58 THE FACTORS OF ORGANIC EVOLUTION. 



between the inner and outer parts of these flattened masses 

 of tissue. In the higher Algse &quot; the outermost layers con 

 sist of smaller and firmer cells, while the inner cells are 

 often very large, and sometimes extremely long ; &quot;* and in 

 the leaves of trees the epidermal layer, besides differing in 

 the sizes and shapes of its component cells from the paren 

 chyma forming the inner substance of the leaf, is itself 

 differentiated by having a continuous cuticle, and by having 

 the outer walls of its cells unlike the inner walls.t 

 Especially significant is the structure of such intermediate 

 types as the Liverworts. Beyond the differentiation of the 

 covering cells from the contained cells, and the contrast 

 between upper surface and under surface, the frond of Mar- 

 chantia polymorpha clearly shows us the direct effect of 

 incident forces ; and shows us, too, how it is involved with 

 the effect of inherited proclivities. The frond grows from a 

 flat disc-shaped gemma, the two sides of which are alike. 

 Either side may fall uppermost; and then of the develop 

 ing shoot, the side exposed to the light &quot; is under all 

 circumstances the upper side which forms stomata, the 

 dark side becomes the under side which produces root-hairs 

 and leafy processes.&quot;! So that while we have undeniable 

 proof that the contrasted influences of the medium on the two 

 sides, initiate the differentiation, we have also proof that the 

 completion of it is determined by the transmitted structure of 

 the type ; since it is impossible to ascribe the development of 

 stomata to the direct action of air and light. On turning 

 from foliar expansions, to stems and roots, facts of like 

 meaning meet us. Speaking generally of epidermal tissue 

 and inner tissue, Sachs remarks that &quot; the contrast of the 

 two is the plainer the more the part of the plant concerned 

 is exposed to air and light.&quot; Elsewhere, in correspondence 

 with this, it is said that in roots the cells of the epidermis, 

 though distinguished by bearing hairs, &quot;are otherwise similar 



* Sachs, p. 210. f Ibid, pp. 83-4. J Rid. p. 185. 



Rid. 80. 



