THE FACTORS OF ORGANIC EVOLUTIOX. 195 



tinuous cuticle, and by Laving the outer walls of its cells unlike the 

 inner walls.* Especially instructive is the structure of such inter- 

 mediate types as the Liverworts. Beyond the differentiation of the 

 raring cells from the contained cells, and the contrast between up- 

 per surface and under surface, the frond of Marchantia polymorph". 

 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 "is under all circumstam 

 the upper side which forms stornata, the dark side becomes the under 

 side which produces root-hairs and leafy processes.' 1 i 80 that while 

 we have undeniable proof that the contrasted influences of the medi- 

 um 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 expansion to stems and roots, facts of like meaning m- 

 08. Speaking generally of epidermal tissue and inner tissue, Sachs 

 remarks that "the contrast of the two is the plainer the more the 

 part of the plant concerned is exposed to air and light." J E^ - 

 where, in correspondence with this, it is said that in roots the cells 

 of the epidermis, though distinguished by bearing hairs, "are other- 

 wise similar to those of the fundamental tissue " * which they clothe, 

 while the cuticular covering is relatively thin ; whereas in stems 

 the epidermis (often further differentiated) is composed of layers of 

 cells which are smaller and thicker- walled : a stronger contrast of 

 structure corresponding to a stronger contrast of conditions. By way 

 of meeting the suggestion that these respective differences are wholly 

 due to the natural selection of favourable variations, it will suffice if 

 I draw attention to the unlikeness between imbedded roots and ex- 

 posed roots. "While in darkness, and surrounded by moist earth, the 

 outermost protective coats, even of large roots, are comparatively 

 thin ; but when the accidents of growth entail permanent exposure to 

 light and air, roots acquire coverings allied in character to the cover- 

 ings of branches. That the action of the medium causes these and 

 converse changes, cannot be doubted when we find, on the one hand, 

 that "roots can become directly transformed into leaf-bearing shoot-. " 

 and, on the other hand, that in some plants certain "apparent roots 

 are only underground shoots," and that nevertheless "they are similar 

 to true roots in function and in the formation of tissue, but have no 

 root-cap, and, when they come to the light above ground, continue to 

 grow in the manner of ordinary leaf-shoots." ] If, then, in highly de- 

 veloped plants inheriting pronounced structures, this differentiating 



* Sachs, pp. S3-4. f Ibid., p. 135. J Ibid., p. 3. * PAd., p. 83. 



I Ibid., p. 147. 



