Finally, when we remember how rapidly analogous modifications of function and structure arise in the superficial tissues of individual plants, the general inference can scarcely be resisted. When we meet with so striking a case as that of the _Begonia_-leaf, a fragment of which stuck in the ground produces roots from its under surface and leaves from its upper surface--when we see that though, in this case, the typical structure of the plant presently begins to control the organizing process, yet the initial differentiations are set up by the differential actions of the environment; the presumption becomes extremely strong that the heterogeneities of surface which we have considered, result, as alleged, directly or indirectly from heterogeneities in the incident forces. CHAPTER IV. DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS.[49] § 277. In passing from plants formed of threads or thin laminæ, to plants having some massiveness, we find that after the external and internal parts have become distinguished from one another, there arise distinctions among the internal parts themselves, as well as among the external parts themselves: the primarily-differentiated parts are both re-differentiated. From types of very low organisation illustrations of this may be drawn. In the thinner kinds of _Laminaria_ there exists but the single contrast between the outer layer of cells and an inner layer; but in larger species of the same genus, as _L. digitata_, there are three unlike layers on each side of a central layer differing from them--augmentation of bulk is accompanied by multiplication of concentric internal structures, having their unlikenesses obviously related to unlikenesses in their conditions. In _Furcellaria_ and various _Algæ_ of similarly swollen forms, the like relation may be traced. Just indicating the generality of this contrast, but not attempting to seek in these lower types for any more specific interpretation of it, let us pass to the higher types. The argument will be amply enforced by the evidence obtained from them. We will look first at the conditions which they have to fulfil; and then at the ways in which the functions and structures adapting them to these conditions arise.