the ground that, in ontogeny, the foliage-leaf generally precedes the sporophyll. The phylogeny of the various floral leaves, for instance, was generally traced as follows: foliage-leaf, bract, sepal, petal, stamen and carpel (sporophylls)—in accordance with what Goethe termed “ascending metamorphosis.” Recent researches, however, more especially those of Celakovsky, tend to prove that the perianth-leaves have been derived from the stamens (i.e. from sporophylls); that is, they are the result of “descending metamorphosis.” Moreover there is the fact that the flowers of nearly all the primitive phanerogams, such as the Gymnosperms, consist solely of sporophylls, having no perianth. There is thus a considerable body of evidence to support Bower's view of the primitive nature of the sporophyll.
Accepting this view of the phylogeny of the leaf, the perianth-leaves (sepals and petals) and the foliage-leaves may be regarded as “modified” or “metamorphosed” sporophylls; that is, as leaves which are adapted to functions other than the bearing of spores. The sepals are generally organs for the protection of the flower-bud; the petals, for attracting insects by their conspicuous form and colour; the foliage-leaves, for the assimilation of carbon dioxide and other associated functions. But this phylogenetic differentiation of the organs was not what Wolff and Goethe had in mind; what they contemplated was an ontogenetic change, and there is abundant evidence that such changes actually occur. Taking first the conversion of members of one morphological category into those of another, this has been actually observed, though rarely. Goebel (Organography) gives several instances of the conversion of the root into a shoot in ferns, and a few in phanerogams (Listera ovata, Neottia nidusavis, Anthurium longifolium). Much more common is the conversion of one form of a member into another form. The most varied changes of this kind have been described, and are generally familiar as “monstrosities”; the study of them constitutes, under the name of teratology, a distinct department of biology. A simple case is that of “double” flowers, in which the number of the petals is increased by the “metamorphosis” of stamens; or again the conversion of floral leaves into green leaves, a change known as “chloranthy.” These changes may be brought about by external causes, such as the attacks of insects or of fungi, alterations in external conditions, &c., or by some unexplained internal disturbance of the morphological equilibrium. They can also be effected experimentally. Goebel has shown that if the developing foliage-leaves of the fern Onoclea struthiopteris be removed as they are formed, the subsequently developed sporophylls assume more or less completely the habit of foliage-leaves, and may be sterile. Similarly bud-scales can be caused to develop into foliage-leaves, if the buds to which they belong are caused to grow out in the year of their formation by the removal of the existing foliage-leaves.
Useful and suggestive as they often are, teratological facts played, at one time, too large a part in the framing of morphological theories; for it was thought that the “monstrous” form gave a clue to the essential nature of the organ assuming it. There is, however, no sufficient reason for regarding the monstrous form as necessarily primitive or ancestral, nor even as a stage in the ontogeny of the organ. For when the older morphologists spoke of a stamen as a “metamorphosed” leaf, it was implied that it originated as a foliage-leaf and subsequently became a stamen. As a matter of fact, a stamen is a stamen and nothing else, from the very beginning. The development of the organ is already determined at its first appearance upon the growing-point; though, as already explained, the normal course of its ontogeny may be interfered with by some abnormal external or internal condition. The word “metamorphosis” cannot, in fact, be used any longer in its original sense, for the change which it implied does not normally occur in ontogeny, and in phylogeny the idea is more accurately expressed by the term “differentiation.” However, it may still be useful in describing “monstrosities,” and perhaps also those cases in which an organ serves first one purpose and then another, as when a leafy shoot eventually becomes a thorn, or the base of a foliage-leaf becomes a bud-scale.
Differentiation.—Any account of the general morphology of living organisms is incomplete if it does not include some attempt at an explanation of its causation; though such an attempt cannot be carried far at the present time. A survey of the vegetable kingdom indicates that evolution has proceeded, on the whole, from the simple to the complex; at the same time, as has been already mentioned, evidence of reduction or degeneration in common. Thus in the series Bryophyta, Pteridophyta, Phanerogamia, whilst the sporophyte presents progressive development, the gametophyte presents continuous reduction.
Evolution means the gradual development of “highly organized” from “lowly organized” forms; that is, of forms in which the “physiological division of labour” is more complete, from those in which it is less complete; of forms possessing a variety of organs, from forms possessing but few. Differentiation means the development and the specialization as organs of various parts of the body. It presents itself in two aspects: there is morphological differentiation, which can be traced in the distinction of the members of the body, root, stem, leaf, &c.; there is physiological differentiation, which can be traced in the adaptation of these members to various functions. But, in actual operation, these two processes are simultaneous; every member is developed as an organ for the performance of some special function.
Factors in Evolution.—Evolution in the race involves progressive differentiation in the individual; hence the causes of evolution and of differentiation must be the same. The evolution of higher from lower plants, it is generally assumed, has proceeded by variation. With regard to the causation of variation Darwin says (Origin of Species, ch. v.): “In all cases there are two factors, the nature of the organism, which is much the most important of the two, and the nature of the conditions. The direct action of changed conditions leads to definite or indefinite results. In the latter case the organization seems to become plastic, and we have much fluctuating variability. In the former case the nature of the organism is such that it yields readily, when subjected to certain conditions, and all or nearly all the individuals become modified in the same way.”
In spite of the statement that the “nature of the organism” is the most important factor in variation, the tendency amongst evolutionists has been to take much more account of the influence of external conditions. Exceptions to this attitude are Lamarck, who speaks with regard to animals (but not to plants!) of “la composition croissant de l'organisation” (Philosophie zoologique, t. i.), and Nägeli, who attributes variation to causes inherent in the “idioplasm,” and has elaborately worked out the view in his Abstammungslehre.
The position assumed in this article is in agreement with the views of Lamarck and of Nägeli. All but the lowest plants visibly tend towards or actually achieve in various degrees the differentiation of the body, whether sporophyte or gametophyte, into stem, leaf, root, &c., that is, the differentiation of parts not previously present. It is inconceivable that external conditions can impart to an organism the capacity to develop something that it does not already possess: can impart to it, that is, the capacity for variation in the direction of higher complexity. The alternative, which is here accepted, is that differentiation is essentially the expression of a developmental tendency inherent in the protoplasm of plants. Just as every crystallizable chemical substance assumes a definite and constant crystalline form which cannot be accounted for otherwise than by regarding it as one of the properties of the substance, so every living organism assumes a characteristic form which is the outcome of the properties of its protoplasm. But whereas the crystalline form of a chemical substance is stable and fixed, the organized form of a living organism is unstable and subject to change.
Influence of External Conditions.—This position does not, however, exclude the influence of external conditions; that influence is undeniable. Darwin's expression “the nature of the organism” has been interpreted in the preceding paragraph to mean an inherent tendency towards higher organization; that interpretation may now be completed by adding that the
