THE OUTER AND INNER TISSUES OF PLANTS. 229 



when tbey consist of more than single rows of cells, the 

 outer cells become unlike the inner, as shown in Fig. 35. 

 Such types as Chrysyinenia 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 Lamina via 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 

 cuticular cells, forming a protecting envelope, diverge physi- 

 cally and chemically from the contained cells of parenchyma, 

 which carry on the more active functions. And the contrast 

 may be observed to establish itself in the course of develop- 

 ment. 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 mean- 

 ing, exists between the bark of every branch and the tissues 

 it clothes. The phsenogamic axis, especially when exogenous, 

 is commonly characterized by an outer 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 un- 

 mitigated actions of forces around — to abrasions, to extremes 

 of heat and cold, to evaporation and soaking with water — its 

 units cannot cease changing until they are in 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 



