From what these forms have in common, let us turn to that which they have not in common, and observe the causes of the want of community. A puff-ball shows us in the simplest way, the likeness of parts accompanying likeness of conditions, along with the unlikeness of parts accompanying unlikeness of conditions. For while, if we cut vertically through its centre, we find a difference between top and bottom, if we cut horizontally through its centre, we find no differences among its several sides. Being, on the average of cases, similarly related to the environment all round, it remains the same all round. The radial symmetry of the mushroom and other vertically-growing fungi, illustrates this connexion of cause and effect still better. But now mark what happens in the group of Agaricus noli-tangere, shown in Fig. [195]. Radially-symmetrical as is the type, and radially symmetrical as are those centrally-placed individuals which are equally crowded all round, we see that the peripheral individuals, dissimilarly circumstanced on their outer sides and on their sides next the group, have partially changed their radial symmetry into bilateral symmetry. It is no longer possible to make two corresponding halves by any vertical plane cutting down through the pileus and the stem; but there is only one vertical plane that will thus produce corresponding halves—the plane on the opposite sides of which the relations to the environment are alike. And then mark that the divergence from all-sided symmetry towards two-sided symmetry, here caused in the individual by special circumstances, is characteristic of the race where the habits of the race constantly involve two-sidedness of conditions. Besides being exemplified by such comparatively undifferentiated types as certain Polypori, Fig. [196], a, b, this truth is exemplified by members of the genus just named. In Agaricus horizontalis, Fig. [196], c, we have a departure from radial symmetry that is conspicuous only in the form of the stem. A more decided bilateralness exists in A. subpalmatus, shown in elevation at d and in section at d´. And Lentinus flabelliformis, of which e and e´ are different views, exhibits complete bilateralness—a bilateralness in which there is the greatest likeness of the parts that are most similarly conditioned, and the greatest unlikeness of the parts that are most dissimilarly conditioned.
Among plants of the second order of composition, it will suffice to note one further class of facts which are the converse of the foregoing and have the same implications. These are the facts showing that along with habitual irregularity in the relations to external forces, there is habitual irregularity in the mode of growth. Besides finding such facts among Thallophytes, as in the tubers of underground fungi and in the creeping films of sessile lichens, which severally show us variations of proportions obviously caused by variations in the amounts of the influences on their different sides, we also, among Archegoniates of inferior types, find irregularities of form along with irregularities in environing actions. The fronds of the Marchantiaceæ or such Jungermanniaceæ as are shown in Figs. [41, 42, 43], illustrate the way in which each lowly-organized aggregate of the second order, not individuated by the mutual dependence of its parts, has its form determined by the balance of facilities and resistances which each side of the frond meets with as it spreads.
§ 219. Among plants displaying integration of the third degree, and among plants still further compounded, these same truths are equally manifest. In the forms of such plants we see primary contrasts and secondary contrasts which, no less clearly than the foregoing, are related to contrasts of conditions.
That flowering plants from the daisy up to the oak, have in common the fundamental unlikeness between the upward growing part and the downward growing part; and that this most marked unlikeness corresponds with the most marked unlikeness between the two parts of their environment, soil and air; are facts too conspicuous to be named were they not important items in the argument. More instructive perhaps, because less familiar, is the fact that we miss this extreme contrast in flowering plants which have not their higher and lower portions exposed to conditions thus extremely contrasted. A parasite like the Dodder, growing in entangled masses upon other plants, from which it sucks the juices, is not thus divisible into two strongly-distinguished halves.
Leaving out of consideration the difference between the supporting part and the supported part in phænogams, and looking at the supported part only, we observe between its form and the habitual incidence of forces, a relation like that which we observed in the simpler plants. Phænogams that are practically if not literally uniaxial, and those which develop their lateral axes only in the shape of axillary flowers, when uninterfered with commonly send up vertical stems round which the leaves and flowers are disposed with a more or less decided radial symmetry. Gardens and fields supply us with such instances as the Tulip and the Orchis; and, on a larger scale, the Palms and the Aloes are fertile in examples. The exceptions, too, are instructive. Besides the individual divergences arising from special interferences, there are to be traced general divergences where the habits of the plants expose them to general interferences in anything approaching to constant ways. Plants which, like the Foxglove, have spikes of flowers that are borne on flexible foot-stalks, have their flowers habitually bent round to one face of the stem: an unlikeness of distribution probably caused by unlikeness in the relation to the Sun’s rays. The wild Hyacinth, too, with stem so flexible that its upper part droops, shows us how a consequent difference in the action of gravity on the flowers, causes them to deviate from their typically-radial arrangement towards a bilateral arrangement.
Figs. 197–199.
Much more conspicuous are these general and special relations of form to general and special actions in the environment, among phænogams that are multiaxial. That when standing alone, and in places where the winds do not injure them nor adjacent things shade them, shrubs and trees develop with tolerable evenness on all sides, is an obvious truth. Equally obvious is the truth that, when growing together in a wood, and mutually interfered with on all sides, trees still show obscurely radial distributions of parts; though, under such conditions, they have tall taper stems with branches directed upwards—a difference of shape clearly due to the different incidence of forces. And almost equally obvious is the truth, that a tree of this same kind growing at the edge of the wood, has its outer branches well developed and its inner branches comparatively ill-developed. Fig. [197], which inaccurately represents this difference, will serve to make it manifest that while one of the peripheral trees can be cut into something like two similar halves by a vertical plane directed towards the centre of the wood—a plane on each side of which the conditions are alike—it cannot be cut into similar halves by any other plane. A like divergence from an indefinitely-radial symmetry towards an indefinitely-bilateral symmetry, occurs in trees that have their conditions made bilateral by growing on inclined surfaces. Two of the common forms observable in such cases are given in Fig. [198]. Here there is divisibility into parts that are tolerably similar, by a vertical plane running directly down the hill; but not by any other plane. Then, further, there is the bilateralness, similar in general meaning though differently caused, often seen in trees exposed to strong prevailing winds. Almost every sea-coast has abundant examples of stunted trees which, like the one shown in Fig. [199], have been made to deviate from their ordinary equal growth on all sides of a vertical axis, to a growth that is equal only on the opposite sides of a vertical plane directed towards the wind’s eye.
From among vegetal aggregates of the third order, we have now only to add examples of the entirely asymmetrical form which accompanies an entirely irregular distribution of incident forces. Creeping plants furnish such examples. They show, both when climbing up vertical or inclined surfaces and when trailing on the ground, that their branches grow hither and thither as the balance of forces aids or opposes; and the general outline is without symmetry of any kind, because the environing influences have no kind of regularity in their arrangement.
§ 220. Along with some unfamiliar facts, I have here set down facts which are so familiar as to seem scarcely worth noting. It is because these facts have become meaningless to perceptions deadened by infinite repetitions of them, that it is needful here to point out their meanings. Not alone for its intrinsic importance has the unlikeness between the attached ends and the free ends been traced among plants of all degrees of integration. Nor is it simply because of the significance they have in themselves, that instances have been given of those varieties of symmetry and asymmetry which the free ends of plants equally display: be they plants of the first, second, third, or any higher order. Neither has the only other purpose been that of showing how, in the radial symmetry of some vegetal aggregates and the single bilateral symmetry of others, there are traceable the same ultimate principles as in the spherical symmetry and triple bilateral symmetry of certain minute plants first described. But the main object has been to present, under their simplest aspects, those general laws of morphological differentiation which are fulfilled by the component parts of each plant.