114 



STRUCTURE OF LEAF. 



[chap. 



Hence we see how beautifully the whole form of 

 these leaves is adapted to the mode of growth of the 

 trees themselves and the arrangement of their buds. 



Before we proceed to consider the next series of 

 species to which I wish to direct attention, it will be 

 necessary for me to say a few words on the micro- 

 scopical structure of the leaf. Although so thin, the 

 leaf consists of several layers of cells. Speaking 

 roughly, and as a general rule, we may say that on 

 each side is a thin membrane, or epidermis, underneath 



Fig. 72. — Maple leaves on Chestnut. 



which on the upper side are one or more layers of 

 elongated cells known from their form as " pallisade 

 cells," beneath which is a parenchymatous tissue of 

 more or less loose texture. The leaf is strengthened 

 by ribs of woody tissue. From this general type there 

 are, of course, numerous variations. For instance, 

 many water plants have no epidermis. The structure 

 of the leaf has been described in a number of memoirs 

 by Bonnet, Haberlandt, Areschoug, Stael, Pick, Hein- 

 richer, Vesque, Tschirch, Hentig, and other writers. 



