230 PHYSIOLOGICAL DEVELOPMENT. 



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

 which it passively protects the parts within. How 



clearly this heterogeneity of structure and function is conse- 

 quent upon intercourse with the environment, every tree 

 and shrub shows. The young shoots, alike of annuals and 

 perennials, are quite green and soft at their extremities. 

 Among plants of short lives, there is usually but a slight 

 development of bark : such traces of it as the surface of the 

 axis acquires being seen only at its lowermost or oldest 

 portion. In long-lived plants, however, this formation of a 

 tough opaque coating takes place more rapidly ; and shows 

 us distinctly the connexion between the degree of differentia- 

 tion and the length of exposure. For, in a growing twig, 

 we see that the bark, invisible at the bud, thickens by 

 insensible gradations as we go downwards to the junction of 

 the twig with the branch ; and we come to still thicker parts 

 of it as we descend along the branch towards the main stem. 

 Moreover, on examining main stems we find that while in 

 some trees the bark, cracked by expansion of the wood, drops 

 off in flakes, leaving exposed patches of the inner tissue which 

 presently become green and finally develop new cortical 

 layers ; in other trees the exfoliated flakes continue adherent, 

 and in the course of years form a rugged fissured coat : so 

 producing a still more marked contrast between outside and 

 inside. Of course the establishment of this he- 



terogeneity is furthered by natural selection, which, where a 

 protective covering is needed, gives an advantage to those 

 individuals in which it has become strongest. But that this 

 divergence of structure commences as a direct adaptation, is 

 clearly shown by other facts than the foregoing. There is 

 the fact that many of the plants which in our gardens 

 develop bark with considerable rapidity, do not develop it 

 with the same rapidity in a greenhouse. And there is the 

 fact that plants which, in some climates, have their stems 

 covered only by thin semi-transparent layers, acquire thick 

 opaque layers when taken to other climates. 



