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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

J

CARNEGIE INSTITUTION OF WASHINGTON, PUBLICATION No. 30.

PAPERS OF STATION FOR EXPERIMENTAL EVOLUTION
AT COLD SPRING HARBOR, NEW YORK. No. 3.

3

PRESS OF

HENRY E. WILKENS PRINTING CO.

WASHINGTON. D. C.

PLATE I.

Stum cicutaefolium Gmel. A jar of seedlings, photographed at the University
of Chicago, October 10, 1903, by Dr. W. J. G. Land ; spring-growth of an old
plant, photographed at Lockport, 111., May 16, 1903, by W. E. Praeger ; and a
flowering-stem photographed at South Chicago, 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.

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. Von 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
occur in both, a given stage appearing earlier in the descendant than in
the ancestor. The idea that larval and juvenile characters agree with
ancestral adult characters was an old conception which was rehabilitated
by Louis Agassiz (1848-1849) and became crystallized in the epigram
of Haeckel (1866), which is now universally known as the "Law of
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

*For a history of the development of the 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)

4 DEVELOPMENT OF SIUM CICUTAEFOLIUM.

growth, modified characters which he believes 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 cicutae folium 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 cicutaefoliwn 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 Urn-
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.

(&) 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 (&) 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 Robey, 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.

O DEVELOPMENT OF SIUM CICUTAEFOLIUM.

(/) 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 were 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), (&), (c) , etc.

The first two classes, (a) and (&), 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 III, 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.

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), (£), (c\ etc., in the text.

PLATE III.

Variation in successive seedling-leaves of Sium cicutaefolium, 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 73 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.

]

Positioi

i of le

if.

i

2

3

4

5

6

7

8

(a) Five-lobed

IQ 5

(&) Three-lobed

*y-D
IQ 4.

1.6

(c) Three-lobed

28 Z

TC 7

37 7

22 6

A 7

7 A

(d) Half 3-lobed

TC 7

*o-t
182

*j'l
22 O

Id. 5

HO
47

O'H

6.Q

(^) Unlobed or irregular

10-O
25 O

61 2

39 0

**rO
22 6

•O

17 O

u.y
7 A

(0 One pair of leaflets. .
(g) Two pairs of leaflets

*o-u
1.6

5-0

15-3

38.7

A/.V»

72.3

2 I

O-t

44.8

4.1 4

27.0
51 4

9-1
182

(,h) Three pairs of leaflets

21 6

72.7

8 DEVELOPMENT OF SIUM 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 (<?). The fifth (e) is composite, but not readily divisible.

2. Some of the distinctions are qualitative, i. e., complexly quantita-
tive, as exemplified by (&), (c), (e), etc.; others are simply quantita-
tive, as, for example, (/), (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 a,ttempt 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 73 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

70

10

FIG. 1. — Graphic representation of the variation in the several " nepionic "
leaves of 5twm cicutaefolium, from the first to the eighth, inclusive.
Ordinates 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 self-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 may 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

IO 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 lamarck-
iana and 0. rubrinervis, the former species being the parent of the
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 lamarckiana.

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
cicutaefolium 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 Sium 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.

Seedlings of Onagra rubrinervis grown at the New York Botanical Garden.
Photograph supplied by Dr. D. T. MacDougal.

PLATE VI.

Large rosettes of Onagra lamarckiana (upper) and O. rubrinervis (lower), grown
at the New York Botanical Garden. Photographs supplied by 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 Proserpinaca 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
Sinm cicutae folium.

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 modification of leaves in the seedlings of
Sium cicutaefolium. Numbers represent the position of each
leaf in the series, beginning with the first nepionic leaf.
Three-fourths natural size.

in so far as this region indicates the ancestry of the species the ancient
Siums should have resembled a fennel (Poeniculum} 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 Sium 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 Sium 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 Sium 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 different from either of tho'se 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 Sium cicutae folium.
Numbered downward from n, the most reduced leaf found in
the inflorescence; y a terminal leaflet showing incomplete
differentiation 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-

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 Slum 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 Sium cicutaefolium 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
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-

c

<0

— Gameto-
phyte

Spores •

Gametes

fertilization

FIG. 4.— Diagram of the life-cycle of a spermatophyte,
showing the relation of the processor rejuvenes-
cence as a "short-cut" across the cycle. Upper
"rejuvenescence" line may represent the rejuve-
nescence of buds at base of stem (fig. 7) ; the lower,
the proliferations of the flower-buds (figs. 8 and 9).

DEVELOPMENT OF SIUM CICUTAEFOLIUM.

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 Slum cicu-
taefolium 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

Fio. 6. — Part of the stem of a specimen of Sium
cicutaefolium with three rejuvenated buds.
Drawn by J. Marion Shull from a photograph.

:6

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 stem of Sium cicutaefo-
lium: n— 6 and n— 4, 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 VII.

Sium cicutaefolinm Gmel. The first four leaves of the spring-growth of an old
specimen. Two-fifths natural size. Photographed April, 1505, from a specimen
taken into the house February 25, 1905.

DEVELOPMENT OF SIUM CICUTAEFOLIUM. I/

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-lobed 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

FIQ. 7.— Leaves of a rejuvenated bud from near the base of a stem of Sium cicutae-
folium. Letters represent in a general way the position of each leaf in the reju-
venation; y, 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 flowering 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 Sium cicutaefo-
lium. Drawn by J.Marion
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 nepionic leaf at the third or
fourth leaf of the proliferation. Some of the proliferations were more

DEVELOPMENT OF SIUM CICUTAEFOLIUM.

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 from an inflorescence of Sium cicutae folium. Numbers
show position of each leaf in its own proliferation. Variously magnified, 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 simplest possible type of leaf.

2O 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 palustris 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 Sium, 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 iMcCallum'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 operative 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 different 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 many 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-
baldiopsis(Potentilla)
tridenata, Artemisia
stelleriana, and San-
guisorba (Poteriitm)
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 the serial homology of
leaflets in a pinnate leaf. Horizontal
lines represent the rachises of the
successive leaves of a stem of Sium
cicutae folium. Nodes assumed to be
homologous are connected by curves.
The proximal leaflets assumed to be
homologous.

DEVELOPMENT OF SIUM CICUTAEFOLIUM.

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. 11.— The same as fig. 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. 10 and n were constructed as
follows : All the leaves of a single stem of Slum cicutaef olium (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. 10 the
proximal leaflets, and in fig. u 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 differ-
entiation and reintegration in the leaf. In fig. n, 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.

26 DEVELOPMENT OF SIUM CICUTAEFOLIUM.

SUMMARY.

The various forms of leaf which occur at different parts of a speci-
men of Sium cicutaefolium 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 Slum 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 plant.

DEVELOPMENT OF SIUM CICUTAEFOLIUM.

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. Willkurliche EntwicMungsanderungeti bei Pflanzen. pp. iv, 166. Figs.
28. Jena : Gustav Fisoher.

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 TINE.

1903. Die Periodioitat morphologiseher Ersdheinungen bei den Pflanzen.
Verhandelingen d. Koninklijke Akad. v. Wetenschappen te Amsterdam,
9 (No. 5) : 1-148.

VON BAER, K. E.

1828. Ueber Entwicklungsgesdhiohte der Thiere. Beobaditungen und Re-
flexion. Konigsberg: Gebr. Borrvtrager.

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