Hitherto we have considered the palaeontological record of plants in
relation to Evolution. The question remains, whether the record
throws any light on the theory of which Darwin and Wallace were the
authors--that of Natural Selection. The subject is clearly one which
must be investigated by other methods than those of the palaeontologist;
still there are certain important points involved, on which the
palaeontological record appears to bear.
One of these points is the supposed distinction between morphological
and adaptive characters, on which Nageli, in particular, laid so much
stress. The question is a difficult one; it was discussed by Darwin
("Origin of Species" (6th edition), pages 170-176.), who, while
showing that the apparent distinction is in part to be explained by our
imperfect knowledge of function, recognised the existence of important
morphological characters which are not adaptations. The following
passage expresses his conclusion. "Thus, as I am inclined to believe,
morphological differences, which we consider as important--such as
the arrangement of the leaves, the divisions of the flower or of the
ovarium, the position of the ovules, etc.--first appeared in many cases
as fluctuating variations, which sooner or later became constant through
the nature of the organism and of the surrounding conditions, as well
as through the inter-crossing of distinct individuals, but not through
natural selection; for as these morphological characters do not affect
the welfare of the species, any slight deviations in them could not have
been governed or accumulated through this latter agency." (Ibid. page
176.)
This is a sufficiently liberal concession; Nageli, however, went much
further when he said: "I do not know among plants a morphological
modification which can be explained on utilitarian principles." (See
"More Letters", Vol. II. page 375 (footnote).) If this were true the
field of Natural Selection would be so seriously restricted, as to leave
the theory only a very limited importance.
It can be shown, as the writer believes, that many typical
"morphological characters," on which the distinction between great
classes of plants is based, were adaptive in origin, and even that their
constancy is due to their functional importance. Only one or two cases
will be mentioned, where the fossil evidence affects the question.
The pollen-tube is one of the most important morphological characters of
the Spermophyta as now existing--in fact the name Siphonogama is used
by Engler in his classification, as expressing a peculiarly constant
character of the Seed-plants. Yet the pollen-tube is a manifest
adaptation, following on the adoption of the seed-habit, and serving
first to bring the spermatozoids with greater precision to their
goal, and ultimately to relieve them of the necessity for independent
movement. The pollen-tube is constant because it has proved to be
indispensable.
In the Palaeozoic Seed-plants there are a number of instances in which
the pollen-grains, contained in the pollen-chamber of a seed, are so
beautifully preserved that the presence of a group of cells within the
grain can be demonstrated; sometimes we can even see how the cell-walls
broke down to emit the sperms, and quite lately it is said that the
sperms themselves have been recognised. (F.W. Oliver, "On Physostoma
elegans, an archaic type of seed from the Palaeozoic Rocks", "Annals of
Botany", January, 1909. See also the earlier papers there cited.) In
no case, however, is there as yet any satisfactory evidence for
the formation of a pollen-tube; it is probable that in these early
Seed-plants the pollen-grains remained at about the evolutionary level
of the microspores in Pilularia or Selaginella, and discharged their
spermatozoids directly, leaving them to find their own way to the
female cells. It thus appears that there were once Spermophyta without
pollen-tubes. The pollen-tube method ultimately prevailed, becoming a
constant "morphological character," for no other reason than because,
under the new conditions, it provided a more perfect mechanism for the
accomplishment of the act of fertilisation. We have still, in the Cycads
and Ginkgo, the transitional case, where the tube remains short, serves
mainly as an anchor and water-reservoir, but yet is able, by its slight
growth, to give the spermatozoids a "lift" in the right direction. In
other Seed-plants the sperms are mere passengers, carried all the way by
the pollen-tube; this fact has alone rendered the Angiospermous method
of fertilisation through a stigma possible.
We may next take the seed itself--the very type of a morphological
character. Our fossil record does not go far enough back to tell us the
origin of the seed in the Cycadophyta and Pteridosperms (the main line
of its development) but some interesting sidelights may be obtained from
the Lycopod phylum. In two Palaeozoic genera, as we have seen, seed-like
organs are known to have been developed, resembling true seeds in the
presence of an integument and of a single functional embryo-sac, as well
as in some other points. We will call these organs "seeds" for the sake
of shortness. In one genus (Lepidocarpon) the seeds were borne on a cone
indistinguishable from that of the ordinary cryptogamic Lepidodendreae,
the typical Lycopods of the period, while the seed itself retained
much of the detailed structure of the sporangium of that family. In the
second genus, Miadesmia, the seed-bearing plant was herbaceous, and much
like a recent Selaginella. (See Margaret Benson, "Miadesmia membranacea,
a new Palaeozoic Lycopod with a seed-like structure", "Phil. Trans.
Royal Soc. Vol." 199, B. 1908.) The seeds of the two genera are
differently constructed, and evidently had an independent origin. Here,
then, we have seeds arising casually, as it were, at different points
among plants which otherwise retain all the characters of their
cryptogamic fellows; the seed is not yet a morphological character of
importance. To suppose that in these isolated cases the seed sprang into
being in obedience to a Law of Advance ("Vervollkommungsprincip"),
from which other contemporary Lycopods were exempt, involves us in
unnecessary mysticism. On the other hand it is not difficult to see how
these seeds may have arisen, as adaptive structures, under the influence
of Natural Selection. The seed-like structure afforded protection to the
prothallus, and may have enabled the embryo to be launched on the world
in greater security. There was further, as we may suppose, a gain in
certainty of fertilisation. As the writer has pointed out elsewhere,
the chances against the necessary association of the small male with the
large female spores must have been enormously great when the cones were
borne high up on tall trees. The same difficulty may have existed in the
case of the herbaceous Miadesmia, if, as Miss Benson conjectures, it was
an epiphyte. One way of solving the problem was for pollination to take
place while the megaspore was still on the parent plant, and this is
just what the formation of an ovule or seed was likely to secure.
The seeds of the Pteridosperms, unlike those of the Lycopod stock,
have not yet been found in statu nascendi--in all known cases they were
already highly developed organs and far removed from the cryptogamic
sporangium. But in two respects we find that these seeds, or some
of them, had not yet realised their possibilities. In the seed
of Lyginodendron and other cases the micropyle, or orifice of the
integument, was not the passage through which the pollen entered; the
open neck of the pollen-chamber protruded through the micropyle and
itself received the pollen. We have met with an analogous case, at a
more advanced stage of evolution, in the Bennettiteae, where the wall
of the gynaecium, though otherwise closed, did not provide a stigma to
catch the pollen, but allowed the micropyles of the ovules to protrude
and receive the pollen in the old gymnospermous fashion. The integument
in the one case and the pistil in the other had not yet assumed all
the functions to which the organ ultimately became adapted. Again, no
Palaeozoic seed has yet been found to contain an embryo, though the
preservation is often good enough for it to have been recognised if
present. It is probable that the nursing of the embryo had not yet come
to be one of the functions of the seed, and that the whole embryonic
development was relegated to the germination stage.
In these two points, the reception of the pollen by the micropyle and
the nursing of the embryo, it appears that many Palaeozoic seeds
were imperfect, as compared with the typical seeds of later times.
As evolution went on, one function was superadded on another, and
it appears impossible to resist the conclusion that the whole
differentiation of the seed was a process of adaptation, and
consequently governed by Natural Selection, just as much as the
specialisation of the rostellum in an Orchid, or of the pappus in a
Composite.
Did space allow, other examples might be added. We may venture to
maintain that the glimpses which the fossil record allows us into early
stages in the evolution of organs now of high systematic importance,
by no means justify the belief in any essential distinction between
morphological and adaptive characters.
Another point, closely connected with Darwin's theory, on which the
fossil history of plants has been supposed to have some bearing, is
the question of Mutation, as opposed to indefinite variation. Arber and
Parkin, in their interesting memoir on the Origin of Angiosperms,
have suggested calling in Mutation to explain the apparently sudden
transition from the cycadean to the angiospermous type of foliage, in
late Mesozoic times, though they express themselves with much caution,
and point out "a distinct danger that Mutation may become the last
resort of the phylogenetically destitute"!
The distinguished French palaeobotanists, Grand'Eury (C. Grand'Eury,
"Sur les mutations de quelques Plantes fossiles du Terrain houiller".
"Comptes Rendus", CXLII. page 25, 1906.) and Zeiller (R. Zeiller
"Les Vegetaux fossiles et leurs Enchainements", "Revue du Mois", III.
February, 1907.), are of opinion, to quote the words of the latter
writer, that the facts of fossil Botany are in agreement with the sudden
appearance of new forms, differing by marked characters from those that
have given them birth; he adds that these results give more amplitude
to this idea of Mutation, extending it to groups of a higher order,
and even revealing the existence of discontinuous series between the
successive terms of which we yet recognise bonds of filiation. (Loc.
cit. page 23.)
If Zeiller's opinion should be confirmed, it would no doubt be a serious
blow to the Darwinian theory. As Darwin said: "Under a scientific point
of view, and as leading to further investigation, but little advantage
is gained by believing that new forms are suddenly developed in an
inexplicable manner from old and widely different forms, over the old
belief in the creation of species from the dust of the earth." ("Origin
of Species", page 424.)
It most however be pointed out, that such mutations as Zeiller, and to
some extent Arber and Parkin, appear to have in view, bridging the gulf
between different Orders and Classes, bear no relation to any mutations
which have been actually observed, such as the comparatively small
changes, of sub-specific value, described by De Vries in the type-case
of Oenothera Lamarckiana. The results of palaeobotanical research have
undoubtedly tended to fill up gaps in the Natural System of plants--that
many such gaps still persist is not surprising; their presence may well
serve as an incentive to further research but does not, as it seems
to the writer, justify the assumption of changes in the past, wholly
without analogy among living organisms.
As regards the succession of species, there are no greater authorities
than Grand'Eury and Zeiller, and great weight must be attached to their
opinion that the evidence from continuous deposits favours a somewhat
sudden change from one specific form to another. At the same time
it will be well to bear in mind that the subject of the "absence of
numerous intermediate varieties in any single formation" was fully
discussed by Darwin. ("Origin of Species", pages 275-282, and page
312.); the explanation which he gave may go a long way to account for
the facts which recent writers have regarded as favouring the theory of
saltatory mutation.
The rapid sketch given in the present essay can do no more than call
attention to a few salient points, in which the palaeontological records
of plants has an evident bearing on the Darwinian theory. At the present
day the whole subject of palaeobotany is a study in evolution, and
derives its chief inspiration from the ideas of Darwin and Wallace. In
return it contributes something to the verification of their teaching;
the recent progress of the subject, in spite of the immense difficulties
which still remain, has added fresh force to Darwin's statement that
"the great leading facts in palaeontology agree admirably with the
theory of descent with modification through variation and natural
selection." (Ibid. page 313.)
XIII. THE INFLUENCE OF ENVIRONMENT ON THE FORMS OF PLANTS. By Georg
Klebs, PH.D.
Professor of Botany in the University of Heidelberg.
The dependence of plants on their environment became the object of
scientific research when the phenomena of life were first investigated
and physiology took its place as a special branch of science. This
occurred in the course of the eighteenth century as the result of the
pioneer work of Hales, Duhamel, Ingenhousz, Senebier and others. In
the nineteenth century, particularly in the second half, physiology
experienced an unprecedented development in that it began to concern
itself with the experimental study of nutrition and growth, and with
the phenomena associated with stimulus and movement; on the other hand,
physiology neglected phenomena connected with the production of form, a
department of knowledge which was the province of morphology, a purely
descriptive science. It was in the middle of the last century that the
growth of comparative morphology and the study of phases of development
reached their highest point.
The forms of plants appeared to be the expression of their inscrutable
inner nature; the stages passed through in the development of the
individual were regarded as the outcome of purely internal and hidden
laws. The feasibility of experimental inquiry seemed therefore remote.
Meanwhile, the recognition of the great importance of such a causal
morphology emerged from the researches of the physiologists of that
time, more especially from those of Hofmeister (Hofmeister, "Allgemeine
Morphologie", Leipzig, 1868, page 579.), and afterwards from the work of
Sachs. (Sachs, "Stoff und Form der Pflanzenorgane", Vol. I. 1880; Vol.
II. 1882. "Gesammelte Abhandlungen uber Pflanzen-Physiologie", II.
Leipzig, 1893.) Hofmeister, in speaking of this line of inquiry,
described it as "the most pressing and immediate aim of the investigator
to discover to what extent external forces acting on the organism are of
importance in determining its form." This advance was the outcome of the
influence of that potent force in biology which was created by Darwin's
"Origin of Species" (1859).
The significance of the splendid conception of the transformation of
species was first recognised and discussed by Lamarck (1809); as an
explanation of transformation he at once seized upon the idea--an
intelligible view--that the external world is the determining factor.
Lamarck (Lamarck, "Philosophie zoologique", pages 223-227. Paris, 1809.)
endeavoured, more especially, to demonstrate from the behaviour
of plants that changes in environment induce change in form which
eventually leads to the production of new species. In the case of
animals, Lamarck adopted the teleological view that alterations in the
environment first lead to alterations in the needs of the organisms,
which, as the result of a kind of conscious effort of will, induce
useful modifications and even the development of new organs. His work
has not exercised any influence on the progress of science: Darwin
himself confessed in regard to Lamarck's work--"I got not a fact or idea
from it." ("Life and Letters", Vol. II. page 215.)
On a mass of incomparably richer and more essential data Darwin
based his view of the descent of organisms and gained for it general
acceptance; as an explanation of modification he elaborated the
ingeniously conceived selection theory. The question of special interest
in this connection, namely what is the importance of the influence
of the environment, Darwin always answered with some hesitation and
caution, indeed with a certain amount of indecision.
The fundamental principle underlying his theory is that of general
variability as a whole, the nature and extent of which, especially
in cultivated organisms, are fully dealt with in his well-known book.
(Darwin, "The variation of Animals and Plants under domestication",
2 vols., edition 1, 1868; edition 2, 1875; popular edition 1905.) In
regard to the question as to the cause of variability Darwin adopts a
consistently mechanical view. He says: "These several considerations
alone render it probable that variability of every kind is directly or
indirectly caused by changed conditions of life. Or, to put the case
under another point of view, if it were possible to expose all the
individuals of a species during many generations to absolutely uniform
conditions of life, there would be no variability." ("The variation of
Animals and Plants" (2nd edition), Vol. II. page 242.) Darwin did not
draw further conclusions from this general principle.
Variations produced in organisms by the environment are distinguished by
Darwin as "the definite" and "the indefinite." (Ibid. II. page 260. See
also "Origin of Species" (6th edition), page 6.) The first occur "when
all or nearly all the offspring of an individual exposed to certain
conditions during several generations are modified in the same manner."
Indefinite variation is much more general and a more important factor in
the production of new species; as a result of this, single individuals
are distinguished from one another by "slight" differences, first in
one then in another character. There may also occur, though this is very
rare, more marked modifications, "variations which seem to us in our
ignorance to arise spontaneously." ("Origin of Species" (6th edition),
page 421.) The selection theory demands the further postulate that such
changes, "whether extremely slight or strongly marked," are inherited.
Darwin was no nearer to an experimental proof of this assumption than to
the discovery of the actual cause of variability. It was not until the
later years of his life that Darwin was occupied with the "perplexing
problem... what causes almost every cultivated plant to vary" ("Life
and Letters", Vol. III. page 342.): he began to make experiments on the
influence of the soil, but these were soon given up.
In the course of the violent controversy which was the outcome of
Darwin's work the fundamental principles of his teaching were not
advanced by any decisive observations. Among the supporters and
opponents, Nageli (Nageli, "Theorie der Abstammungslehre", Munich, 1884;
cf. Chapter III.) was one of the few who sought to obtain proofs by
experimental methods. His extensive cultural experiments with alpine
Hieracia led him to form the opinion that the changes which are induced
by an alteration in the food-supply, in climate or in habitat, are not
inherited and are therefore of no importance from the point of view of
the production of species. And yet Nageli did attribute an important
influence to the external world; he believed that adaptations of plants
arise as reactions to continuous stimuli, which supply a need and are
therefore useful. These opinions, which recall the teleological
aspect of Lamarckism, are entirely unsupported by proof. While other
far-reaching attempts at an explanation of the theory of descent were
formulated both in Nageli's time and afterwards, some in support of,
others in opposition to Darwin, the necessity of investigating, from
different standpoints, the underlying causes, variability and heredity,
was more and more realised. To this category belong the statistical
investigations undertaken by Quetelet and Galton, the researches into
hybridisation, to which an impetus was given by the re-discovery of
the Mendelian law of segregation, as also by the culture experiments
on mutating species following the work of de Vries, and lastly the
consideration of the question how far variation and heredity are
governed by external influences. These latter problems, which
are concerned in general with the causes of form-production and
form-modification, may be treated in a short summary which falls under
two heads, one having reference to the conditions of form-production in
single species, the other being concerned with the conditions governing
the transformation of species.
I. THE INFLUENCE OF EXTERNAL CONDITIONS ON FORM-PRODUCTION IN SINGLE
SPECIES.
The members of plants, which we express by the terms stem, leaf, flower,
etc. are capable of modification within certain limits; since Lamarck's
time this power of modification has been brought more or less into
relation with the environment. We are concerned not only with the
question of experimental demonstration of this relationship, but, more
generally, with an examination of the origin of forms, the sequences of
stages in development that are governed by recognisable causes. We have
to consider the general problem; to study the conditions of all typical
as well as of atypic forms, in other words, to found a physiology of
form.
If we survey the endless variety of plant-forms and consider the highly
complex and still little known processes in the interior of cells, and
if we remember that the whole of this branch of investigation came into
existence only a few decades ago, we are able to grasp the fact that
a satisfactory explanation of the factors determining form cannot be
discovered all at once. The goal is still far away. We are not concerned
now with the controversial question, whether, on the whole, the
fundamental processes in the development of form can be recognised by
physiological means. A belief in the possibility of this can in any case
do no harm. What we may and must attempt is this--to discover points
of attack on one side or another, which may enable us by means of
experimental methods to come into closer touch with these elusive
and difficult problems. While we are forced to admit that there is at
present much that is insoluble there remains an inexhaustible supply of
problems capable of solution.
The object of our investigations is the species; but as regards the
question, what is a species, science of to-day takes up a position
different from that of Darwin. For him it was the Linnean species which
illustrates variation: we now know, thanks to the work of Jordan,
de Bary, and particularly to that of de Vries (de Vries, "Die
Mutationstheorie", Leipzig, 1901, Vol. I. page 33.), that the Linnean
species consists of a large or small number of entities, elementary
species. In experimental investigation it is essential that observations
be made on a pure species, or, as Johannsen (Johannsen, "Ueber
Erblichkeit in Populationen und reinen Linien", Jena, 1903.) says, on
a pure "line." What has long been recognised as necessary in the
investigation of fungi, bacteria and algae must also be insisted on in
the case of flowering plants; we must start with a single individual
which is reproduced vegetatively or by strict self-fertilisation.
In dioecious plants we must aim at the reproduction of brothers and
sisters.
We may at the outset take it for granted that a pure species remains the
same under similar external conditions; it varies as these vary. IT IS
CHARACTERISTIC OF A SPECIES THAT IT ALWAYS EXHIBITS A CONSTANT RELATION
TO A PARTICULAR ENVIRONMENT. In the case of two different species, e.g.
the hay and anthrax bacilli or two varieties of Campanula with blue and
white flowers respectively, a similar environment produces a constant
difference. The cause of this is a mystery.
According to the modern standpoint, the living cell is a complex
chemico-physical system which is regarded as a dynamical system of
equilibrium, a conception suggested by Herbert Spencer and which has
acquired a constantly increasing importance in the light of modern
developments in physical chemistry. The various chemical compounds,
proteids, carbohydrates, fats, the whole series of different ferments,
etc. occur in the cell in a definite physical arrangement. The two
systems of two species must as a matter of fact possess a constant
difference, which it is necessary to define by a special term. We say,
therefore, that the SPECIFIC STRUCTURE is different.
By way of illustrating this provisionally, we may assume that the
proteids of the two species possess a constant chemical difference. This
conception of specific structure is specially important in its bearing
on a further treatment of the subject. In the original cell, eventually
also in every cell of a plant, the characters which afterwards
become apparent must exist somewhere; they are integral parts of the
capabilities or potentialities of specific structure. Thus not only the
characters which are exhibited under ordinary conditions in nature, but
also many others which become apparent only under special conditions (In
this connection I leave out of account, as before, the idea of material
carriers of heredity which since the publication of Darwin's Pangenesis
hypothesis has been frequently suggested. See my remarks in "Variationen
der Bluten", "Pringsheim's Jahrb. Wiss. Bot." 1905, page 298; also
Detto, "Biol. Centralbl." 1907, page 81, "Die Erklarbarkeit der
Ontogenese durch materielle Anlagen".), are to be included as such
potentialities in cells; the conception of specific structure includes
the WHOLE OF THE POTENTIALITIES OF A SPECIES; specific structure
comprises that which we must always assume without being able to explain
it.
A relatively simple substance, such as oxalate of lime, is known under
a great number of different crystalline forms belonging to different
systems (Compare Kohl's work on "Anatomisch-phys. Untersuchungen uber
Kalksalze", etc. Marburg, 1889.); these may occur as single crystals,
concretions or as concentric sphaerites. The power to assume this
variety of form is in some way inherent in the molecular structure,
though we cannot, even in this case, explain the necessary connection
between structure and crystalline form. These potentialities can only
become operative under the influence of external conditions; their
stimulation into activity depends on the degree of concentration of the
various solutions, on the nature of the particular calcium salt, on the
acid or alkaline reactions. Broadly speaking, the plant cell behaves in
a similar way. The manifestation of each form, which is inherent as a
potentiality in the specific structure, is ultimately to be referred to
external conditions.
An insight into this connection is, however, rendered exceedingly
difficult, often quite impossible, because the environment never
directly calls into action the potentialities. Its influence is exerted
on what we may call the inner world of the organism, the importance of
which increases with the degree of differentiation. The production of
form in every plant depends upon processes in the interior of the cells,
and the nature of these determines which among the possible characters
is to be brought to light. In no single case are we acquainted with the
internal process responsible for the production of a particular
form. All possible factors may play a part, such as osmotic pressure,
permeability of the protoplasm, the degree of concentration of the
various chemical substances, etc.; all these factors should be included
in the category of INTERNAL CONDITIONS. This inner world appears the
more hidden from our ken because it is always represented by a certain
definite state, whether we are dealing with a single cell or with a
small group of cells. These have been produced from pre-existing cells
and they in turn from others; the problem is constantly pushed back
through a succession of generations until it becomes identified with
that of the origin of species.
A way, however, is opened for investigation; experience teaches us that
this inner world is not a constant factor: on the contrary, it appears
to be very variable. The dependence of VARIABLE INTERNAL on VARIABLE
EXTERNAL conditions gives us the key with which research may open the
door. In the lower plants this dependence is at once apparent, each cell
is directly subject to external influences. In the higher plants with
their different organs, these influences were transmitted to cells in
course of development along exceedingly complex lines. In the case of
the growing-point of a bud, which is capable of producing a complete
plant, direct influences play a much less important part than those
exerted through other organs, particularly through the roots and leaves,
which are essential in nutrition. These correlations, as we may call
them, are of the greatest importance as aids to an understanding of
form-production. When a bud is produced on a particular part of a plant,
it undergoes definite internal modifications induced by the influence of
other organs, the activity of which is governed by the environment, and
as the result of this it develops along a certain direction; it may,
for example, become a flower. The particular direction of development
is determined before the rudiment is differentiated and is exerted so
strongly that further development ensues without interruption, even
though the external conditions vary considerably and exert a positively
inimical influence: this produces the impression that development
proceeds entirely independently of the outer world. The widespread
belief that such independence exists is very premature and at all events
unproven.
The state of the young rudiment is the outcome of previous influences of
the external world communicated through other organs. Experiments show
that in certain cases, if the efficiency of roots and leaves as organs
concerned with nutrition is interfered with, the production of flowers
is affected, and their characters, which are normally very constant,
undergo far-reaching modifications. To find the right moment at which to
make the necessary alteration in the environment is indeed difficult
and in many cases not yet possible. This is especially the case with
fertilised eggs, which in a higher degree than buds have acquired,
through parental influences, an apparently fixed internal organisation,
and this seems to have pre-determined their development. It is, however,
highly probable that it will be possible, by influencing the parents,
to alter the internal organisation and to switch off development on to
other lines.
Having made these general observations I will now cite a few of the many
facts at our disposal, in order to illustrate the methods and aim of the
experimental methods of research. As a matter of convenience I will deal
separately with modification of development and with modification of
single organs.