50 BOTANY 



which accompanies this opening. The water in the cells 

 of the leaf was in the early stages of development dis- 

 tributed mainly to those of the under side, making them 

 most turgid and causing them to grow most freely. 

 The access of light disturbs this relationship and the 

 cells of the upper side become most turgid; the conse- 

 quent growth causes them to lose the concavity of their 

 upper sides; they become flat or sometimes slightly 

 curved in the other direction. 



If light is not allowed access to them this growth of 

 the upper side is very much interfered with, and the 

 leaves show but little change of curvature, lying close 

 to the stem as it elongates, and in some cases not 

 becoming even flat. 



CHAPTER VI 



THE CONSTRUCTION OF THE SHOOT SYSTEM 



THE behaviour of the plumule in elongating and ex- 

 panding gives rise to the primary shoot. Every succes- 

 sive bud and branch which spring from it increases its 

 dimensions by multiplying the number of twigs it bears. 

 As the number of such buds upon each twig is fairly 

 large, we see that the young branches increase in a kind 

 of geometrical progression, causing the formation of a 

 large shoot system, which constitutes the body of a 

 shrub or the head of a tree. We must next study the 

 construction of such a head. 



To understand it we must inquire what are the 

 purposes for which it exists, and what are the dangers 

 against which it must protect itself. 



We have already drawn attention to the fact that the 

 functions of the root and the shoot are fundamentally 

 different. That being so, it seems clear that the mode 

 of arrangement of the parts of the one need form no 



