CHAPTER IIJ. 

 DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS. 



272. The Protococci and such compound forms as the 

 Volvox globator, 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 hypo- 

 thesis 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. 



Even aggregates of the first order illustrate this truth. 

 Such so-called unicellular plants as those delineated in 

 Figs 4, 5, and 6, show us, on comparing the contents of 

 their fixed ends and their loose ends, that different processes 

 are going on in them, and that different functions are 

 being performed by their limiting membranes. Caulerpa 

 prolifera, which '' consists of a little creeping stem wth 

 roots below and leaves above," originating " in the 



