[32] Here and throughout, the word radial is applied equally to the spiral and the whorled structures. These, as being alike on all sides, are similarly distinguished from arrangements that are alike on two sides only.
[33] It should be added that this change of distribution is not due to change in the relative positions of the insertions of the leaves but to their twistings.
[34] We may note that some of these leaves, as those of the Lime, furnish indications of the ratio which exists between the effects of individual circumstances and those of typical tendencies. On the one hand, the leaves borne by these drooping branches of the Lime are with hardly an exception unsymmetrical more or less decidedly, even in positions where the causes of unsymmetry are not in action: a fact showing us the repetition of the type irrespective of the conditions. On the other hand, the degree of deviation from symmetry is extremely variable, even on the same shoot: a fact proving that the circumstances of the individual leaf are influential in modifying its form. But the most striking evidence of this direct modification is afforded by the suckers of the Lime. Growing, as these do, in approximately upright attitudes, the leaves they bear do not stand to one another in the way above described, and the causes of unsymmetry are not in action; and here, though there is a general leaning to the unsymmetrical form, a large proportion of the leaves become quite symmetrical.
[35] It was by an observation on the forms of leaves, that I was first led to the views set forth in the preceding and succeeding chapters on the morphological differentiation of plants and animals. In the year 1851, during a country ramble in which the structures of plants had been a topic of conversation with a friend—Mr. G. H. Lewes—I happened to pick up the leaf of a buttercup, and, drawing it by its foot-stalk through my fingers so as to thrust together its deeply-cleft divisions, observed that its palmate and almost radial form was changed into a bilateral one; and that were the divisions to grow together in this new position, an ordinary bilateral leaf would result. Joining this observation with the familiar fact that leaves, in common with the larger members of plants, habitually turn themselves to the light, it occurred to me that a natural change in the circumstances of the leaf might readily cause such a modification of form as that which I had produced artificially. If, as they often do with plants, soil and climate were greatly to change the habit of the buttercup, making it branched and shrub-like; and if these palmate leaves were thus much overshadowed by one another; would not the inner segments of the leaves grow towards the periphery of the plant where the light was greatest, and so change the palmate form into a more decidedly bilateral form? Immediately I began to look round for evidence of the relation between the forms of leaves and the general characters of the plants they belong to; and soon found some signs of connexion. Certain anomalies, or seeming anomalies, however, prevented me from then pursuing the inquiry much further. But consideration cleared up these difficulties; and the idea afterwards widened into the general doctrine here elaborated. Occupation with other things prevented me from giving expression to this general doctrine until Jan. 1859; when I published an outline of it in the Medico-Chirugical Review.
[36] It is objected to the above interpretation that “many flowers of sizes intermediate between the Hollyhock and the Agrimony are radially symmetrical and yet grow sideways. I may mention various Liliaceæ, e.g. Chlorophytum, Eucomis, Muscari, Anthericum. Sagittaria, also, has many of its flowers in this position. Further, if the higher insects alight on flowers in a definite way, as they do, the parts of the flower must bear different relations to the visiting insect, however large, so that flowers unvisited ought all to be zygomorphic.” My reply is that in the sense which here concerns us, the different petals of the Hollyhock-flower do not bear different relations to the visiting insect; since, practically, the upper and lateral petals bear no physical relations at all: in so far as the visiting bee is concerned they are non-existent. The argument implies that change in the form of a flower from the radial to the bilateral is likely to take place only when the contact-relations of the petals to the visiting insect, are such as to make some forms facilitate its action more than others; and the large petals of the Hollyhock cannot facilitate its action at all. In respect of the Liliaceæ instanced, it is needful to inquire whether the structures are such that this alleged cause of bilateral symmetry can come into play.
[37] I had intended here to insert a figure exhibiting these differences; but as the Cow-parsnip does not flower till July, and as I can find no drawing of the umbel which adequately represents its details, I am obliged to take another instance.
[38] It has been pointed out to me that “the extreme development of the corolla so often found in the outer flowers or on the outer side of the outer flowers in closely-packed inflorescences, associated as it often is with disappearance of stamens or carpels or both, is usually put down to specialization of these outer flowers for attractive purposes. Since the whole inflorescence is increased in conspicuousness by such a modification, it is supposed that natural selection favoured those plants which sacrificed a portion of their seed-bearing capacity for the supposed greater advantage of securing more insect visits.” But granting this interpretation, it may still be held that increase of attractiveness due to increase of area must be achieved by florets at the periphery, and that their ability to achieve it depends on their having an outer, unoccupied, space which the inner florets have not; so that, though in a more indirect way, their different development is determined by different exposure to conditions.
[39] One of my critics writes:—“This chapter might of course be enormously extended, not only as in the preceding ones by citation of quite similar cases, but by the introduction of fresh groups of cases.”
[40] Natural selection may have operated in establishing a constitutional tendency to other sudden abridgments. Mr. Tansley alleges that this is a part-cause of the varying distribution of leaves. He says:—“I have myself made some observations on the length of internodes in the Beech, and am satisfied that it follows quite other laws, connected with the suitable disposition of the leaves on the branch. Although I have not had the opportunity of following up this line of work so as in any way to generalize the results, I suspect that ‘indirect equilibration’ is a widespread cause of such variation.”
[41] It is but just to the memory of Wolff, here to point out that he was immensely in advance of Goethe in his rationale of these metamorphoses. Whatever greater elaboration Goethe gave to the theory considered as an induction, seems to me more than counter-balanced by the irrationality of his deductive interpretation; which unites mediæval physiology with Platonic philosophy. A dominant idea with him is that leaves exist for the purpose of carrying off crude juices—that “as long as there are crude juices to be carried off, the plant must be provided with organs competent to effect the task”; that while “the less pure fluids are got rid of, purer ones are introduced” and that “if nourishment is withheld, that operation of nature (flowering) is facilitated and hastened; the organs of the nodes (leaves) become more refined in texture, the action of the purified juices becomes stronger, and the transformation of parts having now become possible, takes place without delay.” This being the proximate explanation, the ultimate explanation is, that Nature wishes to form flowers—that when a plant flowers it “attains the end prescribed to it by nature”; and that so “Nature at length attains her object.” Instead of vitiating his induction by a teleology that is as unwarranted in its assigned object as in its assigned means, Wolff ascribes the phenomena to a cause which, whether sufficient or not, is strictly scientific in its character. Variation of nutrition is unquestionably a “true cause” of variation in plant-structure. We have here no imaginary action of a fictitious agency; but an ascertained action of a known agency.