254 PHYSIOLOGICAL DEVELOPMENT. 



that the process follows naturally-induced conditions instead 

 of following artificially-induced conditions. But in the 

 common Ivy which, instead of running along the surface 

 of the earth, runs up inclined or vertical surfaces, we see the 

 process interestingly modified without being essentially 

 changed. The rootlets, here differentiated by their con- 

 ditions into organs of attachment much more than organs of 

 absorption, still develop on that side of the shoot next the 

 supporting surface, and do not develop where the shoot, 

 growing away from the tree or wall, is surrounded equally 

 on all sides by light and air: thus showing, undeniably, 

 that the production of the rootlets is determined by the 

 differential incidence of forces. Though survival of the 

 fittest doubtless furthered this transition yet it clearly did 

 not initiate it. That greenness which may be 



observed in these Ivy-branch rootlets while they are quite 

 young, soft, and unshaded, introduces us to facts which are 

 the converse of the foregoing facts ; and proves that the parts 

 ordinarily imbedded in the soil and adapted to its actions, 

 acquire, often in very marked degrees, the superficial 

 structures of the aerial parts, when they are exposed to light 

 and air. This may be witnessed in Maize, which, when 

 luxuriant, sends out from its nodes near the ground, clusters 

 of roots that are thick, succulent, and of the same colour as 

 the leaves. Examples more familiar to us in England occur 

 in every field of turnips. On noting how green is the un- 

 covered part of a turnip-root, and how manifestly the 

 area over which the greenness extends varies with the area 

 exposed to light, as well as with the degree of the exposure, 

 it will be seen that beyond question, root-tissue assumes 

 to a considerable extent the appearances and function of 

 leaf-tissue, when subject to the same agencies. Let us not 

 forget, too, that where exposed roots do not approach in 

 superficial character towards leaves, they approach in 

 superficial character towards stems: becoming clothed 

 with a thick, fissured bark, like that of the trunk and 



