organism is susceptible to, and can respond to, the action of external conditions. There is every reason to believe that plants are as “irritable” to varying external conditions as they are to light or to gravity. A change in its external conditions may act as a “stimulus,” evoking in the organism a response of the nature of a change in its form. As Darwin has pointed out, this response may be direct or indirect. In illustration of the indirect response, the evolution of the Bryophyta and of more highly organized plants may be briefly considered. It is generally admitted that life originated in water, and that the earliest plants were Algae. The study of existing Algae, that is of plants that have continued to live in water, shows that under these conditions no high degree of organization has been reached, though some of them have attained gigantic dimensions. The primitive water-plants were succeeded by land-plants, a land-flora being gradually established. With the transition from water to land came the progressive development of the sporophyte which is the characteristic feature of the morphology of the Bryophyta and of all plants above them in the scale of life (see Bower, Origin of a Land-Flora). This evolution of the sporophyte is no doubt to be correlated with the great change in the external conditions of life. There is no conclusive ground for regarding the action of this change as having been direct, it is more reasonable to regard it as indirect, having acted as a general stimulus to which the ever-increasing complexity of the sporophyte was the response.
Adaptation.—The morphological and physiological differentiation of the plant-body has, so far, been attributed to (1) “the nature of the organism,” that is to its inherent tendency towards higher organization, and (2) to the “indefinite results” of the external conditions acting as a stimulus which excites the organism to variation, but does not direct the course of variation. The “definite results” of the action of external conditions have still to be considered.
It is a familiar observation that climatic and edaphic (nature of soil) conditions exert an influence upon the form and structure of plants (see Plants: Ecology of). For instance, some xerophytes are dry and hard in structure, whilst others are succulent and fleshy. This so-called direct effect of external conditions upon the form and structure of the body differs from the indirect effect in that the resulting variations bear a relation, of the nature of adaptation, to those conditions, the effect of the conditions is not only to cause variation, but to cause variation in a particular direction. Thus all existing hygrophytes (excepting the Algae) are considered to have been derived from land-plants which have adapted themselves to a watery habitat. The effect can also be demonstrated experimentally: thus it has been observed that a xerophyte grown in moist air will lose its characteristic adaptive features, and may even assume those of a hygrophyte.
Climatic and edaphic conditions are not, however, the only ones to affect the structure and composition of the body or its parts, other conditions are of importance, particularly the relations of the plant to animals and to other plants. For instance, the “animal traps” of carnivorous plants (Drosera, Nepenthes, &c.) did not, presumably, originate as such; they began as organs of quite another kind which became adapted to their present function in consequence of animals having been accidentally caught. It is also probable that the various forms of the angiospermous flower, with its many specialized mechanisms for pollination, may be the result of insect-visits, the flowers becoming adapted to certain kinds of insects, and the insects having undergone corresponding modification. Parasites, again, were derived from normal autotrophic plants, which, as the parasitic habit became more pronounced, acquired the corresponding characteristics of form and structure; there is, in fact, the group of hemi-parasites, plants which still retain autotrophic characters though they are root-parasites.
Though adaptation to the environment seems sometimes to be considered, especially by neo-Lamarckians, as equivalent to, or at least as involving, the evolution of higher forms from lower, there does not appear to be any evidence that this is the case. The effect of external conditions is confined to the modification in various directions of members or organs already existing, and one very common direction is that of reduction or entire disappearance of parts: for instance, the foliage-leaves of certain xerophytes (e.g. Cactaceae, Euphorbiaceae), of parasites, and of saprophytes. Moreover, had the evolution of plants proceeded along the line of adaptation, the vegetable kingdom could not be subdivided, as it is, into the morphological groups Thallophyta, Bryophyta, Pteridophyta, Phanerogamia, but only into physiological groups, Xerophyta, Hygrophyta, Tropophyta, &c.
In endeavouring to trace the causation of adaptation, it is obvious that it must be due quite as much to properties inherent in the plant as to the action of external conditions; the plant must possess adaptive capacity. In other words, the plant must be irritable to the stimulus exerted from without, and be capable of responding to it by changes of form and structure. Thus there is no essential difference between the “direct” and the “indirect” action of external conditions, the difference is one of degree only. In the one case the stimulus induces indefinite variation, in the other definite; but no hard-and-fast line can be drawn between them.
Adaptive characters are often hereditary, for instance, the seed of a parasite will produce a parasite, and the same is true of a carnivorous plant. On the other hand, adaptations, especially those evoked by climatic or edaphic conditions, may only be shown by the seedling if grown under the appropriate external conditions; here what is hereditary is not the actual adaptation, but the capacity for responding in a particular way to a certain set of external conditions.
Summary.—The general theory of differentiation propounded in this article is an attempt at an analysis of the factors termed by Darwin “the nature of the organism” and “the nature of the conditions.” It is assumed, as an inevitable conclusion from the facts of evolution, that plant-protoplasm possesses (1) an inherent tendency towards higher organization, and (2) that it is irritable to external conditions, or to changes in them, and can respond to them by changes of form which may be either indefinite or definite (adaptive). Thus it is that the variations are produced upon which natural selection has to work.
Material Cause of Differentiation.—It may be inquired, in conclusion, if there are any facts which throw light upon the internal mechanism of differentiation, whether spontaneous or induced; if it is possible to refer it to any material cause. It may be replied that there are such facts, and though they are but few as yet, they suffice to suggest an hypothesis that may eventually prove to be a law. Sachs was the first to formulate the theory that morphological differences are the expression of differences in material composition. He considered, for instance, that stems, leaves, roots and flowers differ as they do because the plastic substances entering into their structure are diverse. This view he subsequently modified to this—that a relatively small proportion of diverse substance in each of these parts would suffice to account for their morphological differences. This modification is important, because it transfers the formative influence from the plastic substances to the protoplasm, suggesting that the diverse constituents are produced (whether spontaneously or as the result of stimulation) as secretions by the protoplasm. It is an obvious inference that if a small quantity of a substance can affect the development of an entire organ it probably acts after the manner of an enzyme. Beyerinck has, in fact, gone so far as to speak of “formative enzymes.”
It is not possible to go into all the facts that might be adduced in support of this view: one case, perhaps the most pregnant, must suffice. Beyerinck was led to take up the decided position just mentioned by his researches into the conditions determining the formation of plant-galls as the result of injury by insects. He found that the development of a gall is due to a temporary modification of the part affected, not, as is generally thought, in consequence of the deposition of an egg by the insect, but of the injection of a poisonous substance which has the effect of stimulating the protoplasm to develop a gall instead of normal
