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 Stages in the Development 
 
 OF 
 
 SlUM CICUTAEFOLIUM 
 
 BY 
 
 GEORGE HARRISON SHULL 
 
 WASHINGTON, D. C. : 
 
 Published by the Carnegie Institution of Washington 
 
 May, 1905 
 
 • f 4 3
 
 Stages in the Development 
 
 OF 
 
 SlUM CICUTAEFOLIUM 
 
 BY 
 
 GEORGE HARRISON SHULL 
 
 WASHINGTON, D. C. : 
 
 Published by the Carnegie Institution of Washington 
 May, 1905
 
 Carnegie Institution of Washington, Publication No. 30. 
 
 Papers of Station for Experimental Evolution 
 AT Cold Spring Harbor, New York. no. 3. 
 
 PRESS OP 
 
 HENRY E. WILKENS PRINTING CO. 
 
 WASHINOTON. O. C. 


 
 PLATE I. 
 
 Sium ciciilue/oliuiii (jiiit-l. A jar <il secdIiiiKS, piioto^^raplied at llie University 
 of Chicago, Octolier lo, 1903, by Ur. W. J. G. Land ; spring-growtii of an old 
 plant, piiotograpried at Lockport, ill., May 16, 1903, by W. E. Praeger ; and a 
 flowering-stem piiotograplied at South Cliicago, September 22, 1903.
 
 Stages in the Development of Sium 
 cicutaefolium. 
 
 By George Harrison Shull. 
 
 The juvenile forms of leaves have been believed to be so related to the 
 evolutionary history of a plant as to indicate the form of leaf possessed 
 by its ancestors and to furnish satisfactory evidence of the closeness of 
 relationship between allied species. It is impossible, in most cases, to 
 determine the ancestors of any species, and it is likewise impossible, 
 therefore, to demonstrate a close parallel between ontogeny and phy- 
 logeny. As there is in both processes the development from some simple 
 condition to one and the same complex condition, namely, the climax 
 type of leaf of the present adult plant, we can scarcely escape the belief 
 ^ that such a parallel does exist in many cases ; but how safely or in how 
 "" minute detail we may reason from ontogeny to phylogeny may well be 
 ^] considered an open question. 
 
 en The hypothesis of von Baer ( 1828) has proved a very suggestive one, 
 ^ and, like most suggestive hypotheses, has been given a much wider ap- 
 plication than its author would have been willing to sanction. Yon Baer 
 did not assume that the adult characters of the ancestors occur as larval 
 or juvenile characters in the descendants, but that the same larval stages 
 o occur in both, a given stage appearing earlier in the descendant than in 
 ^ © the ancestor. The idea that larval and juvenile characters agree with 
 fZ C3 ancestral adult characters was an old conception which was rehabilitated 
 2. S by Louis Agassiz (1848-1849) and became crystallized in the epigram 
 
 CO ' "^ 
 
 i^j ^ of Haeckel (1866), which is now universally known as the "Law of 
 
 ij-i ^ biogensis " — that " ontogeny repeats phylogeny."* 
 
 ^ '^ Great stress was laid upon this hypothesis by Hyatt, Cope, and others 
 
 ^ who have used it as a most important principle in disentangling difficult 
 phylogenetic problems. More recently Jackson (1899) has called atten- 
 tion to the fact that in organisms having periodically interrupted 
 
 CD 
 
 *For a [history of the development of tlie idea of repetition see Hyatt, A., 
 " Cycle in the life of the indvidual and in the evolution of its own group." The 
 law of repetition is there called " Agassiz's law of palingenesis." See also 
 Glaser, O. C, " The law of von Baer." 
 
 (3) 
 
 '• !• "3 (T* r* <T "O
 
 4 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 growth, modified characters which he beheves to be atavistic occur in 
 definite parts of the organism, particularly in the region where resump- 
 tion of growth takes place. These local modifications he calls " local- 
 ized stages " and says that the equivalents of these stages " are to be 
 sought in the adults of ancestral groups." In three recent papers in the* 
 American Naturalist, Cushman (1902, 1903, 1904) gives the result of 
 studies similar to those of Jackson, and, besides presenting evidence that 
 at the resumption of growth in perennial herbs in the spring these ata- 
 vistic modifications occur, lays stress upon senescent stages as showing 
 even more primitive conditions than are to'be found in the seedling. It 
 may be said, in passing, that this view of the significance of senescent 
 stages does not fully accord with that of Hyatt (1890, pp. 78, 79; 1897, 
 p. 221), who looked upon senescent stages as indicative of the course 
 any species in question will pursue in the process of degenerative evo- 
 lution. In Hyatt's view the senescent stages have a prophetic instead 
 of an historic significance, though he recognized, of course, that there 
 are many resemblances between the senescent and the juvenile series. 
 
 If the propositions of Hyatt and Cope, of Jackson, and of Cushman 
 are all true, there should be at least three regions in any perennial plant 
 which will agree in presenting ancestral characters — the juvenile leaves 
 following the cotyledons, the earliest formed parts at each renewal of 
 vegetative activity, and the senescent stages approaching and accom- 
 panying the inflorescence. In numerous cases there is a general agree- 
 ment in the forms passed through at these three regions, and no incon- 
 sistency arises when they are all looked upon as atavistic, though the 
 mere fact of agreement in the several regions can not be taken as in 
 itself convincing evidence that these characters agree with the adult 
 characters of some ancestral group. On the other hand, certain plants 
 do not show the same modification of leaf-form in the inflorescence that 
 is found in the " nepionic " leaves of the seedling, and it becomes at once 
 evident that no reliance can be placed upon the forms of leaf occurring 
 in any of these regions as having more than the most general signifi- 
 cance as indications of ancestral characters. 
 
 A plant which most strikingly illustrates this fact is the hemlock 
 water-parsnip {Sium cicutaefolium Gmel.), which presents a great 
 range of leaf-form and passes rather rapidly, sometimes suddenly, from 
 one form to another without repetition, so that each of the regions sup- 
 posed to tell of ancestral conditions tells a different story. This fact is 
 illustrated in a general way by Plate I. 
 
 The seedling of Sium cicutaefolium is so different from the adult 
 plant that, except when the two are associated together, its identity
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 5 
 
 would scarcely be suspected. I am not aware that this seedling has ever 
 been described, and the striking- variations in the form of the juvenile 
 leaves make description difficult. 
 
 The cotyledons are quite similar to those of many other of the Um- 
 belliferae, being narrowly oblong or elliptic oblong, rather acutely 
 rounded at the apex, and tapering gradually into a petiole at the base. 
 The petiole and blade together are 1.5 to 2 cm. long and the blade is 
 2.5 to 4 mm. wide. 
 
 The early " nepionic " leaves are extremely variable, but it is rare 
 that they do not consist of a single blade constructed on a somewhat 
 palmate plan. The blade is usually of rounded form and quite variously 
 notched, cut, or lobed. In order to facilitate a systematic investigation 
 of the variation in these leaves they were divided into several more or 
 less artificial groups. The first nepionic leaf in more than 120 unse- 
 lected individuals* presented forms which were placed in six of these 
 categories, as follows : 
 
 (a) About 10.5 per cent were divided palmately into five nearly equal 
 lobes, one or two of which were sometimes slightly notched. 
 
 (b) Twenty per cent were 3-lobed, with the middle lobe 2-notched 
 and the lateral lobes each bifid. 
 
 (c) The leaves in the third group were likewise 3-lobed, but the lobes 
 were quite variously notched. Over 28 per cent of the seedlings had 
 the first leaf of this description. 
 
 It will be noted that the form of leaf described under (b) is simply a 
 special case of (c), and was separated from it because this was the only 
 form of trilobed leaf which could be so definitely characterized, and 
 because this simple definite type of trilobation is almost wholly limited 
 to the first nepionic leaf. 
 
 (d) Over 15 per cent showed the tendency to trilobation by a single 
 cleft on one side, unmatched by a cleft on the opposite side. These 
 will be spoken of as " half 3-lobed leaves." They pass gradually into 
 a form having one lateral leaflet, and these two conditions were kept 
 together in a single category. 
 
 (e) Nearly 25 per cent had a generally rounded, ovate or cordate 
 form, merely dentate, or irregularly cleft in a manner which did not 
 suggest trilobation. 
 
 *The seedlings were taken up on two small clumps of earth from their native 
 (habitat near Robej'-, south of Chicago, Illinois. The individual seedlings were 
 carefully separated and all were transplanted about 3 cm. apart in a regular pat- 
 tern in glass dishes. The point of departure for each dish was marked and a 
 number assigned to each seedling, so that drawings made from time to time could 
 be labeled in a manner to allow .the progress of each individual to be followed.
 
 6 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 (f) Less than 2 per cent had one pair of lateral leaflets, and may 
 perhaps be looked upon as an extreme form of the 3-lobed condition of 
 category (c), but (c) is essentially palmate, while (/) is essentially 
 pinnate. 
 
 A glance at Plate II will show, better than description, the variability 
 of the first nepionic leaf, and it will also show how truly gratuitous is 
 the division into the categories just described. The leaves were chosen 
 at random from the material at hand, but when more leaves were chosen 
 than could find place in the plate the less dubious forms w^ere discarded 
 in order to show the real difficulties of such a classification and to give 
 a clue to the personal equation of the writer in distinguishing the several 
 categories. The letters A, B, C, etc., illustrate the categories described 
 above under (a), (b), (c), etc. 
 
 The first two classes, (a) and (b), are characteristic of the first nepi- 
 onic leaf. With the exception of these two forms, all the categories de- 
 scribed above are represented in each succeeding leaf up to the sixth, 
 and perhaps occasionally in the seventh, and in these later leaves the 
 various types are much more definitely distinguishable, so that there is 
 rarely any question as to where any leaf should be classed. With rare 
 exceptions every leaf after the sixth is pinnate and shows a marked 
 contrast to the great variability of the first leaf, so that in the eighth leaf 
 a very large majority have three pairs of lateral leaflets which closely 
 resemble the typical stem-leaves of the species, except in texture and in 
 the more ovate form of the terminal leaflet. 
 
 In the second nepionic leaf (Plate 111,2) trilobation was found in 
 only a little over 15 per cent of the seedlings, or about one-third as fre- 
 quently as in the first nepionic leaf, but the half 3-lobed form had in- 
 creased from 15 per cent to 18 per cent, the unlobed or irregularly lobed 
 form had increased from 25 per cent to over 61 per cent, and the form 
 with one pair of lateral leaflets from 1.6 per cent to about 5 per cent. 
 Just as the 3-lobed form was the modal condition of the first nepionic 
 leaf, the unlobed form was the modal condition of the second. 
 
 The third nepionic leaf (Plate III, 3) showed a partial return to the 
 3-lobed condition, and nearly 24 per cent of the specimens were so char- 
 acterized. The half 3-lobed form occurred in over 22 per cent, the un- 
 lobed or irregularly lobed form in 39 per cent, and the number having a 
 pair of lateral leaflets had increased from 5 per cent in the second leaf 
 to over 15 per cent in the third. Here, although there was an increase 
 in each of three categories at the expense of the unlobed form, the latter 
 was still the modal form of leaf. 
 
 The fourth leaf (Plate III, 4) was 3-lobed in 24 per cent, half 3-lobed 
 in 15 per cent, unlobed or irregular in nearly 23 per cent, and pinnate
 
 PLATE II. 
 
 B 
 
 s? 
 <? 
 
 '^ 
 
 ^ 
 
 
 ^ <ii 
 
 O'^o .55* 
 
 Variation in the first "nepionic" leaf of Stum cicutaefolium. Eight-ninths 
 natural size. All drawn from nature with an Abbe camera. A, B, C, etc., cor- 
 respond to (a), (<J), {c), etc., in the text.
 
 PLATE III. 
 
 Variation in successive seedling-leaves of Slum cicutae/olium, from the second to 
 the seventh. Eight-ninths natural size. Drawn from nature with an Abbe 
 camera. Numbers represent the position of each group of leaves in the seedling.
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 with one pair of lateral leaflets in 30 per cent, this simplest type of pin- 
 nation now becoming the modal form of leaf. 
 
 In the fifth leaf (Plate III, 5) the percentages of all the types of single 
 blades had considerably decreased and the pinnate had increased, so that 
 now only 4.3 per cent were 3-lobed and the same number half 3-lobed, 
 17 per cent were unlobed or irregular, and more than 71 per cent had 
 one pair of lateral leaflets, while a little more than 2 per cent had two 
 pairs of leaflets. 
 
 The sixth leaf was 3-lobed in only 3.5 per cent of the specimens ex- 
 amined and the same number were unlobed or irregular. About 7 per 
 cent were half 3-lobed, nearly 45 per cent had one pair of leaflets, and 
 over 41 per cent had two pairs of leaflets. 
 
 In the seventh leaf none were observed which had not at least one 
 pair of leaflets, though it is not improbable that single blades may occur 
 occasionally in the seventh leaf. The specimens in hand showed 27.5 
 per cent with one pair of leaflets, over 51 per cent with two pairs of leaf- 
 lets, and 21.6 per cent with three pairs of leaflets, so that in the seventh 
 nepionic leaf the form with two pairs of lateral leaflets is the modal 
 form. 
 
 The eighth leaf presents the same three forms, but only a little over 
 9 per cent had a single pair of lateral leaflets, 18 per cent had two pairs, 
 and nearly y^ per cent had three pairs. 
 
 For more ready comparison these percentages are brought together 
 in the following table. The modal form (empirical) in each leaf is 
 given in full-faced type. The variation of leaf-form occurring in each 
 succeeding nepionic leaf from the second to the seventh is shown in 
 Plate III. The left and right sides of the figure show the extreme 
 range of form found in each leaf. The index figures at the left-hand 
 side of the plate indicate the position of the leaves with reference to the 
 cotyledons. 
 
 Form of leaf. 
 
 Position of leaf. 
 
 I 
 
 2 
 
 3 
 
 4 
 
 5 
 
 6 
 
 7 
 
 8 
 
 (a) Five-lobed 
 
 (6) Three-lobed 
 
 19.5 
 19.4 
 28.2 
 
 15-3 
 
 25.0 
 
 1.6 
 
 
 
 
 
 
 
 
 
 
 1.6 
 32.6 
 
 14.5 
 22.6 
 38.7 
 
 
 
 
 
 (c) Three-lobed 
 
 15.7 
 
 18.2 
 
 61.2 
 
 5-0 
 
 23-7 
 22.0 
 39.0 
 
 15.3 
 
 4.3 
 4.3 
 17.0 
 
 72.3 
 
 2.1 
 
 34 
 
 6.9 
 
 34 
 
 44.8 
 
 414 
 
 
 
 (d) Half 3-lobed 
 
 (<?) Unlobed or irregular 
 (0 One pair of leaflets. . 
 (g) Two pairs of leaflets. 
 (/») Three pairs of leaflets 
 
 
 
 
 
 27.0 
 51.4 
 
 21.6 
 
 91 
 18.2 
 72.7 
 
 
 
 
 
 
 
 
 ! i
 
 8 DEVELOPMENT OF SIUIVI CICUTAEFOLIUM. 
 
 If the several categories into which these leaves have been divided 
 are examined critically, the following facts will be noted : 
 
 1. They are not all of equal value. The second category (&) is only 
 a special case of the third (c). The fourth (d) is transitional between 
 (c) and (e). The fifth (e) is Composite, but not readily divisible. 
 
 2. Some of the distinctions are qualitative, i. e., complexly quantita- 
 tive, as exemplified by (b), (c), (e), etc.; others are simply quantita- 
 tive, as, for example, (f), (g), and (h). 
 
 3. They are not arranged in a logical linear series and are incapable 
 of being so arranged. They have been arranged in the order in which 
 they become dominant during development and without reference to 
 their logical relations. The undivided or irregularly cleft blade so 
 characteristic of the second nepionic leaf is the logical termination of 
 a line of development from the 3-parted leaf through the half 3-parted, 
 but the pinnate leaf with one pair of lateral leaflets, instead of joining 
 directly to this undivided form which it follows, also comes logically 
 from the 3-parted type. If the progress of the phylogeny of the species 
 is correctly indicated by this ontogenetic series it would appear that 
 after a gradual departure from the ancestral tripartite leaf to the undi- 
 vided leaf there had been a sudden jump or mutation to the opposite 
 extreme in which the clefts of the tripartite leaf became so exaggerated 
 as to leave the lateral lobes separated from the terminal by a space, thus 
 forming a pinnate leaf. It can not rightly be either affirmed or denied 
 that such mutation took place in the history of the species, but it appears 
 to me that any attempt to interpret the details of ontogenetic develop- 
 ment as corresponding so definitely to phylogenetic development would 
 be pressing the analogy to a wholly unwarranted degree. 
 
 In fig. I the variation in the several nepionic leaves from the first to 
 the eighth is represented graphically in the form of curves, the several 
 categories being treated as if of equal value. Although, as we have just 
 seen, this is not strictly true, the figure still allows the derivation of at 
 least one interesting conclusion — there is a progressive lessening of the 
 variability from the first leaf onward. The modal class in the first nepi- 
 onic leaf contained only 28.2 per cent of the individuals, or, if we should 
 include in this class the special form of 3-parted leaf which has been 
 separated as a distinct category, it would still contain only 47.6 per cent 
 of the individuals, while the modal class in the eighth leaf included 
 nearly y;^ per cent of the specimens, and the variability within the class 
 was very much less in the latter than in the former. 
 
 There are several ways in which we may account for the great varia- 
 bility in the earliest leaf and the decreasing variation with subsequent
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 9 
 
 development. If these juvenile leaves do correspond with adult condi- 
 tions of the past we might suppose that at the time when the species was 
 characterized by a tripartite leaf it was much more variable than it be- 
 came later, and that the process of evolution acted to give it gradually a 
 more and more fixed character. Or, on the other hand, it might be sup- 
 posed that the species has been at all times about equally variable, but 
 that by the " law of acceleration " the first nepionic leaf represents a 
 much longer period in the history of the species than does any subse- 
 quent leaf. Again, it may be that these leaf-forms are not atavistic, and 
 
 Fig. 1 .—Graphic representation of the variation In the several " nepionic " 
 leaves of Sium cicutae/olium, from the first to the eighth, inclusive. 
 Ordlnates given in per cents. 
 
 the wider range of variation may merely represent a greater sensitive- 
 ness to minute variations of the internal and external environment. As 
 the plant becomes more and more firmly established it becomes morpho- 
 logically more s6lf-determinative. In other words, the greater variabil- 
 ity and the gradual decrease in variability may be purely physiological 
 facts unrelated in any direct way to the phylogenetic history of the 
 species. It is possible indeed that each of these propositions mat rep- 
 resent a partial truth, as they are not mutually exclusive. The species 
 may have been more variable in its adult leaf-form, the law of accelera- 
 tion may give rise to a greater range of atavistic variation in the earlier
 
 10 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 leaves, and physiological causes may determine which atavistic form 
 shall appear and may in addition give rise to forms or details of form 
 which are in no sense atavistic. 
 
 A point in favor of the interpretation of the tripartite leaf as being at 
 least in a general way atavistic may be found in the fact that ternate 
 division of leaves is notably prevalent among the Umbelliferae ; but it 
 should not be forgotten that a ternate condition is a necessary logical 
 transitional stage between any " simple " leaf and a pinnate leaf, and its 
 significance must rest solely upon this logical relation. To show how- 
 gratuitous is any attempt to draw from the juvenile leaves conclusions 
 regarding the leaf- form in ancestral groups it is only necessary to inves- 
 tigate the conditions found in the few instances in which the ancestry 
 of a species is definitely known. Through the kindness of Dr. D. T. 
 MacDougal, of the New York Botanical Garden, I am enabled to pre- 
 sent photographs of the seedlings and adult rosettes of Onagra laniarck- 
 iana and O. rubrinervis, the former species being the parent of tlie 
 latter. They represent the closest relationship possible between two 
 species, and yet those who are experienced in their culture separate 
 them with unfailing accuracy in the earliest juvenile stages, A com- 
 parison of the seedlings and the adults in Plates IV to VI makes evident 
 the fact that the seedling of Onagra rubrinervis resembles much more 
 closely the adult condition of the same species than either the seedling 
 or the adult condition of its parent species, Onagra laviarckiana. 
 
 Turning now to other regions which have been supposed to present 
 ancestral types, particularly to the two regions most strongly exploited 
 by Cushman (1902, 1903, 1904) in his recent papers in the American 
 Naturalist, we find conditions which are totally different from those we 
 have just considered in the juvenile stages. The first of these regions is 
 that at which a perennial herb resumes growth in the spring after hiber- 
 nation. At the close of the first season from seed the leaflets of Sium 
 cicutaefolimn become more or less dissected into narrow segments, as 
 shown in fig. 2. This modification is very often apparently related to 
 the more aquatic condition of the habitat, and is in line with modifica- 
 tions in many other species whose dissected leaves appear to be corre- 
 lated in some way with aquatic conditions ; but it can be shown that this 
 change in the form of the leaf of Sinm takes place, though less per- 
 fectly, when the plants are not supplied with an unusual quantity of 
 water. Plants taken up as seedlings in June and grown under meso- 
 phytic conditions in the window of a laboratory at the University of 
 Chicago and later in the conservatory of the same showed dissection of 
 the leaves similar to that exhibited by plants that were submerged in 
 imitation of the effects of autumn rains.
 
 PLATE IV. 
 
 Seedlings of Onagra lamarckiana, grown at the New York Botanical Garden. 
 Photograph supplied by Dr. D. T. MacDougal.
 
 PLATE V. 
 
 Seedlinf^s of Onagra rubrinervis grown at the New York Botanical Garden. 
 Photograph supplied by Dr. D. T. MacDougal.
 
 PLATE VI 
 
 Large rosettes ot Onagra lainarckiana (upper) and O. rubriitervis (lower), grown 
 at the New York Botanical Garden. Photographs supplied bv Dr. D. T. 
 MacDougal.
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 II 
 
 Burns (1904) has recently demonstrated a similar independence from 
 aquatic conditions, in the dissected leaf of Proscrpinaca palustris, 
 though McCallum (1902) had looked upon it as a direct response to the 
 dilution of the protoplasm brought about by the abundance of water and 
 the stoppage of transpiration. Burns considers the dissected leaf of 
 Proserpinaca as a juvenile type, but this is certainly not the case in 
 Slum cicutaefoUnni. 
 
 A more pronounced, even extreme, dissection of the leaves character- 
 izes the resumption of growth in the spring (Plates I and VII), so that 
 
 Fig. 2.— Autumnal modlflcation of leaves in the seedlings of 
 Siinn cicutae/olium. Numbers represent the position of each 
 leaf In the series, beginning with the first neplonic leaf. 
 Three-fourths natural size. 
 
 in SO far as this region indicates the ancestry of the species the ancient 
 Slums should have resembled a fennel {Foenicuhim) or a parsley {Lo- 
 matium). These dissected leaves are rarely seen by the systematist who 
 works only with pressed material, as one may convince himself by look- 
 ing through the specimens of Shun in any large hrebarium. Of more 
 than 100 specimens of this genus in the herbarium of the New York 
 Botanical Garden not one showed these dissected leaves. The change 
 from dissected leaves to the simply pinnate leaves with linear, lanceolate
 
 12 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 or lance-ovate, sharply serrated leaflets characteristic of the typical leaf 
 of the stem, is very sudden (see Plates I and VII), and would again 
 indicate a remarkable mutation if the course or ontogenetic develop- 
 ment agrees with the phylogenetic. 
 
 In fairness to Cushman it should be said that while this discussion of 
 the early spring growth is based on a literal interpretation of his papers, 
 it evidently misrepresents his intention. The condition in Siuin is analo- 
 gous to that in the rosette plants, in which the writer had to look care- 
 fully beneath the rosette to find the " primitive " leaves. If he had 
 examined the rosettes at some time during the preceding summer or 
 autumn, when they were just beginning to develop, he would have 
 found less difficulty in securing the leaves for which he was looking. 
 The dissected leaves in old specimens of Shun result from the rejuvena- 
 tion of axillary buds at the base of the flowering stem, and it will be 
 seen later that the first leaves of these buds were of a more juvenile 
 type, but that they belong to the autumn growth instead of the period 
 of renewed vegetative activity in the spring. 
 
 When we examine the senescent stages approaching the inflorescence 
 a series of forms is found entirely diflferent from either of those already 
 considered, and yet according to Cushman it is in the region of the in- 
 florescence that we should be able to trace phylogeny to its earliest 
 stages. During senescence the leaves of Sium cicutaefolium present a 
 marked contrast in every particular to the juvenile stages. The pro- 
 gressive changes (fig. 3) present a simple linear series instead of the 
 complex series seen in the seedlings, and there is an increasing con- 
 stancy of form as senescence progresses instead of the increasing vari- 
 ability which should be expected if the senescent stages repeat the same 
 story as that told by the juvenile stages, but in inverse order. Thus in 
 34 adult plants examined with reference to this point the second leaf 
 below the principal inflorescence (n — 4, figs. 3 and 6) had three pairs of 
 leaflets in 10 per cent, two pairs in 56 per cent, and one pair in 34 per 
 cent ; the leaf subtending the primary umbel (n — 3) had one pair of leaf- 
 lets in more than 70 per cent ; and the peduncle arising axillary to this 
 highest leaf of the principal axis bore a bract (n — 2) having a single 
 lance-linear blade in over 80 per cent. In vigorous plants peduncles of 
 tertiary or still higher orders occur, and the bracts (n — i and n) on 
 these show a regular reduction from the form just described as pre- 
 dominant on the secondary peduncles to small awl-shaped structures 
 which apparently correspond to the base of the petiole and leaf-sheath 
 of the earlier stem-leaves, the blade having completely disappeared.
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 13 
 
 Besides these differences in form between the senescent and the juve- 
 nile leaves, there is quite as marked difference in the texture, venation, 
 etc. The dissected leaf has already been spoken of as having a form 
 not rarely associated with aquatic habitats. Similarly the juvenile and 
 senescent leaves have forms and textures usually associated with meso- 
 phytic and xerophytic habitats, respectively, the reduction of the blade, 
 firm texture, and strong cuticularization of the stem-leaves being in 
 marked contrast with the broad blades and delicate texture of the juve- 
 nile leaves. It will not be surprising to find that each of these three 
 
 Fig. 3.— The senescent series of leaf-forms in 8ium cicutaefolium. 
 Numbered downward from »i, the most reduced leaf found In 
 the Inflorescence; y a terminal leaflet showing Incomplete 
 dlfl'erentlatlon of a pair of lateral leaflets. Two-thirds natural 
 size. 
 
 types of leaves is associated with just the conditions of environment to 
 which it would appear to be adapted. The seedling develops in the 
 shade of other plants during the summer, when the substratum is less 
 wet ; the dissected leaves occur in autumn and spring, when the favor- 
 ite habitats of the plant are apt to be supersaturated or flooded with 
 water; and the stem-leaves are raised above the substratum, where 
 they are more exposed to the drying influences of wind, light, etc., and 
 at the same time farther from their water-supply. 
 
 But too much stress can easily be placed upon external appearance 
 as evidence of adaptation, and the dissected leaves, which suggest by 
 their form an adaptation to hydrophytic conditions, do not bear out the 
 suggestion when put to the test. They are unable to do sufficient pho-
 
 14 
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 tosynthetic work when submerged to allow the plant to continue its 
 growth, and death speedily follows complete submergence. It might 
 be assumed that the dissection is a product of the environment instead 
 of an adaptation to it, and the apparent correlation of these three stages 
 of Siuni with different environmental conditions would accord well with 
 the contention of Klebs (1903) and others that external factors are 
 directly determinative of the form and activities of plants. But the 
 plants which appear to demonstrate this proposition are quite excep- 
 tional. In Siuui cicutaefolinni this direct relation between the form 
 and the environment proves to be only apparent, and this species seems 
 to agree with the vast majority of plants in presenting a rather definite 
 cycle of development which it passes through whenever the environ- 
 ment supplies conditions which are favorable to growth. In such cases 
 the environment is only indirectly determinative of vegetative activities. 
 It supplies the energy, but the mechanism of the protoplasm determines 
 what shall be the product, just as the muscles of the hand and arm used 
 
 in winding a clock supply 
 
 vegetative c// 
 of Sporoph_yt^ « 
 
 the power which is con- 
 verted by the peculiar 
 mechanism of the clock 
 into movement of the 
 hands, ticking of the es- 
 capement, and striking of 
 the hour. 
 
 When a cycle of vege- 
 tative activity comes to a 
 close with the senescent 
 stages it can be repeated 
 only as a result of some 
 process of rejuvenescence. 
 Fertilization and seed pro- 
 duction are to be looked 
 upon as preparatory to or 
 part of the most common 
 process of rejuvenes- 
 cence, as was pointed out 
 long ago by Alexander 
 Braun (1851), but this is 
 not the only manner in 
 which it may be brought about. Whenever a bud grows immediately 
 without a resting period there is no sign of rejuvenescence and it con- 
 
 Gameto- 
 phyte 
 
 Spores 
 
 Gametes 
 
 fertilization 
 
 Fio. 4. —Diagram of the life-cycle of a Rpermatophyte, 
 showing the relation of the process of rejuvenes- 
 cence as a "short-cut" across the cycle. Upper 
 "rejuvenescence" line may represent the rejuve- 
 nescence of buda at base of stem (flg. 7) ; the lower, 
 the proliferations of the flower-buds (flgs. 8 and 9).
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 15 
 
 tinues the cycle on to the close of the senescent series, beginning at or 
 slightly in advance of the stage attained by the axis at the level from 
 which the bud springs. But when it fails to develop at once it can not 
 
 resume activity except 
 through a process of reju- 
 venescence, which throws 
 its development back to 
 some earlier point in the 
 cycle, and always at least 
 as far down 
 the juvenile 
 side of the 
 cycle as the origin of the 
 bud is down the senescent 
 side, a relation which is 
 clearly indicated by the 
 slant of the " rejuvenes- 
 cence " lines in the dia- 
 gram (fig. 4). 
 
 It is this less differen- 
 tiated, juvenile condition 
 which usually occurs when 
 a resting bud is reawak- 
 ened into active develop- 
 ment which suggested to 
 Jackson (1899) his con- 
 ception of localized stages 
 having phylogenetic sig- 
 nificance. 
 
 Some cases of rejuve- 
 nescence were observed in 
 a specimen of Sium cicii- 
 taefolimn which had been 
 torn up by the roots and 
 thrown down in such a 
 way as to submerge the 
 upper portion of the stem 
 and the immature inflores- 
 cence. The axillary buds 
 of the upper part of the stem, which might otherwise have produced 
 secondary branches of the inflorescence, and which would then have 
 
 Yio. 6.— Part of the stem of a specimen of Sium 
 cicutae/oliiim with three rejuvenated buds. 
 Drawn by J. Marlon Shull from a photograph.
 
 i6 
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 borne only the closing members of the senescent series of leaf-forms 
 (fig. 3), were rejuvenated as a result of the submergence and made to 
 begin a new series, the early members of which show the most interest- 
 ing transitions from the senescent to the juvenile forms. Two of these 
 rejuvenations are shown in figs. 5 and 6. The two leaves (n — 4, n — 5) 
 
 Fig. 6.— Two rejuvenated buds from the upper portion of a 8tem of tSiutn cicutaefo- 
 lium: n— 6 and n — I, leaves to which the buds were axillary. Numbers represent 
 the position of each leaf In the bud. Two thirds natural size. 
 
 at the left in fig. 6 are the leaves to which the two rejuvenations shown 
 in the same figure were axillary. Unrejuvenated, the first leaf pro- 
 duced by each of these buds would have been a pinnate leaf of exactly 
 the same character as the leaf to which the bud was axillary, but with a 
 less number of leaflets. Instead of this we see in each case a leaf hav- 
 ing the same arrangement of parts, to be sure, but greatly modified in
 
 PLATE VI 
 
 Sium ciciUacJolium Gmel. The first four leaves of the sprins-srowth of an old 
 specimen. Two-fifths natural size. Photographed April, ic,Ov from a specimen 
 taken into the house February 25, 1905.
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 17 
 
 the direction of the tripartite leaf, which will be recalled as the modal 
 condition in the first nepionic leaf. In the second leaf in the one case 
 a form similarly transitional to the half-tripartite leaf occurred, and in 
 the other case the juvenile type of 3-loJ|pd leaf was perfected. With 
 the third leaf in the one rejuvenation and the fourth in the other, a pin- 
 nate form was produced which corresponded in every way with that 
 seen above to be the modal condition of the fifth nepionic leaf. This 
 was as far as one of these rejuvenations was followed. In the other 
 the fifth leaf had five pairs of leaflets, a degree of differentiation not 
 
 Fig. 7.— Leaves of a rejuvenated bud from near the base of a stem of Sium cicutae- 
 /olium. Letters represent In a general way the position of each leaf In the reju- 
 venation; J/, an abnormally divided blade. Four-ninths natural size. 
 
 usually attained during- the first year from seed. Later these rejuvena- 
 tions produced the dissected leaves characteristic of the autumn and 
 early spring growth. 
 
 Lateral buds at the base of the flo\^(|ping stems are regularly rejuve- 
 nated in the autumn, and the first leaves are pinnate with one or several 
 pairs of undivided leaflets and are followed by a linear series leading 
 more or less quickly to the dissected condition (fig. 7) . It will be noted 
 that these basal rejuvenations never present leaves suggestive of the 
 earliest juvenile stages, as is the case in the rejuvenescence of buds
 
 i8 
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 near the inflorescence, probably because their senescence has not pro- 
 gressed so far. 
 
 Senescence reaches its culmination in the region of the flower. Re- 
 markably interesting conditions were found in the inflorescence men- 
 
 
 Fig 8.— Proliferations (re- 
 juvenations) of an Inflor- 
 escence of Siu77i cicutaefo- 
 Hum. Drawn by J.Marlon 
 Shull from a photograph. 
 
 tioned above which had been 
 submerged before the devel- 
 opment of the flowers. All 
 the flowers had given place to 
 proliferations (fig. 8), i. e., 
 the flower-buds were rejuve- 
 nated by the submergence; 
 and just as the flowers repre- 
 sent the extreme of the senes- 
 cent condition, the prolifera- 
 tions presented extreme juve- 
 nile conditions, showing sev- 
 eral types of leaf which are 
 simpler than any which were 
 found in the seedlings, and 
 usually reaching the condition of the first ncpionic leaf at the third or 
 fourth leaf of the proliferation. Some of the proliferations were more
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 19 
 
 precocious than this, however, and all showed a much more rapid devel- 
 opment of the dissected form than is seen in the seedling. 
 
 The simplest leaf presented by these proliferations (fig. 9) was a 
 minute oblong undivided structure which might be slightly widened 
 and flattened in the distal half, but otherwise showed no distinction of 
 blade and petiole. This might be followed by another undivided leaf 
 
 Fig. 9.— Proliferations froni an inflorescence of Sium cicutaefolium. Numbers 
 show position of each leaf in its own proliferation. Variously magnlfled, x 6 to x 8. 
 
 with a wider, elliptic blade. Then there might be a leaf having one or 
 a pair of simple lateral lobes, followed perhaps by the 5-lobed leaf or 
 the form which has been described as the simplest type of 3-lobed leaf 
 occurring in the seedling, having a 2-notched terminal lobe and bifid 
 lateral lobes. Whether the juvenile stages are atavistic or not, these 
 early leaves of the proliferations logically precede and lead up to them, 
 and carry us down the series to the siinplest possible type of leaf.
 
 20 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 The rejuvenescence of all these buds as the result of a change from 
 an aerial to an aquatic habitat is in perfect agreement with Burns's 
 ( 1904) interpretation of the changes induced in a similar way in Proser- 
 pinaca pahistris and very greatly strengthens his position. The fact 
 that the juvenile leaves of that species are finely dissected in the manner 
 so frequently found in the climactic or ephebolic stages of typical sub- 
 merged plants is either purely a coincidence or may be related in some 
 unknown way to the past environmental relations of the species, as sug- 
 gested by Goebel (1899-1901, p. 546). In Stum, where the juvenile leaf 
 is not at all of the hydrophytic type, submergence in water does not 
 produce a dissected, hydrophytic leaf, but the mesophytic form of leaf 
 characteristic of the Sium seedling. 
 
 This change of view regarding the effect of water in producing the 
 modified leaf-form of these plants does not detract in the least from the 
 value of the negative results of McCallum's (1902) investigation into 
 the nature of the stimulus involved, but gives those results a bearing on 
 the phenomenon of rejuvenescence instead of the change from a terres- 
 trial to an aquatic type of leaf. To ascribe a phenomenon to good or 
 poor vegetative conditions (Burns, 1904, p. 586) does not yet trace it 
 very near to its ultimate cause or causes, but perhaps is as definite as 
 the present state of our knowledge regarding rejuvenescence would 
 warrant. The great vigor of the rejuvenated buds in Sium is scarcely 
 consistent with the view that they are due to starvation or any other 
 poor vegetative conditions, and, indeed, when we find flower-buds, rep- 
 resenting as they do the low ebb of vegetative vigor, suddenly awakened 
 into a new cycle of vegetative development we should assume that they 
 have found good rather than poor vegetative conditions. 
 
 It is evident that a distinction must be made here between the cause 
 of rejuvenescence or the reawakening into the ascending side of the 
 vegetative cycle and the cause which determines just what point in the 
 cycle shall be attained in any specific case. I would offer the tentative 
 suggestions, (a) that a process of senescence resulting in a checking 
 or a cessation of growth is a necessary condition antecedent to reju- 
 venescence, and (b) that the cause of the reawakening may be due to 
 either or both of two complex factors, namely, an increase in available 
 food-equivalent and an increased lability or mobility (perhaps largely 
 fluidity) of the protoplasmic substances. The former may be predomi- 
 nantly o];)crativc when lateral buds are forced into development by 
 checking the growth of a terminal bud, and the latter may be the domi- 
 nant factor in cases of submergence, (c) The causes which determine 
 the point in the cycle which shall be attained under any specific condi-
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 21 
 
 tion may well be the relative degree of one or other of these factors or 
 of their resultant, as compared with that necessary for the produc- 
 tion of the climax type of leaf of the adult plant. This is essen- 
 tially in accord with Burns's (1904, p. 586) suggestion of poor vegeta- 
 tive conditions, and also with the earlier observations of Cushman 
 (1902, p. 885) that weak individuals and those growing on poor soil or 
 with insufficient moisture present earlier stages than do more vigorous 
 specimens or those growing under more favorable conditions with re- 
 spect to soil and water-supply. As applied to Proserpinaca this would 
 mean that the juvenile type of leaf in the submerged plant is primarily 
 the result of protoplasmic dilution, but that the failure to attain the 
 adult condition is due to less favorable relations as to food-supply. Or- 
 dinarily in submerged plants of Proserpinaca these two factors may be 
 conceived to balance each other in such a way as to produce the dis- 
 sected leaf continuously, but an unusually vigorous plant may be able 
 to provide a food-supply sufficient to overbalance the effect of dilution, 
 and so give rise to the adult type of leaf which is occasionally seen 
 even in submerged plants. When the dissected leaf is produced in aerial 
 parts it is primarily due to a condition as to food supply better than 
 that available in the resting condition, but not equal to that necessary 
 for the production of the adult leaf-type. With the return of spring, 
 conditions for food-manufacture are improved and the adult leaf ap- 
 pears at once. 
 
 An explanation of senescence in the same terms would ascribe it to 
 decreasing lability or fluidity unaccompanied by a compensatory in- 
 crease in the available food-equivalent — a condition coming just as 
 fully under the head of poor vegetative conditions as does that pro- 
 ducing the juvenile stages. 
 
 Not only are different regions of the axis supposed to bear leaves 
 characteristic of dift'erent periods in the history of the species, but it 
 is maintained by both Jackson (1899) and Cushman (1902, 1903) that 
 certain parts of the leaf, especially the apical region, likewise show 
 atavistic conditions. Leaving wholly aside for the present the relation 
 between the parts of the present adult leaf and the form of the ancestral 
 leaf, the question involved in this proposition is the localization of the 
 differentiative activities in the leaf. The simpler character of the apical 
 region in many leaves is undeniable, and it may correspond in certain 
 cases, therefore, with ancestral conditions, though this has not been 
 proved in any specific case. But the simplicity of the apical region has 
 led both of these writers into error when they conclude that new char- 
 acters arise at the base of the leaf. After examining: manv leaves and
 
 22 
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 also the figures upon which Jackson and Cushman base their conclu- 
 sions, I am convinced that with possible rare exceptions the new char- 
 acter added to a leaf having a single blade and an entire margin consists 
 in an indentation or an incision rather than an outgrowth, and that, in 
 general, increased complexity is brought about by an increasing number 
 and depth of these incisions, which do not and can not occur at the 
 base, but must occur above it. That part of the base below the lowest 
 incision is characterized by an entire margin and is " primitive," there- 
 fore, in the same sense as is that part of the apex above the highest 
 incision. Just as Cushman (1903, p. 244) noted that emarginate apexes 
 may develop from acute ones in Astragalus adsurgens, so in cases like 
 that of his Arabis albida, in which truncate and cordate bases occur in a 
 series beginning with attenuate bases, these forms are late modifications 
 of a tapering base, due to the excess of marginal growth as compared 
 with that of the mid-rib. Although the basal margin is thus distorted 
 from its original direction, it retains its unbroken "primitive" character. 
 
 The figures which 
 illustrate Cushman's 
 papers show these 
 facts so clearly that 
 it seems strange that 
 he should not have 
 recognized them. 
 Particularly in Sib- 
 bald'wpsis{PotentiUa) 
 tridenata, Artemisia 
 stelleriana, and San- 
 gnisorba (Poteriuui) 
 canadensis is it diffi- 
 cult to harmonize his 
 figures with his state- 
 ment that new charac- 
 ters are produced at 
 base. In the first two 
 of these plants the new 
 character, as I under- 
 stand it, consists in the appearance of two small indentations very near 
 the apex. In Sanguisorba canadensis the leaf is pinnate, and in the 
 later stages of the senescent series each proximal leaflet presents in each 
 leaf a peculiar inequilateral form, though the number of leaflets de- 
 
 FiG. 10.— Diagram to test by the law of 
 periodicity tlae serial homology of 
 leaflets in a pinnate leaf. Horizontal 
 lines represent the rachises of the 
 successive leaves of a stem of Siuin 
 cicutae folium . Nodes assumed to be 
 homologous are connected by curves. 
 The proximal leaflets assumed to be 
 homologous.
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 23 
 
 creases regularly as the inflorescence is approached. In order to make 
 this leaf fit his view of basal differentiation, Cushman (1903, p. 254) 
 assumes that as each proximal leaflet disappears the next higher leaflet 
 takes on this same inequilateral form. 
 
 According to this view the proximal leaflets are the newest. If, on 
 the other hand, it be conceived that the leaflets appear in apical succes- 
 sion by the division of the terminal leaflet, and that they disappear 
 through the loss of one pair of incisions after another, proceeding now 
 basipetally, the proximal leaflets, which have in every late senescent 
 leaf the same peculiar shape, will be the oldest instead of the newest, 
 and the inequilateral leaflets of one leaf will be homologous with those 
 of all of the other leaves. In the latter case it would occasion no sur- 
 prise to find that these leaflets show a well-established form which 
 occurs with considerable constancy in each succeeding leaf. If these 
 proximal leaflets were in a state of perpetual nascence and evanescence 
 we would expect the process to result in frequent imperfect or incom- 
 plete differentiations 
 and consequent great 
 variation, a condition 
 never realized. 
 
 Every one will re- 
 call frequent instances 
 in which the terminal 
 leaflet of a pinnate 
 leaf showed imperfect 
 formation of lateral 
 leaflets by incisions 
 of greater or less 
 depth near its base 
 (n— 4 and y, fig. 3), 
 and every degree of 
 division will have 
 been noted, from a 
 slight notch to the 
 complete formation of 
 a pair of adjacent 
 leaflets. This ap- 
 pears to me to be the best possible evidence that in this region, just 
 above the base of the terminal leaflet, is the place of progressive differ- 
 entiation and reintegration in pinnate leaves, and that it is here and not 
 in the proximal leaflets that new characters are to be looked for. 
 
 Fig. U.— The same as flg. 10, but 
 the distal leaflets are assumed 
 to be homologous.
 
 24 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 As further evidence that this is the correct interpretation of the pin- 
 nate leaf, the diagrams shown in figs. lo and ii were constructed as 
 follows : All the leaves of a single stem of Sium cicutaefolhim (omitting 
 several which had disappeared at the base) were arranged in their 
 normal sequence, but side by side, with their rachises parallel and at 
 equal intervals. These rachises are represented by the horizontal lines 
 of the figures. The point of origin of each pair of leaflets was then 
 marked, and the figures were completed by connecting with lines the 
 points of origin of leaflets assumed to be homologous. In fig. lo the 
 proximal leaflets, and in fig. ii the distal leaflets, are assumed to be 
 homologous. The basis for the interpretation of these figures is to be 
 found in the demonstration by Fraulein Tammes (1903) that the law 
 of periodicity underlies all morphological phenomena in such a way 
 that serially homologous characters increase regularly to a maximum 
 and then decrease, decrease regularly to a minimum and then increase, 
 or that they form a half period, i. e., they begin with the maximum and 
 end with the minimum, or vice versa. Even a most superficial inspec- 
 tion of the two figures discloses the fact that in one there is a simple 
 underlying law which would lend itself to ready formulation, while in 
 the other all is confusion. The characters which are represented in 
 these figures are the interfoliola or portions of the rachis between suc- 
 cessive pairs of leaflets. Homologous interfoliola are represented by 
 the portions of the horizontal lines included between any two consecu- 
 tive curves. In fig. 10 these interfoliola are seen to reach a definite 
 maximum length in the second leaf from the base of the figure and to 
 diminish continuously from that maximum upward until each in turn 
 is reduced to zero, as required by the law of periodicity. The only 
 irregularities appear in the two distal interfoliola of the third leaf from 
 the base of the figure and in the distal interfoliolum of the first and of 
 the fifth leaves from the base, these variations in the distal interfoliola 
 being in full accord with my view that this is the region of active dififer- 
 entiation and reintegration in the leaf. In fig. 11, on the other hand, 
 there is not a single instance in which a series of interfoliola assumed 
 to be homologous shows an increase to a definite maximum followed by 
 a continuous decrease. The law of periodicity is beautifully exemplified 
 in fig. 10, which is based on the assumption that the proximal leaflets 
 of one leaf are homologous with the proximal leaflets of every other leaf 
 of the same stem, while there is no indication of that law in fig. ii, in 
 which the distal leaflets are assumed to be homologous. Certainly no 
 more conclusive proof of the truth of my proposition could be asked. 
 
 As in the case of the juvenile and senescent leaves, the sole basis for 
 the assumption that the base and apex of a leaf exhibit conditions which
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 25 
 
 were characteristic of the whole adult leaf at some time in the past 
 history of the species is the fact of their greater simplicity. The rela- 
 tion is purely a logical one, and the condition of these regions of the leaf 
 are not unlikely to disagree with phylogenetic conditions in every detail 
 except simplicity. 
 
 Although this study makes it evident that no satisfactory inferences 
 can be drawn from the forms occurring at various ontogenetic stages 
 regarding the ancestral adult conditions, these forms do have a bearing 
 upon the relationship of allied species. The similarity of two species 
 as to the characters possessed at any of these stages would lend evidence 
 in favor of their close relationship in the same way that similarity in 
 any other activity or character would, not because of resemblance to a 
 common ancestral adult condition, but because of the presumptive evi- 
 dence of similarity in present protoplasmic structure. 
 
 All the evidence now available indicates that when specific differen- 
 tiation takes place the changed structure of the protoplasm which pro- 
 duces new adult characters also gives rise to new characters in the 
 juvenile and senescent series. The assumption that these stages have 
 a phylogenetic significance tends to obscure the fact that they are the 
 results of present conditions instead of the past history of the proto- 
 plasm and that they are in need of physiological rather than phylo- 
 genetic investigation and interpretation.
 
 2.(i DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 SUMMARY. 
 
 The various forms of leaf which occur at different parts of a speci- 
 men of Slum ciciitaefoliiim are described and their bearing on the phy- 
 logenetic history of the species is considered. The principal conclu- 
 sions are as follows : 
 
 There is great variability in the early nepionic leaves, the first leaf 
 being the most variable and the variability being progressively lessened 
 in subsequent growth. 
 
 The interpolation of an undivided leaf between the 3-lobed and the 
 pinnate leaf is illogical and might be interpreted as representing phylo- 
 genetic mutation. 
 
 To show how wholly gratuitous is the attempt to draw conclusions 
 from juvenile stages regarding ancestral adult conditions, it is pointed 
 out that in the species of Onagra, where the ancestry is definitely 
 known, the juvenile leaves may resemble more closely the adult leaves 
 of the same species than they do either the seedling or the adult leaves 
 of the parent species. 
 
 The senescent stages are totally different from those of the seedling. 
 The leaves show a regular reduction without sudden changes of type, 
 and there is increasing constancy in form as higher stages are reached. 
 
 Although the several stages of Sium present the appearance of being 
 closely correlated with the successive changes in its environment, the 
 plant passes through the same stages whenever the conditions are favor- 
 able for growth, regardless of the characteristics in the environment 
 which have been supposed to determine the several types of structure. 
 
 This cycle of development can be repeated only through rejuve- 
 nescence. This may result from fertilization and seed-production, 
 but can be brought about in other ways. Rejuvenations of axillary 
 buds and of flower-buds are figured and described. Rejuvenescence 
 in these buds was caused by submergence in water of stems in the 
 senescent state. The later the stage of senescence reached the earlier 
 are the juvenile stages produced on rejuvenescence. Proliferations of 
 flower-buds showed several leaf-forms which are simpler than any 
 found in the seedlings. 
 
 Rejuvenescence may be due to increased available food-equivalent, 
 or to increased lability or fluidity of the protoplasmic substances, or a 
 combination of these two factors. The stage attained in any case of 
 rejuvenescence may be due to the relative value of the resultant of these 
 two complex factors, as compared with that necessary to the production 
 of the climax type of leaf of the adult j)lant.
 
 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 2^ 
 
 It has been claimed that the apex of a leaf presents primitive condi- 
 tions and that new leaf-characters appear at the base of the blade. It 
 is shown here that the base is also " primitive " and that the new char- 
 acters appear above the base. 
 
 The proximal leaflets of pinnate leaves on the same stem are homolo- 
 gous, as are all other pairs of leaflets having positions of like order, 
 counting from the proximal pair. 
 
 The sole basis for the assumption that localized stages present ata- 
 vistic characters is the fact of their greater simplicity. No satisfactory 
 inferences can be drawn from ontogenetic leaf-characters regarding 
 the phylogenetic history of the species. The various stages are the 
 result of present protoplasmic structure instead of the past history of 
 the protoplasm, and a change of structure which results in new adult 
 characters may also produce changed juvenile and senescent characters. 
 They are in need of physiological instead of phylogenetic interpretation. 
 
 Station for Experimental Evolution, 
 
 Cold Spring Harbor, N. Y., December, 1904.
 
 28 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 
 
 LITERATURE CITED. 
 
 Agassiz, Louis. 
 
 1848-1849. Twelve lectures on comparative embryology. Boston: Lowell 
 
 Institute. 
 
 Braun, Alexander. 
 
 1851. Reflections on the phenomenon of rejuvenescence in nature, especially 
 in the life and development of plants. Translation from the German, by 
 A. Henfrey. London: Ray Society, 1853. 
 
 Burns, George P. 
 
 1904. Heterophylly in Proserpinaca palustris L. Annals of Botany, 18: 579- 
 587. 
 CusHMAN, Joseph A. 
 
 1902. Studies of localized stages of growth in some common New England 
 plants. Amer. Nat., 36: 865-885. 
 
 1903. Studies of localized stages in some plants of the botanic gardens of 
 Harvard University. Amer. Nat., 37: 243-259. 
 
 1904. Localized stages in common roadside plants. Amer. Nat, 38: 819-832. 
 
 Glaser, Otto C. 
 
 1902. The law of von Baer. Science, N. S., 1 5 : 976-982. 
 
 Goebel, Karl. 
 
 1898-1901. Organographie der Pflanzen insbesondere der Archegoniaten und 
 Samenpflanzen. pp. xviii, 838. Figs. 539. Jena : Gustav Fischer. 
 
 Haeckel, Ernst. 
 
 1866. Generelle Morphologic der Organismen, 2 Bde. Berlin : G. Reimer. 
 
 Hyatt, Alpheus. 
 
 1890. Genesis of the Arietidae. Smithsonian Contribution 673, and Mem. 
 
 Mus. Comp. Zool., 16: 
 1897. Cycle in the life of the individual (ontogeny) and in the evolution of 
 
 its own goup (phylogeny). Proc. Amer. Acad. Arts and Sciences, 32: 
 
 209-224. 
 
 Jackson, R. T. 
 
 1899. Localized stages in development in plants and animals. Proc. Boston 
 Soc. Nat. Hist., 5 : 89-153. 
 
 Klebs, G. 
 
 1903. Willkiirliche EntwicWungsanderungen bei Pflanzen. pp. iv, 166. Figs. 
 28. Jena : Gustav Fischer. 
 
 McCallum, W. B. 
 
 1902. On the nature of the stimulus causing the change of form and structure 
 in Proserpinaca palustris. Bot. Gaz., 34 : 93-108. 
 
 TammES, Fraulein TinB. 
 
 1903. Die Periodioitat imorphologischer Ersdheinungen bei den Pflanzen. 
 Verhandelingen d. Koninklijke Akad. v. Wetenschappen te Amsterdam, 
 9 (No. 5) : 1-148. 
 
 Von Baer, K. E. 
 
 1828. Ueber Entwicklungsgesdhiohtc der Thiere. Beobadhtungen und Re- 
 flexion. Konigsberg: Gobr. Borntrager.
 
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