Every plant, whether an alga or a flowering plant passes, under natural
conditions, through a series of developmental stages characteristic of
each species, and these consist in a regular sequence of definite
forms. It is impossible to form an opinion from mere observation and
description as to what inner changes are essential for the production of
the several forms. We must endeavour to influence the inner factors by
known external conditions in such a way that the individual stages in
development are separately controlled and the order of their sequence
determined at will by experimental treatment. Such control over the
course of development may be gained with special certainty in the case
of the lower organisms.
With these it is practicable to control the principal conditions of
cultivation and to vary them in various ways. By this means it has been
demonstrated that each developmental stage depends upon special external
conditions, and in cases where our knowledge is sufficient, a particular
stage may be obtained at will. In the Green Algae (See Klebs, "Die
Bedingung der Fortpflanzung... ", Jena, 1896; also "Jahrb. fur
Wiss. Bot." 1898 and 1900; "Probleme der Entwickelung, III." "Biol.
Centralbl." 1904, page 452.), as in the case of Fungi, we may classify
the stages of development into purely vegetative growth (growth,
cell-division, branching), asexual reproduction (formation of zoospores,
conidia) and sexual processes (formation of male and female sexual
organs). By modifying the external conditions it is possible to induce
algae or fungi (Vaucheria, Saprolegnia) to grow continuously for
several years or, in the course of a few days, to die after an enormous
production of asexual or sexual cells. In some instances even an almost
complete stoppage of growth may be caused, reproductive cells being
scarcely formed before the organism is again compelled to resort to
reproduction. Thus the sequence of the different stages in development
can be modified as we may desire.
The result of a more thorough investigation of the determining
conditions appears to produce at first sight a confused impression
of all sorts of possibilities. Even closely allied species exhibit
differences in regard to the connection between their development and
external conditions. It is especially noteworthy that the same form in
development may be produced as the result of very different alterations
in the environment. At the same time we can undoubtedly detect a certain
unity in the multiplicity of the individual phenomena.
If we compare the factors essential for the different stages in
development, we see that the question always resolves itself into one
of modification of similar conditions common to all life-processes. We
should rather have inferred that there exist specific external stimuli
for each developmental stage, for instance, certain chemical agencies.
Experiments hitherto made support the conclusion that QUANTITATIVE
alterations in the general conditions of life produce different types of
development. An alga or a fungus grows so long as all the conditions
of nutrition remain at a certain optimum for growth. In order to bring
about asexual reproduction, e.g. the formation of zoospores, it is
sometimes necessary to increase the degree of intensity of external
factors; sometimes, on the other hand, these must be reduced in
intensity. In the case of many algae a decrease in light-intensity or
in the amount of salts in the culture solution, or in the temperature,
induces asexual reproduction, while in others, on the contrary, an
increase in regard to each of these factors is required to produce
the same result. This holds good for the quantitative variations which
induce sexual reproduction in algae. The controlling factor is found to
be a reduction in the supply of nutritive salts and the exposure of the
plants to prolonged illumination or, better still, an increase in the
intensity of the light, the efficiency of illumination depending on the
consequent formation of organic substances such as carbohydrates.
The quantitative alterations of external conditions may be spoken of as
releasing stimuli. They produce, in the complex equilibrium of the cell,
quantitative modifications in the arrangement and distribution of mass,
by means of which other chemical processes are at once set in motion,
and finally a new condition of equilibrium is attained. But the
commonly expressed view that the environment can as a rule act only as
a releasing agent is incorrect, because it overlooks an essential point.
The power of a cell to receive stimuli is only acquired as the result
of previous nutrition, which has produced a definite condition of
concentration of different substances. Quantities are in this case
the determining factors. The distribution of quantities is especially
important in the sexual reproduction of algae, for which a vigorous
production of the materials formed during carbon-assimilation appears to
be essential.
In the Flowering plants, on the other hand, for reasons already
mentioned, the whole problem is more complicated. Investigations on
changes in the course of development of fertilised eggs have hitherto
been unsuccessful; the difficulty of influencing egg-cells deeply
immersed in tissue constitutes a serious obstacle. Other parts of plants
are, however, convenient objects of experiment; e.g. the growing apices
of buds which serve as cuttings for reproductive purposes, or buds on
tubers, runners, rhizomes, etc. A growing apex consists of cells capable
of division in which, as in egg-cells, a complete series of latent
possibilities of development is embodied. Which of these possibilities
becomes effective depends upon the action of the outer world transmitted
by organs concerned with nutrition.
Of the different stages which a flowering plant passes through in the
course of its development we will deal only with one in order to show
that, in spite of its great complexity, the problem is, in essentials,
equally open to attack in the higher plants and in the simplest
organisms. The most important stage in the life of a flowering plant
is the transition from purely vegetative growth to sexual
reproduction--that is, the production of flowers. In certain cases it
can be demonstrated that there is no internal cause, dependent simply
on the specific structure, which compels a plant to produce its flowers
after a definite period of vegetative growth. (Klebs, "Willkurliche
Entwickelungsanderungen", Jena 1903; see also "Probleme der
Entwickelung", I. II. "Centralbl." 1904.)
One extreme case, that of exceptionally early flowering, has been
observed in nature and more often in cultivation. A number of plants
under certain conditions are able to flower soon after germination.
(Cf. numerous records of this kind by Diels, "Jugendformen und Bluten",
Berlin, 1906.) This shortening of the period of development is exhibited
in the most striking form in trees, as in the oak (Mobius, "Beitrage zur
Lehre von der Fortpflanzung", Jena, 1897, page 89.), flowering seedlings
of which have been observed from one to three years old, whereas
normally the tree does not flower until it is sixty or eighty years old.
Another extreme case is represented by prolonged vegetative growth
leading to the complete suppression of flower-production. This result
may be obtained with several plants, such as Glechoma, the sugar beet,
Digitalis, and others, if they are kept during the winter in a warm,
damp atmosphere, and in rich soil; in the following spring or summer
they fail to flower. (Klebs, "Willkurliche Aenderungen", etc. Jena,
1903, page 130.) Theoretically, however, experiments are of greater
importance in which the production of flowers is inhibited by
very favourable conditions of nutrition (Klebs, "Ueber kunstliche
Metamorphosen", Stuttgart, 1906, page 115) ("Abh. Naturf. Ges. Halle",
XXV.) occurring at the normal flowering period. Even in the case of
plants of Sempervivum several years old, which, as is shown by control
experiments on precisely similar plants, are on the point of flowering,
flowering is rendered impossible if they are forced to very vigorous
growth by an abundant supply of water and salts in the spring.
Flowering, however, occurs, if such plants are cultivated in relatively
dry sandy soil and in the presence of strong light. Careful researches
into the conditions of growth have led, in the cases Sempervivum, to
the following results: (1) With a strong light and vigorous
carbon-assimilation a considerably increased supply of water and
nutritive salts produces active vegetative growth. (2) With a vigorous
carbon-assimilation in strong light, and a decrease in the supply of
water and salts active flower-production is induced. (3) If an average
supply of water and salts is given both processes are possible;
the intensity of carbon-assimilation determines which of the two is
manifested. A diminution in the production of organic substances,
particularly of carbohydrates, induces vegetative growth. This can
be effected by culture in feeble light or in light deprived of the
yellow-red rays: on the other hand, flower-production follows an
increase in light-intensity. These results are essentially in agreement
with well-known observations on cultivated plants, according to which,
the application of much moisture, after a plentiful supply of manure
composed of inorganic salts, hinders the flower-production of many
vegetables, while a decrease in the supply of water and salts favours
flowering.
ii. INFLUENCE OF THE ENVIRONMENT ON THE FORM OF SINGLE ORGANS. (A
considerable number of observations bearing on this question are given
by Goebel in his "Experimentelle Morphologie der Pflanzen", Leipzig,
1908. It is not possible to deal here with the alteration in anatomical
structure; cf. Kuster, "Pathologische Pflanzenanatomie", Jena, 1903.)
If we look closely into the development of a flowering plant, we notice
that in a given species differently formed organs occur in definite
positions. In a potato plant colourless runners are formed from the
base of the main stem which grow underground and produce tubers at their
tips: from a higher level foliage shoots arise nearer the apex. External
appearances suggest that both the place of origin and the form of these
organs were predetermined in the egg-cell or in the tuber. But it was
shown experimentally by the well-known investigator Knight (Knight,
"Selection from the Physiological and Horticultural Papers", London,
1841.) that tubers may be developed on the aerial stem in place of
foliage shoots. These observations were considerably extended by
Vochting. (Vochting, "Ueber die Bildung der Knollen", Cassel, 1887; see
also "Bot. Zeit." 1902, 87.) In one kind of potato, germinating tubers
were induced to form foliage shoots under the influence of a higher
temperature; at a lower temperature they formed tuber-bearing shoots.
Many other examples of the conversion of foliage-shoots into runners and
rhizomes, or vice versa, have been described by Goebel and others. As in
the asexual reproduction of algae quantitative alteration in the amount
of moisture, light, temperature, etc. determines whether this or that
form of shoot is produced. If the primordia of these organs are exposed
to altered conditions of nutrition at a sufficiently early stage a
complete substitution of one organ for another is effected. If the
rudiment has reached a certain stage in development before it is exposed
to these influences, extraordinary intermediate forms are obtained,
bearing the characters of both organs.
The study of regeneration following injury is of greater importance as
regards the problem of the development and place of origin of organs.
(Reference may be made to the full summary of results given by Goebel in
his "Experimentelle Morphologie", Leipzig and Berlin, 1908, Section IV.)
Only in relatively very rare cases is there a complete re-formation
of the injured organ itself, as e.g. in the growing-apex. Much more
commonly injury leads to the development of complementary formations, it
may be the rejuvenescence of a hitherto dormant rudiment, or it may be
the formation of such ab initio. In all organs, stems, roots, leaves,
as well as inflorescences, this kind of regeneration, which occurs in
a great variety of ways according to the species, may be observed on
detached pieces of the plant. Cases are also known, such, for example,
as the leaves of many plants which readily form roots but not shoots,
where a complete regeneration does not occur.
The widely spread power of reacting to wounding affords a very valuable
means of inducing a fresh development of buds and roots on places
where they do not occur in normal circumstances. Injury creates special
conditions, but little is known as yet in regard to alterations directly
produced in this way. Where the injury consists in the separation of
an organ from its normal connections, the factors concerned are more
comprehensible. A detached leaf, e.g., is at once cut off from a supply
of water and salts, and is deprived of the means of getting rid of
organic substances which it produces; the result is a considerable
alteration in the degree of concentration. No experimental investigation
on these lines has yet been made. Our ignorance has often led to the
view that we are dealing with a force whose specific quality is the
restitution of the parts lost by operation; the proof, therefore,
that in certain cases a similar production of new roots or buds may
be induced without previous injury and simply by a change in external
conditions assumes an importance. (Klebs, "Willkurliche Entwickelung",
page 100; also, "Probleme der Entwickelung", "Biol. Centralbl." 1904,
page 610.)
A specially striking phenomenon of regeneration, exhibited also by
uninjured plants, is afforded by polarity, which was discovered by
Vochting. (See the classic work of Vochting, "Ueber Organbildung im
Pflanzenreich", I. Bonn, 1888; also "Bot. Zeit." 1906, page 101; cf.
Goebel, "Experimentelle Morphologie", Leipzig and Berlin, 1908, Section
V, Polaritat.) It is found, for example, that roots are formed from the
base of a detached piece of stem and shoots from the apex. Within
the limits of this essay it is impossible to go into this difficult
question; it is, however, important from the point of view of our
general survey to emphasise the fact that the physiological distinctions
between base and apex of pieces of stem are only of a quantitative
kind, that is, they consist in the inhibition of certain phenomena or
in favouring them. As a matter of fact roots may be produced from the
apices of willows and cuttings of other plants; the distinction is thus
obliterated under the influence of environment. The fixed polarity of
cuttings from full grown stems cannot be destroyed; it is the expression
of previous development. Vochting speaks of polarity as a fixed
inherited character. This is an unconvincing conclusion, as nothing can
be deduced from our present knowledge as to the causes which led up to
polarity. We know that the fertilised egg, like the embryo, is fixed at
one end by which it hangs freely in the embryo-sac and afterwards in
the endosperm. From the first, therefore, the two ends have different
natures, and these are revealed in the differentiation into root-apex
and stem-apex. A definite direction in the flow of food-substances
is correlated with this arrangement, and this eventually leads to a
polarity in the tissues. This view requires experimental proof, which in
the case of the egg-cells of flowering plants hardly appears possible;
but it derives considerable support from the fact that in herbaceous
plants, e.g. Sempervivum (Klebs, "Variationen der Bluten", "Jahrb. Wiss.
Bot." 1905, page 260.), rosettes or flower-shoots are formed in response
to external conditions at the base, in the middle, or at the apex of the
stem, so that polarity as it occurs under normal conditions cannot be
the result of unalterable hereditary factors. On the other hand, the
lower plants should furnish decisive evidence on this question, and
the experiments of Stahl, Winkler, Kniep, and others indicate the right
method of attacking the problem.
The relation of leaf-form to environment has often been investigated and
is well known. The leaves of bog and water plants (Cf.Goebel, loc.
cit. chapter II.; also Gluck, "Untersuchungen uber Wasser- und
Sumpfgewachse", Jena, Vols. I.-II. 1905-06.) afford the most striking
examples of modifications: according as they are grown in water, moist
or dry air, the form of the species characteristic of the particular
habitat is produced, since the stems are also modified. To the same
group of phenomena belongs the modification of the forms of leaves and
stems in plants on transplantation from the plains to the mountains
(Bonnier, "Recherches sur l'Anatomie experimentale des Vegetaux",
Corbeil, 1895.) or vice versa. Such variations are by no means isolated
examples. All plants exhibit a definite alteration in form as the result
of prolonged cultivation in moist or dry air, in strong or feeble
light, or in darkness, or in salt solutions of different composition and
strength.
Every individual which is exposed to definite combinations of external
factors exhibits eventually the same type of modification. This is the
type of variation which Darwin termed "definite." It is easy to realise
that indefinite or fluctuating variations belong essentially to the same
class of phenomena; both are reactions to changes in environment. In the
production of individual variations two different influences undoubtedly
cooperate. One set of variations is caused by different external
conditions, during the production, either of sexual cells or of
vegetative primordia; another set is the result of varying external
conditions during the development of the embryo into an adult plant. The
two sets of influences cannot as yet be sharply differentiated. If,
for purposes of vegetative reproduction, we select pieces of the
same parent-plant of a pure species, the second type of variation
predominates. Individual fluctuations depend essentially in such cases
on small variations in environment during development.
These relations must be borne in mind if we wish to understand the
results of statistical methods. Since the work of Quetelet, Galton, and
others the statistical examination of individual differences in animals
and plants has become a special science, which is primarily based on the
consideration that the application of the theory of probability renders
possible mathematical statement and control of the results. The facts
show that any character, size of leaf, length of stem, the number of
members in a flower, etc. do not vary haphazard but in a very regular
manner. In most cases it is found that there is a value which occurs
most commonly, the average or medium value, from which the larger and
smaller deviations, the so-called plus and minus variations fall away in
a continuous series and end in a limiting value. In the simpler cases
a falling off occurs equally on both sides of the curve; the curve
constructed from such data agrees very closely with the Gaussian curve
of error. In more complicated cases irregular curves of different kinds
are obtained which may be calculated on certain suppositions.
The regular fluctuations about a mean according to the rule of
probability is often attributed to some law underlying variability. (de
Vries, "Mutationstheorie", Vol. I. page 35, Leipzig, 1901.) But there is
no such law which compels a plant to vary in a particular manner. Every
experimental investigation shows, as we have already remarked, that
the fluctuation of characters depends on fluctuation in the external
factors. The applicability of the method of probability follows from
the fact that the numerous individuals of a species are influenced by
a limited number of variable conditions. (Klebs, "Willkurl. Ent." Jena,
1903, page 141.) As each of these conditions includes within certain
limits all possible values and exhibits all possible combinations, it
follows that, according to the rules of probability, there must be
a mean value, about which the larger and smaller deviations are
distributed. Any character will be found to have the mean value which
corresponds with that combination of determining factors which occurs
most frequently. Deviations towards plus and minus values will be
correspondingly produced by rarer conditions.
A conclusion of fundamental importance may be drawn from this
conception, which is, to a certain extent, supported by experimental
investigation. (Klebs, "Studien uber Variation", "Arch. fur Entw."
1907.) There is no normal curve for a particular CHARACTER, there is
only a curve for the varying combinations of conditions occurring in
nature or under cultivation. Under other conditions entirely different
curves may be obtained with other variants as a mean value. If, for
example, under ordinary conditions the number 10 is the most frequent
variant for the stamens of Sedum spectabile, in special circumstances
(red light) this is replaced by the number 5. The more accurately we
know the conditions for a particular form or number, and are able to
reproduce it by experiment, the nearer we are to achieving our aim of
rendering a particular variation impossible or of making it dominant.
In addition to the individual variations of a species, more pronounced
fluctuations occur relatively rarely and sporadically which are spoken
of as "single variations," or if specially striking as abnormalities
or monstrosities. These forms have long attracted the attention of
morphologists; a large number of observations of this kind are given
in the handbooks of Masters (Masters, "Vegetable Teratology", London,
1869.) and Penzig (Penzig, "Pflanzen-Teratologie", Vols I. and II. Genua,
1890-94.) These variations, which used to be regarded as curiosities,
have now assumed considerable importance in connection with the causes
of form-development. They also possess special interest in relation to
the question of heredity, a subject which does not at present concern
us, as such deviations from normal development undoubtedly arise as
individual variations induced by the influence of environment.
Abnormal developments of all kinds in stems, leaves, and flowers, may
be produced by parasites, insects, or fungi. They may also be induced
by injury, as Blaringhem (Blaringhem, "Mutation et traumatismes", Paris,
1907.) has more particularly demonstrated, which, by cutting away the
leading shoots of branches in an early stage of development, caused
fasciation, torsion, anomalous flowers, etc. The experiments of
Blaringhem point to the probability that disturbances in the conditions
of food-supply consequent on injury are the cause of the production of
monstrosities. This is certainly the case in my experiments with species
of Sempervivum (Klebs, "Kunstliche Metamorphosen", Stuttgart, 1906.);
individuals, which at first formed normal flowers, produced a great
variety of abnormalities as the result of changes in nutrition, we
may call to mind the fact that the formation of inflorescences occurs
normally when a vigorous production of organic compounds, such as
starch, sugar, etc. follows a diminution in the supply of mineral
salts. On the other hand, the development of inflorescences is entirely
suppressed if, at a suitable moment before the actual foundations have
been laid, water and mineral salts are supplied to the roots. If, during
the week when the inflorescence has just been laid down and is growing
very slowly, the supply of water and salts is increased, the internal
conditions of the cells are essentially changed. At a later stage, after
the elongation of the inflorescence, rosettes of leaves are produced
instead of flowers, and structures intermediate between the two kinds of
organs; a number of peculiar plant-forms are thus obtained (Cf. Lotsy,
"Vorlesungen uber Deszendenztheorien", Vol. II. pl. 3, Jena, 1908.)
Abnormalities in the greatest variety are produced in flowers by varying
the time at which the stimulus is applied, and by the cooperation
of other factors such as temperature, darkness, etc. In number and
arrangement the several floral members vary within wide limits;
sepals, petals, stamens, and carpels are altered in form and colour, a
transformation of stamens to carpels and from carpels to stamens occurs
in varying degrees. The majority of the deviations observed had not
previously been seen either under natural conditions or in cultivation;
they were first brought to light through the influence of external
factors.
Such transformations of flowers become apparent at a time, which is
separated by about two months from the period at which the particular
cause began to act. There is, therefore, no close connection between
the appearance of the modifications and the external conditions which
prevail at the moment. When we are ignorant of the causes which are
operative so long before the results are seen, we gain the impression
that such variations as occur are spontaneous or autonomous expressions
of the inner nature of the plant. It is much more likely that, as in
Sempervivum, they were originally produced by an external stimulus which
had previously reached the sexual cells or the young embryo. In any case
abnormalities of this kind appear to be of a special type as compared
with ordinary fluctuating variations. Darwin pointed out this
difference; Bateson (Bateson, "Materials for the study of Variation",
London, 1894, page 5.) has attempted to make the distinction sharper, at
the same time emphasising its importance in heredity.
Bateson applies the term CONTINUOUS to small variations connected with
one another by transitional stages, while those which are more striking
and characterised from the first by a certain completeness, he names
DISCONTINUOUS. He drew attention to a great difficulty which stands in
the way of Lamarck's hypothesis, as also of Darwin's view. "According to
both theories, specific diversity of form is consequent upon diversity
of environment, and diversity of environment is thus the ultimate
measure of diversity of specific form. Here then we meet the difficulty
that diverse environments often shade into each other insensibly and
form a continuous series, whereas the Specific Forms of life which
are subject to them on the whole form a Discontinuous Series."
This difficulty is, however, not of fundamental importance as well
authenticated facts have been adduced showing that by alteration of the
environment discontinuous variations, such as alterations in the number
and form of members of a flower, may be produced. We can as yet no more
explain how this happens than we can explain the existence of continuous
variations. We can only assert that both kinds of variation arise
in response to quantitative alterations in external conditions. The
question as to which kind of variation is produced depends on the
greater or less degree of alteration; it is correlated with the state of
the particular cells at the moment.
In this short sketch it is only possible to deal superficially with a
small part of the subject. It has been clearly shown that in view of the
general dependence of development on the factors of the environment
a number of problems are ready for experimental treatment. One must,
however, not forget that the science of the physiology of form has not
progressed beyond its initial stages. Just now our first duty is to
demonstrate the dependence on external factors in as many forms of
plants as possible, in order to obtain a more thorough control of all
the different plant-forms. The problem is not only to produce at will
(and independently of their normal mode of life) forms which occur
in nature, but also to stimulate into operation potentialities which
necessarily lie dormant under the conditions which prevail in nature.
The constitution of a species is much richer in possibilities of
development than would appear to be the case under normal conditions. It
remains for man to stimulate into activity all the potentialities.
But the control of plant-form is only a preliminary step--the foundation
stones on which to erect a coherent scientific structure. We must
discover what are the internal processes in the cell produced by
external factors, which as a necessary consequence result in the
appearance of a definite form. We are here brought into contact with the
most obscure problem of life. Progress can only be made pari passu with
progress in physics and chemistry, and with the growth of our knowledge
of nutrition, growth, etc.
Let us take one of the simplest cases--an alteration in form. A
cylindrical cell of the alga Stigeoclonium assumes, as Livingstone
(Livingstone, "On the nature of the stimulus which causes the change
of form, etc." "Botanical Gazette", XXX. 1900; also XXXII. 1901.) has
shown, a spherical form when the osmotic pressure of the culture fluid
is increased; or a spore of Mucor, which, in a sugar solution grows
into a branched filament, in the presence of a small quantity of acid
(hydrogen ions) becomes a comparatively large sphere. (Ritter, "Ueber
Kugelhefe, etc." "Ber. bot. Gesell." Berlin, XXV. page 255, 1907.)
In both cases there has undoubtedly been an alteration in the osmotic
pressure of the cell-sap, but this does not suffice to explain the
alteration in form, since the unknown alterations, which are induced in
the protoplasm, must in their turn influence the cell-membrane. In
the case of the very much more complex alterations in form, such as we
encounter in the course of development of plants, there do not appear
to be any clues which lead us to a deeper insight into the phenomena.
Nevertheless we continue the attempt, seeking with the help of any
available hypothesis for points of attack, which may enable us to
acquire a more complete mastery of physiological methods. To quote a
single example; I may put the question, what internal changes produce a
transition from vegetative growth to sexual reproduction?
The facts, which are as clearly established from the lower as for the
higher plants, teach us that quantitative alteration in the environment
produces such a transition. This suggests the conclusion that
quantitative internal changes in the cells, and with them disturbances
in the degree of concentration, are induced, through which the chemical
reactions are led in the direction of sexual reproduction. An increase
in the production of organic substances in the presence of light,
chiefly of the carbohydrates, with a simultaneous decrease in the amount
of inorganic salts and water, are the cause of the disturbance and
at the same time of the alteration in the direction of development.
Possibly indeed mineral salts as such are not in question, but only in
the form of other organic combinations, particularly proteid material,
so that we are concerned with an alteration in the relation of the
carbohydrates and proteids. The difficulties of such researches are very
great because the methods are not yet sufficiently exact to demonstrate
the frequently small quantitative differences in chemical composition.
Questions relating to the enzymes, which are of the greatest importance
in all these life-processes, are especially complicated. In any case
it is the necessary result of such an hypothesis that we must employ
chemical methods of investigation in dealing with problems connected
with the physiology of form.