Of these, among Fungi, the common Puff-ball is a good example—good because it presents this fundamental differentiation but little complicated by others. In it we have a cortical layer of interwoven hyphæ obviously unlike the mass of spores which it incloses. So far as the unlikeness between external and internal parts is concerned, we see here a relation analogous to that existing in the simple cell; and we see in it a similar meaning: there is a physiological differentiation corresponding to the difference in the incidence of forces.
Under various forms the Algæ show just the same relation. Where, as in Codium Bursa, we have the ramified tubular branches of the thallus aggregated into a hollow globular mass, the outer and inner surfaces are contrasted both in colour and structure, though the tubules composing the two surfaces are continuous with one another. In Rivularia, again, we see the like, both in the radial arrangement of the imbedded threads and in the difference of colour between the exterior of the imbedding jelly and its interior. The more-developed Algæ of all kinds repeat the antithesis. In branched stems, when they consist of more than single rows of cells, the outer cells become unlike the inner, as shown in Fig. [35]. Such types as Chrysymenia rosea show us this unlikeness very conspicuously. And it holds even with ribbon-shaped fronds. Wherever one of these is composed of three, four, or more layers, as in Laminaria and Punctaria, the cells of the external layers are strongly distinguished from those of the internal layers, both by their comparative smallness and by their deep colour.
§ 270. The higher plants variously display the like fundamental distinction between outer and inner tissues. Each leaf, thin as it is, exemplifies this differentiation of the parts immediately in contact with the environment from the parts not in immediate contact with the environment. Its epidermal cells, forming a protecting envelope, diverge physically and chemically from the mesophyll cells, which carry on the more active functions. And the contrast may be observed to establish itself in the course of development. At first the component cells of the leaf are all alike; and this unlikeness between the cells of the outer and inner layers, arises simultaneously with the rise of differences in their conditions—differences that have acted on all ancestral leaves as they act on the individual leaf.
An unlikeness more marked in kind but similar in meaning, exists between the bark of every branch and the tissues it clothes. The phænogamic axis, especially when it undergoes what is known as secondary thickening, is commonly characterized by an outer zone of cells (the cork layer) differing from the inner layers in character and function, as it differs from them in position. Subject as this outer layer is to the unmitigated actions of forces around—to abrasions, to extremes of heat and cold, to evaporation and soaking with water—its units have to be brought into equilibrium with these more violent actions, and have acquired molecular constitutions more stable than those of the interior cells. That is to say, the forces which differentiate the cortical part from the rest are the forces which it has to resist, and from which it passively protects the parts within. How clearly this heterogeneity of structure and function is consequent upon intercourse with the environment, every tree and shrub shows. The young shoots, alike of annuals and perennials, are quite green and soft at their extremities. Among plants of short lives, there is usually but a slight development of bark or none at all: such traces of it as the surface of the axis acquires being seen only at its lowermost or oldest portion. In long-lived plants, however, this formation of a tough opaque coating takes place more rapidly; and shows us distinctly the connexion between the degree of differentiation and the length of exposure. For, in a growing twig, we see that the bark, invisible at the bud, thickens by insensible gradations as we go downwards to the junction of the twig with the branch; and we come to still thicker parts of it as we descend along the branch towards the main stem. Moreover, on examining main stems we find that while in some trees the bark, cracked by expansion of the wood, drops off in flakes, leaving exposed patches of the inner tissue which presently become green and finally develop new bark; in other trees the exfoliated flakes continue adherent, and in the course of years form a rugged fissured coat: so producing a still more marked contrast between outside and inside. Of course the establishment of this heterogeneity is furthered by natural selection, which, where a protective covering is needed, gives an advantage to those individuals in which it has become strongest. But that this divergence of structure commences as a direct adaptation, is clearly shown by other facts than the foregoing. There is the fact that many of the plants which in our gardens develop bark with considerable rapidity, do not develop it with the same rapidity in a greenhouse. And there is the fact that plants which, in some climates, have their stems covered only by thin semi-transparent layers, acquire thick opaque layers when taken to other climates.
Just noting, for the sake of completeness, that in the roots of the higher plants there arises a contrast between outer and inner parts, parallel to the one we have traced in their branches, let me draw attention to another differentiation of the same ultimate nature, which the higher plants exhibit to us—a differentiation which, familiar though it is, gains a new meaning by association with those named above, and makes their meaning still more manifest. Each great plant shows it. When, by the budding of axes out of axes, there is produced one of those highly-compounded Phænogams which we call a tree, the central part of the aggregate becomes functionally and structurally unlike the peripheral part. On looking into a large tree, or even a small one which has thick foliage, like the Laurel, we see that the internal branches are almost or quite bare of leaves, while the leaf-clad branches form an external stratum; and all our experience unites in proving that this contrast arises by degrees, as fast as the growth of the tree entails a contrast between the conditions to which inner and outer branches are exposed. Now when, in these most-composite aggregates, we see a differentiation between peripheral and central parts demonstrably caused by a difference in the relations of these parts to environing forces, we get support for the conclusion otherwise reached, that there is a parallel cause for the parallel differentiations exhibited by all aggregates of lower orders—branches, leaves, cells.
§ 271. Before leaving this most general physiological differentiation, it may be well to say something respecting certain secondary unlikenesses which usually arise between interior and exterior. For the contrast is not, as might be supposed from the foregoing descriptions, a simple contrast: it is a compound contrast. The outer structure itself is usually divisible into concentric structures. This is equally true of a protophyte and of a phænogamic axis. Between the centre of an independent vegetal cell and its surface, there are at least two layers; and the bark coating the substance of a shoot, besides being itself compound, includes another tissue lying between it and the wood. What is the physical interpretation of these facts?
When a mass of something we distinguish as inert matter is exposed to external agencies capable of working changes in it—when it is chemically acted upon, or when, being dry, it is allowed to soak, or when, being wet, it is allowed to dry—the changes set up progress in an equable way from the surface towards the centre. At any time during the process (supposing no other action supervenes) the modification wrought, first completed at the outside, either gradually diminishes as we approach the centre, or ceases suddenly at a certain distance from the centre. But now suppose that the mass, instead of being inert, is the seat of active changes—suppose that it is a portion of complex colloidal substance, permeable by light and by fluids capable of affecting its unstable molecules—suppose that its interior is a source of forces continually liberated and diffusing themselves outwards. Is it not likely that while at the centre the action of the internally-liberated forces will dominate, and while at the surface the action of the environing forces will dominate, there will be between the two a certain place at which their actions balance? May we not expect that this will be the place where the most unstable matter exists—the place outside of which the matter becomes relatively stable in the face of external forces, and inside of which the matter becomes relatively stable in the face of internal forces? And must we not conclude that though part of the adjustment is due to indirect equilibration, the initiation of it is due to direct equilibration?
But we are here chiefly concerned with the more general interpretation, which is independent of any such speculation as the foregoing. These contrasted tissues and the contrasted functions they severally perform are, beyond question, subordinated to the relations of outside and inside. And the evidence makes it tolerably clear that the unlike actions or forces involved by the relations of outside and inside, determine these contrasts—partly directly and partly indirectly.
CHAPTER III.
DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS.
§ 272. The motionless protococcoid forms of lower Algæ, which do not permanently expose any parts of their surfaces to actions unlike those which other parts are exposed to, have no parts of their surfaces unlike the rest in function and composition. This is what the hypothesis prepares us for. If physiological differentiations are determined by differences in the incidence of forces, then there will be no such differentiations where there are no such differences. Contrariwise, it is to be expected that the most conspicuous unlikeness of function and minute structure will arise between the most-dissimilarly circumstanced parts of the surface. We find that they do. The upper end and the lower end, or, more strictly speaking, the free end and the attached end, habitually present the strongest physiological contrasts.