74 COLLEGE BOTANY 



it will be readily seen, that the leaves must have a very definite 

 relationship to both air and sunlight. Looking upward into the 

 top of a tree, it will be seen that the leaves are on or near the 

 tips of the shoots and form a canopy supported by the branches 

 as a framework ; or, looking down on the tree, it will be noted 

 that there is very little overlapping of the leaves, which are so 

 arranged as to form a beautiful mosaic. These primary func- 

 tions will be given further consideration 

 in Chapters XIV and XVI. 



The secondary functions of the leaves 

 are as follows: 



(a) Storage, in which the leaves are 

 fleshy and usually serve to supply nour- 

 ishment for flower and fruit stalks, which 

 are produced later. The so-called cen- 

 tury plant is a. good example of this kind, 

 but the cabbage plant is a much more 

 common example. The cabbage is a 

 biennial which produces its flowers and 

 seeds the second year, using the food 

 that was stored in the leaves during the 



Flo. 55. Barberry leaves 



modified into spines. ft^ ^^3 growt h. 



(&) Leaves serve as bud scales for the protection of the true 

 leaves and the flowers within. By carefully removing the scales 

 there will be found a gradual gradation from true scales on the 

 outside to true leaves within (Fig. 20). 



(c) Leaves are frequently developed as spines, in which the 

 veins become firm and sharp, and the mesophyll is greatly re- 

 duced. The barberry is an excellent example of this type of 

 leaf (Fig. 55). 



