Plants do not display integration in such distinct and multiplied ways as do animals. But its advance may be traced both directly and indirectly—directly in the increasing co-ordination of actions, and indirectly in the effect of this upon the powers and habits.
Let us group the facts under these heads: ascending in both cases from the lower to the higher types.
§ 284. The inferior Algæ, along with little unlikeness of parts, show us little mutual dependence of parts. Having surfaces similarly circumstanced everywhere, much physiological division of labour cannot arise; and therefore there cannot be much physiological unity. Among the superior Algæ, however, the differentiation between the attached part and the free part is accompanied by some integration. There is evidently a certain transfer of materials, which is doubtless facilitated by the elongated forms of the cells in the stem, and probably leads to the formation of dense tissue at the places of greatest strain, in a way akin to that recently explained in other cases. And where there is this co-ordination of actions, the parts are so far mutually dependent that each dies if detached from the other. That though the organization is so low neither part can reproduce the other and survive by so doing, is probably due to the circumstance that neither part contains any considerable stock of untransformed protoplasm, out of which new tissues may be produced.
Fungi and Lichens present no very significant advances of integration. We will therefore pass at once to the Archegoniates. In those of them which, either as single fronds or strings of fronds, spread over surfaces, and which, rooting themselves as they spread, do not need that each part should receive aid from remote parts, there is no developed vascular system serving to facilitate transfer of nutriment: the parts being little differentiated there is but little integration. But along with assumption of the upright attitude and the accompanying specializations, producing vessels for distributing sap and hard tissue for giving mechanical support, there arises a decided physiological division of labour; rendering the aërial part dependent on the embedded part and the embedded part dependent on the aërial part. Here, indeed, as elsewhere, these concomitant changes are but two aspects of the same change. Always the gain of power to discharge a special function involves a loss of power to perform other functions; and always, therefore, increased mutual dependence constituting physiological integration, must keep pace with that increased fitting of particular parts to particular duties which constitutes physiological differentiation.
Making a great advance among the Archegoniates, this physiological integration reaches its climax among Phænogams. In them we see interdependence throughout masses that are immense. Along with specialized appliances for support and transfer, we find an exchange of aid at great distances. We see roots giving the vast aërial growth a hold tenacious enough to withstand violent winds, and supplying water enough even during periods of drought; we see a stem and branches of corresponding strength for upholding the assimilating organs under ordinary and extraordinary strains; and in these assimilating organs we see elaborate appliances for yielding to the stem and roots the materials enabling them to fulfil their offices. As a consequence of which greater integration accompanying the greater differentiation, there is ability to maintain life over an immense period under marked vicissitudes.
Even more conspicuously exemplified in Phænogams, is that physiological integration which holds together the functions not of the individual only but of the species as a whole. The organs of reproduction, both in their relations to other parts of the individual bearing them and in their relations to corresponding parts of other individuals, show us a kind of integration conducing to the better preservation of the race; as those already specified conduce to the better preservation of the individual. In the first place, this greater co-ordination of functions just described, itself enables Phænogams to bequeath to the germs they cast off, stores of nutriment, protective envelopes, and more or less of organization: so giving them greater chances of rooting themselves. In the second place, certain differentiations among the parts of fructification, the meaning of which Mr. Darwin has so admirably explained, give to the individuals of the species a kind of integration that makes possible a mutual aid in the production of vigorous offspring. And it is interesting to observe how, in that dimorphism by which in some cases this mutual aid is made more efficient, the greater degree of integration is dependent on the greater degree of differentiation—not simply differentiation of the fructifying organs from other parts of the plant bearing them, but differentiation of these fructifying organs from the homologous organs of neighbouring individuals of the same race. Another form of this co-ordination of functions which conduces to the maintenance of the species, may be here named—partly for its intrinsic interest. I refer to the strange processes of multiplication occurring in the genus Bryophyllum. It is well known that the succulent leaves of B. calycinum, borne on foot-stalks so brittle that they are easily snapped by the wind, send forth from their edges when they fall to the ground, buds which root themselves and grow into independent plants. The correlation here obviously furthering the preservation of the race, is more definitely established in another species of the genus—B. proliferum. This plant, shooting up to a considerable height, and having a stem containing but little woody fibre, habitually breaks near the bottom while still in flower; and is thus generally prevented from ripening its seeds. The multiplication is, however, secured in another way. Before the stem is broken young plants have budded out from the pedicels of the flowers, and have grown to considerable lengths; and on the fall of the parent they forthwith commence their separate lives. Here natural selection has established a remarkable kind of co-ordination between a special habit of growth and decay, and a special habit of proliferation.
§ 285. The advance of physiological integration among plants as we ascend to the higher types, is implied by their greater constancy of structure, as well as by the stricter limitations of their habitats and modes of life. “Complexity of structure is generally accompanied with a greater tendency to permanence in form,” says Dr. [now Sir J.] Hooker; or, conversely, “the least complex are also the most variable.” This is the second aspect under which we have to contemplate the facts.
The differences between the simpler Algæ and Fungi are so feebly marked that botanists have had great difficulty in framing definitions of these classes. This structural indefiniteness is accompanied by functional indefiniteness. Algæ, which are mostly aquatic, include many small forms that frequent the damp places preferred by Fungi. Among Fungi, there are kinds which lead submerged lives like the Algæ. Besides this indistinctness of the classes, there is great variability in the shapes and modes of life of their species—a variability so great that what were at first taken to be different species, or different genera, or even different orders, have proved to be merely varieties of one species. So inconstant in structure are the Algæ that Schleiden quotes with approval the opinion of Kutzing, that “there are no species but merely forms of Algæ:” an opinion which though now rejected sufficiently implies extreme indefiniteness. In all which facts we see that these lowest types of plants, little differentiated, are also but little integrated.
Archegoniates present a like relation between the small specialization of functions which constitutes physiological differentiation, and the small combination of functions which constitutes physiological integration. “Mosses,” says Mr. Berkeley, “are no less variable than other cryptogams, and are therefore frequently very difficult to distinguish. Not only will the same species exhibit great diversity in the size, mode of branching, form and nervation of the leaves, but the characters of even the peristome itself are not constant.” And concerning the classification of the remaining group, Filicales, he says:—“Not only is there great difficulty in arranging ferns satisfactorily, but it is even more difficult to determine the limits of species.”
After this vagueness of separation as well as inconstancy of structure and habit among the lower plants, the stability of structure and habit and divisibility of groups among the higher plants, appear relatively marked. Though Phænogams are much more variable than most botanists have until lately allowed, yet the definitions of species and genera may be made with far greater precision, and the forms are far less capable of change, than among Cryptogams. And this comparative fixity of type, implying, as it does, a closer combination of the component functions, we see to be the accompaniment of the greater differentiation of those functions and of the structures performing them. That these characters are correlatives is further shown by the fact that the higher plants are more restricted in their habitats than the lower plants, both in space and time. “The much narrower delimitation in area of animals than plants,” says Sir J. Hooker, “and greater restriction of Faunas than Floras, should lead us to anticipate that plant-types are, geologically speaking, more ancient and permanent than the higher animal types are, and so I believe them to be, and I would extend the doctrine even to plants of highly complex structure.” “Those classes and orders which are the least complex in organization are the most widely distributed.”