CHAPTER III. 



DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS. 



§ 272. The motionless protococcoid forms of lower Algce, 

 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 differ- 

 entiations 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, habitu- 

 ally 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. C aider pa 

 prolifera, which " consists of a little creeping stem with 

 roots below and leaves above," originating " in the growth 

 of a body which may be regarded as an individual cell," 

 supplies a still-better example. Among aggre- 



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