlorophyll-grains, &c., their motion is a molecular one of diffusion. When they fill in their elastic condition the intercellular spaces, vessels, cells destitute of sap, or the large air-cavities among the tissues, it is a movement of the mass depending exclusively on expansive force. The movements of diffusion tend to bring about conditions of equihbrium which depend on the coefficients of absorption of the gas by a particular cell-fluid, on ^the molecular condition of the cell-wall, &c., on temperature, and on the pressure of the air. But these conditions are continually varying ; and the equilibrium which is aimed at is being still more ' Prillieux, Comptes Rendus, 1870, vol.11, p. 80. 2 Sachs, Handbuch der Experimental-Physiologie, p. 243. — Miiller, Jahrb. fur wiss. Bot. vol. VII, p. 145. MOVEMENTS. OF GASES IN PLANTS. 615 continually disturbed by chemical transpositions on which depend the metamorphosis of substances in the plant, assimilation, and growth ; so that a state of rest can very seldom occur. The ordinary condition of the gases which are diffused through the cells of plants is that of movement. But even the masses of gas found in the cavities of plants are not generally at rest. By the setting free or absorption of carbon dioxide or oxygen in the cells, the equilibrium is disturbed also in the neighbouring cavities ; and changes in the pressure of the air or in temperature also exert an influence. The flexions again of the stem and leaf-stalk produced by the wind cause pressures and dilatations of the gases which fill the cavities, and these again give rise to currents of gas in the interior. The rapidity of the movement in the cavities varies greatly in proportion to their size ; within the very narrow intercellular spaces of ordinary parenchyma the motion is slow and inconsiderable even under considerable pressure, as contrasted with the rapid currents which are possible in the large intercellular spaces of most foliage-leaves and similar organs, or in the wide air-canals of hollow stems, or in the lacunse of the tissue of water-plants. In attempting to collect the most common phenomena into a more definite arrange- ment from this general point of view, the following appear to be the more important points. (:2 CHEMICAL PROCESSES IN THE PLANT. Sulphur, a constituent of albuminoids, of allyl, and of the essential oil of mustard, is taken up in the form of soluble salts of sulphuric acid, and chiefly (or perhaps always) of calcium sulphate. This salt is probably, as Holzner first pointed out\ decomposed by the oxalic acid which is formed in the plant itself, and the insoluble calcium oxalate is thus formed, while the sulphuric acid parts with its sulphur to the organic compounds which have been mentioned. Iron'^ (often accompanied by very variable quantities of Manganese) is indis- pensable for the production of the green colouring substance of chlorophyll, as is shown by experiments on vegetation; and since the green organs which contain chlorophyll assimilate organic substances out of water and carbon dioxide, the importance of this element for the life of the plant is very evident, although extraordinarily small quantities of it are sufficient for this purpose. It may be taken up by the plant in the form of the chloride or sulphate or of some other compound. If larger quantities of solutions of iron become distributed through the tissues, the cells quickly die. Although small quantities of iron are essential for producing the green colour of chlorophyll, it is nevertheless uncertain whether the green colouring substance itself contains iron as an integral constituent of its chemical formula. Potassium is as essential for the assimilating activity of chlorophyll as iron for its production. Nobbe^ has recently shown that if food-materials otherwise com- plete but possessing no potassium are supplied to plants (as buckwheat), they behave as if they were absorbing only pure water instead of the solution of food- material. They do not assimilate and show no increase in weight, because no starch can be formed in the grains of chlorophyll without the assistance of potassium. The chloride is the most efficacious form in which potassium can be offered to buckwheat ; the nitrate comes next to it. If the potassium is offered only in the form of sulphate or phosphate, a very evident want of health is apparent sooner or later, which results from the starch which is formed in the grains of chloro- phyll not passing into the growing organs and thus becoming available for purposes of vegetation. Sodium and Lithium cannot replace potassium physiologically, be- cause the former is simpl^ useless to the plant, while the presence of the latter in the cell-sap is injurious to the tissues. Phosphorus, Chlorine, Sodium, Calcium, and Magnesiinn, have, as far as is yet known, no definite relation to special physiological purposes. The constant occur- rence however of compounds of phosphoric acid in company with albuminoids, as well as of potassium salts in organs containing starch and sugar, points towards definite relations which they may possess to those chemical processes that imme- diately precede the processes of construction in plants. A large part of the calcium taken up by plants is, as has been mentioned, precipitated by oxalic acid, and ^ Holzner, Ucber die Bedeutung des oxalsauren Kalkes, Flora 1867. — Hilgers, Jahrb. fiir wiss. Bot, vol. VI, p. I. ^ For special proof of the importance of iron see my Handbuch der Experimental-Physiologie, p. 142. ^ Nobbe, Schroder, and Erdmann, Ueber die Organische Lei:.tung des Kaliums in der Pflanze ; Chemnitz, 1871. ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. 623 remains inactive. The importance of calcimn must therefore be sought partly in its serving as a vehicle for sulphuric and phosphoric acid in the absorption of food-material, and partly in its fixing the oxalic acid which is even poisonous to the plant, and rendering it harmless. The elements just named are taken up by the plant when they are offered to it in the form of phosphates, sulphates, nitrates, or chlorides. Si/icon finally is taken up by a very large number of plants in the form of a very dilute aqueous solution of silicic acid ; by some in larger quantities than all the other constituents of the ash. By far the larger part of the silicic acid passes into the insoluble state within the cell-walls, and remains behind after the destruction of its organic substance together with calcium (magnesium and potassium.?) as a skeleton possessing the structure of the cell-wall. In land-plants it accumulates chiefly, though not exclusively, in the tissues exposed to evaporation, and espe- cially in the cuticularised walls of the epidermis. In Diatoms, the cell-wall of which is very strongly silicificd, this arrangement of course does not exist. Since it is possible to cause, by artificial feeding, plants which usually contain abundance of silica (like maize) to grow almost entirely without it, and without any 'obvious departure from their normal structure, silicic acid appears to be of very subor- dinate importance for the chemical and organic processes; and its deposition in the cell-walls docs not take place to any great extent until they are already fully developed. The combinations of food-material must be subject within the tissues to progressive changes of position in addition to and in consequence of their chemical transformations. The equilibrium of diffusion is disturbed by the decomposition of a salt ; immediately round the spot where this takes place the fluid of the tissue contains fewer molecules of the compound ; and the more distant molecules of the same salt in a state of solution move therefore towards the spot where they are wanted. Every cell therefore which decomposes any particular salt acts as a centre of attraction upon the fluids of the tissue surrounding it, and the salt in question is drawn towards this centre. But this process is the same in the case of every other salt dissolved m the same fluid. If, for example, calcium sulphate is decomposed in a cell and crystals of calcium oxalate formed, this itself supplies a cause for the more distant molecules of sulphate to be drawn towards that cell ; but it affords no reason for the molecules of potassium nitrate which are also present to move in the same direction. Every substance dissolved in the cell-sap is set in motion only in so far as the equilibrium of diffusion and the uniform distribution of its own molecules is disturbed. It follows therefore clearly that there can be in general no such thing as a continuous uniform motion of a so-called ' nutritive sap.' It is only w^hen a number of compounds which supply food-material are taken up at one spot such as the root, and are transplanted to another spot as the buds and green leaves, that the direction of movement is nearly the same for all ; but even in this case the rapidity with which the molecules of each particular salt move will vary, because this depends on the rapidity of consumption at the point towards which the movement is directed, and on the special rate of diffusion of each compound. Only when the force of the pressure drives the whole of the cell-sap to one side is the motion uniform for different substances, provided that the fluid moves in open channels such as the laticiferous vessels or sieve-tubes ; but if the pressure causes filtration through closed cell-walls, then in this case also the molecules of different salts are urged forward with a different rate, because the rapidity of filtration of different solutions varies with their composition and degree of concentration. 624 CHEMICAL PROCESSES IN THE PLANT. The same principles hold good also for the absorption of combinations of food- material from without into the absorbing organ. It has already been shown in the previous paragraph how the decompositon of carbon dioxide in the light in a cell containing chlorophyll induces new quantities of the dioxide at once to enter this cell, whether the gas be at the time dissolved in water or present in the atmosphere. If no carbon dioxide were decomposed in the cell, its contents would become saturated with the gas in proportion to the pressure and the temperature, and every cause for further motion would be removed. But the decomposition is constantly providing more space for the entrance of fresh molecules of carbon dioxide ; and this gas, although present in such small quantities in the atmosphere, collects here and supplies the material for the production of compact masses of carbon-compounds. A water-plant acts in the same manner on the salts dissolved in the surrounding water. The external water and the internal cell-sap are in continuous connection through the fluid imbibed in the cell-walls. If the chemical processes within the plant are supposed to be at rest, an equihbrium of diffusion will tend to become established between the external and internal fluid according to the prevailing condi- tions. But the chemical processes in the interior are continually disturbing this equilibrium, the molecules of the salt in question continually streaming from without to the places in the interior where they are to be used. If the molecules of calcium phosphate are even very sparingly distributed through the surrounding water, a dense accumulation will gradually arise in the plant, not of calcium phosphate, but of some other compounds of phosphoric acid and of calcium, because the molecular equilibrium is being continually disturbed by the separation of the phosphoric acid from the calcium, that is, by the chemical process. If the calcium phosphate remained as such within the plant, the movement would cease so soon as the equilibrium of difl'usion was established. It will be at once clear from a consideration of these facts that the accumulation of certain substances in the interior of plants depends in the first place on whether the compound of them which is present in the surrounding water is decomposed in the plant; that moreover the constituents of the different compounds must accumulate in the plant in different quantities according to the extent to which they are needed; and that finally the quantitative composition of the substances in question within the plant usually bears no resemblance to that of the surrounding water. Substances which are present in the water in the form of extremely dilute solutions occur in the plant in great quantities; while others which are abundant in the water are much less so in the plant. Thus, for instance, marine plants take up a much larger quantity of potassium and a smaller quantity of sodium than corresponds to the composition of sea- water; species of Fucus again collect considerable quantities of iodine which is present in sea-water only in extremely small quantities. Since moreover different plants decompose the same compounds with different degrees of rapidity, it is obvious that different plants which draw their food- materials from the same water must exhibit an entirely different composition of their ash. The processes are more compHcated when a land-plant has to take up the saline compounds of its food-material from the soil which contains but little water. By far the greater number of land-plants thrive in soil which usually contains a quantity of water much below its full capacity of absorption, its pores being almost entirely filled with air. The small quantity of water present adheres completely to the minute particles of soil, and for this reason does not flow away ; and this adherent water often covers the surface of the particles of earth in the form of a fine stratum. The roots can only absorb this water when they are in the closest contact with the particles of soil ; hence plants freshly planted wither even in moderately moist ground until a sufficiently large number of particles of earth become attached by means of new root- hairs to the newly formed rootlets. At these points of intimate connection between the root-hairs and the soil the adhering water of the latter is directly continuous with ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. 625 the cell-sap of the root by means of the water imbibed by the cell-walls of the root- hairs. In this manner it is possible for the root to suck up the water of the soil; as this water enters the points of contact, the equilibrium of the strata of water that cover contiguous particles of earth is disturbed, and the water of the soil retained by capillary attraction is set in motion towards the points of contact. This process spreads centrifugally from every root, and thus gradually makes the most distant parts of the soil subserve the nutrition of the plant. If salts, such as calcium sulphate, are present in solution in the enveloping strata of water, these salts follow the movements of the water, and finally enter at the points of contact with the root-hairs. But a large portion of the food-material, especially compounds of ammonia, po- tassium, and phosphoric acid, occur in the ground in a fixed condition, or, as it is generally termed, absorbed ; they are not extracted from the soil even by very large quantities of water; the roots nevertheless take them up out of it with ease. It may be supposed in these cases that the absorbed food-materials occur as an extremely fine coating over the particles of soil, and can therefore only be taken up together with them by the root-hairs at the points of contact; and they are there rendered soluble by the carbon dioxide exhaled by the roots. This action of the root is limited to the points of contact; only those absorbed particles of substance which come directly into contact with the root-hairs are dissolved and sucked up. But since the number and length of the roots is very considerable in all growing land- plants, and since also they are continually lengthening and forming new root-hairs, the root-system comes gradually into contact with innumerable particles of earth, and can thus take up the necessary quantity of the substance in question. This power of the roots of taking up, by means of the acid sap which permeates the walls of even their superficial cells, substances which are insoluble in pure water, presents itself in an ex- tremely evident manner, as I was the first to show, when polished plates of marble, dolomite, or osteolite (calcium phosphate) are covered with sand to the depth of a few inches, and seeds are then sown in the sand. The roots which strike downwards soon meet the polished surface of the mineral and grow upon and in close contact with it. After a few days an impression of the root-system is found corroded in rough lines into the smooth surface ; every root has dissolved at the points of contact a small portion of the mineral by means of the acid water which permeates its outer cell-walls. In taking up both the soluble constituents of the soil as well as those insoluble in pure water, the absorption is therefore first of all accomplished by the plant itself; and it is at the point where solution takes place at the surface of the root that absorption inwards is also effected by endosmose. But in spite of this complication the same principles hold good for the absorption of material from the soil as have been explaia^d in the case of absorption from a solution. Here also it is the consumption, the decom- position of the compounds in the plant, that regulates the absorption of the material. The quantitative composition of the ash has therefore no resemblance to that of the soil ; and the ash of plants of different kinds growing side by side and deriving their nutriment from the same soil may be altogether different \ But the composition of the soil is important to the plant in a secondary degree ; since plants of the same kind, if they grow for example on a soil rich in lime, will take up a greater quantity of lime than if the soil contained but little of it. This is obviously not in contra- ' [Messrs. Lawes and Gilbert's long series of experiments on this subject are of especial value. (See Toum. Roy. Agric. Soc. vol. VIII, p. 496 ei seg., 1S47; Journ. Chem. Soc. vol. X, p. 1, icSg; ; Report Brit. Assoc. 1S61 and 1867.) Their latest publication, « Report of Experiments on the growth of Barley for twentv years in succession on the same land ' (Journ. Roy. Agric. Soc. second series, vol. IX) contains much information as to the power possessed by plants of extracting different substances from the soil. — Ed.] s s ()l6 CHEMICAL PROCESSES IN THE PLANT. diction to the principle laid down, but only shows that the decomposition of a salt in the plant will take place more largely the more easily it is enabled to take it up. Sect. 5. — Assimilation and Metastasis (Stoftwechsel) ^ The food-materials absorbed by the plant are, with a few exceptions, compounds of oxygen containing the highest possible proportion of that element. The assimilated substances, on the contrary, which form the greater part of the dried substance contain but little oxygen, some even none at all. It follows from this that assimilation must be a process of deoxidation. The transformation of food-materials containing a large proportion into the substance of plants containing but little oxygen must necessarily be accompanied by elimination of that element; and since we already know that this takes place only in cells containing chlorophyll and under the in- fluence of sunlight, we have at once the locality, the conditions, and the time of the assimilation thus determined. No organs which are destitute of chlorophyll can assimilate ; and in the dark or when the amount of light is small, even those assimilating organs which contain chlorophyll lose the power of producing organic substances out of water and carbon dioxide with the assistance of other food- materials, — a process to which we shall henceforward exclusively apply the term Assiniilatioit. The products of assimilation of the cells containing chlorophyll may undergo various kinds of chemical metamorphosis either in these cells themselves or after passing into other organs ; and the aggregate of these processes may be distin- guished from assimilation as Metastasis. It is important to bear clearly in mind the difference between these two processes, both in respect to their external conditions and to their results, the following being the chief points: — (i) Assimilation takes place only in those organs that contain chlorophyll; metastasis in all ahke. (2) Assimilation occurs only under the influence of light ; metastasis equally well in the dark. (3) Assimilation is necessarily accompanied by the elimination of a large quantity of oxygen ; metastasis is usually connected with the absorption of small quantities of oxygen and the exhalation of small quantities of carbon dioxide. (4) Assimilation increases the dry weight of a plant ; metastasis only alters the nature of the assimilated materials, and these usually suffer a diminution of their mass, the destruction of a part of the assimilated organic compounds being neces- sarily associated with the inhalation of oxygen and exhalation of carbon dioxide necessary for metastasis. (5) The increase in weight of a plant which contains chlorophyll depends on the accession of assimilated substance in the organs that contain the chlorophyll being greater during the time that they are exposed to light than the loss in the dry weight connected with the exhalation of carbon dioxide during metastasis in all the organs and at all times of vegetation. (6) Organs containing chlorophyll and plants entirely destitute of it (parasites and saprophytes) do not assimilate but absorb substances already assimilated; no pro- cess takes place in them except metastasis ; and since this is associated with ^ See Sachs, Ilandbuch der Experimental-Physiologie, the section on the Transformation of Food-material. ASSIMILATION AND METASTASIS. 627 inhalation of oxygen and exhalation of carbon dioxide, they decrease the entire store of assimilated substances. Growth, I. e. the formation and multiplication of cells, always takes place at the expense of substances already assimilated ; and these therefore must be subject to continual chemical change. Growth is only possible as a result of assimilation; but the two processes do not usually concur either in time or locality. The assimilated substances may remain in the plant for a longer or shorter time without becoming employed in the growth of cell-walls or in the production of protoplasmic substances (protoplasm or grains of chlorophyll) ; and in this case they are termed Reserve- materials. Every cell, tissue, or organ in which assimilated substances are stored up for subsequent use is called a Reservoir of Reserve-material. The assimilating cell may itself serve as a reservoir for reserve-material (as unicellular Algae or the leaves of evergreen plants) ; but usually a physiological division of labour is effected in the plant of such a nature as to transfer the products of assimila- tion from the organs that contain chlorophyll to other organs or masses of tissue which serve as reservoirs of the reserve-material and give it up to the parts destined for the formation of new organs (buds, the rudiments of the roots, or cambium). In Mosses, Vascular Cryptogams, and woody Phanerogams, the tissue of the stem is usually also the reservoir for this purpose ; in perennial herbs and shrubs it is more often the persistent bulbs, tubers, and rhizomes that perform this function. The spores of Cryptogams which have the power of germination always contain a small quantity of reserve-material, at the expense of which the first processes of germination take place; in Rhizocarpege and Lycopodiaceae the whole of the prothallium and embryo is produced in this manner. The seeds of Phanerogams remove a much greater quantity from the mother-plant, which ac- cumulates either in the endosperm or in the cotyledons; the greater the quantity of this reserve-material the more numerous and the larger are the stems, roots, and leaves which the seedling can produce before it begins to assimilate. The minute seedlings, for instance, of Nicotiana and Campanula may be contrasted with the strong ones of the bean, almond, oak, &c. Since no assimilation takes place in the dark, it is only necessary to allow seeds, tubers, bulbs, rhizomes, < ' According to IIoskus, ammonia is also formed during germination ; and Uorscow maintains 632 CHEMICAL PROCESSES IN THE PLANT. the decomposition of the albuminoids under the influence of the energetic oxidation which takes place in the germinating seed, i. e. in the growing parts of the embryo, are again consumed in the formation of albuminoids. The preceding remarks refer to the processes of growth which are associated with the consumption of the substances stored up in the reservoirs of reserve- material. If those plants are now examined in a similar manner whose reserve food- material has been consumed, whose green leaves have begun to assimilate under the influence of light, and which are forming the substances necessary for the growth of their buds, roots, &c., the same substances are found similarly distributed through the conducting tissues of the internodes and the petioles and veins of the leaves as far as the buds and apices of the roots, and subject to the same metamorphoses as in the seedlings. It follows that the assimilating organs which contain chlorophyll perform the same function for the growing parts of the plant that the reservoirs of reserve-material do for the seedling ; but with this difference, that the former produce the formative materials afresh, while in the latter they are not formed but only stored up. The organic compounds originally formed in the cells containing chlorophyll by the decomposition of carbon dioxide and water under the influence of light are generally carbo-hydrates. The most common of these is starch ; sugar occurs less often ; oily matter perhaps occasionally. It has been shown (p. 46) that the starch which so commonly occurs in the chlorophyll-grains of plants that vegetate under normal conditions, can only be produced when the plant is subject to the well- known conditions of assimilation, i. e. when it decomposes carbon dioxide and water under the influence of sunlight. Seedlings which have completely exhausted their supply of reserve-materials by growth in the dark, and are afterwards exposed to the action of light, do not till then develope their chlorophyll. The first grains of starch which are found a little later in the plant are those enclosed in the chlorophyll, and these are at first small, but gradually grow larger. It is only afterwards that starch is found also in the conducting tissues of the internodes and leaf-stalks up to the buds, which then begin to grow anew. It has been shown further that this starch which is formed in the grains of chlorophyll disappears in the dark ; i. e. becomes dissolved and transferred to the conducting tissues. In Allium Cepa the chloro- phyll forms no starch; but a substance similar to grape-sugar is found in large quantities in the green leaves, and is distributed through all the tissues of the plant. Where drops of oil are found in the chlorophyll, they appear to be first of all formed at the expense of the starch which has been produced there; this conclusion being derived especially from the observation of what takes place in Spirogyra. The result of tracing by micro-chemical observation the products of assimila- tion in the conducting tissues leads once more to the conclusion that the starch which is formed in the cells containing chlorophyll is subject to a variety of chemical metamorphoses before it reaches the growing tissues and the reservoirs of reserve-material. Even during the period of vegetation the substances w^hich are that ammonia is set free during the vegetation of Fungi (Melanges bid. tirds du Bullet, de I'Acad. imp. des Sci. Nat., Petcrshourg, vol. VII, 1S68). This is however denied by Wolf and Zimniermann (Bot. Zcitg. 1871, nos. 18, 19). ASSIMILATION AND METASTASIS. 6^Q conducted to the young parenchyma of growing parts as soon as this has been differentiated from the primary tissue, give rise to the formation of fine-grained starch which accumulates there temporarily, and disappears with the final and rapid increase in size of the cells. Starch and other substances are then produced afresh by assimilation in the fully developed leaves ; and starch and the products of its transformation again appear in the conducting tissues, not to be consumed there, but only to be conducted to the still younger parts. The metamorphoses of the formative materials which are conveyed from the assimilating organs to the re- servoirs of reserve-material, generally show a reversed order of succession to that which takes place during germination ; the starch produced in the leaves is trans- formed in the leaf-stalks of growing beet into glucose, from which crystallisable cane-sugar is formed in the swollen tuberous roots ; in the artichoke the starch is converted into inuline which is conducted through the stem to the undergrountl tubers ; in the potato, the mature leaves of which form starch, a substance similar to glucose is chiefly found in the conducting tissues, which is conveyed to the growing tubers, and there evidently forms the material from which the large masses of starch are formed. In ripening fruits and seeds a large quantity of glucose is generally found which disappears from the seeds when they become ripe, starch being formed in the reservoirs of reserve-material; in Ricinus the oil of the endosperm is evidently formed at the expense of the saccharine substance which is conveyed to the seed; in the embryo of the same plant, as well as in that of Crucifers, fine-grained starch is formed temporarily, which disappears when the seeds are ripe, and is replaced by oily matter. Whether the albuminoids also are first formed in the assimilating cells which contain chloroph}ll and whether they can be formed only in them, is still an unde- cided point. It is certain that they are formed in the chlorophyll-containing cells of Algae ; but it cannot be concluded from this that they can only be produced in the corresponding cells of plants with differentiated tissues; at all events experiments on the artificial production of the yeast-fungus show that it is able to form out of sugar and an ammonium-salt or nitrate (with the assistance of the constituents of the ash) not only cellulose but also albuminoids, as may be inferred from the increase of the protoplasm in the rapidly multiplying cells. If the colourless cells of yeast are able to do this, it may be inferred, until the contrary is proved, that those cells of other plants which do not contain chlorophyll can also produce albuminoids, if only a carbo-hydrate or oil (or both) is conveyed to them from the leaves, and an ammonium-salt or nitrate from the roots. That the formation of albuminoids probably takes place in this way within the conducting tissues of internodes and petioles may be concluded from the deposition of calcium oxalate in these tissues ; since in the formation of this salt sulphuric acid becomes separated from the calcium, and its sulphur enters into the chemical formula of albuminoids \ When the cells of the leaves become emptied of their contents at the close of the period of vegetation, and the deciduous parts fall, not only the starch which was formed latest in the latter, but also the material of the grains of chloro- phyll, is itself absorbed and conveyed through the leaf-stalks to the reservoirs of See Sachs, Handbuch der fIxpeiimental-Physiologie. p. 345. 634 CHEMICAL PROCESSES IN THE PLANT. reserve-material ; all the serviceable substances contained in the leaves become in- corporated in the permanent organs. The leaves change colour ; a small quantity of very small shining yellow granules usually remain behind in the cells of the mesophyll as a residue of the absorbed chlorophyll-grains ; and the leaves which ^re emptied in the autumn are therefore yellow. If they are red this is in con- sequence of a red sap which fills the cells in addition to the chlorophyll-grains\ Enormous quantities of crystals of calcium oxalate often remain behind in the deciduous leaves ; the constituents of the ash which are serviceable to the plant, especially phosphoric acid and potassa, are conveyed with the starch and the proto- plasmic structures to the permanent parts ; so that the falHng leaves thus consist only of a skeleton of cell-walls and of the subsidiary products of metastasis which are of no value to the plant. The direction of the Transport of the assimilated substances in the plant is determined by the fact that it must take place from the assimilating organs to the growing parts and to the reservoirs of reserve-material \ while at the com- mencement of every new period of vegetation its direction must be from these reservoirs to the growing organs ; and since new organs are usually formed above as well as below these reservoirs and the assimilating leaves, it is obvious that the movements of the assimilated substances must take place at the same time in opposite directions. The Cojiduciifig Tissue for the transport of the formative materials consists, in plants with differentiated systems of tissue, of the parenchyma and the thin- walled cells of the phloem of the fibro-vascular bundles. By the parenchyma of the fundamental tissue, which always has an acid reaction, are conveyed the carbo-hydrates and oils; by the soft bast the mucilaginous albuminoids which have an alkaline reaction. Only when the conduction is very rapid, as when the leaves are emptied in autumn, and in plants with very rapid growth (as the castor-oil plant and gourd) are small quantities of starch found also in the sieve-tubes. Where there are laticiferous vessels, they furnish an open communication between all the organs of the plant; they contain albuminoids, carbo-hydrates, and oils, as well as the secondary products of metastasis, as caoutchouc and poisonous substances. The mode of motion of the assimilated substances is usually molecular ; /. e. it is a movement of diffusion, especially where the transport takes place through closed cells. The pressure caused by the tension and turgescence of the tissues has in addition a tendency to propel the fluids in the direction of least resistance, which is also that in which they are consumed. In the system of communicating sieve-tubes and laticiferous vessels the movement of the substances is necessarily one of the entire mass, caused by inequalities of pressure, and by the distortions and curvatures which the wind produces. As far as concerns the movements of diffusion, it is a general rule that every cell which decomposes any substance, renders it insoluble, or uses it for its growth, acts upon the dissolved molecules of this substance in the neighbourhood ^ [On the colouring matter of th^ leaves in autumn, see Sorby, Quart, Journ. of Science 1S7] p. 64; and 1873, p. 215.— Ed.] ASSIMILATION AND METASTASIS. 6ot- as a centre of attraction ; the molecules stream to the parts where they are wanted because the molecular equilibrium of the solution is disturbed by its consumption. On the other hand every cell which produces a new soluble compound acts on the dissolved molecules as a centre of repulsion, because the continually increasing: concentration occasions at the point of production a streaming of the molecules away from it towards the point of less concentration, the concentration continually decreasing towards the points where the substances are consumed. When the move ment of diffusion is caused by the production and consumption of definite compounds of this nature, the proximate cause of the molecular movement of the dissolved sub- stances must be the chemical processes involved in their metamorphoses. These metamorphoses take place, as we have seen, not only at the points where the sub- stances are consumed in the process of growth, but also in the conducting tissues ; and this production of transitory compounds must therefore favour movement towards the points of deposition and of growth. The formation of insoluble starch is in this sense a fact of peculiar importance. If for instance the starch produced in the leaves of the potato is required to be transported to the tubers, it must necessarily be conveyed in a soluble form, such as we find in the con- ducting tissues of the stem in the form of glucose. But if this glucose had to undergo no further change in the tubers, a soluuon of glucose of constantly in- creasing concentration would be uniformly distributed through the conducting tissues and the tubers ; and the accumulation of the whole of the reserve-material in the tubers w^ould be impossible. The glucose is used up in the tubers in the formation of starch-grains ; and a fresh quantity therefore continually streams in that direction; the whole mass of the material produced in the leaves is therefore gradually transferred to the reservoirs of reserve-material. The starch is first transformed into glucose, and then back into starch ; and it is in this chemical process that the vehicle for the movement consists. Starch is even produced tem- porarily in the conducting parenchyma, but of course cannot be transported as such from cell to cell ; its movement being effected by the grains being dissolved in one cell, the product of solution diffusing into the adjoining cell, and being there employed in the formation of starch-grains which are then again dissolved, and so on. When again cane-sugar is formed in the tuberous roots of the beet, the movement towards the root of the glucose which is produced from the starch assimilated in the chlorophyll is brought about in this way,— every particle of glu- cose undergoes chemical transformation when it reaches the root, and the mole- cular equilibrium of the solution of glucose is thus disturbed ; the root acting like a centre of attraction on the glucose in the leaf-stalks. But the continual form- ation of the solution of glucose in the leaves at the expense of the starch causes in them an increase of concentration and a streaming of molecules towards the root, where the concentration of the solution of glucose is continually decreasing, while that of the solution of cane-sugar increases. The same is evidently the interpretation of the formation of inuline in the tuberous roots of the dahlia and the tubers of the artichoke, and of that of oil in ripening seeds at the expense of the sugar which is conveyed to them. The co-operation in the movement towards the parts where the substances are consumed of the tissues of the pressure exercised on the cell-sap by the tension e^fi CHEMICAL PROCESSES IN THE PLANT. I infer, even where we have to do with closed cells, from the fact that considerable quantities of the cell - sap appear on the surface of a transverse section of succulent organs, both from the parenchyma and from the cambiform cells, and this is clearly forced up by internal pressure. Since the tension and turgescence of the tissue are always less in the buds and apices of the roots than in the older parts, there must always be a tendency for the filtration of the sap towards the latter, which must act in the same way as diffusion. That the contents of the perforated sieve-tubes and laticiferous vessels are also subject to considerable pressure from the surrounding tissue is shown by the extent to which these fluids flow out when the organ is cut through. The fluid which is subject to pressure will have a tendency to escape from these tubes to parts of the plant where the lateral pressure is less, which is the case in the buds and apices of the roots. The flexions and distortions occasioned in the organ by the wind wifl at the same time cause the fluid contents of the sieve- tubes and laticiferous vessels to be pressed away from the older bent parts towards the buds where the tension is less. The statements here compressed into a very brief space rest on a series of detailed micro-chemical and experimental researches which I have described in the Botanische Zeitung, 1859 and 1862-1865; Pringsheim's Jahrbiicher fiir wissenschaftliche Botanik, Vol. 111. p. 183 et seq.\ Flora, 1862, pp. 129 and 289, and 1863, pp. 33 and 193 ; and have presented in a connected form in the section on the Transformation of Food- materials in my Handbook of Experimental Physiology ^ The reader \\\\\ there find the reasons for the views here given ; and a few examples will now be sufficient to render somewhat clearer the general statements with regard to metastasis and the migration of the assim- ilated substances. In the outset it must be stated that by grape-sugar or simply sugar I understand a substance soluble in the cell-sap, easily reducing copper oxide, and readily soluble in strong alcohol, although it may not always exactly correspond to the grape-sugar of chemists, a point which is of but little importance for our present purpose. The parenchyma of the bulb-scales of the tulip — /. e. the four or five thick colourless leaves which serve as reservoirs of reserve-material — contains, as long as the plant is dormant, in addition to considerable quantities of mucilaginous albluninoids, a very large quantity of coarse-grained starch. The presence of sugar cannot be determined at this time by micro-chemical processes. As soon as the bud of the leaf- and flower-stem which is concealed within the bulb, but had already been formed with all the parts of the flower during the previous summer, begins to elongate in February, and roots make their appearance from the base of the bulb, small quantities of sugar are found with the starch in the parenchyma of the bulb-scales. The whole of the parenchyma and of the epidermis of the leafy stem, of the young foliage-leaves, of the perianth, of the stamens, and of the carpels, becomes filled with fine-grained starch, the substance of which has already been derived from the bulb-scales, where the starch-grains have become transformed into sugar, which diffuses into the growing organs, and there, as far as it is not directly consumed, again supplies material for the formation of starch-grains. Together with its consumption in the growth, at first slow, of the cell-walls, this temporary re-formation of starch at the expense of that contained in the bulb-scales ^ The recent researches of Schroder (Jahrb. fiir wiss. Bot, Vol. VII, p. 261), Soraurer, Siewert, Roestell &c., (collected in Hoffmann and Peters' Annual Report on the Progress of Agricultural Chemistry for i?68 and 1S69, Berlin 1871) contain fresh confirmations of the account here given. ASSIMILATION AND METASTASIS. 637 continues at first in the young internodes, leaves, and flowers. The cells enlarge and become continually more filled up with fine-grained starch till the time when the bud comes above ground (Fig. 441). Then follows the rapid extension of the stem; the leaves expand, and the flower unfolds. With the considerable and rapid increase in size of the cells caused by this unfolding, the fine-grained starch disappears in all these parts, sugar being temporarily produced which furnishes the material for the growth of the cell-wall. When all the parts above ground are fully unfolded, the cells, although much larger, are now devoid of starch. The corresponding loss which the bulb-scales have experienced up to this time is clearly seen from the de- crease of their starch-grains ; they may be found in all stages of absorption. The turgescence of the bulb-scales at the same time decreases, and they become wrinkled ; but the formation of sugar in them still continues at the expense of the starch, even when the parts above ground have already done growing. The starch stored up in the bulb-scales finds in fact still another use; while the flower-stalk is ex- tending, the bud in the axil of the upper- most bud-scale begins to develop rapidly (it had already been formed in the previ- ous summer) ; its cataphyllary leaves swell and become filled with starch ; and the residue of the starch not consumed in the growth of the flower-stalk is transported from the scales of the mother-bulb through its base into the young bulb (Fig. 441,2). These scales become gradually entirely emptied of starch, and while the green foliage-leaves exposed to light are assimi- lating and contributing their share to the growth of the new bulb, they finally wither and dry up from the simultaneous loss of water and of assimilated matters. The reserve-materials which accumulate in the daughter-bulbs are partly derived from those of the mother-bulb ; but are completed by the products of assimilation of the green leaves of the flower-stalk. When the flower-stalk has also died down, nothing remains of the whole plant but the bud which has developed into a new bulb. For a time it does not put out any new organs, but is apparently dormant ; but in the interior the end of the stem continues to grow slowly, and produces new rudi- ments of leaves and the flower-bud for the next year ; when the process now described is repeated. So far we have only pointed out the relation of the starch and of the sugar produced from it to the growth of the plant ; there are formed however along with it, and probably likewise at the expense of these carbo-hydrates, other substances, such as the colouring matter of flowers, the oil in the pollen-grains &c. The albuminoids at first contained in the bulb-scales become transported to a distance from them. Fig. 441-— Longitudinal section through a germinating bulb of Tulipapyacox: h the brown enveloping membrane, k the flattened stem which forms the base of the bulb and bears the bulb-scales sh ; si the elongated part of the stem which bears the foliage-leaves ^'/', and terminates in the flower; c the ovary,/ perianth, a anthers; 2 a lateral bulb in the axil of theyoungesf bud-scale, which developes into the bud of next year's bulb ; -w the roots which spring from the fibro-vascular bundles of the base of the bulb. 638 CHEMICAL PROCESSES IN THE PLANT, and furnish the material for the formation of the protoplasm in the young cells of the growing flower-stalk ; a large part is evidently employed in producing the grains of chlorophyll in the foliage-leaves as they become green. Its function is now to pro- duce at least as much formative material by assimilation as is required to build up the transitory flower-stalk, and to supply it to the bulb. The ripe seed of Rkinus communis contains a very small embryo in the middle of a very large endosperm ; neither contains starch, sugar, nor any other carbo-hydrate, if we exclude the very small amount in weight of the cellulose of the thin cell-walls. The reserve food-material consists of a great quantity of oil (as much as 60 per cent.) and albuminoids, the admixture and composition of which have already been described on p. 52. The very small quantity of these substances contained in the embryo would only suffice for the first and very inconsiderable development of the seedling; its enormous increase in size during germination must therefore be attributed almost entirely to the substances deposited in the endosperm. The endosperm of Ricinus enlarges very considerably, as Mohl first showed, during germination, and the material required for its growth must therefore be diverted from the embryo. The tw^o thin broad cotyledons remain in the endosperm, with their surfaces in contact with one another, long after the root and the hypocotyledonary part of the stem have emerged from the seed ; they are in contact by their backs with the tissue of the endosperm which surrounds them on all sides, and absorb the reserve-materials from it, while they keep pace slowly with its enlargement. When the parts of the seedling have increased very considerably and the root has developed a number of lateral roots, the hypocotyledonary portion of the stem elongates so that the cotyledons are drawn out of the endosperm which is then completely emptied and reduced to a thin mem- branous sac. They now rise above the ground, become expanded to the light where they continue to grow rapidly and become green, to serve from this period as the first assimilating organs. In this case, as in the germination of all oily seeds, sugar and starch are produced here in the parenchyma of every growing part, disappearing from them only when the growth of the masses of tissue concerned has been completed. Since the endosperm grows also independently, starch and sugar are, in accordance with the general rule, temporarily produced in it. The cotyledons apparently absorb the oil as such out of the endosperm, whence it is distributed into the parenchyma of the hypocotyledonary portion of the stem and of the root, serving in the growing tissues as material for the formation of starch and sugar, which on their part are only precursors in the pro- duction of cellulose. But in these processes of growth tannin is also formed which is of no further use, but remains in the separate cells, where it collects apparently un- changed until germination is completed. It can scarcely be doubted that the material for the formation of this tannin is also derived from the oil of the endosperm, although perhaps only after a series of metamorphoses. The absorption of oxygen, which is an essential accompaniment of every process of growth and especially of germination, has in this case, as in that of all oily seeds, an additional significance, inasmuch as the formation of carbo-hydrates at the expense of the oil involves the appropriation of oxygen. Since the metamorphoses of material proceed pari passu with the grow'th of the separate parts, the distribution of the products of metastasis through the tissues is continually changing, and can only be understood by a consideration of all the sur- rounding circumstances. The micro-chemical investigation of seedlings in the state represented in Fig, 442 //, gives, for instance, the following result : — in the endo- sperm is found a great deal of oil and a little starch, with sugar at the outside; the epidermis and parenchyma of the slowly growing cotyledons are filled with drops of oil ; a large number of the epidermal cells contain tannin ; starch-granules are found only in the parenchyma of the leaf-veins; the parenchyma of the hypocotyledonary portion of the stem, which is at present growing the most rapidly, contains only ASSIMILATION AND METASTASIS. 639 con parat.vely little oil but much starch and sugar; and a number of the cells of the ep.dernus and parenchyma are filled with tannin. The primary root has first of all completed its growth in length and thickness (after germination it begins afresh); in Its lower part it contains neither starch nor sugar (the former is present in the root- cap j ; in Its upper part from which the lateral roots spring and in the lateral roots tnemselves sugar is also present, which is conveyed into the growing apices of the latter. When the hypocotyledonary portion of the stem has subsequently taken a direction straight upwards and ceases to grow, the oil, starch, and sugar have almost entirely disappeared from it, and in their place the cell-walls have become thick, and the vessels and first cells of the wood and bast are already thickened. After the stem ot the young plant has become upright, the cotyledons expand and grow rapidly, and the remainder of the oil which they had taken up from the endosperm now also disap- pears trom them together with the starch and sugar. The seedling has now entered on a state in which the non-nitrogenous reserve-materials are consumed; a framework Fig. AA^.-RiciHUS communis; I lonjjitudinal section of the ripe seed; // germinnting seed with the cotyledons still «n the endosperm (shown more distinctly in A and B), s testa, e endotsperni, c cotyledon, Ac hypocotyledonary portion of the stem, w primary root, w' secondary roots, x the caruncle. of large and solid cell-walls is produced in their place ; and a quantity of tannin remains behind in some of the cells as a secondary product, as well as various other substances not present in the seed. The albuminoids which form so peculiar and intimate a mixture with the oil in the ripe seed, and which are partially contained in the aleurone-grains of the endosperm in the form of crystalloids, are, during the processes which have been described, transferred to the embryo, where they produce the protoplasm. During the whole of the period of germination the cells of the fibro-vascular bundles are found to be densely filled with albuminous mucilage, subsequently only those of the phloem ; these substances are evidently in motion towards the apices of the roots \vhere new cells are continually being formed. Every young rudiment of a lateral root behaves to reagents as an accu- mulation of albuminous substance in contact with the fibro-vascular bundles of the primary root. But a very considerable portion of this material remains in the upper part of the stem of the seedling where new leaves are formed, and a still larger portion in the cotyledons themselves, where it furnishes the material for the formation of the numerous grains of chlorophyll. 640 CHEMICAL PROCESSES IN THE PLANT. After the consumption of the reserve-material at the end of the period of germi- nation, the cells — with the exception of the youngest parts of the buds and the apices of the roots — are destitute of any formative material ; although it has grown to a large size and contains a great quantity of water, the dried weight of the plant is very small and even less than that of the seed, because a portion of the substance has been destroyed in the process of respiration. But active organs are formed from the earlier inactive store of material; the roots absorb water and dissolved food- material ; the green cotyledons begin to assimilate ; they produce starch in their chlo- rophyll ; and the same substance is subsequently found also in the parenchyma of the petioles and in the stem as far as the bud, the young leaves of which grow from the products of the assimilation of the chlorophyll. At first the unfolding of new leaves and the increase in length and thickness of the stem and roots are very slow ; but the capacity for work possessed by the plant increases with every freshly developed leaf and every new absorbing root ; on each successive day it can produce a larger quantity of formative material than on any preceding one, and thus the rate of growth also increases. If a castor-oil plant is examined at the time when vegetation is most active, when the green leaves supply the material for metastasis in all the organs, starch is found in their chlorophyll and distributes itself from them through the parenchyma of the veins and petioles downwards into the stem as far as the root, and upwards to the young leaves which are not yet in a condition to assimilate. The excess which is not immediately required for the purposes of growth becomes deposited in the pith and medullary rays, where (as well as in the chlorophyll) it is always accompanied by sugar ; and it is evidently this latter substance which brings about the diffusion from cell to cell, and at the same time furnishes the material for the formation of new starch-grains. The sugar is the migratory product which takes part in the diffusion ; the starch-grains are the stationary transitional product. The distribution of starch and sugar shows moreover that they move from the primary stem through the rachis of the inflorescence and the pedicels into the paren- chymatous tissues, and penetrate into the young tissue of the flower, the growing fruit, and the ovules, there to be employed in the production of cellulose. The distributed starch collects more abundantly especially in the immediate neighbourhood of those layers of cells which afterwards form the hard endocarp and the solid testa of the seed, in consequence of its being required here in greater quantity, disappearing also from them after the complete development of these layers of tissue. The sugar and starch are conveyed through the funiculus to the ovules ; they are distributed through the integuments and the parts surrounding the nucleus ; and a large quantity of sugar enters the growing endosperm, which supplies the material for the formation of the oil which gradually accumulates, while fresh supplies of sugar are constantly entering from without. In the growing embryo the cells are filled at a certain period with fine-grained starch, which then entirely disappears and is replaced by oil. All this indicates that the oil of the ripe seed of Ricinus is produced from the starch and sugar which were transported to it from the assimilating organs during the period of repose ; and even the hard woody pericarp and the testa obtain their formative material from those substances. The albuminoids which collect also in the young leaves and from which the chlorophyll-grains are formed, as well as that portion of these substances which accumulates in the seed as reserve food-material, are transported from the stem by the sieve-tubes and the cambiform cells of the fibro-vascular bundles. ' In the Leguminosae^ a very important part in the transport of the reserve proteinaceous substances is played by Asparagin. To demonstrate this, moderately thin sections are placed in alcohol, and the saturation assisted by shaking. This mode * What follows is taken from a letter from Dr. Pfeffer. (Compare Book I, Sect. 8, p. 51). ASSIMILATION AND METASTASIS. 641 is however applicable only when the asparagin is abundant ; when it is present in small quantities it can still be demonstrated by placing a thin cover-glass on the sections, and running in underneath a little absolute alcohol. In this case the asparagin crystallises out round the section ; while in the former case it is precipitated in the cells in the form of crystals. These can easily be recognised ; they are comparatively large, and cannot be mistaken for other crystals which are formed in all plants on treatment with alcohol, even where no asparagin is present, since these — which belong to various salts, among others to nitrates — always remain very small and have an entirely different appearance. ' Lupinus liiteus is a good object for examination, and possesses the great advantage that we have in its case an analytical investigation of Beyer's^ in which the organic constituents and especially the asparagin hav-e been determined in the root, hypocoty- ledonary portion of the stem, and cotyledons, at two stages of germination, the last shortly before the cotyledons have thrown off the testa. ' The following is what is known respecting the movements of the non-nitrogenous reserve-substances. Starch is first of all formed in the hypocotyledonary portion and root, then disappears and remains only in the endoderm, the rest being transformed into sugar. Asparagin is first formed in the hypocotyledonary portion and root when they are about 10 mm. long, but then rapidly increases in quantity while these parts elongate ; and it is now formed also in the petiole of the cotyledons, and in the coty- ledons themselves before they have become green and thrown off their testa, especially in their lower part. The conditions remain the same during the whole of the time that the reserve albuminous substances are being consumed. Asparagin is now found in large quantities in the petiole of the cotyledons, almost to the extent of a saturated solution (i part dissolves in 58 parts of water at 13° C), as well as in the hypo- cotyledonary portion and in the stem as soon as it begins to grow. The asparagin extends from the root and stem towards the punctum -vegetationis almost exactly as far as the sugar, becoming finally, like the latter, less abundant. Beneath the coty- ledons it is wanting in the pith, while in the stem it is as abundant there as in the cortical tissue; it is never found in the vascular bundles. The asparagin also extends into the petiole of young leaves as far as the base of the unfolding pinnae, as well as into the lateral roots. As long as asparagin is formed out of the albuminous substances in the cotyledons, it may also be found in the plant distributed as has been described ; but when the cotyledons have been entirely emptied, the asparagin also disappears ; but this does not happen in the case of Lupinus luteiis until several leaves have completely unfolded. 'The process is quite analogous in Tetragonobolus purpureus and Medicago tuber- culata : in Ficia sati-va and Plsum sati'vum the presence of asparagin in the cotyledons themselves cannot be proved with certainty, but is found at their base and usually also in their petiole, although these plants produce decidedly less of it than Lupinus luteus. Since moreover chemical analysis has established the production of great quantities ot asparagin on germination in the case of a large number of other species of the order, we may regard this substance as the form of transport for the albuminous substances characteristic of all Leguminosse. Albuminous substances are moreover found m these plants also in the thin-walled elongated cells of the vascular bundles ; and it is quite possible that they are at the same time also transported by these structures. It is evident that the source of the asparagin must be the albuminous substances, be- cause the absolute amount of nitrogen remains the same during germmation ; and the nitrogen of seeds is all or nearly all contained in their albuminous ingredients. 'As to the influence of darkness on the formation of asparagin, we have diame- trically opposite statements from Piria and Pasteur. The only certain pomt is that 1 Landwirthschaflliche Versuchsstationen, vol. IX. T t '>^Z CHEMICAL PROCESSES IN THE PLANT. light has no influence at all on the formation of asparagin, but has upon its transform- ation into albuminous substances ; it therefore accumulates in plants germinating in the dark, and remains unaltered till their death. The influence of light can however only be indirect, as is shown by the fact that in Tropaeolum asparagin is formed tempo- rarily in the dark during the first stages of germination, and then again disappears ; and even in Leguminosae appears to undergo subsequent metamorphosis into albuminous substances. The explanation is now quite simple. ' The following numbers show the percentage composition of asparagin, and the composition of an amount of legumin, containing an equivalent quantity of nitrogen. Asparagm. Legumin. C = 36"4 G = 64'9 H- 6-1 H= 8-8 N = 2I*2 N = 21'2 = 36-4 = 30-6 ' It is seen at once that in the formation of asparagin out of legumin a large quantity of carbon becomes available. The exact mode in which this comes to pass must be left in doubt, like the fixation of carbon in the re-formation from asparagin of albu- minous substances (albumin is probably formed, and not legumin ; their composition does not vary greatly, but the latter gradually disappears almost entirely in growing plants). But when a plant growing in the dark uses up its non-nitrogenous reserve- material, and even the carbon and hydrogen set at liberty by the conversion of legumin into asparagin, the material which would be produced in the light by assimilation is wanting for the re-formation of the albuminous substances out of asparagin. In Tropaeolum, where asparagin occurs only in the first stage of germination, it may com- pletely disappear in the dark. The amount of asparagin formed is however only moderate, and it disappears before the reservoir of reserve-material is exhausted ; it therefore at most always plays only a subsidiary part, as is also the case in Silybum Marianum, Helianthus tuberosus, and Zea Mais. In Ricinus on the other hand I could not find, either in the dark or in the light, any asparagin at all ; and Dessaignes and Chautard looked for it in vain in the seeds of the gourd, buckwheat, and oat, germi- nating in the dark. Its physiological significance remains therefore at present limited to the Leguminosae ; and in them it is confined to the consumption of the reserve albuminous substances, since it is, according to Pasteur, never present in the flowers. When the lateral buds are put out, this substance is not formed in Leguminosae any more than in other plants. Hartig maintains that the production of asparagin, or at least of a trace of a substance identical with it, is a general phenomenon ; but I think that he had before him the small crystals mentioned above which he mistook for true asparagin. He has moreover not contributed any evidence as to the physiological significance of this substance. ' The existence of asparagin has also been proved in the leaves and stems of some plants (see Husemann, Pflanzenstoffe) ; and its presence in the underground perennial parts of Stigmaphylloyi jatrophoefoUum almost gives the impression of its being there also a reserve-material.' The absorption of assimilated substances into the plant from without takes place in seedlings, the reserve-materials of which are contained in the endosperm, in parasites \ and in saprophytes which contain no chlorophyll. Seedhngs, which ^ Parasites which contain chlorophyll, like the Loranthacere, can themselves assimilate, and only require therefore to draw water and mineral substances from their host (^see Pitra in Bot. Zeitg. ASSIMILATION AND METASTASIS. 643 are best known in this respect, show how the reserve-materials of the endosperm may pass into the absorbing organs (in this case ahnost always foliar structures) without there being any actual cohesion of the absorbing organ with the endosperm ; they only lie in close apposition, and can be separated without any injury (as in Ricinus, Fig. 442). It cannot be doubted that the metamorphoses which take place in the nutrient endosperm are brought about by the absorbing organ, that is by the embryo itself; the behaviour of the endosperm of the germinating date, which is absorbed by the delicate tissue of the absorbing organ belonging to the coty- ledon, shows clearly that the hard thickening-layers of the cell-walls of the endo- sperm are first of all transformed into sugar under the influence of this organ, and then absorbed. A substance evidently passes out of the absorbing organ into the endosperm which causes this metamorphosis of the cellulose. The oil and albuminoids of the endosperm are at the same time taken up into the embryo, where all the conducting parts of the parenchyma are filled with sugar and starch as long as the endosperm is not entirely absorbed. In the same manner also in Grasses substances possibly pass out of the embryo into the endosperm, and there bring about the chemical metamorphosis and solution of the starch and albu- minoids before they are absorbed by the scutellum which is applied to the surface of the endosperm. It is possible however that in this case there may be agents capable of bringing about the solution of the starch and gluten in the presence of water independently of any chemical action of tlTe embryo. The absorbing roots of parasites penetrate into the tissue of the host, and often grow into it in the most intimate manner. It is certain that the exciting cause of the transport of the products of assimilation from the host to the parasite resides in the latter; the parasite acts on the conducting masses of tissue of the host like a growing bud of the host itself; the food-materials penetrate into it because it consumes and changes them. The influence exerted by the absorbing organ of the embryo on the substances in the endosperm, dissolving and chemically changing them, points to the way in which the absorption of food-material is effected by saprophytes which possess no chlorophyll, their absorbing organs probably first causing the solution and chemical transformation of the decaying organic constituents of the humus. The decaying foliage in which Monotropa, Epipogium, and Corallorhiza grow, does not give up to water the serviceable materials which are still present in it, any more than the cellulose of the endosperm of the date, or the starch of the endosperm of Grasses, or the oil of the seed of Ricinus, can be extracted by water ; but these saprophytes nevertheless obtain their nutriment from them. The fact that the roots of plants of this kind are so few in number and so diminutive in length, as in Neottia, or are entirely wanting, as in Epipogium and Corallorhiza, is very remarkable in connection with this. These plants are concealed in the nutrient substratum till the time of flowering, and may act upon it by their whole surface ; and it is important to note that the absorbing surface of seedlings is very small in proportion to the great i86i,p. 3). Those parasites which are apparently destitute of chlorophyll (like Orobanche), and saprophytes (as Neottia) contain, according to Wiesner (Bot. Zeitg. 1871, p. 37), traces of chloro- phyll, which however can hardly be taken into account in assimilation. T t 2 644 CHEMICAL PROCESSES IN THE PLANT. amount of work done, as is also the case with the absorbing roots of Cuscuta, Orobanche, &c. Sect. 6. — The Respiration of Plants ^ consists, as in animals, in the continual absorption of atmospheric oxygen into the tissues, where it causes oxidation of the assimilated substances and other chemical changes resulting from this. The formation and exhalation of carbon dioxide — the carbon resulting from the decomposition of organic compounds— may always be directly observed ; the production of water at the expense of the organic substance in consequence of the process of respiration is inferred from a comparison of the analysis of germinating seeds with the composition of those which have not yet germinated. Experiments on vegetation show that growth and the metastasis in the tissues necessarily connected with it only take place so long as oxygen can penetrate from without into the plant. In an atmosphere devoid of oxygen no growth takes place ; and if the plant remains for any time in such an atmo- sphere it finally perishes. The more energetic the growth and the chemical changes in the tissues, the larger is the quantity of oxygen absorbed and of carbon dioxide exhaled ; hence it is especially in quickly germinating seeds and in un- folding leaf- and flower-buds that energetic respiration has been observed; such organs consume in a short time many times their own volume of oxygen in the production of carbon dioxide. But in all the other organs also — in every indi- vidual cell — respiration is constantly going on ; and it is not merely the chemical changes connected with growth that are dependent on the presence of free oxygen in the tissues ; the movements of the protoplasm also cease if the sur- rounding air is deprived of this gas ; and the power of motion possessed by periodically motile and irritable organs is lost if oxygen is withheld from them ; but if this happens only for a short time the motility returns when the oxygen is again restored. The respiration of plants is, like that of animals, associated with a loss of assimilated substance, this loss being always a great deal smaller in assimilating plants than the gain of substance by the activity under the influence of light of the cells which contain chlorophyll ; but when, as in the germination of seeds, an energetic growth is combined with powerful respiration, no new products of as- similation replacing the loss, the loss in weight of the growing plant may be very considerable. Seeds which germinate in the dark may in this way lose almost one-half of their weight when dry, and it would seem that this loss is occasioned exclusively by the decomposition of the non-nitrogenous reserve-material and its combustion into carbon dioxide and water. If the rest of the non-nitrogenous reserve-material consists of oil, 2*. e. of a substance containing very little oxygen, a portion of the inhaled oxygen remains in the germinating plant, carbo-hydrates containing a large quantity of oxygen such as starch and sugar being formed at the expense of the oil. * The special references for what is said on this subject will be found in my work on Expe- rimental Physiology, sect. 9, On the action of atmospheric oxygen. Of more recent works may be mentioned especially, Borscow, On the behaviour of plants in nitrogen (Melanges biologiques tires du Bulletin de I'Acad. Imp. des Sci. Nat. de St. Petersbourg, vol. VI, 1867); also Wiesner Sitzungsber. der Wiener Akad. vol. LXVIII, 1871. RESPIRATION OF PLANTS. 645 The loss of assimilated substance caused by respiration would appear purpose- less if we had only to do with the accumulation of assimilated products ; but these are themselves produced only for the purposes of growth and of all the changes connected with life ; the whole life of the plant consists in complicated movements of the molecules and atoms ; and the forces necessary for these movements are set free by respiration. The oxygen, while decomposing part of the assimilated sub- stance, sets up important chemical changes in the remaining portion, which on their part give rise to diffusion-currents, and these bring into contact substances which again act chemically on one another, and so on. The dependence on respiration of the movements in protoplasm and motile leaves is very evident, since, as has been mentioned, they lose their motility when oxygen is withheld from them. These considerations lead to the conclusion that the respiration of plants has the same essential significance as that of animals ; the chemical equilibrium of the substances is being continually disturbed by it, and the internal movements maintained which make up the life of the plant. Respiration is, it is true, a source of loss of sub- stance; but it is also in addition the perpetual source from which flow the forces necessary to the internal movements ^ The combination into carbon dioxide of the inhaled oxygen with a portion of the carbon of the assimilated substance is, like all combustion, accompanied by the production of a corresponding amount of heat; but this only rarely leads to a sensible increase of temperature of the masses of tissue, because respiration, and in consequence the production of heat, is not in general very copious, while the circumstances are very favourable to the loss of heat by the plant. In this respect also plants. may be compared with cold-blooded animals. When an amount of heat is set free in the cells by the process of respiration, it first of all distributes itself over the large mass of water which permeates the cells and the adjoining tissue. In the case of a water-plant the least excess of temperature is at once equalised by the surrounding water ; while in the case of a land-plant evaporation has a powerful cooling effect on the atrial parts, quite independently of the action of the radi- ation of heat which is favoured by the large superficial development of most plants, and especially by their hairiness. With these causes of a rapid loss of heat, it is not surprising that the parts of a plant which are expanded in the air are even colder than it, although their respiration is continually producing small quantities of heat. But if the causes of the loss of heat are removed, it is possible to observe with the thermometer the increase of temperature caused by respiration. This can be done by accumulating rapidly germinating seeds, as is shown m the ^ [M. Corenwinder, from a series of observations on the maple and lilac, has confirmed the view to a certain extent held by Mohl, that the process of respiration is always gomg on m a plant even when concealed by the greater activity of the decomposition of the carbon ^ •o'^^de by the parts containing chlorophyll. He distinguishes two periods in the vegetative season ^^ ^^^ P^^"^- the first period, when nitrogenous constituents predominate is that ^^-^^^^^f /-^^P^'- """neTo active; the second, when the proportion of carbonaceous substance is relaUvely ^-^^ ' ^J^^^^^^^^^^^^^^ when respiration is comparatively feeble, the carbon dioxide evolved bemg -S-"J^^^^^^^^ taken up by the chlorophyll, decomposed, and the carbon fixed m the process «{j^— ^^^^^ found that the proportion of nitrogenous matter in leaves gradually ^^.'-^-^^'^^^^ ^^';^![^^^^';7 aceous matter increases, between auUimn and spring. (See R^vue sc.entihque, Aug. i. 1874.-ED.] 646 CHEMICAL PROCESSES IN THE PLANT. considerable elevation of temperature of grains of barley in the manufacture of malt ; and this elevation can also be proved in the case of other germinating seeds, or growing bulbs and tubers. The proof is more difficult in plants with green leaves. In some flowers and inflorescences the production of carbon dioxide which accompanies the inhalation of oxygen is very energetic, the radiation of the heat produced being at the same time diminished by the small superficial extent of the organ and by protecting envelopes ; and in such cases a very considerable elevation of temperature of the masses of tissue has been observed. The best illustration of this is the spadix of Aroidese at the time of fertilisation, where (especially in warm air) an excess of temperature of from 4° to 5° or even of 10° C. or more has been detected. Less considerable elevations of temperature have also been observed in the separate flowers of Cucurbita, Bignonia radicmts, Victoria regia, &c. In the few cases in which up to the present time the development of light or Phosphorescence has been observed in living plants, this phenomenon is also dependent on the respiration of oxygen. In Agaricus olearius (of Provence) this has been definitely proved by Fabre. This Fungus emits light only so long as it is alive, and ceases to do so at once when it is deprived of oxygen ; the respiration is in this case also very copious. Besides this Fungus, Agaricus igneus (of Amboyna), A. nocii- lucens (of Manilla), A. Gardneri (of Brazil), and the Rhizomorphs are known to emit light spontaneously ; the statements with respect to the light emitted from various flowers are of extremely doubtful value ■. The apparatus described in my Handbook of Experimental Physiology, p. 271, may be easily employed, with the necessary modifications, for the observation of the pro- duction of carbon dioxide and the elevation of temperature of germinating seeds. The following experiment is also adapted for the demonstration of these points in a lecture. One-third of a glass cylinder of 2 litres capacity is filled w^ith soaked peas or some other seeds or with flowers in the act of unfolding {e,g. small flower-heads of Com- positae, as Matricaria or Pyrethrum), and closed with a well-fitting glass stopper. If the vessel is opened carefully after several hours, the air contained will be found to extinguish a burning taper let down into it, as if it had been filled with carbon dioxide. In order to observe the development of heat also in small quantities of seeds and even in single flowers of larger size, I use various forms of the apparatus represented in Fig. 443. The flask/* contains a strong solution of potash or soda / which absorbs the carbon dioxide set free from the plants. In the opening of the flask is placed a funnel r, containing a small filter-paper perforated with a needle. The funnel is filled with soaked seeds or with cut flower-buds in the act of opening ; and a bell-glass g is now placed over it, through the tube of which a thermometer graduated to tenths of degrees is let in so that the bulb is surrounded on all sides by the plants. A loose pad of cotton- wool , and loses its turgidity. This phenomenon is seen with remarkable clearness in the large leaf-stalks of Cyjjara Scolymus when they freeze slowly. The succulent parenchyma separates from the epidermis, which surrounds the former like a loose sack ; the parenchyma itself splits apart in the interior so that each fibro-vascular bundle is enclosed in an envelope of parenchyma. Fig. 444 shows how the coatings of ice project from the masses of parenchyma. From pieces of the leaf-stalk which weighed 396 grammes I have collected 99 gr. of ice, which, when evaporated to dryness after thawing, left only slight traces (about I p. c.) of solid substance. I have often observed similar phenomena in other plants; the formation of ice is however not so regular as here. In the cavities of the Fig. 444.— Transverse section of a slowly frozen leaf-stalk of Cyna}-a Scolymus: e the detached epidermis ; i^ tlie parenchyma in which lie the transverse sections of the fibro-vascular bundles (left white). It forms a tousjh but pliant mass, which is ruptured during- the process of freezing ; a peripheral layer has become separated from the inner parts which surround the bundles; the surface of each portion of the parencliyma is covered with a crust of ice A" A' con- sisting of densely crowded prisms (the cavities of the ruptured tissue are left black in the figure). ruptured tissue (as in the succulent stems of the cabbage) small irregular flakes of ice are formed ; sometimes the ice splits the epidermis and projects in the form of combs above the surface of succulent stems (Caspary). I have already shown elsewhere'- that when sections of succulent parts of plants (such as the beet) are protected from evaporation and allowed to freeze slowly, continuous coatings of ice are produced on the surfaces of the section, consisting of prisms growing at the base. The formation and growth of these ice-crystals may be explained in this way. The temperature of the tissue falls to a certain point, thereby causing the freezing of an extremely thin stratum of water which overspreads the outside of the uninjured cell-walls. A new very thin stratum of water then immediately passes from the thickness of the cell-wall to its surface and also freezes, thickening the stratum of ice already formed ; and thus it goes on. The cell- wall is constantly absorbing cell-sap-water from within, and at the same time allows the outermost molecular stratum of its water of imbibition to freeze. The first thin layers of ^ When this contraction operates unequally on different sides of a leaf or branch, it is easy to see that curvatures must result which are indeed actually frequently observed. The splitting of the trunks of trees in consequence of frost is probably only the result of changes of this nature. 2 Sachs, Formation of Crystals in the Freezing, and change of the Cell-walls in the Thawing of Succulent Parts of Plants (Bericht der kon. siichs. Ges. der Wiss. i860). I have already mentioned in the first edition of this work the formation of crystals in the interior of frozen plants described above, and applied it to the explanation of freezing. Prillieux (Ann. des Sci. Nat. vol. XII, p. 128) afterwards, in 1S69, also described similar phenomena in a variety of plants. 6-6 GENERAL CONDITIONS OF PLANT-LIFE. ice on the exterior of the uninjured cells form polygonal plates in contact with one another; each plate becomes a prism by growth on its lower side; and the closely crowded prisms form a coating of ice which easily crumbles. These processes cause the cell-sap to become a more and more concentrated solution, while the cell-wall and the protoplasm contain a gradually diminishing quantity of water. It can now be to a certain extent understood why a rapid thawing kills the cells, while a slow thawing does not ; for if the thawing take place slowly, the ice-crystals melt at their base where they touch the cell ; the water as it becomes fluid is at once absorbed into the cell ; and the original con- ditions of the cell-sap, cell-wall, and protoplasm may be re-established, if they have not been permanently impaired during the freezing. If on the contrary the coating of ice melt off" very quickly, a portion of the water runs into the interstices of the tissue before it can be absorbed ; the original normal degree of concentration of the cell-sap and degree of imbibition of the cell-wall and protoplasm cannot be re-established in the cells ; and this may be fatal. It is evident, on the view here taken, that the danger of freezing increases with the amount of water in the tissue ; for the less watery the tissue the more concentrated is the cell-sap and the larger is the proportion of water retained by the force of imbibition ; only a small portion of the water can therefore form ice- crystals, and when they thaw the injurious effects are not so great. We can now also understand why some plants are killed by being thawed too quickly when they have been frozen by very severe cold, while freezing by a moderate amount of cold is not injurious to them ; for the lower the temperature falls the larger is the proportion of the cell-sap and water of imbibition that is converted into ice ; the dis- turbance of the degree of concentration of the sap and of the imbibition of the cell- wall is always greater with the increase of the cold ; and therefore the restoration of the normal condition on thawing more difficult. That the splitting asunder of whole masses of tissue during freezing such as has been described has but little effect on the continuance of the life of the organ after thawing, is shown by the fact that even the leaf-stalks of the artichoke, the frozen state of which is represented in Fig. ^44, remain uninjured till the following summer if thawed slowly. These internal rupturings have as little to do with the sudden destruction of the life of the cells from cold as the splitting of the trunks of trees caused by frost, which, when the temperature falls very low, is produced by the contraction of the bark and outer layers of wood, the crevices again closing when the temperature rises. The idea that growing plants, especially those which require a high temperature for their growth, can be directly killed by the cooling of their tissues for a short time nearly to the freezing-point is shown by H. de Vries' experiments (/. c.) to be fallacious. The older observations of Bierkander and Hardy that some plants of this description {e. g. Cucurbitaceae, Impatiens, the potato, Bixa Ore/ /ana, Crescentia Cujete, &c.) freeze when exposed to the air at low temperatures above the freezing-point, may nevertheless be explained if it is recollected that the temperature of their tissues may fall below the freezing-point from radiation, even when that of the air is 2° or 3° or even 5^ C. above it. But there is another way in which low temperatures above zero are injurious to plants from southern climates, 'viz. when the soil about the roots remains for a consider- able time at this low temperature while the leaves continue to transpire. In this case the absorption of water through the roots becomes so slow that they are no longer able to replace the loss caused by evaporation from the leaves, which in consequence wither, and at length altogether dry up. It is then sufficient to warm the soil about the roots in order to revive the withered leaves; as I found in the case of plants of Nicotiana, Gucurbita, and Phaseolus grown in pots\ In England the branches of a vine which were made to grow into a hothouse, while the roots stood in the ground outside, withered in winter, evidently only from the low temperature of the ground ; for when this was watered with warm water, the branches in the hothouse recovered. * Sachs, in Landwirthschaftliche Versuchsstationen, 1865, Heft i, p. 195. INFLUENCE OF TEMPERATURE ON VEGETATION. 657 Among the changes caused in plants by long-continued depression of temper- ature, one of the most striking is the change in colour of leaves which persist through the winter, originally observed by MohP, and recently more minutely studied by Kraus'-. This change is of two kinds ; the leaves either merely lose their colour and become brownish, yellowish, or rusty brown, as in Taxus, Abies, Pinus, Juniperus, and Buxus ; or turn a decided red on the upper surface, as in Sedum, Sempervivum, Ledum, Ma- honia, Vaccinium, &c. The loss of colour of the first group depends, according to Kraus, on a change in the grains of chlorophyll, which lose their form and definition, a cloudy mass of protoplasm of a reddish brown or brownish yellow colour being formed, while the nucleus remains colourless. These changes are usually more complete in the 'pallissade-cells' on the upper side than in the parenchyma which lies deeper. A spectroscopic examination shows that of the two pigments, a mixture of which forms, according to Kraus, the colouring substance of chlorophyll, the golden-yellow one re- mains unchanged, while the spectrum of the blueish-green substance undergoes a slight change. The winter-leaves of the second group, which are coloured red or purplish-brown on the upper side, owe this colour to a rounded hyaline strongly refractive mass lying in the upper part of the pallissade-cells, which appears of a beautiful carmine-red where the leaves are red, but elsewhere of a pale yellow, and consists mainly of tannin. The grains of chlorophyll, intact and of a beautiful green, are all crowded together in the inner end of these cells. In the spongy parenchyma of the mesophyll a colourless or red mass of tannin occurs in the centre of each cell, while the chlorophyll-grains, also intact, are collected in roundish or irregular lumps, sometimes in one place sometimes in several, but always on the sides towards the adjoining cells. In these cases the colour- ing matter of the chlorophyll is unchanged with regard to either of its constituent pigments. The red colouring matter is soluble in water, and cannot be distinguished by spectrum-analysis from the red colouring substances of flowers. In all leaves which persist through the winter, and in the green parts of bark, Kraus found that the grains of chlorophyll had removed from the walls to the interior of the cell, and had collected there in lumps (see Sect. 8). When the weather has become sufficiently warm in the spring, the normal condition is restored ; the red colouring sub- stance disappears, and the grains of chlorophyll again take up their normal position on the cell-walls. Kraus shows that the winter change of the leaves depends on the fall of the temperature, since it is restored to the normal state by a simple rise in the temper- ature, whether in the dark or the light. By taking cut branches of box into a warm room when the cold was severe and placing them in water, he found that the proto- plasm of the cells, which had become homogeneous after one or two days, collected on the walls, and then divided into grains (as in the formation of grains of chlorophyll in the dark); the red colouring matter being changed first to a yellowish-green and finally to pure green. After the lapse of three, five, or at most eight days, the walls of the cells became lined with bright green sharply-defined grains of chlorophyll. In Thuja the process required two to three weeks (with me however only a few days). The restoration is therefore rather a slow process ; while, according to Kraus, a single frosty night suffices to bring about the change in the form and colour of the chloro- phyll-grains in the case of Buxus, Sabina, and Thuja. That light has no share in the restoration of the normal condition of the chlorophyll, is shown by the fiict that it takes place also in branches which are kept in a dark room. On the other hand the fact that the parts protected by bemg covered by other leaves show no change of colour would seem to indicate that the whole phenomenon has less to do with the low temper- ature of the air than with the cooling produced by radiation. 1 Mohl, Vermischte Schriften ; Tubingen, 1845. p. 375. ■' Kraus, Observations on the winter colouring of evergreen plants ; in the Sitzungsber. der phys.-med. Societut zu Erlangen, Dec 19, 1871, and March 11, 1872. U U GENERAL CONDITIONS OF PLANT-LIFE. Convenient contrivances for observing the action of particular higher or lower tem- peratures on plants or parts of plants of considerable size are easily arranged ^ It is more difficult to expose microscopic objects to a particular higher or lower temperature in such a manner that it can easily and certainly be observed, and that the temperature of the object is also that indicated by the thermometer, or nearly so. All these require- ments are fulfilled by the very cheap heating apparatus for the microscope represented in Fig. 445. Since I have net only made great use myself of this apparatus for three I'IG. 445. — Heatintj apparatus for the microscope. years, but have also recommended it to others, a description is the more in place here as it is well adapted for demonstrations in lecture rooms. The size of the heating apparatus must vary with that of the microscope ; mine is constructed for one of Hartnack's ordinary instruments. The box is nearly cubical, and has double walls of sheet-zinc at the bottom and sides, enclosing a space 25 mm. thick, which is filled with water through the hole /. It is quite open above ; but in the front side-wall is an opening /, which is closed by a glass plate well fitted but not other- wise fixed. This window is sufficiently large, and is so placed that it allows enough light to fciU on the mirror of the microscope which stands in the box. The height of the ^ See Sachs, Handb. der Exp.-Phys. pp. 64, 66. ACTION OF LIGHT ON VEGETATION. 659 box is so arranged that the upper rim of the double wall is on a level with the arm b of the microscope. The opening of the box is closed by a thick cardboard cover d d, in which an opening is cut exactly to fit the arm b. By the side of the tube of the micro^ scope a round hole is cut in the cover through which a closely fitted small thermometer t is passed, so that its bulb hangs near the object. The box is painted on the inside with black varnish, and a piece of cardboard moistened with water lies beneath the foot of the microscope in order to prevent its moving and to keep the air within moist. The focus is easily adjusted to the object by means of the fine adjustment s which pro- jects above the cover; two openings in the side, one of which is shown at 0, enable the slide bearing the object to be moved, when necessary, by a pair of forceps. It is still more convenient to fix the slide on a wire which goes through a cork fitted to the opening 0. If observations are required at a higher temperature, the water in the box is heated by a spirit-lamp placed underneath. When the temperature has reached nearly the de- sired point, the spirit-lamp is replaced by an oil-lamp with a floating light ; the temper- ature will after a time become constant. In order to obtain higher or lower constant temperatures, one two or three floating night-lights are placed in the lamp. If care is taken that the combustion be uniform, the temperature in the box remains for several hours so constant that it will vary only about i" C. This constancy of temperature en- sures that the temperature of the object itself is that indicated by the thermometer. It is easy by means of this heating apparatus to observe and demonstrate the in- fluence of temperature on protoplasm-currents. To take observations at lower temper- atures it is sufficient to enlarge the hole /, in order from time to time to place pieces of ice in the cold water\ Sect. 8. — Action of Light on Vegetation^. A. General. The entire life of the plant depends on the action of light on the cells that contain chlorophyll, this being the essential condition under which new organic compounds are formed out of the elements of carbon dioxide and water. The amount of oxygen evolved in this process is nearly the same as that required for the combustion of the substance of the plant ; and the amount of work equivalent to the heat produced by this com- bustion gives a measure for the amount of work performed by light in the chloro- phyll-containing cells of the plant. After a certain quantity of assimilated substance has been produced under the influence of light, a long series of vegetative processes may be carried on at its ex- pense without any further direct action of light. The growth of new organs and the metastasis connected with it kept up in the organs by means of respiration is entirely' or to a certain extent independent of light, and can even be carried on in absolute darkness. This is the case in the germination of seeds, bulbs, and tubers, the development of buds from woody branches and underground rhizomes, &c. Even leafy plants which have accumulated a sufficient quantity of reserve-material in the light, put out shoots and even flowers and fruits when placed in the dark. As the parts of chlorophyll-containing plants which are underground or other- 1 [For further arrangements for maintaining a constant temperature under the microscope, see Strieker and Burdon-Sanderson, Quart. Journ. Micr. Sci. 1870 ; Schafer, ibid. 1874-— Ed.] 2 A. P. De Candolle, Physiologic vegetale, i832.-Sachs, Ueber den Einfluss des Tages-hchtes auf Neubildung u. Entfaltung verschiedener Pflanzenorgane ; Bot. Zeit. 1863, Supplement.-Sachs, Wirkung desLichtes auf die Bluthenbildung u. Vermittlung der Laubbl^tter ; Bot. Zeit. 1865, p 117. — Sachs, Handb. der Exp.-Phys. 186^, p. i. U U 2 6f:0 GENERAL CONDITIONS OF PLANT-LIFE. wise excluded from light are nourished by the products of assimilation produced in the light, so also parasites and saprophytes destitute of chlorophyll live, as has already been explained, on the work performed by plants that contain chlorophyll, and are therefore dependent indirectly on light, even though the whole of their development may be completed in darkness, as in the truffle ; in other instances they only emerge to unfold in the air the flowers already formed underground, and disseminate their seeds, as is the case with Limodorum aboriivum., Epipogium, Coral- lorhiza, IMonotropa, Lathraea, Orobanche, &c. Even many plants which do contain chlorophj'Il and which live on inorganic food complete their growth and the pro- cesses connected with it in complete darkness, only putting forth their green leaves at certain times for the purpose of again accumulating beneath the ground fresh formative material. This is the case with the autumn crocus, tulip, crown imperial, terrestrial orchids, and many others, and especially with plants which form bulbs, tubers, and rhizomes. If the growing end of a stem of a green-leaved plant {e. g. Cucurbita, Tropeeolum, Ipomoea, or Hedera) is secluded from all light while the green leaves remain exposed to it, the buds develope in the dark ; leaves and flowers are produced, M'hich latter attain their full size and beauty of colour, are capable of fertilisation, and produce fruits and even fertile seeds at the expense of the substance assimilated in the light in the green leaves and carried to them by the stem. These and a number of other facts show that growth, /. e. the processes by which the form of the plant is attained, and metastasis are not necessarily dependent, or only to a subordinate extent, on the influence of light, if only the necessary quantity of assimilated material has previously been accumulated. This is a general statement of the case. If however the various separate processes of vegetation are observed — the behaviour of protoplasm, the formation, arrangement, activity, and destruction of chlorophyll, the growth of the younger and older parts, the movements resulting from the tension of the tissues, &c. — a long series of very varied facts presents itself which requires detailed consideration, because the rays of different refrangibility which are mingled in white daylight affect vegetation in a manner altogether different ; certain functions are induced only by the strongly refrangible rays, others only or chiefly by those of less refran- gibility. These effects moreover vary not only with the temperature but also with the intensity of the particular rays. Finally it must be observed that light affects plants only when its rays penetrate into their organs ; this however modifies them in intensity and to a certain extent also in refrangibility. In every investigation of the action of light these points must therefore be kept in view. The following summarises what is at present knowm as to the general facts. (i) Action of 1- ays of different refrajigibility. The rays of difl"erent refrangibility commingled in white sunlight which appear as variously coloured bands in the spectrum, vary in their physiological action on the processes of vegetation. Chemical changes, so far as they are in the main dependent on light, are produced chiefly or solely by rays of medium or low refrangibility {viz. the red, orange, yellow, or green). This is the case for instance with the production of the green colour of chlorophyll, the decomposition of carbon dioxide, and the formation in chloro- phyll of starch, sugar, or oil. ACTION OF LIGHT ON VEGETATION. 66 1 On the other hand the rays of high refrangibility (the blue or violet, as well as the invisible ultra-violet rays) are the principal or the only ones which produce mechanical changes so far as these are dependent on light. It is these rays which influence the rapidity of growth, alter the movements of the protoplasm, compel swarm-spores to adopt a definite direction in their motion, and change the tension of the tissues of the motile organs of many leaves and hence affect their position. These two laws, the result of careful observation, are only in apparent contradiction to the division of the rays of light which is current in chemistry and physics into those called chemically active, including the highly refrangible blue, violet, and ultra-violet, and the chemically inactive, or at least less active, including the less refrangible red, orange, and yellow, and partly also the green rays. This division has long been familiar; silver-salts, nitrogen terchloride, and other inorganic compounds, are powerfully acted on by the former, scarcely at all by the latter. But when it was shown that the organico-chemical processes in plants were caused mainly or solely by the latter kind of rays, it was seen that this classification into chemical and non-chemical rays resulted from an imperfect induction, and that the correct statement of the fact is rather that there are chemical processes (generally dependent on light) which are related to rays of particular refrangibility. As far as concerns the mechanical effect on the plant of the highly refrangible rays, it is at present uncertain whether they are not ultimately due to chemical changes. In any case the action is visible to the observer only in the form of mechanical effect (movements, tensions, &c.) ; and this is in harmony with the classification given above. If sunlight is made to pass through sufficiently thick strata of solutions of potassium bi-chromate and ammoniacal copper oxide ^, the first only permits the passage of light consisting of the less refrangible half of the spectrum (red, orange, yellow, and some green), while the blue solution allows, in addition to some green, only the blue, violet, and ultra-violet rays to pass through. The sunlight is therefore in each case halved by the absorption in such a way that the spectrum beneath the orange solution extends from the red to the green, that beneath the blue solution from the green to the ultra-violet. If the light after passing through these fluids is directed on plants capable of decomposing carbon dioxide, and pieces of very sensitive photographic paper are at the same time exposed by their side, it is seen that the less refrangible rays of light (transmitted through the potassium bichromate) effect the decomposition of carbon dioxide and the colouring of the chlorophyll almost as energetically as white daylight, while they produce only a very slight effect on the photographic paper. The growth of seedlings, on the con- trary, proceeds in this light exactly as in the dark, although the leaves turn green. Conversely the light which had passed through the ammoniacal copper oxide had very little effect in decomposing carbon dioxide, although the action on photo- graphic paper was very vigorous. The growth of seedlings was on the coritrar\- the same as in white light ; and the mechanical process of heliotropic curvature was ' Sachs, Bot. Zcit. 1864, p. 253 ei seq., where the labours of previous observers are referred to ill detail. 662 GENERAL CONDITIONS OF PLANT-LIFE. very manifest. A number of more recent observations have confirmed and extended the results previously obtained ^ (2) Variation in the action of light on plants in proportion to its ifitensiiy"-. That the action of light on vegetation varies with its intensity, as that of temperature with its elevation, does not admit of a doubt, and agrees with the observed facts of vegetative physiology. There can scarcely be said, however, to be any exact investigations on this point; and the great obstacle to their accomplishment is that we have at present no method of measuring the intensity of rays of light of any particular refrangibility in terms of a fixed unit which can be applied to plants. As far as concerns the highly refrangible rays, i.e. those which have the greatest mechanical effect, we are compelled to adopt the photo-chemical method of Bunsen and Roscoe^ which however gives no information respecting the different intensity of the red, orange, and yellow light, and can only be applied with great difficulty to experiments on vegetation. In the photometry of the less refrangible rays, on the contrary, we can always have recourse, according to the ordinary method, to the sensitiveness of the eye, i. e. to brightness, which cannot be con- sidered in itself an actual objective measure of the intensity of the light, though it must under certain circumstances depend upon it. In describing the relation between the intensity of light and vegetation, we have therefore at present, with a few exceptions, to employ the ordinary expressions dark, dull, bright, dazzlingly bright, &c. There is one case in which this relation between the sub- jective sensitiveness of the eye and the action upon vegetation of the light which causes it can be very strikingly proved; Pfefifer has shown that the curve of the subjective sensitiveness of the eye for the colours of the solar spectrum coincides exactly with the curve expressing the power of different regions of the spectrum in decomposing earbon dioxide''. This coincidence must however at present be considered purely accidental ^^ and cannot be extended to other pheno- mena. If the sunlight or diffused daylight which reaches the observer were always of the same intensity, it would be easy to regulate artificially, according to definite gradations, the intensity of the light that acts on the plant. But since the light of incandescent bodies (such as the Drummond's light ''^) contains the same rays as sunlight and acts similarly on the functions of plants, constant sources of light of a definite intensity can in this way be arranged, which will admit of gradual adjustment, in order to study the influence on vegetation of light of different intensities. ^ I have replied, in the second part of the 'Arbeiten des botan. Inst, in Wiirzburg,' 1872, to the objections urged by Prillieux to this statement, which rest on an entire confusion of the ideas Intensity of Light (objective), Brightness (subjective), Refrangibility (an objective), and Colour (a subjective property of light). ^ With respect to the distinction which must here be borne in mind between the objective intensity of light and its brightness to the eye, see the paper quoted above and the literature there referred to. 2 See the admirable paper by Wolkoff in the Jahrb. fiir wiss. Bot. vol. V, p. i. * Pfeffer in vSitzungsber. der Ges. zur Beforderung der ges. Naturwiss. fiir Marburg. 1S72, May 16. ^ See note on p. 669. '"' See Herve Mangon, Comp. rend. 1861, p. 243.— Prillieux, ibid. 1869, p. 408. ACTION OF LIGHT ON VEGETATION. 66? If we now turn to the observations on record, those of Wolkoff are the only ones in which actual measurements have been made. With the assistance of the photometric method contrived by Bunsen and Roscoe\ he showed first of all that changes in the intensity of the highly refrangible light do not stand in any appreciable relation to the exhalation of gas by water-plants. This is an ad- ditional proof that these rays play only an extremely small part in this process, so small indeed that in the experiments the actual effect might be concealed by other causes (see p. 667). He next used as the source of light a dull glass plate illuminated by daylight, at different distances from which he exposed the plants (Ceratophyllum, Potamogeton, Ranunculus Jluitans) in a dark room ; and he ascer- tained that the exhalation of gas was, within certain limits, nearly proportional to the intensity of the light I There is probably however some particular intensity of the efficient rays at which a maximum of gas is exhaled, and above which the rapidity of the process again decreases and the plant suffers injury; but whether this maximum intensity of light is attained or exceeded by the sunlight as it falls on the surface of the earth cannot at present be determined. In reference to the smallest degree of intensity of light at which exhalation of gas can still take place, we have only the statement of Boussingault that a leaf of oleander ceased to ex- hale oxygen after sunset^. The green colour of the chlorophyll of Monocotyledons and Dicotyledons is not produced in the dark, as may be seen by enclosing plants in closely shut- ting boxes of wood or metal, or in a dark cellar. The colouration begins however when the amount of light is barely sufficient to read a book by ; and when it in- creases to the ordinary brightness of a sunny summer day, the rapidity of the change increases, and the colour becomes a deeper green than that produced when plants are placed for a longer time in places not so strongly illuminated. Famintzin nevertheless showed*, in the case of Lepidium sativum and Zea Mais, that bleached seedlings become green more slowly in direct sunlight than in dif- fused daylight. The small intensity of light which suffices for the formation of chlorophyll is not sufficient for assimilation or for the formation of starch in the chlorophyll. Plants (such as Dahlia, Faba, Phaseolus, Cucurbita, &c.), which rapidly become green in the normal condition of full daylight, as well as in the diffused light of the back of a room, still form no starch in their chlorophyll. They do however produce chlorophyll in a window where, at the most, they enjoy but half the direct sun- light and diffused daylight ; but, in harmony with this, the assimilation of these plants is much less active in the window than in full daylight in the open air'\ The following experiment gives a somewhat more precise result. Four plants of TropcEolum majus grown from seed in the back of a room, all gave, when dried at iio°C., a smaller weight than the seed; they had not assimilated, and died after Bunsen and Roscoe: Pogg. Ann. vol. loS. See also Pfefter: Arbeiten des botan. Inst, in Wiirzburg, Heft i, p. 41. Comp. rend. vol. 68, p. 410. Famintzin, Melanges biologiques ; Pelersbuig. vol. VI, p. 94, 1S66. Sachs, Bot. Zeit. 1862, No. 47; and 1864, p. 289 et seq. 664 GENERAL CONDITIONS OF PLANT-LIFE. consuming the reserve-material, although in the shade of the room they all pro- duced green leaves. Four other plants of the same species which germinated at the same time grew for three months, exposed for only seven hours each day to the diffused light of a west window in the forenoon ; they weighed when dry nearly 5 grammes. Four other plants which were exposed in a WTSt window from I p.m. till the following morning, and therefore to the afternoon sunshine, weighed also only 5 grammes ; while four other plants which stood in the window during the same time day and night produced nearly 20 grammes each of dry substance \ It is a necessary conclusion from the increase in weight of these plants, that in the diffused daylight of the window of a room carbon dioxide is decomposed by the cells which contain chlorophyll ; although this does not take place with great activity. The same conclusion is drawn from the observation that ValUsncria spiralis and Elodea canadensis give off bubbles of gas when the light falls on them for only a rather short time from the northern sky on a clear day, although the exhalation is much more rapid in direct sunlight. In the case of most plants which grow in full daylight, especially our cultivated plants, the increase of weight by assimilation is greatly diminished when they are grown in a window. Within a room itself they usually become exhausted by their own growth in consequence of the defective assimilation, which is not sufficient to replace the material con- sumed in growth and in respiration ; and the plant ultimately dies. IMany Mosses on the other hand, and wood-plants of various kinds which grow in the deep shade (as the wood-sorrel), are killed by constant exposure to broad daylight; but whether in these cases it is the intensity of the light or the transpiration that is too great, and which of the two is the direct cause of injury, is unknown. Stems Mhich attain an enormous length in complete darkness remain perceptibly shorter in the shade of a room ; in a window their growth is still less, and least of all in the open air in full daylight. The reverse is the case with the leaves of Dicotyledons and Ferns ; in the dark they are often very small ; in deep shade they are con- siderably larger, and still more so in a light window; in this position they even appear in many plants (Phaseolus, Begonia, &c.) to attain their maximum of super- ficial development, remaining smaller in the open air^. (3) Penetratio7i of the rays of light into the plant. In order to determine the dependence on light of certain phenomena of vegetation, it is of special interest to know the depth to which rays of a given refrangibility can penetrate any tissue of a plant, and the intensity with which the different elements of daylight act on particular internal layers. With the exception of the underground parts of plants, stems enveloped in bark, young organs enclosed in leaf-buds, and the like, which are in complete darkness, the assimilating and growing organs are penetrated by ^ Sachs, Exp.-Phys. p. 21. It must however be observed that the shorter the duration of the light in these cases, the longer was the time of their exposure to the dark in which they again lost a portion of the assimilated substance by respiration. ^ The statement made by Famintzin (Mel. biol. vol. VI, p. 73, 1866) that the motile Algse, Chlamydomonas pulvisciihis., Eitglena viri-dis, and Oscillatoria insignis turn both from direct sunlight and deep shade to a light of medium intensity, is contradicted by Schmidt (quoted infra), who found that they always turn to light of greater intensity, and even to direct sunlight. Tlie method of observation of both authors was however very imperfect. ACTION OF LIGHT ON VEGETATION. 665 light. The deeper the light penetrates, the more does it lose in intensity by ab- sorption, reflexion, and dispersion. This loss however affects the different elements of white light in very different degrees, as was shown by my investigations made in 1859^ at present the only ones on this subject. The rays of greatest rc- frangibility are in general almost entirely absorbed by the superficial layers of tissue, while the red light penetrates most deeply. Of successive layers of an apple, gourd, succulent stems, &c. only the outermost receives the light that falls on it unchanged (independently of the reflexion from the surface) ; each deeper layer is penetrated by light less intense than the preceding one, and of a different composition. This change in the light which penetrates the tissue is principally caused by colouring materials, especially chlorophyll, which have an absorptive power for particular groups of rays, allowing others to pass through, and producing in addition rays by fluorescence which v/ere not contained in the incident light. But the relations of these changes of light in the tissues to the changes which the light causes are not yet accurately known; not even in reference to chloro- phyll, to which we shall again recur. What we have now said is intended only to draw the attention of the student to the subject ; more exact investigations must be made in working out the different questions which arise. B. Special, (i) Chemical Action of Light on Plants, (a) Formation of Chloro- phylP. By the formation of the grains of chlorophyll the protoplasm becomes differentiated into a colourless homogeneous part which forms the proper motile or protoplasmic body of the cell, and into smaller distinct green portions which remain imbedded in the former, the grains of chlorophyll. This process, as far as concerns the differentiation, is independent of light, at least in flowering plants, where the chlorophyll-grains are formed in the cells of the leaves even in the dark. The chemical process, on the contrary, by which the green colour is pro- duced has a complicated dependence on light. If, for instance, the temperature is sufficiently high, the green colouring substance is formed in the cotyledons of Conifers and in the leaves of Ferns in complete darkness as well as under the influence of lights In Monocotyledons and Dicotyledons, on the contrary, the grains of chlorophyll which are formed in the dark remain yellow, until they are exposed to light even of small intensity, when they become green if only the temperature is sufficiently high ; and the nearer, as I have shown, the temperature approaches a definite maximum (25 to 30° C.) the quicker does the chlorophyll of Angiosperms become green in the light. Provided therefore that the temperature is favourable, the chlorophyll in the cotyledons of Conifers and the leaves of Ferns does not require light in order to assume its green colour; while that in Angio- 1 Sachs, Ueber die Durchleuchtung der Pflanzentheile ; Sitzungsber. der Wicn. Acad. iSrio. Vol. 43 ; and Handb. der Exp.-Phys. p. 6. 2 Sachs, Bot. Zeitg. 1862, p. 365, and Exp.-Phys. pp 10 and 318.— Sachs, Flora 1862, p. 21.^, and 186+, no. 32 —Mohl, Bot. Zeitg. 1S61, p. 238.-Bohm, Sitzungsber. der \Vic»ncr Akad. vol. II. Compare also Book I. sect. 6 of this work. ■' P. Schmidt (Ueber einige Wirkungen des Lichts auf Pflanzen ; Dissertation, Breslau 1870, p. 22) believes that these facts can be at least partially combated; but his experiments only prove that the chlorophyll which is formed in the dark is again destroyed by long exposure to dark at a high temperature (33"7 C), as is also the case with other plants. 666 GENERAL CONDITIONS OF PLANT-LIFE. sperms does require it; and in both cases the change does not take place at low temperatures (see p. 651). It may be concluded from such observations as have been made that all the visible parts of the solar spectrum have the power of turning the etiolated grains of chlorophyll of Angiosperms green ; but that the yellow rays and those nearest to them on each side are the most powerful ; and that this is also the case with the exhalation of oxygen from cells containing chlorophyll ^ (d) The Decomposition of carbon dioxide in cells containing chlorophyll, on which depends the assimilation of plants, and which is perceptible externally by the exhalation of a volume of oxygen nearly equal to that of the carbon dioxide absorbed, is brought about under favourable circumstances (see p. 651) by rays of light. In submerged water-plants the gas (always mixed with a larger or smaller quantity of nitrogen) escapes in the form of bubbles from wounds, espe- cially transverse cuts of the stem ; and it has been shown by Pfefifer and myself that when their size is constant the rapidity of these bubbles^ i. e. the number of them formed in a unit of time, may even be used to give an exact measurement. In observations on land-plants it is on the other hand necessary to expose the leaves to light together with air containing carbon dioxide in glass vessels of a suitable size and form, and to measure the quantity of gas by a eudiometer. The smallest intensity of light necessary for the evolution of oxygen is — judged by the subjective measure of its brightness to our eye — rather considerable (see p. 664). This evolution is always takings place with considerable energy in diffused daylight, even when the rays reach the plant only from a small portion of the sky ; but it is much stronger in direct sunlight. The specific effect on the evolution of oxygen of the variously refrangible elements of sunlight, in other words of the different coloured bands of the solar spectrum, has been carefully in\-estLgated by Draper and very recently again by Pfeffer^. The observations were made partly with the solar spectrum, partly Mith solutions of different colours which transrrtitted light of a particular refrangibility. The amount of gas exhaled was measured partly by the eudiometer, partly by the number of bubbles. Pfeffer showed fij-st of all * that each portion of the spectrum exercises a specific quantitative influence on the power of assimilation ; and that this remains unchanged whether the particular rays act separately on the parts of plants that contain chlorophyll, or combined with some or with all the other rays of the spectrum.' The following additional result was also obtained from Draper's and Pfeffer's observations, and from mine already quoted : — ' Only those rays of the spectrum which are visible to our eye have the power of decomposing carbon dioxide ; and indeed those which appear brightest to the eye, the yellow rays, are alone as ^ See in particular Guillemin, Ann. des Sci. Nat. 1857, vol. VII, p. 160. ^ Draper, Annales de chimie et de physique 1844, p. 214 et seq. — Pfefifer, Arbeiten des Botan- ischen Instituts in Wiirzburg, Heft I, p. 48, where reference is also made to the whole of the rest of the literature. — Pfeffer, Sitzungsber, der Gesellsch. zur Beforderung der gesammt. Naturwiss. zu Marburg 1872, May 16; and Bot. Zeitg. 1872, no. 23 et seq., where the paper by Midler, Botanische Untersuchungen, Heft I, Heidelberg 187 1, is also discussed. ACTION OF LIGHT ON VEGETATION. 667 efficacious in this process as all the others put together. The most refrano-ihle rays of the visible spectrum, and those which act most energetically on silver chloride &c., play a very subordinate part in the process of assimilation.' Draper placed glass tubes filled with water saturated with carbon dioxide in which he had placed green parts of plants, in the different coloured portions of a solar spectrum. Seven of these tubes were exposed simultaneously in the same spectrum. The following table gives the result of two experiments of this kind: — Part of the Spectrum. Gas evolved, Experiment Dark-red • 033 Red-orange . . 2 0-00 Yellow-green . . .^6-oa Green-blue O-iO- Blue 00 Indigo . o-o Violet . O-Q Experiment II. GO 24-75 4375 4-10 I '00 00 O'O Pfeffer experimented chiefly on leaves of the cherry-laurel and oleander, which were placed in air containing carbon dioxide (shut off by mercury) in suitable glass vessels, and received the sunlight through coloured solutions (tested by the spectro- scope). The following was the result of sixty-four experiments : — If the amount of gas evolved in light which has passed through a stratum of water of standard thickness is represented by 100, the numbers here given are the corresponding quantities of carbon dioxide decomposed in light which has passed through equal thicknesses of the solutions named. Solution. Amount of carbon dioxide Colour of light. decomposed. Potassium bichromate Red, orange, yellow, green 88 6 Ammoniacal copper oxide Green, blue, violet 76 Orcin Red, orange-green, blue, violet 53*9 Aniline-violet Red, orange-blue, violet 38-9 Aniline-red Red, orange 32-1 Chlorophyll Red-orange, yellow, green 1 5 g Iodine solution Quite dark 14-1 (carbon di- oxide produced). From a comparison of these numbers Pfeffer deduced the following values for the decomposing power of the different regions of the spectrum, the action of white light being again placed at 100 : — For Red-orange . . . • 32"i Yellow . . . .46-1 Green i5'o Blue-violet . . • •7'^' :oo-8 and from these is deduced the first statement of Pleffer given above. 668 GENERAL CONDITIONS OF PLANT-LIFE. If these values are erected as ordinates upon the solar spectrum, taking its corresponding parts as abscissae, the result, as shown in Fig. 446, is that the curve of the different powers of light for causing evolution of gas corresponds in the main with the curve of subjective brightness of the same regions of the spectrum; but has no relation to the curve of heating power. Pfeffer's experiments had shown that the method first employed by me for determining the intensity of the action of light on water-plants, viz. counting the number of jthe bubbles of gas given off in a unit of time, gave nearly the same results as actual measurement of the gas, the result being in fact somewhat too great, and inexact in inverse proportion to the amount of gas given off. I then applied this method to determine the amount of oxygen given off from a small- water-plant {Elodea canadensis) when exposed to a portion 13 mm. in breadth of a Fig. 446. — Graphic representatfon of the efficacy of rays of dffierent refrangibility in causing- the evolution of oxygen, com- parcti with tlieir brightness and Iieating power. Tlie solar spectrum A — //serves as a base, on wliieh lines to represent the tliree different effects are erected as ordinates ; the three curves are thus obtained which represent assimilation, brightness, and heat. very intense solar spectrum 23 cm. long. In this experiment I had the advantage of being able to determine the amount of gas given off by the same plant in all the regions of the spectrum in successive very short spaces of time, and thus avoiding various errors of observation which inevitably accompany eudiometric ob- servations, or at least are very difficult to get rid of. A number of observations conducted in this manner gave the following result as the mean capacities for decomposition possessed by the different regions of the solar spectrum, yellow being placed at 100 : — Red Orange ^5 4 63-0 Yellow . lOO'O Green • 37-2 Blue 221 Indigo • 135 Violet 71 IS ACTION OF LIGHT ON VEGETATION. 669 If allowance is made for the small error mentioned above incident to the method of counting the number of bubbles, we find that the curve of capacity for exhaling oxygen agrees still more exactly with the curve of brightness than represented in Fig. 446, which was drawn from only a few data obtained difficulty. Since a comparison of the curve of brightness with that of the evolution of oxygen, otherwise convenient, has turned the attention of observers in a wrong path, and has led to many erroneous theories, it will be convenient to state the only relation between the two with which we have to do here, in the following terms : — The evolution of oxygen caused by chlorophyll is a function of the length of the waves of light ; only those wave-lengths which are not greater than 0-0006866 mm. and not less than 0-0003968 mm. being able to produce this efi'ect. Starting from the two extremes, the capacity of light for causing evolution of oxygen rises till it reaches its maximum at a wave-length of 0-0005889 mm. It will be at once seen that we have here a similar phenomenon to that of the relation of vegetation to temperature; for we found (see p. 652) that this function also rises with the rise of temperature, attains a maximum at a definite temperature, and again decreases as the temperature rises still higher ^ {c) Formation of Starch in the Chlorophyll'^ . The yellow chlorophyll-grains formed in the dark are small ; after turning green on exposure to light they become considerably larger, corresponding to the increase in size of the cells in which they arc contained. It is only after they have assumed their green colour and under the continued action of more intense light, in other w^ords under conditions favour- able to assimilation, that the formation commences of the starch which is enclosed within the chlorophyll-grains (see p. 46). When cells whose chlorophyll has already produced starch on exposure to light are again placed in the dark, the starch is absorbed and disappears completely from the chlorophyll-grains, and does so the quicker the higher the temperature. If light is again allowed access starch is again formed in the same chlorophyll-grains ; and the formation of starch is there- fore a function of chlorophyll exposed to light, its absorption a function of chloro- phyll not exposed to light. If complete or partial darkness is continued for a length of time, the chlorophyll is usually itself destroyed; it first loses its form, is then absorbed, and finally disappears from the cells together with the colourless protoplasm ; in the case of leaves of rapidly growing Angiosperms this takes place after a few days when the temperature is high. Cactus-stems with slow growth and the shoots of Selaginella on the contrary remain green for months in the dark. The absorption and re-formation of starch in the chlorophyll— a process which 1 was the first to demonstrate in the leaves of Phanerogams— can be seen more readily in Algoe of simple structure like Spirogyra, which may therefore serve for purposes of investigation. I had already shown that the formation of starch in ' The same law of dependence is also evidently applicable to the sensitiveness of the eye to brightness; and this is the cause of the curve of the brightness of light running nearly parallel to that of the evolution of oxygen. '' Sachs, Ueber die Auflosung und Wiederbildung des Amylums in den Chiorophyll-kurncrn bej wechselnder Beleuchtung: Bot. Zeitg. 1864, p. 289. 6;o GENERAL CONDITIONS OF PLANT-LIFE. chlorophyll depends on conditions which favour assimilation, and that assimilation proceeds vigorously in light transmitted through potassium bichromate, and consists therefore of red, orange, yellow, and to a certain extent green rays ; while the more strongly refrangible half of the spectrum, consisting of green, blue, violet, and ultra-violet rays, obtained by passing the light through ammoniacal copper oxide, has only a very slight effect. The conclusion at once followed from this, that the formation of starch must take place in the set of rays first named to the same extent that it does in full sunlight, but only to a very small extent in the latter set. This was confirmed by Famintzin's experiments \ in which he found that in Spirogyra the formation of starch in the chlorophyll took place only in the mixed yellow light (that had passed through potassium bi-chromate), and not in the mixed blue light (that had passed through ammoniacal copper oxide), in which the starch already formed even disappears. Since however a small exhalation of oxygen takes place even in the mixed blue hght, it must be supposed that a small production of starch occurs in it. Kraus's experiments ^ with Spirogyra, Funaria, and Elodea, confirm this. He also found that in plants of Spirogyra which had lost their starch from exposure to dark, the formation of this substance in the grains of chlorophyll recommenced in five minutes in direct sunlight, in two hours in diffused daylight. In funaria the formation of starch recommenced in the same manner within two hours in direct sunlight, within six hours in diffused daylight ; and similar results were obtained from leaves of Elodea, Lepidium, and Betula^. (2) Mechanical Action of Light on Plants, (d) The influence of light on the movement of protoplasm varies according to the nature of the motion. Those movements which are the cause of the formation of new cells are not in general directly dependent on light (see p. 673) ; since they take place, in the great majority of cases, in partial or complete darkness. The ' streaming ' motion of the proto- plasm in older cells, or rotation and circulation, also goes on in continuous dark- ness as well as in alternate davli^ht and nio^ht : and even in the hairs of etiolated ^ Famintzin, Action of Light on Spirogyra; Melanges biologiques, Petersburg 1865, Dec; and 1867, P- 277. '^ Kraus, Jalirb. fiir wissensch. Bot. vol. VII, p. 511. ^ In accordance with the theory propounded by me that the starch formed in the chlorophyll- grains under the influence of light is the first product of assimilation produced by the decom- position of carbon dioxide, Godlewski has found (Flora, 1873, p. 383), as the result of experiments as simple as ingenious, that in an atmosphere devoid of carbon dioxide no starch is produced in the chlorophyll-grains even in the dark ; that the starch contained in the chlorophyll disappears when the carbon dioxide is removed from the surrounding atmosphere, not only in the dark, but even in bright light. It may be inferred from this that the starch which is at any time found in the chloro- phyll is only the excess of the whole product of assimilation which has not yet been taken up. Of especial importance is his observation, which agrees with his eudiometrical experiments, that an increase in the proportion of carbon dioxide in the atmosphere to 8 p.c. in a bright light increases the rapidity of the formation of starch four or five fold, while in a diffused light the action is much less. A very large quantity of carbon dioxide in the atmosphere, on the contrary, retards the form- ation of starch in inverse proportion to the intensity of the light. Godlewski's experiments, made on the cotyledons of seedlings of Raphanus sativtis, are also opposed to the statement of Bohm (Sitz- ungsber. der Wien. Akad. March 6, 1873), that the starch contained in the chlorophyll is not a product of assimilation, a view which has already been sufficiently refuted by my earlier investi- gations. ACTION OF LIGHT ON VEGETATION. 67 1 shoots which are developed in darkness \ It has not been ascertained whether in these cases the rapidity and direction of the movement, the mode of distri- bution of the currents, and the accumulation of the protoplasm at particular spots, are influenced by the direction of the rays of light. An influence of this kind is apparently exercised by light on the plasmodia of Aethalium. As long as the Plasmodia are still in motion and not ripe for the production of spores, they appear on the surface of the tan when it is dark ; but in the light, as in a sunny window, they again conceal themselves in the dark parts of the tan, — a process which the plant may be made to repeat two or three times in a day. It is not till the Plasmodium has collected into a thick firm mass, and is preparing for the produc- tion of spores, that it comes to the surface in places exposed to light, but appa- rently only in the night or early morning. The protoplasm which envelopes the grains of chlorophyll in the green leaves of Mosses and Phanerogams and in the prothallia of Ferns, is induced, by the varying intensity of the light, to accumulate to a greater or less degree at different parts of the cell-walls, carrying the grains of chlorophyll along with it, and thus altering their distribution in the cell. It is still uncertain whether in this case the light affects the protoplasm only, the grains of chlorophyll being carried passively along with it ; or whether the influence of the light is not first of all on the latter, which then give the impulse to the protoplasm. In either case it appears certain that the grains of chlorophyll do not of themselves possess any power of free motion, but are carried about by the motile protoplasm. Famintzin and Borodin^ found that under the influence of continued partial darkness the chlorophyll-grains in various Mosses and in the prothallia of Ferns collect on the side-walls of the cells (those at right angles to the surface of the organ) ; and that when these parts are exposed to light they leave them and distribute themselves over the parts of the cell- walls which are parallel to the surface of the organ. Prillieux^ and Schmidt have confirmed these statements. The view which I adopted long ago (see the first and second editions of this work), that these changes of position in the chlorophyll- grains are caused by the protoplasm, is confirmed by Frank's recent researches'*. He shows that when the light falls only from one side, the protoplasm and the grains of chlorophyll collect mostly on those parts of the cell-walls on which the strongest rays fall, if the cells are sufficiently large to allow the light to be so arranged, and these changes to take place in the position of their contents (as in the prothaUia of Ferns and leaves of Sagittaria). Frank brought under a general point of view the changes in position of the grains of chlorophyll described by Famintzin and Borodin; he shows that the protoplasm in these cells is capable, according to circumstances, of adopting two different modes of distribution. In one mode, which he calls Epistrophe, the protoplasm and chlorophyll-grains collect on the free cell-walls, z.e, those which do not immediately adjoin other cells; for ^ Sachs, Bot. Zeitg., 1863, Supplement. 2 Bohm, Sitzungsber. der Wien. Akad. 1857, p. 510.— Famintzin, Jahrb. fiir wissenf.ch. Bot. vol. IV, p. 49.— Borodin, Melanges biologiques; Petersburg, vol. VI, 1867. 3 Prillieux, Compt. rend. 1870, vol. LXX, p. 60.— Schmidt I.e. * Frank, Bot. Zeitg. 1872, Nos. 14, 15 ; and Jahrb. fur wissensch. Bot. vol. VIII, p. 216 et sej. 67Z GENERAL CONDITIONS OF PLANT-LIFE. instance, next the surface in the superficial cells of organs consisting of several layers (the leaves of Sagittaria, Vallisneria, and Elodea) ; on the upper and under walls in organs consisting of only one layer of cells (leaves of Mosses, prothallia of Ferns) ; and on the parts that bound the intercellular spaces in internal cells. This is the position assumed in the normal conditions of vegetation and the mature state of the cells, but before they become too old. The second mode, or Apostrophe, takes place under unfavourable external condidons ; as for instance in small fragments of tissue, when respiration is defective, turgidity diminished, the tem- perature too low, the cells too old, or — what is of most interest here — when light is cut off for a considerable time. Under these circumstances the protoplasm and chlorophyll-grains collect chielly on the walls that are not free, i. e. on those adjacent to other cells. The occurrence of apostrophe under direct sunlight which Borodin asserts^ (in various Phanerogams as Lemna, Callitriche, and Stellaria), is denied by Frank, who maintains that what takes place in these cases is rather a collection of the protoplasm at the spots where the light is strongest, which may happen to be at the sides. It is evidently these aggregations of chlorophyll- grains on the side-walls of the cells caused by sunlight which were observed by Borodin that produced the phe- nomenon pointed out by IMarquard and more exactly described by myself^, viz. that green leaves {e.g. those of Zea, Pelargonium, Oxalis, Nicotiana, &c.) when exposed to sunlight assumed a bright green colour in a shorter time than in diffused light or in deep shadow. This can be made very evident by shading particular parts by pressing closely on them a strip of lead or tinfoil; if this strip is removed after five or ten minutes, the parts that were shaded show a dull green, those exposed to the sun a bright green colour. It is obvious that the tissue will appear to the eye a deeper green in proportion as the green grains are distributed uniformly over the surfaces facing the eye, a less deep green in propor- tion as they collect on the side-walls. Borodin's observations directly confirm this hypothesis. This alteration in the grouping of the grains of chlorophyll which accompanies a change in the intensity of the light is caused only by the highly refrangible rays ; the less refrangible ra}'S (the bright and red ones) have the same effect as darkness^ It results therefore, as I showed in 1859, that if a strip of blue glass is laid on a leaf exposed to sunshine, it will produce no change of colour, while one will be caused by a strip of red glass. Since these movements of the grains of chlorophyll are produced by the colourless protoplasm in which they are imbedded, it might be expected that the protoplasm of hairs which contain no chlorophyll or only a small quantity would be similarly influenced by the colour and intensity of the light. But the state- ments of Borscow and Luerssen^ which might be interpreted in this direction at least to some extent have not been conflrmed by the observations of Reinke ^. ^ Borodin, Melanges bid., Petersburg, 1S69, vol, VII, p. 50. ^ Sachs, Berichte der math.-physik. Klasse der k. sJichs. Ges. der Wiss. 1859. ^ Borodin, I.e.; Frank, Bot. Zeitg. 1871, p. 238. * Borscow, Melanges biol., Petersburg 1867, vol. VI, p. 312.— Luerssen, Ueber den Einfluss des rothen u. blauen Lichts u. s. w.. Dissertation, B:emen, 1S68. 5 Reinke, Bot. Zeitg., 1871, Nos. 46, 47. ACTION OF LIGHT Oi\ VEGETATION. 673 The swarming of zoospores is also connected with protoplasmic movements. Their motile organs, the cilia, are supposed to be themselves only slender threads of protoplasm, by the vibration of which both the rotatory and the advancing move- ment of the zoospores is caused. The axis of rotation becomes subsequently the axis of growth; the anterior end in the advancing motion (where the zoospore is usually narrower, hyaline, and provided with cilia), becomes the base of the germinating plant when the zoospore has come to rest. These movements of zoospores and the very similar ones of Volvox are affected by light to this extent, that when the light comes from one side they either tend towards or away from the source of light, this depending apparently partly on the species and partly on the age of the individual. Here also Cohn states that the less refrangible rays have the same effect as darkness, while the direction of the motion is determined by the blue and therefore more highly refrangible rays\ (e) Cell-Division and Growth''-'. The first formation and early growth of the new organs in the higher plants consisting of masses of tissue is the result of a great number of cell-divisions, which usually take place in complete darkness; as, for example, in the roots of land and marsh-plants, the buds on underground rhizomes, and leaves and flowers which are produced within the dense envelopes of the bud. Cell-formation of the same kind may however take place under the influence of light which may even be intense, as is shown by the growth of the roots of land-plants in water exposed to light, or that of the aerial roots of Aroideae (which are highly transparent at their cell-forming apex). The formation of stomata and hairs which is the result of cell-division may take place either in the light or in complete darkness within the bud, without any essential difference being observable in the two cases. In the same manner the cambium of the trunks of trees is covered by completely opaque envelopes, such as bark; while that of many annual stems (as Impatiens) is exposed to the light which penetrates the thin succulent cortex. Similar phenomena are presented in the formation and ripening of ovules within transparent or completely opaque ovaries. They are most obvious when shoots or even flowers which under ordinary circumstances are deve- loped in the light are made to grow in complete darkness from bulbs, tubers, or seeds. The small variations from the normal condition which occur in such cases do not affect the early development of the organs; but their later growth which does not depend on cell-division is necessarily interfered with, as well as the development of chlorophyll. An obvious and necessary condition of these processes of growth, whether in the dark or the light, is the presence of a supply of assimi- lated reserve-materials, at the expense of which the formation of new cells can take place. In the case of the buds of the higher plants their reservoirs of reserve- material are the bulbs, tubers, rhizomes, parts of the stem, cotyledons, and endo- sperm ; after the complete exhaustion of these growth ceases in the dark but 1 Cohn, Schles. Ges. fur vaterl. Cultur, Oct. 19, 1865. The facts have however recently been questioned by Schmidt. ^ Sachs, Ueber den Einfluss des Tageslichtes auf Neubildung u. Entfaltung verschiedener Pflanzen-organe, Bot. Zeitg. 1863, Supplement. If I here consider cell-division and growth as essentially mechanical processes, this does not imply that chemical changes do not also accompany every process of growth. x x 6/4 GENERAL CONDITIONS OF PLANT-LIFE. continues in the light, because the assimilating organs can then produce new material. This relation of growth which is dependent on cell- division to assimi- lation, is especially clear in Algae of simple structure (as Spirogyra, Vaucheria, Hydrodictyon, Ulothrix, &c.), which assimilate in the day-time under the influence of light, while cell-division proceeds exclusively or at least chiefly at night. The swarmspores are also formed in the night, but swarm only with access of daylight. In some Fungi also, as Pilohohis cry stall inns, the splitting up of the protoplasm in the sporangium into a number of spores takes place only in the night, the spores being thrown out on access of light. While therefore in the larger and more highly organised plants assimilation and the construction of new cells out of the assimilated substances is carried on in diff'erent parts but at the same time, in smaU transparent plants in which the parts where these functions are effected are not surrounded by dark envelopes, they take place at diff'erent times. We have here a case of division of physiological work which shows us that the cells which have to do with chemical work (assimilation) cannot at the same time perform the mecha- nical labour of cell-division ; the two kinds of labour are distributed in the' higher plants in space ; in very simple plants in time. Provided there is a supply of assimilated reserve-material, cell- division can therefore take place either in the light or the dark. Whether there are special cases in which light promotes or hinders cell-division is not known with certainty. We might suppose wt have such a case when Fern-spores and the gemmae of Marchantia^ germinate in the light but not in the dark ; but Borodin has shown that the less refrangible rays are alone active in this process of growth, mixed blue light (passed through ammoniacal copper oxide) acting like complete darkness. But since the less refrangible rays, as we have seen, have exactly the same effect on growth as the absence of light, but on the other hand are the efficient agent in assimilation, it may be supposed that these spores and gemmae do not contain certain substances necessary for germi- nation, which must therefore be produced by assimilation. On the other hand it has not yet been explained on what depends the formation in long-continued dark- ness from many stems (as those of Cactus, Tropseolum, Hedera, &c,), of roots which are not produced under the ordinary amount of light. Whether the degree of humidity is an element in this is uncertain but not improbable. When the young organs emerge from the bud-condition, an active growth commences, which is chiefly occasioned by the absorption of water into the cells and by a corresponding superficial extension of the cell-walls, cell- division still taking place only occasionally or not at all. This process of elongation takes place, in the case of aerial stems and foliar structures, in the daylight which penetrates deep into the transparent succulent tissues. In order to estimate the amount of its influence on these processes, it is best to grow seedlings or shoots of the same species of plant in continuous complete darkness, and others under an alternation of day and night, especially in the height of summer. Independently of the fact that chlorophyll (with the exceptions already named) does not assume its green colour in the dark but remains yellow, difl'erences of form which are often ^ Borodin, Melanges biol., Petersburg, 1867, vol. VI; Pfeffer, Arbeiten des bot. Inst, in Wiirzburg, vol I, 1871, p. 80. ACTION OF LIGHT ON VEGETATION. (^n r very striking are exiiibited by plants grown in the dark, and constitute the bleached or etiolated condition. The internodes of etiolated plants are in general much longer than those of plants of normal growth; and the long narrow leaves of Monocotyledons are subject to the same change. On the other hand the leaves of Dicotyledons and Ferns usually (but not always) remain very small and do not completely outgrow their bud-condition, or exhibit peculiar abnormalities in their expansion. These peculiarities will be explained more in detail in Chap. IV. It is not necessary however to contrast etiolated plants with those of the normal green colour, in order to establish the influence of light on their growth. If plants of the same species are compared when grown in more or less deep shade with others grown in full daylight, these differences are still more conspicuous, varying ac- cording .to the intensity of the light. Different species are however affected to a different extent by etiolation ; the internodes of climbing plants, which are very long even under normal conditions, become much longer still in the dark; anil some leaves of Dicotyledons, as for instance those of the beet, become tolcrabJv large under the same circumstances, while on the other hand the abnormal! \ elongated internodes of etiolated potato plants put out leaves of only a very small size. It is remarkable that etiolation, as I have already shown ^, does not extend to the flowers^. As long as sufficient quantities of assimilated material have been previously accumulated, or are produced by green leaves exposed to the light, flowers are developed even in continuous deep darkness which are of normal size, form, and colour, with perfect pollen and fertile ovules, ripening their fruits and producing seeds capable of germination. The calyx however, w^hich is ordinarily green, remains yellow or colourless. In order to observe this it is only necessary to allow tulip-bulbs, the rhizomes of Iris, or the like planted in a pot, to put up shoots in complete darkness, when perfectly normal flowers are obtained with completely etiolated leaves. Or a growing bud on a stem of Cucurbita, Tropaeolum, Ipomcea, &c., with several leaves, is made to pass through a small hole into a dark box, the leaves which remain outside being exposed to as strong light as possible. The bud developes in the dark a long colourless shoot with small yellow leaves and a number of flowers, which, except in the colour of the calyx, are in every respect normal ^. The extremely singular appearance of these abnormal shoots with normal flowers shows in a striking manner the difference in the influence of light on the growth of different organs of the same plant. The retarding effect of light on the growth of the shoot is evident even in a short time ; and, as I have already briefly shown ^ a periodical oscillation in the rapidity of growth is caused by the alternation of day and night (when the tem- perature is nearly constant). This variation is shown by the growing internode ^ Sachs, in Bot. Zeitg. 1863, Supplement; and 1865, p. 117. 2 [An exception to this rule is afforded by the coloured kinds of lilac which are forced during the months of February and March by the market-gardeners of Paris, at a temperature of from '33° to 35° C, and in almost complete darkness. The flowers expanded under these conditions are completely white. See Duchartre, Journ. de la Soc Imp. et cent, d'hort. de France, i860, pp. 272- 280.— Ed.] 3 Sometimes however abnormal flowers appear in the dark as well as the normal ones. See Sachs, Exp.-Phys. p. 35. * Sachs in Heft II of the Arbeiten des Bot. Inst, in Wiirzburg, 1S72. X X 2 6/6 GENERAL CONDITIONS OF PLANT-LIFE. exhibiting a maximum of hourly growth towards sunrise, decreasing gradually from the advent of daylight till mid-day or afternoon, when it reaches its minimum, and increasing from this time till morning, when it again attains its maximum. Since the leaves of etiolated plants are much smaller than in the normal state, it might be expected that they would grow much more quickly in the day than in the night, or that the mechanical laws of their growth would be opposed to those of the internodes with respect to the influence of light. But it would be too hasty to come to this conclusion; for the objection might be made that normal leaves assimilate in the day, while they grow chiefly in the night \ One of the best-known phenomena occasioned in plants by light is the fact that growing stems and leaf-stalks, when the amount of light which they receive is very diff'erent on different sides, bend or become concave towards the side exposed to the most intense light. This curvature is caused by the slower growth in length of the illuminated than of the shaded side; and parts of plants which show this behaviour to light are called helioiropic'^. From the fact of heliotropic curvature towards the side which receives the most light, it is obvious that the plant would grow more quickly if shaded on all sides than if the light were more intense. The observation that leaves, some roots. Fungi, filamentous Algae (like Vau- cheria), &c., curve heliotropically, indicates that their growth is retarded by light. That the chlorophyll has no share in causing this heliotropism is shown by the fact that organs which contain none, like some roots, or Fungi, as the peri- thecia oi Sordaria fimiseda (according to Woronin), the stipes of the pileus of Clavi- ceps (according to Duchartre^), and colourless etiolated stems, bend towards a stronger light. Since most heliotropic parts of plants are highly transparent, the light which falls on one side must penetrate more or less to the other side, on which also some light falls ; it follows therefore that even inconsiderable differences in the intensity of the light which falls on the two sides must cause heliotropic curvature ; i. e. difference in the rate of growth *. If plants which show heliotropic properties are grown in a box which receives light from one side that has passed in one case through a solution of potassium bichromate, in another case through one of ammoniacal copper oxide, the internodes of the first remain quite straight and lengthen considerably as if they were in the dark, while those exposed to the mixed blue light grow less and at the same time bend strongly towards the light. It follows from this that only rays of high refrangibility, the blue, violet, and ultra-violet, cause the curvature by retarding growth^. ^ Compare infra, Chap. IV. Sect. 20. 2 Further details on heliotropism will be given in Chap. IV. [See also p. 190.] 3 Duchartre, Compt. rend. 1870; vol. LXX, p 779. * It must however be noted that in the case of parts containing chlorophyll the light in pene- trating the tissues loses its more refrangible rays which are the only ones that produce the effect ; as has been already shown, only the less refrangible rays pass through the superficial layers (see p. 665). ^ See Sachs, Bot. Zeitg. 1865, On the action of coloured light on plants, where the literature is also quoted. I consider experiments with absorbent fluids more decisive than those with the spec- trum ; in this latter Guillemin states that not only do all the rays act heliotropically, but that there is even a lateral curvature towards the blue end of the spectrum. When the light is sufficiently strong the spectrum is certainly never free from diffused white light, which will cause heliotropism even when its intensity is very small. ACTION OF LIGHT ON VEGETATION, 677 In addition to the large number of the parts of plants which, when illumi- nated unequally bend so as to make the more strongly illuminated side concave, there are a much smaller number which bend in the opposite direction, t. e. become concave on the shaded side. In order to distinguish between them the former are termed positively, the latter negatively heliotropic. Both positive and negative heliotropism occur not only in organs containing chlorophyll, but also in those that are colourless ; among the former in the green tendrils of Vitis and Ampelopsis^ ; among the latter in the colourless root-hairs of Marchantia^, the aerial roots of Aroidese, Orchideae, and Chlorophytiim Gayarnwi, and the rootlets of some Dicotyledons, as Brassica Napus and Siftapis alba^. From the fact that' positive heliotropism depends on a retardation of the growth of the organ exposed to the stronger light, it might be inferred that negative heliotropism is occasioned conversely by a more vigorous growth of the side exposed to the stronger light. This conclusion would be confirmed by a superficial examination of the phenomena; but if the attendant circumstances are observed more closely, some considerations arise which I shall examine in detail in Chap. IV. It need only be mentioned here that according to a theory started by Wolkoff, two different explanations are possible : — Very transparent organs, like the apices of the roots of Aroidese and of Chlorophytum refract the light which falls upon them in such a manner, that the shaded side of the organ may actually be more strongly illuminated than the other; and its negative heliotropism is then only a special case of positive heliotropism. But in other cases, as in the ivy and TropcBohim ??iajus, the internodes are positively heliotropic when young, but negatively when old before growth ceases ; and Wolkoff supposes that the curvature which is in these cases convex on the illuminated side is caused by the more vigorous assimilation and consequent longer duration of growth. It depends there- fore upon nutrition which only affects the mechanism of growth in a secondary degree. (/) Aciio7i of Light on the tension of the tissue of the coiitractile organs of leaves endowed zvilh motion^. The leaf-blades of Leguminosse, Oxalideae, Marantaceae, Marsileaceae &c., are borne on modified petioles which serve as contractile organs, bending upwards or downwards under various external and internal influences, and thus giving a variety of positions to the leaf-blades. If these plants are placed in permanent darkness, the curvatures due to internal changes alternate upwards and downwards. Light exercises an immediate influence on these peri- odically contractile organs ; any increase of its intensity tends to give the blade an expanded position, such as it occupies in the day-time ; any diminution tends to cause it to assume a closed position upwards or downwards such as it has m the night. This sensitiveness, which I previously termed ' paratonic,' is not the cause of the periodic movements ; but rather counteracts the periodicity caused by the internal forces. In most leaves endowed with periodic movements the paratonic 1 Knight, Phil. Trans. 181 2, Pt. I, p. 314- 2 Pfeffer, Arbeiten des hot. Inst, in Wiirzburg 1871, Heft I, Div. 2. 3 For the literature on this subject see Sachs, Exp.-Phys. p. 41- * See Sachs, Ueber vorhergehende Starrezustande, &c., Flora, 1S63. — Further details will be given in Chap. IV. 6jH GENERAL CONDITIONS CF PLANT-LIFE. influence of light is so strong that it neutralises them, and induces in their place a periodicity dependent on the alternation of day and night. In the lateral leaflets of the leaves of Desmodium gyrmis on the contrary the internal causes of the rapid periodic oscillations are so powerful as to overcome the paratonic sensi- tiveness; and these leaflets move upwards and downwards when the temperature is high even in spite of changes in the amount of light. My earlier researches^ show that it is only the more refrangible rays that excite paratonic sensitiveness, while red rays act like darkness. The influence of light on the position of the contractile organs is not however only of this direct character; the motile condition is also indirectly dependent on it. Both the periodic and paratonic movement, as well as th^.t of INIimosa when mechanically irritated — in fact, the power of movement in plants — is lost when they have remained in the dark for a considerable time, such as a whole day ; in other words, they become rigid by long exposure to darkness. From this rigid condition they do not immediately recover when again exposed to light; the ex- posure to light must continue for a considerable time, some hours or eveii days, before the motile condition which I have termed ' Phototonus ' is restored. It is only in this condition that the leaves are motile and sensitive to changes in the intensity of the light or to mechanical irritation. The paratonic curvatures of fully developed contractile organs caused by sensitiveness to light are distinguished from the heliotropic curvings of growing organs by the fact that, firstly, they are connected with phototonus, while the latter are not ; and secondly, that they ahvays take place in a plane determined by the bilateral structure, while the plane of heliotropic curvature depends only on the direction of the rays of light. Optical Properties of the Colouring matter of Chlorophyll. If the parts of plants that contain chlorophyll are repeatedly boiled in water and then quickly dried at a temper- ature not too high and pulverised, a substance is obtained which is easily examined and can be preserved for a long time unchanged. From this powder the green colouring matter can be extracted by alcohol, ether, or oil. The green solution is speedily changed by the action of light in proportion to its intensity, the less refrangible rays of the spectrum acting most actively and rapidly. It then assumes a dirty brownish yellow- green colour, the green colouring matter having become modified or lost its colour. If sunlight that has passed through a stratum of the pure green solution not too thick or too dark is decomposed by a prism, an extremely characteristic spectrum is obtained in which rays of very various refrangibility appear to have been more strongly absorbed the darker the solution or the thicker the stratum. This chlorophyll-spectrum has been the subject of much research ; the most recent and comprehensive being that of Kraus, from whose description I borrow the following^: — The spectrum of an unchanged alcoholic solution of chlorophyll shows seven absorption-bands, four of which are narrow (Fig. 447 A, I, II, III, IF), and are situated in the less refrangible half; while three {F, FI, FII) are broad and are situated in the more refrangible half. The latter, distinguishable as distinct bands only in very dilute solutions, coalesce, even in the solutions of medium concentration which are ^ Sachs, Ueber die Bewegungsorgane von Phaseohis und Oxalis, Bot. Zeit. 1857, P- Sii et seq. ^ Kraus. Sitzungsb.der yjhys.-med. Soc. in Eilangen, June 7 and July 10, 1871. See also Askenasy, Bot. Zeit. 1867, P- 225; Gerland und Rauwenhoff, Archives neerlandaises, vol. VI 1H71 ; and Gerland, Pogg. Ann. 1871, p. 585. [Kraus, Zur Kenntniss der Chloroph) llfarbstoffe u. ihrer Vcrwandten; Stuttgart, 1872. F"or reference to Mr. Sorby's papers see Sect, b a.] ACTION OF LIGHT ON VEGETATION. 679 ordinarily examined, into a single continuous absorption-band occupying the whole of the more refrangible half of the spectrum. The bands /, //, ///, and /rare situated in the red, orange, yellow, and yellow-green. The deep black band /, sharply defined on both sides, lies between Fraunhofer's lines B and C; the three others, shaded off on both sides, diminish in strength in the order of their numbers. Between these bands the illumination is dim, and progressively in the order of the numbers ; i. e. is less dim between // and III than between / and //, &c. To the left of /the light is undiminished. The bands T, VI, and FIl in the more refrangible half of the spectrum are shaded on both sides; Vis situated to the right of Fraunhofer's line F; FI, which is dark in the Fig. 447. — Absorption-spectra of the colouring matter of chlororophj-II (after Kraus). A the spectrum 01 the alcohohc extract of green leaves ; B that of the blue-green constituent soluble in benzol ; C that of the yellow constituent. The absorption-bands of A and A' are indicated in the less refrangible (left-hand) portion as they would be produced by a more concentrated, in the more refrangible (right-hand) portion of the spectrum as they would be produced by a less concentrated solution ; the letters B—G indicate the well-known Fraunhofer lines of the spectrum ; the figs. / — /V/ Kraus's absorption-bands in succession from the red to the violet end ; the spectra are divided into twenty equal parts. middle, to the left of and on the line G ; VII may be regarded as the total absorption of the violet end. The Spectrum of li'ving leaves agrees with that of the solution in its main character- istics ^ The bands / — F ?ire, according to Kraus, easily made out in all ordinary leaves of Dicotyledons, Monocotyledons, and Ferns. But this spectrum differs constantly from that of the solution in all its bands being ahvays nearer the red end ; a point which was determined by Kraus by the use of Browning's micro-spectroscopic apparatus. This difference in position of the absorption-bands of the spectrum is, as he shows, an illustration of the universal rule that the absorption-bands approach nearer to the red end in proportion to the specific gravity of the solvent of the colouring substance. It follows from this that the green colouring matter is distributed in such a manner in the colourless matrix of the chlorophyll-grains that it must be considered in a state of solution. In no case can the colouring matter of chlorophyll in living cells be in a solid state, or equivalent to the residue left behind when the solution is evaporated. * For further evidence of this very remarkable fact see Geiland und Rauwcnhoff, I.e., p It is not easy to understand how any physicist could maintain the contrary. 604. uSo GENERAL CONDITIONS OF PLANT-LIFE. If an alcoholic solution of chlorophyll (according to Conrad it must be very dilute^) is agitated with any quantity of benzol (say double its volume) two very sharply separated strata are formed after the fluid comes to rest, a lower alcoholic stratum of a pure yellow colour, and an upper blue-green stratum of benzol. Kraus considers this process to be a dialytic one ; there are, according to him, two colouring substances in the ordinary chlorophyll-solution, a blue-green and a yellow one, soluble in very different degrees in alcohol and benzol^. Kraus therefore holds the spectrum of chlorophyll to be a combination-spectrum, i. e. that it arises from the superposition of the two spectra of the blue-green and the yellow colouring-matter. The blue-green substance gives the four narrow absorption-bands in the less refrangible half of the spectrum (Fig. 447, B), and part of the band VI which is situated at G in the more refrangible half. The band F (Fig. 447 C) results from the yellow colouring matter which has absorption-bands in the more refrangible half of the spectrum. The band VI of the chlorophyll-spectrum is the result of partial superposition of corresponding bands in the spectra of the yellow and the blue-green substances, which however do not perfectly coincide. Both colouring substances alike produce the absorption-band VII at the violet end. The yellow colouring m.atter is soluble in alcohol, ether, and chloroform, but not in water. On addition of hydrochloric or sulphuric acid (as Micheli had already 5hown) it becomes first emerald-green, then verdigris-green, and finally indigo-blue; the spectrum of the yellow substance which has in this manner become gi-een shows altogether different absorption-phenomena to those of chlorophyll. The spectrum of the yellow ingredient of chlorophyll is identical with that of most yellow flowers (as Ranunculus, Mimulus, Gentiana lutea, Brassica, Taraxacum, Matricaria, &c.), and agrees with it also in the reactions just named, as also does that of the yellow colouring substance of fruits and seeds (Euonymus, Solanum, Pseudocapsicum &c.) ; this yellow substance is, like chlorophyll, combined with protoplasm. The substance present in the cells in the liquid form, as for instance in the flowers of the dahlia, is different ; it is soluble in water, and does not give a spectrum consisting of bands, but a continuous absorption of the blue and the violet. The colouring substance of some orange flowers, e.g. Eschscholtzia, also soluble in alcohol, is again different, possessing a fourth band in the blue-green to the left of the three bands of the ordinary yellow substance. The colouring matters of bright-coloured lower organisms which are soluble in alcohol are not identical with either of the two which constitute chlorophyll, but are related to them. According to Kraus, the yellow substance of etiolated leaves also exactly resembles the yellow constituent of chlorophyll; he considers the green colour produced by exposure to light to be the result of the formation of the blue-green constituent. The Fluorescence of the colouring-matter of chlorophyll is seen from the fact that a sufficiently dark concentrated solution appears dark-red by reflected but green by transmitted light. The fluorescence is much more decided if the pencil of converging rays of the sun is made to fall on the green fluid through a condensing lens. If the solar spectrum is thrown upon the surface of a solution of chlorophyll^, it may be ^ Kraus obtained a solution of chlorophyll by pouring alcohol upon boiled leaves which have contained water. Conrad shows that it is only such dilute solutions of chlorophyll (alcohol of 65 p. c. or less) that show Kraus's reactions ; and that on the contrary a solution obtained from dried leaves by absolute alcohol and then mixed with benzol does not separate into yellow and blue-green strata. ^ This is however rendered doubtful by Conrad's more recent researches. If a solution of chlorophyll in absolute alcohol is evaporated, the residue extracted with water does not contain a yellow constituent as it does when the solution is prepared with dilute alcohol. It is therefore not improbable that chlorophyll-green is decomposed by dilute alcohol, and that the two con- stituents of which Kraus supposes chlorophyll to consist did not exist before the operation an) more than those imagined by Fremy. •■ Hagenbach, Pogg. Ann. vol. 141, p. 245; Lommel, ih. vol. 143, p. 571. ACTION OF LIGHT ON VEGETATION. ' 68 1 ascertained w hich rays of the sunlight cause the fluorescence ; the red begins a Httle to the left of the line B of the solar spectrum, and stretches, although varying in inten- sity, over the violet end. On the dark-red ground are seen seven intensely red bands, each corresponding exactly both in position and in strength to an absorption-band in the spectrum of chlorophyll. If the fluorescence caused by the solution of chloro- phyll is itself observed through a prism, it is seen to consist only of red rays, the refrangibility of which coincides with the strongest absorption-band of chlorophyll between B and C. Every ray produces by fluorescence only such as correspond in their refrangibility to the absorption-band /. Whether the chlorophyll contained in living cells is subject to the same fluorescence is not certain, from the imperfect observations at present made ; but it is probable, from the absorption-phenomena and their connection with fluorescence. The question whether the absorption-bands of the spectrum of the colouring- matter of chlorophyll have any causal connection with the function of the chlorophyll- grains in decomposing carbon dioxide has recently been answered by Lommel in the affirmative, on purely theoretical grounds, in support of which he brings forward the following facts ^ : — ' The most efficacious rays in promoting assimilation in plants are those which are most strongly absorbed by chlorophyll, and which at the same time possess a high mechanical intensity (heat-action) ; these are the red rays between B and C But a glance at the carefully prepared tables given at pp. 667, 668, shows that this theoretical reasoning is incorrect. If Lommel's hypothesis were con-ect, the evo- lution of oxygen would be seen, on observing the solar spectrum, to attain its maximum between B and C^, which however, as Pfeffer has shown, is by no means the case. The second of Lommel's statements is : — ' The yellow rays can produce only a small effect notwithstanding their considerable mechanical intensity, because they are absorbed only to a small extent ; and the same is the case with the orange and green rays.' This statement is again entirely opposed to observation; for it is these very rays that are the most efficacious in promoting evolution of oxygen. Lommel says indeed Q.c. p. 584) that 'this inference is incorrect'; it is however no inference, but the result of actual observation. That the light which has passed through a solution of chlorophyll causes only an inconsiderable evolution of oxygen is easily explained when it is recollected that even the yellow is considerably weakened in the spectrum of chlorophyll. But according to Lommel's theory there ought to be no evolution of oxygen at all when light has passed through a solution of this kind if it shows the absorption-bands very dark, since those rays which according to him are alone efficacious are wanting. There is however no need for this direct contradiction; for a correct estimate of known facts leads to the conclusion that it cannot be those rays which are ab- sorbed by the colouring matter of chlorophyll that cause the evolution of oxygen; for the rays absorbed in such a solution are the same as those absorbed in a green leaf (see p. 679). In the former there is however no evolution of oxygen (and ap- parently also no oxidation) ; and there is nothing to justify the supposition that the same rays which the colouring matter of chlorophyll absorbs in solution without causing evolution of oxygen should cause it in the living leaf. It must certainly be right to suppose, as a necessary result of the principle of the conservation of energy ^ that the rays which are efficacious in causing evolution of oxygen must be absorbed, 1 Lommel, Pogg. Ann. Vol. 143, p. 581 et seq. 2 Muller, (Botan. Beobachtungen ; Heidelberg 1871, Heft I) has adduced a great array of figures in support of this conclusion. But any one who knows how such observations should be made knows also what value is to be attached to these. See also Pfeflfer, Bot. Ze.t. 1872, No. 23 el seq. ■^ See also what I said on this subject seven years ago in my Experimental Physio.ogy, p. 2^7. 0(^: GENERAL CONDITIONS OF PLANT-LIFE, inasmuch as they perform chemical work ; but observation shows that it is not the rays absorbed by the green colouring matter that perform this work either in the solu- tion or the living plant \ T/6f Relation of Cell-di-vision to Light has, as I have already explained, been completely misunderstood by Famintzin. In my paper 'On the influence of daylight on the formation and unfolding of various organs of plants' (Bot. Zeit. 1863, Supplement), I described in detail a long series of processes which show that' the fresh formation of parts connected with cell-division is in general independent of light as long as there is a supply of reserve food-material to support growth. The main results were again collected in my 'Handbook of Experimental Physiology,' p. 31, referring also to that paper. Notwithstanding this, Famintzin^ commences his paper quoted above (three years later than one, and five than the other of my works) with the words: ■ — ' The action of Hght on cell-division has not yet been carefully examined by any one. All that I have been able to find on this subject is limited to a remark of A. Braun's on Spirogyra and a statement of Sachs relating to cell-division in general.' He then quotes a passage from Braun cited also by me, and continues:— 'Basing his remarks on these statements, Sachs expresses himself as follows,' and then quotes some passage from my Handbook, p. 31, no reference being made to the earlier paper or its conclusions. He then maintains that his own observations lead to entirely different results; but it is easy to show that they rather lead to the same as mine. At the end of his memoir (p. 28) he says: — 'The cell-division of Spirogyra is not prevented by light, as has hitherto been supposed, but on the contrary is promoted by it' (which is incorrect). According to Famintzin's observations, this acceleration of cell-division by light depends on the fact that light induces the assimilation of food- material ; which is obviously a different question from that argued by me and opposed by him ; since, presupposing the presence of a supply of food-material, I only argued the question whether light exerts any influence on the physical fact of cell-division. 'The cell-division of Spirogyra,' continues Famintzin 'has been proved to be de- pendent on light to the same extent as the formation of starch; but the relationship in the former case diff"ers from that in the latter in the following respect:— the formation of starch is induced by a very brief exposure to light (about half an hour) and re- quires that its action be direct; starch is formed only under the influence of light; in its absence the formation at once ceases. Cell-division, on the other hand, is induced only after light has acted for some hours; it then commences in the cells whether these have been exposed to light for some time or have been removed into the dark.' This shows therefore that when food-materials are formed cel'-division takes place in the light as in the dark ; a fact w^hich I had proved five years before by a great number of observations. Better in more than one respect is Batalin's treatise ' On the action of light on the development of leaves' (1871)^. Starting from the facts discovered by himself and by Kraus that cells have the same size in small etiolated leaves as in large leaves of the same species grown in light, he concludes with justice that the number of cells is larger in the normal than in the etiolated leaf, and that the size of leaves is proportional to the number of cells in them. But from this he draws the following erroneous conclusion : — ' The leaf grows so long as it produces new cells ; and the growth of the leaf does not depend on the increase in size of the cells.' It should rather be, — ' The growth of the leaf depends firstly and directly solely on the increase in size of the cells, and is proportional to this ; but the cells, when they have grov/n larger, divide so that they are actually of about the same size in the small etiolated ^ Gerland (/. c. p. 609) has also arrived at a similar conclusion. ^ Famintzin, Melanges phys. et chim., Petersbourg 1868, vol. Vll, On the action of light on the cell-division of Spirogyra. 3 Batalin, Bot. Zeit. 1871, p. 670. ACTION OF LIGHT ON VEGETATION. 683 as in the large green leaf.' He continues : — ' Leaves do not grow in the dark because their cells cannot divide without the assistance of light;' while the exact converse is the fact,— they do not divide because they do not grow. This error prevails throughout the whole treatise, which in other respects contains a number of instructive observations. It must be observed in addition that the very small growth of leaves in the dark is not a universal phenomenon even amongst Dicotyledons. The leaves produced from the tuberous roots of the dahlia and beet grown in the dark, and even those of Phaseolus attain very considerable dimensions, and sometimes, especially when the temperature is high, almost the size of those developed in the lights Contri'vances for observing plants in light of deferent colours (or of different refrangi- bility). In order to allow light of different degrees of refrangibility to act upon plants, three methods may be adopted:— (i) The use of the spectrum; (2) The removal of particular rays by absorbent media (glass or fluids); and (3) Coloured flames. (i) If a ray of light is decomposed by passing it through a prism, it is possible to expose small plants or parts of plants to the action of narrow zones of the spec- trum ; and hence to allow light of approximately equal refrangibility to act upon them. Draper, Gardner^, Guillemin, and Pfeffer, have worked in this manner. In using the spectrum it must however be observed that the intensity of the light in its different parts is less than that of the light that passes through the slit in pro- portion to the length of the spectrum. If the spectrum at the distance from the prism where the observation is made is, for instance, 200 mm. long, but the slit only 1 mm. broad, the mean intensity of light of the whole spectrum is only 7200 of that which passes through the even slit, if no light is otherwise lost, which is seldom the case. Only a small luminous intensity must therefore be expected in the spectrum. In order to obviate this difficulty, it is necessary that very intense light pass through the slit, which may be effected by the use of condensing lenses. If, as is usually the case, sunlight is employed, the ray to be decomposed must be kept in a fixed position by a heliostat, or at least by a moveable mirror. (2) Absorbent tnedia. The defects which have been mentioned in observations with the spectrum, as well as the considerable cost of a heliostat, are avoided when coloured light is obtained by means of absorbent media. For this purpose discs of coloured glass or strata of fluids enclosed between colourless glass plates may be used. These last possess the advantage that almost any required amount of space may be illuminated by the light in question, and that the transmitted light only loses so much in intensity as is due to the small amount of absorption of the transmitted rays by the coloured medium. It is a mistake, though a very common one, to think that observations made with coloured screens are less exact than those made with the spectrum; in general it is just the reverse; and which method should have the preference must be decided in each case. The use of absorbent media is always subject to the disadvantage that they do not generally transmit light of a single colour, but several different kinds of rays. This disadvantage is especially the case with coloured glass plates; and, with the exception of the deep red ruby and the very dark blue cobalt glass, there are scarcely any kinds which answer our purpose. It is more practicable to obtain coloured fluids of the desired quality, although here also the number that can be used is small. The two which have been already mentioned are particularly useful, inz. a saturated solution of potassium bichromate, and a dark solution of ammoniacal copper oxide ; by means of these, with the right concentration and thickness of the stratum, experi- ments can be contrived so as to split white daylight exactly into two halves, the first solution transmitting the less refrangible rays from the red to the green, the ^ See infra. Sect. 20. 2 Gardner, Froriep's Notizen 1S44, vol. 30, No. 1 1. —Guillemin, Ann. des Sci. Nat. 1S57, vol. VII, p. I Co. 684 GENERAL CONDITIONS OF PLANT-LIFE. blue solution all the more refrangible rays from the green to the ultra-violet. Those fluids also are of great use which transmit the whole spectrum with the exception of a few groups of rays as sharply limited as possible. If certain phenomena occur when plants are exposed to light transmitted through these solutions, it is certain that they are not caused by rays of that particular refrangibility which are absent, and 'vice 'versa. It is obvious that absorbent media are of use in experiments only when the spectrum of the light that passes through them is accurately known. Glass plates are employed as windows in dark boxes closed on all sides in which plants are placed ; coloured fluids can also be employed for the same purpose by enclosing them between parallel plates of glass and using these as a window. When it is not necessary to allow light to fall in parallel rays upon the plant, the most convenient use of coloured fluids is to fill with them the space between the two walls of a double glass bell which is then placed like an ordinary bell-glass over the plants to be observed. For microscopic observations in coloured light I employ boxes like that represented in Fig. 445 (p. 658); only that instead of the colourless plate of glass, a double window is used, the space betw^een the two panes being filled with coloured fluids. (3) Coloured Flames — i.e. the light of bodies in a finely divided state heated to incandescence in a flame which is itself non-luminous — have not hitherto been employed for accurate observations on plants. I know only of one statement by Wolkoff"^ ; that etiolated seedlings of Lepidium sati-vum became green when placed for seven or eight hours at eight inches distance from a non-luminous gas flame in which sodium carbonate had volatilised and become incandescent. This light, as is well known, consists only of rays which correspond to Fraunhofer's line D. The red light of the flame of lithium or the blue light of that of indium &c., may be employed in the same manner as this yellow flame, if sufficient intensity and the necessary perma- nence can be attained with these flames-. [The foregoing account would be incomplete without some statement of the results attained on this subject by Mr. H. G. Sorby. The following is a brief abstract, sup- plied by him, of investigations which will be found reported in detail in his published papers-^: — Vegetable colouring-matters may be divided into two principal classes, fundamental and accidental. The fundamental are those which are essential to the healthy growth of the plant ; and by carefully studying the position of the absorption-bands in living leaves these substances are often found in a free and solid state, even when they are soluble in water, or could easily combine with the closely associated oils or wax. When set free by boiling in water or by decomposition, they dissolve according to their pro- perties in this respect in water, or combine with oil or wax if these be present. The petals and other portions of the organs of reproduction often contain some of the fundamental colouring-matters of the leaves, but frequently others are developed. Accidental colouring-matters are those which may be present or absent without apparently interfering wMth the healthy growth of the individual plant, and are often so conspicuous as to make mere colour of very little importance if it depend upon them, and not on the diff'erence in the kind or relative proportion of the fundamental colouring- matters. These non-essential substances are far more common in the petals than in the leaves, and if of any use to the plant, are only indirectly advantageous, as, for in- stance, in attracting insects. It is doubtful to which of these two divisions certain ^ Wolkoff, Jahib. fur wiss. Bot. 1866, vol. V, p. 11. ^ [The most recent researches on the spectrum-analysis of the green colouring matter of plants is by Chautard in Ann. de Chim. et de Physique, Sept. 1874. — Ed,] ^ [Proceedings of the Royal Society, vol. XV. 1867, p. 433. — Quarterly Jonraal of Microscopical Science, vol, IX, 1869, p, 358; vol. XI, 1871, p. 215. — Monthly Microscopical Journal, vol III, 1870, p. 229; vol. VI, 1871, p, 124.— Proceedings of the Royal Society, vol. XXI, 1875, p .142.] ACTION OF LIGHT ON VEGETATION. 685 substances should be referred, and perhaps some may not be essential for the healthy performance of vital functions, but merely necessary products; and some may be essential to one plant and not to others. It has been found convenient to arrange the colouring-matters of plants in the following groups, which are as it were of generic value, and include several different species.- Chlorophyll group.— The green substance described as chlorophyll by many writers must often have contained two perfectly distinct green substances, and the product of the action of acids on one of them, mixed with one, and in some cases with three, different species of xanthophyll, and one or two of lichnoxanthine. These two green substances are b/ue chlorophyll and yellow chlorophyll^. Blue chlorophyll dissolved in alcohol is of a splendid blue-green colour, the whole of the green part of the spectrum and a considerable part of the contiguous blue being readily transmitted. Tello^o chlorophyll absorbs the whole of the blue and the blue end of the green, so that the general colour is a bright yellow-green. Chlorofucine is of a clear yellow-green colour. It has many properties in common with the above-named two kinds of chlorophyll, being, like both of them, highly fluorescent and easily decomposed into another modifi- cation by acids. All three are insoluble in water and soluble in absolute alcohol, but not always in carbon bisulphide. The difi"erence between their spectra will be better understood by means of the fol- lowing figure, 447 b, which represents the absorption-bands as seen in solutions diluted so as to show those at the blue end, and only the darkest and most characteristic of those in the red. Red end. Blue end. Blue chlorophyll Yellow chlorophy Chlorofucine. Fig. 447 b. — Spectra of the chloropliyll group compared. Xanthophyll group. — This group includes a number of yellow or orange-coloured substances, insoluble in water but soluble in carbon bisulphide, giving spectra with two more or less well-marked absorption-bands in different positions, according to the particular species. They are not fluorescent, and when dissolved in absolute alcohol, after addition of a little hydrochloric acid, they all gradually become colourless, but two of them are first changed into a blue substance. Nearly all green leaves contain three perfectly distinct fundamental species, which Mr. Sorby has named orange-xanthophyll, xanthophyll and yellow xanthophyll. The spectrum given in Fig. 447 (P- 679), copied from Kraus, must have been due to a mixture of the latter two. Olive Algae contain another fundamental species, fucoxanthine. In many Fungi, and in the petals of flowers, occur other more orange-coloured species, of which that in Peziza aurantiaca is a good example. Sorby adopted the name proposed by Kraus ^ for a still more red orange. 1 [The spectrum given by Kraus (Fig. 447 B, p. 679), is due to a mixture of these with some of the products of the action of acids. — Ed.] ^ Chlorophyllfarbstoffe, p. 109. 6cS6 GENERAL CONDITION OF PLANT-LIFE. coloured species; but what Kraus describes as phycoxanthine must have been a mixture of this substance with fucoxanthine and lichnoxanthine. The difference between the spectra of some of the above-named species \\\'\ be better understood by means of the following figure (447 f), which represents those of the solutions in carbon bisulphide. Red end. Blue end. Phycoxanthine Peziza xanthine .... Orange xanthophyll. . Xanthophyll Yellow xanthophy Fig. 447 r. — Spectra of tlie xauthophyll group compared. ^Vhen these various substances are dissolved in benzol, their absorption-bands are all equally raised towards the blue end, so that we appear to have a remarkable series of very closely related substances. Lichnoxanthine group. — The colouring-matters belonging to this division are insoluble in water, soluble in absolute alcohol, and sometimes also in carbon bisulphide. They all give spectra without bands, and absorb more or less from the blue end. Some are yellow, and others so red that they may be .called lichnoerythrines. Lichnoxanthine occurs in both the highest and lowest classes of plants, but the whole group is more especially developed in Lichens and Fungi. It is not yet possible to say what part they play in the economy of plants, and in some cases they are probably only products of the oxidisation of chlorophyll and resins, from which they may be prepared artificially. We now come to a number of different groups, soluble in water but insoluble in carbon bisulphide. Phycocyan and Phycoerythrine groups. — There are at least five distinct colouring- matters included in these two groups, which differ from one another in many well-marked particulars. The phycocyans are highly fluorescent, but the phycoerythrines little if at all. They give remarkable spectra with one main absorption-band. Some are con- nected with albuminous substances in much the same manner as the hccmoglobin of blood, being like it decomposed at exactly the same temperature as that at which albumen coagulates, whilst the others appear to be associated with some different but related substance. They are especially characteristic of red Algae, but also occur in a few Lichens. Erythrophyll group. — The colouring-matters belonging to this group are very numer- ous, and their production often depends upon obscure and accidental causes, easily modified by slight variations in the internal or external conditions. They may be divided into three well-marked sub-groups, according as they are changed by the action of sodium sulphite. They are soluble in water, and are usually, if not always, dissolved in the juices of the plant, and disseminated in cells of various kinds. A greater number ACTION OF LIGHT ON VEGETATION. 6(S7 of different species occur in the petals than in the leaves. They are usually indicative of low constructive energy, but yet are not products of merely chemical decomposition. Chrysotannin ^ro«/>.— Much remains to be learned with respect to these more or less pale yellow or even colourless substances, and the part they play in plant-life. The most striking fact connected with them is that when oxidised they give rise to the various brown substances which are the cause of many of the characteristic tints of autumnal foliage. These changes are mainly, if not entirely, due to chemical action, and can easily be imitated artificially. Exposure to a greater or less degree of light may produce a great quantitative or even qualitative difference in the colouring matters. Rudimentary petals and rudi- mentary leaves correspond closely, but subsequently development takes place in two different directions; and very often when the petals of the more highly developed varieties are only partially grown, the constituent colouring-matters are both qualitatively and quantitatively the same as those in some other variety, as though this were due simply to a natural arrest of development. By growing almost in the dark flowers coloured by more or less of the orange species of the xanthophyll group, the petals are obtained of the full size, but only yellow and corresponding exactly to the normally yellow variety ; and there is this remarkable peculiarity, that the relative proportion between the different colouring-matters approximates more or less closely to what is obtained by exposing to light a solution of those found in the normal petals ; that is to say, absence of light tends to prevent the formation in the petals of those more orange-coloured substances which are the most readily decomposed by exposure to light when they are dissolved out from the petals.] ^ Sect. 9. — Electricity^ The chemical processes within the cells of a plant, * [The occasional occurrence of ' chlorophylloid green colouring-matters' in the tissues of animals is a matter of considerable significance. Mr. E. R. Lankester has obligingly drawn up the following list of such cases. Those marked with an asterisk have been observed by him with the spectroscope for the first time: — Infusoria; S/enlor Mulleri and others. Foraminifera. Radiolaria; Rhaphidiophrys viridis, Helerophrys myriapoda (^Quart. Journ. Micr. Sc. 1S69). Coelenttrata ; *Spon- giUa fliivia'ilis (Journ. Anat. and Phys. 1869), ^Hydra viridis, Anthea cereiis var. sma7'agdina (chloro- fucine). Vermes ; Mesostoinum viride (Planariae), ^ Bonellia viridis (in the skin), "^ChiEtqpfenis Valenciennesii (in the walls of the alimentary canal). Crustacea ; *Idotea viridis (Isopoda). The chlorophylloid substance is not present in the same physical or chemical condition in all these cases. In Rhaphidiophrys, lieterophrys, Spongilla, and Hydra, it is localised in granules imbedded in the protoplasm : this is also the case in Bonellia, but the granules are finer. In Idotea it is not in granules but diffused in the chitino-calcareous integument. In all cases the chlorophylloid sub- stance agrees in having a strong absorption-band in the red— a little to the right or left; and, except in Idotea, in being soluble in alcohol ; and in having strong red fluorescence and in finally losing its colour when dissolved. In Bonellia, Choetopterus, and Spongilla, the absorption-spectrum presents differences in other respects in each case, and the green tint is itself different — being black olive-green in Chaetopterus, bluer but equally dark in Bonellia, and apple-green in Spongilla and Idotea. In Spongilla the green colour is not developed if the animal grows in the dark. But like etiolated vegetable tissues, Spongilla, when immersed in strong sulphuric acid, gradually deve- lopes a strong leaf green colour, fully as intense as that of the naturally green specimens (Quart. Journ. Micr. Sc. 1874, P- 40o)- Bonellia, on the other hand, always lives in a dark hole excavated by it in calcareous rock, and Chaetopterus lives in a thick opaque tube. — Ed.] 2 Villari, Pogg. Ann. 1868, vol. 133, p. 425.— Jiirgensen, Studien des phys. Inst zu Breslau, 1S61; Heft I, p. 38 et seq.—Ueidenhain, ditto 1863, Heft 2, p 65.— Briicke, Sitz-ungsb. der Wien. Akad. 1862, vol. 46, p. I. — Max Schultze, Das Protoplasma der Rhizopoden; Leipzig, 1863, p. 44.— Kiihne, Untersuchungen iiber das Protoplasma, 1864, p. 96.— Cohn, Jahresber. der schles. Ges. fur vater- landische Cultur 1S61; Heft i, p. 24.— Kabsch, Bot. Zeit. i86r, p. 358.— Riess. Pogg. Ann. vol.69, p. 288.— Buff, Ann. der Chem. u. Pharm. 1854, vol. 89, p 80 et seq.—[]. Ranke, Untersuchungen iiber Pflanzenelektricitiit, Akad. der Wissen. Miinchen, Math -Phys. Klasse, July 6, 1872.] 6.S8 GENERAL CONDITIONS OF PLANT-LIFE. the molecular movements connected with the growth of the cell-wall and protoplasm, and the internal changes on which the activity of the protoplasm depends — whether exhibited in the formation of new cells or in movements of rotation — are probably connected with disturbances of the electrical equilibrium, although no actual em- pirical proof of this has yet been obtained. The fluids with different chemical pro- perties in adjoining cells, the diffusion of salts and of assimilated compounds from ce'.l to cell, and their decomposition, must also bring electromotive forces into play ; but even this has not yet been observed directly. Even the electrical currents which must no doubt be set up by the evolution of oxygen from cells containing chlorophyll, by the formation of carbon dioxide in growing organs (as in seed- lings), and by the transpiration of land-plants — although investigated by a few physicists — has not yet been actually established or accurately determined. Accord- ing to Buff's careful observations, which have been confirmed by Jiirgensen and Heidenhain, the internal tissue of land-plants is always electro-negative to its strongly cuticularised surface ; the surface of roots, saturated with sap (like a trans- verse section of the tissue), is also electro-negative to the surface of the stems and leaves. If a plant or a cut part of a plant is placed, with the necessary precautions, in the circuit of a very sensitive galvanometer, a current passes from the external surface to the cut surface or to the surface of the root ; this is in consequence of the contact of the cell-sap of the surface of the root or of a cut surface with the pure water employed to complete the circuit. The alkaline fluids of the thin-walled phloem of the fibro-vascular bundles are surrounded by the acid fluids of the paren- chyma, and become completely mixed by diffusion-currents. This behaviour, which must certainly produce electromotive effects, has not hitherto been investigated with this object \ The leaves and branches of plants present a large surface to the air ; and the tissue of the whole plant is permeated with electrolytic fluids. These phenomena appear to adapt plants to be the medium for equalising electrical differences between the earth and air by means of currents traversing the plant. Since therefore the electrical tension of the air is generally different from that of the earth, and the relationship of the two is constantly varying with changes of weather, it may be assumed that in all probability constant electrical interchanges are going on through the agency of plants^. Whether these have a favourable effect on the processes of vegetation has at present, like the whole subject, not been investigated scientifically. The destructive discharges of atmospheric electricity which are effected through trees by means of flashes of lightning"^, at least show that smaller differences of electrical equilibrium between the air and earth may also be equalised by means of plants*. ^ Sachs, Ueber saure, alkalinische, und neutrale Reaction der Siifte lebender Pflanzen ; Bot. Zeit. 1862, No. 33. ^ [Becquerel thought that the evaporation from leaves forms an upward current of vapour which acted as a conductor to electricity. In this way, by destroying the necessary electrical conditions, he thought forests tended to dissipate hail-clouds. Mdm. de I'lnst. vol. XXXV, pp. 806, 807. — Ed.] ^ [The disruptive effect of lightning upon trees is probably due to the sudden conversion of moisture into steam. See Osborne Reynolds, Proc. Phil. Soc. Manch. 1874, p. 15. — Ed.] * [Edwin Smith (Chemical News, Dec. 17, 1869) has detected constant currents of electricity passing in certain diiections in plants, as follows : — In a cut piece of leaf-stalk (rhubarb) from the ACTION OF LIGHT ON VEGETATION. 689 The researches on the action of the electric stimulus on the movements of protoplasm and of leaves the motion of which is caused by tension of the tissues, have not at present led to any important result from a physiological point of view, although distinguished observers have paid attention to this subject. It can only be said in a general way that very weak constant currents or induction-shocks (for a short time) produce no perceptible effect ; that sufficiently strong electromotive force produces effects on the protoplasm and in the contractile tissues similar to those produced by a high temperature and by mechanical means ; and that finally, when the strength of the current is still further increased, the protoplasm is killed and the motility of the leaves permanently destroyed, but sometimes in the latter case without causing death. Jurgensen allowed the current from a battery of small Grove's elements, the force of which was regulated by a rheochord, to act under the microscope on the tissue of a leaf of Vallisneria spiralis. A constant current from one element produced no perceptible action ; two or four elements caused a retardation of the protoplasmic movement, and when continued for a longer time completely stopped it. When the current was interrupted, the movement, if it had only been retarded, was restored to its original rapidity after the lapse of a short time ; if it had entirely ceased, it was not recom- menced even if the current was at once stopped. When the movement is thus arrested, the grains of chlorophyll which are carried along by the very watery protoplasm accu- mulate at different spots. A current from thirty elements causes permanent cessation of the movement even if the connection is only momentary. Induced currents act like constant ones ; but the number of induction-shocks which pass through the cells in a unit of time appears to have no considerable influence on the action. The changes of form of protoplasm under the influence of a sufficiently strong elec- tric current are, according to the observations of Heidenhain, Briicke, Max Schultze, and Kiihne, similar to those caused by a high temperature near the extreme limit or beyond it. From those of Kiihne it appears to result that protoplasm is a very bad conductor of electricity, and that the excitement caused by a current at particular spots in the protoplasm is not easily transferred to other spots. Cohn, Kabsch, and others, state that weak induction-currents act on the sensitive parts of the leaves of Mimosa, the stamens of Berberis, Mahonia, and Centaur ea Scabiosa, and the gynostemium of Stylidium graminifoUiim like concussion or contact, the parts moving as if under the influence of these agencies. According to Kabsch, stronger induction-currents, which permeate the whole plant, destroy the sensitiveness of the gynostemium of Stylidium even for mechanical excitation ; but after half an hour the sensitiveness again returns. The statement of Kabsch is noteworthy that the move- ment of the leaflets of Desmodium gyrans are permanently prevented by stronger in- duction-currents, which however do not kill them. end nearest die root to the end nearest the blade of the leaf; from the outer side of the leaf-stalk nearest the cuticle to the inner axis ; from the lower end of the flower-stalk (p?eony) to the biact or petal ; from the upper to the under surface of the leaf; in the stem diawthorn)from the cambium to the outer cuticle; in the root (several plants) from the outside to the axis, and from the root-stock towards the apex; in the hollow stems of monocotyledonous plants (grass) from the inner to the outer surface ; in the potato from the centre to the outside ; but in the lemon, pear, gooseberry, and turnip from the outside to the centre ; in a living plant (^Tropoeolum) from the plant itself to the soil. Dr. Burdon-Sanderson has made a remarkable series of observations on the electric currents in DioTKEa muscipula (see Report of British Association for 1873; also Nature, vol. VIII, p. 479 and Proc. Roy. Soc. vol. XXI, p. 495). By the aid of Thomson's galvanometer he has shown that these currents are subject, in all respects in which they have been as yet investigated, to the same laws as tl.ose of animal muscle and nerve. — Ed.] Y y '^'V^^ GENERAL CONDITIONS OF PLANT-LIFE. Sect, i o. — Action of Gravitation on the Processes of Vegetation ^ Since the attraction of the earth acts uninterruptedly on all parts of the plant, the entire vegetable organisation must be so contrived that the weight of the separate parts of the plant is serviceable, or at least not injurious to the various purposes of the life of the plant. In observing these relaiionships the first thing is to distinguish between those contrivances which l^ave for their object to bring the weight of the parts of the plant into harmony with the purposes of its life — gravitation itself not taking any direct recognisable part in the attainment of these objects' — and those phenomena of vege- tation on the other hand which are brought into existence by the direct influence of gravitation on the mechanism of growth. To the first of these groups belongs the fact that the branches and foliage of upright stems are distributed nearly equally on all sides, and. that in larger plants the firmness and elasticity of the masses of tissue in the stem is promoted by the form- ation of wood, or is brought about by other means, as for instance in the trunk of Musa. But since it is very common in the organic world for the same purpose to be attained by very different means, slender delicate stems with but little wood can protect themselves from sinking down and can expose their foliage to the light by twining round firm supports, or by climbing with the help of tendrils, hooks, spines, &c. The same purpose is evidently served by the various floating contriv- ances of water-plants and those of fruits and seeds ; in all these cases the structure is obviously adapted to make the weight of the part of the plant serviceable or at least not injurious to its life ; although it cannot be maintained that gravitation takes any part in the formation of wood, in the sensitiveness of tendrils, or in the produc- tion of a floating apparatus. The only explanation of these arrangements lies in Darwin's Theory of Descent ; viz. that, under the influence of long-continued natural selection, only those structures are finally able to maintain their existence which, while sufficient for the other requirements of life, are so arranged that the weight of the part is not injurious or is even useful. It must not be inferred from this, nor does observation render it probable, that gravitation takes any direct part in these phenomena. Gravitation however exerts a direct influence on the growth of young parts of plants as soon as the longitudinal axis of the growing organ is inclined obliquely to the perpendicular and therefore to the action of gravitation. In this case the growth in length of the oblique organ is different on the upper and under sides, and the more so the more nearly horizontal the axis of growth. According to the nature of the organ and its purpose in the economy of the plant, either the upper side grows more strongly than the under side, or the reverse. A curvature concave either downwards or upwards is thus caused by the influence of gravita- tion and growth, and this curvature increases until the free-growing end is directed vertically either downwards or upwards ; the former, for example, in primary roots, the latter in many primary stems. In lateral branches, leaves, and secondary roots. ' These statements are intended in the first place to draw the attention of students to the pro- cesses of vegetation which are especially influenced by gravitalion. Its action on the mechanism of growth will be fully described in Chap. IV, where also the literature is (quoted. ACTION OF GRAVITATION ON VEGETATION. 69 1 similar phenomena occur, though not so strongly. Internal processes of vegetation, the weight of the upper parts, or the influence of light, act in opposition to that of gravitation, so that conditions of equilibrium arise which cause the organs to stand horizontally or obliquely to the perpendicular. Thus the vertical direction of primary roots and stems, and the oblique direction of their lateral branches, are determined solely by gravitation, or at any rate to some extent, so long as these parts are still growing ; when they subsequently become lignified or cease to grow, they maintain the position once acquired. If therefore a growing plant rooting in the ground (inside a pot) is placed horizon- tally, the mature parts remain in this position ; but the apex of the primary root turns downwards, and the growing internodes of the end of the stem turn upwards, the leaves, branches, and secondary roots also bend until they make about the same angle with the horizon that they did before the change in their position. The parts which were actually growing when the change was made are shown by the curvatures caused by the influence of gravitation. Although we must defer till the fourth chapter the consideration of the internal changes which accompany these curvatures, the proofs that they are really caused by gravitation may be presented in the two following forms : — (i) Individuals of the same species have everywhere on the earth's surface the same position with respect to the horizon, and therefore also with respect to the earth's radius. Upright stems therefore, such as pines, grow in South America in totally different directions from what they do with us ; if their axes of growth were elongated downwards, they would intersect in the centre of the earth, and coincide with its radii. It follows therefore that their direction of growth must be determined by a force which stands in a perfectly definite relation to the position of the earth's centre of gravitv. But there is only one such force, viz. gravitation or the attraction of the mass of the earth. The same argument holds for horizontal or oblique branches, leaves, and roots, since these form a constant angle with the primary stem. (2) Gravitation differs from other forces in acting independently of the chemical or other properties of the body, being regulated only by its mass ; but the same property is also possessed by centrifugal force. If, as Knight^ first showed, a growing seedling is made to rotate with a rapidity sufficient to bring centrifugal force into play, this force acts on the different parts like gravitation; t. e. the parts which would otherwise be influenced by gravitation (as the primary root), now follow the direction of the centrifugal force and grow outwards from the centre of rotation, while the stem, which would otherwise grow upwards con- trary to the direction of gravitation, now assumes a direction towards the centre of rotation, i. e. in a direction opposite to that of the acting force. This law is strikingly illustrated when seedlings, the roots and stems of which had previously grown^'in one straight hne, are fixed upon a rotating disc (protected from evaporation by a bell-glass) in such a manner that the axis of growth has a tangential direction. The mature parts maintain this direction during the rotation, while those which are still growing bend so that the apices of the roots point Knight, Phil. Trans. 1806, part I, p. 99. y \' 2 /,(.)2 MECHANICAL LAWS OF GROWTH. outwards and the apices of the stem inwards (towards the centre of rotation). If the rotation takes place in a horizontal plane, gravitation acts, in addition to centrifugal force, on the growing parts, and the direction of the stem and root becomes oblique. But when the rotation is very rapid, it is possible to increase the centrifugal force to such an extent that the axis of growth remains nearly horizontal. If, on the contrary, the seedlings are fixed to a disc rotating in a ver- tical plane, each side of the growing part is in turn directed for a short time upwards, downwards, to the right, and to the left. The action of gravitation there- fore affects all sides equally ; /. e. the growth of the organ is practically inde- pendent of gravitation. Centrifugal force is therefore the only force that acts on the growing parts; and the root takes an outward radial direction even when the disc is not rapidly turned, the stem an inward radial direction. If however the disc is made to turn very slowly in a vertical plane (round a horizontal axis), so that there is in fact no centrifugal force (as by intermittent turns, one revolution in ten to twenty minutes with a radius of from 5 to 10 cm.), I have shown^ that the organs then grow neither in the direction of gravitation nor in that of the centrifugal force, but just in those direcdons in which they had happened to be placed when fixed in the vessel. Under such conditions parts which normally grow straight often curve in a- plane quite independently of external forces, and this can only be due to internal causes of growth which are distributed unequally round the axis of growth. Thus, for example, primary roots and stems of germinating seeds (Faba, Pisum, Fagopyrum, Brassica), will not lie in a straight line, but their respective axes of growth will intersect at any angle up to a right angle, the anterior side of the base of the stem growing more rapidly than the posterior side, and thus causing a curvature. It is clear that the direction of the secondary roots which spring from the primary root, as well as that of the leaves on the stem, is also, under these conditions, affected only by internal causes of growth. It is only in this way that we can explain the directions and forms assumed by parts of plants when unin- fluenced by gravitation, centrifugal force, or heliotropic curvatures, which could not occur in these experiments. CHAPTER IV. THE MECHANICAL LAWS OF GROWTH. Sect. ii. Definition. The growth of crystals consists in an increase of their volume by the apposition of homogeneous particles in definite directions. In plants the process which we call growth is much more complicated ; and the term is employed in different senses, according as we are speaking of the grov»th of a grain of starch or of chlorophyll, of part of a cell-wall, of a whole cell, or of a multi- Wiirzburger Med.-Phys. Gcsellschaft, March 16, iS; DEFINITION. H?> cellular organ. The common point in all these processes is that they depend at last on the intercalation of new molecules between those already in existence, in other words on intussusception, as has already been explained in the first section of Book III. But even in structures so simple as grains of starch or parts of cell-walls, we are met with insurmountable difficulties when we attempt to explain the mechanical process of growth in all its details ; and the present state of our knowledge by no means enables us to propound a connected theory of the growth of the entire cell or of a multicellular organ. We are in fact at present able only to follow empirically the processes of growth in detail, their causes and results. After this we may attempt to form definite ideas of the separate pro- cesses, taking for granted at the outset the purely formal phenomena of mor- phology, and regarding as the ultimate object of the enquiry the obtaining an insight into the mechanism of growth. If the solution of this difficult problem must be deferred to a distant future, it at any rate lies within the scope of this work to collect together the ascertained phenomena. But even here we meet with the difficulty that no one has as yet undertaken to limit the term Growth to a definitely circumscribed idea. The term is however always employed in the case of plants and animals to designate changes in form or volume or both brought about by internal causes, themselves the result of organisation, and in their turn excited and maintained by definite external causes, as heat, light, gravitation, the supply of food-materials, water, &c. Changes in the form or volume of parts of plants that remain quite passive to external forces, and in which changes no organic process cooperates, ought not to be included in the term Growth. Thus, for example, there is no growth when the form or length of an internode or root is altered by simple stretching, pressure, twisting, or bending (it may be by the hands). It is quite possible however that under certain circumstances internal changes might be brought about by external influences to which the part of the plant is at first altogether passive, but which, combined with organic processes, cause true growth or changes of growth. By organic processes I understand those internal changes which fulfil the two following conditions : — firstly, they are caused by the specific organisation of the part of the plant, which is of such a nature that any external influence can only effect changes in accordance with it ; secondly, they result in a permanent change of the organised part which is not at once re- versed by opposite external influences. If, for example, the elevation of the temper- ature above the inferior limit (see Sect. 7, p. 651) has caused an increase in volume of the embryonic structures already saturated with water, the parts will not contract to their previous volume when the temperature again falls below this point, but will retain the increase acquired during the higher temperature; in other words the process is not reversed, it only ceases. Microscopic as well as other kinds of ex- amination also show that the internal organisation has undergone permanent change varying with the specific properties of the plant. If on the contrary a stem is allowed to wither from want of water, it becomes shorter and ceases to grow ; when it again absorbs water it becomes longer and thicker and begins to grow. The contraction on withering and the lengthening on the absorption of water are mere physical phenomena ; but the lengthening and thickening of a part resulting from continuad turgescence mav actually depend upon growth, the organisation of the 694 MECHANICAL LAWS OF GROWTH. plant bein^ altered permanently and to an amount varying with the species by the operation of the turgescence. It is again the result of permanent and specific change of organisation when a tendril, in consequence of the light pressure of the body to which it clings, lengthens less on the side in contact, more on the opposite exposed side ; the curvature thus caused does not disappear if the pressure has lasted long enough ; the whole phenomenon is therefore one of growth. When, on the contrary, the motile organ of a Mimosa-leaf bends downwards in consequence of irritation, and afterwards again bends upwards, this is, it is true, caused by the peculiar organisation of the plant; but the movement induces no change in the organisation itself, and its effects are not permanent, the leaf soon returning to its original condition. The sensitiveness of the leaves of Mimosa does not therefore depend on a change of growth caused by the irritation ; while the power of ten- drils to curl round supports depends, it is true, on sensitiveness, but of such a character as to cause a change in the processes of growth. If increase in volume is included in the idea of growth, as is the case in ordinary language, the rigorously scientific use of the word would require special care ; for if we simply say that a plant or a part of a plant of considerable size grows, this may be accompanied actually by a decrease of the whole volume. Thus, for example, when bulbs sprout or seeds germinate in the air, the whole does not grow, but only the younger parts develope at the expense of the older, which in addition give off aqueous vapour and carbon dioxide. It is therefore necessary to distinguish accurately the growing parts from those which do not grow. There are however changes of form in the parts of plants which are not asso- ciated with increase, and which may even be attended with decrease in volume, but which nevertheless depend on a permanent and irreversible change of organisation. Thus, for instance, the pith, after removal from the internodes, increases in length for days even while it loses water by evaporation in air that is not saturated. It would scarcely seem convenient to exclude these and similar phenomena from the idea of growth; and it is therefore necessary to distinguish between growth with and growth without increase in volume ; in the latter case growth consists in a mere change of form which again depends on an alteration of position of the smallest particles. Every case of increase in volume of a grain of starch or of a cell must not be regarded as growth, inasmuch as it may be caused by absorption, and may be reversed by loss of water ; nor is it necessary that growth in a single cell should be associated with increase in volume, since particular parts of the cell may furnish material for the increase of other parts. In this case the cell considered as a whole only changes its form ; and if this change is caused by internal organic forces, it must be considered as a kind of growth. Those changes in the form and volume of cells must, on the other hand, be excluded from the idea of growth which occur only occasionally and admit of being completely reversed, as is the case with the contractile organs of sensitive and periodically motile leaves. An error which is constantly made by those who are unacquainted with physiology is to confuse the ideas Growth and Nutrition, or to consider them identical. It is no doubt true that all growth must be associated with the conveyance of food-materials to the growing parts ; but these food-materials are usually withdrawn from older parts where they were previously inactive ; the whole organism, consisting of both growing VARIOUS CAUSES OF GROWTH, 695 and non-growing parts (as a bulb suspended and putting out leaves in the air), is not nourished as such from without. The growth of certain parts is therefore no indication of nutrition of the whole. Still less necessary is the connection between growth and nutrition from without ; the special organs of nutrition, the green leaves, do not grow after they are mature, although they carry on the process of nutrition. The two pro- cesses may coincide both in place and time, /'. e. in the same cell; but may also be separated in both space and time ; and this is indeed usually the case, as has been suffi- ciently shown in Sect. 5. Sect. 12. Various causes of Growth. Growth, like vital activity, takes place only when certain favourable external conditions coexist. These are the presence of assimilated food-material, water, oxygen, and a sufficiently high temperature. Under these conditions individual cells or masses of tissue may grow, provided that (heir organisation permits it. But independently of these conditions there are others, as we have seen in the last chapter, which, without absolutely causing or arrest- ing growth, nevertheless influence it; as light, gravitation, and pressure. The first- named may be called the necessary, the last the secondary conditions of growth. In all growth all the necessary conditions must concur w^hile the secondary condi- tions intervene only in certain cases, and exert their modifying influence very differ- ently on the corresponding parts of diff'erent plants. The conditions spoken of as Necessary and Secondary Conditions depend upon ihe environment of the plant, and act upon it from without. They may therefore be described as External Conditions or causes of growth, in contradistinction to the Internal Conditions dependent on the organisation of the plant. The existence of the latter conditions is most strikingly manifested in the fact that all parts of plants arc able to grow only during a certain time ; when this time — the period of youth and development — is past, they no longer grow, even when all the favourable conditions concur. This shows that the internal organisation undergoes changes, which at length render the continuance of growth impossible. But even in organs which are still growing a certain independence of external circumstances may be perceived ; an oak-leaf invariably grows differently from an elm-leaf, an oak-fruit Irom an oak-root. The diff"erences of these processes of growth is at once manifest in the diff"erence of form and of the other properties of the organ; and no com- bination of external circumstances has the power of giving to a root, by change in its growth, the form of a leaf, or to an oak-leaf the structure of an elm-leaf. There are also certain internal conditions of growth which do not decide, like the age of an organ and the necessary external conditions, whether growth shall take place, or at what rate; but determine how^ it shall proceed, and what specific and determinate organisation shall be attained by it. This latter circumstance depends only on the parent plants, or in other words on the species or variety to which it belongs. Descent determines the specific character of the growth ; all the other conditions determine only whether growth shall take place at all, and with what rapidity and energy. The innate internal conditions that regulate the nature of the growth of the plant, when once present cannot again be destroyed or reversed ; while the ex- ternal conditions may be at one time brought into action, at another time set aside. The internal and external conditions of growth may therefore be distinguished as the historical and the physical ; but those properties of a plant which have been 696 MECHANICAL LAWS OF GROWTH. obtained historically are generally termed hereditary. The term is not open to objec- tion unless heredity be considered, as has recently been done by many, as a kind of natural force requiring no further analysis. For in distinguishing hereditary con- ditions of growth — /. ^. those that have been acquired historically — from physical ones, it is not meant that the former do not also owe their existence to physical phenomena, but only that besides the accidental concurrence of physical conditions, it is also necessary to take into account certain characters which the plant has acquired when in the embryonic condition (in the broadest sense of the term) in the form of definite specialities of organisation through the influence of its parents. These remarks must suffice here. The extremely difficult question w-hich has been raised may be illustrated by protracted and elaborate explanations, but cannot be satisfactorily answered. The external or physical causes of growth are the only ones that can be submitted to direct experimental investigation ; the internal hereditary causes must be considered simply as something that exists and that is in the main unalterable ; for if it were possible to change some of the mechanical and chemical properties of a tissue by means of external influences, this could not affect the true kernel of the hereditary characteristics ; and again conversely changes in these hereditary peculiarities, or variatmis, are never brought about by direct external influences, but only by unknown internal changes. Since therefore the specific peculiarities in the organisation of a plant are something in its nature that is entirely unknown, any investigation of the processes of growth must rest satisfied with showing the mode in w^hich they are always associated with constant internal conditions, and what visible changes are produced in the processes of growth by physical influences. We cannot therefore be astonished if in the action of known external causes — light, gravitation &c. — on plants, effects are produced which appear altogether strange to one accustomed to examine purely physical processes ; but this aston- ishment disappears when it is borne in mind that the specific organisation of a plant itself represents a complexity of causes which we cannot analyse, and there- fore are unable to estimate. It is in the constant recognition of this unknown factor — which causes physiological effects to turn out so entirely different from purely physical ones — that the difference between physiology and physics consists. The most striking mode however in which the aggregate of conditions of growth manifests itself in the inherited organisation, is w'hen the same external causes pro- duce entirely opposite effects on plants belonging to different species and even on different parts of the same plant. To understand correctly the phenomena of vegetation, it is also necessary to distinguish between the direct and indirect action of external causes on growth. For since growth is always dependent primarily on the presence of assimilated food-materials, light, temperature, of other external conditions may indirectly in- fluence growth by affecting the formation and transport of the food-materials. But it is also possible and even probable that the mechanical process of intus- susception itself on which growth is directly dependent, may be modified by those and other causes the influence of which on growth is therefore in that case a direct one. The growth of one part may also be indirectly promoted or retarded by the growth or the removal of another part. GENERAL PROPERTIES OF GROWING PARTS OF PLANTS, 697 The unknown factor which exists in the inherited properties of organisms is by no means without analogy in inorganic nature. Chemists and physicists have also to assume peculiar properties of elementary substances. The. aggregate of properties by which a particle of iron is absolutely distinguished from a particle of oxygen is as unknown and much more invariable than the aggregate of physiological causes which distinguish the inherited properties of an oak from those of a pine. So far as the definition given above of historical properties concerns the inherited specific peculiarities of plants, the term is not metaphorical from the point of view of the Theory of Descent, but must be taken in its literal signi- fication. The specific properties which determine qualitatively the growth of each organ have sprung up successively in the course of time, i. e. in a series of genera- tions. The chief evidence in favour of this view will be given in the last chapter of this work. It need only be mentioned now that this theory of the genesis of specific properties indicates the only possibility of arriving at an understanding of them in accordance with the laws of causality. At the present time this is possible only in the most general outline. The use here made of the terms 'historical' and 'physical' may also be illustrated from another subject in the following manner. The nature of the geological form- ations of which the crust of the earth consists can be understood only from a historical point of view, because it is only at particular spots and at particular times that the conditions have concurred which produced, for example, the Chalk or Old Red Sandstone. The formation of these rocks was dependent on chemical and physical processes, which must however have been preceded by other physical changes in the crust of the earth, in order that these rocks should be formed exactly at particular spots and particular periods. A crystal of sodium chloride can, on the contrary, be produced at any time if the necessary conditions are artificially brought together. Pseudomorphosis of crystals can again be explained only from a historical point of view, although it is certain that the chemical and physical properties of the substances are alone concerned in the process. We see therefore — and this is the object of these remarks — that the historical explanation of a natural phenomenon does not exclude its explanation from a physical point of view, but on the contrary includes it where we have to do with natural phenomena ; and this principle is equally applicable to those properties of vegetable species which have been acquired hereditarily or historically, even when the application is practically much more diffi- cult than in the case of inorganic nature. Sect. 13. General Properties of the G-rowing Parts of Plants \ From the consideration of this subject the true crystals which are found in cells may be entirely excluded, since they do not differ in their general properties from those which occur elsewhere. The organised elementary structures on the contrary, the proto- plasm, the nucleus, chlorophyll- and starch-grains, and the cell- walls, exhibit proper- ties which distinguish them from all inorganic bodies. These organised bodies are, in the first place, all capable of swelling ; i. e. they have the power of absorbing water or aqueous solutions between their solid * See Nageli u. Sclivvendener, Das Mikroskop, p. 403 et seq. 698 MECHANICAL LAWS OF GROWTH. particles with such force that the particles are forced apart; the whole structure increases in size, and can thus exercise considerable pressure on the surrounding parts. If water is by any means withdrawn from the body which has thus swollen up, its particles again approach one another, and with such force that considerable strains may be exerted on the adjoining parts connected with it ; as, for example, is shown in the bursting of dry capsules. The swelling and dessication of organised parts may therefore cause change of form in the surrounding parts, i.e. in other oro-anised parts. This power of swelling is of still greater importance, since it is this process that renders possible the interchange of sap between the individual cells as well as between whole masses of tissue. In order that growth by intussusception mav take place, the dissolved food-materials must be able to enter by imbibition be- tween the part'cles of the growing structure, and the chemical processes must take place there which construct from the dissolved food-materials solid particles to be intercalated between those already in existence, and in consequence of w'hich the organic mass alters its volume and form (see Book III, Sect. i). A second general property of the organised parts of plants is that they change thei)' form when the external conditions remain perfectly unaltered, internal changes being the only efficient cause. Almost every process of growth is associated with change of form. These facts may be more briefly described by ascribing to organised structures endowed with the power of growth internal forces or plastic tendencies, if it is clearly understood that the term is only used to express a still unresolved aggregate of causes. As a result of these internal forces, organised structures have the power of overcoming resistance. Thus, for example, plasmodia which are constantly altering their form, are able, notwithstanding their gelatinous and very soft nature, to overcome their own weight, and to creep up solid bodies. In the same manner the growth of wood takes place with such force as to overcome the very considerable pressure of the surrounding bark. But although the internal causes of these plastic tendencies are able to over- come certain obstacles, it is on the other hand certain that growth is also influenced by external forces, such as pressure, traction, stretching, bending, &c., which are able to alter the form of solid bodies. The observations w^hich have been made on this subject will be collected in the following sections ; but it is in the first place necessary to define certain terms which will frequently be employed. Like unorganised €olid bodies, those which are organised oppose a greater or less resistance to the external forces which tend to alter their form ; and are hence divided into hard and soft bodies. A hard body is one which offers con- siderable resistance, like many lignified or silicified cell-walls ; a soft body is one which offers very little resistance, like protoplasm, chlorophyll-grains, or swollen cell- walls which have ceased growing, as gum-tragacanth. Structures which become disintegrated under pressure and traction rather than undergo any considerable change of form, are brittle, like grains of starch or crystalloids of aleurone. If, on the contrary, they are capable of undergoing considerable changes of form, whether this take place by pressure or traction, they are extensible. It is clear that flexibility depends to a certain extent on extensibility, since the side of the bent part which becomes concave is compressed, the convex side stretched. All these properties are relative, and the sam^e body may exhibit different phenomena according to the GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 699 nature of the external forces which act upon it. Thus, for example, under a sudden blow the apex of a root behaves like a brittle body, and breaks easily, while it is flexible if slowly bent. If the form of an extensible body has been changed by pressure, traction, or bending, and if, when then left to itself, it retains the form to which it has been forced, it is called inelasiic ; if, on the other hand, it resumes its original form, it is elastic. If the changes of form produced by external causes are small, they are usually completely reversed when the body is left to itself, and within these limits the body is perfecdy elastic; but if the change of form exceeds certain limits dependent on the nature of the body and the length of time during which the force has been acting, it does not again assume exactly its previous form. The greatest amount of change which yet permits a complete restoration of the original form determines the Limit of Elasticity o{ the body; when this is exceeded, the stretched substance partially retains the form which it has been made to assume, and the less complete the return to its primitive shape the more imperfect is its elasticity. It would appear as if all organised bodies were imperfectly elastic to any long-continued stretching or alteration of form, and as if there were no limit of elasticity in the case of very long-continued but weak external influence. In all these points organised bodies, especially the growing parts of plants, exhibit the same phenomena as inor- ganic bodies. It must however be remembered that the terms explained above have reference only to eff'ects visible externally ; the internal changes which bring about the external effect may be very different in different bodies. Rigidity, /. e. resistance to bending, depends, for example, evidently on very different internal conditions in the case of a woody cylinder and of a succulent stem or root consisting mainly of parenchyma. This is at once experimentally proved by the woody cylinder becom- ing less flexible and even brittle from loss of water, while the flexibility of succulent parenchyma is thereby increased. This is readily understood on recollecting that the flexibility of the woody cylinder depends on that of the walls of the wood- cells, which are not closed cavities, and therefore cannot become turgid, while the flexibility of parenchymatous tissue depends on the change of form of the closed turgescent cells, the extensibility and elasticity of the celhwalls taking only a sub- ordinate part. Changes of form take place however more easily the less the turgidity of the cells ; a parenchymatous tissue may be compared to an aggre- gation of bladders each of which is full of water ; if they are all turgid with water, each bladder is tense and rigid, as also is the w^hole ; if, on the contrary, they contain only enough water to fill without distending them, each separate bladder is flaccid, as also is the whole, which can therefore be bent in any direction. A mass of parenchyma may therefore be stiff and rigid even if its cell-walls are thin and very flexible, if only they are firm enough not to give way from the pressure of the water which stretches them or to allow it to filter through. The flexibility and elas- ticity of the moist cell-wall cannot however be compared directly with these pro- perties in a perfectly dry cell-wall or a strip of metal, as Nageli and Schwendener (/. X>C>E. It is at once evident from this that every layer was before the separation in a state of negative tension towards the next one inside, of positive tension towards the next one outside. The epidermis alone i.j in a state of passive tension ; the pith alone is passively compressed, or rather prevented from extending. The extensibility and elasticity of tissues are altered during the growth of an internode, as may be seen by comparing internodes of various ages ; the exten- sibility of the wood decreases rapidly, that of the epidermis and cortex more slowly, as may be inferred from the decreasing rapidity with which these tissues contract on their isolation, and from the thickening of the cell-walls ^ The pith from internodes of different ages shows on isolation at first an increasing, afterwards a decreasing- amount of elongation. If the tendency of the pith to expand remained the same at all ages, it would, when isolated, elongate more in older than in younger inter- nodes, in consequence of the increasing resistance of the tissues which are in a state of passive tension ; but when the growth in length has ceased, or soon after, the pith loses its tendency to expand, as may be concluded from the fact that on isolation from such internodes it elongates less, and finally not at all^, although the ^ The decrease in the extensibility of the epidermis was determined by Kraus (/. c, tables, p. 9), by attaching weights to strips of epidermis. 2 The relation between the tension of tissues and the state of growth of the internode (2. e. the phase of its greatest period of growth) requires fresh and detailed investigation. Kraus's Table III (Bot. Zeitg. 1867), shows that the greatest difference of length between cortex and pith does not always occur at the time of the greatest growth ; and that even after growtli has ceased, 7i8 MECHANICAL LAWS OF GROWTH. resistance of the wood has greatly increased ; were the pith now as elastic as before, it would expand more rapidly when freed from the very great resistance of the wood. The following table will now be understood ; the length of the entire internode being always placed at loo, and the amount of contraction indicated by negative, of expansion by positive percentages. Number of the internode, countino; from the youngest. Change of length of the isolated tissue in percentage of the entire internode. NicofiiVia Tahncum do. Sambuciis nigra do. do. Cortex. Xj'lem. Pith. I- - IV -5"9 -I "5 + 2*9 V- -vn -3"i — II + 3*5 VIII- - IX -3-5 -1-5 + 0-9 X- - XI -0-5 -0-5 + 2-4 I- - II -2-2 + 2-3 III- - IV — 1-2 + 4-2 V- -VII — I'D + 2-8 VIII- -IX -1-8 + 27 I -2-6 -2-6 + 4-0 II — 2 O -2-8 + 5-5 III -1*5 — Q-Q + 1-5 I -0-6 + 37 II -1-6 + 5-1 III — Q-O + 0-9 I -IS + 6-5 II -15 + lo-i III -0-6 + 2-3 These numbers, taken from my Handbook of Experimental Physiology, may be supplemented by some others, calculated from the statements of Kraus^ (/. c. Table i). Number of the inter- node, counting from the youngest. Change of length of the isolated tissue in percentage of the entire internode. Epidermis. Corte.K, Xylem. Pith, icoiiana Tahacum III— IV -2-9 — 1*4 + 3-5 V— VI -2-9 -1-3 -0-8 + 27 VII— IX -2-7 — 2-r — Q-Q + 3-4 X— XII — 1*4 -o*5 -Q-O + 34 XIII— XV -1-05 — O'O -o-8(.?) + 4"o itis vinife7'a I -3'i -1-6 -f 6-0 II -17 -O'O + 87 III -2-5(?) -ro(?) + 7-1 IV — Q-Q — O'O + 6-0 V — O'O — Q-O + 27 tensions may still continue. It must however be remarked that the method by which these num- bers have been obtained is liable to considerable suspicion. ^ Kraus has only given the absolute numbers ; but a correct notion can be obtained only by comparing them with the length of the internode. PHENOMENA DUE TO THE TENSION OF TISSUES. 7J9 Number of the inter- node, counting from th youngest. m mge of length of the isolated tissue percentage of the entire internode. Epidennis. Cortex. Xyleni. I'itii. Samhucus nigra I -31 —0.0 + 0-0 II -1-5 -ro + 6-4 III -1-6 + 6-5 IV -1-6 +0-3 (?) + 6-1 V — 0'2 -f 0-2 (?) + 07 VI ^us I- IV -4'3 -0-5 -0-5 + 0-1 Helicvithiis liihero -17 + 6-8 V— VI -17 — O'O -^(i-d VI— VII -0-9 -0-4 + 4-4 VIII -05 — O'O + 3-2 IX— XI — 0"O + o-9(?) + 2-0 It is easy to establish the existence of similar contractions of the outer tissues and elongations of the parenchyma in the case of growing leaf-stalks, as those of Beta, Rheum, Philodendron, &c. If a growing internode or a leaf-stalk is split by two longitudinal sections at right angles to one another, the parts will bend concavely outwards, evidently in consequence of the lengthening of the pith and contraction of the outer tissue. This phenomenon is seen most clearly if a thin longitudinal slice is taken from the middle of the internode, laid flat, and the pith then halved lengthwise ; as the knife advances the two halves will bend concavely outwards. If, instead of cutting it in two, thin strips of tissue are cut proceeding from without inwards, first one including the epidermis, next one including the cortical tissue, and finally one including the wood, they will all bend concavely outwards, because the adjacent layers are all in a state of negative tension on the outside, of positive tension on the inside, and when separated, the outer side always becomes shorter, the inner side longer. That this bending is caused by simultaneous contraction of the outside and lengthening of the inside, is at once clear from the measurements already given, but may also be observed directly, as will be seen from the following table. Longi- tudinal slices of considerable thickness were cut from the middle of growing inter- nodes, laid flat, and the pith then halved by a longitudinal cut ; the radius of the curvature which each half at once assumed was determined, and the length of the convex inner and the concave outer side measured by means of a strip of card oraduated in millimetres. Silphium perfoliaimn. Left half Right half Silphmm perfoliatwn. Left half Right half Macleya cor data. Cavitv. Length of the entire internode. 69-5 mm. 69*5 190 190 134-5 Radius of curvature of the segment. 4 cm. 4 3—4 3—4 .^—6 Contrac- tion of the concave outer side. 2'8 p.c. 2-4 2-8 2-6 Lengthen- ing of the convex inner side. 9-3 p.c. 93 95 IO-8 71 Semidiameter of the internode. 3 mm. 3 3'5 4-5 33 ■JIQ MZCHANICAL LAWS OF GROWTH. As we have already seen from the measurements of the layers when entirely isolated, it was also evident from the curvature of the two halves of the longitudinal slice that the contraction of the epidermis is less than the elongation of the pith. Since this slice is somewhat longer than the entire internode, the proportionate con- traction of the outside would be greater, the lengthening of the inside less. A rapid rate of growth, united with a certain amount of physical differentiation of the different layers of tissue, such as occurs in erect leafy shoots, stout leaf-stalks, and tendrils, appears generally to be favourable to the production of the tensions in tissues of which we have been speaking, as they are not found in stems of very slow growth, like stout rhizomes, the thick stolons of Yucca and Dracaena, &c* That the existence of tension has more to do with a physical differentiation in the elasticity and extensibihty of the layers than with a morphological one, is shown by the fact that very considerable tensions are found even between the outer and inner layers of the hyphal tissue of the stems of the larger Hymenomycetous Fungi, which are morphologically similar. Within the growing apical region of roots, on the con- trary, where we have a combination of two layers of tissue sharply differentiated morphologically, viz. an axial fibro-vascular bundle surrounded by a parenchymatous cortex, we do not find any considarable tension when the part is split by two longi- tudinal cuts at right angles to one another, or when the layers are completely isolated. But since it is easy to prove that the cortex of the root grows more rapidly and for a longer time than the axial bundle ^ it may be assumed that in an uninjured grow- ing root there is nevertheless a small tension between them, positive in the case of the cortex, negative in that of the axial bundle ; but it is only rarely that this tension becomes strong enough to be perceptible by the parts bending inwards when cut lengthwise ; probably because the axial bundle, although entirely composed of procambial tissue, is so extensible that it yields almost without resistance to the traction of the cortex. The case is different in the older parts of the root behind the growing end (which does not exceed lo mm. in length). If this portion is split, the parts generally gape concavely outwards, although much less so than the grow- ing part of erect stems. The curvature is however considerable in the aerial roots of Aroideae, where the opposite curvature which takes place at the apex is also sometimes well-marked. The description now given of the states of tension in the case of stems is also applicable to all expanded internodes and leaf-stalks. Within the bud itself, and especially at the punctum vcgetationis, there appears to be no tension of the tissues, or only one as slight as in the apices of roots. It is only when the epidermis is becom- ing cuticularised and the walls of the bast-cells are beginning to thicken that the tensions become perceptible. The individual parts of fully mature organs, especially leaves, not unfrequently retain the tensions acquired during growth, which are in such cases often particu- larly strong. This is the case, for instance, in the contractile organs of the sensitive or periodically motile leaves of Papilionacese, Mimoseae, Oxalidese, &c., to which we shall recur. While in these cases the true leaf-stalks and the internodes from which ' The halves of roots split lengthwise continue lo grow for days, and bend concavely on ihe cut surface. PHEXOMENA DUE TO THE TEXSION OF TISSU7.S. 72 1 they spring have long become rigid, and no longer show any considerable tension of the tissues, an extraordinary elongation of the parenchymatous cortex occurs in the contractile organs, if they are separated from the solid axial fibro-vascular bundles ; and considerable flexion results when these organs are split lengthwise. The opposite to this occurs in the nodes of the stems of Grasses, i. e. in the annular thickenings at the base of the leaf-sheaths ; no perceptible tension is observable in these. If a median longitudinal section is made and divided into its inner and outer layers, they exhibit none of the curvatures which are so striking in portions of the internodes. This flaccidity of the tissue, or at least the insignificance of the tension, must depend on the concurrence of two causes ; on the one hand on the cessation of the growth of the parenchyma in the node (although it remains in a state capable of growing, and under certain circumstances begins to grow again), and on the other hand on the extensibility of the fibro-vascular bundles which do not become lignified within the node, or not till a late period when the cells of the same bundles, where they lie in the leaf-sheath and the internode, have long become lignified and rigid. While, therefore, the parenchyma of the node continues to grow, it stretches the unresisting fibro-vascular bundles, and when its growth ceases no perceptible tension remains. In the contractile organs of sensitive and periodi- cally motile leaves, on the contrary, the axial fibro-vascular bundle becomes elastic and resistant before the growth of the surrounding parenchyma has ceased ; and when this is the case a tension remains which is further increased by the extra- ordinary capacity of the parenchyma for becoming turgid. If we now attempt to give an account of the causes which render the tension at first (when in the bud) imperceptible in the internodes of erect rapidly growing stems, and make it subsequently increase and finally altogether disappear when the internodes are fully mature, we find that we must content ourselves with probable conjectures rather than with fully demonstrated propositions. The origin of tension between the layers must in any case be referred mainly to differences in the rate of growth of the cell-walls, it may arise from the inter- calation of fresh material taking place less rapidly in one layer than in another ; and it is especially manifest when the cell-walls in the one case subsequently undergo thickening. From the first of these causes the layers which lengthen more slowly are placed in a state of passive tension by those that grow more rapidly ; while the second cause diminishes their extensibility to an increasing extent, especially when, as in the xylem of the fibro-vascular bundles, the cell-walls become lignified, which renders them capable of resisting extension. The more quickly, on the other hand, the thin cell-walls in the pith and parenchyma generally increase in size (especially in length) by superficial growth, the stronger becomes the tension of the passively distended layers of tissue. To this must be added the peculiar power of the medul- lary cells to absorb water from the older parts with great force and rapidity, and thus to maintain themselves in a state of the highest turgidity. This distends the pith independently of the superficial growth of its cell-walls, and besides influencing the more slowly growing layers of tissue, also contributes to increase the superficial growth of the cell-walls of the pith. If the woody bundles then become lignified as the tissues become more developed internally, and the resistance of the epidermis, which is constantly becoming more cuticularised, becomes too great, these tissues .1 A 722 MECHANICAL LAWS OF GROWTH. oppose an insuperable resistance to the further distension of the pith by growth and turgidity, and no further elongation of the internode is possible. The tendency of the pith to expand ceases ; its cells lose their turgidity, they give off their water to adjacent tissues, and become filled with air. According to this view, which has been fully established in the main, the actual motive power of growth in internodes emerging from the bud-condition is the pith, and the thin-walled parenchyma generally. It is only the force thus exercised that causes the other tissues to increase in length as long as they are sufficiently extensible. The extraordinary absorbent power possessed by the pith enables it when growing to withdraw the water from the surrounding layers of tissue, and thus prevents its cells from becoming more strongly turgid, neutralising by this means one of the causes of the superficial growth of the cell-walls. It must also be remembered, as has already been shown in Fig. 448, that the turgidity of the cells of the dilated layers is even diminished, while that of the compressed cells (in the pith) is increased by the tension ; and we consequently have here another Cause of diff'erences in the superficial growth of the cell-walls. Finally,- it must be borne in mind that the internodes, at least of land-plants, are exposed to transpiration as soon as they emerge from the bud ; but this cause of diminished turgidity will affect chiefly the epidermal cells and the subjacent layers, least of all the pith. The great importance which is here attached to turgidity as a cause of growth is justified by the fact that the growth of the internodes is at once stopped by its decrease, i. e. by the withering of the shoot ; while it is promoted by its increase, i. e. the growth of the shoot in water or damp air. The first and most efficient cause of the tension of tissues in a growing inter- node is therefore the different capacity for turgidity of the diff"erent tissues ; this depending partly on the nature of their fluids, partly on the structure of their cell- walls, and partly on their relative position in the internode. A more secondary place must be assigned to the swelling of the cell-walls caused by imbibition ; since it may be assumed that even when the turgidity of the cell is slight, the cell-wall still obtains sufficient water to satisfy its capacity for imbibition. If it were directly dependent on this, all the layers of tissue would grow equally rapidly, even when the turgidity was small, or had entirely disappeared. I rather hold the state of the case to be that when the cell-wall is passively distended by turgidity or by the tension of the sur- rounding layers of tissues, it is only enabled to deposit fresh substance in the direc- tion of its surface when perfectly saturated ; this does not however imply that other causes do not cooperate in promoting the intercalation. The importance of turgidity as a cause of growth may be very strikingly illus- trated in the case of isolated cylinders of pith, as we shall show presently. When, in consequence of their separation, the tissues which were in a state of passive tension become suddenly shorter, and the pith which was in a state of posi- tive tension suddenly longer, this process must be connected with a corresponding change in the form of the cells ^ ; the cells which contract must at the same time ^ Any considerable change in the vohime of the medullary cells when isolated must not indeed be expected, when it is recollected that neither the water contained in the cells nor the cell-walls PHENOMENA DUE TO THE TENSION OF TISSUES. 723 become wider in diameter, while those of the pith which lengthen must on the con- trary become narrower. It is impossible however to measure directly these changes of diameter, which are so small that ordinary methods are inapplicable. It is, however, a necessary consequence of what has been said that the passive lengthening of the epidermal cells, &c., in a growing internode makes them narrower ; the young epidermis must therefore be too narrow, besides being too short for the inner masses of tissue. Similarly the pith, being prevented from elongating in the growing internode by the surrounding layers, must in consequence have a tendency to enlarge transversely; besides being too long for the elongated tissues, it will also be too thick for them, and must have a tendency to force them apart. It follows therefore from the longitudinal tension which has been observed in the layers of tissue of a growing organ, that a transverse tension must also exist in it of such a nature that the outer layers are in a state of passive tension, w^hile the medullary cells which are prevented from lengthening have a tendency to dilate transversely. If thin transverse segments^ are cut radially from somewhat older growing stems, they gape open, evidently because the epidermis contracts in the peripheral direction, having been previously of too small circumference for the inner tissue, in other words, in a state of passive tension. The tendency of the medullary cells which are pre- vented from lengthening to become broader transversely, does not appear, on the other hand, to be always hindered by the surrounding wood and cortical tissue, but often to be even promoted by them ; so that these layers of tissue which surround the pith grow more rapidly in the peripheral direction than does the pith itself, and therefore exercise a radial traction upon it. A striking proof of this phenomenon is afforded by the frequent formation of cavities in stems and leaf-stalks at the time and place where the growth in length is most rapid. The increase in thickness of the pith is not sufficient to fill up the space which is enclosed by the surrounding tissues, and which increases in size ; its cells separate in the longitudinal direction, and the woody cylinder remains clothed on the inside by a layer of pith, the longi- tudinal tension of which still continues. The existence of an outward traction upon the pith can also be demonstrated in the case of internodes with solid cylinders of pith which are growing and at the same time increasing rapidly in diameter {e.g. Nicotiana, Silphium per/oliaiu77i), by dividing a fresh transverse segment (laid on glass) through the centre. The two cut surfaces of the pith now become curved outwardly and separate from one another, while the cortical parts of the segment still touch. This is an indication of the outward traction of the pith, and of the tendency of the cortical envelope to dilate peripherally. These statements rest however at present on but a small number of observ- ations, and better results may be expected from their repetition. It may nevertheless be assumed that in young internodes, before the fibro-vascular system has begun to become lignified, the pith exerts an outward pressure. This is accompanied later, permeated with water alter their volume from the forces exerted in this case. An alteration in the volume of the entire pith could at most arise from a change in the size of the intercellular spaces in consequence of the change in form of the cells. ^ Sachs, Experimental-Physiologic, p. 471. 3 A 2 724 MECHANICAL LAWS OF GROWTH. when the tangential growth of the wood and cortex is more rapid, by an outward traction, which at length becomes so strong as to overcome the tendency of the pith to dilate transversely. The pith is therefore now actually in a state of passive tension transversely (and at the same time compressed longitudinally), until at length the cells in the centre of the pith become detached from one another, and a hollow is formed, if the whole does not lose its sap and become dried up, as for example in the elder. Kraus observed^ that the medullary cells of an internode are very slightly longer when it is growing than when mature ; but this may be explained, in accordance with what has been said, by the cells of the pith finally losing their power of elongating when isolated. In the internode they are certainly not at first longer, and are afterwards shorter ; but the difference is only observable on isolation, and indicates that these cells at length lose the property of changing their form when isolated, or in other words have become rigid. The views here brought forward respecting the tension of the tissues of growing internodes and leaf-stalks are, I think, supported by the fact that the sudden and very considerable lengthening of the pith at the moment of its separation from the surrounding layers of tissue is followed by a slow lengthening which lasts for some days, while, on the contrary, the cortex and epidermis, which are in a state of passive tension, scarcely experience afterwards any perceptible contraction (but, according to Kraus, do not become longer even when placed in water). This subsequent length- ening of the isolated pith takes place with extreme force when it absorbs water, as Kraus has already shown; but the lengthening also continues in dry air when the pith even loses small quantities of its water, a point which had been previously overlooked. The isolated cylinder of pith of a growing internode is very flaccid, flexible, and extensible ; but if placed in water it soon becomes tense, rigid, and elastic, longer and apparently also thicker. The lengthening may amount in a few hours to as much as 40 p. c, or even more. These phenomena are explained if we suppose the medullary cells to be very strongly endowed with endosmosc^, by which they become in a high degree turgid, and thus not only increase considerably in size, but also become more rigid. The considerable increase in size presupposes, however, from the rapidity with which it takes place, great extensibility in the cell- walls. Isolated prisms of pith exposed to the air become shorter even than the length they possessed in the internode^; the cell-walls which were previously in a state of tension evidently contract elastically, as the turgidity diminishes from loss of water. But if care is taken that isolated cylinders of pith do not absorb any water, while at the same time they can only lose a very small quantity of it, by enclosing them in a glass tube containing about i litre of dry air, they nevertheless continue to lengthen perceptibly for some days, although not so considerably as when they absorb water; and this lengthening affects chiefly the older parts, while the ' Bot. Zeitg. 1867, p. 112. 2 Notwithstanding this powerful endosmose, the amount of solid substance dissolved in the cell-sap of the parenchyma is very small, as is shown by the fact that in cylinders of pith of this kind I found the dry weight only from 2 to 5 p. c, a considerable portion of which belonged to the cell-walls and protoplasm. " Kraus /. c, Tables, p. 29. PHENOMENA DUE TO THE TENSION OF TISSUES. 725 youngest parts sometimes contract. The whole cylinder becomes dry and rigid on the surface. Out of a large number of observations the following may be chosen to elucidate this point. A prism of pith from a part of a shoot of Sciiecio umhrosus 235*5 mm. long, lengthened about 57 p. c. when isolated, and weighed 5-3 grammes. It was divided into three parts by marks of Indian ink; their lengths being: — i. (the oldest) 100 mm., ii. 100 mm., iii. (the youngest piece) 49 mm. The prism of pith was now fixed in a dry glass tube, which was then corked at both ends. After fourteen hours the parts had lengthened as follows : — part i. about 4-5 mm., part ii. about 6-5 mm., part iii. about 2 mm. or 4*1 p.c, while the pith had lost 0*15 grm. of water. After re- maining for twenty-six hours more in the glass tube the following further changes had taken place ; part i. had again lengthened about 2*5 mm., part ii. about 0*5 mm., while the length of part iii. had diminished about 05 mm. No further loss of water had taken place, because the glass tube had become covered with moisture. The pith was now placed in water, and after six hours the following increase of length had taken place:— in parti, about 18 mm. or i6-8 p.c, in part ii. about 23 mm. or 21-6 p. c, in part iii. about 11 mm. or 2i'6 p.c. (as compared with the length before placing in water). The pith had also become considerably thicker, having absorbed 6 grammes of water. The estimation of the dry weight showed that the pith contained only 0*22 grm. of solid substance ; this was combined, when the pith was isolated, with 5*08 grm. of water ; it subsequently lost 0*15 grm., but by the end of the experiment had again absorbed 6 grm. At first therefore the pith con- tained 4*23 p.c, at last only 1*97 p.c. of solid substance. Experiments of this kind show that the pith of the youngest internodes loses its water most easily by evaporation, as is shown by its decrease in length. Kraus was led by other ex- periments to the same conclusions ; and he also showed — not in contradiction, as he thought, but in harmony with these results {I.e. p. 123) — that the older pith of growing internodes attracts water more eagerly and expands more than that of younger internodes. If the question is now asked how the lengthening of the pith can take place in spite of the loss of water (though this may be small), it must first of all be noted that its surface becomes remarkably dry under the circumstances described. It is scarcely possible to attribute this significant desiccation of the surface to the small loss of water indicated by the weight ; it is probably rather caused by the inner cells of the pith withdrawing water from the outer cells, and thus lengthening; but the outer cells would become shorter if they were not stretched by the inner ones. That this is actually the case is shown by the rigidity of the pith under these circumstances, caused by the tension that subsists between the dry outer layer and the moister inner mass. If the prism of pith is divided lengthwise, the parts curve outwards; and sometimes the outer surface becomes even strongly concave. If the inner cells of the pith are able to withdraw water from the outer ones, it may be inferred that the outer cells are also able to withdraw it from the surrounding wood and especially from the peripheral tissues, preventing these from becoming strongly turgid ; their growth being thus retarded in favour of that of the pith, by which they are now placed in a state of passive tension. It is noteworthy that the medullary cells which contain a minimum quantity of dissolved substances nevertheless absorb water 726 MECHANICAL LAWS OF GROWTH, sufficiently powerfully to abstract it from the surroundinf^ tissues which must evi- dently contain a much greater quantity of dissolved substances \ It is now clear from the observations which have been described, why portions of shoots cut lengthwise in half or in four and placed in water curve outward to such a remarkable extent ; and why a curvature, which may be small but continues to increase for some time, takes place when such pieces are placed in a closed glass tube in dry air. (2) Transvei'se tension caused by stihsequcnt thickening of the wood. It has already been shown that transverse tensions also arise during growth caused by the longitudinal tension, a more exact knowledge of which is still a desideratum. With the commencement of the increase in thickness of the stem caused by the cambium- ring, a new cause of tension arises, acting in both a radial and peripheral direction ; and this transverse tension generally continues as long as the cambium-ring re- mains active. The layers of tissue formed from the cambium-ring have at first a tendency to expand in the tangential direction to an extent greater than the space between the epidermis and the primary cortex permits. These outer tissues therefore become stretched in the peripheral direction ; and, since they are elastic and have a tendency to contract, they exert a pressure in the radial direction on the cambium and the tissue formed from it, viz. the wood and the layers of secondary cortex. It happens however also that the rings of wood produced on the inside of the cambium grow more strongly in the tangential direction than the phloem produced on the outside, which is therefore passively distended. A tension is hence set up in the transverse diameter of the stem during its increase in thickness of such a kind that each layer is stretched peripherally on its outside and compressed radially on its inside ; in other words, is in a state of negative tension on its outside, of positive tension on its inside. If the separate layers of a transverse segment — epidermis, primary cortex, secondary cortex (phloem) and xylem — are separated, and their peripheral length compared, we get the following expression for the transverse tension : — E < C < Ph < X. As the increase in thickness proceeds the transverse tension increases, as is shown by Kraus's very complete experiments ; i. e. if the rings of tissue in a transverse segment of the stem or in a woody branch are separated from one another, by dividing it longitudinally and then separating the rings, they contract the more the nearer they lie to the circumference, and the contraction is the more considerable, compared with the original circumference of the whole, the older the original segment. The traction upon the cells of the epidermis and of the primary cortex caused by the transverse tension is easily observed by the microscope in the transverse segment, if young internodes of plants which increase rapidly in thickness, as Helianthus, Ricinus, or Ribes, are compared with those which have already been forming wood for some weeks or months. The form of the cells shows that they have been strained in the peripheral (see Fig. 56, p. 69), and have in consequence grown ^ I must content myself here with this preliminary sketch, which I shall carry out more in detail in the Proceedings of ihe Wiirzburg Botanical Institute. Absorbent root-hairs and cortical cells behave in the same manner as the pith. PHENOMENA DUE TO THE TENSION OF TISSUES. 72/ rapidly in the tangential direction ; the cells which have been thus altered in form are divided by radial septa. But at length the epidermis and primary cortex are no longer able to obey the peripheral traction; longitudinal fissures occur in the cortical tissue, generally after the commencement of the formation of cork. When the periderm and cork have been formed on the older parts of stems, these secondary epidermal tissues undergo a continuous strain in the peripheral direction, and exert in turn a radial pressure on the living phloem, cambium, and xylem. The first result of this pressure exerted by the growing inner tissues is the splitting of the layers of bark, especially longitudinally. The form of the fissures depends, however, on the course of the bundles of bast which take part in the formation of the bark, and on other relations of the tissues to one another. If a stem does not in its growth take the form of a cylinder or slender cone but of a spherical tuber, as in Beaucarnea and Testudinaria, the layers of periderm split apart in the form of tolerably regular polygons which cover the spherical surface of the stem like shields. These examples show at the same time that even in Monocotyledons tensions are produced by the subsequent increase of the stem in thickness similar to those caused by the activity of the true cambium-ring ; for in this case it is re- placed by a thickening-mantle, in which new layers of fibro-vascular bundles and intermediate parenchyma are constantly being produced. (See Fig. 91, p. 107.) It is evident that before the bark splits or fissures already in existence become wider and penetrate inwards, the transverse tension must attain a certain intensity, which, from the great firmness of the bark, cannot be inconsiderable. At the moment when the splitting takes place at least a portion of the tension must, how- ever, be destroyed. This is clearly the reason why the transverse tension attains its maximum (measured in the way described above), as Kraus has pointed out, above the part of the stem where the scaling-off of the bark begins. But even in annual stems which increase rapidly in thickness, as HeHanthus, Dahlia, &c., the transverse tension does not progressively increase from the apex to the root, but attains its maximum at an intermediate height, below which it diminishes. An explanation of this phenomenon is afi"orded by the fact that the limit of the elas- ticity of the bark is gradually exceeded by the long-continued pressure to which it is subject from within, and that the cell-walls which are strained grow at the same time by intussusception, and thus a portion of their tension becomes neutralised. While we may consider the turgidity of the pith and its enormous endosmotic power as the principal cause of the longitudinal tension of growing internodes and leaf-stalks before they become lignified, it is on the other hand probable that the imbibition and swelling of the cell-walls are the chief cause of the transverse tension. The wood, where the transverse tension chiefly originates, is, when mature, scarcely adapted for any distension by turgidity; while at all events in cells or vessels with bordered pits it is altogether impossible. Closed wood-cells, when turgidity is possible in them, cannot however distend greatly ; since their own wall, and the woody substance which surround them are far too inextensible to stretch to any considerable extent under the influence of hydrostatic pressure. It has, on the other hand, been already shown (Sect. 13) what considerable alterations of dimension the wood experiences especially in the peripheral and radial direction simply by imbi- bition. Every layer of wood freshly formed on the inside of the cambium-ring has 728 MECHANICAL LAWS OF GROWTH. a. tendency to grow wider in the peripheral direction, as long as the supply of water is sufficient to cause a decided swelling of the cell-walls. But the cambial tissue is by this means stretched tangentially, and the enlargement of its cells thus caused is increased by turgidity ; and from the thinness of their walls it may be assumed that it is their turgidity that protects them from becoming destroyed by compression between the wood and the bark. The elements of the secondary cortex — the bast- cells and the phloem-parenchyma— can scarcely experience any great change of dimensions owing to the swelling of their cell-walls; the former are indeed thick- walled, but their position does not allow them to form a layer which increases in size from this cause. Finally, the periderm and the bark dry up and contract, if not to any great extent, yet with considerable force. The experience of every year shows that the fissures in the bark — especially of thick trunks at the end of winter in February and March — become deeper and wider, evidently in consequence of the great swelling of the wood which at this time contains the greatest quantity of water; while the bark had time to dry up and contract during the dry weather in winter. If the fissures increase in width by the strong tension thus produced — which can be easily seen when fresh — the damp weather in spring causes the bark to swell ; the tension between it and the wood becomes much less, and the production of wood now begins afresh in the cambium. While the wood is becoming thicker during the summer, the bark dries up and shrinks, and the tension between the outside and inside again increases, to cease once more in the following spring. Not only does an annual period of transverse tension thus arise, but this is also the cause, as we shall see presently, of the difference between the spring and autumn layers of wood. The statements made in this section may be briefly summed up as follows:— The tissues, at first homogeneous, become first of all differentiated in such a manner that chemico-physical differences cause certain layers, especially the pith, to absorb the water in the tissues more strongly than the others, and consequently to grow more rapidly ; and the layers which are less turgid and grow more slowly are exposed to a passive traction which promotes their growth. After growth has ceased it is principally the stronger imbibition and swelling of the wood that presses the surrounding layers of tissue outwards and promotes their peripheral growth. The intensity of the longitudinal and transverse tensions consequently depends mainly on the addition of water to the turgescent pith and the swelling wood ; any decrease of the turgidity of the pith must cause it to contract, and hence the whole shoot to become shorter and flaccid. This is in complete accord with observation, since withered shoots, i. e. such as have lost water by transpiration, have not only become shorter but also flaccid. Any diminution of the amount of water absorbed by the wood must in the same manner diminish the transverse tension and the diameter of the shoot. A small loss of water in the peripheral tissue when in a state of passive tension does not on the other hand usually cause directly any considerable increase in its tendency to con- tract ; since the increase in its size from turgidity and imbibition are generally much less considerable than in the pith and wood. If now there are circumstances which cause a daily periodic change in the quantity of water contained in, the tissues, the result will be also a periodic increase and decrease in the intensity of the longitudinal and transverse tensions. Such a daily periodicity of the tension has been actually discovered by Kraus (/. c. p. 122), who has observed that the longitudinal tension estimated by the difference in length of the pith and the bark, as A\ ell as the transverse tension estimated by the contraction of the bark when detached MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION. 729 from wcody stems, decrease, mider the normal conditions of life, from early morning till midday or early in the afternoon, when they reach their minimum, and then again increase, attaining their maximum early the next morning. Millardet determined this periodicity in quite a different way ; and since the objects on which he experimented permitted an exact measurement, he detected in addition an increase, usually small, of the tension in the afternoon. Notwithstanding the statements of Kraus — which are partly opposed to this conclusion, but on the whole confirm it — I am inclined to attri- bute this periodicity chiefly or altogether to the variation in the amount of water contained in the tissues of the plant at different periods of the day. When transpiration is greatly diminished during the night, the quantity of water in the plant must in- crease, and with this the tension ; and conversely the increase of transpiration during the early part of the day must diminish the tension. Space does not permit me to give in detail the opposing statements of other observers ; but this will be done in part further on. Here I need only point out that the periodicity, especially of the longitudinal tension, may possibly be also directly dependent on light, i. e. independent of the heat which accompanies the light and of the increase of transpiration caused by it (although this cannot be proved by Kraus's experiments. I.e. p. 125). As far as concerns a daily periodicity independent of temperature, light, and the amount of water contained in the tissues, I could only admit it when any other explanation of the phenomena was shown to be impossible. At present this is not the case. From the intimate dependence and correlation of growth and tension, from the fact discovered by me^ that the daily periodicity of growth coincides in every particular with the daily periodicity of tension observed by Millardet and Kraus, and finally from the fact that the periodicity of growth is caused simply by changes in temperature and light, I con- sider it very probable that the daily periodicity of tension is also dependent on these agencies. On the one hand they influence growth and through it the tension, while on the other hand they affect the amount of water contained in the tissues by modifying transpiration and its conduction from the roots. Like all other periodic phenomena of vegetable life, that of tension requires a very careful investigation of its external causes before we resort to the last expedient of assuming internal periodic changes, of which no explanation can be given in the present state of our knowledge. Sect. 16. — Modification of Growth caused by Pressure and Traction. Cells or whole masses of tissue may be subjected to pressure and traction in very different ways. On the one hand these forces may result, in a perfectly normal manner, from the tension of the tissues; on the other hand, external and more accidental circumstances may cause single cells or masses of tissue to be com- pressed or stretched in particular places by solid bodies, or tissues to become accidentally freed from the pressure and traction to which they are normally subject. The numerous phenomena which indicate or prove that growth is altered in this way have however at present been exactly investigated from this point of view in only a few cases. The following will therefore only serve to draw attention to a subject in which further discoveries must contribute to the establishment of a mechanical theory of growth. I. Every cell-wall is subject to Pressure from within, by which it is distended, so long as the cell is turgid. But since the daily experience of microscopists teaches us that all growing cells are turgid; and that on the other hand no cell which is unable to become turgid in consequence of openings in its cell-walls has any power of growth ; and that moreover withered internodes, leaves, and roots Arbeiten des Bot. Inst, in Wiirzbiug 1S72, Heft IT, p. 168. 73 O MECHANICAL LAWS OF GROWTH. do not grow, while these organs grow more rapidly the more strongly turgid they are, it may be inferred that turgidity is an essential condition of the growth of the cell-wall. This appears to a certain extent intelligible if Nageli's theory of growth and Traube's experiments on artificial cells described in Sect, i of Book III are accepted. It may then be assumed that the interstices between the solid particles of the cell-wall which are occupied by water increase slightly in consequence of the distension of the cell-wall caused by the hydrostatic pressure of the sap ; and that space is thus obtained for the intercalation of fresh particles of solid substance ; the distension caused by turgidity then begins afresh and pro- duces the same effect. The distension which takes place at any particular spot of the cell-wall and the consequent intercalation of fresh solid substance, depend however chiefly on the internal properties of the cell-wall itself. Not only do different parts of the cell- wall differ in their extensibility, but they may even vary at the same spot in this respect in the longitudinal and in the tangential or the oblique direction, as may be seen from the swelling of the cell-wall. But that there is actually such a general difference in the extensibility in different directions is at once shown by the fact that growing cells assume the most various forms, — cylindrical, stellate, &c. ; while, if the extensibility of the cell-wall were the same in all directions, the cells must all become spherical as the result of turgidity, or polyhedral under that of mutual pressure. This little is nearly all that we know at present with reference to exten- sibility, turgidity, and growth by intussusception. It must be borne in mind that the rapidity of the growth of cells is in proportion to the thinness and therefore the extensibility of their walls. The growth in thickness of the cell-wall usually begins when the increase of the cell in volume begins to diminish or has altogether ceased. If then the distension of the cell-wall caused by turgidity is the origin of its superficial growth, something similar must also occur when the cell-wall is stretched in some other way by external forces, the turgidity being less. This is the case with the epidermis and cortex of shoots as a result of the tension of the tissues. Since in long internodes and leaves these cells usually grow principally in the longi- tudinal direction, while in broad leaf-blades they assume the form of polygonal plates, this may be referred in the first case partly to the disturbance to which they are subject being chiefly in the longitudinal direction, in the second case to its being in all directions parallel with the surface \ It has already been stated that the cells of the primary cortex of shoots which are increasing rapidly in thickness are not merely stretched but also grow rapidly in the tangential direction^. 2. Pressure from ivitJiout on the ceU-ivaU which is distended by turgidity occurs in a very simple form when the apices of growing cells come into contact with solid bodies; as the root-hairs of land-plants with the particles of the soiP. The very thin * For further details on the possible influence of tension on the formation of stomata, see pfitzer, Jahrb. fiir wiss. Bot. vol. VII, p, 542. 2 On the connection of the radial and peripheral arrangement of rows of cells in a transverse segment with the increase in diameter, see the lucid description of Nageli in his Dickenwachslhum des Stengels bei den Sapindaceen, Munich 1864, p. 13 et ieq. Sachs, Experimental-Physiologic, p. 186. MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION. 73 1 and extensible cell-walls are in close contact with the irregular surface of the par- ticles, just as when an elastic bladder filled with water is pressed externally by an angular body, only that they retain, after the pressure is removed, the form which has thus been given them, evidently in consequence of the intercalation of fresh particles of solid matter which perpetuates the form at first acquired only by distension. The reverse takes place when the external pressure on the cell-wall is removed. Avery simple instance of this is afforded by the formation of the so-called 'Tiillen' in vessels ^ These appearances are produced where the thin non-lignified wall of a cell of the wood-parenchyma, sdll capable of growth, adjoins the bordered pits of a vessel. The portion of wall which is stretched over the opening is forced through it by the pressure of the sap of the cell and swells out in the form of a papilla into the cavity of the vessel. As long as the vessel contained sap and was in a turgid state, its turgidity was in equilibrium with that of the adjoining cell ; but as soon as the cell-sap of the vessel was absorbed, the portion of cell-wall which covers the bordered pit was subject to pressure on one side only, and was therefore forced in the opposite direction. These phenomena can be produced artificially by the removal of the pressure to which the cells are subject from the adjacent tissues ; thus, for example, the cambium swells up on the cut surface of woody branches when placed in moist sand or air, in the form of a cushion between the bark and the wood. This ' Callus,' as it is termed, results from the growth of the uninjured cambial and adjoining cortical cells next the cut, where their growth was previously prevented by the cells which have now been removed. When once projecting beyond the cut, they grow more rapidly than before in a lateral direction in conse- quence of the turgidity, and become divided by transverse and longitudinal walls 2. The further development of such a callus where branches have been cut off leads to the well-known overgrowth on the stumps. In internodes of seedlings of Phar seolus which had accidentally become hollow, 1 found the medullary cells which surrounded the cavity to have grown into it in the form of spherical or club-shaped papillae ; divisions ensued, and nuclei were formed in the cells thus produced. The medullary cells which exhibited this active growth on the free surfaces of their walls would have retained their polyhedral form had the pith remained solid, because every surface of the cell-wall would have been exposed to the pressure of the two adjoin- ing cells ; but in consequence of the formation of the hollow, the pressure was removed on one side, and the turgidity, being no longer neutralised, caused the cell- wall to swell out, and induced in it an active superficial growth^ These phenomena and others of the same kind show that it is often sufficient merely to remove the pressure to \yhich tissues or individual cells are subject in order to bring about an active growth of the free surfaces of their cell-walls. The first cause at least of the new growth is the distension of the free surfaces of the cell-walls in consequence of the turgidity of their cells which was previously neutralised by that of the adjoin- ing cells. But that a very small pressure from without is sufficient to prevent the growth of softer tissues at the points of contact is seen in the case of many large ' See Book I, p. 27 [and references in foot-note]. ^ Further details on this point will be given in a yet unpublished memoir by Prantl. ^ Prantl succeeded in artificially inducing similar phenomena in the tubers of Dahlia. iy- MECHANICAL LAWS OF GROWTH. Fungi which develope among the vegetable mould of woods, and enclose in the margin of their pileus light loosely lying leaves, pieces of stick, and the like. The small pressure from without clearly prevents in these cases the superficial growth of the walls of the cells with which these bodies are in contact, while the adjoining cells extend laterally and enclose them. But the most remarkable illustration of this law is seen in the effect produced by a slight pressure on the growth of tendrils, the longitudinal growth of the cells being thus gready hindered and sometimes even stopped, while the cells of the opposite free side elongate rapidly, as is seen even at the first glance without measurement by making a longitudinal section of a tendril curling round a slender support. In what way the slight pressure which acts in a radial direction, and is generally combined with friction, exerts an influence on the longitudinal growth is however entirely un- known. Very similar phenomena are exhibited by the primary and secondary roots of seedlings (as Zea, Faba, and Pisum). If they are allowed to grow in a damp Fig. 449.— Growth of the pollen-tube o^ Campanula rapunciiloides : Kp the pollen-grain; ps the pollen-tube closely applied to the stigmatic hair nh. locality, and the growing parts are made to press on one side some solid body as a pin or another root, the root bends like a tendril round the body with which it is in contact, this side growing more slowly than the opposite one. It is evidently in consequence of a similar influence of pressure on growth that the aerial roots of Aroideae and Orchideae become closely attached to solid bodies, following exactly their inequalities. But even unicellular tubes, such as the hyphae of Fungi and pollen-tubes (Fig. 449) are induced by contact with a solid body to grow closely applied to it. In this simplest case, where the hydrostatic pressure is uniform over the cell and distends the cell-wall, it does not admit of a doubt that the pressure from without impedes the growth of the cell, independently of turgidity, while the growth proceeds unhindered on the side which is not in contact. But the mechanical processes by which pressure on an organ in the radial direction impedes its growth on that side are unknown. The solution of the question must depend in the first place on whether the pressure acts on the cell-wall directly or in some way or other through the protoplasm^ ^ If the relation between protoplasm and the growth of the cell-wall were better known, stress might be laid on the fact that even a very slight pressure on the cell-wall disturbs the movement of the protoplasm, and may even cause it to become detached from the cell-wall (see Hofmeister, Lehre von der Pflanzenzellc, p. 51). MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION, 733 But in contrast to the phenomena which have now been described, external pressure also sometimes causes growth at places where otherwise there would be none. Thus Pfeffer has shown ^ that certain hyaline superficial cells on both of the flat sides of the gemmae of Marchantia possess the power of growing out into tubular root-hairs when they remain in contact for some time with a moist solid body ; while contact with water produces no effect of the kind. These cells usually develope into root-hairs only when their outer surface is directed downwards, while those on the upper side, not being in contact with a solid body, do not grow out. This, as we shall see presently, is an eifect of gravitation, which is however over- come by the action of the slight continuous contact, since this causes the cells on the upper side of the gemmae also to grow out into root-hairs. The ' haustoria' of Cuscuta and Cassytha and the adhesive discs on the tendrils of the Virginian Creeper are only formed, as was shown by v. Mohl, on the continuous contact of the surfaces of the tissue with a solid body ; and this has been confirmed by recent ex- periments of Pfefifer's {I.e. p. 96)^. In these cases a growth combined with cell- division and differentiation of tissue is caused by contact or slight pressure on a part of the organ, and would not take place without this pressure. These haustoria and adhesive discs thus formed are altogether indispensable for the life of the plant ; for Cuscuta is nourished exclusively by the haustoria which penetrate into the tissue of the host ; and it is by the formation of adhesive discs on the tendrils that the Virginian Creeper is enabled to climb up walls. If the tendrils do not meet with any solid body to which they can attach themselves by means of these discs, they dry up and fall off, while those which have formed discs increase in thickness and become woody. The injurious effect on growth of an external pressure on the cells is very evident in the formation of the annual rings in wood. In the earlier editions of this work I called attention to the fact that the larger radial diameter of the wood-cells in the portion of the rings formed in the spring, and their smaller radial diameter in the por- tion formed in the autumn, may possibly depend on a difference in the pressure from the surrounding bark to which the cambium and the wood are subject, this pressure being less, as we have showm, in the spring, and constantly increasing during the sum- mer. This hypothesis has been fully confirmed by H. de Vries's recent investigations^. In branches two or three years old he increased the pressure of the bark in the spring by firmly winding strings round them at particular places. ' The experiment showed in all cases, firstly, that the absolute thickness of the annual ring was less beneath the liga- ture than the mean thickness of the same annual ring at some distance above or below that spot. In several instances the difference was so considerable that the spot where the experiment was made appeared of considerably less diameter even to the naked eye, and this effect was increased by the formation of cushions of wood immediately above and below the ligature. Secondly, the absolute thickness of the ' autumnal layer ' of wood (up to the middle of August, when the increase in diameter of the tree on which the observations were made ceased), was always greater, and generally considerably so, than the normal thickness at the spot where the experiment was made. In the trees * Aibeiten des Bot. Inst, in Wiirzburg, Heft I, p, 22. ^ [See also Darwin, On the Movements and Habits of Climhing Plants, London 1865, p 84 et seq. — Ed.] ' H. de Vries, Flora 1872, No. 16. 34 MECHANICAL LAWS OF GROWTH. examined {Acer Pseudo-platatius, Sa/ix cinerea, Popuhis albn, Pavia) the autumnal wood was formed at this spot of fibres flattened radially, between which were a smaller number of vessels than in the normal wood ; its composition was therefore the same as that of the normal ' autumnal wood.' The normal autumnal wood of Ailanthus glandu- losa consists almost entirely of wood-parenchyma-cells flattened radially; while the autumnal wood beneath a ligature made in May consists of a thicker layer of flattened^ fibres, between which only a few vessels could be seen. These results show that when the pressure is increased, the formation of the autumnal wood begins at a time when, under normal pressure, a large-celled woody tissue is still being formed. ' A diminution of pressure is obtained by making radial longitudinal incisions into, the bast-tissue. The strips of bast contract somewhat tangential) y, since their tension ceases. Near the incisions the pressure of the bast upon the wood is entirely removed; but in the middle between two adjacent incisions a considerable pressure always remains. The fresh portions of tissue which are formed next to the w^ounds differ to the greatest extent in their composition from the ordinary structure of the wood. A layer of wood of the ordinary structure is formed, on the other hand, in the portions of the cambium at the greatest distance from the incisions, and afterw^ards also outside these abnormal portions of tissue. But it is only the tissue consisting of wood formed under artificially- diminished pressure that we have at present to consider.' Incisions 4 to 6 -cm. in cir- cumference, and mostly 2 to 3 cm. long, w^ere made in two- to three-year-old branches in the middle of June and the middle of July, and therefore after the formation of the normal autumnal wood had already begun. ' The effect of the decrease of pressure was first of all shown, after the branches had been cut oft' in the middle of August, by an increase in thickness considerably greater at the spots than above or below them. On the transverse sections the thickness of the annual ring was greatest near the incision and decreased gradually from there to the middle points between two incisions. The layer of wood formed after the commencement of the experiment was often more than twice as thick at the former as at the latter spots.' For a more exact investigation only those pieces were used in which a layer of distinctly flattened fibres of autumnal wood had been formed before the incision was made. ' But in all cases (the trees already named) the wood outside this layer of autumnal wood— and therefore all that formed after the decrease of pressure — consists of fibres which are not at all flattened radially, but have the same diameter, or even one somewhat greater, than those in the middle of the normal annual ring ; it contains also as many vessels, or even more, than the normal wood. At the time therefore when autumnal wood is being formed in the normal parts of the branches, a woody tissue is produced, if the pressure is artificially diminished, agreeing in its structure with the ordinary wood formed in the middle part of the annual ring. For the normal production of autumnal wood it seems therefore necessary for the bark and the bast to exercise a considerably greater pressure on the cambium and the young wood.' These results explain the older experiments of Knight in 1801. He fastened young apple-trees with a stem of about one inch diameter so that the lower part, about three feet long, was immoveable, while the upper part with the foliage could bend under the pressure of the wind. During the period of vegetation the upper moveable part of the stem increased considerably in diameter, the lower fixed part only slightly. This is easily explained if we bear in mind that the swaying of the upper parts of the stem in different directions by the wind must always stretch the bark on the convex side, and therefore eventually relax it ; it must thus become looser, and therefore the pressure of the bark at these points is always somewhat less than at the lower and immoveable parts of the tree. This explanation is completely confirmed by the fact that in one of the trees which could be swayed by the wind only in a northerly and southerly direction, the diameter of the stem increased so much in this direction as to bear the proportion of 13 to 11 as compared with the diameter in the easterly and westerly direction. It is obvious that this explanation is much more probable GROWTH IN LENGTH UNDER CONSTANT EXTERNAL CONDITIONS. 735 than that given by Knight himself, who thought the movement of the sap in the wood was promoted by the swaying of the stem caused by the wind. The great assistance to the increase in diameter of trees afforded by the diminution of the pressure of the bark on the cambium was long ago employed in horticulture. The bark of young trees is split from above downwards in summer; cushions of wood are formed at the edges of the incisions, which soon close up the wounds. The use of this process is that from the more rapid increase of the wood in thickness, the conduction of water to the leaves becomes more copious and the loss by transpir- ation is more easily replaced. The development of the buds and hence the formation of the organs of assimilation will be promoted by the increase of turgidity in the young branches. Sect. 17. Course of the growth in length under constant external con- ditions \ It has already been explained in the morphological portion of this work that the organs of a plant do not grow simultaneously and uniformly at all points ; but that roots and stems always increase slowly in size at the apex, as leaves also do at least at first. The growing cells not only multiply by cell-divisions which take place regularly, but do not as a whole exceed a certain size, which is always small. Below this punciiim vegeiationis, consisting of primary meristem, not only does the differentiation of the homogeneous tissue into layers of different kinds begin, but also a more rapid increase in size of the cells, which do not now divide so often as before. In the parts of the organ which lie further from the pjinctiu7i vegelationis cell-division ceases altogether (but at different periods in the different layers of tissue), while the growth of the cells still actively continues, until at length, when they have attained their ultimate form and size, the growth of the whole ceases. The cells are then several hundred or even thousand times larger than at the time of their formation beneath the punctum vegetatioiiis. When the growth of stems, leaves, and roots has reached a sufhciendy advanced stage of development, we are able therefore to divide their tissue into three regions : — (i) the piinchim vegeiationis, where new cells are chiefly formed and increase in size is slow; (2) the portion where the main part of the increase in size takes place, but where there is no longer any cell-division or only to a subordinate extent ; this is the elongating portion of the organ; and (3) the portions which no longer grow, at least in length, i.e. the mature portions of the organ. When growth entirely ceases at the punctum vege- tatiojiis, as is usually the case with leaves, all the cells continue to enlarge until the whole is mature. If the stem produces a number of closely crowded leaves, as it usually does at its growing end, the whole of the region in which the chief part of the cell-division takes place is clothed with young leaves, which also themselves consist of cells undergoing division. But as soon as the leaves enter the second stage of development and begin to lengthen, they incline outwards; and when the stem is growing rapidly in length and forming evident internodes (which is by no means always the case) the lengthening begins at those points where it bears ^ Ohlert, Langenwachsthum der Wurzel, Linn?ea 1837, vol XI, p 615. — Miinter, Bot. Zeitg. 1843, p. 125, and Linneea 1841, vol. XV, p. 209. — Griesebach in Wiegmann's Archiv. 1843, p. 267. — Sachs, Jahrb. fiir wissensch. Bot. i860, vol. IT. p. 339. — Miiller, Bot. Zeitg. 1869, No. 24. — Sachs, Arbeit, des Bot. Inst, in Wurzburg 1872, Heft II p. 102; ditto, Heft III, 1873, and Flora 1873, No. 21. — Askenasy, Flora 1873, No. 15. 736 MECHANICAL LAWS OF GROWTH. the leaves, which also begin to lengthen at the same time ; the older mature leaves are generally placed on mature internodes. If the internodes are clearly marked out from one another, as is especially the case when the leaves are verticillate or sheathing at their base, each internode forms a more or less individualised whole as soon as it emerges from the bud, and different stages of growth may be distin- guished in it, advancing from below upwards. This may take place in two different ways, according as the uppermost or lowermost part of an internode remains in an undeveloped condition, the other end being completely mature. This zone which continues for some time in an undeveloped state — cell-division taking place actively in it — is more commonly found at the lower than at the upper end of the inter- node (as in Phaseolus), especially when it is enveloped by closely adpressed leaf- sheaths or by a bulb, as e. g. in Equisetacese (especially E. hyemale), Umbelliferae, the bulbous Liliaceae, the haulms of Grasses, &c. If the internodes are not sharply distinguished, as in stems with small leaves and the floral axes of Dicotyledons, the various states of growth which have been described pass insensibly into one another on the stem ; and this is always the case with roots. If leaves when once expanded continue to grow for some time, the process is the same as with flower-stalks or branches ; while the lower portion of the leaf-stalk is fully mature, the upper parts present successively younger or less developed states. The formation of cells finally ceases at the apex and all the parts then become fully mature. This is strikingly the case in Ferns, less so in the pinnate leaves of Papilionaceae or the incised leaves of Araliaceae. But very often the activity of the piinctum vegctationis of the leaves lasts for only a short time and its tissue matures while cell-divisions still continue at the base of the leaf, and all the transitional states of growth are to be found between the base and the apex. This occurs, for instance, in the long leaves which grow from the bulbs of Liliaceae and allied Monocotyledons. When a cell-producing zone of this kind occurs at the base of an internode or of a leaf, with more mature tissue lying above it, the whole organ behaves as if this zone were a punctiim vegetatioiiis ; the states of growth succeeding one another in the reverse order. Such a zone, in- tercalated between mature portions of tissue may be called an Intercalary vegetative zone. The growth of the internode or leaf may be termed basipetal, in contrast to the acropetal development where the pimdum vegetationis lies at the apex of the internode or leaf. According as the conditions of growth — temperature, the supply of water, and light — are favourable, these phenomena proceed more or less rapidly and uniformly. Every young cell formed at the pimctum vege/alw?n's grows and matures more rapidly the more favourable these conditions are. But if the organs are observed under the most constant possible conditions as they emerge from the bud, it is seen that the growth of the organ both in length and thickness, de- pendent on the gradual development of the cells, does not advance by any means uniformly. The growing portion of a root, internode, or leaf does not lengthen to an equal amount in equal consecutive intervals of time ; and the same is the case with stems consisting of a number of internodes, and with each zone, however small, of a growing organ. It is seen in fact that the growth of each part begins at first slowly, becomes gradually more rapid, and finally attains a maximum of GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. y ]7 rapidity, after which the growth becomes again slower, and finally ceases when the organ is fully mature. If successive equal intervals of time are represented by T^, T,^ .. .T„, and the increments during these intervals by I^, I2 .. . !,„ then it may be stated as a general rule that — for T, T, T3 T, T, T, T, we shall have I^ < I^, < I3 < I, > I, > Ig > zero. This rule holds good for the separate zones of roots, internodes, and leaves, as well as for the entire organs, and for whole stems from their first formation to the time of their full maturity. This course of growth I have termed The Grand Pen'ocP, or Grand Curve of Growth; since it is at once evident that if the values I^, I2 ....I„ are drawn as ordinates with the intervals of time as abscissae, a curve will be obtained which, starting from the axis of abscissae, reaches a maximum of elevation, and returns again to the axis. The following examples will render this more clear. Koppen^ found the following increase of length attained in periods of twenty- four hours with a nearly uniform mean temperature : — Roots ^ of Luphius albus. Increase f length. Mean temperature. First three days : per diem 10 mm. IV2°C. Fourth day 18 i6'6 Fifth day 44 17-1 Sixth day 32-6 16-9 Seventh day 27-9 171 Eighth day 28 i6'4 In an internode of the flowering stem of Fritillaria imperialis I found the fol- lowing increase of length in each period of twenty-four hours* : — Mean temperature. io-6°C. 10-5 11-4 12*2 13-4 i3"9 14*6 150 '^ ' Grand periods,' in contrast to the small periodic oscillations of growth which, if represented graphically, would appear as smaller elevations and depressions on the grand curve. ^ Kiippen /. c. p. 48. I have calculated the daily growth from the lengths given in his tables. ^ That is, the root together with the hypocotyledonary portion of the stem. * A few irregularities in the course of the growth are explained by the temporary acceleration of the growth from the soaking of the ground. Compare the curve in pi. i of the Arbeiten des bot. Inst, in Wiirzburg, Heft II. p. 129. 3 B Normal plant Etiolated plant in the 1 ght. in the dark. March 20 2"o mm. 21 5'3 ' 22 61 23 6-8 24 9'3 7-5 mm. 25 13-4 12-5 26 12*2 12-5 27 8-5 ii'5 7i^ MECHANICAL LAWS OF GROWTH. April March 28 29 30 31 I 2 3 4 5 6 7 8 9 10 Normal plant in the light. io*6 mm. 103 6-3 47 5-8 4*4 3'8 2*0 I'2 0-7 00 Etiolated plant in the dark. 14*2 mm. 12*6 15-9 i6-6 18-2 15-5 i4'o 13-8 11-9 8-8 4'4 2-1 0-6 0"0 Mean temperature. i4-3°C. 12*4 I2*0 II'2 io*7 IO*2 9'4 IO-6 io"7 II'O IIO 112 11-5 12-5 An internode of Humulus Lupidus gave — Increase of length in 24 hours. April 22 19-0 mm. 23 24 25 26 25-0 260 172 4-8 Mean temperature. 149° c. i4"5 14-3 i3'9 141 Harting found that a hop-stem consisting of a number of internodes which was 492 millimetres long on May 15th, had attained by the end of August a length of 7-263 metres, this growth being distributed as follows over the different months: — 0*492 metres in April. 2*230 May. 2*722 Jun«. 1-767 July. 0-052 August. These observations and a number of others show that the grand period of growth manifests itself even when the course of the changes of temperature acts in opposition to it ; i. e. when the temperature rises while the rapidity of growth de- creases owing to internal causes, and vice versa. The course of growth may no doubt be so modified by great changes of temperature that the curve of the grand period can no longer be recognised in the measurements. In order to determine the grand period of growth in a piece of a growing root, internode, or leaf-stalk, it is sufficient to mark a zone of the organ at the part where growth begins by two lines of Indian ink, and to measure the daily (or half daily) growth of this piece until it ceases. By applying this method to the primary root of Vicia Faha, the temperature varying each day between i8° and 2r5°C., I found the following increase to take GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. 739 place in each period of twenty-four hours in a piece originally i mm. long situated immediately above the piinctiim vegetationis : — I St day 1-8 mm 2nd 37 3rd »7-5 4 th 1 6-5 5th 170 6th 14-5 7th 70 8th 00 In the same way I found that a piece at first 3-5 mm. long of the first inter- node of Phaseolus muUiflorus beneath the first pair of foliage-leaves, with a daily variation of temperature between 1275° and 1375° C, showed the following in- crease : — I St day 1*2 mm. 2nd '5 3rd 2*5 4th 5'5 5th 70 6th 90 7th 14-0 8th lo-o 9th 7-0 roth 2-0 Since every organ that is growing in length consists of zones of different ages, which are produced in succession from the primary meristem of the pmtctum vegeta- tionis (or of an intercalary vegetative zone), the successive zones of an internode or a root indicated by ink-marks must show different increments of growth in equal times. While the zone nearest i\iQ puncium vegetationis is beginning to grow, the next one has already entered on a later phase of its grand period, while one at a greater distance would have attained the maximum of its rapidity of growth, and a still further one would have ceased to grow. In other words, a number of zones below the cell-producing puiictum vegetationis are in the ascending phase, while those lying further backwards are in the descending phase of their grand period ; or again, each zone is in a later phase of its period of growth the greater its distance from the pwicium vegetationis. If the successive zones of a growing organ are indicated by the figures I, II, III, &c., and the increments of growth observed at the same time in each of them by Ij, Ij, I3, &c. ; then we have the following relationship : — I II III IV V VI VII VIII \ < I, < I3 < I, > I5 > Ig > I, > zero. There is therefore in the organ a region of maximum rapidity of growth. Thus, for example, 14'ound in the first internode oiPhaseolus multijiorus^ which was divided into twelve zones, each 3-5 mm. long, in the first forty hours: — 3 B 2 740 MECHANICAL LAWS OF GROWTH. Zone. Increment, ist 2*0 mm. 2nd 2*5 .^^ 3rd 4*5 4th 6-5 5th 5*5 6th 3'o 7th 1-8 8th ro 9th i-o loth o'5 nth o"5 1 2th o*5 The maximum rapidity of growth lay therefore in the fourth zone, which was originally situated at a distance of about io'5 mm. from the upper end of the inter- node. As it is usual for several contiguous internodes of stems to be growing at the same time, and the maximum rapidity of growth occurs, according to circumstances, in the second, third, fourth, or fifth internode beneath the bud, the region of most rapid growth is at a considerable distance from the apex of the stem, and especially when the internodes attain a considerable length and several are growing at the same time. In roots, on the other hand, the maximum rapidity of growth occurs much nearer the punclum vegefatiom's, usually at a distance of only a few millimetres ; and the portion of the root beneath its apex in which the chief part of the growth takes place is consequently only a few millimetres long, while in stems with long internodes it is often many centimetres in length. If therefore a root and stem with long internodes are divided into zones of equal lengths, e.g. 1 mm., com- mencing from the punctum vegeiaiiom's, the law of growth, as expressed by the general formula given above, is the same in both cases, but with this difference, that in the stem the number of zones that are increasing in length at the same time is much greater than in the root, in consequence of the fact that in the last case each zone completes its period of growth more quickly^; its curve is shorter and more abrupt. Thus, for example, in a primary root of Vici'a Faha which grew in damp air and which was divided, starting from the punclum vegeiatioftis, into zones each i mm. in length, I found the following increments of growth in the first twenty-four hours at a temperature of 20'5'' C. : — Zone. Increment. 1 0th o*i mm. 9th 02 8th 0-3 7th 0-5 6th 1-3 , * It by no means however follows from this that the root grows more rapidly, t. e. attain? in the same time a greater length than the stem. GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. 74 1 Zone. Increment. 5th 1-6 4th 3-5 3rd 8-2 2nd 5-8 apex 1*5 In this case, therefore, the third zone, where the maximum increase of growth took place, was at first at a distance of only 2 mm. from the apex. It is clear that if an organ is divided into zones of small length, each zone will in general contain a larger number of cells the nearer it is to the punctu77i vegetationis, since the cells are longer the further they are from the apex. But from the point where growth ceases the number of cells in the successive zones of an organ of uniform structure will be the same. If therefore the zones are again designated by the numbers I, II, III, &c., the number of cells in them by N^, Ng, Ng . . . Nn, then we have : — I II III IV V VI VII VIII N, > N^ > N3 > N, > N, > N, > N, = N3. But the difference in the number of cells in the zones is very far from being the cause of the difference in the rapidity of growth that prevails in them ; as is seen at once if it is recollected that the number continually decreases from the apex throughout the growing region, while the rapidity of growth first increases and then decreases. This may be expressed by the following formula : — 1 II III IV V VI VII VIII \ < I2 < I3 < I^ < I5 > Ig > I, > zero. If it were possible to divide in the same manner a filament of Vaucheria, a root-hair of Marchantia, or a similar unicellular organ, into small zones, it can scarcely be doubted (as we may conclude from other circumstances dependent on growth) that we should find the same law to regulate the distribution of the rate of growth in individual cells endowed with a power of apical growth. Since the same law applies to roots and stems — whether zones i or 2 millimetres or stems i or 2 centimetres in length are observed — it is to be expected that this formula would hold good also if zones of only a tenth or hundredth, or even thousandth of a millimetre could be marked out and measured. In other words, we should find that the law of the grand period holds good for each single minute particle of the surface of the wall of a young cell. If the power of any particular zone to attain a definite length is called its' Energy of Growth, then a zone which up to the time when its growth ceases reaches a length of 10 mm. would have a smaller energy than one which continues to grow until it has reached a length of 100 mm. Thus, for example, the successive inter- nodes of most stems each of which was at one period i mm. long, differ very greatly in length when mature ; the internodes first formed are short, the next longer, and finally we have one the longest of all, followed again towards the apex 742 MECHANICAL LAWS OF GROWTH. by shorter ones. If we designate the energy of growth of the internodes I, II, III, &€., by Ej, Eg, E3, &€., we get the series — I II III IV V VI VII VIII E, < Eg < E3 < E, > E, > Eg > E, > Eg. With this increase and decrease in the energy of growth of the various inter- nodes of a stem is usually associated a similar relationship between the size of their leaves, the lower ones forming smaller, the upper ones larger leaves, and then a largest of all (or whorl of largest leaves), usually followed again by smaller ones\ The secondary roots also which spring from the same primary root show similar relationships, the first attaining a smaller length than those that follow, and these being again followed by a graduated succession of shorter ones. The same is the case also with the lateral branches of an annual stem, as well as of trees, especially when the order of development is distinctly monopodial. It seems probable that an investigation of the zones of a root, stem, or leaf, would also show that the energy of growth of successive zones first increases, then reaches a maximum, and finally decreases. The cells in the zone in which the maximum energy of growth prevails would also be the largest, while their number would be least. This hypothesis is in harmony with Sanio's measurements^ of the wood-cells of Pinus sylvestris ; for he found that the final constant size of the wood- cells of the stem varies, increasing gradually from below upwards, till it attains a maximum at a definite height, and then again decreases towards the apex. The same is the case with the branches. If it were possible to predicate the exact energy of growth of every separate zone of an organ, it would also be possible, from the fact that every zone has its separate period of growth, to determine a grand period for the whole organ itself The maxima of rapidity of growth attained in the successive zones first rise and then fall ; the duration of growth also of the zones probably at first increases and afterwards diminishes. Consequently the measurements of the whole organ represent the sum at first of only few and small partial increments, later of more numerous and larger ones ; finally the sum of the partial increments diminishes, because the number of zones growing at any one time and the energy of their growth alike diminish. Further investigation will show whether this hypothesis, which is at least an approximate one, is correct. If the increments of length of an internode, stem, or leaf, in short intervals of time such as half-an-hour or an hour, are compared, it is usually found that they do not increase and then decrease regularly, but irregularly, the growth being sometimes greater, sometimes smaller. If the grand curve of growth is constructed directly from them, it does not assume the form of a continuous curve, but shows a number of small zigzags, which however disappear, if, for example, the interval is extended ' This phenomenon has not at present been sufficiently investigated. In many stems, especially creeping ones, when the leaves have reached a certain size, this size remains constant in a long series of leaves before any decrease occms. 2 Jahrb. fiir wissensch. Bot. 1872, vol. VII, p. 402. By a 'constant' size of the wood-cells I understand that which they possess in their later annual growths ; in the inner annual rings they gradually increase, until in the following ones they attain a constant size. DAILY PERIODICITY OF GROWTH IN LENGTH. 743 from one to three hours or more. These phenomena I call irregular variations of growth \ They appear to result from the plant being subjected to continual small variations of temperature, air, light, and moisture of the soil, which alter the turgidity, and therefore the extensibility and elasticity of the growing cells. I come to this conclusion from observing that irregular variations of growth become less the more the plant is protected from variations in the surrounding conditions. Partial irregular neutralisations of the tension of the tissues may also cooperate to produce this result. Sect, i 8.— Periodicity of Growth in length caused by the alternation of day and night. The alternation of day and night implies varying combina- tions of the conditions of plant-life, especially of those that affect growth. Day and night are distinguished not only by the presence and absence of sunshine, but also by a consequent higher and lower temperature, which again causes variations in the moisture of the air. Independently of special meteorological phenomena, the temperature falls daily with the diminishing elevation of the sun till sunrise the next day, that of the air rapidly, that of the ground more slowly ; at sunset the fall is sudden, as is the rise at sunrise. In general the atmosphere approaches a state of saturation as the temperature falls, /. e. the hygrometric difference decreases, as it increases with the rising temperature. But these general daily alternations act in a variety of ways, and even in opposite directions on the growth of plants ; the increasing intensity of the light after sunrise retards growth, while the increasing temperature promotes it, as long as the other conditions remain the same ; but the increase of the hygrometric difference caused by the increasing temperature of the air occasions also an increase of transpiration, which effects a diminution of the turgidity of the tissues, and this again retards growth. It is uncertain which of these variable causes may have the greatest influence on growth ; and it will depend on this whether the growth of the plant is most rapid by day or by night. On a cloudy but warm and damp day the weak light has only a slightly retarding effect, but the temperature and the great amount of moisture greatly promote growth ; under these circumstances the growth may be greater than in the succeeding night (equal spaces of time being compared), when the total absence of light promotes growth, but the lower temperature is less favourable to it. But the proportion may be reversed ; the plant may grow more slowly by day than by night when the difference in the temperature and moisture of the air during each is but small and very bright days intervene between dark nights, the intense light retarding growth by day more than the depression of the temperature by night. The greatest variety of combinations may be imagined in this respect ; and from the extreme changeableness of the weather the plant will, according to circumstances, sometimes grow more quickly by day, sometimes by night, without exhibiting any exactly recurrent periodicity. The numerous observations which have been made in this direction do not therefore point to any general law^ It has however ^ For further details see Reinke, Verhandl. des bot. Vereins fiir die Provinz Brandenburg, Jahrg. VII; and Sachs, Arbeit, des bot. Inst, in Wiirzburg, Heft II, p. 103. 2 These will be found described by me in detail in the Arbeiten des bot, Inst, in Wiirzburg, 1872, p. 170. 744 MECHANICAL LAWS OF GROWTH. been ascertained that, especially when long spaces of time such as entire days are compared, all the other conditions of growth are outweighed by the effects of the variations of temperature, so that in general the rapidity of growth increases with a rising and decreases with a falling temperature. The result of a number of measure- ments made by Rauwenhoff during several months in the most changeable weather was that the mean growth was greater in twelve hours of the day than in twelve hours of the night ; viz. — By day. By night. in Bryonia 59-0 p.c. 4i-op.c. Wistaria 57*8 42*2 Vitis 55-1 44-9 Cucurbita 567 43-3 do. 57-2 42-8 Dasylirion 55-3 447 A similar tabular statement shows that the favourable influence of a higher temperature by day outweighs the retarding influence of daylight. Rauwenhoff' s measurements show accordingly that the mean growth during six hours of the fore- noon is less than that during six hours of the afternoon ; since, while the average amount of light is the same, the temperature is higher in the afternoon than in the forenoon. If the afternoon growth is placed at 100, then the morning growth is — in Bryonia 86 Wistaria 71 Vitis 67 Cucurbita 79 do. 81 If however we calculate from Rauwenhoff 's measurements the daily and nightly and the morning and afternoon values for shorter periods in which the changes of the weather do not neutralise one another, it will be found that the growth by night sometimes exceeds that by day, and that the afternoon is not always more favourable than the morning. It is clear from what has been said that it is impossible to determine from observations in the open air, where the variations of temperature, light, and moisture are very great and are combined in a great variety of ways, in what manner each separate condition of growth affects the plant, and whether the alternation of day and night causes a similar alternation of growth, or whether there exist in the plant itself causes of daily periodicity independently of external changes. In order to decide this question, it is necessary first of all to make the observations independent of the accidents of weather, which is only possible by carrying them on in well-closed rooms where the temperature is kept constant or made to vary, and where the amount of light can be increased or decreased, and the moisture regulated in the air and in the soil of the flower-pot. Under these circumstances it is possible to study the action of an increasing or decreasing amount of light upon a plant exposed to constant conditions of humidity and temperature, and therefore exhibiting a con- stant degree of turgidity ; it is sufficient to measure and compare the increments of growth during short periods of time. DAILF PERIODICITY OF GROWTH IN LENGTH. 745 A long series of observations of this kind on internodes has given me the fol- lowing results^ : — (i) The more exactly a constant temperature is maintained in a constantly- dark space, the amount of moisture being also constant, the more uniform is the course of growth at different periods of the day. There does not appear to be any daily periodicity of growth independent of external influences. The irregular variations of growth mentioned above were however observed. (2) If great variations of temperature are allowed to act on a plant growing in darkness and with a constant amount of moisture, to such an extent that the tem- perature of the air round the plant alters some degrees C. from hour to hour, the rate of growth of the internodes rises and falls with the rising and falling temperature. If the hourly increments are taken as ordinates, and the intervals of time as abscissae, the curve of growth follows all the elevations and depressions of the curve of temperature, without however any actual proportion being observable between the growth and temperature ; the curves do not run parallel but are only of the same description. (3) If care is taken that during the period of observation the temperature undergoes only slight and gradual changes, while (the temperature being sufficiently uniform) the amount of light changes in the ordinary manner, increasing from morn- ing till midday and decreasing from midday till evening, to complete darkness at night, it will be found that the increments of growth are always greater from even- ing till sunrise, diminishing suddenly after sunrise, and then more slowly till evening. The alternation of day and night causes therefore under these circumstances a periodical rising and falling of the curve of growth of such a nature that a maxi- mum occurs in the morning at sunrise and a minimum before sunset. A second rising of the curve of growth usually takes place also in the afternoon ; but this, as I have shown, is a consequence of the higher temperature in the afternoon which overcomes the influence of light. The retarding influence of light is therefore strong enough to overbalance the favourable influence of the slight elevation of temperature in the forenoon, but not sufficient to overcome that of the stronger elevation of temperature in the afternoon. The fact is of great interest that when a plant has been exposed to light during the day, its curve of growth after sunset, or if placed in the dark in the evening, does not immediately rise abruptly; i.e. that the most rapid growth which is independent of light is not at once attained when it is suddenly placed in the dark ; but that — as is shown by the curve rising slowly till morning — the growth which has been retarded during the day only becomes gradually more rapid in the course of some hours, until the light to which the plant is again exposed in the morning causes a fresh retardation of growth, which again increases from hour to hour till the slowest rate is attained in the evening, if the temperature remains constant. In other words, the two internal conditions of the plant w^hich correspond to darkness on the one hand and to daylight on the other hand pass over only gradually into one another. Light ^ Sachs, Arbeit, des bot. Inst. Wiirzburg, 1872, p. 168 et seq. The plants observed were chiefly Fritillaria imperialis, Humulus Lupulus, Dahlia variabilis, Polemonium reptatis, and Richardia ccthiopica. 746 . MECHANICAL LAWS OF GROWTH. requires a considerable time in order to overcome the nocturnal, darkness a con- siderable time to overcome the diurnal condition of the plant. If this were not the case, the curve of growth would at once rise abruptly in the evening when the room is suddenly darkened, would then continue at the same elevation till morning, fall abruptly when light is again let in, and continue at the same height till the evening. But this does not correspond to the observed phenomena. In order to study more closely the changes of growth occasioned by internal causes, or the dependence of these changes on external conditions, it is necessary to measure Fig. 450. — Arc-indicator, or apparatus for measuring the development of an internode of a growing plant a during short periods of time. the increments in short spaces of time such as an hour or two or three hours. In the case of internodes or leaves of large plants which are growing very rapidly, as the flower- stems of Agave or the leaves of Musaceae, this can be done with a certain degree of exactness by simple measurement with a measuring-rod. But for the purpose of more exact observations it is more convenient to make use of smaller plants which do not grow so rapidly, the growth during an hour not amounting to more than a millimetre, or even less. In such cases a simple measuring-rod is not sufficiently exact ; and I have employed in its place three different methods. In each of them a thin but strong thread of silk is fixed to the upper end of the stem or internode of the plant growing in a pot, the thread passing vertically over an easily moveable pulley and moving an index fixed to the free end of the thread or to the pulley. DAILY PERIODICITY OF GROWTH IN LENGTH. 7A7 I. The Thread-indicator is a simple contrivance in which the free end of the thread which hangs down from the pulley and is kept tight by a weight of a few grammes, carries a horizontal needle which moves freely over a graduated scale as the end of the thread which is fixed to the plant rises with its growth. Fig. 451.— Autographic Auxanometer, fof recofding- the development during short periods of an internode of a growing plant y,- ss' the lines scored by the index z on the blackened paper / fixed to the cylinder C which is made to rotate eccentrically by means of the clock-work D. 2. In the Arc-indicator the thread cf(¥\g. 450) fixed to the plant a is carried over the pulley d and fixed to a pin which is attached to a second pulley g. An index z made of a straight and firm straw is fastened to this second pulley in the radial direction, its 74 B MECHAXICAL LAWS OF GROWTH, free end pointing to a graduated scale on the arc of a circle m n. The equilibrium of the index is secured by the small weight / which tends to turn the pulley in the opposite direction with a force which keeps the thread cf'va a state of tension. As the internode below the hook h lengthens, the weight / sinks, and a piece of the thread cf of equal length is rolled off the pulley^, thus raising the index on the arc. If the index is, for example, ten times as long as the radius of the pulley, the portion of the arc which it will pass over represents ten times the increase in length of the internode. But since it is not usually required to know the absolute amount of the increase but only the relative amount in different times, it is sufficient merely to read off and compare the movements of the index on the graduated scale. By this instrument we are able to measure very small increments of growth ; but, like the first process, it has the disadvantage that the observer must watch it during the whole time, which renders the investigation very difficult, especially at night. 3. The Autographic Auxanomeier gets rid of this difficulty. It consists of a simpler form of the instrument already described. The thready fastened to the plant sets directly in motion the pulley which carries the index 2, being fixed to it by a pin at r. The tension of the thread caused by the index itself is still further increased by the weight g. By this contrivance the point of the index falls as the stem grows below the point to which the thread is fastened. By means of the clock-work D the cylinder C fixed upon the vertical axis a is made to rotate slowly, the rotation being arranged by adjusting the length of the pendulum / so that a revolution is completed in exactly an hour. The cylinder is however fixed eccentrically on the axis «, so that during the rotation one side describes a larger circle that the other side. On the former side is fastened a piece of smoked paper pp. When the index is properly adjusted, its point touches the paper and describes on it a white line s s during the rotation of the cylinder. But after the rotation has continued for some time the index is no longer in contact with the paper owing to the eccentricity of the cylinder, but becomes so again afterwards when it in- scribes another line lower down. The distances between the lines described by the cylinder evidently depend on the rapidity of growth of the plants When, in consequence of this growth, the index has, after say twenty-four hours, reached the lower margin of the paper pp, the clock-work is stopped, the paper removed and replaced by a fresh piece, the index being again set by raising the pulley, and the observation repeated. The lines on the blackened paper are fixed by a varnish of collodion and dried, and the distances between them are proportional to the hourly growths of the internode. It is clear that the apparatus not only magnifies the increments, but also records them in the absence of the observer, which is very convenient, especially for observing the nocturnal growth. It is however necessary even in this case for the observer to note the temper- ature and the hygrometric conditions, at least between morning and evening. Fig. 451 shows in addition a tin vessel 5, consisting of two halves united by a hinge, which may be used for shutting out the light from the plant, even after the thread has been attached to it. At E the thermometer / is placed in a similar vessel near the plant. Sect. 19. — Effect of Temperature on Growth^. It has already been shown in Sect. 7 that the life of a plant generally and its growth in particular is carried on only within certain limits of temperature (in general between zero and 50° C), and that each function has apparently in every plant its inferior and superior limits ; so that, for example, the lovv'est temperature at which a plant of wheat can grow is * See Arbeiten des Wiirzburg. hot. Inst. , Heft II. 2 F. Burkhardt in Verhandl. der naturf. Ges. in Basel, 1858, vol. II, i, p. 67.— Sachs, Jahrb. fiir wissensch. Bot. i860. Heft II, p. 338. — Alph. De Candolle in Biblioth. univ. et rev. Suisse, Nov. 1866. — H. de Vries, in Archiv. neeilandaises 1870, vol. V. — Koppen, Warme und Pflanzen-Wachsthum, Dissertation, Moskow 1870. EFFECT OF TEMPERATURE ON GROWTH. 749 different from the lowest at which a gourd can grow, &c. It has also been shown that growth, like other phenomena, is more active the higher the (constant) tem- perature above the inferior limit, but that there is a certain temperature at which growth reaches its maximum activity, and above which any further rise of temper- ature causes a diminution of its rapidity. There is not, in the mathematical sense of the term, any proportion between the rapidity of growth and the height of the temperature, and the more accurately the relation between the two has been investi- gated, the more difficult is it to express this relation by any mathematical formula. It cannot, on the other hand, be doubted that it is of the utmost importance for any future theory of the mechanical laws of growth to ascertain the extent to which growth depends on temperature, at least in a few particular cases. The difficulties of investigations of this kind are however much greater than is generally thought ; and the results obtained hitherto, valuable as they are, go no further than what is stated above, and give us no deeper insight into the way in which that particular mode of jnotion of the molecules which we call heat is con- nected with that mode of motion which causes growth. Restricting ourselves to the results at present obtained, it will be seen that they have a great practical value in addition to their theoretical significance. A know- ledge of the cardinal points of temperature, I'l'z. its superior and inferior limits and the particular temperature at which the maximum of action takes place, is indis- pensable to investigations of various kinds, in order to get at a correct interpretation of the phenomena. On this account a few of the more trustworthy observations may be given here. In order to determine the cardinal points of temperature to which allusion has been made, observations are of value only when conducted at a nearly constant temperature ; the means deduced from very variable temperatures may, as I have shown, lead to very erroneous conclusions. It is however by no means easy to maintain a sufficiently constant temperature for a whole day even by artificial heating or cooling. Special difficulty is met with in the determination of the inferior limit or specific zero, since the observation must extend over a considerable time — in the case of germination, several weeks — to be certain that growth does not take place. It would be possible, by means of the apparatus already described, to determine in the course of a few hours whether growth still takes place in an internode at a very high or at a very low temperature, and at what temperature it is the most rapid, if it were not extremely difficult to regulate the temperature of the plant in the apparatus with sufficient exactness. The auxanometer will however be very useful even in this case. The observations on this point hitherto made, at least those which have any physiological value, have been on germinating seeds, as the temperature and moisture of the soil in which they grow can be more easily regulated than of the air in the case of internodes. Special facilities are offered by the roots of seedlings, as they do not emerge from the soil, and are more easily measured, from their simpler and more regular form. The following figures refer only to the roots of seedlings, the hypocotyledonary portion of the stem being also, in the case of Dicoty- ledons, included in the root. That exactly the same figures are not always obtained by different observers is the result of differences in the mode of observation, the amount of water, the nature of the soil, the inaccuracy of thermometers, &c. The first point to determine is, whether germination — /'. e. the growth of the embryo at the expense of the reserve materials in the seed — takes place only at certain temper- atures, and at what temperature it takes place most quickly. Observations of my own gave the following results : — 750 MECHANICAL LAWS OF GROWTH. Inferior limit. Most rapid s jrowth. Superior limit. Jriticum Tulgare 5^C. 28-7° G. 42-5°C. Hordeum 'vulgare 5 28-7 377 Cucurbita Pepo i3"7 337 46-2 Phaseolus multt/lorus 9*5 337 46-2 Zea Mais 9*5 337 46-2 This table shews, if the ascertained temperatures are correct, that grains of wheat cannot germinate below 5° G, or seeds of the gourd below i3°*7, &c., however long they may lie in moist earth ; and that they no longer germinate, but quickly perish at temper- atures above those named in the third column ; while at the temperatures named in the second column germination takes place in a shorter time than at either higher or lower temperatures. It may however be taken for granted, from the great difficulty of obtain- ing these numbers, that the result of further observations will not be identical, though probably approximate. It is clear that many series of experiments will be necessary in order to determine each of the cardinal points. The following figures, obtained by Koppen, agree moderately well with mine, as far as they relate to the same plants. Inferior limit. Most rapid growth. Triticum "vulgare 7'5°C. 297° c. Zea Mais 9-6 32-4 Lupinus albus 7-5 28-0 Pisum sati'vum 67 26-6 The following figures were obtained by H. de Vries :— Most rapid growth. Superior limit. Phaseolus indgaris 3i-5°G. above 42*5 Helianthus annuus 31*5 below 42*5 Br as sic a Napus 31*5 » 42-5 Cannabis sativa 31-5 above 42*5 Cucumis Melo 37-5 Sinapis alba 27-4 above 37-2 Lepidium sati'vum 27*4 below 37*2 Linum usitatissimum 27*4 above 37*2 The following results ', obtained by Alphonse de Gandolle, are moderately trustworthy as far as relates to the inferior limit, but hardly so much so with respect to the superior limit and the temperature of most rapid growth, as may be concluded by comparing with those of other observers. Inferior limit. Most rapid growth. Superior limit. Sinapis alba o-o° C. 2I°G. 28° G. Lepidium sati'vum 1-8 21 28 Linum usitatissimum 1-8 21 28 Collomia coccinea 5*o 17 about 28 Nigella sati'va 57 above 2 1 (?) „ 28 Iberis amara 57 TrifoUum repens 57 21-25 below 28 Zea Mais 9-0 21-28 about 35^ Sesamum orientale 13-0 25-28 below 45 I take the figures from the table of curves in De Candolle's treatise, with the assistance of the text. ^ De Candolle remarks that the seeds of maize, melon, and Sesamum become brown, the first as if burnt at 40° C, a phenomenon which has not been noticed by others. These ' burnt ' seeds however germinated afterwards at a lower temperature. EFFECT OF TEMPERATURE ON GROWTH. 75 1 When De Candolle's inferior limits are below 5° C, they are most probably correct ; his superior limits and temperatures of most rapid growth are, on the other hand, for the most part certainly too low. Those figures deserve a more careful study which give the lengths attained by roots in the same periods of time at different temperatures, and express therefore the rate of the growth of the roots of seedlings at different constant temperatures. These numbers increase from the inferior limit to the temperature of most rapid growth, and fall again from it to the superior limit. In Zea Mais, for example, I found — Temperature. Length attained by the root. in 2 X 48 hours 17*1° C. 2*5 mm. 48 48 48 48 48 Temperature. i4-i°G. i8-o 23-5 26-6 28-5 30-2 33'5 36-5 Temperature. 21*6 27-4 30*6 33*9 37*2 The importance of maintaining a constant temperature during each experiment for the determination of these cardinal points is especially evident from the fact observed by Koppen, that the same part of a plant grows with very different degrees of rapidity even though the mean temperature be the same ; if, for example, in one case the mean remains nearly constant, while in the other case it varies repeatedly above or below the mean. It is obvious therefore that if the mean tem.perature is that of most rapid growth, every oscillation either upwards or downwards must retard growth. Koppen shows however in addition (/. c. p. 1 7 et seq.) that growth is retarded by considerable oscillations even below this most favourable temperature. He found, for example, that after a seed of Pisum satiinim had germinated for 144 hours at a constant temperature of 15-1° C, the root had attained a length of no mm. ; when the temperature was variable, while the earth had twice been heated to 20° C. but had fallen between times to 15° C, the mean being 16° C, the roots grew only to 88 mm.; when the temperature varied between 15-0° and 30*0°, the mean being i8*o°, the length attained by the roots was only 56 mm. Although therefore the calculated mean temperatures were higher than 115° C, the growth was retarded, and the more so the greater the oscillations. The following table of the lengths attained by the roots in ninety-six hours in each case is taken from a copious list of Koppen's. 26-2 24*5 35*2 39-0 34-0 55-0 38-2 25-2 42-5 5*9 ■ollowing length of the roots in periods of forty-eight hours :— Lupinus albns. Pisum satiiiiim. Zea Mais. 9' I mm. 5-o° mm. 11-6 8-3 IT mm. 31-0 30*0 IO-8 54T 53-9 29*6 50-I 40-4 26-5 43-8 38-5 64-6 14-2 23-0 69-5 12-6 8-7 20*7 Vries' results, also in periods of forty-eight hours : — cinnis Melo. 'zuapts alba. Lepidium sativum Li^mm tcsitatissintum 3*8 mm. 5-9 mm. IT mm. 24-9 38-9 20-5 i8-2 mm. 520 71-9 44-8 27-1 44-1 44-6 39'9 38-6 30-2 26-9 28T 70-3 lO'O O'O 9-2 J^^l MECHANICAL LAWS OF GROWTH, lean temperature. Hourly change of temperature. L%tpi7t%is albiis. Vicia Faba. I4-4^G. o-o6° G. 3o'omm. 14*0 mm. 14-1 0-28 19-0 9-8 i6-6 0*04 44-0 31-2 17-2 0*26 31-9 17-8 It appears therefore that the growing part of a plant must be subjected for a con- siderable time to any particular temperature in order that its growth may attain the greatest rapidity corresponding to this temperature. Koppen's results are only in apparent contradiction to my own, according to which the curve of growth rises and falls with that of temperature; for it is possible that the entire growth in a given time may be greater when the temperature remains at a con- stant elevation than when it oscillates above and below it. Sect. 20.— Action of Light on Growth. — Heliotropism \ Since we shall now pay exclusive attention to the questions whether and in what way light pro- motes or retards quantitatively the superficial growth of the cell-wall, w^e may for the time leave entirely out of consideration those cases where it changes or may possibly change qualitatively the physiological and morphological nature of the newly formed organs. The dependence of growth on light has already been spoken of in general terms in Sect. 8 ; and it was there especially insisted on that, in order to avoid serious misconceptions, this must be distinctly separated from the question of the part taken by light in assimilation. Here also we are concerned only with the processes of growth itself, since we always start from the point at which the cells or organs concerned have already obtained a sufficient quantity, or even excess, of formative materials. It has been already stated that the various parts of the flower grow as readily in permanent darkness as in light. Most internodes, on the contrary, as has been explained in Sect. 18, grow more slowly when exposed to light on all sides, and remain shorter than when growing in the dark ; when the light reaches them from one side only, they curve concavely towards the source of light. Other organs how^- ever, as root-hairs, tendrils, and some internodes, become longer on the side exposed to light than on that left in the dark. We have seen also that the leaves of Ferns and Dicotyledons soon cease growing in the dark and remain small. These observ- ations show clearly enough that different cells and organs are differently affected by fight as respects their growth. Since the light itself remains the same and there is a supply of formative materials, any explanation of these differences must aim at showing how the inherited organisation of the plant must have been altered just in this way and no otherwise by the oscillations of the ether. It is however at pre- sent quite impossible to give such an explanation^, since far too little is yet known ' A. P. De Candolle, Physiologic vegetale, Paris 1832, vol. Ill, p. 1079. — Sachs, Bot. Zeitg. 1863, Supplement, and 1865, p. 117. — Ditto, Experimental-Physiologic, Sect. 15. — Hofmeister, Lehre von der Pflanzenzellc, Sect. 36. — Kraus, Jahrb. fiir wissensch. Bot. vol. VII, p. 209 et $eq. — Batalin, Bot. Zeitg. 1871, No. 40. ^ If Miiller, in the second part of his Botanischc Untcrsuchungen (Heidelberg 1872), gives the impression of having achieved this with but little difficulty, this only shows how far he is from a true method of investigation. ACTION OF LIGHT ON GROWTH. 75^ of the phenomena themselves ; the ascertained facts cannot yet even be reduced to a general law, especially in consequence of the obscurity which involves the action of light on leaves and on negatively heliotropic organs. If these difficulties, which were referred to in Sect. 8, were solved, the organs of plants might be divided in respect of their behaviour towards light into three kinds: — (i) those the growth of whose cells is in general independent of light ; as petals, stamens, fruits, and seeds ; (2) those whose growth is retarded by light; the positively heliotropic organs which become abnormally elongated by absence of light; and (3) those whose growth is promoted by light. To this last category would belong negatively heliotropic organs if we could be certain of the relation in which negative stands to positive heliotropism ; whether, as has elsewhere been mentioned, it is not, at least in many cases, a modi- fication of the positive form depending on the chemical action of light which is essential to growth ; although recent researches render this very improbable. The question in what manner light affects the mechanical laws of growth of the cell-wall can therefore, in the present state of our knowledge, have a definite meaning only in reference to positively heliotropic organs ; inasmuch as it is in these cases certain that the growth of the cell-wall in the direction of the axis of growth of the organ is retarded and limited by light. But even in this case the question cannot at present be answered, since several others must first be solved. It must first of all be decided whether light acts in this manner on the cell-wall only when its incidence is oblique to the axis of growth. A similar problem, as we shall see, is presented in the action of gravitation on growth. The various phenomena of positive heliotropism allow in fact of the supposiUon that rays of light which penetrate the cell-wall in a direction parallel to the axis of growth of the organ do not hinder growth, while they do so more strongly the more nearly vertical they are to it, whether the organ be multicellular or a simple tube. Light therefore acts more intensely the more nearly the transverse vibrations of the ether are parallel to the surface of the cell-wall. But the solution of these quesdons would by no means explain the action of light on the growth of the cell-wall; in the first place we must know whether light acts directly on the cell-wall, or indirectly by means of the protoplasm, or by chemical changes in the cell-sap. But since we know that the cell-wall only grows so long as it is in contact on the inside with living protoplasm, and that the protoplasm itself is set in motion by light, in consequence of which it accumulates at particular parts of the cell-wall (see Sect. 8); and since this, like the growth of the cell-wall, is caused by the highly refrangible rays — the hypothesis must not at once be set aside. The question may moreover be asked whether light does not influence the growth of the cell-wall by means of chemical efi'ects which it brings about in the cell-sap or the protoplasm, which however cannot be referred to assimilation, since they take place even in cells destitute of chlorophyll, as for instance in the positively heliotropic neck of the perithecium of Sordaria fimiseda, the stems of Claviceps, and in many roots of seedhngs ; and since the leaves of Dicotyledons exhibit relations to light {vide infra) which indicate a chemical action on assimilated substances, but not on the process of assimilation itself. So long as we take into account multicellular organs alone, great weight might be allowed to the hypothesis of a change in the turgidity caused by light (brought about by some chemical alteration in the cell-sap and the consequent change in 3 c 754 MECHANICAL LAWS OF GROWTH. diosmose^). But the fact that even unicellular tubes like those of Vaucheria and the internodal cells of Nitella are positively heliotropic, forbids this hypothesis, since in these cases the side exposed to light grows more slowly than the other, although all the parts of the cell-wall are subject to the same hydrostatic pressure from the sap. The examples already given of positive heliotropism in submerged unicellular tubes, as well as the heliotropic curvings of multicellular internodes under water, show at once that they have nothing to do with a more rapid transpiration in- duced by light or its results. The hypothesis would appear on the contrary to be worth more attention whether the reason why light retards the superficial growth of positively heliotropic cells is not because it first of all promotes increase of thickness, and therefore diminishes the extensibility of the cell- wall under the influence of the pressure of the sap on the side exposed to the strongest light. This hypothesis w^ould be confirmed by Kraus's observations, according to which the cuticularising of the epidermis as well as the thickening of the walls of the cortical and bast-cells is in fact materially hindered in etiolated internodes, and the extensibility of these cell- walls consequently increased by the want of light. This explanation would apply not only in the case of the shaded side of a multicellular internode which curves towards the light, but also in that of a Vaucheria-tube or internode of Nitella ; since it may be supposed that the w^all is in the first place more strongly thickened on the side exposed to light and hence becomes less extensible, and therefore yields less to the pressure of the sap, and, in consequence, grows more slowly. We have at present no observations on heliotropic unicellular tubes. If then it is proved, as the recent researches of Wolkoff give ground for be- lieving, that the negative heliotropism of organs which contain chlorophyll depends as little as that of roots on the stronger power of assimilation possessed by the side exposed to the source of light, it must be assumed that all the actions which have been mentioned as possible in one direction may take place also in an opposite direction ; and this will show the great difficulty of the investigation. A complete account of the mode in which growth depends on light is scarcely possible at present ; what has now been said will call the attention of the reader to the most important questions involved in the investigation. It may be desirable however to collect some of the more important facts at present known, and to add some critical remarks. (a) Organs ^vbose growth is retarded by light. To take first the case of those inter- nodes (including, according to Hofmeister, the unicellular ones of Nitella) which, when the light is unequal on the two sides, curve so that the side facing the source of light is concave while the other side is convex, or in other words are positively heliotropic. These exhibit a periodicity in their longitudinal growth corresponding to the alternation of day and night, when the temperature is sufficiently constant. The growth is more rapid from evening to morning, and less so from morning to evening. Both these facts are however consistent with the phenomenon that the same internodes often grow longer, and even considerably so, in permanent darkness than they would under normal conditions. These three results lead naturally to the conclusion that it is the direct action of light (and only in fact of its more refrangible rays, see Sect. 8), which retards the growth of See Dutrochet, Memoires pour servir, Paris 1S37. vol. II, p. 60 et seq. ACTION OF LIGHT ON GROWTH. J ^^ these internodes. In the case also of positively heliotropic roots (as those of Zm Mais Lemna, Gucurbita, Pistia, &c.), it may be supposed that if exposed to daylight they would exhibit the same alternation as internodes ; but this is not yet fully established. Wolkoff has, on the other hand, already shown in the case of some roots that when they develope in water behind a transparent glass plate they grow more quickly in per- manent darkness than under the alternation of day and night. Twelve primary roots of seedlings of Pisum snti-vum gave, for example, the following results:— Day. ' Successive increments. In the dark. In (Uffusetl litcht. ist 195 mm. 161 mm. 2nd 239 153 3rd 250 210 4th 126 113 5th 113 78 In the 5 days. 923 mm. 715 mm. The increments of growth of primary roots of seedlings of Ficia Fnba were as follows : — In the dark. In diffused light. In 5 roots as 309 to 272 II 743 ^'2 9 612 416 In these cases a tendency of the roots was observed, though not a very decided one, to positive heliotropic curvature. The difference in the rapidity of growth would no doubt have been greater \i the increments in the same time had been compared during the day only. The long narrow leaves of many Monocotyledons exhibit the same phenomena as internodes and roots, becoming considerably longer in permanent darkness than under normal conditions, and showing positive heliotropic curvature when the light from the two sides is unequal. The plane of curvature may coincide with the plane of the leaf, so that one margin may be considerably longer than the other, and the whole leaf there- fore unsymmetrical. I have observed this very evidently in a plant of Fritillaria impe- rialis grown in a window ; those leaves only which sprung exactly from the side of the stem exposed to light being symmetrical like those growing in the open air. We have at present no observations on the daily periodicity in these leaves caused by light. Observation of the broad netted-veined leaves of Dicotyledons is much more difficult. From the fact that in the dark they remain smaller, and often very much so, than under normal conditions, it might be concluded that their superficial growth presents exactly opposite phenomena to those of internodes and the long leaves of Monocoty- ledons. But Batalin has shown that it is sufficient to expose etiolated plants now and then to light— the time not being long enough for them to become green — for their growth in the dark to be afterwards considerably promoted. This leads to the suppo- sition that light causes in etiolated leaves a change which does not consist in chemical assimilation, by which they are enabled to grow further in the dark. In any case this phenomenon shows that there is no real contradiction between the growth of these leaves and that of internodes, and that the reason why they become larger under the normal conditions of light than in permanent darkness is not because light has a directly favourable influence on the growth of the cells of these leaves. The recent experi- ments of PrantP rather favour the hypothesis that green — and therefore healthy and normal — leaves exhibit the same diurnal periodicity of growth as positively heliotropic internodes. He succeeded, by a number of measurements both in breadth and length ^ Compare also Sachs, Arbeit, des hot. Inst, in Wiirzburg, Heft II, p. il 3 C 2 /J 6 MECHANICAL LAWS OF GROWTH. of the leaves of Cucurbita Pepo and Nicot'iana Tabaciofi, taken at intervals of three hours, to construct curves of growth, which in spite of adverse fluctuations of temper- ature, rose from evening to morning, attained a maximum after sunrise, and then fell during the day till evening ; exactly what I showed to be the case with positively helio- tropic internodes. If this general law is established, it results that the broad netted - veined leaves of Dicotyledons grow more quickly in the dark than in the light, and are therefore hindered in their growth by light. But when such leaves remain nevertheless smaller in permanent darkness because they cease growing earlier, this must be inter- preted as an unhealthy condition depending on the suspension of certain processes of metastasis which must precede growth and which are induced by light. In conformity with this hypothesis we must suppose that in leaves which unfold under the alternate influence of day and night, growth is directly hindered by light ; but that at the same time certain chemical changes take place which in general make growth possible, and enable it to continue in the succeeding darkness, if it does not last too long. That this has nothing to do with assimilation is shown by Batalin's experiments with leaves desti- tute of chlorophyll. If we now enquire what are the mechanical changes which light causes in the organs we have been considering, and by which their growth is retarded, it is to be regretted that no experiments have yet been made as to their effect on unicellular organs which ex- hibit positive heliotropism, as Vaucheria-tubes and internodes of Nitella, since they present the most simple case from a mechanical point of view. In the case of the internodes of Phanerogams which consist of tense layers of tissue, Kraus found in the etiolated state a smaller tension between the medullary and cortical layers, and therefore that the cell- walls of the layers of tissue placed in a state of passive tension by the pith were less thickened, lignified, and cuticularised. It follows that these last are more extensible than in the normal internode, and therefore offer less resistance to the tendency of the pith to elongate. If we suppose that in unicellular tubes light also increases the cuticularis- ation and thickening of the cell-wall, the wall will offer greater resistance to the pressure of the cell-sap will become less stretched, and will therefore grow more slowly. But little can be inferred as to the mechanical influence of light on growth from the changes in the tension of the tissues on the convex and concave sides of internodes with positive heliotropic curvature. If such an internode is split lengthwise so that the side exposed to light is separated from the other side, the former becomes more concave, while the latter becomes less convex or even somewhat concave towards the shaded side. In other words, the tension between the outer and inner layers is greater on the concave side exposed to light than on the convex shaded side. But the same phenomenon occurs also in internodes with an upward geotropic curvature, and with negatively heliotropic internodes, as well as with twining tendrils; and could not in fact be otherwise. (b) Of Negatively heliotropic organs'^ only a comparatively small number are at present known. Among those which contain chlorophyll may be named the hypocotyledonary portion of the stem of the seedling of the mistletoe, the older nearly mature internodes of the ivy and Trcpoeolum majus, and the basal portions of the tendrils of the vine, Vir- ginian creeper and Bignonia capreolata. I pass over at present the doubtful negative heliotropism, as I think, of the thallus of Marchantia and the prothallia of Ferns, as well as of other decidedly bilateral organs. Among organs which are not green must be especially mentioned the negatively heliotropic aerial roots of Aroideae and epidendral Orchids^; but, beyond all others, the roots of Chlorophytum guayanum, which are ex- tremely sensitive to light coming from one side. Negative heliotropism has, in addition, been stated to occur in the roots of seedlings of Cichoriaceae, Cruciferae, &c., and has recently been certainly determined by Wolkoff" in the case of Brassica Napus and Sinapis * Knight, Phil. Trans, 1812, p. 314. — Dutrochet, Memoires, &c„ vol.11, p. 6 et 557.— Durand and Payer's statements. — Compare Sachs, Exper.-Phys., p. 41. ^ According to a great number of observations of my own and statements of others. ACTION OF LIGHT ON GROWTH, y ^y alba. Among unicellular organs destitute of chlorophyll the only ones known at present with certainty to be negatively heliotropic are the root-hairs of IVIarchantia. The remark that a number of organs destitute of chlorophyll and endowed with negative heliotropism, and in particular the highly sensitive roots of Chlorophytum, are very transparent, led Wolkoff to the hypothesis that the rays of light may be refracted by their cylindrico-conical shape, so as to produce a more intense illumination of the tissue on the side removed from the source of light than on that exposed to it ; and that therefore the concave curvature on the former side is in fact a form of positive helio- tropism. The apices of roots, when separated by a transverse section, if illuminated from one side and viewed from above, exhibit exactly such differences of luminosity as might be expected on this hypothesis. It must however not be forgotten that the apices of roots which are by no means negatively but at an earlier period even positively helio- tropic, like those of Vicia Faba, manifest the same phenomenon, though perhaps to a lesser degree. Whether, on the other hand, it is possible to suppose a similar refraction of light in the case of the very thin-walled negatively heliotropic root-hairs of Mar- chantia, is still in doubt. Further researches must show whether Wolkoff's happy idea is tenable or not. In the cases of the older internodes of the ivy which are only very slightly trans- parent, the older and lower parts of tendrils, &c., the existence of an actual focal line on the shaded side cannot be admitted, because this would evidently imply that they received more intense blue and violet light than, from the fact that the tissue which is penetrated by the light contains chlorophyll, it is probable they do. The negatively heliotropic curvature takes place however, at least in the ivy as well as in the roots of Chlorophytum, only in highly refrangible light (after passing through an ammoniacal solution of copper oxide), not in yellow^ light (which has passed through potassium bichromate). If, as Wolkoff at one time supposed, the more vigorous nourishment, /. e. accumulation of assimilated substances, were the cause of the more rapid growth on the side exposed to light in this class of negatively heliotropic organs, they ought to curve much more strongly in the less refrangible (red, orange, or yellow) than in the more refrangible rays. This hypothesis would moreover fail to explain why the same internodes which when young showed decided positive heliotropism, at a later period when their growth has almost ceased manifest the opposite behaviour towards light. The experiments which Wolkoff is now carrying on in the botanical laboratory at Wiirzburg, and which are not yet completed, lead at present to the conclusion that there are two kinds of negatively heliotropic organs. In one kind are included roots, in which the negatively heliotropic curvature takes place near the apex at the spot where growth is most rapid ; to the other kind belong internodes where the negatively helio- tropic curvature takes place only at the older parts whose growth is completed, while the young quickly growing parts manifest positive heliotropism. In these latter cases the additional peculiarity occurs that the older parts, after being exposed to light on one side, will continue for some time to curve in the dark so that the side exposed to light becomes still more convex. This is a property which appears to be wanting in organs of the first kind as well as in those that are positively heliotropic. It is evident that we are here confronted with an unsolved problem; and when all the facts have been taken into consideration, the theory that there are two kinds of cells, the growth of one of which (positively heliotropic) is retarded by light, whilst that of the other kind (negatively heliotropic) is promoted by it, may be the simplest and most in accordance with facts. This difference is the less remarkable since in the behaviour of growing cells with respect to gravitation we find a precisely similar difference, but much more strongly marked ^ * Schmitz, Linnrea, 1843, p. 513^/5^7. If, as can scarcely be doubted, Schmitz's statements with regard to Rhizomorphs are confirmed, it results that no certain inference can be drawn as to the positive heliotropism of an organ from the fact that its growth is more rapid in the dark. We 75 y MECHANICAL LAWS OF GROWTH. Sect. 21. — Influence of Gravitation on Growth: — Geotropism^ It has already been shown in Sect. 10 that, when the access of Hght is equal on all sides or when heliotropisra is prevented by the exclusion of light, gravitation is the cause of certain organs turning downwards, others upwards, and others again in a direction oblique to the horizon. At present we shall speak only of those which take a direc- tion directly upwards or downwards, since other causes co-operate to bring about an oblique growth. Just as organs, according to their internal nature, grow either more rapidly or less rapidly on the side which faces the source of light than on the other side, so also gravitation effects, in accordance with the nature of the organs, either an acceleration or a retardation of growth on the side which faces the earth. Those organs which are thus retarded in their growth are called positively geoiropic, those which are accelerated negatively geotropic organs. Positively geotropic organs consequently become concave on the under side, and direct their growing apex downwards if their axis of growth is brought into a horizontal or oblique direction ; negatively geotropic organs, on the contrary, become convex on the under side under similar conditions, and elevate their growing apex until it stands erect. It has not yet been ascertained whether positively geotropic organs would mani- fest a different rapidity of growth if entirely withdrawn from the influence of gravita- tion (like positively heliotropic organs when withdrawn from the influence of light) from that displayed when gravitation acts in a direction parallel to the axis of growth. It would seem however as if gravitation only affects the rapidity of growth when its direction cuts that of the axis of growth at an angle, and the more so the nearer the angle approaches a right angle. The positive or negative character of geotropism depends as little as that of heliotropism on the morphological nature of the organ. Not only, for example, are all the primary roots of the seedlings of Phanerogams positively geotropic, and most secondary roots which spring from underground stems, as tubers, bulbs, or rhizomes ; but also many leafy lateral shoots, especially those which are destined to produce rhizomes or to form new bulbs {e.g. Tulipa, Physalis, Polygonum, &c.), and even fohar structures, like the cotyledonary sheaths of Allium, Phoenix, and many other Monocotyledons. Among positively geotropic organs must also be included the lamellae and tubes of the hymenium of Hymenomycetous Fungi. All axes which grow upright (and are not bilateral), petioles, and the stipites of many Hymenomycetous Fungi, exhibit, on the other hand, decidedly negative geotropism. The geotropism, like the heliotropism, of different organs varies in all degrees. It is, for example, manifested very strongly in the primary roots and upright could scarcely have a better proof of the necessity for a fresh and more accurate investigation of all the phenomena of heliotropism. ^ Knight, Phil. Trans. 1806, vol. I, pp. 99-108. — Johnson, Edinburgh, Phil. Journ. 1828, p. 312. — Dutrochet, Ann. des Sci. Nat. 1S33, p. 413. — Wigand, Botan. Untersuch. Braunschweig 1854, p. 133. — Hofmeister, Jahrb. fiir wissensch. Bot. vol. Ill, p. 77. — Ditto, Bot. Zeitg. 1868, Nos. 16, 17, and 1869, Nos. 3-6. — Frank, Beitriige zur Pflanzen-Phys. Leipzig 1868, p. 1. — Midler, Bot. Zeitg. 1869 and 1871.— Spescheneff, Bot. Zeitg. 1870, p. 65. — Ciesielski, Untersuch. iiber die Abwiirts- kriimmung der Wiirzeln, Breslau 1871. — Sachs, Arbeit, des bot. Inst, in Wiirzburg 1872, Pleft 2, Abh. 4 and 5. — Ditto, Exper.-Phys., p. 505. — Ditto, Flora, 1873, No. 21. INFLUENCE OF GRAVITATION ON GROWTH. 759 primary stems of seedlings ; much less strongly in the secondary roots which spring from rhizomes, climbing stems, &c. The secondary roots of the first and of higher orders which spring from the primary roots of seedlings display this phenomenon in different degrees. It appears to be the general rule that when lateral shoots of the same kind spring from a vertical and therefore decidedly geotropic organ, the branches of the first order are less geotropic, and the further ramifications still less so the higher the order to which they belong ; the exceptions to this rule may be caused by special circumstances. This gradation is very obvious in roots. From the primary root or a strong root springing from the stem with decidedly positive geotropism, proceed secondary roots of the first order which exhibit the phenomenon much less decidedly; and from these again secondary roots of the second order which apparently are not at all geotropic, and therefore grow in all directions as they may chance to originate. Geotropism, like hehotropism, does not depend on the organ containing or not containing chlorophyll, nor on whether it consists of masses of tissue or of a simple row of cells or of a single cell. To this last category belong, for example, the positively geotropic radical tubes of the Mucorini and the negatively geotropic sporangiophores of the same family and of numerous other Mould-fungi. In the same manner the rhizoids of Chara display positive, the stems negative geo- tropism, both consisting of unicellular segments, the former destitute of chlorophyll, the latter green. Whether and how strongly an organ is positively or negatively heliotropic or geotropic depends altogether on its importance in the economy of the plant, and hence on its physiological functions. From the remarkable fact that there are organs endowed with positive and negative hehotropism and geotropism, and from many similarities exhibited by the two phenomena, the question presents itself whether all positively heliotropic organs must not possess one description of geotropism either positive or negative, or vice versa ; in other words, whether the two properties do not stand in some definite relation to one another. This does not however appear to be the case. Of primary roots, all of which are positively geotropic, some display positive, others negative hehotropism ; and again, the aerial roots of Chlorophytum, Aroidese, and Orchideae, display very distinct negative hehotropism, but are scarcely at all geotropic. There appears therefore to be no necessary connection between the two phenomena. It is clear that organs which are both heliotropic and geotropic, and on which, since they lie obliquely to the horizon, the light falls from above or from below, are subject to changes in their growth dependent both on light and on gravitation. Thus, for example, the bending upwards of a branch placed horizontally on which the light falls from above may be caused at the same time by positive hehotropism and by negative geotropism. An erect stem, on the other hand, which turns helio- tropically towards a source of light at the side and thus makes a curvature which is concave below, will have a tendency to become erect in consequence of its negative geotropism, just as if there were no light falling on it from one side. Stems there- fore which in the evening were bent by positive hehotropism, will stand upright in the morning. These considerations are evidently of the first importance in making observations on the two phenomena. We have already seen that no clear idea has yet been obtained of the mode in which light acts in influencing growth in heliotropic organs. As little are we at 760 MECHANICAL LAWS OF GROWTH. present in a condition to affirm how the acceleration or retardation of the growth of the cell-walls results from the action of gravitation. The hypotheses and considerations there stated may be repeated here mutatis mutandis. Particular stress must be laid on the fact that movements are induced in protoplasm by the action of gravitation just as by the action of light. Thus Rosanoff showed^ that the plasmodia oi ^tha- lium sepiicum are negatively geotropic, creeping, under the influence of gravitation, over steep moist walls, and turning, under the action of centrifugal force, towards the centre of rotation ; they take therefore those directions which would be least expected from their apparently fluid condition. The question suggests itself whether there is not also protoplasm which behaves in this respect in an exactly opposite manner ; and from the dependence of the growth of the cell-wall on the activity and probably also on the disposition of the protoplasm in the cell, the hypo- thesis must not be altogether set aside that all geotropic phenomena are in the first place caused by the protoplasm taking up definite positions in the cells under the Fig. 452. — Diagram for illustrating geotropic upward and downward curvature. influence of gravitation, and thus accelerating or retarding the growth of the cell-walls on the under sides. Since nothing is known on this subject, we must direct our attention solely to the growth of the cell-walls, leaving it undecided whether the efTect of gravitation be direct or indirect. In order to state clearly the problem how gravitation acts on the growth of the cell-wall^, we may consider as the simplest example a unicellular tube, such as we find in Vaucheria, the posterior end of which developes as a positively geotropic root, the anterior end as a negatively geotropic stem. Fig. 452 ^ may represent this, assuming that the whole tube grew at first in a vertical direction either upwards or downwards, but was then placed in a horizontal position, as shown by the light out- lines -S* and W. After some time the radical end would show a downward curvature, like W\ the part 6" on the contrary the upward curvature, as S\ It is self-evident that each of these curvatures can only result from the growth, equal on all sides when the organ is erect, having now become unequal on the upper and under sides, the convex growing in both cases more quickly than the concave side. * Rosanoff, De Tinfluence d'attraction terrestre sur la direction des plasmodia des Myxomycetes (Memoires de la Societe imperiale des sciences de Cherbourg, vol. XIV). ^ Duchartre's assertions on geotropism in his Observations sur le retournement des champignons (Compt, rend. 1870, vol. LXX, p. 781), show that he has not clearly comprehended the question. INFLUENCE OF GRAVITATION ON GROWTH. 76 1 If we now apply the results of my experiments on internodes and nodes of Grasses which curve upwards to the simple tube, the growth is found to be more rapid on the convex under side, less rapid on the upper side of the upwardly curved part, than when it grew erect. It may be assumed, from Ciesielski's measurements of roots, that when the tube curves downw^ards the growth is more rapid on the convex upper side, less rapid on the concave under side, than when the curved part grew for a longer time in a vertical direction. In other words, when the tube is placed in a horizontal position the growth is accelerated on the upper side of the positively geotropic part and on the under side of the negatively geotropic part, but always retarded on the opposite sides. If therefore we assume that in Fig. 452 ^ the two side walls of a transverse disc of the part 6" of the tube when in an upright position had lengthened in a definite time to the equal lengths and u u, it would have remained upright ; and if the tube had been placed horizontal during this time, the lower side would have attained the greater length 11 ii, the upper side the shorter length 0' o\ and the piece must in consequence become curved. Exactly the opposite would be observed, as shown in Fig. 452 C, if the growing piece belonged to the part fFof the tube. If now the unicellular tube A were supposed divided by transverse and longi- tudinal divisions into a tissue consisting of a number of layers of cells ; or if, what amounts to the same thing, a stem of a seedling were supposed substituted for the part 6" of the tube, and a root for the part W, the same phenomena w^ould occur, as experiments have shown, in every cell of the growing part, as those previously observed in the tube. In the part S every cell would grow^ more rapidly on the under side, less rapidly on the upper side than if the part were upright, the reverse in the part W. We should find that in -S" both the upper and under sides of any cell {i. e. upper and under in relation to the radius of the earth) are longer than those of the cells situated above it, the reverse in W; in other words, that every individual cell of a part which shows geotropic curvature behaves in the same way as if the part previously straight were held firmly by the two ends and then bent. This will be made clearer to the student if in the portion of the curved part included in A lines are drawn parallel both to the straight and the curved outlines, and the septa of the cells are then indicated in the straight piece simply by parallel lines crossing the first at right angles, in the curved part by lines corresponding to the radii of curvature. The cells exposed by longitudinal sections through nodes of Grasses and roots endowed with geotropic curvature exhibit this phenomenon, although with many irregularities. When the facts connected with the geotropism of the cell-wall have thus been made clear, we may proceed to the question, w^hy or by what effect of gravitation these differences are occasioned in the growth on the upper and under sides of every cell of a geotropic organ when placed in a horizontal position. We have at present however no answer to this question, any more than in the case of helio- tropism, the same diagram availing, mutatis mutandis^ for the two phenomena. The view brought forward by Hofmeister, and for some time adopted by me, that positive geotropism occurs only in those organs and in those parts of organs in which there is no tension in the tissues, while the organs in which there is strong tension are negatively geotropic, rested on imperfect induction. On the one hand 7^2 -r-'j MECHANICAL LAWS OF GROWTH. the parts of the roots of seedlings which curve downwards (as I have shown else- where), are not entirely without tension between the cortex and the axial bundle ; while, on the other hand, in the nodes of Grasses, although they display a high degree of negative geotropism, there is no or very little such tension. Even in the negatively geotropic contractile organs of the petioles of Phaseolus^ the tension between the cortex and the axial bundle is of a similar character to that which occurs in positively geotropic roots, but extremely intense. If therefore the tension of tissues and the alteration effected in it by the influence of gravitation cannot be considered as the cause of the upward curvature, it may still be admitted that it is only useful to upright organs by increasing their rigidity and elasticity, and thus making them more adapted for the erect condition ; while this would be quite un- necessary in those that grow downwards. A good illustration of the part played by rigidity and elasticity in producing the erect position of negatively geotropic organs, is afforded by the pendent pedicels of many flowers and flower-buds, in which the tendency to bend upwards is altogether obscured, the weight of the flower being sufficient to bend the pedicel downwards. If in such cases the flower-buds are cut off, the pedicel becomes erect ^ from the stronger growth of the under side, as e. g. in Cle?)iatis integrifolia, Papaver pilosiwi and dubiiim, Geinn rivale, and Anemone pratensis. The tension in the tissue of such pedicels is not sufficient to give them the rigidity needful to overcome the weight of the flower by their geotropic curvature upwards ; this weight, on the contrary, overcomes the tendency of the pedicel to curve convexly on the lower side, which tendency comes into play when the weight is removed. The same is the case in very long but not very rigid shoots, as those of the weeping willow, weeping ash, &c. Since geotropic like heliotropic curvatures take place only during growth^, the position of the parts that will curve in the various organs is known beforehand if the course of their growth is known (see Sect. 17); and conversely the part where growth is at any time taking place may be ascertained by this rule from the fact of its curving. From causes which we cannot go into here more in detail, the curvature does not generally take place in the form of an arc of a circle ; but there is in organs of considerable length — whether they curve upwards or downwards — a spot where the curvature is greatest, i. e. where the radius of curvature is least. It would appear, from all that is at present known, that when organs are laid in a horizontal position the strongest curvature is always found at the spot where growth is most rapid. But since in erect stems a piece of considerable length (often 20 cm. or more) is actually growing, a long and flat arc is formed when the stem erects itself from a horizontal position, the maximum curvature of which is at a considerable distance from the apex of the stem. In primary roots, on the contrary, growth exists only in a space of a few mm. from the apex, the maximum increment of growth taking place at * Sachs, Experimental-Physiologie, p. 105. ^ See De Vries, in Arbeiten des Wiirzhurg Bot. Inst., Heft II, p. 229. ^ It must be noted that some organs, if grown in a normal position and then placed horizontal, begin then to grow like the nodes of Grasses and the contractile organs of Fhaseolus. INFLUENCE OF GRAVITATION ON GROWTH. 763 the most from 2 to 3 mm. from the apex, and the strongest curvature is therefore at this spot, or very near the apex ; and when the organ lies in a horizontal position the curvature is very strong, or the radius only very small (a few millimetres). It is easy to see that when a very strong curvature takes place near the apex of a root, it serves to fix it in the ground; while it is mechanically useful for the erection of stems that they curve in larger flatter curves. In the jointed haulms of Grasses the work of flexion is distributed over two or three nodes, a portion of the curvature taking place in each node until the haulm again stands erect. Knight, the discoverer of the fact that gravitation is the cause of geotropic curvature, thought that the curving upwards of the stem was occasioned by the food-materials collecting in greater quantities on the under side and hence causing a more powerful growth. Hofmeister, who called attention to the relation of the tension of the tissues to the various curvatures of the parts of plants, explained the action of gravitation in caus- ing an upward curving in the first place by an increase of the extensibility of the tissue on the under side, which is in a state of passive tension. I have, on my part, directed attention to the fact that the growth of the under side of organs placed horizontally which have a tendency to curve upwards is accelerated, while that of the upper side is retarded ; but whether this is caused by a corresponding distribution of the food- materials, or by a change in the extensibility of the passive layers, or in any other way, 1 leave for the present undecided. The curving downwards of the roots of seedlings was explained by Knight In an unsatisfactory way as a result of the softness and flexibility of the growing apex, a view which was adopted by Hofmeister in a less crude form, and for some time also by myself. It was assujued on this theory that the tissue of growing roots may be compared to a tough piece of dough, which tends, from the force of its own weight, to curve down- wards at the free unsupported end. I thought that by the excess of weight of the free apex a traction was exerted on the growing cell-walls of the parts of the upper side which curve, by which growth or deposition of food-material is promoted on this side, while the reverse must be the case on the under side ; and I think that Hofmeister explained the process in a similar manner. Frank therefore did not hit the nail on the head when he merely insisted that the downward curving of the apex of the root depends on growth being stronger on the upper side ; this we had admitted. It would have been more to the purpose had he said why growth is more rapid on the upper than on the under side of the apex of a root placed horizontal. Frank, on the other hand, was right in maintaining that our explanation was untenable, because, as Johnson had already shown, the apex of a root turns downwards even when its own weight is counterbalanced by an equal or slightly greater one, and because the root, even when it rests on a horizontal solid support, shows the same phenomena of growth which cause its apex to point down- wards. The statements of Frank and the subsequent ones of Miiller were however inadequate on the points in question. If I relinquish Hofmeister's view, which I had previously in the main adopted, it is in consequence of more comprehensive experiments on the growth of roots, and especially on their geotropic curvature. It would carry us too far here to give the reasons for and against the theories which have been alluded to ; and it would serve as little purpose to go into an explanation of particular phenomena, as for example the fact that roots penetrate to a depth of from 2 to 3 mm. into mercury, whether they impinge upon it vertically or obliquely \ It seems to me that any theory of geotropism can only be adequate if it is able to 1 See Pinot u. Mulder, Ann. des Sci. Nat. 1829, vol. XVII, p. 94, and Bydiagen for de natuur- kund. Wetensch. 1829, vol. VI, p. 429 ; also SpeschenefT (Bot. Zeitg. 1870, No. 5), whose statements I am able lo confirm in tlie main by a number of experiments of my own. 764 MECHANICAL LAWS OF GROWTH. explain equally the positive and the negative descriptions, and to show why the same external cause produces opposite results in cells and organs of precisely similar structure, acceleration or retardation of growth on the under side and the reverse on the upper side. If a number of organs grow in a horizontal or oblique direction without curving either upwards or downwards, this may result from their not being geotropic and grow- ing straight forward in the direction of their first origin, as rootlets of a high order which grow downwards from the under side of their parent root, upwards from the upper side, horizontally from the sides, or continue to grow straight and oblique according to the direction of the primary root. To this must be referred, among other phenomena, the striking one described by me that plants which grow in uniformly moist soil emit a large number of fine roots out of it with their apices pointing upwards ; there are even rootlets of the first or second order which spring from the upper side of Fig. 453. — Apparatus to illustrate the mode in which the geotropism of the roots h i k m of seedlings^ g g is overcome when they come into contact with a moist surface. horizontal or oblique parent roots and grow straight upwards without being geotropic. If the air is able to enter the ground freely, its surface is often dry, and the fine roots which are directed upwards die off, as I have ascertained by growing plants in glass vessels filled with earth. But even geotropic organs may grow obliquely or horizontally when other causes oppose or counterbalance their geotropism. One of the most common of these causes is the bilateral structure which makes an organ grow more strongly on one side from internal causes. Since I shall recur to this subject in the next section, only a single example need be given here. In the case of seedlings, rootlets of the first order not unfrequently appear above the surface of the soil obliquely when it is uniformly moist ; and I have convinced myself that this is the result in cases which have been observed {e. g. Victa Faba) of a stronger growth of their lower side altogether independent of geotropism, in consequence of which they always grow in a flat curve concave upwards. But external causes may also act in opposition to geotropism even when this is very strongly developed. Thus Knight and Johnson have shown, as I have recently described UNEQUAL GROWTH. - 765 more in detail, that primary roots with strong positive geotropism, as well as secondary rootlets, when growing in moderately damp air, deviate from their vertical or oblique direction when there is a moist surface near them. Under these circumstances a curvature concave to the moist surface takes place at the region below the apex where there would otherwise be a downward curvature, the apex being by this means con- ducted towards the moist surface so that it may penetrate into the moister soil or grow in contact with it. The apparatus represented in Fig. 453 is well adapted to exhibit this phenomenon. It consists of a zinc frame a a covered below with wide-meshed network, thus forming a sieve hanging obliquely and filled with moist sawdust ff. The seeds ggg germinate in the sawdust, their roots penetrating at first vertically downwards into it. When the apex of a root escapes through the network into air which is not too dry, it turns towards the moister surface h-m^ its geotropism being thus evidently overcome'. Sect. 22. Unequal G-rowth ^. Our observations have hitherto had reference almost exclusively to the growth of multilateral or polysymmetrical organs, such as erect stems and descending roots. Organs of this kind offer the simplest example of growth taking place equally on all sides. But they form only a small minority, since not only a large number of primary stems like those of Hepaticoe, Rhizocarpeoe, and Selaginellea?, but also by far the greater number of erect stems, and all leaves, display a decidedly bilateral structure, i. e. two sides of their axis of growth exhibit different characters. With this bilateral organisation is also usually connected a difference in the growth of the two unequal sides, which causes curvatures and hence changes in the position of the apex. The two unequal sides of bilateral organs must also be acted on differently by external agencies which affect growth, such as light, gravitation, and pressure. We do not attempt here to solve the question of the causes which produce the bilateral structure in any particular case ; it need only be shown incidentally that this structure of lateral organs (as we have already seen in Book I, Sect. 27) is probably always brought about by internal causes, and is independent of the action of external circumstances. This is in general at once evident from the fact that the median plane of bilateral appendi- calar organs has always a perfectly definite geometrical relation to the axial structure which bears them, and that moreover in the dark and under the influence of slow rotation round a horizontal axis, which eliminates the effect of gravitation, the bilateral structure and relation to the axis remain unchanged. But before we proceed to the consideration of the growth of bilateral organs, it must be premised that even in multilateral erect stems and vertically descending roots growth does not always proceed equally and with equal rapidity on all sides of the longitudinal axis ; it is much more common for first one side and then another of the organ to grow more rapidly than the rest, curvatures being thus caused the con- vexity of which ahvays indicates the side that is at the time growing most rapidly. ^ [For a further detailed series of experiments on the influence of gravitation on growth, see Sachs, Flora, 1873, No. 21, and Arb. des bot. Inst. Wiirzburg, 1873, Heft 3. — Ed.] 2 A. B. Frank, Die natiirhche wagerechte Richtung von Pflanzentheilen (Leipzig, 1870). The views propounded in Frank's treatise are opposed by H. de Viies in the second Heft of the Pro- ceedings of the Wiirzburg Bot. Inst. 187 1, p. 223 et seq — See also Hofmeister, AUgemeine Morpho- logic der Gewdchse, Leipzig, 1868, Sect. 23, 24. 766 MECHANICAL LAWS OF GROWTH. If another side then grows more rapidly, it becomes convex, and the curvature changes its direction. Curvatures of this kind caused by the unequal growth of different sides of an organ may be called Nutations. They occur most commonly and evidently when growth is very rapid, and consequently in organs of consider- able length, and are produced under the influence of a high temperature either in darkness or when the amount of light is very small. When two opposite sides of an organ grow alternately more and less rapidly, curvatures are caused first on one side and then on the other ; it will, for example, bend first to the left, then become erect, and then bend to the right side ; as occurs, e.g. in the long fiow^er-scapes oi Allium Porum, which finally take an erect position when their growth is ended. It is much more common for the apices of erect stems above the curved growing part to move round in a circle or ellipse, the region of most active growth moving gradually, as it were, round the axis. This kind of nutation may "be termed a Revolving Nutatioji. Since the apex of the stem is constantly rising higher during the nutation owing to the elongation of the part below it, its revolving motion does not take place in a plane, but describes an ascending spiral line. This form of nutation occurs in many flower-stalks before the unfolding of the flowers, as in those of Brassica Napus, wher^ the movement ceases when growth is completed, and the stem finally becomes erect. It is very general in climbing stems and in almost all erect stems that bear tendrils ; but bilateral tendrils also revolve at the time when they are about to take hold of a support \ In bilateral appendicular organs nutation does not usually take the form of a revolving motion, or only to a subordinate extent, as in tendrils. The outer or dorsal side more often grows more rapidly so that the organ is curved concavely to the primary axis, and the inner side afterwards begins to grow more quickly, so that the organ finally becomes straight, or even concave on the dorsal side. This is the case in all strongly developed foliage-leaves, very strikingly in those of Ferns, which are at first rolled up towards the axis, and then unroll, often bending over backwards, becoming finally straight. The same phenomenon occurs in the tendrils of Cucur- bitaceas, which are also at first rolled up inwards, then become straight, and are finally rolled backwards. Other tendrils are at first straight or only slightly con- cave inwards like leaves in vernation, but are afterw-ards rolled backwards. Move- ments of nutation are very common and easily observed in stamens with long fila- ments, as TropcBolimi majus^ Dictamuiis Fraxinella (Fig. 454), Parnassia palustris^, &c., and in long styles like those of Nigella saliva, Sec. They occur at the time of the maturity of the sexual organs, and serve to place the stigmas and anthers in the positions adapted for the conveyance of pollen by insects from one flower to another ^ Most lateral shoots behave in the same manner as ordinary leaves, grow- ing at first only quickly enough on the outer side to become adpressed to the primary axis in vernation, afterwards more rapidly on the inner side, by which they become straight and diverge at a greater angle from the primary shoot. * See Sect. 25, On the Twining of Tendrils. ^ [On the stamens of Parnassia, where there is not properly any movement of nutation, see Gris, Comp. rend, Nov. 2, 1868; and A. W. Bennett, Journ. Linn. Soc. vol. XI, p. 24, 186^. — Ed.] ^ Vide infra under Fertilisation, Chap. VL UNEQUAL GROWTH. 767 These movements of nutation of bilateral appendicular organs take place mostly in one plane which coincides with the median plane of the organ. As long as the organ grows most rapidly on the dorsal side, it may be termed, after de Vries, hyponastic, afterwards, when it grows most rapidly on the inner or upper side, epiuasiic. Since in the later stages of development of an organ growth ceases at certain places — while at different distances from these places it presents different stages of growth, until it finally ceases everywhere — it is clear that in the same organ, by the side of spots where growth is completed and nutation no longer takes place, others occur with hyponastic and others again with epinastic growth, until at length nutation and growth alike cease altogether, as in Fern-leaves. Seedlings of Dicotyledons are a remarkable illustration of bilateral structures which nutate in one plane ; although their stem and primary root become afterwards multilateral and grow vertically upwards and downwards. The stem terminates in a pendent or nodding bud ; and the curvature, which is generally very great, exhibits FIG. 454. — Nutation of the filaments of Dzctanntus Fraxniella ; the filaments of the stamens whose anthers have not yet opened are bent downwards ; those with anthers already burst are bent upwards. itself also in germination when it takes place out of the ground, in a vessel that rotates slowly round a horizontal axis; it is a true curvature of nutation inde- pendent of light and gravitation. But the older portions of the stem become straight as they develope from the curved portion ; and in proportion as the stem increases in length, the straight part which bears the nodding bud also lengthens. When germination takes place in a feeble light, or better in a slowly rotating vessel, a more rapid growth occurs of the older portion of the stem which was at first con- cave, causing it to become convex on this side ; and hence the older and younger parts of the stem form together a letter S, as in Phaseolus, Vi'cia Fada, Polygoniijji Fagopyriwi, Cruciferae, &c. But the primary roots of dicotyledonous seedlings also manifest a tendency to a bilateral structure ; since, when they develope under slow rotation round a horizontal axis, they seldom continue to grow straight, but curve concavely either in front or behind, sometimes even becoming rolled up. These and other instances of nutation are not clearly seen when the development takes place under normal conditions, because the growth of the stem of the seedling is retarded by light, and the curvature both of stem and root prevented by geotropism. 7^8 MECHANICAL LAWS OF GROWTH, A knowledge of the different capacity for growth possessed by the anterior and posterior sides of bilateral organs lies at the root of an understanding of the fact that leaves, lateral shoots, and many secondary roots, although they are heliotropic and geotropic, yet assume definite positions with respect to the horizon, but without growing vertically upwards or downwards. When multilateral primary stems and roots grow vertically, the essential cause is their growth being uniform on all sides of the axis of growth ; the different sides of the organ are in equilibrium with one- another. Every deviation from the vertical position, to the right, left, front, or back, is counterbalanced by geotropism ; the growing part continues to grow until the free apex stands erect, in which position the action of gravitation is again equal on all sides. In the same manner light acts equally strongly on all sides of such organs. If therefore one side is exposed to stronger light, a heliotropic curvature takes place which finally brings the free part into a position in which all sides receive equally strong light on all sides, and therefore grow uniformly without any further curvature. The case is different with bilateral organs the anterior and posterior sides of which possess independently different capacities for growth, and which therefore exhibit a tendency for their more rapidly growing side to become convex. If the growth is very strongly hyponastic or epinastic, the curvature thus caused may take place in spite of the opposing action of light and gravitation, supposing the organs to be actually heliotropic or geotropic. Organs which grow horizontally or obliquely to the horizon must not be assumed to be on that account wanting in heliotropism or geotropism ; still less is it necessary to suppose in these cases any special or altogether abnormal relations to light and gravitation. It is sufficient, as de Vries has clearly shown, to suppose that light and gravitation act in the ordinary way on the growth of bilateral organs, in order to explain their directions of growth, if only it is borne in mind that their heliotropism and geotropism cooperate with their hyponastic and epinastic properties, and thus bring about positions of the organs which must be considered as the resultants of these different forces. The weight of the overhanging part must however also be taken into account, its tendency being always to change the lateral direction of the organ into a more horizontal or even pendulous one ; and this must occur more decidedly the less the elasticity of the organ. When large leaves assume oblique or horizontal positions, it is because their epinasty tends to make them con- cave downwards as they unfold, while their positive heliotropism tends to make them concave upwards. The result is consequently a more or less flat expansion of the leaf, the position of which depends on the relation of the weight of the lamina to the flexibility of the petiole and mid-rib. The same phenomena are observable in horizontal or oblique lateral shoots, in which however the hyponasty of the axis often counterbalances the greater mass of the pendent parts (as in Primus avium^ Ubnus awipcstris, Corylus Avellana^ Picea nigra, &c.). As soon as the position re- sulting from these forces is attained, it becomes permanent, from the mature parts becoming Hgnified, rigid, and hard, and thus in a condition to maintain the weight of the pendent parts. If leaves which are unfolding or still growing have their under side turned upwards or towards the light, very strong curvatures take place, generally combined with torsions, by which the lamina finally resumes more or less completely its normal position ; and the impression is given as if the under side were more UNEQUAL GROWTH. 769 sensitive to the influence of light, and the upper side to that of gravitation than the reverse. But this hypothesis is superfluous if it is borne in mind that in this case epinasty works concurrently with heliotropism and geotropism, and hence much stronger curvatures must take place than in the normal position where the former acts in opposition to the two latter forces. The results here described are derived from the experiments of de Vries, which have been already quoted. For the following I am also indebted to him. (a) Leaves. If a strongly developed mid-rib is separated from a leaf in active growth, it curls up concavely on the under side, showing that a tension exists between it and the mesophyll. De Vries found this to be the case in nearly two hundred species, with only a few exceptions. This curvature does not take place equally strongly at all ages; in leaves which have but just emerged from the bud it does not occur at all ; it increases with age, and attains its maximum when the leaf is nearly fully grown, then again decreases, and altogether disappears when the leaf has reached full maturity. This tendency to curve is at first apparent along the whole length of the mid-rib ; it dis- appears first of all at the base, the part capable of curvature becoming constantly smaller and smaller towards the apex. If mid-ribs of leaves are separated in this last stage of growth and fixed upright in a damp and dark place {e.g. in wet sand in a spacious closed zinc box), they will continue to grow for some time ; and since growth is more vigorous on the inner (anterior or upper) side, they will curve concavely on the posterior (or under) side, the curvature being however partially counteracted by geo- tropism. If separated mid-ribs of leaves are suspended horizontally in wet sand, so that the median plane lies horizontal, the epinastic curvature will take place without hindrance in a horizontal direction ; but a geotropic curvature will at the same time ensue in a vertical plane, so that the two kinds combine to produce an obliquely ascending position. If, on the other hand, two similar mid-ribs are separated and placed horizontally in wet sand, with the posterior side in one case above, in the other case below, geotropism will act in the former in opposition to epinasty, while in the latter the two will cooperate ; and the consequence will be that in the former case the epinastic curvature will be more or less neutralised, while in the latter a strong curvature will take place upwards, the two forces acting in unison. Phenomena of the same kind are produced by a combination of epinasty with helio- tropism, if the separated mid-rib is placed vertically in wet sand in a closed vessel into which light is admitted from one side through a glass plate. Heliotropism is generally but not always exhibited, and is then always positive ; but in all the cases hitherto ob- served is too weak to overcome epinasty. It will be seen from what we have said that all these movements of the mid-rib will be much less considerable when it is still in connexion with the mesophyll. Petioles show in general the same phenomena as mid-ribs, but their m.otions which result from heliotropism, geotropism, and epinasty are unimpeded. (b) Bilateral secondary shoots, such as branches of an inflorescence, horizontal or erect leaves, and stolons, would exhibit precisely similar phenomena. It may be proved also that the branches of the inflorescence of Isatis tinctoria, Archangelica officinalis, Crambe cordifoUa, and all others that have been observed, the horizontal branches of Pyrus Malusy Asperugo procumbens, &c., as well as the runners of Fragaria, Potentilla reptans, Ajuga reptans, &c., are epinastic. When placed horizontally in wet sand, they all curl upwards, whether the side that normally faces downwards (the posterior side) was placed below or above, but in the latter case more strongly, because geotropism and epinasty then cooperate. In some species (as Tilia and Philadelphus) a branch, when stripped of leaves and placed in its normal position, did not curl upwards, while one placed in a reverse position did so, proving that there was in these cases an equilibrium between geotropism and epinasty. The horizontal branches of Prunus auium, JJlmus campestris, 3 D 770 MECHANICAL LAWS OF GROWTH. Corylus At^ellana, and some other plants were found on the other hand to be hyponastic ; when laid horizontally in their natural position they curved upwards, but downwards if reversed, because their hyponasty was stronger than their geotropism. Similar experiments to those made on petioles with respect to heliotropism, showed in many cases the absence of this phenomenon, especially in the case of stolons; and that in other cases it was always positive, but too feeble to overcome the influence of their epinasty. In the case of branches, especially such as are long and slender, more account must be taken of weight in modifying the direction of growth than in that of leaves. The removal of the leaves (iatitis : \\\ perianth cut through longitudinally. A before, B after pollination (magnified). and stigmas which occupy the same position in different flowers are made mutually to act on one another. But there are besides many other contrivances, most variable in their nature and often perfectly astonishing, for effecting the conveyance of pollen by insects. A few examples may suffice. (i) Dichogamous Flowers^ are oxWi^x protandrous oy protogynous'^. In the former the stamens are developed first, their anthers opening at a time when the stigmas are still undeveloped and not yet receptive ; the stigmatic surface is only developed later, and ^ F. Delpino, Ulteriori osservazioni sulla dicogamia nel regno vegetabili, Atti della soc. Ital. di sci. nat. vol. XIII, 1869, and Bot. Zeit. 1871, No. 26 et seq.; ditto, in Bot. Zeit, 1869, p. 792. ^ [For a list of British protandrous, protogynous, and 'synacmic' plants (or those in which the male and female organs are mature at nearly the same time), see A. W. Bennett in Journal of Botany, 1870, p. 315, and 1873, p. 329.— Ed.] INFLUENCE OF THE ORIGIN OF CELLS ON FERTILISATION. 813 usually not till the pollen has been carried away from the anthers by insects ; they can then only be fertilised by the pollen of younger flowers. To this category belong the various species of Geranium, Pelargonium, Epilobium, Malva, Umbelliferae, Compositae, Campanulaceae, Labiatae, Digitalis, &;c. The phenomena referred to, especially the movements of the stamens and stigmas, are so readily observed in these cases, e.g. in Geranium and Althaea, that no further description is necessary. In protogynous flowers the stigma is receptive before the anthers in the same flower are mature ; when these subsequently open and allow the pollen to escape, the stigma has already been pollinated by foreign pollen or has even withered up and fallen off" (as in Parietaria diffusa) ; and the pollen of these flowers can therefore only be apphed to the fertilisation of younger flowers. To this class belong Scrophularia nodosa^ Mandragora 'vernalis, Scopolia atropoides, Plantago media, Luzula pilosa, Ant box ant hum odoratum, &c. Among protogynous flowers Aristolochia Clematitis is characterised by striking and peculiar contrivances. In Fig. 458 A is shown a young flower cut through lengthwise; the stigmatic sur- face n is already in a receptive condition, but the anthers are still closed ; a small fly /, which has brought on its back a mass of pollen from an older flower, is forced in through the narrow throat of the perianth, and runs about in the globular swelling k ; as many as from six to ten flies are not unfrequently found in one flower. They are shut up and cannot escape, because the throat of the perianth r is furnished with long hairs moving as on a hinge, which present no impediment to the entrance of the insect, but pre- vent its escape like a trap. While the insect is moving about in the cavity, its back laden with pollen comes into contact with the stigmatic surface and pollinates it, in conse- quence of which the lobes of the stigma curve upwards, as is shown in Fig. 458 5, «. As soon as this has taken place, the anthers, previously closed, open ; they are laid bare by the change in the position of the stigmas, and are rendered accessible by the withering up of the hairs at the bottom of the cavity of the flower, which has now become wider. The flies which have now carried their pollen on to the stigmatic surface, can therefore creep down to the open anthers, where the pollen again becomes attached to them. By this time the throat of the perianth r has again become passable, the net-work of hairs in it having died and withered away after the pollination of the stigma. The insect, laden with the pollen of this flower, can now escape, and again performs the same work in another flower. But while the changes which have been described are taking place inside the flower, its position has also altered. As long as the stigma is still receptive, the pedicel is erect and the perianth open outwards (Fig. 457 //), so that the visiting flies find a door hospitably open. But as soon as the pollination of the stigma has been eff"ected, the pedicel bends sharply downwards just beneath the ovary, and when the flies, again laden with pollen, have flown out of the flower, the standard-like lobe of the perianth above the mouth of the tube (Fig. 458 5) closes, preventing the entrance of the flies, whose visits would now be useless. (2) Floivers in o. — Viola tricolor: A longitudinal section through the flower (natural size) ; B the ovary fertilised and swollen ; the filaments have been ruptured and the anthers drawn up by the growth of the ovary ; C the stigma with its orifice o and lip //, on the style gr (magnified) ; / sepals, Is prolonged base of the sepals, c petals, cs spur of the inferior petals or nectary; fs appendages of the two inferior Stamens projecting into the spur, which secrete the nectar, a the anthers, 11 stigma, V bracts; D horizontal section through the ovary with the three placenta; sp and ovules sK\ E horizontal section through an unripe anther. the arrow, the lower arm of the connective is pressed down, and the upper arm c is made to move forward, and thus to strike the back of the insect. In the pansy {Viola tricolor) we have quite a different contrivance for preventing the possibility of self-fertilisation. In Fig. 460, A and B, is shown the position and arrange- ment of the parts of the flower. The cavity of the flower enclosed by the petals is completely filled up by the anthers and ovary, with the exception of the tubular spur of ^ For further details, see HildeLrand, Jahrb. fiir wiss. Bot. vol. IV, 1S65, p. i. INFLUENCE OF THE ORIGIN OF CELLS ON FERTILISATION. 815 the inferior petal in which the nectar collects, secreted by the appendages of the two inferior stamens. The only entrance to this nectary, which therefore lies behind the reproductive organs, is through a deep channel in the inferior petal, lined with hairs. The upper and lateral petals incline towards one another in front of the ovary which is surrounded by the anthers, and above the channel in such a manner that the entrance to it is entirely filled up by the capitate stigma B, n. The stigma is seated on a flexible style (Cfgr), is hollow and opens by an orifice which faces the hairy channel of the lower petal ; the lower and posterior margin of this orifice has a lip-like appendage. The anthers open of their own accord, and the pollen in the form of a yellow powder collects below and behind the stigma among the hairs of the channel. An insect which has already brought pollen on its proboscis from another flower inserts its pro- boscis beneath the stigma through the channel into the nectary. The foreign pollen, which is attached to the proboscis, is thus rubbed off" on to the lip of the stigma, it is de- tained by the viscid secretion which fills up the hollow of the latter, and subsequently emits its pollen-tubes through the canal of the style (see also Fig. 364, p. 499). While the insect is sucking the nectar in the spur, the pollen of this flower, which lies in the channel behind the stigma, becomes attached to the proboscis ; when the proboscis is again drawn out, this poflen does not come into contact wuth the viscid stigma, the lip being drawn forward by the motion of the proboscis, and the orifice of the stigma protected. The pollen that is removed from this flower is now carried, in the manner described to the stigma of another flower. If the insect were to insert its proboscis again into the nectary of the same flower, the pollen would be detached into the cavity of its own stigma ; but, as Hildebrand has remarked, insects do not usually do this, but suck up the nectar only once, and then visit another flower. The proceeding of the insect may be imitated by inserting a fine sharp pin beneath the stigma into the channel and again withdrawing it, and filling with the pollen thus removed the stigmatic cavity of another flower. The contrivances for cross-pollination in Orchids, as numerous as they are compli- cated and ingenious, have been described in detail by Darwin in the work already named \ One of the simpler cases, and the most frequent in its main features, may be briefly described in the case of Epipactis latifolia. At the time when the reproductive organs are mature, the flower stands, in consequence of a torsion of its pedicel, so that the true posterior leaf of the six that form the perianth (the labellum) hangs in front and downward ; it is hollowed out in its lower part, and is thus transformed into a receptacle for the nectar which it secretes (Fig. 461, B, D, /). The sexual organs, borne on the gynostemium S (in C) project obliquely above this nectary; the stigma forms a disc with several lips hollowed out and viscid in the centre, the surface of which is in- clined obliquely above the nectary. The two gland-like staminodes xx stand right and left beside the stigma ; above the stigma and covering it like a roof lies the single fertile anther, of considerable size, which is again on its part protected above by its cushion-like connective en; the lateral walls of the two anther-lobes burst lengthwise right and left, so that their pollen-masses (pollinia) became partially exposed, the pollen-grains remaining attached to one another by a viscid substance. In front of the middle of the anther and above the stigmatic surface is the rostellum >6, a peculiarly metamorphosed part of the stigma (see yi) ; the tissue of the rostellum is transformed into a viscid substance covered only by a thin membrane. The flower of Epipactis is not fertilised if left to itself ; the pollinia do not fall of their own accord out of the anther, and would even then not reach the stigmatic surface ; they must be carried away by insects to the stigma of other flowers. The mode in which this is eff'ected is ex- plained by inserting the point of a black-lead pencil into the flower in a direction towards the bottom of the labellum and beneath the stigmatic surface ; if it is then pressed ^ See also Wolff, Beitriige zur Entwickelungsgeschichte cler Orchideen-bliilhe, in Jahib. fiir \vi • vol. IV, 1865. 8i6 PHENOMENA OF SEXUAL REPRODUCTION. slightly against the rostellum, and again withdrawn slowly in this position (Z)), the viscid mass of the rostellum or adhesive disc of the pollinia to which the pollen-masses are attached, remains sticking to the pencil. The pollinia are now completely removed from the two anther-lobes by the withdrawing of the pencil, as is shown in E and F. If the pencil with its pollinia attached is now again inserted into another flower in the direction of the bottom of the labellum, the pollinia necessarily come into contact with the viscid stigmatic surface and adhere firmly to it ; when the pencil is again withdrawn they are left behind, being partially or entirely torn from the pencil. In consequence of the form and position of the parts of the flower, an insect which settles on the anterior part of the labellum would in the same mianner be able to creep into the bottom of the nectary with- out disturbing the rostellum ; but when it again crept out after obtaining the nectar, it YlQ,. \'o\.—Epipactis latifolia: A longitudinal section through a flower-bud; B open flower after removal of the perianth with the exception of the labellum / ; C the reproductive organs after removal of the perianth seen Irom below and in front ; Z> as -5, the point of a lead-pencil b inserted after the manner of the proboscis of an insect ; E and F the lead-pencil with the pollinia attached ; fK ovary, I labellum, its bag-like depression serving as a nectary, n the broad stigma, oi the connective of the single fertile anther,/ pollinia, h the rostellum, X X the two lateral gland-like staminodes, i place where the labellum has been cut off, s the columnar style. would strike against it and carry off" the pollinia ; and on crawling into a second flower, these would come into contact with the viscid stigma, and would remain attached to it. In some other Orchideae the contrivances are much more complicated. Sect. 32. — Hybridisation ^ In the preceding paragraphs we have spoken only of the union of the reproductive cells of the same plant, or of two individuals of the same species. We learn however from experience that a fertile sexual union can ^ J. G. Kolreuter, Vorlaufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden Versuchen u. Beobachtungen, Leipzig, 1761; Appendices in 1763, 1764, and 1766. — W. Herbert, On Amaryllidaceoe, with a treatise on cross-bred vegetables; London, 1837. — Gartner, Versuche u. Beobachtungen iiber die Bastarderzeugung im Pflanzenreich ; Stuttgart, 1849. [See notice by Berkeley, Journ. Roy. Hort. Sec. vol. V, 1850, p. 156.] — Wichura, Die Bastardbefruchtung im Pflanzenreich, erlautert an den Bastarden der Weiden (with two nature-printed plates) : Breslau, 1865. [See abstract by Berkeley, Journ. Roy. Hort. Soc. new series, vol. I, 1850, p. 57.] HYBRIDISA TION. 8 J 7 lake place between plants which are specifically distinct. A union of this kind is called Hybridisafwn, and its product a Hybrid. According as the union takes place between different varieties of one species, different species of one genus, or be- tween two species belonging to different genera, the resulting hybrid may be termed a variety-hybrid, species-hybrid, or genus-hybrid. Among Cryptogams only a few instances of hybridisation are known with certainty. Thuret (Ann. des sci. nat., 1855) obtained hybrid plants by bringing spermatozoids of Fucus serratus into contact with oospheres of F. vesiculosus. In some other families of Cryptogams forms have been found which have been sup- posed, from their characters, to have a hybrid origin. Thus A. Braun (Verjiingung, p. 329) adduces instances of hybrids between ]\Iosses\ Physcomilrium pyriforme and Funaria hygrometrica, and between Physcomitrhim /asciculare and Funaria hygro- meirica, and between the following species of Ferns — Gynmogr amine chiysophylla and G. calomelaiia, G. chrysophylla and G. distans, and Aspidiinn Fi/ix-mas and A. spinulosinn'^ . The most important observations from a scientific point of view, which have given us the clearest insight into the nature of the difference of sex, are however those made on hybrids between flowering plants, resulting from the artificial convey- ance of pollen from one species to another. Nageli has collected the results of many thousand experiments on hybridisation made by Kolreuter in the last century, and more recently by Knight, Gartner, Herbert, Wichura, and other observers. The following facts are taken chiefly from Nageli's resume^. Only those forms which are closely related genetically can produce hybrids. They are formed most easily between different varieties of the same species ; with greater difficulty — but are still possible in a great number of cases — between two species of the same genus ; of hybrids between species which belong to different genera only a very few instances are known, and it is probable that in these cases the species ought to be included in the same genus. The facility with which hy- brids can be produced varies extremely in different orders, families, and genera of Angiosperms. The phenomenon is frequent among Liliacese, Iridese, Nyctagineae, Lobeliacese, Solanaceae, Scrophulariaceae, Gesneraceae, Primulaceae, Ericaceae, Ranun- culacese, Passifloraceas, Cactaceae, Caryophyllaceae, Malvaceae, Geraniaceae, (Enoihe- reae, Rosaceae, and Salicineae. It does not occur at all, or only very exceptionally, in Gramineae, Urticaceae, Labiatae^ Convolvulaceae, Polemoniaceae, Grossulariaceae, Papaveraceae, Cruciferae, Hypericineae, and Papilionaceae. Even genera of the same order or family differ in this respect. Among Caryophyllaceae, the species of Dianthus hybridise easily, those of Silene only with difficulty; among Solanaceae, the species of Nicotiana and Datura have a tendency to produce hybrids, while those of Solanum, Physalis, and Nycandra have not; among Scrophulariaceae, ^ [See also H. Philibert, L'Hybridation dans les Mousses (Grimmia) Ann. des sci. nat. 1873, vol. XVII, p. 225.— Ed.] 2 [See also T. Moore on Adianhim farleyense, Journ. Roy. Hort. Soc. new series, I, p. 83 ; Berkeley on Asplefiwni ebenoides, Scott, ibid. p. 137. — Ed.] 3 Nageli, Sitzungsber. der kais. bayer. Akad. der Wiss. in Mlinchen, Dec 23, 1865, and Jan. 13, 1866. * [Stachys mnhigua Sm. is considered to be a hybrid between S. sylvatica and S.palustri&—Ex).'\ 3 G 8i8 PHENOMENA OF SEXUAL REPRODUCTION. Verbascum ^ and Digitalis, but not Pentstemon, Linaria, or Antirrhinum ; among Rosaceae, Geum, but not Potentilla. Hybridisation between species belonging to different genera has been observed between Lychnis and Silene, Rhododendron and Azalea, Rhododendron and Rhodora, Azalea and Rhodora, Rhododendron and Kalmia, Rhododendron and Menziesia^ ^Egilops and Triticum, and between Echinocactus, Cereus, and Phyl- locactus, to which must be added a few wild forms which appear to be genus- hybrids. Besides the near genetic relationship, the possibility of the production of hybrids depends also on a certain relationship between the parent-plants, which is manifested only in the result of hybridisation, and which Nageli calls ' Sexual Affinity.' This kind of affinity is not always concurrent with the external resemblance of the plants. Thus, for example, hybrids have never been obtained between the apple and pear^, Anagallis arvensis and ccerulea, Primula officinalis and elalior, or Nigella damascena and saliva, nor between many other pairs of species belonging to the same genus which are very nearly allied to one another ; while in other cases- very dis- similar forms unite, as Jigilops ovala with Trilicum vulgare, Lychnis diurna with L. Flos-ciictili, Cereus speciosissimus with Phyllocaclus PJiyllanlhus, the peach with the almond. A still more striking proof of the difference between sexual and genetic affinity is afforded by the fact that varieties of the same species will sometimes be partially or altogether infertile with one another, as e.g. Silem inflala var. alpina with var. angusli/olia, var. latifolia with var. lilloralis, «fec. When a sexual union is possible between two species A and B, A can usually produce hybrids when fertilised by the pollen of B, and B when fertilised by the pollen of A (reciprocal hybridisation). But there are cases in which A can only be the male and B only the female parent plant, the pollination of A by B yielding no result. Thus Thuret found, as has already been mentioned, that Fuctis vesiculosus produces hybrids with the spermatozoids of F. serralus, while the oospheres of the latter species could not be fertilised by the spermatozoids of the former. Gartner states that Nicotiana paniculata produces seeds when acted on by the pollen of N. Langsdorfii, while the latter does not under the influence of the pollen of the former. Kolreuter easily obtained seeds of Mirahilis Jalappa with the pollen of M. longiflora, while more than two hundred experiments on pollinating the latter by the former species extending over eight years produced no result. Sexual Affinily presents a great variety of gradations. At one extreme we have complete infertility under the influence of the pollen of another variety or species, the pollen-tubes not even entering the stigma, and the pollinated flower behaving pre- cisely as if no pollen had reached it ; the other extreme is shown in the production ^ [On hybridity in the genus Verbascum, see Darwin, Journ. Linn. Soc. 1868, p. 437. — Ed.] "^ [The history of the plant which is here intended is given in the Botanical Gazette, vol. Ill, p. 82. It was raised from seed of Bryanthus {Menziesia) empetriformis, supposed to be fertilised by the pollen of Rhodothanuins (Rhododendron) ChamcEcistus. It is figured under the name of Bryanthiis erechis in Paxton's Flower Garden, vol. I, 1. 19 ; but it agrees well with specimens of its female parent from the Rocky Mountains, and is probably therefore not a hybrid at all. — Ed.] ^ [An instance to the contrary is recorded in the Proc. Acad, Philadelphia, 1S71, vol, I, p. 10. —Ed.] HVBRIDISA TION. 819 of numerous hybrids, which not only grow vigorously, but are themselves fertile. The lowest degree of the action of pollen of a different kind consists in various changes taking place in the parts of the flower of the mother-plant, the ovary or even the ovules also growing, without any embryo being produced. A higher degree is manifested in the production of ripe normal fruits and seeds containing embryos, but these embryos having no power of germination. Further steps are indicated by the number of embryos which have the power of germination that are produced in the ovary \ When pollen from different species is applied simultaneously to the same stigma, only one kind is potent, viz. that from the species which has the greatest sexual affinity to the one that is pollinated. And since, as a general law, pollen is most efficacious on a different flower of the same species — in other words, the highest degree of sexual affinity occurs between different individuals of the same species — when a stigma is pollinated at the same time with pollen of the same and of another species, the first only is potent. But since, on the other hand, hybrids are sometimes more easily produced between varieties than between individuals of the same variety, in this case the foreign pollen may be prepotent over that of the same kind. When the pollen of different species reaches the stigma at the same time, and if that which reaches it later has a greater sexual affinity, it can only be potent when the first is not potent or acts injuriously. In Nicotiana the production of hybrids can no longer be prevented by its own pollen after two hours, in Malva and Hibiscus after three hours, in Dianthus after five or six hours. The hybrid is possessed of external characters intermediate between those of its parent-forms, usually nearly half way between ; less often it resembles one of the parent-forms more nearly than the other, and this is more often the case with variety- hybrids than with species-hybrids. It follows that in reciprocal hybrids from the species A and B, the hybrid A B is generally similar externally to the hybrid B A, though the two forms may differ somewhat internally. Thus, according to Gartner, the hybrid Nicotiana paniculato-rustica is more fertile than the reciprocal hybrid Nicotiana riistico-panicidata'^. An internal difference between reciprocal hybrids is also show^n by the fact that one is more variable than the other ; thus, according to Gartner, the progeny oi Digitalis purpureo-liitea is more variable than that oi D.lu'co- piirpiirea^ the progeny of Dianthus pidchello-arenarius more variable than that of D. arenario-pulchellns. When two species A and B hybridise, and the one species A exercises a greater influence on the form and properties of the hybrid than the other species B, the hybrid or its descendants, if fertilised by A, will revert more quickly to the parent- form A than it will to the parent form B if fertilised by it. Thus Gartner states that the hybrid of Dianthus chinensis and D. Caryophytltis reverts to the latter form after three or four generations if repeatedly fertilised by it, while it requires fertilisation for five or six generations by D. chinensis in order to revert to that form. ^ See Hildebrand, Bastaidirungsversuche an Orchideen, Bot. Zeit. 1865, No. 31. 2 In this mode of designating hybrids, the name of the male parent-plant stands first ; thus Nicotiana riistico-paniculata is the product of the fertilisation of N. pafiiaJata by the pollen of N. rui>(ica. 3 G 2 820 PHENOMENA OF SEXUAL REPRODUCTION. The characteristics of the parent-forms are as a rule so transmitted to the hybrid that the influence of both is manifested in all its characters, producing a fusion of the different pecuharities. This is more evident in the species- than in the variety-hybrids ; in the latter some of the non-essential characters of the parents sometimes present themselves in the offspring uncombined side by side ; e. g. various kinds of streaks and blotches instead of a mixing of the colours of the flowers. Thus a hybrid which Sageret obtained from Cucumis Chate (female) with C. Melo Canta- lupiis (which had a reticulated flesh) had a yellow fiesh, a reticulate marking of the rind and moderately prominent ribs like the male parent, but white seeds and an acid flavour like the female parent. Another hybrid from the same species had, on the contrary, the sweet flavour and yellow flesh of the male, with the white seeds and smooth rind of the female parent. To this category belongs also the hybrid of Cytisiis Lahurnwn and purpureus [known as Cylisus Adami\ some of the branches of which partially or entirely resembled one and some of them the other parent-form. I have found what seemed to be a hybrid Aiitirrhinum majus, in which the inflor- escence bore on one side of the axis only dark-red, on the other side only yellow flowers, while between the two halves stood a single flower which was half red and half yellow. In addition to its inherited properties, the hybrid usually possesses characters of its own by which it is distinguished from both its parent-forms. One of these new characters, which occurs especially with variety-hybrids, is the tendency to vary more strongly than its parent-forms. Species-hybrids are usually weak in their sexual properties; those derived from nearly related parent-species are, on the other hand, more vigorous in their growth than their parent-forms, while hybrids resulting from the union of species less nearly related are generally feebler in their development. The luxuriant growth of the hybrids from nearly allied species is displayed in their taller and stouter stems, more copious root-system, and larger number of shoots (stolons, scions, &c.). Hybrids have also a tendency to a longer duration of Hfe ; those of annual or biennial parent-forms often live a number of years, probably in consequence of their producing a smaller number of seeds. Hybrids are also characterised by commencing to flower earlier, and continuing to do so longer and more abundantly, than the parent-forms ; sometimes they produce an extraordinary number of flowers, which are also larger, more enduring, and of brighter colour and stronger odour. They have also a tendency to become double, their staminal and carpellary leaves to increase in number and develope into petals. Along with this luxuriant vegetative growth, the sexual organs are usually weak, and this in every possible degree. ' The stamens,' says Nageli, ' are, it is true, in some cases perfect externally, but partially or altogether infertile, the pollen-grains not attaining their proper development ; while in others the stamens are altogether abortive and reduced to rudiments. The pistils (gynaeceum) of hybrids are in most cases not distinguish- able externally from those of the parent species, but their ovules have no power, or only to a slight degree, of becoming fertilised ; no embryonic vesicles are formed, or the embryo which begins to be developed from the embryonic vesicles perishes sooner or later. Under favourable circumstances, when fertile seeds are produced, their number is smaller, and they manifest a certain degree of feebleness in their slow germination and the short duration of this capacity.' The feebleness of the sexual HYBRIDISA TION. 8 Ji I function is in some variety-hybrids scarcely perceptible, in others but small ; in general it is the more marked the more distant the genetic and sexual aflmity of the parent- forms. When species-hybrids have the power of producing seeds by self- pollination, and this is repeated in the progeny, their fertility generally diminishes from generation to generation ; though this phenomenon probably depends less on the sexual feebleness of hybrids than on the circumstance that their flowers have probably been generally self-fertilised, instead of being pollinated by other flowers or other individuals of the same hybrid. Nageli's rule holds true in the general way, that the male organs of species-hybrids are functionally weak to a higher degree than the female organs, although the rule is not without exceptions. ' Hybrids usually vary less in the first generation, the less the degree of affinity between their parent-forms ; species-hybrids therefore less than variety-hybrids ; the former are often characterised by a great uniformity, the latter by a great variability. When hybrids are self-fertilised, the variability increases in the second and succeeding generations the more completely it was absent from the first ; and three difl'erent varieties arise more certainly the less the affinity between the parent-forms; viz. one corresponding to the original (hybrid) type, the two others bearing a greater resemblance to the two parent-forms. But these varieties show but little constancy, passing easily into one another, at least in the earlier generations. An actual re- version to one of the two parent-forms (with pure breeding-in) takes place especi- ally when the parent-forms are very nearly related, as in variety-hybrids and those from species that approximate to varieties. When this reversion occurs in other species-hybrids, it appears to be limited to those cases where one of the parent- species exercised a preponderating influence in the hybridisation.' (Nageli, /. c). When a hybrid is made to unite with one of its parent-forms, or with another parent-form, or with a hybrid of different origin, the product is termed a ' derivative hybrid'; and this may again on its part unite with one of the parent-forms or with a hybrid of different origin. When a union is effected between a hybrid and one of its own parent-forms, and the hybrid thus obtained unites again with the same parent- form, and so on through several generations, the derived progeny ap- proach more and more nearly in their characters to those of this parent-form, until they come to resemble it in all respects. According as one or the other of the parent-forms is taken, a larger or smaller number of generations are required to effect the perfect reversion ; and this behaviour has been reduced by Nageli to a numerical expression (formula of heredity), which indicates in numbers the amount of influence exercised by a species in reference to the hereditary transmission of its qualities in hybridisation. In proportion as the derivative hybrid approaches one or the other of its parent-forms, its hybrid nature gradually decreases, and its fertility at the same time increases. When a hybrid unites with a new parent-form or with a hybrid of another species, a derivative hybrid results in which three, four, or more species (or varie- ties) are combined ; Wichura has even united six different species of willow in one such derivative hybrid. Hybrids of this kind, which may conveniently be termed * combined hybrids,' usually follow the same rules with reference to their form and other characters as hold good in the case of simple hybrids. Combined hybrids become less fertile the larger the number of different parent-forms that are united in 82 2 ORIGIN OF SPECIES. them ; and they are usually very variable. Wichura showed, from his own observ- ations and those of Gartner, that hybrid pollen produces a greater variety of forms in its progeny than does the pollen of true species. The results of hybridisation are important with respect to the theory of sexuality, because there is no boundary-line or essential distinction between the self-fertilisation of pure species or varieties and their fertilisation by other species or varieties ; and be- cause in the latter case — in other words in hybridisation — certain peculiarities of sexual differentiation and union are rendered more evident. The two extremes of the con- ditions under which a fertile union of sexual cells is possible lie at a great distance from one another, but are connected by very numerous transitions. One extreme is presented in the genus Rhynchonema and in some Saprolegnieae, where a fertile sexual union of sister-cells takes place regularly ; the other extreme is furnished in genus-hybrids, where the uniting cells belong to very different forms of plants whose descent from a common ancestor dates back to a remote antiquity. But the great majority of phenomena in the vegetable kingdom show that sexual union is usually most productive when the cells stand neither in too close nor in too remote an affinity to one another ; self-fertilisation is in the vast majority of cases as carefully avoided as the hybridisation of different species or genera. The phenomena may be comprised in the statement that the original form of sexual differentiation was probably the simultaneous formation of male and female organs in close juxtaposition on the same plant, but that sexual union is more potent and more favourable for the maintenance of the race when the closely contiguous cells do not unite, but those of different descent, a certain mean amount of difference of descent being established as the most favourable. This mean of the difference of descent associated with a maximum of sexual potency is obtained when the sexual cells belong to different individuals of the same species \ The phenomena of structure described in the preceding paragraphs which are manifested in polygamy, diclinism, dichogamy, dimorphism, the impotence of pollen on the stigma of the same flower (as in Corydalis and Oncidium), the mechanical contrivances for rendering self-fertilisation impossible (as in Aristolochia Clematitis, many Orchidess, &c.), are different means for promoting the cross-fertilisation of individuals belonging to the same species or for rendering it alone possible. CHAPTER VII. THE ORIGIN OF SPECIES. Sect. 33. — Origin of Varieties. The characters of plants are transmitted to their descendants, or, in other words, are hereditary. But, in addition to the inherited properties, new characters may arise in a smaller or larger number of the descend- ants of a plant which were not possessed by the parent-plants. Thus, for example, Descemet obtained in 1803^, among the seedlings from Rohinia Pseud-acacia, an ^ [See Darwin, Variation of Animals and Plants under Domestication, vol. II, chap, xvii, where several illustrations of the law are given. — En.] ' See Chevreul, Ann. des sci. nat. 1846, vol. VI, p. 157. [Journ. Roy. Hort. Soc. vol. VI, 1851. p. 61.] ORIGIN OF VARIETIES. 83a individual without spines; Duchesne, in 1761^, among seedlings of the strawberry, one with simple instead of trifoliolate leaves ; and Godron^, among seedlino-s of Datura Tatula, one with smooth instead of spiny capsules. The characters which arise in single descendants are often only individual, i. e. they are not again transmitted to their descendants. Thus the seeds of the un- armed Robinia produced again spiny plants resembling, not their immediate ancestor, but more remote ones ; while in other cases the new character is hereditary, thouo-h at first perhaps only partially so, the new form making its appearance only in a certain proportion of the descendants, while the others revert to the original form, as in Duchesne's unifoliolate strawberry. When a new character is transmitted by inheritance to new generations, the number of individuals that revert to the primitive form often decreases from gener- ation to generation, or the hereditary permanence of the new character increases ; they become more and more constant, and sometimes even as much so as those of the primitive form. Such new constant forms are termed Varieties^. The same parent-form may produce a smaller or larger number either simul- taneously or in succession, sometimes even hundreds of new forms ; and this is especially the case with cultivated plants. The enormous number of varieties of the dahlia, differing in the colour, size, and form of the flowers and in their mode of growth, now cultivated in our gardens, have been derived since 1802 from the simple yellow-blossomed primitive form of Dahlia variabilis. The great variety of pansies, distinguished chiefly by the colour of their flowers, have resulted since 1687 from the cultivation of the Viola tricolor of our fields with small flowers almost uniform in colour"*. Still more numerous are the varieties of Cticiirbita Pepo, differing not ojily in the form of their fruit but also in all other characters ; and the same is the case with the cabbage {Brassica oleracea) and a vast number of other cultivated plants. Some plants have a special tendency to variation ; among native species, for example, the fruticose Rubi, and those of Rosa and Hieracium ; others, on the con- trary, are distinguished by great constancy in their characters, as for example rye, which has as yet produced no hereditary varieties, notwithstanding long cultivation ; while the nearly related species of wheat (especially Triticimi vtilgare, amyleum and Spelta) are distinguished by a number of old varieties and an ever-increasing number of new ones. By far the greater number of hereditary varieties are the result of sexual repro- duction ; among Phanerogams the new characters appear suddenly in individual seedlings, which differ at once from the parent-plant in these respects. Sometimes however it happens that particular buds develope differently from the other shoots of the same stock ; and of this Bud-variatio7i^ two different cases must be carefully ^ For further details, see Usteri, Annalen der Botanik. vol. V, p. 40. ^ See Naudin, Compt. rend. 1867, vol. LXIV, p. 929. For examples, see Hofmeister, Allgemeine Morphologie, p. 565. * Darwin, The Variation of Animals and Plants under Domestication, vol. 1, p. 368 et seq. ^ [T. Meehan adduces a number of remarkable instances of bud-variation in which hybrid- isation could not have taken any part; — in Rubus which rarely produces seeds in the wild state, Convolvulus Batatas, which seldom flowers in America, &c. See Proceedings of the Philadelphia Acad, of Nat. Sci. Nov. 29, 1870. — Eu.] 824 ORIGIN OF SPECIES. distinguished, since their significance is altogether different. In one case the ab- normal shoot of a stock which itself belongs to a variety resembles or reverts to the primitive form ; and this therefore is an instance not of the production but of the cessation of a new form. In the botanic garden at IMunich there is, for example, a beech-tree with divided leaves, itself a variety, a single branch of which bears the ordinary undivided entire leaves, or has reverted to the primitive form. In the second case new characters not previously displayed arise on particular shoots of a stock. Thus, for instance, single shoots of the myrtle are sometimes found with leaves in alternating whorls of threes, instead of pairs ; but these shoots again produce from the axils of their leaves the ordinary branches with decussate leaves. Knight (see Darwin, /. c. vol. I, p. 375) observed a cherry (the May Duke) with one branch bear- ing fruit of a larger shape which always ripened later. The common 'moss-rose' is considered by Darwin (/. r. p. 379) to have probably arisen by 'bud- variation' from i?. ceiitifolia ; the white and striped moss-roses made their appearance in 1788 from a bud of the common red moss-rose ; Rivers states that the seeds of the simple red moss-rose almost always again produce moss-roses ^ Those changes which are produced in a plant by the nature of its food and other external conditions must not be confounded with variation. Specimens of the same plant often differ conspicuously in the size and number of their leaves, shoots, flowers, and fruits, according as their supply of food has been abundant or deficient ; deep shade frequently occasions the most striking changes in the habits of plants that usually grow in sunshine ; but these changes are not hereditary ; the descendants of such individuals revert, under normal conditions of light and nutrition, to the original characters of the species. Those characters, on the contrary, which may become hereditary or form the groundwork of varieties, arise independently of the direct influence of soil, locality, climate, or other external influences ; they appear seemingly wdthout any cause. We must therefore assume either that external impulses which are altogether imper- ceptible first cause an imperceptible deviation in the process of development, which is always extremely complicated, and that this variation gradually increases until it becomes perceptible, or that the processes in the interior of the plant itself react upon one another in such a manner as to cause sooner or later an external change. The fact that wild plants, when cultivated, usually begin to produce hereditary varieties, shows that the change in the external conditions of life disturbs to a certain extent the ordinary process of development ; but it does not show that par- ticular external influences produce particular hereditary varieties corresponding to them ; for under the same conditions of cultivation the most different varieties arise simultaneously or successively from the same parent-form. The same is the case also in nature with wild plants ; in the same locality under precisely the same vital conditions a number of varieties are often found by the side of their parent-form, and the same variety is often found in the most diverse localities^. It is for this very ' [See also M, J. Masters, On a pink sport of the Gloire de Dijon rose, Journ. Roy. Hort. Soc. new series, vol. IV, p. 153. — Ed.] - Further details on this important subject are given by Niigeli in the Sit/ungsberichte der kon. bayer. Akad. der Wiss. Dec. 15, 1865, ORIGIN OF VARIETIES. 825 reason — because varieties are to so great an extent independent of external influences — that they are hereditary. A change produced in a plant by moisture, shade, or any similar cause, is for the same reason not hereditary, because its descendants, when placed under other vital conditions, acquire again other non-permanent characters. That hereditary characters, or those which may become so, are not produced by ex- ternal influences, is proved most conclusively by the fact that seeds from the same fruit produce diff'erent varieties, either entirely so or together with the inherited parent-form. Although the production of varieties and the form they assume are not the direct results of external influences, yet the continuance of the existence of a variety may be determined by these influences. When a variety has arisen, the question arises whether it will thrive best in damp or in dry ground, in sunny or shady places, and so forth ; whether it can reproduce itself under these circumstances, or whether it will perish. The conclusion follows that hereditary varieties arise independently of direct external influences, but that the continuance of their existence depends on external causes. A variety which occurs only in a particular locality is not produced by the conditions of this particular locality ; but it alone furnishes the peculiar vital conditions which this particular variety requires, while other varieties which have arisen at the same place disappear. It has already been shown in Sect. 32 that hybrids show in general a tendency to the production of varieties. Two diff*erent sets of hereditary characters are com- bined in a hybrid, and there is hence a strong tendency towards the formation of new characters which may be more or less hereditary. Hybridisation is therefore one of the most important means at the command of the horticulturist for disturbing tjie constancy of inherited characters and producing a number of varieties from two dis- tinct ancestral forms \ But even the ordinary sexual union of two individuals of a species, as in dioecious, dichogamous, or dimorphic plants, may be considered as a kind of hybridisation ; in these cases also the individuals which unite must cer- tainly be diff"erent, since otherwise their cross-fertilisation would be no more pro- ductive than self-fertilisation. In these cases therefore two sets of characters which diff"er, though it may be but slightly, also unite in the descendants ; and if a hybrid from two difterent species exhibits a strong tendency to variation, the cross-fertil- isation of two diff"erent individuals of one and the same species may at least give rise to a slight tendency in the same direction. It is therefore probable that in the cross-fertilisation of different individuals — towards which there is always a tendency in nature even in hermaphrodite flowers — we have a perpetual cause of variation in plants. But this is by no means the only cause of variation, as is shown by the existence of bud-variation, and by the reflection that there must always be a slight diff"erence between individuals which produce a variable progeny. A great number of facts point to the conclusion that almost every plant has a tend- ency to vary continually and in different directions, while every new character which is not produced directly by external agencies tends at the same time to become hereditary. ' See also Naudin, Compt. rend. 1S64, vol. LTX, p. S37. [Jouni. Roy. Ho.i. Soc. new series, vol. 1, p. I.] 8^6 ORIGIN OF SPECIES. If notwithstanding this many wild plants and some cultivated ones are very constant and produce no varieties which can be distinguished externally, this is mainly the result of the fact that the newly produced varieties are unable to exist in the conditions by which they are surrounded, or at least soon disappear, a point to which I shall recur more in detail. The hereditary transmissibility of acquired characters exhibits itself in a most marked way when it does not affect the whole of the parent-plant, but only a particular branch. A still more remarkable case was observed by Bridgman. He noticed that the spores from the lower inner part of the lamina of the leaves of the varieties Scolo- pendrium indgare laceraium and S. vulgare Crista-galli, which were of the normal form, uniformly produced plants of the normal parent-form, w'hile those produced on the outer abnormal part of the leaf reproduced the special varieties \ Sect. 34.— Accumulation of new characters in the reproduction of varieties. The difference between a variety and its parent-form, or between the varieties of a common parent-form, is usually at first small and affects only a few characters. But the descendants of the variety may again vary, the new characters may thus become intensified, and other new characters of a different kind may be added to them. The amount of difference between parent-form and variety and between the various varieties of the same parent-form thus becomes greater : and if the tendency to become hereditary of the characters increases with the increase of their difference, the variety comes at length to differ so greatly from the parent-form that their genetic connection can only be proved historically or by the existence of transitional forms. This is the case with many of our cultivated plants, as e.g. the pear, which varies much even in the wild state, but in cultivation has altered its mode of growth, form of leaf, flower, and especially its fruit, to such an extent that it would be impossible to suppose the finest sorts of pears to be descendants of the wild Pyrus communis, if Decaisne had not proved their genetic connection by the study of the transitional forms (Darwin I.e. vol.1, p. 350). In the same manner it scarcely admits of a doubt that all the cultivated kinds of gooseberry are descended from the wild Ribes Grossularia of Central and Northern Europe ; and Darwin brings forward historical evidence to show that the size of the fruit has been con- tinually increased by cultivation since 1786, so that in 1852 it had attained the weight of 895 grs. Darwin found that a small apple 6| inches in circumference weighed as much (/. ^- p. 356). The different varieties of cabbage are all descended from one parent-species, or, according to Alph. De Candolle, from two or three closely related ones still growing in the neighbourhood of the Mediterranean. In this case hybridisation has also cooperated; the varieties are for the most part hereditary but without any great constancy. The extent of the variation which has taken place under cultivation is shown by the existence on the one hand of shrubby forms with branching woody stems, 10 to 12 or even 16 feet high, on the other hand of the round cabbage with a short stem and a spherical, pointed, or broad head consisting of leaves closely packed one over another ; and again of the savoy with its curled blistered leaves, the kohl-rabi with its stem swollen below, the cauliflower with its crowded monstrous flowers, &c.^ * Ann. and INIag. Nat. Hist, third series, vol. VIII, 1861, p. 490 ; Darwin /. c. vol. II, p. 379. 2 See Metzger, Landwirthschaftliche Tflanzenkunde, Frankfiirt a. M. 1851, p. 1000 ; and Darwin /. c. vol. I, p. 323. ACCUMULATION OF NEW CHARACTERS. 827 In the case of many cultivated plants the original wild form is unknown. It is possible that in a few cases it may have disappeared ; but it is more probable that the varieties w^hich have arisen in cultivation have gradually acquired such a number of new characters that their resemblance to the wild parent-form can no longer be traced. This is probably the case with the cultivated Cucurbitacese, gourds, bottle-gourds, melons, water-melons, &c., the hundreds of varieties of which were traced back by Naudin to three primitive forms, Cuciirbita Pepo, maxima, and moschata, neither of which how^ever is known in the wild state. These original forms have been as it were evolved from the resemblances and differences of the number- less varieties, and have only an ideal existence ; it is doubtful whether either of them ever actually existed, or whether these ideal parent-forms do not merely correspond to three principal varieties which arose from a single primitive form which possibly still exists, or from the hybridisation of several. The characters of many of these varieties are perfectly hereditary, and all the organs show the greatest degree of vari- ation ; how great and various these differences are is seen from the fact that Naudin has divided the group of forms which he included under the name Ciiciirbita Pepo into seven sections, each of which again includes a number of subordinate varieties^ The fruit of one variety exceeds that of another variety more than two thousand fold in size ; the original form of the fruit is probably ovoid, but in some varieties it is elongated into a cylinder, in others abbreviated into a flat plate ; the colour of the rind varies almost infinitely in the different varieties ; in some it is hard, in others soft; some have a sweet, others a bitter flesh ; the seeds vary in length from 5 or 7 to 25 mm. ; in some the tendrils are of enormous size, in others they are altogether wanting ; in one variety they are transformed into branches which bear leaves, flowers, and fruits. Even characters which are normally constant throughout entire natural orders become extremely variable in the gourds; thus Naudin (Compt. rend. 1867, vol. LXIV, p. 929) describes a Chinese variety o'i Cucurbiia maxima which has a per- fectly free or superior ovary, whereas it is inferior elsewdiere in the Cucurbitaceae and in the nearly allied orders I The varieties of melon {Cucumis Meld) Naudin divides into ten sections, which differ also not only in their fruit, but also in their leaves and their entire habit or mode of growth. Some melons are no larger than small plums, others weigh as much as 66 lbs. ; one variety has a scarlet fruit ; another is only i inch in diameter but 3 feet long, and is coiled in a serpentine manner in all directions, the other organs being also greatly elongated. The fruits of one variety can scarcely be dis- tinguished externally or internally from cucumbers ; one Algerian variety suddenly splits up into sections when ripe (Darwin, I.e. vol. I, p. 357). The behaviour of the genus Zea is similar to that of Cucurbita. The cultivated varieties of maize are probably descended from a single primitive wild form which has been cultivated in America for a very long period ; but it seems doubtful whether the native Brazilian species, the only one known in the wild state, with long glumes ^ See Metzger, Landwirthschaftliche Pflanzenkunde, p. 692, and Darwin, /. c. vol. I, p. 358. ^ Hooker states that a specimen oi Begonia frigida at Kew produced, in addition to male and female flowers with inferior ovary, also hermaphrodite flowers with superior ovary. This variation was the product of seeds from a normal flower. (Darwin /. c. p. 365.) 82 cS ORIGIN OF SPECIES. enveloping the grains, is the primitive form ; if it is not, then no plant is now known which can be considered as the ancestral form of our numerous and extremely diverse varieties of maize. In this case also continued cultivation has increased the amount of difference between the different varieties, as well as to a prodigious extent that between them and the primitive form ; and the separate varieties are distinguished from one another by a number of different characters. Some are only i^ feet high, others as much as 15 to 1 8 feet; the grains stand on the rachis in rows varying from six to twenty in number ; they may be white, yellow, red, orange, violet, streaked with black, blue, or copper-red ; their weight varies sevenfold ; their form also varies extremely ; there are varieties with three kinds of fruit of different form and colour on one rachis; and a great number of other differences also occur\ These instances may suffice to show to what an extent the amount of deviation of the varieties of a primitive form may increase under cultivation^. It is much more difficult, and to a great extent impossible, to prove directly to what extent the variation of wild forms can increase without cultivation, because historical evidence is in this case generally impossible, or can only be obtained indi- recdy or conjecturally. But since the laws of variation are unquestionably the same in the case of wild as of cultivated plants — although they operate in the two cases under different conditions — we may for the time at least assume as probable that plants vary as greatly in the wild as in the cultivated state. We shall however in the sequel have to examine a number of weighty considerations which lead to the con- clusion that variation has produced infinitely greater effects in originating the various wild forms of plants than those which we perceive in cultivated varieties^. The variation of cultivated plants shows that there is only one cause for the internal and for the external hereditary resemblance between different plants, and that this cause is the common origin of similar forms from one and the same ancestral form. When we meet with corresponding phenomena in wild forms, and when we find that with them as with cultivated plants dissimilar forms are connected by a series of intermediate forms, just as we find to be the case between the primitive forms of cultivated plants and their most abnormal varieties, we are forced to the conclusion that in wild plants also a similar affinity is the only cause of resemblance. The extraordinarily numerous forms, for example, of the widely distributed genus Hieracium present phenomena similar in many respects to those of culdvated gourds, cabbages, &c. In addition to a number of forms which are considered to be species, there are a still greater number of intermediate forms, some of which only are hybrids, the greater part perfectly fertile varieties. Nageli *, who has made this genus ^ See Darwin /. c. vol. I, p. 365, and Metzger I.e. p. 207. No great value with reference to variation and the constancy of varieties must be set on the result of experiments on cultivated plants, since the possibility of hybridisation was not excluded. Some varieties of maize appear to hybridise with difficulty. 2 Further material will be found collected in Darwin's and Metzger's works already quoted, and in De Candolle, Geographie botanique ; Paris, 1855. 3 [H. Hoffmann gives in the Bot. Zeit. for April 27 and May i, 1874, an account of an inter- esting series of experiments on the extent to which the characters which distinguish the allied species Papaver Rhceas and dtibunn and Phaseolus vulgaris and multifloriis can be made to vary by cultivation, and on the tendency of the cultivated varieties to revert to the parent-form. — En.] * Sitzungsberichte der kon. bayer, Akad. derWiss. March 10, 1866. ACCUMULATION OF NEW CHARACTERS. 829 the subject of close study, says:— 'If an attempt is made to unite into a single species all the types which are connected by perfectly fertile transitional forms, we should find only three species of native Hieracia, which have been erected by some authors into distinct genera :— Pilosella (Piloselloidese), Hieracium (Archieracium), and Chlorocrepis {H. staticifoliwn). Between these three groups we have, at least in Europe, no transidonal forms ; hybrids between Piloselloide^ and Archieracium have been erroneously stated to occur, but the alleged hybrids are either true Pilo- selloidese or true Archieracia. . . . According to the present state of our knowledge, no other hypothesis is possible but that all the various species of Hieracium have sprung from the transmutation (descent with variation) of forms which have either disappeared or are still in existence ; and a large number of the intermediate forms still occur which must have had their share in producing several new species by the splitting up of one original species, or which would have occurred in the transform- ation of a still living species into one derived from it. In the case of Hieracium the species have not become so completely separated by the suppression of the inter- mediate forms as is the case in most other genera.' By the term Species is meant the aggregate of all the individual plants which have the same constant characters, these characters being different from those of other somewhat similar forms. It is clear from what has already been said that the only distinction between varieties of a known primitive form which have become constant, and the wild species comprised within a genus, is that in one case their descent is known, in the other it is not. The various cultivated varieties of a primitive form which have become con- stant are linked together by intermediate forms in which the progressive accumulation of new varietal characters may be perceived ; but these intermediate forms may dis- appear, and then there is a more or less wide chasm between the various varieties them- selves, as well as between them and the primitive form. Both of these cases occur also in wild plants. In some genera, like Hieracium, species the extreme forms of which differ greatly are connected together by a number of intermediate forms which occur along with them. The analogy of cultivated plants justifies us in considering these intermediate forms (so far as they are not hybrids) as varieties in a progressive state of development, some particular descendants of which have advanced furthest in the accumulation of new properties. But usually the intermediate forms, the connecting links so to speak between the ancestral and the derived forms, have disappeared ; and the species of the same genus are then completely separated from one another, and their characters are at once distinguishable ; while the different species of the same genus agree among one another in a number of inherited characters, and are distinguished only by single constant characters ; the amount of resemblance is much greater than the amount of difference. The same relationship therefore exists, but in a greater degree, betv.een the various species of one genus as between different varieties of the same primitive form. And since noother explanation is known of this relationship than com- mon descent with variation and the heredity of the new characters, so we are entitled to consider the species of a genus as varieties of a commjon ancestral form which have developed further and become constant,— the original form having possibly actually dis- appeared or being no longer recognisable as such. There is therefore no natural bound- ary-line between variety and species ; they differ only in the amount of divergence of the characters and in the degree of their constancy. Just as a number of varieties are included in the idea of a species — the varietal characters being neglected in the diagnosis of the species — so several species are united into a genus by including in the diagnosis of the genus a maximum of their common characters. But since it is im- possible either to determine by measure or by weight which are the most important 830 ORIGIN OF SPECIES. characters of a plant, so it is difficult and to a certain extent impossible to define, i.e. to determine by comparison, what amount of differentiation is necessary in order to classify two different but similar forms as species rather than varieties. In the same manner it is left to a great degree to personal judgment to decide w^hether tw'o different but similar groups of forms should be classed as varieties of two species or as species of two distinct genera. The only object actually presented to the eye is the individual (and even this not always as a whole) ; the ideas Variety, Species, Genus are abstract ideas, and indi- cate a progressive scale of the differences between individuals which is small in the variety, larger in the species, and still larger in the genus. But in all these cases points of difference are of less importance than the amount of resemblance ; and since in the phenomena of variation we learn that forms w^hich are similar but are constantly becom- ing more different are derived from the same ancestor by the continual accumulation of differences, so we assume that the higher degree of variation of similar forms which we express by the terms Species and Genus have resulted from the accumulation of new characters in the variation from one ancestral form. Sect. 35. — Causes of the progressive development of varieties. The characters of the cultivated varieties of one parent-form show, as Darwin was the first to point out, a constant striking and remarkable relation to the purpose for which the plant was cultivated by man. The varieties of w^heat differ from one another only slightly in the form of the haulm or leaves, w^hich are of but small im- portance to mankind ; but they show^ a great variety and extent of difference in the form and size of the grains, and the quantity of starch and proteine contained in them, 2. e. in the characters of that part of the plant for the sake of which wheat is cultivated, and in those properties of this part which under various circumstances are especially useful to mankind. The varieties of the cabbage, on the other hand, scarcely differ at all in their seeds or even in their seed-vessels or flowers, the external pro- perties of w^hich are useless to man, and the internal properties only of value because the seed has to reproduce the variety ; the varieties of cabbage differ exclusively in the development of those parts which are used as vegetables, and to which therefore cultivation is directed. The object of cultivation is therefore, retaining the taste and value as food for man, sometimes to increase the succulence of the tissues, sometimes to attain as large a size as possible, sometimes to alter the time of the year at w^hich the vegetable can be used. These and a number of other properties are furnished by the different varieties. The varieties of beet differ only slightly in their flowers, more in their leaves, according as they are growm in the garden as ornamental foHage-plants or as agricultural crops ; the varieties in the latter case differ from one another in the size and shape of the roots and the amount of sugar they contain, properties which make the plant valuable on the one hand as food for cattle, on the other hand for the manufacture of sugar. Fruit-trees of the same kind differ but little in general in their roots, leaves, flowers, or stems, but to an extraordinary extent in the size, shape, colour, smell, taste, period of maturity, and keeping-pro- perties of the fruit, according to the special purpose or prevalent mode in which it is employed. In garden-flowers it is generally the flowers and especially the corolla and inflorescence that differs in the varieties of a species, because the greater number are cultivated only for the shape, size, colour, or odour of the flowers. This relation of cultivated varieties to the requirements of man is explained if we suppose that only those varieties were cultivated, at first undesignedly afterwards CAUSES OF THE PROGRESSIVE DEVELOPMENT OF VARIETIES. 83 1 designedly, in which some character useful to man was more strongly manifested than in the others ; those individuals were selected which best answered to a definite requirement ; they alone were further cultivated ; the particular character was again strongly displayed in some of their descendants, and only these individuals were again selected for reproduction ; and the desired character was thus continually in- creased in strength. Other characters of the plant also varied at the same time, but they were disregarded, and the individuals in which they occurred were not preserved for reproduction, and no increase of these characters consequently took place from generation to generation. The greatest service which Darwin has rendered to science is to have shown that wild plants are also subject to vital conditions the effect of which consists in this, that only some of the varieties of one primitive form maintain themselves and increase their peculiarities, while others perish. The relationship of the varying wild plant to its environment in the broadest sense of the word is however different from that of the cultivated plant to man ; man protects his charges in order to preserve them ; he places them under favourable conditions in order that those properties which are useful to him may become freely developed. Wild plants, on the contrary, have to protect themselves against all injury from without ; their existence is con- tinually threatened by other plants or animals or by the hostility of the elements ; and in this Struggle for Existence^ as Darwin has appropriately termed it, only those individuals are able to maintain themselves which are best able to resist the prejudicial influences to which they are exposed; and only those varieties which happen to be the best endowed in these respects will reproduce themselves and further develope their special properties. Hence the characters of wild plants, as far as they are not of a purely morphological nature, always show a perfectly definite relationship to the conditions in which they are placed ; the form and other characters of the organs have essentially for their object to secure the existence of the plant under the local conditions of its habitat ; varieties and species which are not endowed with qualities to endure the struggle for existence perish. The struggle for existence acts there- fore in a certain sense similarly to the selection of the breeder ; as the breeder de- velopes only that which is suited to his own purposes, so in the struggle for existence only those varieties survive and reproduce their kind which are better adapted, through some property which they possess, to endure the struggle. Thus, finally, through imperceptible variation, through the destruction of those characters which are not beneficial, and through the further development of the useful ones — in one word, through what may be termed metaphorically Natural Selection in the struggle for existence, — forms are produced which are as well or even better adapted for the purpose of self-preservation than cultivated plants are for the purposes of man. By the undesigned reciprocal influences of plants and of their living and physical environment, specialities of organisation finally arise which could scarcely be better adapted for the preservation of the plant under its special local conditions, and which give the impression of being the result of the greatest ingenuity and foresight. In order to understand clearly how the struggle for existence has caused the existing wild forms of plants to be so admirably adapted to their specific vital con- ditions, it must be borne in mind that all plants are continually varying to a very slight extent, and that the variation affects all their organs and all their characters, 832 ORIGIN OF SPECIES. although usually to an imperceptible amount. On the other hand, the struggle for existence in plants (as well as in animals) is a perpetual and never-ceasing one, in which the smallest advantage that the plant has obtained through variation in any one direction may be of the utmost importance for its perpetuation. The struggle which the plant carries on by means of its capacity for variation has two different aspects. On the one hand its tendency is to adapt the organisation of the plant completely to the conditions of food and growth afforded by the climate and the soil. It is evident that the organisation of a submerged water-plant must be different from that of a land-plant ; that the assimilating organs of a plant that grows in the deep shade of a wood must be differently constructed from those of a plant exposed daily to bright sunshine, and so forth. The vital conditions of all plants growing at a great elevation and in Arctic countries must be different from those growing in the lowlands of the Tropic and Temperate zones. If we had to do only with the general conditions of plant-life, the struggle for existence would be a com- paratively simple process. It would be easy to imagine how, among the varieties of a primitive form which grew in water, there would be some which would be occasionally subjected to a subsidence of the water, and how these would give birth to descend- ants which would gradually assume the character of marsh- and finally of land-plants, as is well illustrated in the case of Nasturtium amphibium^ Polygonum amphibiuniy &c.^ It may also be supposed that some of the descendants of a plant exhibit a somewhat greater power of resisting frost, that this property increases in the course of generations, and that thus a form which can at first only bear a temperate climate gradually produces varieties which can endure a more and more severe climate ; and so forth. But these comparatively simple relationships must lead to a great diversity in the varieties which claim descent from one ancestral form ; for each adaptation to new conditions of climate or locality would act in different ways ; z*. e. varieties of different descriptions would take up and carry out in different ways the struggle against the influences of the elements. But the struggle for existence and the changes occasioned by it in the organ- isation of plants are greatly complicated by the fact that every plant, while struggling to adapt itself to its special vital conditions, has also to protect itself at the same time against a number of other plants and against the attacks of animals ; or, what is more to the point, its capacity for variation enables it to make use of particular favourable conditions which are offered to it by other plants and animals in order to take ad- vantage of them ; as parasites of their hosts, dichogamous and other flowering plants of the visits of insects, &c. These relationships are endless in their diversity, and can only be illustrated by examples. But we must here call special attention to a remark of Darwin's ; that the indi- viduals of the same species or variety are competitors for position, food, light, &c. The fact that plants of the same species have the same requirements itself gives rise to a struggle for existence among them ; and the same is the case, though to a some- what smaller but still to a great extent between the different varieties of the same ^ A special interest attaches in this connection to Hildebrand's observations on Marsilea in Bot. Zeit. 1870, No. I, and Askenasy's on Ranunmlus aquatilis and divamatus in Bot. Zeit. 1870, p. 193 et seq. CAUSES OF THE PROGRESSIVE DEVELOPMENT OF VARIETIES. 833 primitive form, to a less extent between different species and genera. The result of these relationships is seen on the one hand in the fact that with plants which live socially only the most vigorous seedlings arrive at full maturity, while the weaker ones are smothered, as may be seen in any young plantation ; on the other hand, that species and genera which differ greatly from one another can thrive side by side, because their requirements are different and the competition between them is less. From the fact that plants whose organisation differs can thrive better side by side on the same soil in consequence of the diminished competition between them, Darwin drew the important and pregnant conclusion that in the propagation of the varieties of one primitive form those new forms must be the best able to maintain themselves in the wild state which differ most from the primitive form and from one another, while the intermediate forms are gradually dispossessed. This is the reason why the connecting forms between the different species of a genus are so often want- ing, although the conclusion cannot be avoided that the species arose by variation from a single ancestral form, and by the propagation of varieties. In its larger features (but on that account more conspicuously) the struggle for ex- istence between the various forms of plants, the competition for space, food, and light, are manifested in the luxuriant growth of what we term weeds in our gardens and fields. Our cultivated plants are able to bear our climate, and the soil supplies what they require for their vigorous growth. But a number of wild plants are still better adapted to the climate ; and they grow still more vigorously, rapidly, and luxuriantly on cultivated soil, and their seeds or rhizomes are everywhere present in enormous quantities. If the cultivated plants are not carefully protected from the weeds, the latter soon dis- possess them of the ground which was set apart for them. Every country and every soil has its own peculiar weeds ; i.e. under any particular external conditions there are always certain forms of plants which thrive best and drive out the cultivated plants. To a certain extent we have a measure of the amount of advantage which weeds have over cultivated plants in the amount of labour bestowed by man on their destruction in order to preserve and maintain his nurselings. The primitive forms of our cultivated plants are mostly natives of other countries, where they are not only sufficiently adapted for the climate, but are able to sustain competition with their neighbours. The number of species or of individuals of any species which we find in a meadow, a marsh, &c. is not a matter of chance ; it does not depend merely on the number of seeds of one or another species produced or brought to the locality ; every one of these spe:ics would, if it alone existed there or were protected by cultivation, of itself cover the space of ground in a short time ; and yet there is a definite relationship between the numbers of individuals of the different species when left to themselves, a relationship which de- pends on the specific power of each particular species to maintain itself in the struggle with the rest^. How complicated may be this relationship in the cases of only two nearly related forms of plants in their struggle for existence in particular localities, has been described as exhaustively as clearly by Niigeli in the case of various Alpine plants. ' The interne- cine war,' he says^, Ms obviously most severe between the species and races that are most nearly related, because they require the same conditions of existence. Achillea * [How the relationship subsisting between the species in permanent pastures may be disturbed by the application of different manures, may be seen in Lawes and Gilbert's paper on this subject in Journ. Roy. Agric. Soc. vol. XXIV, 1863.— Ed.] 2 Sitzungsber. der Icon, bayer. Akad. der Wiss. Dec. 15, 1865. 3 H 334 ORIGIN OF SPECIES. moschata drives out A. atrata, or is driven out by it ; they are seldom found side by side ; while each grows along with A. Millefolium. It is clear that Achillea moschata and atrata, being extremely similar to one another externally, make similar demands on their en- vironment, while A. Millefolium, which is less nearly allied to both, does not properly compete with them, because it requires other conditions of existence. Still less do plants of different genera or orders compete with one another ... In the Bernina Heuthal (Upper Engadin) Achillea moschata, atrata, and Millefolium occur in profusion, A. moschata and Millefolium on slate, A. atrata and Millefolium on limestone ; where the slate ends and limestone begins, A. moschata always ceases and A. atrata takes its place. Both species are therefore here strictly circumscribed as to soil, and this I have found to be the case also at various spots in Bilndten, where both species occur together. But where one species is absent the other is widely distributed, and is then found indiscriminately on slate or limestone. Although A. moschata does not apparently grow so readily on limestone as A. atrata does on slate, yet in the neighbourhood of the primary rocks it is found on a distinctly calcareous formation along with the vegetation characteristic of it. In the Bernina-Heuthal I found in the midst of the slate which was thickly covered with A. moschata a large erratic block of limestone covered with a crust of soil scarcely an inch thick, upon which a patch of A. moschata had established itself, because it did not here meet with any competition from A. atrata. . . . A similar relationship was observed in certain districts between Rhododendron hirsutum Sind ferrugineum, Saussurea alpina and discolor, and between species of the genera Genti- ana, Veronica, Erigeron, Hieracium, &c.' The obvious objection, that there cannot possibly be any struggle between two forms of plants as long as there is space for both in the area in question, rests on an incorrect basis, and is disposed of by Niigeli as follows : — ' Upon a slate slope are a million plants of A. moschata ; they obviously do not occupy the whole space, for a hundred millions or more could find room there ; but the rest of the space is occupied by other plants. There is here a condition of equi- librium, which has been produced in reference to the nature of the soil and the preced- ing climatic influences. The number one million gives us also the proportion which A. moschata is able to maintain in relation to the rest of the vegetation ; and the objection that there would still be plenty of room for A. atrata is an untenable one. If the space were accessible to species of Achillea generally, it would be occupied by the species which is already present, and which in any case has the advantage, A. moschata. If we now imagine that the two species happened for once to be intermixed on the slate slope, perhaps in consequence of artificial transplanting, in equal quantities, say 500,000 plants of each, A. moschata would thrive the better of the two, as the soil contains but little hme; A. atrata would become weaker and its tissue less matured, and would in conse- quence have less power to withstand external prejudicial influences, as summer frosts, long-continued rainy weather, or persistent drought, &c. If we suppose, for example, that every twentieth or fiftieth year a severe frost occurs at the time of flowering which destroys half the plants of A. atrata, while the more vigorous A. moschata re- sists it, the voids are again filled up by the dispersion of the seeds ; but more plants of A. moschata spring up than of A. atrata, because the number of individuals of the latter was reduced by the frost to 250,000, while that of the former remains at 500,000. The million plants of Achillea on the slope will in consequence be composed of say 670,000 A. moschata and 330,000 A. atrata. After a second frost, which again destroys one half of the individuals of A. atrata, we should have about 800,000 of A. moschata to 200,000 of A. atrata. In this manner the number of the latter would decrease with every un- usual summer frost, until at length it entirely disappeared, a nearly-allied hardier species becoming distributed over the locality in its place.' In conclusion, the following remark by the same author may be added : — ' From such a course of reasoning the conclusion might perhaps be drawn that this result would always take place, and that one of two plants would always be crowded out, because the two could hardly be precisely equally hardy. But this conclusion would be unsound, because it would hold good only for CAUSES OF THE PROGRESSIVE DEVELOPMENT OF VARIETIES. S^^^ plants whose conditions of existence were as nearly as possible alike. We can imagine another case in which the two species suffer injury from altogether dissimilar external influences (one, e. g., from spring frost, the other from dry heat), so that sometimes the number of individuals of one species, sometimes that of the other species diminishes, and where moreover the production and the germination of the seeds are affected by altogether dissimilar external influences, so that sometimes the one sometimes the other species increases most rapidly and occupies the vacant spots. The numerical proportion of the two species must in this case be variable, but neither is able to expel the other.' Just as the struggle between two species is the result of their thriving more or less vigorously on a soil of a particular chemical nature, so also the need for more or less water, light, heat, &c. can determine also the nature of the struggle for existence. NageU gives some examples of the first case. When Primula officinalis and elatior occur together in a district, they are sometimes sharply separated from one another, P. offici- nalis preferring the dry, P. elatior the damp spots. Each is most vigorous in its own locality, and may expel the other. But when only one species occurs, it is not so par- ticular ; P. officinalis will choose damper, P. elatior drier situations, than if they were in company. Prunella 'vulgaris and grandijiora behave in the same manner in reference to poorer and more fertile soils ; as also do Rhinanthus Alectorolophus and minor, Hieracium Pilosella and hoppeanum. These examples may suflftce to show what is meant by the Struggle for Existence. It must however be borne in mind that such a struggle must arise in reference to every vital phenomenon of a plant, and to each of its relationships to the external world, especially to the animal kingdom ; and that its course must vary for the same plant in different localities. An understanding of the Theory of Descent, and especially na insight into the causes of the perfect structural contrivances adapted to the vital conditions of the plant which are often extremely local, depends essentially on a clear comprehension of the struggle for existence. Sect. 36. — Relationship of the morphological nature of the organ to its adaptation to the conditions of plant-life. Every plant is very accurately adapted (though not absolutely so) to the conditions and circumstances in which it grows and is reproduced ; its organs have the shape, size, mode of develop- ment, power of movement, chemical properties, &c. needful for this purpose. If this were not the case, the plant would inevitably perish in the struggle for existence. But the vital conditions are extremely various, and undergo, in the course of time, endless changes. The diversity in the characters of plants corresponds to this infinite variety in the conditions of life ; and yet even in the more highly differentiated classes there are only three or four morphologically distinct forms of structure, axis (cau- lomes), leaves (phyllomes), roots, and trichomes, which suffice for these conditions, while maintaining a constant morphological character through numberless variations in their physiological properties. This relationship has already been described in chap, iii of Book I as the metamorphosis of the morphological members of a plant, understanding by metamorphosis the adaptation to various physiological purposes of morphologically equivalent members. The diversity in the physiological development is directed to the vital conditions of the plant ; and to this extent Metamorphosis is synonymous with what we have here termed Adaptation, and which has also been described as Accommodation. When we speak of Purpose in the structure of a plant, we mean in fact nothing more than that the form or other characters of the organ are adapted to its conditions of life, which may be at once inferred from the 3 H 2 836 ORIGIN OF SPECIES. very survival of the plant in the struggle for existence. The terms Purpose, Adapt- ation, and Metamorphosis express therefore the same thing, and may be used as synonymous, as we have already repeatedly done. For the purpose of the questions to be treated of in the following paragraphs it is important to have as clear a conception as possible of the relationship of adapt- ation to the morphological nature of the organs, and of the great constancy of morphological characters and the infinite diversity of metamorphosis ; for such re- lationship can be explained by no other theory than that of descent. In its most general features the relationship of adaptation to the morphological nature of organs is manifested in the fact that all the various morphological members perform the most different functions and in an infinite variety of ways ; in other words, that the morphological nature of the parts of a plant is not directly determined by their function, nor is the function of an organ determined directly by its morpho- loo-ical nature. Thus, for example, trichomes sometimes take the form of a pro- tective envelope (mostly in buds), sometimes of glands, sometimes of absorptive organs (as root-hairs), sometimes of asexual organs of reproduction (as the sporangia of Ferns), &c. The leaves again are usually organs of assimilation containing chlo- rophyll ; but they may also be employed as protective envelopes to winter-buds (in most of our native woody plants), as reservoirs for reserve food-materials (in the seedlings of flowering plants and in bulbs) ; in Vascular Cryptogams they bear the sporangia. In flowering plants the organs of reproduction and their envelopes are peculiarly metamorphosed leaves ; in many slender-stemmed Angiosperms the leaves are transformed into tendrils, in order to raise up the slender stem and fix it to neighbouring supports ; the leaves of Nepenthes produce at their apex an append- age which forms a pitcher provided with a moveable lid and filled with the fluid which it itself secretes ; some of the leaves contained in the flowers are developed into nectaries and then perform the function of glands ; not unfrequently they are transformed into hard woody spines ; in other cases they are sensitive to irritation, contractile, and so forth. The parts of the axis are scarcely less varied in their development ; sometimes they cling round upright supports ; sometimes they are woody and able to retain themselves in an erect position ; sometimes they are slender swaying branches, or thick fleshy succulent masses (Cactus), or round tubers filled with food-materials (Arum, potato), or they become tendrils (the vine), or spines (Gleditschia) ; sometimes they assume the form of foliage-leaves (Ruscus, Xylo- phyllum, &c.). The adaptations of roots are less numerous ; usually filiform, slender, cylindrical, and provided with root-hairs for absorbing water and dissolved mineral substances, they become tuberous reservoirs for reserve food-materials in the dahlia ; their tissue is loose and contains air and they resemble swimming-bladders in jussiaea ; in the ivy, Ficus repens, &c., they are simple organs of attachment for the stem ; in Vanilla aromatica they play the part of tendrils ; but they never produce sporangia or sexual organs. According to the definition already given of Purpose in the vegetable organ- isation, its relationship to the morphological nature of the organ can also be illus- trated by keeping in view the purpose to be served, i. e. the character of the plant which is serviceable in the struggle for existence, and then observing the means employed for attaining this purpose, /. e. what members of the plant are RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 837 adapted for the purpose, and what metamorphosis they undergo. A few examples will explain this'. It is obviously useful for the greater number of flowering plants — in other words advantageous in the struggle for existence — that their stem should grow rapidly to a certain height, because the conditions of assimilation (light and warmth) are thus most perfectly fulfilled, and because — which is perhaps of greater importance — the flowers are more easily detected by insects on the wing, and the pollen trans- ferred by them from one flower to another. Even where (as in many Coniferae, &c.) the light pollen is carried by the wind to the female flowers, this is accomplished better when the flowers are at a greater height from the ground ; and finally by this means the dissemination of the seeds by the wind or by frugivorous birds is pro- moted, or their scattering by the bursting of the fruits. That these arrangements for propagation are especially promoted by the upright growth of the stem is evident from the large number of plants which develope their leaves in a rosette close to the ground or on a stem that creeps along it, a rapidly ascending flower-stem being formed only just before the unfolding of the flower-buds. Still more strikingly is this the case in parasites and saprophytes (Orobanche, Neotda, &c.), which vegetate below and blossom above ground. If we concede these and other special purposes of upright growth, it is of interest to see in what various v;ays this one purpose is attained in diff"erent species of plants. In many shrubs the growing stem is endowed with sufficient firmness and elasticity to support in an upright position the weight of the leaves, flowers, and fruits ; if it happen to be broken down, or if it must raise itself from a previously creeping position, advantage is taken of the property of geotropism. But the slender haulms of Grasses are not themselves endowed with this power ; and in their case the basal portion of each leaf-sheath forms a thick ring the tissue of which retains for a long time its power of growth ; and when the haulm is bent by the wind, or is in its early stage prostrate on the ground, the elevation into an erect position is brought about by the surface of the node which faces the ground growing rapidly and strongly; a knee-shaped bend is thus formed by which the upper part of the haulm is raised up. If, on the contrary, the stem is perennial, and has to bear a great weight of branches, leaves, and fruits, contrivances of this kind are not sufficient, and then the tissue becomes woody ; if the weight of the crown increases year by year, the stem also becomes thicker each year, as in dicotyledonous trees and Conifers ; if the weight of the foliage does not increase, as in Palms, the stem only retains the same thickness. In such cases a considerable quantity of as- similated food-material is necessary in order to produce the massive solid stem, while in many other cases the elevation is attained at the expense of a very small amount of organic substance, as in climbing and twining plants, such as are found in the most widely separated families of Angiosperms. Plants with a twining stem like the hop presuppose in general the existence and proximity of other plants which are able 1 In these examples I am compelled to confine myself to the most important points. Most of the adaptations are so complicated that a detailed description of them in even a single plant would require a great deal of space. What was said in the . fourth chapter of this book on climbing plants and in the sixth on the adaptation of the foliar organs of a flower to the purpose of cross- fertilisation may be . consulted. .: .■"'■:.-' 838 ORIGIN OF SPECIES. themselves to grow upright and round which they twine ; and in order that such a neighbouring support may be more easily and certainly taken hold of, the slender stem of climbing plants is endowed with a power of revolution by which the apex is carried round in a circle and occasionally pressed closely to the stem of an upright plant, up which it then climbs. The greater number of plants provided with tendrils are also dependent on the proximity of erect plants round which they can climb ; they are characterised by an extreme parsimony in the employment of organic substances for the purpose of an erect growth. Sometimes (as in the grape-vine) the tendrils are axial structures furnished with minute leaves and branching from the axils of these ; but much more commonly (as in Clematis or Tropseolum) the petioles, or (as in Fumaria) the branched narrowly -divided lamina, or most often the metamorphosed apical parts of the foliage-leaves {Cobcea sca?idens, the pea and other Papilionaceae) are developed in a filiform manner and perform the function of tendrils. The morphological signi- ficance of the tendrils of Cucurbitacese is not yet perfectly determined; but they are probably metamorphosed branches. Tendrils occur only in those plants whose stem is not able to bear in an erect position the weight of the foliage, flowers, and fruits ; in the genus Vicia, for example, all the slender-stemmed species have leaf- tendrils ; but in the thick-stemmed erect V. Faha they are rudimentary. The office of tendrils is to twine round the slender branches and the leaves of other neigh- bouring plants, and thus to fix the apex of the stem as with cords on various sides while it is growing upwards. The mode of development of tendrils, /. e. their endowment with useful properties corresponding to their purpose, is, as Darwin has shown, not only extremely diverse, but exhibits also very different grades of perfection, like climbing stems. Some tendrils are only of slight use ; sometimes (as in some species of Bignonia) they are rather helps to an imperfectly climbing stem ; but where they are perfectly adapted to their function, a variety of properties concur in a remarkable way to increase to a maximum the mode of adaptation to the use of the plant. The tendrils radiate in different directions from the growing apex of the shoot, which makes movements of revolving nutation by which the tendrils are brought into the greatest variety of positions, they themselves also revolving at the same time, so that within a certain area, often not a very small one, they assume an infinite number of positions, by which they must almost inevitably be brought into contact with some support, such as a branch or leaf, lying within this area. The supports are, so to speak, sought out in the most industrious manner ; when one is touched by a tendril, the tendril bends and twines firmly round it ; and when several tendrils do the same in different direc- tion from the stem, it hangs suspended between the points of support. If this were all, the attachment would be a very weak one, and the elevation of the stem would only take place slowly ; but the whole contrivance is perfected in the most ingenious way. When the tendrils have fixed themselves by their extremities, they draw the stem towards the support by twisting themselves spirally. When several tendrils do this in different directions, the stem which is suspended between them is tightly stretched, and the tenacity of the tendrils is at the same time con- siderably increased by the twisting. Many tendrils, while very tender at the time when Ihey are sensitive, become afterwards hard and woody, and some become RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION, 839 much thicker ; this is strikingly the case in Clematis glandulosa and Solanum jasmi- noides. But the most perfect adaptation is shown in the tendrils of the Virginian creeper, Bigjionia capreolata, and some other plants. As in the grape-vine, the tendrils are here branched axial structures, and are to a much greater extent nega- tively heliotropic ; their power of twining round slender supports is but slightly developed, but when, in consequence of their negative heliotropism, they come into contact with a wall, or in the wild state with a rock, trunk of a tree, &c., there is formed in the course of a few days on each branch of the tendril which touches the support with its curved and hooked apex, a cushion-like swelHng which afterwards expands into a red flat disc, and becomes firmly attached by its surface to the support. The adhesion of this organ of attachment is probably at first occasioned by an exudation of viscid sap ; but the attachment to the support is caused mainly by this organ of attachment forcing itself into all the depressions in the surface of the support and growing over the slight elevations. After this has taken place the whole tendril becomes thicker; it contracts spirally, the stem to which it belongs being thus drawn towards the wall, rock, &c. ; then it becomes woody, and the firmness of its tissue and the power of retention of the disc are so considerable that, according to Darwin \ a tendril ten years old and furnished with five of these discs can support a weight of lolbs. without giving way and without the disc becoming detached from the wall. Since a shoot which is growing upwards forms a number of tendrils, this attachment to the flat support is a very effectual one, and enables the plant to endure the annually increasing weight of the stem which is gradually becoming thicker and more woody; and in this way it climbs over the walls and roofs of buildings more than 100 feet high. The fact is very interesting that those tendrils of the Virginian creeper which do not come into contact with the wall or rock die after some time, and wither up into slender threads which then fall off, no adhesive disc having been formed on them. But in order that these peculiar tendrils may more readily come into contact with the support, even the upright shoot is scarcely positively heliotropic, since this property would cause it and its tendrils to move further away from the supports ; while the young shoots which exhibit such very slight heliotropism become erect under the influence of gravitation ; otherwise the whole of the contrivances connected with the tendrils would be purposeless. If looked at merely from the outside, the mode in which the Virginian creeper climbs up rocks, walls, and thick trees, presents a certain resemblance to the climbing of the ivy ; but in fact the adaptations of the two are altogether difl"erent. It has already been shown how negative heliotropism causes the leafy branches of the ivy to become closely pressed to the support, and how the summit of the branch at first exhibits slight positive heliotropism, so that it is attached to the support with a slight convexity. At this point of pressure rows of aerial roots afterwards arise (not in consequence of pressure, for they make their appearance also on branches which hang free) which apply themselves to the inequalities of the bark of the tree or the rock which serves as a support, and thus fix the ivy-stem to it. Other weak-stemmed plants attain the same object (that of elevating their assimilating and flowering shoots) by apparently much simpler means, as the bramble, rose, [Movements and Habits of Climbing Plants ; Journ. Linn. Soc. vol. IX, 1865, p. 87.] 840 ORIGIN OF SPECIES. and some climbing Palms like Calamus, &c., whose long shoots spread over neighbouring plants and are supported by them, their hooked prickles and other similar contrivances assisting in this. It is of service to many plants in the struggle for existence that they should keep firm possession of the piece of ground they have once occupied, without formino- for this purpose large woody masses, like trees and shrubs. The under- ground parts of such plants are perennial, and they send up separate shoots in each veo-etative period to be exposed to the light and air where they will be able to assimilate, to produce flowers, and to scatter their seeds. This persistence of the underground parts has the advantage that the plant, although it assimilates and grows only at particular times of the year, is not compelled to seek each year, like annual plants, a new locality in which its seeds may germinate. The collection of reserve food-materials underground gives strength to the plant ; it developes its buds beneath the soil to such an extent that at the right time they can grow up quickly at the expense of the rich supply of food. Every year very strong shoots are put forth, while in annual plants a number of feeble seedlings perish annually before some of them attain sufficient strength to protect themselves from the shade and humidity to which their neighbours subject them. Plants whose underground parts are perennial have in particular the power of resisting long and severe frost and the greatest variations of temperature, because these only penetrate slowly beneath the soil. It is for this reason that so large a number of Alpine and Arctic plants belong to this class. They are also able to grow in localities which are much too dry for the germination of the seeds of annual plants, because moisture is retained at a great depth for a longer period than near the surface. Numerous other advan- tages might also be mentioned which are of course secured to annual plants by other adaptations \ This permanence of the underground parts is attained in the greatest variety of ways. Sometimes the plant possesses slender creeping underground shoots in which the reserve food-materials are collected and which themselves rise above the surface at a particular time, as in many Grasses ; or sometimes the leafy stems are developed from lateral buds, as in Equisetum ; or there are thick stout stems from which shoots appear each year at the same place. In some cases the whole plant is annually renewed; all the parts which existed the previous year die off, and a complete rejuvenescence of the individual is accomplished underground. In the potato and artichoke only the apical parts of the underground lateral shoots swollen into tubers remain over till the next year, the whole of the rest of the plant having perished. In many of our native Orchids the rejuvenescence takes place in a similar way (see p. 199 and fig. 150); and one of the most interesting cases of annual rejuvenes- cence occurs in Colchicum auiumnale (see fig. 391, p. 545). In these cases, with the exception of the Orchids, the reserve food materials accumulate in underground parts of the axis ; in other cases this takes place in the swollen roots, which remain in connection with the underground part of the stem that bears the new buds, as ^ [This subject — and especially the relation of peculiar habits of life to the power of resisting great cold — is very fully discussed in Kerner's treatise Die Abhangigkeit der Pflanzengestalt von Klima und Boden, Innsbruck, 1869. — Ed.] RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 84I in the hop, dahlia, and bryony. In bulbs again the reserve accumulates in the leaves (bulb-scales) which surround the bud that developes into the new plant. The reserve often collects in cataphyllary leaves of peculiar development; in Allium Cepa in the lower part of the leaf-sheaths, which persist through the winter, while the upper parts of the leaves die off. We have already in the last chapter spoken of the immense variety of the contrivances which have for their object the partial or entire prevention of the self- fertilisation of plants, in order to produce a stronger and more numerous off- spring by the sexual union of different individuals ; and only a few examples need now be mentioned. Since the form, size, colour, position and movements of the parts of the flower are almost invariably adapted to facilitate the conveyance of pollen from one flower to another, generally by insects, and often even to render self-fertilisation impossible ; and since a great diversity even of those forms of flowers which are constructed on the same morphological type results from this, so the properties of ripe seeds and fruits are no less adapted^ to bring about the disse- mination of the seeds. Fruits which are very similar from a morphological point of view may nevertheless assume physiological properties which are altogether different, and fruits which are very different morphologically may become extremely similar in consequence of their adaptation to the purposes of dissemination. The service rendered by insects in the fertilisation of diclinous, dichogamous, dimorphic, and many other flowers, is performed by birds in the dissemination of a number of seeds which are concealed beneath fleshy edible pericarps ; in some cases, as the mistletoe, it is scarcely possible to imagine any other mode of dissemination than the eating of the berries by birds. Dry fruits or the seeds which are shed by dry fruits are often provided with an apparatus adapted for transport by the wind, the morpho- logical value of which is as various as possible. The wings on the seeds of species of Abies are a superficial layer of the tissue of the scale (carpel), those on the seed of Bignojiia muricata originate from the integument of the ovule ; the wings of the indehiscent fruits (samarse) of Acer, Ulmus, &c., are outgrowths of the pericarp ; the crown of hairs on the seed of Asclepias syriaca evidently performs a similar service ; as does the pappus of many Compositse which is a metamorphosed calyx. In these cases it is obvious that the wind carries the seeds or fruits ; in other cases animals of considerable size perform this office involuntarily, the hooked or rough fruits becoming attached to them and afterwards falling off^. In most of these adaptations, both their purpose and the mechanical con- trivances for its attainment are easily recognised; but not unfrequently the latter require a closer examination and some reflection in order to understand them. Among many other cases of this kind one only may be mentioned here which any one can easily observe for himself. The fruit of Erodium grm'num and other Geraniacese^ splits up into five mericarps each of which has the form of a cone with 1 It is scarcely needful to mention again that this mode of expression has only a metaphorical meaning from the stand-point here assumed, and is only used for the sake of convenience. - [A remarkable instance of this is recorded by Dr. Shaw (Journ. Linn. Soc. vol. XIV, 1874, p. 202), in the introduction into South Africa and enormously rapid distribution of a European plant, Xanthium sphwsiim, by the spiny achenes clinging to the wool of the Merino sheep.— Ed.] 3 See Hanstein, Sitzungsber. der niederrheinischen Ges. in Bonn, 1868. 842 ORIGIN OF SPECIES. the apex pointing downwards, containing the seed and bearing above a long awn. When moist this awn is stretched out straight, but if it becomes dry while lying on the ground the outer side of the awn contracts strongly, causing the upper end to describe a sickle-shaped curve, which brings its point against the ground, the cone being thus placed with its apex downwards. The lower part of the awn now begins to contract into narrow spiral coils, causing the cone to turn on its axis and to penetrate the ground, and the erect hairs on it which point upwards retain it there like grappling-hooks. After the cone has penetrated the ground, the twisted part of the awn does the same, driving the part which contains the seed further and further into the soil. If the mericarp now becomes moistened, the coiled part attempts to straighten itself, but its coils are held by the hairs which stand on the convex surface ; and thus this movement also contributes to drive the cone deeper into the soil. Whether therefore the moisture is greater or less, the me- chanical contrivance produces the same effect, namely, to drive the part of the mericarp which contains the seed into the soil. Some of the contrivances found in plants are extremely striking, from the concur- rence of the most different qualities for the attainment of a perfectly definite purpose corresponding only to certain specific vital conditions, as the adaptation of the Virginian creeper to climbing up vertical walls, the contrivance to prevent self-fertilisation in the flowers of Aristolochia Clematitis, the bursting of the fruit of Momordica Elaterium, and a thousand similar structures. The most beautiful instances are generally connected with the ordinary structure, or even with other extreme cases, by a number of the most diverse intermediate or transitional forms. These transitional forms have been described in detail by Darwin in the case of climbing and twining plants, and the fertilisation of Orchids, in his works already mentioned, and by Hildebrand in the case of the fertilis- ation of Salvia^. Sect. 37. — The Theory of Descent. The facts and conclusions which have been indicated rather than described are the foundation of the Theory of Descent. This theory consists in the hypothesis that the most unlike forms of plants have a relationship to one another of the same kind as that which the varieties gradually developed from one ancestral form bear to it and to one another. It supposes that the different species of a genus are varieties derived from one progenitor which have undergone further development ; and that in the same manner the various genera of an order owe their common characters to their descent from one and the same older ancestral form, and their differences to variation and to the accumulation by their descendants of new characters in the course of a long series of generations. The theory of descent goes still further, and assumes the same mutual relationship between the various orders of a class, and finally between the various groups. It considers variation with descent to be the cause of all the differences among plants; and the inheritance of these characters to be the cause of the agreement which subsists even between the most diverse forms of plants. What we call the common law of growth of a class, or in other words its Type, is the result of all the plants of this class being descended from one ancestral form or Archetype, as Darwin terms it. That which was long since termed in a merely metaphorical sense Jahrbuch fiir wiss. Bot. vol. IV, 1865. THEORY OF DESCENT, 843 the affinity between different forms of plants is, according to the theory of descent, an actual affinity or blood*relationship in various degrees. The differences have arisen in the course of a long series of generations^ by the descendants of the same archetype continuing to vary ; and the different individuals varying in different ways, the difference between them continually increases, and must continue to increase under diverse conditions of climate, and especially under those imposed by the struggle for existence, in order that they may still be capable of maintaining them- selves. At the same time numberless varieties, species, and genera are constantly disappearing, because they are not sufficiently adapted for the struggle for existence under the new conditions caused by geological changes, and in consequence of the appearance of other forms which are better adapted to resist it. The scientific basis for the theory of descent rests in the fact that it alone is able to explain in a simple manner all the mutual relationships of plants to one another to the animal kingdom, and to the facts of geology and palaeontology, their distribu- tion at different times over the surface of the earth, &c. ; since it requires no other hypothesis than descent with variation and the continued struggle for existence which permits those forms only to persist that are endowed with sufficiently useful pro- periies, the others perishing sooner or later. But both these hypotheses are sup- ported by an infinite number of facts. The theory of descent involves only one hypothesis that is not directly demonstrated by facts, namely that the amount of variation may increase to any given extent in a sufficiently long time. But since the theory which involves this hypothesis is sufficient to explain the facts of morphology and adaptation, and since these are explained by no other scientific theory, we are justified in making this assumption. The theory of descent explains intelligibly how plants have obtained their extraordinarily perfect adaptations for resisting the struggle for existence; this struggle has itself been the means of their obtaining them by the ' Survival of the Fittest,' that is, by permitting the existence and propagation of those newly-formed varieties alone which are endowed with the various characters that render them best fitted to the climate and to resist the rivalry of competitors, the attacks of animals, &c. In this manner adaptations are gradually developed from a slight and imperceptible beginning by the accumulation of useful characters which have the appearance of being the result of the most careful and far-sighted calculation and deliberation, or sometimes even of the most cruel caprice (as in the fertilisation of Apocyniim androscEmifolium by flies which are tortured to death in the process). The fact that members which are morphologically similar are adapted for the most various functions is explained when we consider that the morphological features in the structure of plants are those which are most certainly transmitted unchanged to posterity, either because they are useless in the struggle for existence, or because they have proved useful in the various relations of life ; as for example the differentiation into stem, root, leaves, &c., and into the different tissue-systems^ by which the division of physiological labour and the acquisition of the most various properties useful for the struggle for existence are facilitated. The structure of Thallophytes, Characeae, and Hepaticae, shows that these morphological differentia- tions do not exist in the first or lowest forms of plants, but that they come gradually into existence ; but when once fully developed they are preserved by 844 ORIGIN OF SPECIES. further variations, because they are never prejudicial, but often on the contrary advantageous for the purposes of adaptation. The perfect mode in which morphological characters are inherited gives rise to a very remarkable phenomenon, the production of functionless members. It is obvious that hereditary peculiarities may have lost their use under the new vital conditions of the descendants, because the physiological requirements of the plant are supplied by other means, by fresh adaptations. Of this nature are, for example, the minute leaves on the root-like shoots of Psilotum, the formation of endosperm in the embryo-sac of many Dicotyledons whose embryo afterwards grows so vigo- rously as to supplant the endosperm, w^hile it becomes itself filled wdth reserve food- materials which in other cases are stored up in the endosperm for the seedling. The most striking illustration however is the behaviour of parasites and saprophytes destitute of chlorophyll, which are found in various orders of plants, and the near allies of which form large green leaves containing chlorophyll, w^hile these produce leaves similar in a morphological sense, but which are neither large nor green, and sometimes degenerated so as to have become obsolete. The explanation of this phenomenon is at once afforded by the theory of descent, viz. that the parasites and saprophytes which contain no chlorophyll are the transformed descendants of leafy ancestors which did form chlorophyll, but which gradually became accustomed to take up the assimilated food-materials of other plants or their available products of decomposition ; and the more they did this the less needful did it become for the plants themselves to assimilate. The green leaves therefore became meaningless and ceased to form chlorophyll; but without chlorophyll the leaves were of little or no service to the new form, and therefore as little substance as possible was em- ployed in their development, and they gradually degenerated. Looked at from the point of view of the theory of descent, the natural system of the classification of plants represents their blood-relationship to one another. A species consists of all the varieties which are descended from a common ancestral form.; a genus of all the species which were produced from an older progenitor, and became in the course of time further difi"erentiated ; an order includes all the genera which are descended with variation from a still older ancestral form ; and the first primitive form of all the orders comprised in a group belongs to a still older past; and finally there must have been a time when a primordial plant originated the whole series of development ; and this must have produced in its varying de- scendants the primitive types of all the later forms. The relationships of the various classes and groups described at length in Book II, might be represented by lines, which should express their actual affinity to one another ; and the system of diverg- ing lines which would thus be obtained might be compared to an irregular system of branching. In a plan of this kind we should proceed, starting from the lowest Algae, along a number of lines of descent towards the various and more highly developed classes of Algae. From the Siphonese a branch would shoot, beginning with the Phycomycetes, itself branching copiously, and leading to the various forms of Fungi. From a higher section of Algae another line would branch out which would represent the Characeae; and in its neighbourhood another would be given off which, splitting into two twigs, the Hepaticae and Mosses, would represent the Muscineae. From the same neighbourhood another Hne would start which would THEORY OF DESCENT. 845 represent the ancestors of the Vascular Cryptogams, and from this branch of the tree the Ferns, Equisetaceae, Ophioglossaceae, Rhizocarpese, and Lycopodiaceae, would proceed as branches which themselves further ramify. Where the branch is given off for the heterosporous Vascular Cryptogams would be situated the primitive forms of Phanerogams, beginning with the Cycadeas, and producing by further ramifications the Coniferse, Monocotyledons, and Dicotyledons \ There is still much uncertainty in this plan, but the greater the progress made by a severe method of investigation and with the light of the theory of descent, the more nearly will it be possible to build up the family-tree and to give it a distinct form. The theory of descent requires that the various forms of plants must have arisen at different times, that the primitive forms of the separate classes and groups existed at an earlier period than the derived ones ; and palaeontological research, although at present it has but a very small amount of material at its disposal, supports this view. In the same manner it is a necessary consequence of the theory that each plant- form must have originated at a definite spot, that it must have spread gradually more widely from that spot, that its change of locality in the course of generations must have depended on climatic conditions, the competition of rivals, &c., and that its distribution must have been impeded by hindrances or assisted by means of transport^. The geographical distribution of plants has already determined in the case of many forms the spots on the surface of the earth or centres of distri- bution from which they gradually spread ; it has shown how the distribution has been hindered sometimes by climate, sometimes by chains of mountains, sometimes by seas ; how more recently formed islands have been peopled by the plants from the neighbouring continents which have become the ancestors of new species^; how some species when transported to a new soil (as European plants in America and vice versa) have sometimes carried on a successful struggle for existence with the native plants and have increased enormously. In the distribution of plants at present existing, as for instance Alpine plants, it is possible to recognise the influences of the last great geological changes, of the entrance and disappearance of the glacial epoch and of earlier periods. ' [In the fourth edition of his ' Lehrbuch,' recently published, Sachs has united Algre and Fungi into one group (see Appendix, p. 847). He has also withdrawn the pedigree of the vegetable king- dom sketched in the text, and has substituted for it (p. 9 1 8) the following remarks : — ' Frequent attempts have been made to draw up such a so-called " genealogical tree" either for the whole or some part of the vegetable kingdom. Up to the present time these attempts have not proved very satisfactory. Our knowledge of the true relationships is still very imperfect ; too much room is consequently left for fanciful speculation and the influence of subjective impressions. I shall content myself therefore with pointing out that in drawing out such a genealogical tree the closest attention must be paid to the simplest existing forms of the different types or classes ; the relationship to the common primitive parent-forms will reveal itself most distinctly in these. From each of these simplest forms, however slightly different, a ramifying series may be derived ; variation, proceeding independently in each series, will separate the series themselves still further ; and the most perfect forms of the different types will therefore differ the most widely from one another.' — Ed.] 2 Kerner has given an illustration of what can be accomplished in this direction in the rela- tionships, geographical distribution, and history of the species of Cytisus from the primitive form Tubocytisus, in his pamphlet Die Abhangigkeit der Pflanzengestalt von Klima und Boden ; Inns- bnick, 1869. 3 See Dr. Hooker, On Insular Floras, Gardener's Chronicle, Jan. 1867 ; Ann. des sci. nat. 5th series, vol. IV, p. 266. 846 • ORIGIN OF SPECIES. When we reflect ^vhat a number of generations our cultivated plants must have passed through before any considerable amount of new properties were manifested in their varieties, and how long it takes for these new properties to become hereditary, and further how enormous is the diversity of hereditary properties, we are forced to the conclusion that an inconceivably long period must have elapsed since the appearance of the first plants on the earth. But geology and the physical nature of the globe require as great a space of time for the explanation of other facts ; and a few millions of years more or less is a matter of but little consequence in the expla- nation of facts which require lapse of time in order to reach a given magnitude. The first rudiments of the Theory of Descent, which holds good for the animal as for the vegetable kingdom, may be traced to Lamarck, at the commencement of the century, in his Zoologie Philosophique (Paris, 1801); it was afterwards advocated by Geoffroy St. Hilaire ; but it is only since the publication of Darwin's work ' On the Origin of Species by means of Natural Selection' (London, 1859), that it has become an integral part of science. Darwin's great service to science is to have established as a fact the struggle for existence which all living beings have to fight, and to have proved' its action in the maintenance or destruction of new forms. It is only in the struggle for existence that the motive principle is recognised, and that the theory of descent is enabled to solve the great problem why parts which are morphologically similar are adapted for such different functions; and conversely, to show how purpose in organisation can be ex- plained, and at the same time the relations of affinity among plants, Darwin considers the Natural Selection which the struggle for existence brings about as the sole cause of the increasing diff'erentiation of plants which are undergoing variation ; he starts with the hypothesis that every plant varies in all directions without any definite tendency to become further developed in any one particular direction. He attributes to the struggle for existence alone the power of securing the perpetuation of one or more varieties among the countless numbers which are produced, and is convinced that in this way not only is a perfect adaptation of the new forms effected, but morphological differentiation is also carried further. Niigeli^ assumes, on the contrary, that each plant has in itself a tendency to vary in a definite direction, to increase the morphological difterentiation, or, as it is commonly expressed, to perfect itself. The great differences of a purely morphological nature between the classes and smaller divisions of the vegetable kingdom can then owe their existence to this internal tendency towards a higher and more varied differentiation; while the struggle for existence brings about the adaptation of the separate forms. Weighty arguments can be brought forward for and against this theory of Nageli's ; but in the present state of science I think it is impossible to decide either way; the great services of the theory of descent remain in either case; Nageli's view does not exclude Darwin's ; but the latter includes the former as a more special case. The first and simplest plants had no ancestors ; they arose by spontaneous generation or special creation. W^hether this took place only once ; whether only one or a number of primitive plants were produced simultaneously, giving origin in the latter case to different series of development, or whether, as Nageli supposes, spontaneous generation has taken place at all times, and is now taking place, giving rise to new series of de- velopment, are questions which still await solution, and which we cannot follow out further here. Nageli, Entstehung und Begriff der naturhistorischen Art ; Munich, APPENDIX. [In Hedwigia (1872, p. 18 ; see also Journ. of Bot. 1872, p. 114) Cohn has published a classification of Cryptogams in which, as respects Thallophytes, the distinction between Algae and Fungi is abandoned. In the fourth edition (pp. 248-340) of the present work Sachs has however proposed and adopted a new classification which, except in this respect, has little in common with Cohn. In each class the names on the left hand belong to forms containing chlorophyll (so-called Algae); those on the right to forms destitute of chlorophyll (so-called Fungi). The numbers refer to the pages in the present edition where the groups are described or mentioned. CyanophycesB. Chroococcaceae (216). Nostocaceae (215). Oscillatorieae (215). Rivularieae (215). Scytonemeae. PalmellaeeaB (in part). CLASS I. Protophyta. Schizomycetes (214). Sphaerobacteria. Microbacteria. Desmobacteria. Spirobacteria. Saccharomyces (254). CLASS IL Zygospore.e. Conjugating cells locomotive. VolvocineaB (217). Myxomycetes (274). (HydrodictyeeB) (217). Conjugating cells stationary. Zygomycetes. ConjugataB. INIesocarpeae (220). Zygnemeae (220). Desmidieae (221). Diatomaceae (222). Mucorini (245). Piptocephalidae (246). SpheBroplea (231). Vaucheria (223). GEjdogoniesB (229). Fucaceae (226). CLASS in. Oospores. CoBloblastaa. f Saprolegnieae (242). I Peronosporeae (244). 848 APPENDIX. CLASS IV. Carpospore^. ColeochaetsB (231). FlorideaB (233). Characeae (278). Ascorayeetes (254). Gymnoascus ^ Discomycetes (259). Erysipheae (256). Tuberaceae (255). Pyrenomycetes (256). Lichenes (262). M cidiomycetes (Uredineae, 246). Basidiomycetes (249). Exobasidium (249). Tremellini (249). Hymenomycetes (249). Gasteromycetes (251). — Ed.] 1 [Baranetzky, Bot. Zeit. 1872.] INDEX. Abies, 448, 452. Abietineae, 452, 460. Abortion, 198, 479, 563. Absorption of assimilated substances, 642. Acalypheae, 583. Acanthaceae, 580. Accumulation of characters, 826. Acetabularia, 64, 226. Achenium, 538. Achillaea, 833. Achlya, 12, 13. Acorn, 536. Acrocarpous Mosses, 319. Acropetal order of develop- ment, 149. Acrosticheas, 361. Actinostrobeae, 460. Acyclic, 523, 565. Adaptation, 835. Adhesion, 198, 479. Adiantum, 161,343, 344, 345. Adventitious formations, 150, 152, 35I5 563- Aecidium, 241, 246. Aesculineae, 582. Aesculus, 567. Aethalium, 276, 760. Agaricus, 249. Aggregatae, 581. Aggregates of cells, 68. Akebia, 467. Albumen, 428. Albuminoids, 629. Albuminous, 513. Aleurone, 51, Algae, 208. Alisma, 514, 549. Alismaceae, 549, 554. Allium, 17, 112, 546, 547. Almond, 560. Aloe, 171, 552. Alpinia, 548. Alsineae, 583. Alternate arrangement, 168, 524. Alternation of generations, 202, 421, 805. Althaea, 43, 86, 478, 483. Amaranthaceae, 569, 583. Amentiferae, 578. Amoeba-movement of proto- plasm, 39, 276. Amorphophallus, 162. Ampelideae, 568, 582. Ampelopsis, 781, 839. Amphigastria, 306. Amygdaleae, 585. Amygdalus, 560. Amyrideae, 582. Anacardiaceae, 567, 582. Anagaliis, 496. Ananasineae, 555. Anaptychia, 267, 270. Anatropous, 427, 501. Andreaea, 322. Andreaeaceae, 329. Androecium, 426, 473. Androspore, 229. Aneimia, 89. Anemophilous, 810. Aneura, 296. Angiocarpous Lichens, 268. Angiosperms, 466. Angleof divergence, 167, 181. Anisocarpae, 580. Anisostemonous, 565. Annonaceae, 579. Annual ring, 574. Annular vessels, 23. Annulus, 330, 356. Anterior, 523. Anthela, 521. Anther, 427, 475. Anther-lobes, 473. Antheridium, 212, 258, 299, 321, 342, 363, 803. Antherozoid, 203, 336, 384, 803. Anthoceros, 302, 303. Anthoceroteae, 302. Antipodal cells, 507. Apetalous, 565. Apex, 155, 182. Aphanocyclae, 579. Apical cell, 118, 153, 348, 424. Apical growth, 137, 410. 31 Apocynaceae, 112, 580. Apopetalous, 471. Apophyllous, 471. Apophysis, 334. Aposepalous, 471. Apostasiaceae, 556. Apostrophe, 672. Apothecium, 268. Apple, 537. Aquifoliaceae, 582. Aquilegia, 500, 567. Araliaceae, 584. Araucarieae, 460. Arbutus, 475. Arc-indicator, 746. Archegonium, 203, 336, 434. Archetype, 843. Archidium, 330. Arcyria, 275. Aril, 428, 501. Aristolochia, 812. Aristolochiaceae, 578. Aroideae, 112, 549, 554, 646. Arrangement of leaves, 173. Artocarpeae, 578. Asarineae, 578. Asarum, 468. Asclepiadeae, 112, 580. Ascobolus, 262. Ascogonium, 257, 803. Ascomycetes, 254. Ascophorous hyphae, 269. Ascospore, 240, 254, 258. Ascus, 240, 258. Asexual generation, 203, 336, 423, 432, 805. Asexual reproductive cells, 203. Ashes, 36, 618. Asparagin, 640. Aspergillus, 257. Aspidieae, 361. Aspidium, 351, 357, 359- Asplenieae, 361. Asplenium, 123, 151, 358. Assimilation, 626, 651, 666. Aurantiaceae, 569, 582. Automatic periodic move- ments, 784, 801. 8^0 INDEX. Autumnal layer of wood, 733. Auxanometer, 748. Auxospore, 223. Axial fibrovascular bundle, 146, 418 Axial longitudinal section, 182. Axile placentation, 495. Axillary branching, 155, 425. Axis, 129, 166. Axis of growth, 138, 182,186. Azolla, 398. Bacillarieae, 222. Bacteria, 214. Balanophora, 557. Balanophoreae, 505, 585. Balsamia, 255. Balsamineae, 583. Bambusa, 525. Barbula, 316. Bark, 20, 81, 717. Base, 182. Basidiomycetes, 249. - Basidiospore, 18, 240. Basidiuni, 18, 240, 251. Basifugal growth, 138, 350. Bast, 94. Bast-cells, loi, 592. Batrachospermum, 235. Begonia, 189, 563. Begoniaceae, 585. Benthamia, 537. Berberideae, 570, 579, 787. Berberis, 246, 570, 787. Berry, 459> 539- Betulaceae, 578. Bicornes, 581. Bifurcation, 156, 161. Bignoniacese, 580. Bilateral structure, 183, 765. Biscutella, 500. Bisexual, 426. Bixaceae, 582 Blackberry, 537. Blasia, 307. Blastocolla, 115. Bleeding of wood, 601. Bloom on plants, 84. Boletus, 81, 249. Bordered pits, 20, 25, 464. Borragineae, 522, 580. Bostrychoid cyme, 159. Bostrychoid dichotomy, 157. Botrychium, 378, 379, 380. Botrydium, 225. Bract, 519. Bracteole, 291, 426, 519. Branching, 148, 155. Branching of leaves, 161. Branching of roots, 160. Branching of stem, 163. Bromeliaceae, 555. Bryaceae, 330. Bryonia, 777. Bryophyllum, 152, 563. Bryopsis, 226. Bryum, 81, 314. Bud, 135. Bud-rudiment, 282. Bud-variation, 823. Bulb, 196, 841. Bulbil, 151, 282. Bulbochaete, 230. Bundle-sheath, 105, 106. Burmanniaceae, 556. Burseraceae, 582. Butomus, 489, 549. Buttneriaceae, 583. Buxineae, 583. Cabombeae, 579. Cactaceae, 585. Caesalpineae, 584. Calamite, 374, 376. Calanthe, 497. Calcium, 622. Calcium carbonate, 64. Calcium oxalate, 52, 64, 112. Calhtrichaceas, 585. Callitris, 451. Callus, 731. Calothamnus, 476. Calycanthaceae, 584. Calycereae, 581. Calyciflorae, 584. Calyculus, 472. Calypogeia, 309. Calyptra, 300. Calyx, 470. Cambiform tissue, 100. Cambium, 79, 93, 463. Cambium-ring, 432, 573. Camellia, 21. Campanula, 566. Campanulaceae, no, 566, 581. Campylotropous, 427, 501. Canal of the style, 498. Canal-cell, 336, 344, 387. Candollea, 569. Canna, 548. Cannabineae, 578. Cannaceae, 556. Cap-cell of root, 124. Capillary attraction, 608. Capillitium, 255, 275. Capitulum, 520. Capparideae, 527, 579. Caprifoliaceae, 566, 581. Capsella, 515. Capsule, 294, 538. Carbon, 619, Carbon dioxide, 644, 666. Carboniferous fossils, 376. 420. Carcerulus, 537. Carpellary leaf, 429. Carpogonium, 257. Carpophore, 537, Caruncle, 540. Caryophylleae, 583. Caryophyllineae, 583. Caryopsis,.538. Casuarina, 473. Casuarineae, 585. Cataphyllary leaves, 165, 193. Caulerpa, 137, 226. Cauline bundles, 134, 417, 575- Caulome, 129, 136. Cedreleae, 582. Celastrineae, 582. Celastrus, 524. Cell, Primordial, 5. Cell, Structure of, i. Cell-division, 8, 12, 673, 682. Cell-families, 68, 209. Cell-multiplication, 8. Cell-nucleus, 2, 18, 37, 44. Cell-sap, 2, 62. Cell- tissue, i, 8. Cell-wall, 2, 19. Cells, Formation of, 7. Cells, Formation of the com- mon wall of, 70. Cells, Forms of, 5, 98, 576. Cellulose, 2, 19, 631. Celosia, 569. Celtideae, 578. Centaurea, 797. Centradenia, 475. Central cell, 293, 336, 342, 434, 802. Centranthus, 566. Centrifugal force, action of, 691. Centrifugal inflorescence, 520. Centripetal inflorescence, 520. Centrospermae, 553, 583. Ceramiaceae, 237. Ceratonia, 36. Ceratophyllaceae, 585. Ceratozamia, 440. Cercis, 187. Cerorchideas, 488. Chalaza, 501. Chara, 132, 279, 291. Characeae, 278 Characteristic forms of leaves and shoots, 190. Chelidonium, 571. Chemical processes, 618. Chenopodiaceae, 583, Chenopodium, 469. Chimonanthus, 557. Chlaenaceae, 582. Chlamydccoccus, 218. Chlamydomonas, 218. Chloranthe^, 578. Chlorine, 622. INDEX, Chlorofucine, 685. Chlorophyll, 6, 665, 678. Chlorophyll-bodies, 45. Chlorophytum, 756. Chorisis, 528. Chroococcaceae, 216, 263, 273. Chrysobalaneae, 585. Chrysotannin, 687. Cichoriaceae, no, 787. Cichorium, 23. Cicinal cyme, 159, 522. Cicinal dichotomy, 157. Cicinus, 160. Cilia, 211, 331, 334. Cinnamomum, 566. Circulation of protoplasm, 39. Cistineae, 582. Citrus, 113, 569. Cladonia, 265, 273. Claviceps, 258, 259, 260. Claw, 471. Cleistogamous flowers, 810. Clematis, 154. Cleome, 527. Climbing stems, 197, 772. Clusiaceae, 582. Coalescence of cells, 73. Cocoa-nut, 511. Ccelebogyne, 805. Coenogonium, 268. Coffee-berry, 512. Coherence, 201, 471. Colchicum, 545. Coleochaete, 209, 231. Coleorhiza, 143, 541. Collema, 264. Collenchyma, 24, 80, 83, 105, 576. CoUeter, 115. Colloids, 594. Colours of leaves in autumn, 657. Columella, 275,295, 303, 324, 33ij 359- Columnea, 534. Columniferae, 583. Combined hybrids, 821. Combretaceae, 585. Gommelynaceae, 112, 555. Common bundles, 134, 369, 431, 463. Compositae, 115, 566, 581. Compound spores, 241. Conceptacle, 22/, 271. Concussion, Irritability to, 784. Condition of aggregation of organised structures, 587. Conducting tissue for the assimilated food-materials, 634. Conducting tissue of style, 499- Confervaceae, 231. Conidia, 244, 256. Coniferae, 115, 442. Conjugates, 10, 220. Conjugation, 8, 9, 203, 212, 221, 245, 261, 802. Connective, 427, 473. Contortae, 580. Contractile organs, 677, 783. Convolvulaceae, 580. Corallorhiza, 194, 542, 643. Coriaria, 166. Cork, 80, 90. Cork-cambium, 90. Cormophytes, 130. Cornaceae, 584. Corolla, 470. Corolliflorse, 555, 584. Corona, 471. Corpuscula, 422, 434, 802. Corrosion by roots, 625. Cortex, 91, 280, 573. Cortical sheath, 574. Coryanthes, 601. Cosmarium, 221. Cotyledons, 435, 513, 557. Crassulaceae, 584. Cremocarp, 537. Crest, 540. Crocus, 546. Crown, 286. Crozophora, 567. Crucibulum, 69, 251. Cruciferae, 527, 570, 579. Cruciflorae, 579. Crustaceous Lichens, 263. Crystalloids, 49, 596. Crystals, 64. Cucurbita, 24, 32, 33, loi, 477, 484, 779, 827. Cucurbitaceae, 566, 576, 581, 776, 838. Cunninghamieae, 460. Cunoniaceae, 584. Cupressineae, 451, 459. Cupule 473. Cupuliferae, 578. Curvatureof concussion, 707. Cuscuta, 197, 241, 557, 561, 572, 733- Cuscutese, 580. Cuticle, 34, 83. Cuticulai isation of the cell- wall, 20, 34. Cyatheaceae, 360. Cycadeae, 436. Cycas, 438. Cyclantheae, 554. Cyclic, 523, 531, 565. Cyclomyces, 249. Cyme, 158, 160, 521. Cymose branching, 158. Cymose inflorescence, 520. Cymose umbel, 158, 521. 312 Cynara, 655, 798. Cynaraceae, 787, 797. Cyperaceae, 548, 555- Cypripedium, 479, 526. Cystocarp, 213, 235. Cystolith, 64. Cystopus, 243, 244. Cytineae, 579. Dahlia, 26, 63, loi, 823. Daily periodicity of growth, 743. Dammara, 453. Davallieae, 361. Decussate, 168, 177. Dedoublement, 48 1, 528, 568. Definite inflorescence, 520. Degradation - products, 48, 628. Dehiscent fruits, 5 3 8, 539. Delphinium, 531. Deposition in the cell-wall, 31. Derivative hybrid, 821. Dermatogen, 126. Descent, Theory of, 842. Desmidieae, 221. Desmodium, 678, 785. Development of the mem- bers of one branch-system, 155. Diagonal plane, 523. Diagram, Floral, 524. Dialypetalae, 581. Diandrae, 580. Dianthus, 472. Diatomaceae, 222. Diatomine, 223. Dichasium, 158, 159, 521. Dichogamy, 808. Dichotomy, 148, 156, 406. Diclinous, 426. Dicotyledons, 433, 556. Dictamnus, 114, 154, 493) 528, 767. Dictyota, 156. Dictyoteae, 237. Didymium, 275. Differentiation of cell-wall, Differentiation of tissues, 117. Dilleniaceae, 569, 579. Dimorphism, 809. Dioecious, 426, 804. Dioecism, 807. Dionaea, 689. Dioscoreae, 555. Diosmeae, 583. Diosporineae, 581. Dipsacaceae, 581. Dipterocarpeae, 582. Directions of growth, 155, 182, 186. 852 I N DE X. Discomycetes, 259. Discophorae, 583. Displacement, 198. Diurnal and nocturnal posi- tions of organs, 784. Divergence, Angle of, 167, 181. Dracaena, 107, 552. Dried substance of plants, 618. Drosera, 522, 796. Drupe, 539. Dryadeae, 585. Dudresnaya, 213, 237, Dwarf males, 229. Ebenaceae, 581. Elaeagnaceae, 584. Elaeagnus, 490. Elacis, 54. E'aphomyces, 255. Elasticity, 699. Elater, 23, 294, 373. Elatineae, 585. Electricity, 687. Elementary constituents of the food of plants, 618. Eleutheropetalae, 581. Eleutheropetalous, 471. E!eutherophyllous, 47:. Eleutherosepalous, 471. Elodea, 664. Embryo, 203, 205, 421, 432, 434, 513. Embryo, Cell-division in, 17, 511. Embryo-sac, 422, 432, 454, 506, 802. Embryonic vesicles, 422,432, 458, 507, 803. Emergences, 140. Empetraceae, 585. Enantioblastae, 555. Endocarp, 518, 537. Endogenous formations, 141, 149, 370. Endosmotic force, 597. Endosperm, 205, 421, 432, 434, 510, 805. Endospore, 32, 294. Endostome, 501. Energy of growth, 741. Entomophilous, 810. Epacrideae, 567, 581. Epen, 103. Epenchyma, 103. Ephebe, 266. Ephedra, 461. Epicalyx, 472. Epicarp, 518, 537. Epidermal tissue, 78, 79. Epidermis, 80, 81, 82, 717. Epigynae, 581. , Epigynous, 490. Epilobium, 486. Epimedium, 500, 570. Epinasty, 767. Epipactis, 814, Epipetalous, 524. Epiphragm, 253, 33 1- Epipogium, 542, 620, 643. Episepalous, 524. Epistrophe, 671. Equisetaceae, 362. Equisetum, 14, 122, 153, 363. Equivalent members, 148. Er.^ot, 258. Ericaceae, 567, 581. Eriocauloneae, 555. Erodium, 841. Eryngium, 490. Erysiphe, 256. Erythrophyll, 686. Erythroxylaceae, 582. Escallonieae, 584. Eucyclae, 581. Eucyclic, 524. Euphorbia, 168. Euphorbiaceae, iii, 567, 583. Euphorbieae, 583. Eurotium, 257. Everina, 272. Exalbuminous, 513. Excipulum, 268. Exobasidium, 249. Exogenous formations, 133, 149. Exospore, 32, 294, 397. Exostome, 501. Extensibility, 698, 703. Extension, 138. Extine, 34, 485. Extra - axillary branching, 562. False dichotomy, 158. Fascicular tissue, 79. Female reproductive cell, 203, 802. Ferments, 254. Ferns, 340. Fertilisation, 203, 430, 509, 803. Fertilisation of hybrids, 821. Fibrovascular bundles, 79, 92, 353, 431. Ficus, III, 200. Fig, 200, 518, 537. Filament, 427, 473. Filices, 340. Filiform apparatus, 507. Fissidens, 313. Flat pro-embryo, 318. Flexibilityof internodes, 703. Floral diagram, 524. Floral formulae, 529, 565. Florideae, 233. Flower, 319, 425, 523, 564. Flowers of tan, 276. Fluorescence of chlorophyll, 680. Foliaceous Lichens, 264. Foliage-leaves, 193. Foliose Hepaticae^ 297, Follicle, 538. Fontinalis, 132, 331. Food-materials, 619, 626. Foot, 346, 389. Foramen, 427. Formative materials, 628. Fossil Equisetaceae, 376. Fossil Lycopodiaceae, 420. Four-fold pollen- grains, 488. Fovea, 408. Foveola, 408. Fovilla, 486. Francoaceae, 584. Frangulineae, 582. Frankeniaceae, 582. Freecell-formatipn,8,i 1,507. Freezing, effects of, 653. Frenela, 451. Fritillaria, 172, 544. Fruit, 430, 518, 536. Fruticose lichens, 265. Fucaceae, 226. Fucoxanthine, 685. Fucus, 3, 227. Fumariaceae, 526, 535, 570, 579- Funaria, 47, 82, 312, 320, 321, 331. Fundamental tissue, 78, 102, 355. Fungi, 238. Funiculus, 427, 501. Funkia, 15, 23, 482, 503, 508. Gamopetalae, 580. Gamopetalous, 201, 471, Gamophyllous, 471. Gamosepalous, 201, 471. Gases, movements of, 614. Gasteromycetes, 251. Gelatinous lichens, 265. Gemmae, 151, 298, 318. General vital conditions of plants, 647. Generating tissue, 79. Generations, Alternation of, 202, 421, 805. Genetic spiral, 169, 774, Gentianaceae, 580. Genus, 829, 844. Genus-hybrid, 817. Geographical distribution of plants, 845. Geotropism, 758. Geraniaceae, 583, 842. Germ-cell, 203, 802. Germinal vesicles, 203, 422, 507. INDEX. 853 Germination of Phanero- gams, 422, 541, 558, 638, 651. Gesneraceae, 580. Gemn, 201. Glands, no, 113. Glandular hairs, 86, 139. Glans, 538. Gleba, 253. Gleicheniaceae, 360. Globoids, 52. Globulariacese, 580. Globule, 284. Glomerulus, 237. Glume, 554. Glumiflorae, 554. Gnetacese, 460, Gnetum, 461. Gonidium, 263, 272. Goodeniaceae, 581. Gramineae, 525, 555. Grand curve of growth, 737. Granulose, 57, 60. Graphis, 264. Grass, flower of, 525. Gravitation, action of, 187, 690, 758. Grossulariaceae, 584. Growth, 692, 695, 712. Growth, directions of, 182. Growth in length, 735. Growth in length of the root, 124. Growth in thickness of the cell-wall, 22. Growth in thickness of the stem, 107, 572. Growth of starch-grains, 58. Gruinales, 583. Guard-cells of stomata, 75, 87. Gum-passages, 77, 115. Gum-resin, 116. Guttiferae, 582. Gymnocarpous lichens, 268. Gymnosperms, 423, 433. Gymnostachys, 549. Gymnostomum, 331. Gynaeceum, 426, 488, 491. Gynandrae, 556. Gynobasic style, 498. Gynophore, 479. Gynostemium, 479, 796. Haemodoraceae, 555. Hairs, 84, 130, 138. Haloragideae, 585. Haplomycetes, 238. Haustoria, 244, 733. Head, 284. Heat, Action of, 647. Heat, Conduction of, 648. Heat, Production of, 646. Heat, Radiation of, 648. Heat-expansion, Coefficients of, 649. Heating apparatus for the microscope, 658. Hedera, 76, 859. Hedychium, 548. Helianthus, 42, 63, 69, 133, 497. Helicoid cyme, 159, 521. Helicoid dichotomy, 157. Heliotropism, 190, 676, 752. Helobiae, 553. Helvelleae, 259, Hemicyclic, 523, 565. Hepaticae, 295. Heracleum, 533. Hereditary characters, 696, 821, 822. Hermaphrodite, 426. Herminium, 198. Herpothamnion, 236. Hesperideae, 582. Hesperidium, 539. Heterocyst, 215. Heteroecism, 241, 246. Heteromerous Lichens, 265. Heterosporous Vascular Cryptogams, 339. Heterostylism, 809. Hieracium, 829. Hilum, 540. Hippocastaneae, 567, 582. Hippocrateaceae, 582. Hippurideae, 585, Hippuris, 133, 470. Homoomerous Lichens, 265. Hoya, 29, 592. Humiriaceas, 582. Humulus, III, 115. Hyacinthus, 75, 87. Hybrid, 817. Hybridisation, 816. Hydnora, 505. Hydnoreae, 579. Hydnum, 249. Hydrangeae, 584. Hydrilleae, 554. Hydrocharideae, 554. Hydrodictyeae, 217. Hydrodictyon, 217. Hydrogen, 620. Hydropeltidineae, 579. Hydrophyllaceae, 580. Hymenium, 240. Hymenomycetes, 249. Hymenophyllaceae, 359. Hymenophyllum, 341. Hypericineae, 582. Hypericum, 476, 524. Hyphae, 238. Hypoderma, 80, 83, 105. Hypodermal tissue, 376. Hypodermiae, 246. Hypogynae, 580. Hypogynous, 489. Hyponasty 767. Hypophysis, 515. Hypothecium, 269. Hypsophyllary leaves, 193, 519. Ice, Formation of, 655. Ilex, 35. Imbibition, 710. Incombustible deposits in cell- wall, 36. Indefinite inflorescence, 520. Indehiscent fruits, 538, 539. Indusium, 356. Inferior ovary, 497. Inflorescence, 426, 431, 519, Inherited characters, 822. Innovation, 292. Insect-agency in pollination, 429, 808. Insertion, 167. Insertion of leaves, 134, Integument, 427, 501. Intercalary growth of cell- wall, 22, 137. Intercellular spaces, 71, 73. Intercellular substance, 70, 74, lOI. Interfascicular cambium, 552, 573. Intermediate tissue, 105. Internodes, 135. Internodes, Elongation of, 737. Interposed members, 524, 568. Intine, 32, 485. Intrapetiolar buds, 562. Intussusception, 31, 58, 590. Inuline, 63. Involucel, 520. Involucre, 473, 520. Irideae, 548, 555, Iron, 622. Irritability, 776, 781. I satis, 155. Isoetes, 161, 401, 402, 407. Isocarpae, 581. Isosporous Vascular Crypto- gams, 338. Isostemonous, 565. Ivy, 76, 859. Jasminiaceae, 580. Juglandeae, 585. Juliflorae, 578. Juncaceae, 555, Juncagineae, 549j 554- Jungermannia, 310. Jungermannieae, 306. Juniperineae, 460. Juniperus, 447, 45 1- 854 INDEX. Knight's experiments on the influence of gravitation, 763. Labiatae, 580. Labiatiflorae, 580. Labium, 408. Lamina, 191, 471, 564. Lamium, 480. Lateral arrangement, 184. Lateral plane, 523. Lateral roots, 144. Lateral shoots, 152, 166. Latex, no. Lathraea, 50, 572. Laticiferous vessels, 74, 109. Latticed cells, loi. Lauracese, 566, 579. Leaf, 109, 131, 136, 161, 173, 190. Leaf-blade, 191, 564. Leaf- forming axis, 131, 151. Leaf-sheath, 365. Leaf'stalk, 191, 564. Leaf-tendrils, 194, 775, Leaf- thorns, 194. Leaf-trace, 134, 431. Leaf-veins, 192. Leaflet, 191. Legume, 538. Legumin, 642. Leguminosse, 584, 640. Lejolisia, 235. Lemnaceae, 553. Lentibulariaceae, 581. Lenticels, 91. Lepidodendron, 420 Lepidostrobus, 421. Leptogium, 265. Levisticum, 162. Leycesteria, 566. Libriform tissue, 35, 100. Lichens, 262. Lichina, 272. Lichnoerythrine, 686. Lichnoxanthine, 686. Light, Action of, 659, 752, 784, 790. Light, Intensity of, 662. Light, Refrangibility of, 666. Lignification of the cell-wall, 7, 20, 35. Ligule, 192, 408, 547. Liliaceae, 524, 555. Liliiflorae, 555, Limnanthaceae, 583. Linaceae, 583. Loasaceae, 582. Lobelia, 566. Lobeliaceae, in, 581. Loculicidal dehiscence, 538. Lodicule, 471. Loganiaceae, 580. Lomentum, 537. Lonicera, 566. Loranthaceae, 505, 513, 557, 585. Lunularia, 298. Lupinus, 52, 641. Lychnis, 472. Lycogala, 276. LycopodiacesE, 400. Lycopodieae, 416. Lycopodium, 70, 400, 404, 416. Lygodium, 197, 360, 772. Lythrarieae, 585. Macrosporangium, 393, 396. Macrospore, 335, 396, 403, 432. Magnesium, 622. Magnoliaceae, 579. Mahonia, 475, 787. Malaxis, 544. Male reproductive cells, 203, 426, 802. Malpighiaceae, 582. Malvaceae, 583. Manglesia, 478. Manubrium, 284. Marattia, 361. Marattiaceae, 361. Marchantia, 23, 76, 89, 298, 301, 305. Marchantieae, 305. Marcgraviaceae, 582. Marsilea, 140, 163, 384, 388, 393, 398, 399- Mechanical laws of growth, 692. Mechanical structure of irri- table parts, 792. Median plane, 167. Medullary rays, 573. Medullary sheath, 463, 574. Megaclinium, 785. Melanosporeae, 229. Melastomaceae, 585. Meliaceae, 582. Melobesiaceae, 64, 234. Members, 130. Menispermaceae, 566, 579. Mericarp, 537, 538. Merispore, 241. Meristem, 79. Mesembryanthemeae, 585. Mesocarp, 518, 537. Mesocarpeae, 220. Mesophyll, 192, 356. Metamorphosis, 631, 835. Metamorphosis of organs, 128, 183, 194. Metaplasm, 37, 41. Metastasis, 626. MetzgerJa, 120, 160, 297. Micranthae, 554. Microcyst, 277. Microgonidia, 219. Micropyle, 427, 501. Microsporangium, 393, 396. Microspore, 335, 396, 401, 432. Mid-rib, 192. Mimosa, 694, 793. Mimoseae, 584. Mineral substances in food of plants, 622. Mistletoe, 557. Mnium, 151, 313. Moist surfaces, Growth of roots in, 764. Molecular forces, 587. Molecular structure, de- struction of, 591. Molecules, 588. Monoblepharidae, 243. Monocarpellary, 491. Monocarpic, 519. Monochlamydeae, 578. Monocleae, 304. Monocotyledons, 433, 541. Monoecious, 426, 804. Monoecism, 807. Monopodial inflorescence, 520. Monopodium, 156. Monosymmetrical, 183, 533. Monotropa, 194, 557, 620, 643. Monotropeae, 581. Moreae, 578. Mosses, 295, 311. Mother-cells of pollen, 32, 484. Motility, 788. Movement of food-materials, 623. Movement of protoplasm, 39, 276, 651, 670. Movement of water, 598, 652. Movements of gases, 614. Mucilage, conversion of the cell-wall into, 20, 36. Mucor, 245, 246. Mucorini, 245. Mulberry, 518, 537. Multilateral structure, 184. Musaceae, 548, 556. Muscari, 154. Muscineae, 292. Mushroom, 249. Mycelium, 239, 249. Myricaceae, 585. Myristica, 512. Myristicaceae, 579. Myrsinaceae, 581. Myrtaceae, 585. Myrtiflorae, 585. Myxoamoebae, 10, 39. Myxomycetes, 10, 274. INDEX, ^55 Naiadeae, 504, 553. Naias, 473, 495, 504. Nardus, 525. Natural Selection, 831, Natural System, 844. Nectar, 500. Nectary, 430, 500. Negative heliotropism, 677, 756. Nelumbiaceae, 579. Nemalieae, 237. Neottia, 194, 620, 693. Nepentheae, 578. Nepenthes, 601. Nidularieae, 251. Nitella, 16, 285, 287, 288, 289. Nitrogen, 621. Node, 135. Nostoc, 215. Nostocaceae, 215, 273. Nostochineae, 214. Nucleoli, 38, 44. Nucleus, 2, 18, 37, 44, 252, 422. Nucule, 284, 289. Nuphar, 539. Nut, 538. Nutation, 766. Nutmeg, 512. Nyctagineae, 583. Nymphaeaceae, 576, 579. Ochnaceae, 583. CEdogonieae, 229. CEdogonium, 9, 22, 230. CEnothereae, 585. Oil, 55, 115. Oleaceae, 566, 580. Onygenaceae, 256. Oogonium, 3, 203, 212, 802. Oosphere, 203, 212, 8cr2. Oospore, 213, 803. Opening and closing of flowers, 798. Operculum, 330. Ophioglossaceae, 378. Ophioglossum, 379, 381. Ophrydeae, 488. Orchideae, 488, 505, 506, 526, 556, 814. Orchis, 502, 536, Order of development of roots, 143. Order of succession of the parts of the flower, 530. Organic centre, 182. Organic processes, 693. Organs of plants, 128. Origin of species, 822. Original forms of plants, 128. Orobanche, 194, 241, 557. Orobancheae, 580. Orthostichy, 167. Orthotropous, 427, 501. Oscillatoria, 215. Oscillatorieae, 215. Osmunda, 341. Osmundaceae, 360. Ovary, 429, 466, 488. Ovule, 203, 427, 501, 504. Oxalideae, 583. Oxalis, 787, 796. Oxygen, 621. Paleae, 129, 347, 356, 554. Pallisade-tissue, 465, 657. Palmaceae, 552, 554. Palmellaceae, 263, Pandanaceae, 554. Pandorina, 219. Panicle, 520. Papaver, 571. Papaveraceae, iii, 571, 579. Papayaceae, no, 582. Papilionaceae, 584. Pappus, 471, 540, 841. Paraphyses, 251, 293, 356. Parasites, 194, 241, 557,572, 620, 643, 844. Parasitism of Lichens, 263, 273. Parastichy, 173. Paratonic condition, 677. Parenchyma, 78, 100. Parietales, 582. Paris, 167. Parnassia, 566, 766. Paronychieae, 569, 583. Parthenogenesis, 805. Passiflora, 776, 780. Passifloraceae, 582, Pastinaca, 162. Pediastrum, 68, 217. Peduncle, 426. Peltigera, 264. Perianth, 293, 309, 426, 469. Periblem, 126. Pericambium, 144, 353. Pericarp, 236, 518, 537. Perichaetium, 293, 309, 320. Periderm, 81, 90. Peridium, 239, 251, 253. Perigonium, 320. Perigynae, 584. Perigynous, 489. Periodic movements of or- gans, 782. Periodicity of growth in length, 743. Perisperm, 428, 512. Peristome, 331. Perithecium, 256, 258. Permanent tissue, 79. Peronosporeae, 244. Pertusaria, 264, 271. Petal, 470. Petiole, 191, 564. Peziza, 11, 261. Phaeosporeae, 229, Phalloideae, 253. Phallus, 254. Phascaceae, 329. Phaseolus, 24, 126, 146, 492. 559- Phelloderm, 91. Phellogen, 90. Philadelpheae, 584. Phloem, 94, 100. Phloem-layers of fibrovas- cular bundles, 100. Phloem-sheath, 419. Phlomis, 494. Phoenix, 542. Phosphorescence, 646. Phosphorus, 622. Phototonus, 790. Phycocyanine, 216, 686. Phycoerythrine, 234, 237, 686. Phycomycetes, 240, 242. Phycophaeine, 226. Phvcoxanthine, 223, 226, 686. Phyllanthaceae, 583, Phyllantheae, 583. Phyllocladus, 450. Phyllode, 202, 408. Phylloglossum, 407. Phylloid, 211. Phyllome, 130, 136. Phyllopode, 420. Phyllophyte, 130. Phyllotaxis, 167, 173, 348. Physarum, 275. Physcia, 272. Phytelephas, 512. Phytolacca, 569, 576. Phytolaccaces, 569, 583. Pileus, 249, 254. Pilularia, 32, 392, 395, 396, 397. Pine-apple, 537. Pineae, 460. Pinus, 25, 31,70,72, 89, 105, 442, 449, 465. Piperaceae, 495, 504, 578. Piperineae, 578. Pisum, 52. Pitcher-like organs, 601. Pith, 108, 717. Pitted vessels, 26. Pittosporeae, 582. Placenta, 427, 488. Plane of insertion, 167. Plane of symmetry, 183. Plantagineae, 566, 580. Plasmodium, 10, 39, 275, 760. Platanaceae, 578. Platycerium, 347. 856 Plerome, 127. Pleurocarpous Mosses, 319. Plumbagineae, 581. Plumule, 518. Podocarpeae, 460. Podocarpus, 450. Podophyllum, 570. Podostemoneae, 585. Point of insertion, 167. Polarized light, 588. Polemoniaceae, 580. Pollen, Formation of, 14, 15, 34, 440, 481. Pollen-grain, 23, 203, 423, 426, 432, 449, 485. Poilen-sac, 426,440,448,482. Pollen-tube, 33, 450, 485, 509. Pollination, 429. Pollinium, 488. Pollinodium, 258. Polycarpae, 554, 579. Polycarpellary, 492. Polycarpic, 519. Polyembryony, 459. Polygala, 535. Polygalacese, 582. Polygamous, 467, 807. Polygonatum, 165, 542. Polygonaceae, 495, 585. Polypodiaceae, 341, 360. Polypodieae, 361. Polysymmetrical, 183, 533. Polytrichum, 333. Pomeae, 584. Pontaderiaceae, 555. Pore-capsule, 539. Porphyreae, 237. Portulacaceae, 583. Posterior, 523. Potamogeton, 548. Potamogetoneae, 554. Potassium, 622. Potato, 59. Pressure, Effect of, on growth, 729. Prickle, 85. Primary cortex, 574. Primary meristem, 79, 117. Primary root, 142, 144. Primary tissue, 78. Primary wood, 574. Primine, 501. Primordial cell, 5. Primordial epidermis, 126. Primordial utricle, 42. Primulaceae, 495, 531, 568, 581. Primulineae, 581. Procambium, 93. Products of degradation, 48, 628. Pro-embryo, 205, 279, 311, 432, 434, 458, 513- Pro-embryonic branch, 282. INDEX. Prolification, 426, 439. Promycelium, 239. Prosenchyma, 78. Protandrous, 812. Proteaceae, 584. Proteine-grains, 51. Proten, 103. Protenchyma, 103. Prothallium, 205, 335, 432. Protogynous, 812. Protomyces, 255. Protonema, 205, 311, 341. Protoplasm, 2, 37. Prototaxites, 226. Pseudaxis, 157, 159. Pseudocarp, 201, 518. Pseudopodia, 318, 329. Psilotum, 406, 411. Psoralea, 113. Pteris, 24, 27, 28, 30, 35, 92, 96, 105,123,196, 343, 345, 347, 348, 350, 354- Puccinia, 246. Pulvinus, 783, 786. Punctum vegetationis, 117. Pycnidium, 256, 271. Pyrenomycetes, 256. Pyrola, 493, 557. Pyrolaceae, 581. Pyxidium, 538. Quercus, 559. Raceme, 520. * Racemose branching, 158. Racemose inflorescence, 520. Radicle, 518, 558. Radula, 309. Rafflesiaceae, 557, 579. Ramondieae, 580. Ranunculaceae, 567, 579. Raphe, 427, 501. Raphides, 65, 112. Reaumuriaceae, 582. Receptacle, 239, 249, 426, 489. Receptive spot, 344. Reciprocal hybrids, 818. Refrangibility, Action of rays of diff'erent, 666. Regular flowers, 533. Rejuvenescence of the cell,8. Reproductive cells, 202, 426, 802. Reseda, 166. Resedaceae, 582. Reserve-materials, 627. Resin-passages, 77, 115, 465. Respiration, 644. Restiaceae, 555. Retardation of growth by light, 675, 754. Revolving nutation, 766, Rhamnaceae, 582. Rheum, 496, 507. Rhizantheae, 579. Rhizine, 264. Rhizocarpeae, 383. Rhizoid, 211, 282, 317. Rhizome, 196. Rhizophore, 147, 411. Rhizopus, 245. Rhodoraceae, 581. Rhus, 567. Rhodospermine, 51. Ribes, 91. Riccia, 304. Riccieae, 304. Ricinus, 95, 97, 108, 161, 476, 558, 638. Rivularia, 215. Rivularieae, 215. Roccella, 272. Root, 129, 140. Root-cap, 123, 127, 140. Root-hairs, 139. Root-pressure, 600, 609. Root-sheath, 143. Root-system, 142. Roots, Branching of, 160. Roots, Growth of, 124, 740. Rosa, 200. Rosaceae, 584. Rose-hip, 201, 537. Rosiflorae, 584. Rotation of protoplasm, 39, 43. Rubiaceae, 566, 581. Rutaceae, 583. Ruteae, 583. Sabal, 169. Sabina, 451. Saccharomyces, 254. Sagittaria, 72. Salicineae, 582. Salisburia, 446. Salvia, 814. Salvinia, 170, 384, 386, 387, 390, 391, 394. Samara, 538. Sambucus, 573. Samydaceae, 582. Sanguisorbeae, 585. Santalaceae, 505, 585. Santalum, 507. Sap, 62. Sap-conducting intercellular passages, 76, 115, 464. Sap-vesicles, 42. Sapindaceae, 575, 582, Sapotaceae, 581. Saprolegnieae, 242, 805. Saprophytes, 194, 542, 557, 572, 620, 643, 844. Sarcocarp, 537. Saurureae, 578. Saxifraga, 492. INDEX. 857 Saxifragaceae, 566, 584. Saxifragine ae, 584. Scalariform vessels, 27, 98, 355. Scale-leaves, 193. Schizaeaceae, 341, 360. Schizandrese, 579. Schizocarp, 537, 538. Schizomycetes, 214. Schizosporeae, 214. Schultz's solution, 67. Schvvendener's Lichen- - theory, 262. Scirpus, 548. Scitaminese, 555. Sclerantheae, 583. Scleranthus, 569. Sclerenchyma, 35, 104, 106. Sclerotium, 239, 259, 277. Scolecite, 262. Scorpioid cyme, 160, 522. Scorpioid dichotomy, 157. Scorzonera, iii. Scrophularia, 99. Scrophulariaceae, 580. Scutellum, 144, 541. Scutiform leaf, 389. Secondary cortex, 573. Secondary products of me- tastasis, 628. Secondary roots, 142. Secondary wood, 574. Secundine, 501. Seed, 423, 432, 518, 540. Segmentation of the apical cell, 118. Selagineae, 580. Selaginella, 47, 78, 104, 403, 405> 409* 410^ 414* 415* 416, 417, 419. Selaginelleae, 416. Sensitiveness, 651, 776, 781. Sepal, 470. Septicidal dehiscence, 538. Septifragal dehiscence, 538. Serpentariese, 578. Seta, 294, 324, 330, 471. Sexual affinity, 818. Sexual generation, 203, 335, 400, 432, 805. Sexual reproduction, Pheno- nomena of, 801. Sexual reproductive cells, 203. 426, 802. Sheath-teelh, 370. Shells,' Formation of, in the cell-wall, 33, Shield, 284. Shoot, 136, 194. Sieve-cells, 23. Sieve-discs, 25, loi. Sieve-tubes, loi, 418. Sigillaria, 420. Sileneae, 583. Silicon, 623. Siliqua, 538. Simarubeae, 583. Simultaneous whorls, 166. Siphoneae, 223. Skeleton of cell-wall, 37. Skeleton of starch-grain, 60, 589. Skin of protoplasm, 38, 41. Sleep of plants, 799. Sodium, 622. Soft bast, 10 1. Solanaceae, 198, 522, 580. Solitary arrangement, 167. Solorina, 271. Solubility of starch-grains,6 1 . Sorby's researches on chlo- rophyll, 684. Soredial branch, 266. Soredium, 271. Sorus, 356. Spadiciflorae, 554. Spadix, 520. Spathe, 473. Special mother-cells, 32, 88, 485. Species, 829, 844. Species, Origin of, 822. Species-hybrid, 817. Spectrum of chlorophyll, 678. Sperm-cell, 203, 802. Spermogonium,246,256,27i. Spermatia, 240, 246, 256. Spermatozoid, lo, 203, 212, 803. Sphacelia, 259. Sphaeroplea, 231. Sphagnaceae, 326. Sphagnum, 326, 327, 328, 329. Sphere-crystals, 63, 65. Spicular cells, 66. Spike, 520. Spine, 197. Spiraeeae, 585. Spiral arrangement, 169. Spiral flowers, 523, 531. Spiral theory of phyllotaxis, 180. Spiral vessels, 23, 97. Spirogyra, 9, 10, 17, 220. Splitting of the cell- wall, 71. Spontaneous periodic move- ments, 784, 801. Sporangium, 129, 252, 356. Spore, 203, 240. Spores, Mode of formation of, 14. Sporidia, 248. Sporocarp, 392. Sporogonium, 292, 324, 338. Spur, 500. Stamen, 426, 473. Staminode, 479. Staphyleaceae, 582. 3 K Starch, 6, 57, 635, 669. Starch-cellulose, 57. Stem, 130, 136, 575. Stem-tendrils, 196, 775. Stephanosphaera, 218. Sterculia, 478. Sterculiaceae, 583. Sterigma, 257, 271. Sticta, 264. Stigeoclonium, 4, 8. Stigma, 429, 488, 499. Stigmaria, 421. Stigmatic cells, ■?.^i, 323. Stipule, 192, 281, 564. Stolon, 196. Stomata, 75, 86, 603. Stone-fruit, 104, 106, 539. Stratification of the cell- wall, 20, 29. Stratioteae, 554. Strawberry, 518, 536. Streaming of protoplasm, 38, 42. Striation of the cell-wall, 20, 29. Stroma, 256. Strophiole, 540. Struggle for existence, 831. Strychnaceae, 580. Style, 488, 498. Stylidieae, 581. Stylospore, 254, 256. Stypocaulon, 118. Styracaceae, 581. Subepidermal tissue, 80, 105. Subhy menial layer, 269. Successive whorls, 166, 170. Sulphur, 622. Superficial glands, 114. Superior ovary, 491. Surface-growth of the cell wall, 21. Superposed members, 168,, 524, 568. Survival of the Fittest, 843. Suspensor, 405, 424, 513. Swarmspore, 5, 39, 211. Swartzieae, 584. Swelling, 36, 698. Swelling of starch-grains, 61, 592. Swimming of swarmspores and spermatozoids, 39. Symmetrical, 183, 469, 533. Sympetalae, 580. Sympetalous, 201, 471. Symphoricarpus, 566. Symphyllous, 471. S y mpodial inflorescence, 521, Sympodium, 157, 159. Synandrae, 581. Syncarp, 537. Synsepalous, 201, 471. System, Natural, 844. 8^8 Taccaceae, 555. Tamariscineae, 582. Tap-root, 561. Targioneae, 306. Taxineae, 460. Taxodineae, 460. Taxodium, 446. Taxus, 447, 450, 455. Teeth, 331, 334. Teleutospore, 248. Temperature, Action of, 591, 651, 748, 784, 789. Temperature, Dependence of vegetation on, 647. Temperature, Limits of, 651. Tendril, 194, 196, 766, 775. Tension of tissues, 708, 713. Terebinthaceae, 116, 582. Terebinthineae, 582. Terminal branching, 155. Ternstromiaceae, 582. Testa, 42ij 427, 512. Tetracyclae, 580. Tetragonieas, 585. Tetraphis, 319. Tetraspore, 234. Thalloid Hepaticae, 296. Thallome, 130, 137. Thaliophytes, 130, 207. Thallus, 130. Theca, 294, 324. Theory of descent, 842. Thickening-ring, 107, 109, 575. Thread-indicator, 747. Thuja, 449. Thujopsidae, 460. Thunbergia, 34. Thymelaeaceae, 584. Thymelaeineae, 584. Tiliaceae, 569, 583. Tissues, Forms and Systems ofj 77, 431- Tissues, Morphology of, 68. Torsion, 770. Torus, 426, 489. Trabeculae, 413. TracheYdes, 73, 98. Traction, Action of, on growth, 729. Trama, 250. Transfusion-tissue, 466. Transpiration, 602. Transport of assimilated sub- stances, 634. Trapa, 517, 557. Traube's artificial cells, 594. Tree-ferns, 355. Tremellini, 249. Trichia, 275. INDEX. Trichogyne, 212, 235. Trichomanes, 341. Trichome, 129, 138, 356. Trichophore, 213, 236. Tricoccae, 583. Triglochin, 549. Tropaeolaceae, 582. Tropaeolum, 14, 776. Tube connecteur, 237. Tuber, 196, 255. Tuberaceae, 255. Tubiflorse, 580. Tulipa, 637. Tiillen, 27. Turgidity, 700, 708. Turneraceae, 582. Twining of climbing plants, 772. Twining of tendrils, 775. Twining stems, 197, 772. Type, 842. Typha, 473, 495. Typhaceae, 504, 555. Udotea, 226. Ulmacese, 578. Ulvaceae, 231. Umbel, 520. Umbel, Gymose, 158. Umbelliferae, 584. Umbelliflorae, 584. Unequal growth, 765. Unguis, 471. Unicellular plants, 77, 209, 615. Unilateral cicinal cyme, 522. Unilateral helicoid cyme, 521. Uredineae, 246. Uredo, 248. Urn, 294, 324. Urticaceae, iii, 578. Urticeae, 578. Urticineae, 578. Usnea, 264, 266, 272. Vacciniaceae, 581. Vacuoli, 38, 41. Vaginula, 309, 324. Valeriana, 566. Valerianaceae, 566, 581. Vallisneria, 664, 689. Vallisnerieae, 554. Variation, 696. Variation of hybrids, 820, Variety, 823. Variety-hybrid, 817. Vasa propria, loi. Vascular bundle-sheath, 355. Vascular Cryptogams, 335. Vascular portion of fibro- vascular bundle, 98. Vaucheria, 41, 223, 224, 225. Vegetable ivory, 512. Vegetative cone, 117. Velum, 249, 408. Venation, 103, 192, 547, 564. Verbenaceae, 580. Verticillate flowers, 523. Vesicular vessels, no. Vessels, 98. Vicia, 558. Viola, 499, 511, 814. Violaceae, 582. Virginian creeper, 781, 839. Viscum, 506, 557. Vitis, 562, 780. Volvocineae, 217. Volvox, 219, Waking and sleeping of plants, 786. r^ater, Asce root, 608. Water, Currents of, in the wood, 603. Water, Exudation of, 600. Water of crystallisation, 32. Water of organisation, 32, 62. Water, Movements of, 598. • 652. Watsonia, 507. Wax, 84. Welwitschia, 461. Wendungszellen, 286, 289, Whorl, 149, 166. Whorl, Spurious, 149. Wood, 98, 574, 717. Xanthophyll, 685. Xanthoxylaceae, 583. Xylem, 94, 717. Xylem-portion of fibrovas- cular bundle, 98. Xyrideae, 555. Yeast-fungi, 254. Yucca, 552. Zamia, 437. Zea, 42,61, 71, 94, i33> Mi? 143, 144, 147, 827. Zingiberaceae, 548, 556. Zoosporangium, 13. Zoospore, 13, 211. Zygnema, 46. Zygnemeae, 220. Zygomorphic, 183, 533. Zygophyllaceae, 583. Zygospore, 10, 212, 220, 245, 802. BOOKS LATELY PRINTED AT THE CLAEENDON PRESS, OXFORD, AND PUBLISHED FOR THE UNIVERSITY BY MACMILLAN AND CO., 30, BEDFORD STREET, COVENT GARDEN, LONDON. Alt Elementary Treatise on Heat, with nu- merous Woodcuts and Diagrams. By BALFOUR STEWART, LLD., F.R.S., Professor of Natural Philosophy in the Owens College, Man- chester. Second Edition. (Ext. fcap. 8vo., cloth, price "js, 6d.) ' The publication of this manual is exceedingly well-timed ; it includes within narrow limits the leading facts and principles of this younger-born of the Sciences, and for the mastery of the greater portion of the contents only requires ordinary intelligence on the part of the reader.' — Spectator. Chemistry for Sttidents, By A. W. Williamson, Phil. 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