THE STRUCTURE OF THE SHOOT 71 



the periblem, is the cortex. Its composition is very 

 varied as the plants grow older. In the young condition 

 it is only noteworthy because its outer layers of cells 

 contain the green bodies we have called chloroplasts. 

 The great development of branching which takes 



FIG. 28. Epidermal cells of leaf showing three stomata in 

 various stages of opening. 



place necessitates a considerable enlargement of this 

 primary structure. The increase in number of the 

 leaves makes it important to increase the means of 

 transport of water; the slender cylindrical tube of the 

 young stem soon becomes unable to support the weight 

 resulting from its greater size and the number of its 

 branches. The transport of food to its different parts 

 makes increasing demands upon its bast. We must 

 examine the way in which these necessities are supplied. 



