§ 268. The simplest plant presents a contrast between its peripheral substance and its central substance. In each protophyte, be it a spherical cell or a branched tube, or such a more-specialized form as a Desmid, a marked unlikeness exists between the limiting layer and that which it limits. These vegetal aggregates of the first order may differ widely from one another in the natures of their outer coats and in the natures of their contents. As in the Palmella-form of one of the lower Algæ, there may exist a clothing of jelly; or, as in Diatom, the walls may take the form of silicious valves variously sculptured. The contained matter may be partly or wholly here green, there red, and in other cases brown. But amid all these diversities there is this one uniformity—a strong distinction between the parts in contact with the environment and the parts not in contact with the environment.

When we remember that this trait is one which these simple living bodies have in common with bodies that are not living—when we remember that each inorganic mass eventually has its outer part more or less differentiated from its inner part, here by oxidation, there by drying, and elsewhere by the actions of light, of moisture, of frost; we can scarcely resist the conclusion that, in the one case as in the other, the contrast is due to the unlike actions to which the parts are subject. Given an originally-homogeneous portion of protoplasm, and it follows from the general laws of Evolution (First Principles, §§ 149–155), first, that it must lose its homogeneity, and, second, that the leading dissimilarities must arise between the parts most-dissimilarly conditioned—that is, between the outside and the inside. The exterior must bear amounts and kinds of force unlike the amounts and kinds which the interior bears; and from the persistence of force it follows inevitably that unlike effects must be wrought on them—they must be differentiated.

What is the limit towards which the differentiation tends? We have seen that the re-distribution of matter and motion whence, under certain conditions, evolution results, can never cease until equilibrium is reached—proximately a moving equilibrium, and finally a complete equilibrium (First Principles, §§ 170–175). Hence, the differentiation must go on until it establishes such differences in the parts as shall balance the differences in the forces acting on them. When dealing with equilibration in general, we saw that this process is what is called adaptation (First Principles, § 173); and, in this work, we saw that by it the totality of functions of an organism is brought into correspondence with the totality of actions affecting it (§§ 159–163). Manifestly in this case, as in all others, either death or adjustment must eventually result. A force falling on one of these minute aggregates of protoplasm, must expend itself in working its equivalent of change. If this force is such that in expending itself it disturbs beyond rectification the balance of the organic processes, then the aggregate is disintegrated or decomposed. But if it does not overthrow that moving equilibrium constituting the life of the aggregate, then the aggregate continues in that modified form produced by the expenditure of the force. Thus, by direct equilibration, continually furthered by indirect equilibration, there must arise this distinction between the outer part adapted to meet outer forces, and the inner part adapted to meet inner forces. And their respective actions, as thus meeting outer and inner forces, must be what we call their respective functions.

§ 269. Aggregates of the second order exhibit parallel traits, admitting of parallel interpretations. Integrated masses of cells or units homologous with protophytes, habitually show us contrasts between the characters of the superficial tissues and the central tissues. Such among these aggregates of the second order as have their component units arranged into threads or laminæ, single or double, cannot, of course, furnish contrasts of this kind; for all their units are as much external as internal. We must turn to the more or less massive forms.

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.