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.