THE OUTER AND INNER TISSUES OF PLANTS. 247 



unlikeness very conspicuously. And it holds even with rib 

 bon-shaped fronds. Wherever one of these is composed of 

 three, four, or more layers, as in Laminaria 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 

 epidermal cells, forming a protecting envelope, diverge physi 

 cally and chemically from the mesophyll cells, which carry 

 on the more active functions. And the contrast may be 

 observed to establish itself in the course of development. 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 con 

 ditions 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 phamogamic axis, especially when it under 

 goes what is known as secondary thickening, is commonly 

 characterized by an outer zone of cells (the cork layer) differ 

 ing from the inner layers in character and function, as it 

 differs from them in position. Subject as this outer layer is 

 to the unmitigated actions of forces around to abrasions, to 

 extremes of heat and cold, to evaporation and soaking with 

 water its units have to be brought into 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 

 from the rest are the forces which it has to resist, and from 

 which it passively protects the parts within. How 



