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 STRUCTURE AND DEVELOPMENT OF 
 SPORANGIA AND SPOROPHYLLS 
 OF ISOETES 
 
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 SCHOOLS OF ARTS, LITERATURE, AND SCIENCE, IN CANDIDACY 
 FOR THE DEGREE OF DOCTOR OF PHILOSOPHY 
 
 DEPARTMENT OF BOTANY 
 
 BY 
 
 R. WILSON SMITH 
 
 CHICAGO 
 
 £be mnlversitg of Chicago iPress 
 
 1900 
 
Digitized by the Internet Archive 
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 University of Illinois Urbana-Champaign Alternates 
 
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VOLUME XXIX 
 
 NUMBER 4 
 
 Botanical Gazette 
 
 APRIL , igoo 
 
 THE STRUCTURE AND DEVELOPMENT OF THE 
 SPOROPHYLLS AND SPORANGIA OF ISOETES. 
 
 CONTRIBUTION FROM THE HULL BOTANICAL LABORATORY. 
 
 XVIII. 
 
 R. Wilson Smith, 
 (with plates xiii-xx) 
 
 Few plants have excited more interest than Isoetes, a small 
 genus of about fifty species, which has been variously classified, 
 and the histology and development of which have been described 
 in the most contradictory manner. It was with the purpose of 
 obtaining, if possible, some data by which to clear up its homol¬ 
 ogies and relationships, and especially of examining the founda¬ 
 tion of a claim made in recent years of its being the point of 
 contact between monocotyledons and vascular cryptogams that 
 the following investigation was undertaken. 
 
 The intention at first was to have the work include not only the 
 reproductive parts of the sporophyte, but also the development 
 of the female gametophyte and of the embryo. But so small a 
 proportion of the spores was found capable of germination that 
 the study of the prothallium had to be abandoned; and my 
 
226 
 
 BO TANICAL GAZE TTE 
 
 [APRIL 
 
 observations of the embryo agree so closely with those of Pro¬ 
 fessor Campbell (4) that it did not seem worth while to publish 
 any drawings. One difference may be noticed here and this 
 part of the subject may be dismissed at once. Campbell says 
 that but three archegonia are formed at first, and only in case 
 none of these are fertilized do others appear. I have found new 
 archegonia appearing even after three embryos had begun to 
 develop, two of which had made considerable growth. It thus 
 appears that sometimes new archegonia may arise even after 
 fertilization. 
 
 The species selected for study were /. echinospora and /. 
 Engelmanni, the former of which was examined more carefully. 
 The material was collected by Mr. Raynal Dodge, of Newbury- 
 port, Mass. Part of it was fixed at once, and part after it had 
 been cultivated for some time in the laboratory. The fixing 
 reagents employed were 1 per cent, chrom-acetic acid, and 
 Flemming’s weaker solution. After remaining twenty-four hours 
 in one of these fluids it was washed twenty-four hours in water and 
 transferred through graded alcohols and chloroform or xylol to 
 paraffin. The sections were cut 5, 10, or 15//, in thickness and 
 stained in the ordinary way on the slides. Delafield’s haema- 
 toxylin and erythrosiri, safranin and gentian violet, Heiden- 
 hain’s iron-alum-haematoxylin, and cyanin and erythrosin were 
 all used with good results except in the case of the megaspore 
 mother cell. 
 
 THE STEM. 
 
 The technique which is best adapted to an investigation of 
 the development of the sporangia is not very suitable for an 
 examination of the histology of the stem. Accordingly I have 
 not attempted to make an exhaustive study of the latter, or of 
 the vascular bundles of the leaf. Still the arrangement of the 
 stem tissues is so peculiar that a few remarks will not be out of 
 place. There is probably little variation in this respect in the 
 different species. /. echinospora and /. Engelmanni agree very 
 closely with /. lacustris as figured and described by Farmer (1), 
 whose account is the latest and best dealing with the structure 
 
1900 ] SPOROPHYLLS AND SPORANGIA OF ISOETES 227 
 
 of the vegetative organs. The center of the stem is occupied 
 by a mass of short spiral and reticulated tracheids interspersed 
 especially near the periphery with less numerous parenchymatous 
 cells. The peripheral parenchyma is not sufficiently aggregated 
 or continuous to form a xylem sheath such as occurs in the 
 leaves. The xylem region is surrounded by a ring of tabular 
 cells of glistening white appearance, thick-walled and empty 
 towards the center but thin-walled towards the outside and more 
 or less banded with incomplete layers of starch-containing cells. 
 The cells are arranged in pretty regular radial rows, whether 
 examined in longitudinal or transverse section. This ring is 
 usually designated the prismatic layer, and very frequently, after 
 Russow (1), the phloem. Russow claimed to have traced a con¬ 
 tinuity between the prismatic layer and the phloem of the leaf. 
 I have not been able to satisfy myself of any organic continuity, 
 but even did it exist it seems to me very questionable whether 
 that would be a sufficient reason to justify Russow’s view. No 
 clearly defined sieve-tubes, the essential elements of the phloem, 
 have ever been found either in the stem or in the leaf; and 
 besides the inner cells of the prismatic zone are known to become 
 secondarily thickened and transformed into xylem tracheids. 
 The cells marked 0 in jig. 5 are in this process of transformation. 
 Does not this indicate, if not a xylem character, at least the 
 undifferentiated nature of the prismatic cells ? A transforma¬ 
 tion of phloem into xylem would be, to say the least, an anomaly. 
 In view of these difficulties to which may be added another — 
 the relation to the cambium — it seems better to drop this appli¬ 
 cation of the word phloem until its justification shall be estab¬ 
 lished on physiological grounds. A small portion of the prismatic 
 layer is shown in jig. 5. 
 
 Immediately outside the prismatic layer and indistinguishable 
 from it except in the staining and size of the cells, is a'zone of 
 meristem which by its active division gives origin outwardly to 
 an immense mass of cortex, and internally adds slowly to the 
 prismatic layer. This zone is the so-called cambium. Its cells 
 contain deeply staining plasmic contents in addition to starch 
 
228 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 (fig- 5 ) - Whether or not the dividing cylinder is more than 
 one cell-layer in thickness I could not determine. 
 
 The cortex is very bulky and consists throughout of iso- 
 diametric parenchymatous cells abundantly filled with starch 
 together with a little oil. The cells undergo no divisions, but 
 as they are forced outward by the activity of the cambium 
 they increase very greatly in size {figs. 6 and 7). In this way 
 by the growth of the cells in all directions the cortex expands 
 upwards and downwards as well as outwards, and as a result 
 carries up the older leaves to a height considerably above the 
 stem apex. This is illustrated in fig. 68, in which the unbroken 
 lines represent the form which the cortex would take if its cells 
 underwent no enlargement as they are pushed out from the 
 cambium, and the dotted lines the form and dimensions which it 
 actually assumes. 
 
 The stem apex lies at the base of the conical depression 
 formed in the manner just explained. In small plants it is dis¬ 
 tinguishable in longitudinal sections as a slight elevation {fig.j)', 
 in older plants it is merely a flattened area between the bases of 
 the young leaves {fig. 4). 
 
 The method of growth of the apical meristem was first cor¬ 
 rectly described by Hegelmaier (2). Hofmeister (1) had erro¬ 
 neously ascribed it to the segmentation of an apical cell, having 
 been led to that conclusion probably by too exclusive study of 
 young plants. There is neither an apical cell nor such a group 
 of initials as might result from the division of a rectangular apical 
 cell like that of the Marattiaceae. Only two or three layers of cells 
 show their meristematic nature by their contents. The superficial 
 layer appears to divide only in an anticlinal direction except when 
 young leaves are about to be formed ; but this layer, as Hegelmaier 
 showed, can on no account be regarded as a dermatogen. 
 
 Although all the species of Isoetes are perennial, only a 
 small portion of the plant persists from year to year. The roots, 
 the leaves, and the bulky cortex are shed or decay annually, 
 and are as often renewed from the stem apex and the meristem¬ 
 atic zone which surrounds the small central permanent cylinder. 
 
1900] 
 
 SPOROPHYLLS AND SPORANGIA OF ISOETES 
 
 229 
 
 THE LEAF AND LIGULE. 
 
 In its earliest recognizable form the leaf rudiment seen from 
 above is a crescent-shaped band of meristematic cells, curved 
 about the stem apex. Sections show that it arises from the 
 superficial cells of the stem apex, and is soon pushed up into a 
 low broad mass, highest in the middle and inclined inwards. 
 The ligule appears very early, and the leaf becomes distinguish¬ 
 able into a proximal part somewhat triangular in section and 
 destined to bear the sporangium, and a distal part approximately 
 circular in section and destined to become the chlorophyllous 
 region. In correlation with the rapid development of the spo¬ 
 rangium, the growth of the leaf is at first almost confined to the 
 basal region. Compare, e. g., the three leaves shown in fig. 8 ; 
 tranverse sections would show the rapid growth of the basal 
 region in a still greater degree. This region continues to widen 
 as the leaf is pushed outward, by the formation of new leaves 
 and the diametral enlargement of the stem ; but longitudinally, 
 except for a slight addition below the sporangium, there is 
 only sufficient growth to accommodate the sporangium, velum, 
 and ligule. 
 
 When the sporangium is well under way the region of rapid 
 multiplication and growth of cells is transferred to the part above 
 the ligule. The cells here are arranged with beautiful regular¬ 
 ity, and growth is so rapid that this soon becomes the most 
 prominent part of the leaf. The maximum diameter, so far as 
 the number of cells is concerned, is speedily attained, and 
 growth thereafter is only in the longitudinal direction. At first 
 every part of the leaf rudiment is meristematic, but in a short 
 time the apex passes over into permanent tissue. This change 
 into permanent tissue progresses gradually downward until 
 finally the whole leaf is involved. For some time a region of 
 ever narrowing extent above the ligule continues in active 
 division, but there is present no sharply marked or persistent 
 meristematic zone, as seems to be implied in Farmer’s account. 
 The leaf is still quite small when all cell divisions have practi¬ 
 cally ceased, and its further elongation, which may amount to 
 
230 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 400 or 500 per cent., is accomplished by th*e growth of the indi¬ 
 vidual cells. 
 
 The formation of the air cavities is interesting, since it is 
 comparable in some respects to the differentiation of trabecular 
 and sporogenous tissue in the sporangium. In a leaf such as is 
 represented in cross section in fig. 9, there is yet no sign of the 
 air chambers. Increase of diameter is actively going on and 
 the whole leaf is still meristematic. In the leaf shown in fig. 
 10 the position of the future air chambers is indicated by four 
 symmetrically placed groups of cells which have lost most of 
 their contents. The peripheral cells of the leaf, the central 
 cells, and four radiating bands which appear in cross section as 
 spokes arranged in the form of the sign -\- continue to grow and 
 are distinguishable by their larger more densely filled cells. 
 Stained with Delafield’s haematoxylin and erythrosin these 
 cells show deep red cytoplasmic contents and large nuclei in 
 which the red staining predominates ; while in the areas which 
 are to become air chambers the cytoplasmic contents have 
 almost entirely disappeared, but the nuclei still retaining their 
 chromatin stain intensely with haematoxylin. When only a 
 nuclear stain is employed, such as iron-alum-haematoxylin, the 
 four non-protoplasmic areas are rendered very prominent by 
 their black nuclei. Longitudinal sections show that the regions 
 which are thus sharply distinct in cross section run lengthwise of 
 the leaf in unbroken bands from just above the ligule nearly to 
 the apex, and there are as yet no air cavities. 
 
 The air chambers are formed lysigenously. The growing 
 tissues generate a tension in the empty cells, and as a result 
 these are ruptured irregularly, and small cavities appear, sepa¬ 
 rated by diaphragms or plates of cells extending across from 
 the central to the peripheral growing regions. As the leaf elon¬ 
 gates, the air cavities increase in size, while the diaphragms drawn 
 farther and farther apart lose their protoplasm to the surrounding 
 cells. When once this splitting into diaphragms and cavities 
 has occurred, it is not repeated ; there remains no meristem in 
 which they may be generated. Occasionally single diaphragms 
 
1900J SPOROPHYLLS AND SPORANGIA OF ISOETES 231 
 
 of unusual thickness may be again ruptured, but no considerable 
 increase in their number ever occurs after their first formation. 
 
 It is easy with the low power of the microscope to count the 
 diaphragms in leaves floating upon a little water on a slide. 
 The usual number is from fifty to seventy, and is quite as many 
 in young leaves three fourths of an inch long as in leaves fully 
 formed. It is instructive, too, as proving the absence of a defi¬ 
 nite meristematic zone, to count the average number of super¬ 
 ficial cells which intervene between the diaphragms. In very 
 young leaves this is from three to six or eight throughout the 
 whole length, but in older leaves it is much greater, varying 
 from twelve to twenty in the tip region to forty to sixty in the 
 middle and basal regions, which remain longest in the meristem¬ 
 atic condition. 
 
 The diaphragms, I think, are quite functionless, and their 
 existence merely incidental to the manner of origin of the air 
 chambers. They are too delicate to serve for mechanical sup¬ 
 port, which is sufficiently secured by the four longitudinal bands 
 already described. The position of the air chambers and longi¬ 
 tudinal bands between them in relation to the axis of the plant 
 is always the same as that indicated in figs. 10, //, 44. Near 
 the ligule the air spaces are less regular, and instead of four 
 of them symmetrically placed we find many irregular ones. 
 Behind the sporangium the dorsal longitudinal band of living 
 cells, and sometimes the two lateral ones, are well marked, but 
 there are no large distinct air spaces. The vascular bundle of 
 the leaf is as characteristic as that of the stem. My observations, 
 referring chiefly to the changes of form of the bundle, were made 
 with the view of discovering whether there is any definite relation 
 between it and the sporangium or the ligule, and whether it 
 presents any evidence that the leaf of Isoetes has been reduced 
 from a more complex type. The leaf trace can first be recognized 
 inthebaseof the young leaf and in thestem region below it towards 
 the central bundle. The xylem elements are first differentiated,, 
 and consist of five or six tracheids grouped into a cylinder and 
 surrounded by a sheath of parenchymatous cells with dense 
 
232 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 contents. These parts can be traced later to the corresponding 
 parts of the axial bundle. Behind the sporangium the xylem 
 spreads out into a broad band in which the amount of xylem 
 parenchyma is greatly increased, and the tracheids are.in five or 
 six scattered groups. Above the sporangium the xylem con¬ 
 tracts again into a cylinder, and lies between the cornua of the 
 ligule base. A more striking change occurs above the ligule 
 where the xylem elements suffer an extreme diminution, there 
 being in that region in I. echinospora only a single imperfect 
 central tracheid surrounded by a sheath of parenchyma {Jigs, p, 
 10). Occasionally in /. echinospora , and usually in I. Englemanni, 
 two, sometimes three, other such groups can be traced up the 
 leaf. 
 
 The phloem is best represented in the chlorophyll-bearing 
 portion of the leaf. It there consists of two strap-shaped bands 
 on the dorsal side, more or less united by their edges, so as partly 
 to surround the xylem. In less distinct form the phloem may 
 be traced downwards to the region of the central bundle. 
 
 The development of the ligule was accurately described by 
 (Hofmeister i), and also by Hegelmaier (]). The latter refers 
 its origin to more than one cell. Since the former gives few 
 figures, however, and the latter none, I shall again briefly 
 outline the course of growth and illustrate it with a few 
 drawings. The ligule originates from a single large vesicular 
 cell protruding from the ventral face of the leaf rudiment. Pro¬ 
 vision for its rapid growth is shown in the large size of the nucleus 
 of this cell, and the density of the cytoplasm {jigs. 12 , ij). The 
 first division is always parallel to the face of the leaf {Jigs. 14, 
 ij ), and usually the second division is parallel to the first. 
 The ligule of I. lacustris is described as passing through a fila¬ 
 mentous stage ; but in I. echmospora and I. Engelma7ini it is 
 hardly worth while to distinguish such a stage, for the filament 
 never consists of more than three cells. The terminal cell then 
 divides in a vertical plane at right angles to the first wall ( figs. 
 16 , 18). Other vertical divisions follow until the ligule has 
 become a plate of cells of very regular arrangement. Figs. 18 
 
1900] 
 
 SPOROPHYLLS AND SPORANGIA OF ISOETES 
 
 233 
 
 and ig are median sections of the ligule made tangentially to 
 the face of the leaf. Longitudinal sections are shown in figs. 26 , 
 2j, 28 , 55, 55. Growth in length and breadth continues very 
 rapid, and the ligule soon overtops the leaf (fig. 8). For 
 some time it remains a single layer of cells in thickness, but 
 eventually it becomes double throughout most of its extent. 
 The doubling begins in the middle region near the base and 
 extends in all directions, never reaching the apex or margin 
 however, which remain to the last but one layer in thickness 
 (fig. 21). The expanded part soon reaches its maximum 
 growth. Not so the foot region ; this becomes quite massive 
 and deeply embedded in the tissue of the leaf, especially at the 
 sides which grow upward and downward into two prominent 
 cornua. Figs. 22-25 may help to explain the form of the base of 
 the ligule. Fig. 25 is a transverse section of the leaf cutting 
 across the cornua above the main place of union of the ligular 
 and leaf tissues. Sections below it show the cornua connected 
 by a transverse band embedded in the leaf ; and sections still 
 lower would show portions of the cornua only. The other 
 figures need no fuller explanation than that accompanying the 
 plates. 
 
 Along with the growth of the ligule there comes about a 
 differentiation of the cells composing it. There may be said to 
 be four regions. The base is closely surrounded by a layer of 
 small deeply-staining gland-like cells (.? in figs. 22,58) which 
 we may call the sheath. It forms a conspicuous layer, every¬ 
 where investing the base of the ligule, and becoming continuous 
 with the superficial cells of the leaf. Next to the sheath is an 
 irregular layer or band of large empty cells, the glossopodium 
 (g in figs. 22 , 58; see also figs. 25-25). The glossopodium 
 appears to form the base of the ligule, but the true base includes 
 the sheath which, as a study of the development shows, is 
 derived from the lowermost cell of the young ligule (fig- 58). 
 Above the glossopodium are smaller cells containing protoplasm 
 and forming the greater part of the ligule. The apex and 
 margin of older ligules constitute the fourth region ; the cells are 
 
234 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 shrunken and contorted, their nuclei broken down, and the cyto¬ 
 plasm disorganized. 
 
 A study of the ligule of Isoetes to be complete must be 
 accompanied by a comparative examination of the ligule of 
 Selaginella. With this in view I have studied the origin and 
 growth of the ligule in .S. Marte?isii and vS. apus , and compared 
 my sections with the excellent drawings of Professor Harvey 
 Gibson (2). Professor Farmer (1) has expressed the view that 
 the ligules of Isoetes and Selaginella have little in common 
 except their position and name. I have been led to quite the 
 contrary conclusion, to hold in fact that there is a very close 
 homology between the two. What has appealed most to me, 
 in addition to the position of the organs, is the similarity of the 
 regions of which both are seen to consist. The ligule of Selag¬ 
 inella has a glossopodium of large empty cells, sheathed by a 
 gland-like layer, and shows also two upper regions, one of living 
 and one of disorganizing cells. The two are alike also in the 
 absence of chlorophyll, starch, and intercellular spaces; and 
 both show their embryonic character by passing their maximum 
 of growth before the leaf has reached its greatest functional 
 activity. Differences are to be expected, of course, and are 
 chiefly these: the ligule of Isoetes arises from a single cell, that 
 of Selaginella from a group of cells ; and, whereas the ligule 
 of Isoetes is almost from the beginning a conspicuous part of the 
 leaf, that of Selaginella is rather late in making its appearance, 
 no trace of it being discoverable unt.il after the sporangium rudi¬ 
 ment is plainly perceptible. 
 
 THE SPORANGIUM. 
 
 The sporangium has repeatedly been made the object of 
 investigation during the last fifty years. Hofmeister (1) was 
 the first to make a careful study of its origin and development. 
 Though his view that the sporangium can be traced back to a 
 single cell has been discredited by later observers, I hope to show 
 that his error was largely due to his exclusive dependence upon 
 longitudinal sections. Except for his failure to see the true 
 
1900] 
 
 SPOROPHYLLS AND SPORANG/A OP ISOETES 
 
 235 
 
 nature of the sporangium rudiment as a transverse row of cells, 
 his account is surprisingly accurate when the imperfect methods 
 of sectioning and staining of that time are taken into considera¬ 
 tion. 
 
 According to Hegelmaier (2) and Tschistiakoff (1) the 
 sporogenous tissue is differentiated out of a considerable mass 
 of deep-lying meristem between the epidermis and the vascular 
 bundle. 
 
 Goebel (1) agrees substantially with the two preceding 
 authors, but is more explicit in his description. The Anlage of 
 the sporangium according to him is a group of cells of the leaf 
 base, chiefly the three upper layers. The outer layer gives rise 
 to the sporangium wall, and the hypodermal layer to the arche- 
 sporium from which all the spore mother cells, trabeculae, and 
 tapetum are derived. Goebel’s account, as confirmed and 
 restated by Sadebeck (1) in Schenck’s Ha?idbuch der Botanik, has 
 formed the basis of all the text-book descriptions of the sporan¬ 
 gium of Isoetes written since that time. 
 
 The latest student in this field is Bower (5), whose descrip¬ 
 tion is confirmatory of Goebel’s except that he traces the origin 
 of the sporangium to a group of superficial cells. This differ¬ 
 ence, however, is of the very greatest importance. For whereas 
 the derivation of the archesporium by periclinal divisions of 
 superficial cells is the rule in Pteridophytes, the origin of the 
 sporogenous tissue from a hypodermal layer separated from the 
 beginning from the epidermis is a spermatophyte character. The 
 result of Bower’s work then is to put Isoetes in line with other 
 Pteridophytes in respect to the origin of the archesporium. 
 
 My own results are in the main confirmatory of Bower’s as 
 to the origin of the sporangium, though with variations in minor 
 details which may be due to specific differences (Bower studied 
 /. lacustris ); but as to the later stages of development, especially 
 of the megasporangium, I cannot make my observations harmo¬ 
 nize with any accounts hitherto written. 
 
 It will, of course, be apparent, when so many discrepancies 
 appear in the descriptions of different investigators, that the 
 
236 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 study must be one which involves considerable technical diffi¬ 
 culty. This is attributable (1) to the absence of an elongated 
 axis and internodes and the consequent crowding of the sporo- 
 phylls, and (2) to the early appearance of the sporangium and 
 the consequent difficulty of distinguishing it from the other 
 meristematic tissues in which it is placed. The kinds of evidence 
 on which I have relied in my interpretations may be stated briefly 
 as follows : 
 
 1. Study was made of sporangia whose sporogenous tissue 
 was already distinct and unmistakable. Then by comparisons 
 with successively younger sporophylls the attempt was made to 
 trace the sporangium to its earliest rudiment. 
 
 2. A careful comparison was made of sections in the three 
 planes, longitudinal, transverse, and tangential. This involved 
 the waste of a great deal of material. For it will be made clear 
 by a glance at fig. 4 that sections made longitudinal to the stem 
 could give longitudinal sections of very few young leaves, and 
 oftener than not would fail in this altogether, since the leaves 
 have a spiral arrangement ; while, in order to obtain transverse 
 and tangential sections, one must cut obliquely to the stem with¬ 
 out possessing any clue by which to determine the proper angle 
 of obliquity. 
 
 3. The position of the vascular bundle enables one to 
 determine whether the sections are truly longitudinal, and which 
 of a number of serial longitudinal sections is exactly median. 
 This help is available only after the sporangium is distinctly out¬ 
 lined, and somewhat advanced in development, for in case of 
 very early stages of the sporangium, the vascular bundle has not 
 yet been differentiated. 
 
 4. In such early stages one must depend very largely upon 
 the ligule, which in position and outline is so definite, and in 
 manner of growth so regular as to make it of the highest impor¬ 
 tance in assisting one to orient the sections. 
 
 5. The sporogenous tissue is often distinguishable from 
 vegetative tissue by a difference in staining. There are three 
 periods when this difference is most manifest. The superficial 
 
igoo] SPOROPHYLLS AND SPORANGIA OF ISOETES 237 
 
 cells which form the earliest rudiment of the sporangium fre¬ 
 quently take a distinctive cytoplasmic staining, especially in 
 material fixed in Flemming’s solution. It must be confessed 
 that this means of recognizing sporogenous tissue is not so trust¬ 
 worthy as one could wish, for at this period of the leaf’s history 
 all the tissues are meristematic, and hence readily susceptible to 
 protoplasmic stains. One who studies the origin of sporangia 
 in Lycopodium or Selaginella meets with the same difficulty in 
 those plants, a difficulty in my experience quite as great in these 
 cases as in Isoetes. When the superficial layer of the sporan¬ 
 gium has assumed its character as an epidermis, the deeper lying 
 sporogenous cells are easily distinguishable by stain reactions 
 from the surrounding tissues. At a later period the spore 
 mother cells selected out of the general internal mass of the 
 sporangium become quite distinct on account of their denser con¬ 
 tents and more intense staining. 
 
 Longitudinal sections of young leaves show no space between 
 the base of the ligule and the stem. At this time there is still 
 an active uplifting of cells above the general stem level, a con¬ 
 tinuance of the process by which the leaf first emerged. When 
 the ligule has grown sufficiently to contain eight or ten cells in 
 longitudinal section the space below it is occupied by one large 
 cell with dense cytoplasmic contents ( fig. 26 ). The next change 
 which takes place is a transverse division of this cell as shown 
 in figs. 27, 28. Comparisons of successive serial sections 
 show that the two cells shaded in fig. 27 form the middle of a 
 group of cells arranged transversely to the leaf. This group of 
 cells, distinguishable in good preparations by their deeper stain¬ 
 ing and larger nuclei, constitute the rudiment of the sporangium. 
 In order to learn its extent and arrangement recourse must be 
 had to transverse and tangential sections. 
 
 Most transverse sections of this early stage of the sporangium 
 show that it is five cells in width. Whether or not these can be 
 traced back to a still smaller number I am in doubt. Fig. 2(7 
 certainly shows an example where the transverse row consists of 
 only three cells, and it is clear that the shaded cells of fig. jo 
 
238 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 may have had their origin in three similar to those of fig. 2g . 
 But I have succeeded in getting only two such cases as that of 
 fig. 2Q , one in I. echinospora and one in I. Engelmamii , and have 
 failed altogether to obtain a tangential view. 
 
 Tangential sections of the leaf at this early stage are almost 
 uninterpretable. The face of the leaf is so closely pressed 
 against the back of the next younger one that it is quite impos¬ 
 sible in most instances to distinguish the tissues of the two leaves 
 or to determine what is a truly tangential section. That shown 
 in fig. 31 was such as to admit of certain interpretation. The 
 shaded cells occupy the surface of the leaf and clearly corre¬ 
 spond to the group which we have already examined in longi¬ 
 tudinal and transverse sections. It is probable that another cell 
 seen in the adjacent section to the left of those figured belongs 
 to the same group, making the total number of cells seven. 
 
 It is evident from a comparison of my figs. 26-28 with figs. 
 104-106 of Professor Bower’s plates, that the longitudinal 
 growth of the leaf base of I. lacustris is much more rapid than 
 that of I. echinospora; and his figures though not his text sug¬ 
 gest that the six superficial cells which make up the sporangium 
 Anlage are derived from not more than three rows and probably 
 from but two. If this suggestion be correct, it would bring 
 Bower’s and Hofmeister’s accounts, so far as regards longitudinal 
 sections, into harmony with each other, and with the foregoing 
 account of I. echhiospora. 
 
 The young sporangium, situated as it is on the hollow side of 
 the leaf crescent, projects little if at all from the surface. By its 
 rapid growth, however, it soon forms an oval prominence at first 
 wider than long, then nearly circular in surface view, and finally 
 considerably longer than wide. In its development I have not 
 been able to establish any regular order of sequence. Starting 
 from such a beginning as figured in fig. 26 , it is certain that 
 transverse and longitudinal divisions are the first to occur. 
 Then periclinal walls appear ( fig . 36). The middle cells of the 
 sporangium rudiment are at first most active in dividing, not 
 only in respect to surface growth, but in periclinal divisions also. 
 
1900] 
 
 SPOROPHYLLS AND SPORANGIA OF ISOETES 
 
 239 
 
 Sections adjacent to that represented in fig. 31 show three or 
 four hypodermal cells which have been cut off from the middle 
 cells of the group and evidently belong to the same series. 
 
 There is at no time a single complete hypodermal layer 
 which may properly be termed an archesporium. For when the 
 middle cells have just completed their periclinal divisions the 
 lateral cells are still undivided, and by the time the lateral cells 
 have undergone their first periclinal division the middle of the 
 sporangium is at least three layers deep. A very good example 
 of this is seen in fig. 42, which represents the side of quite a 
 large sporangium. 
 
 The growth of the sporangium is carried on most actively by 
 the two or three outer layers of cells, as is evidenced by their 
 large size and deeper staining, and the frequency with which 
 they are found in karyokinesis. The divisions of the superficial 
 layer are by no means limited to those in anticlinal planes, as 
 is usually the case with the external cells of sporangia, but for 
 a long time they continue to add to the inner mass by periclinal 
 divisions. In the sporangium of which fig. 39 shows a section, 
 as many as eight or ten of the external cells were in the act of 
 periclinal division. Even in so old a sporangium as that shown 
 in fig. 43 the same process is still in continuance. The cells 
 marked with a cross have evidently been derived from the 
 external layer. Though in older sporangia the additions so 
 made go to form part of the sporangium wall, there can be no 
 question that in the younger sporangia they add to the true 
 sporogenous tissue. The bearing of this fact upon the question 
 of what constitutes an archesporium will be considered further 
 on. 
 
 It seems necessary to digress at this point in order to make 
 clear some features in which the preceding account differs 
 from what has been recorded by previous observers. Both 
 Hegelmaier (2) and Tschistiokoff (1) assert that the wall of the 
 sporangium is from the beginning (“von Anfang an gesondert”) 
 separated from the inner complex, and emphasize with great 
 distinctness the deep-*seated origin of the sporogenous tissue. 
 
240 
 
 BOTANICAL GAZETTE 
 
 [APRIL. 
 
 Goebel (i) states his approval of Hegelmaier’s view, but the 
 occasional periclinal division of the external cells does not 
 escape his notice, though he considers it as merely adding to 
 the thickness of the wall. Bower (5), on the other hand, 
 observed both the superficial origin of the sporangium and the 
 failure of the first periclinal divisions to completely delimit the 
 archesporium. 
 
 As already stated, I do not find the outer wall separate 
 from the sporogenous complex from the beginning. On the 
 contrary, it is distinctly active in increasing the dimensions, 
 of the sporangium. Ultimately the superficial layer loses 
 some of its protoplasmic contents, and assumes the appearance 
 of an epidermis. It sometimes happens that this separation of a 
 wall layer occurs quite early {fig. 41), but oftener it is not till 
 the sporangium has come to consist of many hundred cells. 
 Even then periclinal divisions do not entirely cease. 
 
 According to my observations there is no regularity in the 
 arrangement of the cells within the sporangium. The discovery 
 of this was a great surprise to me, for Goebel’s statement is very 
 explicit: “Each of the cells composing the archesporium has 
 an independent growth,” and in this he has been corroborated 
 by Sadebeck and Farmer. Bower has not traced the history 
 of the sporangium with any fullness; he merely states that his 
 results are confirmatory of Goebel’s and his figures certainly 
 convey the impression that each cell of the archesporium has an 
 independent growth. But he has made use of the same style of 
 drawing in representing the sporangia of other genera (Lycopo¬ 
 dium, Selaginella, Equisetum), in which no such claim is made. 
 In view of my own observations and of Bower’s drawings, it is 
 difficult to know just how much is meant by the phrase “inde¬ 
 pendent growth.” 
 
 In the case of bryophyte antheridia the primary spermatog- 
 enous cells are clearly distinguishable throughout the whole 
 development of the antheridium, although each may become 
 divided up into a hundred or more sperm mother cells. The 
 individuality of the original cells is marked in several ways: 
 
igoo] SPOROPHYLLS AND SPORANGIA OF ISOETES 241 
 
 their outer walls remain straight and become thicker than those 
 which subsequently appear within them; and the incomplete 
 separation of the derivatives of any single primary sperm cell 
 from one another and their complete separation from those of 
 other primary cells are shown by their dividing concurrently. 
 I have frequently observed in the antheridia of Polytrichum, 
 Porella, Marchantia, and Asterella that all the cells derived 
 from one of the primary sperm cells enter into karyokinesis 
 together, finish their division, and enter into the resting condi¬ 
 tion together, quite independently of what may be going on in 
 the derivatives of other primary cells. In such cases it is quite 
 proper to speak of an independent growth ; for the separation 
 and isolation of each group by thickened walls are sufficient to 
 insure a simultaneous exposure and obedience of all the cells to 
 the physiological stimulus which induces karyokinesis. 
 
 Are there any indications of such independent growth in the 
 sporangium of Isoetes ? I can find none, either in the arrange¬ 
 ment of the tissues or in the presence of thickened walls which 
 mark the boundaries of the original archesporial cells, or in the 
 simultaneous entrance of the cells of each group into the phases 
 of division. All the mature cell walls of a growing sporangium 
 are of equal thickness; and in marked contrast to what is seen 
 in the leaves there is no regularity of stratification or lining-up 
 of the cells. I am forced to conclude that the sporangium 
 of Isoetes (at least of I. echinospora and I. EngeImamu ), just as 
 the microsporangium of angiosperms, grows as a unit and not as 
 a number of individual segments. 
 
 Before continuing the subject of the development of the 
 sporangium it will be convenient to consider the formation of 
 the velum. The velum makes its appearance very early in the 
 history of the sporangium, almost as soon in fact as the first 
 periclinal divisions of the superficial cells. It is formed immedi¬ 
 ately below the ligule. Hofmeister (1) says: “Of the two cells 
 into which by a transverse septum the cell underneath the place 
 of insertion of the ligule is divided the upper one becomes the 
 primary cell of the velum and the lower the primary mother cell 
 
242 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 of the sporangium.” It has already been said that Hofmeister 
 was in error because of failure to notice the lateral extension of 
 the sporangium rudiment. Even allowing for this, however, I 
 am not able to agree fully with his account. It appears rather 
 that the upper tier of cells while giving rise to the velum makes 
 some additions at the same time to the sporangium. In other 
 words, the velum is a sterilized portion of the sporangium. Some 
 sections seem to admit of this interpretation only, though others, 
 such as fig. 32 , are not unfavorable to the view that the separa¬ 
 tion of velum and sporangium proper is accomplished by the 
 first transverse division of the sporangium rudiment. 
 
 Early stages of the velum may be seen in figs. 32-36; it is 
 at this time a transverse row of slightly projecting cells. Its 
 cells soon become comparatively empty, contrasting strongly 
 with the young sporangium. Growth is very rapid and in an 
 upward oblique direction; in some cases there is a tendency to a 
 downward growth also, such as obtains among terrestrial 
 species. 
 
 The velum reaches its full size much sooner than the spo¬ 
 rangium, and is not affected by the changes which determine the 
 character of the latter. The cells of the interior become large 
 and lose their contents; those of the inner surface layer — that 
 adjacent to the sporangium — are smaller and more regular in 
 size and outline, and have- a semi-glandular appearance. In 
 many species of Isoetes many of the cell walls of the velum and 
 of the leaf region adjoining the ligule become lignified and take 
 on spiral and annular thickenings. I. eclimospora and I. Engel- 
 mamii offer no exception in this respect, the thickenings being 
 much more pronounced in the latter species. The change first 
 appears in proximity to the ligule, and spreads thence into the 
 remoter parts of the velum and of the leaf. The thickened cells 
 never have any connection with the vascular bundle {figs. 23 , 24). 
 
 FURTHER DEVELOPMENT OF THE MICROSPORANGIUM. 
 
 In origin the two kinds of sporangia are identical, and for 
 a considerable period of their development they exhibit no 
 
1 9 oo] SPOROPHYLLS AND SPORANGIA OF ISOETES 243 
 
 observable difference. The general statement of the text-books, 
 following Goebel and Sadebeck, is that they follow the same 
 course of development only so far as the formation of the 
 archesporium, and thereafter may be distinguished by their 
 manner of growth. It is said that in the megasporangium certain 
 archesporial cells divide only by periclinal walls, but in the 
 microsporangium all the archesporial cells divide both anticli- 
 nally and periclinally, and that in this respect the two are dis¬ 
 tinguishable from the archesporial stage on. Such is not the 
 case in the forms which I have studied. In these all the arche¬ 
 sporial cells, whether of megasporangium or microsporangium, 
 undergo divisions in all directions, and the similarity of the two 
 kinds of sporangia continues much beyond the archesporial 
 stage. Not only do they agree in origin, but up to a time when 
 they are eight or ten cells deep, they agree absolutely in manner 
 of growth, and exhibit no histological features by which one may 
 determine whether a given sporangium will bear microspores or 
 megaspores. 
 
 As an example, consider the sporangium of which fig. 43 
 represents a section. It had advanced so far beyond the arche¬ 
 sporial stage as to contain about 8000 cells. From the position 
 of its sporophyll we may infer it was destined to become a 
 megasporangium. But there is nothing in the arrangement or 
 character of the cells or in their mode of division to warrant 
 that prediction, or to enable us to say such a group of cells will 
 become a trabecula, and such a group will produce spores. It 
 has the characters neither of a megasporangium nor of a micro¬ 
 sporangium, but is as yet quite undifferentiated. 
 
 The first changes which occur to mark the microsporangium 
 are those which lead to the differentiation of the spore mother 
 cells from the trabeculae, sporangium wall, and tapetum. Pre¬ 
 viously there has been no essential difference in the cells as to 
 size, form, or contents, excepting the external layer. But when 
 the sporangium is approaching a limit of cell multiplication, that 
 is, when the number of cells is 15,000-20,000, certain regions 
 begin to lose their power of division and reaction to stains, while 
 
244 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 other regions become more active in division and more deeply 
 stainable. The former may be called the sterile regions, since 
 they form the walls, trabeculae, and tapetum, and the latter the 
 fertile region, since they give rise to the spores. Even in 
 unstained sections the difference is noticeable as one of relative 
 abundance of protoplasmic contents. 
 
 At first it is difficult to see clearly the limits of the regions 
 or to make out their arrangement. But in older sporangia they 
 are seen to be disposed in irregular bands extending from the 
 base of the sporangium outwards to the wall. The published 
 drawings, and unfortunately in some cases the written descrip¬ 
 tion also, are calculated to convey an erroneous idea of the 
 trabeculae. They are not partitions, but, though irregular in 
 outline and frequently branched and anastomosed, are com¬ 
 parable rather to pillars. It is accordingly incorrect to speak 
 of the sporangium as chambered, for the fertile cells are not 
 segregated into loculi, but form a continuous mass pierced here 
 and there by the trabeculae. It is hoped that figs. 44-47 will 
 make the relations of the trabeculae clear. The shaded por¬ 
 tions of these drawings represent the fertile regions, and the 
 unshaded portions the trabeculae and walls. The continuity of 
 the sporogenous mass is clearly seen in the tangential section 
 
 {fig- 46). 
 
 A more detailed account of the development of the micro¬ 
 sporangium will now be given. Fig. 48 shows a small portion 
 of a microsporangium in which the differentiation into sterile 
 and fertile regions has just begun. The fertile cells stain deeply 
 and are still rapidly multiplying, as is evidenced by the many 
 karyokinetic figures. The sterile cells have almost entirely 
 ceased divisions, though here and there a dividing cell may be 
 found. It is important to notice that the one character in which 
 the two regions differ is in the relative abundance of protoplasm, 
 the fertile cells being densely filled with deeply staining cyto¬ 
 plasm, while the cytoplasm of the sterile cells is beautifully 
 vacuolated. In all other respects the cells of the two regions 
 are essentially alike. They are not markedly different in size, 
 
1900 ] SP 0 R 0 PHYLLS AND SPORANGIA OF ISOETES 245 
 
 or in the size and appearance of their nuclei, nor is there anything in 
 their arrangement to suggest a difference in their origin or growth. 
 In fact, as Professor Bower has pointed out, there is here a most 
 excellent illustration of the sterilization of sporogenous tissue. 
 
 The trabeculae at this age show about 15-25 cells in cross- 
 section (tangential section of the sporangium), and are more or 
 less cylindrical. There is as yet no tapetum. Towards the outer 
 and inner sides of the sporangium the trabeculae are continuous 
 with about three layers of cells which form the sporangium wall 
 (fig. 49 ). That the trabeculae and walls are of the same nature, 
 both being the result of sterilization of potentially sporogenous 
 tissue, is proved not only by the similarity of their cells, and 
 their passing uninterruptedly into one another, but also by their 
 relation to the tapetum, which is formed out of the layer that 
 lies next to the spore mother cells. 
 
 The inner cells of the trabeculae, those which become the 
 trabeculae proper (i. e. } exclusive of the tapetum), are at first 
 isodiametric and in no way different from the outer ones. But 
 while the latter are undergoing a transformation into tapetum, 
 the former undergo changes which are dependent on the growth 
 of the sporangium. As the dimensions of the sporangium 
 increase — a change which goes on rapidly at the period when 
 the sporogenous cells are multiplying — the trabeculae are nec¬ 
 essarily lengthened. This is accomplished, not by division of 
 the cells, but merely by their elongation. At the same time 
 they suffer a lateral compression from the growing sporogenous 
 cells and become flattened (fig. 50). The tabular form of the 
 cells doubtless furnishes the ground for the common view, which 
 ascribes the form of the cells to the direction of their division 
 planes. Such a view is incorrect, however, for divisions have 
 entirely ceased in this region before the elongated form of the 
 cells is attained. The shape of the cells is easily accounted for 
 by their growth in the one direction possible for them while 
 yielding to the pressure of the turgescent mother cells. 
 
 In this connection it may be remarked that with the possible 
 exception of the tapetum all the cells of the sporangium, after 
 
246 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 losing their power of division, enter upon a period of growth 
 which is quite comparable to that occurring in vegetative meri- 
 stems. The difference in size of the sporangia represented by 
 figs. 43 and 63, which are drawn under the same magnification, 
 is due partly, it is true, to increase of the number of cells, but a 
 glance at the two figures shows there has been also a decided 
 growth of the individual cells. 
 
 Accompanying the modification of the trabecular cells, there 
 is a change of form of their nuclei. These become first elon¬ 
 gated and oval (fig. 30), and finally spindle-shaped, suggestive 
 of the changes which attend the development of the vascular 
 strand out of the tissues of a growing point. Instances of much 
 greater elongation than that shown in fig. 31 are frequently met 
 with, though in other cases the changes are comparatively slight. 
 The nuclei at this time are relatively large and prominent, and 
 appear to form the center of aggregation of what little cytoplasm 
 still remains in the trabecular cells. In old sporangia the cells of 
 the trabeculae are nearly or quite empty, and much compressed. 
 
 Bower has discussed the function of the trabeculae. They 
 may serve for mechanical support of the sporangium, or to afford 
 a larger nutritive surface, or, since the two functions are not 
 incompatible, for both. The relation of the trabeculae to the 
 base of the sporangium where it is closest to the vascular bun¬ 
 dle, and the resemblance of the nuclei to those of plerome regions 
 in general, suggested to me that the trabeculae might be the chan¬ 
 nels through which nutriment is supplied to the spores; but the 
 suggestion is not borne out by observation. It is clear that in a 
 hydrophytic plant no elaborate apparatus is needed to provide 
 the sporangium with water, which can easily enter directly from 
 the outside; and an examination of my sections shows that 
 the organized food stuffs, such as starch and oil, pass to the 
 spores through the inner wall of the sporangium, and not through 
 the trabeculae. 
 
 The tapetum, as already stated, is organized out of that layer 
 of the sterile cells, whether of wall or trabeculae, which is in contact 
 with the fertile cells. At a stage between those shown in figs. 48 
 
igooj SPOROPHYLLS AND SPORANGIA OF ISOETES 247 
 
 and 50, the cells of this layer multiply rapidly. They are fre¬ 
 quently found in mitotic division, with the axis of the spindle 
 always perpendicular to the surface of the trabeculae or sporan¬ 
 gium wall. Divisions may still go on here after the spore 
 mother cells have reached maturity, and the changes of the 
 trabeculae are nearly complete. In this way the tapetal cells 
 become very numerous, but reduced in size. They form but a 
 single layer except in limited areas, where a doubling may some¬ 
 times occur. 
 
 At first the tapetum is not deeply stained {fig. 50), but as 
 the spore mother cells prepare for their tetrad division, the 
 tapetal contents increase in density, and they continue to do so 
 until they surpass young spores in this respect. 
 
 From what has been said, and from figs. ^7, etc., it will be 
 understood that the tapetum completely invests the trabeculae 
 and sporangium wall, forming a lining layer everywhere between 
 the spore mother cells and the sterile regions. It is a persistent 
 layer, and in this respect is to be contrasted with that of most 
 ferns and angiosperms. In these latter the walls of the tapetum 
 break down and are dissolved, the cells become disorganized, 
 and their materials, mingling with the other contents of the 
 sporangium, are used to nourish the mother cells or young spores. 
 In Isoetes, however, as in Lycopodium and Selaginella, no such 
 disorganization of the tapetum occurs. Its cells do not fall 
 apart and its walls are not absorbed. In old sporangia it is still 
 recognizable, though often its contents have been lost and the 
 walls are pushed nearly together. 
 
 Probably the tapetum can best be regarded as a gland or 
 layer of glandular cells. If so, the manner of action in a per¬ 
 sistent tapetum, such as that of Isoetes, Lycopodium, and Selagi¬ 
 nella, must be quite different from what it is in a tapetum which 
 is regularly disorganized and absorbed. In the one case the 
 nutrient substances secreted by the cells must be passed on 
 through the walls into the cavity in which the young cells are 
 growing. In the other case there can be little or no passing of 
 nutrient substances through the walls, but at the proper time the 
 
248 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 secreted materials are rendered available by the total collapse of 
 the cells. 
 
 In many plants also, especially in those in which the tapetum 
 undergoes complete disorganization, it is common for the tapetal 
 cells to become multinucleate, the division of the nuclei being 
 sometimes accomplished by karyokinesis, but mostly by amitosis. 
 The cells of the tapetum of Isoetes, in this respect again agreeing 
 with Lycopodium and Selaginella, are uniformly uninucleate. 
 
 In almost every sporangium examined the number of layers 
 of cells outside the fertile regions when they first become dis¬ 
 tinct is three. In a very few cases there were four layers. As 
 already shown, the innermost of these becomes tapetum. Of 
 the other two layers, one, apparently the hypodermal, usually 
 undergoes division, so that the wall region ultimately consists of 
 three layers outside the tapetum. 
 
 At the base of the sporangium, between it and the vascular 
 bundle, are a few layers of cells which may be regarded as the 
 inner wall of the sporangium. The exact origin of these I have 
 not been able to make out. Whether, like the outer wall, they 
 are derived from the sterilization of sporogenous tissue, or 
 whether they are derived from the tissues underlying the original 
 archesporium, I cannot say. It is always difficult in all sporangia 
 except the very youngest to define the exact inner limits. 
 Between the vascular bundle and the three or four outer layers 
 where growth and division are most actively carried on, there is 
 a mass of small cells staining deeply. Such a section as fig. 38 
 makes it probable that all the tissues between the parenchyma¬ 
 tous sheath of the xylem and the outside arises from the sporan¬ 
 gium Anlage , and that therefore the inner wall arises also by 
 sterilization. 
 
 The formation of the microspores in Isoetes takes place in 
 much the same way as in other vascular plants. After the 
 fertile regions have ceased their cell divisions, the cells and 
 their nuclei pass through a period of rest and enlargement. 
 The nuclei especially increase in size and become rich in chro¬ 
 matin. At the same time the cytoplasm remains dense and never 
 
1900] SPOROPHYLLS AND SPORANGIA OF ISOETES 249 
 
 shows the vacuolated appearance of the sterile cells. Shortly 
 afterwards the mother cells break away from the tapetum, which 
 from this time on gains in density and apparent activity. The 
 mother cells, at first in a continuous mass, soon break up into 
 smaller and smaller groups of cells by the enlargement of the 
 cavity in which they float. Finally the individual cells fall apart 
 and round up, and pass rapidly through the two divisions by 
 which the microspores are formed. 
 
 No attempt was made to follow closely the cytology of these 
 divisions because it was found impossible to make any satisfac¬ 
 tory observations on the corresponding divisions of the mega¬ 
 spore mother cells. The following notes may however be of 
 interest. The achromatic figures appear to have a polycentric 
 origin, and the chromatin passes through a synapsis stage. All 
 the nuclei make their preparation for division and begin to 
 divide almost simultaneously, and this notwithstanding their 
 immense number. It is possible to find a better series of 
 karyokinetic figures in a single sporangium of many ferns, 
 where there are but sixteen mother cells, than in an Isoetes 
 microsporangium where the mother cells number three or four 
 times as many thousand. This 1 think may be regarded as an 
 additional proof of the growth of the sporangium as a unit, and 
 not as an aggregation of segments. 
 
 In the majority of cases the two divisions are of the type 
 which is characteristic of cycads and monocotyledons, and has 
 been called “successive;” that is, the first division of the nucleus 
 is followed by the formation of a cell wall before the immedi¬ 
 ately following division of the daughter nuclei {fig. 3j). The 
 spores in this case are bilateral and may have their nuclei in one 
 plane or in two planes at right angles to each other. But it is 
 not at all infrequent to find the divisions of the simultaneous 
 type; that is, the first division of the nucleus is not attended by 
 cell division, but before a wall is formed between the daughter 
 cells each new nucleus begins its second division {fig. 34). In 
 this case the spores may be of the bilateral type, as in fig. 33 
 a and b , or they may be tetrahedral as in fig. 33 c. Much 
 
250 BOTANICAL GAZETTE [april 
 
 diversity may be found within a single sporangium. Figs. 53 a, 
 b , c, and 54 a , b, c , were all taken from the same section of the 
 same sporangium. Probably the variation in this respect is not 
 of great importance except as indicating that the divisions of 
 Isoetes have not acquired so definite and settled a character as 
 those of most other plants. 
 
 Although the nuclei of the young spores may arrange them¬ 
 selves in typical tetrahedral fashion, there is an important differ¬ 
 ence between their relation here and in the tetrahedral divisions 
 of dicotyledons, Lycopodium, etc. In these it is well known that 
 all four nuclei (of such a stage as fig. 54) become connected by 
 spindle fibers, and that the walls separating the spores are 
 formed in connection with the thickening of the cell plates of 
 the six spindles. In spite of careful search I have been unable 
 to find in Isoetes any such sextuple spindles. The daughter 
 nuclei are connected only in pairs, as in fig. 33 or 34. In what 
 way the spore walls originate in such cases I cannot conjecture. 
 It seems certain they are not formed in connection with the 
 achromatic figures, unless it is possible that the cell plate, which 
 is always present in the first division, may make its influence felt 
 later on, and ultimately serve as the basis of the wall. 
 
 The young tetrads soon fall apart, and the individual spores 
 lose their angularity and round up, still retaining traces, how¬ 
 ever, of the bilateral shape impressed upon them by their 
 manner of origin. When once the permanent form is assumed 
 there is little further increase of size. The mature spores of 
 fig. 36 are little larger, it will be seen, than the newly formed 
 spores of fig. 51. 
 
 An interesting phenomenon in connection with the micro¬ 
 spores is the extreme smallness of their nuclei in comparison 
 with those of the mother cells. One would naturally expect the 
 relative volumes to be about 1:4, or the relative diameters to be 
 about 3 : 5 (since ^ \ = f nearly). But the volume of the micro¬ 
 spore nucleus is really no more than one twelfth of this esti¬ 
 mate ; or to express the comparison in another way, it would 
 need the nuclei of fifty microspores combined to equal the 
 
1 9 oo] SPOROPHYLLS AND SPORANGIA OF ISOETES 251 
 
 volume of one mother cell nucleus. Very likely similar reduc¬ 
 tions in the volume of the microspore nuclei occur during the 
 tetrad division of other plants, but I have not seen any other 
 case where the disparity of size is so great, nor do I remember 
 to have read any record of such a reduction. 
 
 The number of spores formed within a microsporangium is 
 enormous — much greater than in any other living plant. In some 
 species it is said to exceed a million. But the largest number I 
 have found in /. echinospora is 300,000. My estimates place the 
 average number from 150,000 to 250,000. 
 
 As is well known, no provision is made for the dehiscence of 
 the sporangium wall. The spores are set free only by the decay 
 of the tissues enclosing them. 
 
 FURTHER DEVELOPMENT OF THE MEGASPORANGIUM. 
 
 My observations on the development of the megasporan¬ 
 gium differ very much from those of previous investigators, so 
 very much, indeed, that I would be loath to present them at all 
 had I not confirmed them again and again by long and careful 
 study. These differences are concerned not only with the origin 
 •of the archeSporium and early growth of the sporangium, 
 which have been already spoken of, but they involve also the 
 manner of selection of the mother cells and the origin and 
 behavior of the tapetum. A discussion of the points at issue will 
 be reserved until the general history of the megasporangium has 
 been considered. 
 
 One of the first megasporangia which I sectioned presented 
 the appearance shown diagrammatically in fig. 6j . The two large 
 cells M and M are evidently megaspore mother cells, but what 
 is the group of cells a , corresponding to them in outline and 
 position ? It consists of six cells in all, three in the section 
 under examination, and three others in the adjacent section. A 
 little search discovered other similar groups of a variable num¬ 
 ber of cells, sometimes but two or three, often five or six. If 
 the number had been constantly four the groups might have 
 been regarded as spores resulting from a precocious division of 
 
252 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 the mother cells. But that explanation being precluded it 
 became necessary to determine their relation to the single large 
 mother cells, and to learn their later and earlier history. In 
 attempting to do so I have become convinced that a very large 
 number of cells are potentially megaspore mother cells, that a 
 considerable number of these make a start to differentiate them¬ 
 selves fully from the sterile cells, but that comparatively few 
 are finally successful in reaching the large size and well-nour¬ 
 ished condition necessary for the production of megaspores. 
 
 The changes which first distinguish the megasporangium 
 occur relatively earlier than those which mark the microsporan¬ 
 gium. In the latter, as we have seen, the first change is the 
 separation of certain sterile regions from the fertile cells as indi¬ 
 cated by a difference in cell contents. In the former, however, 
 changes occur at a considerable time before there is any possibil¬ 
 ity of distinguishing the trabeculae. When the megasporangium 
 has reached a stage of development considerably more advanced 
 than that shown in fig. 43, a change is discernible in many of the 
 cells which form the third and fourth layers approximately. 
 The whole sporangium has at this time entered upon the period 
 of enlargement due to the growth of the individual cells. But 
 in fig, 63 it is clear that certain cells have greatly outgrown 
 their fellows. Their well-nourished condition is attested by the 
 density of their cytoplasm and their large nuclei, which contain 
 many nucleoli. All these enlarged cells are engaged in the 
 struggle to become mother cells. Which and how many will be 
 successful will probably depend upon their holding an advan¬ 
 tageous position with respect to the supply of nutriment, pei- 
 haps also to their having obtained an earlier start. 
 
 It does not always happen that a considerable group of cells, 
 enlarge together. Indeed, it is a comparatively rare case when 
 all the cells of the third and fourth layers enlarge to any con¬ 
 siderable extent. Sometimes the enlarging cells are in more or 
 less isolated groups separated by cells of ordinary size. Fig. 
 64 shows such a group of cells, taken from the side of a spo- 
 rangium. 
 
igoo] SPOROPHYLLS AND SPORANGIA OF ISOETES 253; 
 
 Quite often, too, it happens that one cell gets the advantage 
 almost from the beginning. But it may be stated as the rule 
 that there is a selection and partial enlargement of many more 
 cells than can ultimately become mother cells, and these enlarg¬ 
 ing cells belong mostly to the third and fourth layers of the 
 sporangium, either extending continuously across the sporan¬ 
 gium or occurring in groups separated by ordinary cells. That 
 this condition is associated with the selection of megaspore 
 mother cells is proved, I think, by the fact that enlarging cells.,, 
 comparable to those of fig. 6j , are never found in the sporangia 
 formed late in the season, that is, in those which are to bear 
 microspores. 
 
 What becomes of the defeated cells ? This is a difficult ques¬ 
 tion to answer, for since there is so much variation in the early 
 condition of the megasporangium it is impossible when examin¬ 
 ing one of the later stages to tell just what the antecedent con¬ 
 ditions in that sporangium may have been. From the frequency 
 with which karyokinetic figures appear in the cells surrounding 
 the nearly mature megaspore mother cells, it seems pretty cer¬ 
 tain that the cells which have been left behind in the struggle 
 simply divide until their products have the general size and 
 appearance of the other cells of the sporangium. If the enlarge¬ 
 ment has not gone very far the cells retain their angular con¬ 
 figuration ; if it has gone further the cells may round up while 
 exerting a considerable pressure on those adjacent. So I inter¬ 
 pret the group c in fig. 6j. 
 
 Fig. 66 will furnish a good illustration of the behavior of the 
 unsuccessful mother cells, although no single section can be so 
 convincing as a series of them. The tissues are somewhat con¬ 
 tracted, but this defect does not hide the rounded form of cer¬ 
 tain groups of cells, and their marked resemblance, except in 
 being multicellular, to the mother cells. The section contains 
 but one fertile mother cell, the one labeled m. One other is 
 situated in the opposite end of the sporangium, just beyond the 
 limit of the figure. The cell a is undergoing division, the mitotic 
 figure being seen in the adjacent section. An interesting fact 
 
254 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 which goes far to explain the division of the groups b, b , is the 
 occurrence in adjacent sections of larger undivided cells (fertile 
 mother cells), similar to m, and so situated as to be almost or 
 quite in contact with the dividing groups. Their proximity 
 accounts for the failure of the groups b, b , to produce spores. 
 Some of the smaller and less rounded groups probably repre¬ 
 sent mother cells which suffered an early defeat, while the 
 larger groups represent those which held out almost to the last. 
 Such cases as these, which can be easily duplicated in rapidly 
 growing sporangia of the right age, are conclusive, it seems to 
 me, when considered in conjunction with the manner of growth 
 of the sporangium, to show that the fertile mother cells are 
 selected by their advantageous environment and not by any 
 strict morphological position. 
 
 The fertile mother cells increase enormously in size before 
 dividing into spores. Their nuclei maintain a proportionate 
 growth, and their cytoplasm remains dense though not homog¬ 
 eneous, and frequently contains grains of starchy matter and 
 drops of oil. 
 
 Notwithstanding the large size of the mother cells and of 
 their nuclei I was unable to make any detailed study of their 
 division. About the time when division occurs, the cells seem 
 to be peculiarly liable to suffer plasmolysis, for under the action 
 of the fixing agent they are contracted to a mere fraction of 
 their proper volume. When sectioned in this condition they are 
 seen to lie free in large cavities which presumably they filled 
 completely when living, and they stain so intensely that it is 
 impossible to make out any details of the karyokinetic process. 
 I have not once had the good fortune to see karyokinesis in an 
 uncontracted megaspore mother cell, although the corresponding 
 phase of the microsporangium offers no technical obstructions 
 to cytological study. The liability of the megaspore mother 
 cells to suffer contraction in the process of fixation was noticed 
 by Kienitz-Gerloff (i) and other investigators; it is possibly 
 associated with the entrance of the nuclei into the synapsis 
 stage. 
 
SPOROPHYLLS AND SPORANGIA OF 1 S 0 ETES 
 
 255 
 
 1900] 
 
 The young megaspores almost invariably have the tetra¬ 
 hedral arrangement, as in fig. 5Q. Occasionally the bilateral 
 arrangement is found, in which case the divisions so far as 
 observed are successive (figs. 60 , 61). 
 
 The further growth of the megaspores, the manner in which 
 their walls are laid down, and the storing of reserve material, 
 were not investigated. 
 
 The arrangement and subsequent development of the trabec¬ 
 ulae and tapetum of the megasporangium offer, as is to be 
 expected, a rather close homology to what is seen in the 
 microsporangium. The trabeculae are formed out of the same 
 kind of cells as compose all the other parts of the young sporan¬ 
 gium. I do not discover any grounds for considering them the 
 product of a peculiar kind of growth. They are altogether 
 unrecognizable in the young sporangium, and their position 
 when first outlined seems to be determined by that of the 
 mother cells. Not until these have been selected and con¬ 
 siderably enlarged is it possible to distinguish the trabeculae, 
 which then appear as feebly-staining bands extending from 
 front to back across the sporangium midway between the fertile 
 cells. 
 
 The cells of the trabeculae proper undergo the same process 
 of elongation and flattening, attended by elongation of their 
 nuclei, that has been described as occurring in the microsporan¬ 
 gium. The only noticeable difference is that in the megasporan¬ 
 gium the trabeculae are relatively fewer in number and more 
 massive. For example, in one cas^, an exceptional one, I 
 counted 400 cells in a cross section of a trabecula, whereas in a 
 microsporangium the number of cells in a cross section of a 
 trabecula rarely exceed fifty, and is oftener under twenty-five. 
 This is only another way of saying that the process of steriliza¬ 
 tion has gone much further in the megasporangium than in the 
 microsporangium. The total mass of the megaspore mother cells 
 in a sporangium is only a small fraction of that of the combined 
 microspore mother cells, though doubtless the total volume of 
 the mature spores in the two cases is about equal. 
 
2 56 
 
 BOTANICAL GAZETTE 
 
 [APRIL 
 
 The tapetum is formed in this case also out of those layers 
 of the sterile cells which border upon the fertile cells. No doubt 
 a considerable part of it is derived from the unsuccessful mother 
 cells; but as these are the homologues of the trabecular cells of 
 the microsporangium, being merely sterile sporogenous cells, the 
 homology of tapetum and trabeculae in the two sporangia is 
 complete. The only difference which it is necessary to notice 
 is the greater abundance of the tapetum in the megasporangium. 
 Instead of being a single layer it is several layers in thickness 
 {figs. 57, 58), and often projects into the sporangial cavity 
 in the form of irregular papillae, especially from the base of the 
 sporangium. A rounding up of the cells immediately about the 
 megaspore mother cells, such as is described and figured by 
 Goebel, I was never able to find. 
 
 Though the megaspore mother cells do not lie in contact 
 with one another as the microspore mother cells do, but are 
 isolated in groups of one or sometimes two, the cavities in which 
 they lie become continuous in the older sporangia. This is 
 brought about by a very great enlargement of the cavities after 
 the formation of the spores. The enlargement seems to be due 
 to turgescence, induced probably by the osmotic activity of the 
 substances surrounding the spores. It cannot be accounted for 
 by mere growth of the wall cells, nor by that of the young 
 spores, for these do not completely fill the cavities. I have 
 computed the enlargement of the megasporangium after all cell 
 divisions have ceased to amount to an increase of three or four 
 times in volume. A similar change of size, though less in 
 extent, occurs in the microsporangium. 
 
 If the preceding account of the development of the spo¬ 
 rangia, especially of the megasporangia, be compared with the 
 account given by Goebel (1) and Sadebeck ( 1), it will be seen that 
 the differences are considerable, and of much theoretical impor¬ 
 tance. According to these writers certain cells of the arche- 
 sporium divide only by the periclinal walls which serve to 
 cut off the primary tapetal cells. In these no anticlinal divisions 
 occur. One cell of each of the rows formed in this manner, 
 
1 9 oo] SPOROPHYLLS AND SPORANGIA OF ISOETES 257 
 
 apparently the innermost one, though that point is not made 
 clear in the descriptions, becomes the megaspore mother cell. 1 
 In certain other archesporial cells divisions take place in all 
 planes, but more particularly in the anticlinal direction. The 
 products of these latter cells give rise to the trabeculae. Vines 
 in his text-book gives nearly the same description, but says 
 that the archesporial cell from which the megaspore mother cell 
 arises undergoes but a single division. 
 
 If the assertion be correct that certain archesporial cells 
 develop only into trabeculae and certain others only into mother 
 cells and tapetum, it is clear that there must be two categories 
 of archesporial cells, one set destined to become sterile, the 
 other to become fertile; and these, although indistinguishable in 
 appearance and size, are quite unlike in their mode of division 
 and growth and in the ultimate fate of their derivatives. It is 
 impossible, too, to escape the inference that the megaspore 
 mother cells are already determined in position and number 
 when the sporangium has got no further in its development than 
 to the differentiation of an archesporium. Further, the spo¬ 
 rangium must be regarded as compound, each fertile archesporial 
 cell representing a separate sporangium, and each sterile one an 
 imperfect wall. These conclusions, which I think are logical and 
 necessary deductions from Goebel’s description, are all incon¬ 
 sistent with the development of the sporangium as I have found 
 it in I. echinospora. 
 
 In order to bring out the points of contrast more clearly 
 I will summarize them. I do not find any difference among 
 the archesporial cells either in manner of development or of 
 growth. I find no flattened tapetal cells overlying the megaspore 
 mother cells. I find no grounds whatever for the assertion that 
 each archesporial cell follows an independent growth, or that 
 each megaspore mother cell represents one archesporial cell. I 
 do not even find a single definite hypodermal archesporium 
 which can stand as the starting point of the inferences above 
 enumerated. On the other hand, I find the derivatives of all 
 
 *See, however, Schenck’s Handbuch 3 : 392. 
 
2 5 8 BOTANICAL GAZETTE [april 
 
 the archesporial cells dividing in various planes, and blending 
 indistinguishably. The sporangium is single, not multiple, and 
 the megaspore mother cells are not morphologically predeter¬ 
 mined but are physiologically selected from among a large num¬ 
 ber of potentially sporogenous cells. 
 
 Though the certainty of the matter must depend upon 
 observation, it may be pointed out that the number of mega¬ 
 spores has a bearing upon the question. A megasporangium 
 contains from 150 to 250 megaspores. If we take 200 as the 
 average, it represents fifty mother cells, that is, according to the 
 current view, fifty archesporial cells. To this we must add at 
 least fifty others for the trabeculae, giving a total of one hundrc d 
 .archesporial cells. It does not need a very careful examination 
 of I. echinospora to demonstrate the impossibility of there being 
 so large an archesporium, for when the sporangium has a super¬ 
 ficies of one hundred cells it is far past the archesporial stage. 
 It is, I think, absolutely certain that each archesporial cell gives 
 rise to several megaspore mother cells, as well as to trabeculae 
 and tapetum. In the microsporangium, too, the trabeculae alone 
 outnumber the archesporial cells (cf figs. ji } 46 ); and their 
 extreme irregularity and frequent branching and anastomosis 
 make their origin each from a single cell exceedingly improbable. 
 
 [To be concluded .] 
 
SMITH on ISOETES 
 

 
 « : 
 
 LIBRARY 
 
 OF THE 
 
 UNIVERSITY e» ILLINOIS. 
 
 
 , / 
 
 
 
BOTANICAL GAZETTE, XXIX 
 
 PLATE XIV 
 
 SMITH on ISOETES 
 
/ 
 
 
 LIBRARY 
 
 
BOTANICAL GAZETTE, XXIX 
 
 PLATE XV 
 
 SMITH on ISOETES 
 
LIBRARY 
 
 UN Wrr T tfiu.iN0is. 
 
07 AN 1 CAL GAZETTE, XXIX 
 
 SMITH on ISOETES 
 

 LIBRARY 
 
 UNIVERSITY^ ILLINOIS. 
 
BOTANICAL GAZETTE, XXIX 
 
 PLATE XVII 
 
 SMITH on ISOETES 
 
muSB'**' 
 
 
BOTANICAL GAZETTE, XXIX 
 
 PLATE XVIII 
 
 SMITH on ISOETES 
 

 k 
 
 
 
 
 
 
BOTANICAL GAZETTE, XXIX 
 
 PLATE XIX 
 
 SMITH on ISOETES 
 
UiiRr\iiY 
 
 UNIVfcflWTY rf VU-VHOIS. 
 
BOTANICAL GAZETTE, XXIX 
 
 PLATE XX 
 
 SMITH on ISOETES 
 

 LIBRARY 
 
 OF THE 
 
 UNIVERSITY of ILLINOIS. 
 
 
 
THE STRUCTURE AND DEVELOPMENT OF THE 
 SPOROPHYLLS AND SPORANGIA OF ISOETES. 
 
 CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY. 
 
 XVIII. 
 
 R. Wilson Smith. 
 
 (with plates xiii-xx) 
 
 ( i Concluded from page 258) 
 
 THE SUCCESSION OF SPOROPHYLLS. 
 
 The order of succession of the sporophylls is subject to some 
 variation. It is not at all uncommon, especially in I. Engel- 
 tnanni , to find the regular sequence interrupted by the occurrence 
 of several megasporophylls among the microsporophylls. Occa¬ 
 sionally, also, sporangia are found containing both megaspores 
 and microspores. This is rarely the case in wild plants, though 
 quite common, along with other irregularities, in those cultivated 
 in the laboratory. Some plants taken in December, after grow¬ 
 ing rapidly for seven or eight months in the laboratory, had 
 formed only megasporangia; some others, though producing a 
 few microsporophylls, had failed to bring any microspores to 
 perfection. 
 
 The sterile leaves of I. echinospora differ from the fertile ones 
 chiefly in their smaller size, the reduction of the sheathing base, 
 and the absence of a developed sporangium. They remain green 
 throughout the winter; while the sporophylls, set free by decay 
 of the base and buoyed up by the gas within the numerous air 
 cavities, are borne away by currents or waves. A close study of 
 the sterile leaves almost always reveals the presence of aborted 
 sporangia. These range in size from a few to many hundred 
 cells ; they are often of irregular shape and have lost their pro¬ 
 toplasmic contents, though now and then one is found in which 
 a few spores have matured. A longitudinal section of a typ¬ 
 ical sterile leaf is shown in fig. 62 , in which the shaded part 
 1900] 323 
 
3 2 4 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 represents the undeveloped sporangium. The sterilization of the 
 sporangium does not affect the development of the velum, a 
 fact which supports Hofmeister’s view of the primary separation 
 of velum and sporangium. The occurrence of aborted sporangia 
 on so many of the sterile leaves shows that all the leaves are 
 potentially sporophylls, and suggests the probability that Isoetes 
 has retained a more primitive form of the sporophyte than any 
 other vascular plant. 
 
 HOMOLOGY OF THE ARCHESPORIUM. 
 
 The term “ archesporium ” was first employed by Goebel (i), 
 who defined it as a cell, cell-row, or cell-plate, from which all 
 the spore-producing cells are formed, and who concluded that in 
 all sporangia the archesporium occupies a hypodermal position. 
 Allusion has been made to the difficulty of accepting this con¬ 
 clusion in such a case as that of Isoetes, but the difficulty is not 
 peculiar to Isoetes. Bower has shown that in several pterido- 
 phytes (Selaginella, Equisetum, Lycopodium) the archesporium 
 is not delimited by the first periclinals of the outer layer. How 
 shall we define the archesporium in cases where there is no 
 single hypodermal layer from which the whole mass of sporog- 
 enous tissue is derived, and to which the term can be correctly 
 applied as required by etymology and definition ? We must 
 either modify our conception of the archesporium or abandon 
 the term altogether as failing to express the facts. It appears 
 to the writer that by changing our notion of the necessary posi¬ 
 tion of an archesporium we could not only avoid this difficulty 
 but would also be enabled to make a more consistent compari¬ 
 son of the sporangia of seed-plants and pteridophytes than is 
 possible with the present nomenclature. 
 
 It is pretty generally recognized that there is no true epi¬ 
 dermis in pteridophytes. The so-called epidermis is physiologi¬ 
 cally but not morphologically equivalent to that of seed-plants, 
 for a true epidermis is traceable to a primary layer of the embryo, 
 the dermatogen, which is distinctly present only in seed-plants. 
 As the dermatogen is not represented in pteridophytes, unless it 
 
1900 ] SPOROPHYLLS AND SPORANGIA OF ISOETES 325 
 
 be in the root-tip, there is consequently no true epidermis, and 
 the part which performs the functions of an epidermis is the 
 outermost layer of the periblem derivatives. Thus, while sper- 
 matophytes have three embryonic tissue regions, the plerome, 
 the periblem, and the dermatogen, pteridophytes, with the possible 
 exception of the root-tip, have only the two first-named regions. 
 
 The archesporium of all spermatophytes is hypodermal. In 
 no case is it known to be derived from the superficial cells. The 
 epidermis is from the beginning distinct from the inner cells of 
 the sporangium. Though there may be periclinal divisions in 
 the superficial cells, as in gymnosperms, many Ranunculaceae, 
 etc., these occur only after the differentiation of the archespor¬ 
 ium, and the cells so added merely increase the thickness of the 
 wall or apex of the sporangium, but never become part of the 
 sporogenous complex. The true epidermis, set apart at a very 
 early period from the inner tissue of the embryo, is incapable of 
 producing spore-forming cells. That role is played by special 
 cells of the outer layer of the periblem. 
 
 It is otherwise with the “hypodermal archesporium” of 
 pteridophytes. This is cut off by periclinal divisions from a 
 superficial cell in the case of ferns and Equisetum, and from a 
 group of such cells in the case of the Lycopodiales and Isoetes. 
 
 But if the absence of a true epidermis in pteridophytes and 
 the homology of periblem with periblem in all vascular plants 
 are conceded, then the hypodermal cells of spermatophytes are 
 comparable, not to the hypodermal cells of pteridophytes, but 
 to the superficial cells. As we have seen, the sporogenous mass 
 in pteridophytes can always be traced to superficial cells, but in 
 spermatophytes to hypodermal cells and no further ; that is, in 
 all cases the function of giving rise to spore-producing cells is 
 localized in certain cells of the outer layer of the periblem. 
 
 These facts serve to show the inconsistency of undertaking 
 to limit the archesporium to a hypodermal position in all cases. 
 The cell or group of cells whether superficial or hypodermal, to 
 which in a last analysis all the sporogenous portion of a sporan¬ 
 gium can be traced, ought to be called the archesporium. The 
 
3 2 6 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 change of nomenclature which I would propose, therefore, refers 
 only to the position of the archesporium, and not at all to the 
 meaning of the term. It is simply this: let the term archespo¬ 
 rium continue to be used as at present in descriptions of seed- 
 plants, but let it be understood in the case of pteridophytes to 
 signify the superficial cell or cells from which the spore-forming 
 tissue is derived. In this way the difficulties which have been 
 pointed out will all be met, and a better system of homologies 
 can be made for sporangia in general. The archesporium will 
 always occupy the same position relative to the primary meri- 
 stematic regions, and will be the only part from which the spore¬ 
 forming tissue arises. 
 
 The nomenclature here proposed can be more easily under¬ 
 stood by reference to the accompanying diagrams. Figs. 69 and 
 jo represent two early stages of the sporangium of a common 
 fern (Pteris), but for the present purpose may be taken as typ¬ 
 ical of any pteridophyte sporangium. The shaded cell ( a ) would 
 be regarded as the archesporium ; it divides into an inner fertile 
 cell (/), from which all the spore mother cells are derived, and 
 an outer cell (5) which gives rise to a large part of the sporan¬ 
 gium wall. In some instances, as we have seen, the separation 
 into fertile and sterile cells is not accomplished by the first divi¬ 
 sion. In such cases there is no contradiction of terms, since all 
 the spores arise from the archesporium. The final condition is 
 the same in all cases, the difference consisting simply in the 
 earlier or later sterilization of the wall region. 
 
 Fig. ji represents a young microsporangium and fig. jj a 
 young megasporangium of an angiosperm. The outer layer ( e ), the 
 epidermis, takes no part in the formation of the spore producing 
 cells ; a is the archesporium, which usually, as in pteridophytes, 
 divides into an outer sterile region (5) called the primary tape- 
 turn, and an inner fertile region (/) called the primary sporog- 
 enous cell or cells [figs. J2 , J4). The name primary tapetum 
 was given to the sterile region to express its supposed function 
 of giving origin to the true or functional tapetum. Enough is 
 now known of the origin of the true tapetum to enable us to say 
 
1900 ] SPOROPHYLLS AND SPORANGIA OF IS 0 E 7 ES • 327 
 
 it has no definite relation to the primary tapetum, and that in 
 fact the term “primary tapetum” is a misnomer. The true 
 tapetum, in many cases at least, is not represented by any mor¬ 
 phological structure in the young sporangium. 
 
 Is it not possible that the cells s s of figs. J2, represent the 
 wall layer oi fig. 70, and that the “primary tapetum,” in addition 
 to the protective and sometimes nutritive purpose which its 
 derivatives subserve, has also a phylogenetic meaning as a sur¬ 
 vival of the pteridophyte sporangium wall which has been in 
 great part replaced by the true epidermis ? Such at least is the 
 view suggested by a comparison of embryonic organs in general, 
 and of the relations of the primary sporogenous cells. 
 
 RELATIONS OF THE VELUM. 
 
 On the question whether the velum has any homologue 
 among other plant structures my observations do not furnish 
 any information. It has been compared on the one hand 
 with the indusium of ferns, and on the other hand with the 
 integument of an ovule. The possibility of the latter relation 
 certainly has not been disproved, but the evidence for it is so 
 scant that it must remain merely an interesting suggestion. As 
 to the other relationship, it ought to be borne in mind that the 
 only ferns which can be at all closely related to Isoetes are the 
 euspoiangiate families, and all of these bear naked sporangia. 
 The indusium appears in fact to be a special organ of the higher 
 leptosporangiate ferns, without representation in the lower fami¬ 
 lies, such as the Osmundaceae or in the eusporangiates. This 
 absence of an indusium in the intermediate orders, and the 
 doubtfulness of the homology of the various outgrowths known 
 as indusia, make it impossible to regard the velum and indusium 
 as more than homoplastic structures. 
 
 THE AFFINITIES OF ISOETES. 
 
 The systematic position of Isoetes has been discussed again 
 and again. By Linnaeus it was placed among the vascular crypto¬ 
 gams, where most later taxonomists have been content to leave it. 
 During the first half of the present century it was most frequently 
 
328 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 grouped with the Marsiliaceae and Salviniaceae, chiefly on the 
 grounds of their heterospory and hydrophytic habit. DeCan- 
 dolle was the first to suggest a connection with Lycopodium. In 
 this view he was followed by Brogtiiart, Endlicher, Hofmeister, 
 and the later German botanists. A summary of the various 
 relationships which have been assigned to Isoetes was given in 
 1888 by Vines (i),who in the same article put forth the opinion 
 that its affinities are with the eusporangiate ferns, rather than 
 with the Lycopodiales. More recently Farmer (1) and Camp¬ 
 bell (4) have expressed their concurrence with this disposition 
 of the genus. 
 
 Since this classification has been retained by Vines in his 
 Text-Book of Botany, and adopted by Campbell in his Mosses and 
 Ferns , it will not be unprofitable to re-examine the evidence, 
 with the purpose of seeing what light can be thrown upon the 
 subject by the present and other recent investigations. 
 
 In any discussion of relationships, and especially when there 
 is so great diversity of opinion as in the present case, the con¬ 
 clusion is likely to be a personal one merely, dependent on the 
 kind of evidence which the examiner holds most weighty, rather 
 than on its absolute nature. There are some general principles, 
 however, to which everyone will probably assent, and which 
 ought to govern one in estimating the relative value of the con¬ 
 flicting evidence on which the taxonomist relies. In the first 
 place, the larger the number of characters in which there is 
 agreement, the closer is the relationship, especially if the charac¬ 
 ters are such as are known to have great taxonomic value in 
 groups related to the one under consideration. Of single 
 characters, those which are most constant are of most value, 
 even though we are not able to detect their special utility. It is 
 generally accepted, too, that those characters which appear in 
 the embryonic stages of an organism serve best to mark its 
 wider relationships, as of class or family, while characters which 
 do not display themselves till later in the individual life are 
 better adapted to distinguish the near relationships of species 
 and genus. This principle applies not merely to the organism 
 
1900] 
 
 SPOROPHYLLS AND SPORANGIA OF ISOETES 
 
 3 2 9 
 
 as a whole, but quite as fully to the embryonic stages of its dif¬ 
 ferent organs, such as leaf, root, sporangium, and the like. 
 
 In conformity with these principles it is proper, in the deter¬ 
 mination of natural affinities, to place great emphasis upon the 
 reproductive parts, for such parts are found to show very great 
 constancy in their form and occurrence. The sporangia espe¬ 
 cially, and the form and arrangement of the sporophylls, have 
 long been recognized as of the highest importance. Thus, the 
 classification of the Filicales is largely based on sporangial 
 characters ; and the position of the Salviniaceae and Marsiliacese, 
 which was formerly as unsettled as that of Isoetes, was estab¬ 
 lished beyond doubt as soon as the development of the sporan¬ 
 gia was fully understood. 
 
 It is chiefly on the basis of the superficial resemblances of 
 the sporangia of Isoetes with those of Lycopodium and Selagi- 
 nella that it has been so long associated with them. If we 
 enumerate the chief differences between the sporangia of Lyco- 
 podiales and of ferns, we shall see that in every particular Isoetes 
 agrees with the former. While the Filicales bear numerous 
 sporangia on the dorsal surface of the leaf, Isoetes and the 
 Lycopodiales, with the exception of the Psilotaceae, the exact 
 relation of whose sporangia to the leaf is still in dispute, bear 
 but one sporangium to a sporophyll, and that on the ventral 
 surface at the base. Such exceptional forms as the Ophioglos- 
 saceas and Marsiliaceae do not help us in this inquiry. Though 
 it may be true that the whole sporangiophore of the Ophioglos- 
 saceae is, as several morphologists have suggested, the homo- 
 logue of the single sporangium of Lycopodium or Isoetes, the 
 suggestion is so hypothetical in itself as to give no support to 
 any view based upon it. It is only in the position of the sporan¬ 
 gium that these families approach Isoetes ; in other sporangial 
 characters, such as number and development, they are like other 
 ferns. 
 
 The relative age of the leaves, when the sporangial rudi¬ 
 ments first make their appearance, is of considerable significance. 
 In the Filicales, with the exception of the heterosporous forms, 
 
33° 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 which being leptosporangiate cannot be closely related to 
 Isoetes, the sporangia appear late in the history of the leaf. 
 There is an enormous development of the midrib with its con¬ 
 ductive tissues, and of the expanded pinnae, before the sporan¬ 
 gia are recognizable. Nothing is more striking, however, than 
 the quickness with which in the Lycopodiales and Isoetes the 
 rudiment of the sporangium follows the inception of the leaf, 
 which when the sporangium first comes into view is no more than 
 a mere papilla of undifferentiated tissue, without a sign of photo¬ 
 syntactic or conductive tissue. 
 
 Still more far-reaching is the agreement of Isoetes with the 
 Lycopodiales in the character of the sporangium rudiment. 
 Goebel (i) in his celebrated paper of 1880-1 classified sporangia 
 as leptosporangiate or eusporangiate according as they arise 
 from single cells or from groups of cells. Though the two 
 classes are connected by transitional forms, such for instance as 
 the Osmundacese, in which the sporangia, though always classi¬ 
 fied as leptosporangiate, do not arise strictly from single cells, 
 the distinction has been approved by all later morphologists. 
 The leptosporangiate plants make a well-defined and consistent 
 group, but the eusporangiates comprise very diverse forms, 
 including the several divisions of seed-plants, the Lycopodiales, 
 the Equisetales, and part of the Filicales. If, however, we leave 
 out accessories, and turn our attention entirely to the essential 
 part of the sporangium, that is to the sporogenous tissue, we 
 find a distinction which has the merit of leaving the Filicales 
 an unbroken group, and of agreeing closely with what is required 
 by a consideration of other characters. This distinction pertains 
 to the origin of the archesporium. The spore-forming part of the 
 sporangium of Isoetes and Lycopodiales can be traced back to a 
 number of cells placed transversely to the leaf, but of all other 
 pteridophytes to a single cell. Is not this distinction as valid as 
 that which pertains to the origin of the whole sporangium? If 
 so, it tends strongly to justify the inclusion of the Lycopodiales 
 and Isoetes within a distinct group set apart from all other vas¬ 
 cular cryptogams. 
 
igooj SPOROPHYLLS AND SPORANGIA OF ISOETES 33 1 
 
 Certain other features of the sporangium of Isoetes find 
 duplication only among members of the Lycopodiales. In all 
 the higher leptosporangiate ferns there is an elaborate mechan¬ 
 ism for the bursting of the sporangium and the scattering of the 
 spores. This device, consisting of a row of peculiarly thickened 
 cells (the annulus), and a group of cells which form an easy 
 place of rupture (the stomium), is very rudimentary in the 
 lower leptosporangiates (Osmundacese), and in the Ophioglos- 
 saceae and Marattiaceae, but it is not altogether absent. There 
 is at least a predetermined line along which dehiscence shall 
 take place. The elaboration of this dehiscence apparatus is one 
 of the chief peculiarities of the higher leptosporangiates. When 
 we turn to the Lycopodiales and Isoetes, however, we find posi¬ 
 tively no contrivance for dehiscence, and no vestige of an 
 annulus or stomium. The sporangium wall is simple, and 
 bursts by desiccation in Lycopodium and Selaginella, and by 
 decay in Isoetes; and neither method can be regarded as a 
 specialization. 
 
 Another analogy has been brought to light by Bower’s dis¬ 
 covery in Lepidostrobus of certain radiating strands or processes 
 in the sporangium which are regarded by him as very probably 
 of the nature of trabeculae. Since the relationship of Lepidostro¬ 
 bus to Lycopodium can hardly be doubted, there is here a point 
 of contact with this group of plants in a feature in which other¬ 
 wise Isoetes stands alone. 
 
 Again, Selaginella and Isoetes agree very nearly in the man¬ 
 ner of selection of the megaspore mother cells. The unselected 
 mother cells do not divide at all, and all the spores resulting 
 from the division of the fertile ones as a rule reach maturity. 
 In heterosporous ferns all the mother cells divide into spores, of 
 which but one becomes a megaspore. The contrast may be 
 expressed in the statement that the megasporangium is differ¬ 
 entiated in Isoetes and Selaginella before the tetrad division, but 
 in heterosporous ferns not until after that division. 
 
 The persistence of the tapetum in Lycopodiales and Isoetes 
 is a character to which no great importance is to be attached, for 
 
33 2 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 tapetal characters are notoriously variable. Such bearing as it 
 has, however, is in harmony with what may be inferred from 
 other features of the sporangium. It involves no disorganization 
 of the cells, no multiplication of nuclei except as related to cell- 
 division, and no mingling of naked protoplasm with the young 
 spores. 
 
 One of the facts which Vines advanced as an argument 
 against the usually accepted classification of Isoetes is the 
 absence of a strobilus, the characteristic arrangement of the 
 sporophylls in the Lycopodiales. He contrasts also the elon¬ 
 gated, slender, branched stem of Lycopodium or Selaginella with 
 the short unbranched stem of Isoetes, which much more closely 
 resembles that of some eusporangiate ferns. It may be doubted 
 whether such superficial characters, unless accompanied by 
 internal features of which they are the outward expression, have 
 any value in settling the relationship of distinct genera or families. 
 At all events, their usefulness in angiosperm taxonomy is limited 
 to the distinction of species ; they would be of no use in decid¬ 
 ing the family to which an undetermined species ought to belong. 
 I am inclined to think the whole plant-body of Isoetes can best 
 be explained as a shortened strobilus, just such as Lycopodium 
 would become by suppression of the stem and axis, while allow¬ 
 ing a normal development of the leaves and sporangia. 
 
 The most obvious diagnostic character of the three groups 
 of pteridophytes is furnished by the leaves. The leaves of the 
 Isoetes are siri generis, and afford little ground for associating it 
 with any one group rather than another. Though they are rela¬ 
 tively few and large, as is the case among ferns, their 
 unbranched outlines and simple tissues show an analogy with 
 the leaves of Lycopodiales ; while their peculiar vascular bundles, 
 and chambers, and diaphragms remove them as effectually from 
 either group. There is record, it is true, of a fossil Isoetes with 
 a branched leaf, indicating, when taken in conjunction with the 
 sudden reduction of the vascular bundle just above the ligule, 
 the possibility that the present form of the leaf may be a 
 reduced one representing a more complex ancestral type. But 
 
1900] SPOROPHYLLS AND SPORANGIA OF ISOETES 333 
 
 we must admit, so far as mature leaf structures are concerned, 
 that Isoetes occupies an isolated, and in no sense an intermediate 
 position. 
 
 The testimony of the young leaves, however, is not so neu¬ 
 tral. The form of the leaf rudiments, their manner of growth, 
 and arrangement about the axes are the same in Isoetes as in 
 Lycopodium and Selaginella, and quite different from what is seen 
 among ferns. The difference is not fully expressed in saying 
 that in one case the leaf originates from a single apical cell, and 
 grows by means of it, and that in the other case the initiative is 
 from a group of cells. The leaves of ferns are distinctly acroge- 
 nous, which method of growth gives them the power of assum¬ 
 ing complex forms and allows the successive and often slow 
 formation of stipe, pinnae, and pinnules, and their gradual 
 unfolding. A leaf which grows as does that of Isoetes has its 
 power to assume a complex form limited to the time when it is 
 meristematic throughout ; as soon as the apex becomes perma¬ 
 nent tissue the outline of the leaf is determined. The difference 
 between such leaves is fundamental and far-reaching. A Lyco¬ 
 podium leaf could easily attain the size of an Isoetes leaf by 
 retaining the meristematic power for a longer time, for they dif¬ 
 fer only in degree. The leaf of a fern could become like that 
 of Isoetes, or vice versa , only by a radical change in the manner 
 of growth. 
 
 The similarity of the leaf rudiments of Lycopodium and 
 Isoetes is only a particular instance of a general likeness which 
 extends to all their embryonic organs. We have already seen 
 how this is true of the sporangia ; and it holds equally good for 
 the roots 1 and stem apex. In none of these organs is there ever 
 an apical cell or any concentric segmentation of the apices, such 
 as are characteristic of all the Filicales and Equisetum. A dif¬ 
 ference in this respect in the case of apical-growing organs, like 
 the roots and stem, may not lead to important differences in the 
 mature structures, as the variation in the stem apices of Selag¬ 
 inella suffices to show. But a comparative examination of 
 
 I Van Tieghem (i), but Bruchmann (i) entertains a different view. 
 
334 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 meristems was shown by Bower to possess considerable phyloge¬ 
 netic value, jn the case of ferns, and to lead to results which 
 agree with those arrived at by a comparison of other characters. 
 The fact that Bower has since changed his view with regard to 
 which type of fern is more primitive does not in any way lessen 
 the value of his previous conclusions. If we extend the series 
 made out by him it would be in this order : typical leptosporan- 
 giate ferns, Osmundaceae, eusporangiate ferns, Selaginella, Iso- 
 etes, and Lycopodium. In this connection the dichotomy of the 
 roots of Isoetes, Lycopodium, and Selaginella ought not to be 
 overlooked. 
 
 The ligules of Selaginella and Isoetes were by Goebel made 
 the ground for grouping the two genera into one order, the 
 Ligulatae, though the classification was recognized by its pro¬ 
 poser as merely one of convenience. In the former part of this 
 paper I have made a comparison of these organs, and expressed 
 the view that their similarity is sufficient to demonstrate their 
 homology. If this view is correct, it furnishes additional sup¬ 
 port to the relationship of Isoetes and the Lycopodiales, 
 especially in consideration of the discovery of a ligule in the 
 vegetative leaves and the sporophylls 2 of Lepidostrobus, another 
 lycopodiaceous plant. 
 
 Turning now to the gametophytes, we notice that when Vines 
 suggested the connection of Isoetes and ferns, it was supposed 
 that important differences existed between the female gameto¬ 
 phytes of Isoetes and Selaginella; but the later and more com¬ 
 plete investigations of Heinsen (i) and Arnoldi (i) have demon¬ 
 strated their close resemblance. The diaphragm of the female 
 gametophyte of Selaginella is not a true septum, and does not 
 arise as Pfeffer (i) supposed it did, by the division of the spore 
 into two cells. In both Isoetes and Selaginella, the free division of 
 nuclei, their parietal placing, and the gradual extension of cell 
 division from-the periphery to the center of the spore are the 
 same, and have no counterpart in the germination of the mega¬ 
 spores of heterosporous ferns. The gametophytes agree also in 
 the absence of chlorophyll. 
 
 2 Maslen (i). 
 
1900] 
 
 SPOROPHYLLS AND SPORANGIA OF ISOETES 
 
 335 
 
 Nearly similar evidence is furnished by the male gameto- 
 phyte. Though Belajeff (i), to whom we owe the most exact 
 investigation of the subject, says the male gametophytes of 
 Isoetes and Selaginella afford little ground for relating the two 
 genera, he. has shown several points of resemblance, such as the 
 separation of the prothallial (or rhizoidal) cell from the single 
 antheridium by a cellulose wall, and the final dissolution of the 
 non-cellulose septa of the antheridium wall, so that the sperma- 
 tozoids float free in the cavity of the spore. 
 
 Though not disposed to place much dependence as a clue to 
 the working out of phylogenetic relationships among hetero- 
 sporous plants on such structures as archegonia and antheridia, 
 which must necessarily conform more or less in shape to the 
 space in which they are confined, I find some interest in the 
 fact that Isoetes and Lycopodium are the only genera of pterido- 
 phytes in which the occurrence of more than two neck canal 
 nuclei has been reported, and that in Isoetes, as in Lycopodium 
 Phlegmaria and Equisetum, the plane of the division of the primary 
 neck canal nucleus is at right angles to the archegonium axis. 
 
 The two characters which stand most in opposition to the 
 inclusion of Isoetes in the group Lycopodiales are its multicil- 
 iate spermatozoid and the embryogeny of its sporophyte. 
 Campbell has very properly emphasized the similarity of the 
 Isoetes spermatozoids to those of ferns. It requires only a brief 
 survey of the plant kingdom to show the great constancy of the 
 form and behavior of male cells in different classes of plants. 
 Consider, for example, the non-motile spermatozoids of the Flori- 
 deae, or the biciliate spermatozoids of bryophytes. Accord¬ 
 ingly, if we still classify Isoetes among Lycopodiales, we must 
 admit that the multiciliate spermatozoids make an exception to 
 a constancy which is otherwise remarkable. Unfortunately, we 
 have only Lycopodium and Selaginella for comparison, and are 
 still in ignorance as to what the gametophytes of the other 
 genera may have to tell us. 
 
 The embryo of Isoetes finds its nearest approximation in 
 Botrychium 3 though the resemblance may be only an external 
 
 3 Jeffrey (i). 
 
336 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 one, due to the late differentiation of the embryonic organs 
 and the suppression of the stem, rather than to any deep-seated 
 likeness. However that may be, the suspensor of Lycopodium and 
 Selaginella is a positive morphological character separating them 
 from Isoetes. Probably the embryos of Isoetes and Botrychium 
 can be looked upon as generalized types, the specialization taking 
 the form in ferns of a very early demarcation of the embryonic 
 organs, and in Lycopodium and Selaginella of a suspensor. 
 
 Two other possible reasons for relating Isoetes to ferns 
 deserve a passing mention. Of the connection between the 
 velum and the indusium enough has already been said ; and of 
 the agreement of the stems of Isoetes and Botrychium it is suf- 
 ficent to say that the agreement is simply in the fact of second¬ 
 ary thickening. 
 
 The claim that Isoetes is the genus of modern pteridophytes 
 which makes the closest approach to angiosperms, particularly 
 to monocotyledons, gives it an interest quite out of proportion 
 to its numerical representation. It is not clear, however, that 
 the claim is well supported by facts. Unquestionably Isoetes 
 and Selaginella, in their heterospory, and their intrasporic and 
 reduced gametophytes, exhibit features of life history which 
 run closely parallel to that of seed-plants ; but such features 
 really foreshadow monocotyledons no more than they do other 
 seed-plants. The hypodermal archesporium, and the origin of 
 the megaspore mother cell as the lowest of a row resulting from 
 periclinal divisions of an archesporial cell—two points which my 
 observations disprove — would, if established, be as strong proof 
 of a gymnosperm as of an angiosperm attachment. Some facts 
 distinctly favor the gymnosperm connection ; these are the man¬ 
 ner of germination of the megaspore, and the method of selection 
 of the megaspore out of a large mass of potentially spore- 
 producing cells, as in Cycas, Callitris, etc., to which may be 
 added whatever favors the relationship of Isoetes with the Lyco- 
 podiales. 
 
 Professor Campbell has shown that the embryo of Isoetes 
 bears a likeness to that of a monocotyledon in having a lateral 
 
900] 
 
 SP0R0PHYLLS AND SPORANGIA OF ISOETES 
 
 337 
 
 stem apex and a terminal cotyledon, and suggests a comparison 
 with the embryo of Alisma for instance. The resemblance in 
 form is undoubtedly very close, but we ought not to overlook 
 some equally important differences. The entire absence of a 
 suspensor in Isoetes, which has been brought forward as an 
 objection to its close affinity with Lycopodium, militates quite 
 as strongly against an affinity with monocotyledons; and the 
 foot, which is particularly well developed in Isoetes, cannot 
 be said to have any clear representative in monocotyledon 
 embryos. 
 
 In general habit Isoetes has been compared to some grasses, 
 rushes, and the like ; this is a mere external resemblance in one 
 of the most adaptive features of plants, and not supported by 
 internal and essential similarities. A similar objection can be 
 raised to the comparison of the stelar regions of Isoetes and of 
 such monocotyledons as Dracaena. In the latter it is true there 
 is a secondary thickening carried on by means of an extra 
 stelar “ cambium,” but this cambium merely adds parenchyma¬ 
 tous tissue within which separate vascular bundles are organ¬ 
 ized ; there is nothing strictly comparable to the prismatic zone 
 or central xylem cylinder of Isoetes. Even were the likeness 
 much closer than it is, the peculiar stem of Dracaena, Yucca, 
 etc., is so certainly a newly acquired, and not a primitive charac¬ 
 ter, that it affords no sound reason for deriving monocotyledons 
 through an Isoetes-like type. 
 
 To one who has followed this discussion thus far it will be 
 evident that in the writer’s opinion the balance of evidence is in 
 favor of relating Isoetes to Lycopodium and Selaginella rather 
 than to eusporangiate ferns. Of course the facts are not all in 
 hand as yet, and new discoveries may materially affect the 
 aspect of the case. The facts which the present investigations 
 have brought to light certainly tend in the one direction. The 
 mode of origin, development, position, and general characters of 
 the sporangia, the development of the leaf, and the nature of 
 the ligule point to the correctness of including Isoetes among 
 the Lycopodiales; while the form of the spermatozcids and 
 
338 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 embryo show the necessity of making it a separate family. The 
 Lycopodiales so constituted comprise six genera pretty widely 
 separated in morphological characters, as from the antiquity of 
 the group one might naturally expect. But the extreme differ¬ 
 ences are not greater than in the Filicales. If we can include 
 Azolla, Marsilia, the common ferns, Hymenophyllum, the Marat- 
 tiacese, and the Ophioglossaceae in one group, it ought not to 
 appear inconsistent to include Psilotum, Lycopodium, Phyl- 
 loglossum, Selaginella, and Isoetes in a group of coordinate rank. 
 A fuller knowledge of the three little-known genera may tend 
 to confirm this view, especially if they depart as widely from 
 the remaining genera in other characters as in general habit 
 and sporangia; but if their gametophytes, spermatozoids, and 
 embryos agree very nearly with Lycopodium and Selaginella, it 
 will probably be better to make of Isoetes a fourth group ,of 
 pteridophytes equivalent in rank to the three now universally 
 recognized. If an affinity with seed plants must be sought, the 
 evidence points to a connection with gymnosperms rather than 
 with monocotyledons, 
 
 SUMMARY. 
 
 1. The stem apex lies at the bottom of a funnel-shaped 
 depression, around the sides of which the leaves are arranged 
 spirally. This depression is produced by the expansion of the 
 cortical cells of the stem in all directions. 
 
 2. The leaves arise as crescent-shaped bands of meristematic 
 tissue. At first the basal part of the leaf (the sheath) grows 
 most rapidly ; afterwards the region of growth is transferred to 
 the part above the ligule. There is no persistent or sharply- 
 marked zone of meristem. The whole leaf is meristematic at 
 first; it then gradually passes into permanent tissue, the change 
 beginning at the apex and extending gradually downwards. 
 
 3. The air-cavities are formed out of four longitudinal bands 
 of cells, which after losing their contents and power of multipli¬ 
 cation are ruptured into transverse partitions by the growth of 
 the other parts of the leaf. The size, but not the number of the 
 air-cavities, increases with the age and growth of the leaf. 
 
1900] 
 
 SPOROPHYLLS AND SPORANGIA OP ISOETES 
 
 339 
 
 4. The ligule originates in a single vesicular cell as described 
 by Hofmeister. The mature ligule can be distinguished into 
 four regions: (1) the sheath which has its origin in the lower¬ 
 most cells of the young ligule, (2) the glossopodium, (3) a 
 region of living cells, and (4) a region of disintegrating cells. 
 
 5. The rudiment of the sporangium is a transverse row of 
 superficial cells below the ligule ; the upper part of this gives rise 
 to the velum, the lower part to the sporangium proper. 
 
 6. There is no definite hypodermal archesporium. The middle 
 cells of the sporangium Anlage are the first to undergo periclinal 
 divisions. Additions to the sporogenous complex are made from 
 the superficial cells of the sporangium. 
 
 7. The general direction of growth of the sporangium is at 
 right angles to the face of the leaf, with a slight tendency in 
 young sporangia to an upward direction. The cells are not in 
 well-arranged rows or stratified layers. 
 
 8. There is no evidence that certain of the archesporial cells 
 give rise to trabeculae only, and certain others to mother cells 
 only. The trabeculae and megaspore mother cells or groups of 
 microspore mother cells greatly outnumber the archesporial cells. 
 
 9. There is no evidence that each of the primary cells of the 
 sporangium pursues an independent growth. On the contrary, 
 their derivatives blend indistinguishably. 
 
 ic. The microsprangia and megasporangia are indistinguish¬ 
 able until they have attained a volume of 15,000-25,000 cells. 
 
 11. The sporangium becomes recognizable as a microspo¬ 
 rangium by its differentiation into irregular deeply-staining and 
 feebly-staining radial bands. The deeply-staining regions after 
 a period of active multiplication become the mother cells. The 
 feebly-staining regions become the trabeculae, walls, and tapetum, 
 
 12. The tapetum is organized out of the layer of sterile cells 
 adjacent to the mother cells; its cells are small, densely cyto¬ 
 plasmic, and persistent. 
 
 13. The middle cells of the trabeculae become elongated by 
 compression and growth; their nuclei also become elongated 
 and spindle-shaped. 
 
340 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 14. The outer wall of the microsporangium is usually four 
 layers thick, the innermost layer being part of the .tapetum. The 
 inner wall, that is the cells between the base of the sporangium 
 and the vascular bundle, is probably formed by sterilization of 
 cells derived from the primary cells of the sporangium. 
 
 15. The divisions of the microspore mother cells may be 
 either successive or simultaneous. The two spindles of the sec¬ 
 ond division do not become connected by secondary fibers. The 
 microspores are usually bilateral but sometimes tetrahedral. 
 
 16. The number of microspores in a sporangium is 150,000— 
 300,000. 
 
 \ 
 
 17. A sporangium first becomes recognizable as a megaspo¬ 
 rangium by the marked enlargement of many or most of the cells 
 of about the third and fourth layers. All such enlarged cells 
 are to be regarded as potential mother cells, and the number 
 of them which succeed in producing megaspores is probably 
 dependent on nutrition. No tabular tapetal cells are cut off in 
 connection with the development of the megaspore mother cells, 
 nor is the megaspore mother cell the innermost of a row of cells 
 formed from a single archesporial cell in a manner comparable to 
 what is seen in the ovules of seed-plants. 
 
 18. Many cells which enlarge almost to the size of mature 
 mother cells are finally unable to give rise to spores, but divide 
 up into smaller cells which ultimately form part of the tapetum. 
 
 19. The trabeculae, tapetum, and walls arise in the megaspo¬ 
 rangium as in the microsporangium, the chief difference being 
 the greater massiveness of the single trabeculae in the former 
 and the much greater abundance of the tapetum. 
 
 20. No details of the division of the megaspore mother cell 
 were obtainable. The megaspores are usually tetrahedral in 
 arrangement, but occasionally bilateral. The number in a spo¬ 
 rangium is 150-300. 
 
 21. The first leaves of a season are megasporophylls, and 
 these are succeded by microsporophylls. There is occasionally 
 some irregularity in the order of succession, and sometimes a 
 sporangium is found which bears both kinds of spores. 
 
1900] SPOROPHYLLS AND SPORANGIA OF ISOETES 341 
 
 22. The sterile leaves in a majority of cases have aborted 
 sporangia. When these have made any considerable develop¬ 
 ment they are usually found to show the characters of megaspo¬ 
 rangia. 
 
 23. The sporangia after all cell divisions have ceased continue 
 to increase in volume, apparently by the osmotic properties of 
 the substances surrounding the young spores. 
 
 24. An attempt to relate the change from megasporophylls 
 to microsporophylls to an exhaustion of the nutritive cortical 
 cells formed in the preceding year was unsuccessful. 
 
 25. To secure a more consistent nomenclature it is proposed 
 to employ the term archesporium in speaking of a pteridophyte 
 sporangium to designate the superficial cell or cells from which 
 the sporogenous tissue takes its origin. 
 
 I am indebted to Professor John M. Coulter, of the University 
 of Chicago, at whose suggestion the investigation was under¬ 
 taken, for helpful suggestions and criticisms. Acknowledg¬ 
 ments are also due to Dr. C. J. Chamberlain. 
 
 McMaster University, 
 
 Toronto, Canada. 
 
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 505. 513-524. 1874. 
 
 Treub, M. (i) Etudes sur les Lycopodiacees. Ann. Jard. bot. Buit. 4 : 1884 ; 
 5 : 1886 ; 7 :1888 ; 8:1889. 
 
 Tschistiakoff. (i) Notice sur le developpement des sporanges de 
 /’ Isoetes Durieui. Nuovo Giornale bot. Ital. 1873. 
 
344 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 Van Tieghem, Ph. (i) (et Duliot) Recherches comparatives sur l’origine 
 des membres endogenes dans les plantes vasculaires. Ann. Sci. Nat. Bot. 
 VII. 8 : [-656. 1888. 
 
 (2) Traite de Botanique. 1891. 
 
 Vines, S. H. (i) The systematic position of Isoetes. Ann. Bot. 2 : 117-123, 
 223-224. 1888-9. 
 
 (2). A text-book of botany. 1895. 
 
 EXPLANATION OF PLATES X 1 II-XX. 
 
 The drawings, except Jig. 1, have been made with the aid of a camera 
 lucida, and all have been reduced by photography to two fifths of their origi¬ 
 nal size. Both in the text and in . the explanation of the plates, the terms 
 transverse, longitudinal, and tangential, when used to describe sections of the 
 ligule and the sporangium'are to be understood as indicating the planes in 
 which the sporophylls were sectioned. 
 
 Fig. 1. Base of microsporophyll, inner face showing the sporangium (V), 
 the velum (v), and the ligule (/). X 4. 
 
 Fig. 2. Radial longitudinal section of base of sporophyll; s, v, /, as in Jig. * 
 
 1. X 5. 
 
 Fig. 3. Longitudinal section of apex of a small plant. X 48. 
 
 Fig. 4. Longitudinal section of apex of a larger plant ; x, the tracheids, 
 p , the prismatic layer, t, the leaf traces, m, the cambium (semidiagrammatic). 
 
 X 48. 
 
 Fig. 5. Part of the prismatic ring as seen in transverse section of the 
 stem ; x,p, t, m , as in Jig. 4. X 160. 
 
 Fig. 6. Cortical cells from the neighborhood of the cambium. X 240. 
 
 Fig. 7. Cortical cells from the outer region of stem. X 240. 
 
 Fig. 8. Longitudinal section of a number of young sporophylls ; the spo¬ 
 rangia are indicated by the dotted outlines. X 48. 
 
 Fig. 9. Cross section of a young leaf above the ligule. X 300. 
 
 Fig. 10. Cross section of a young leaf more advanced than that shown in 
 fig. 9 ; the position of the future air chambers is shown by the groups of nearly 
 empty cells ; the small circles of Jigs. 9 and 10 indicate the side towards'the 
 axis of the plant. X 300. 
 
 Fig. 11. Part of a longitudinal section of a leaf more advanced than that 
 of Jig. 10 , showing origin of the air cavities; p, the phloem, x, the xylem. 
 
 X 300- 
 
 Figs. 12-13. Transverse section of leaf with the vesicular cell from which 
 the ligule originates. X 300. 
 
 Fig. 14. First division of the ligule, sectioned in plane a — a of Jig. 15. 
 
 X 490 . 
 
 Fig. 15. First division of the ligule seen in radial longitudinal section. 
 
 X 490 . 
 
SPOROPHYLLS AND SPORANGIA OF ISOETES 
 
 345 
 
 1900] 
 
 Figs. 16, 18. First division of terminal cell of ligule sectioned in plane 
 a—a of fig. 15. X 49 °- 
 
 Fig. 17. The same sectioned in plane b—b of fig. 15. X 490 - 
 
 Fig. 19. Tangential section of older ligule. X 490. 
 
 Fig. 20. Transverse section of ligule of same age as that of fig. 19. 
 X 490. 
 
 Fig. 21. Transverse section of still older ligule. X 300. 
 
 Fig. 22. Median radial longitudinal section of base of half-grown ligule ; 
 s, the sheath, g, the glossopodium, v, the velum. X 490. 
 
 Fig. 23. Radial longitudinal section of base of mature ligule at the posi¬ 
 tion indicated by a—b of fig. 25, showing the thickened cells of the velum 
 and leaf adjacent to the ligule. X 48. 
 
 Fig. 24. The same in I. Engelmanni. X 48. 
 
 Fig. 25. Transverse section of ligule and leaf at the position indicated by 
 a—b oifig. 23. X 30 - 
 
 Fig. 26. Median radial longitudinal section of young sporophyll, showing 
 rudiment of the sporangium. X 490. 
 
 Figs. 27-28. The same more advanced. X 490. 
 
 Fig. 29. Transverse section of young sporophyll with sporangium. X 490. 
 
 Fig. 30. Transverse section of sporangium more advanced. X 490. 
 
 Fig. 31. Tangential section of young sporangium. X 490. 
 
 Figs. 32-38. Radial longitudinal sections of young sporophyll ; fig. 37 is 
 a section through the side of the sporangium of which fig. 36 is a median sec¬ 
 tion ; v, the velum, g, the glossopodium, s, the sheath. X 490. 
 
 Fig. 39. Transverse section of sporangium more advanced. X 490. 
 
 Fig. 40. Transverse section of leaf with sporangium of same age as that 
 o t fig- 39 - X 490 - 
 
 Fig. 41. Median radial longitudinal section of a sporangium of about the 
 same age as the last. X 490. 
 
 Fig. 42. Longitudinal section of side of sporangium. X 490. 
 
 Fig. 43. Transverse section of a sporangium still older but in which the 
 trabeculae are not yet recognizable. X 490. 
 
 Fig. 44. Transverse section of young leaves and of microsporangium at 
 the time of the first differentiation of fertile and sterile regions ; the shaded 
 portion represents the fertile region ; v, the velum, /, the ligule, f, the vas¬ 
 cular bundle. X 48. 
 
 Fig. 45. Oblique nearly tangential section of microsporangium. X 48. 
 
 Fig. 46. Tangential section of microsporangium. X 30. 
 
 Fig. 47. Cross section of sporophyll and microsporangium, showing the 
 trabeculae, tapetum, t, and a few microspores ; v, the velum (semidiagram- 
 matic). X 30. 
 
 Fig. 48. Portion of microsporangium at the time of the first differentiation 
 of fertile and sterile regions. X 490. 
 
 / 
 
346 
 
 BOTANICAL GAZETTE 
 
 [may 
 
 Fig. 49. The same, showing portion of outer wall. X 490. 
 
 Fig. 50. Portion of an older microsporangium, showing differentiation of 
 sterile regions into trabeculae and tapetum ( 7 ). X 490. 
 
 Fig. 51. Portion of trabecula, tapetum (/), and 3'oung spores of a micro¬ 
 sporangium. X 490. 
 
 Fig. 52. Part of outer wall and tapetum (/) of nearly mature microspo¬ 
 rangium. X 490. 
 
 Figs. 53-55. Division of mother cells to form microspores; fig. jj illus¬ 
 trates successive division ; fig. 54, simultaneous division ; fig. jj, the shape 
 of the spores, bilateral in a and b, tetrahedral in c. X 490. 
 
 Fig. 56. Young microspores. X 490. 
 
 Fig. 57. Cross section of megasporangium with young spores and tape¬ 
 tum ( 7 ). X 48. 
 
 Fig. 58. Portion of a trabecula and tapetum (7) of a megasporangium, 
 X 490. 
 
 Fig. 59. Tetrahedral arrangement of young megaspores. X 490. 
 
 Figs. 60-61. Successive division of megaspore mother cells, spores 
 bilateral. X 490. 
 
 Fig. 62. Median radial longitudinal section of sterile leaf with aborted 
 sporangium (shaded). 
 
 Fig. 63. Transverse section of megasporangium first distinguishable as 
 such. X 490. 
 
 Fig. 64. Part of transverse section of a megasporangium with a group of 
 potential mother cells. X 490. 
 
 Fig. 65. The same with a single mother cell. X 490. 
 
 Fig. 66. Part of a transverse section of a megasporangium ; for explana¬ 
 tion see text. X 490. 
 
 Fig. 67. Diagram of young megasporangium. 
 
 Fig. 68. Diagram of a longitudinal section of the stem. See p. 228. 
 
 Figs. 69, 70. Early stages of the sporangium of a fern. Diagrammatic. 
 a, archesporium ; f, fertile sporogenous cell ; s, sterile wall cell. 
 
 Figs. 71, 72. Early stages of a microsporangium of an angiosperm. 
 Diagrammatic, a, archesporium ; e, epidermis; s, primary tapetum ; /, pri¬ 
 mary sporogenous cell or cells. 
 
 Figs. 73, 74. Early stages of a megasporangium of an angiosperm. 
 Diagrammatic. Letters as in figs. 77, 12.