THE STRUCTURE OF THE SHOOT 



75 



crinkled and split up through the action of the weather 

 and the storms the tree is exposed to. 



We must now examine the interior arrangements of 

 the leaf. We have already learnt that its special work 

 is mainly twofold. It is the chief agent in transpira- 



FIG. 33. Section of bark of Quercus. pe, cork layers arising at 

 different depths in the cortex. (After Kny.) 



tion or the evaporation of the water which the plant 

 does not permanently retain; while it also is the chief 

 seat of the construction of organic food substance. 

 These two duties are discharged mainly by the cells of 

 the lower and of the upper halves of the leaf respectively. 

 The petiole or stalk of the leaf has a general structure 

 not unlike that of the stem, except that its vascular 

 strands do not form a complete cylinder, but only half 



