448 PLANTS AND THEIR RELATION TO MAN 



When a leaf is held up to the light, a close network of smaller, 

 finer veins is visible (Fig. 315). Veins form the framework of 



the leaf and give support to 

 it ; they also serve as channels 

 for the passage of water and 

 cell sap. Different leaves are 

 characterized by different 

 arrangements of veins : lily 

 leaves are parallel veined ; 

 beech and birch leaves are 

 feather veined ; maple leaves 

 are palm veined. 



FIG. 315. Every Leaves as food for man. - 

 leaf has a network T ne leaves of green plants 



of veins. . 



always contain rood and man 

 has learned to use many kinds to satisfy his hunger. Such 

 widely known vegetables as cabbage, spinach, water cress, 

 kale, and lettuce are merely clusters of leaves rich in sub- 

 stances agreeable to man's taste and capable of satisfying 

 his hunger (Fig. 316). When we think of the great variety of 

 green leaves and the few that are used for food, we see that we 



FIG. 314. Many 

 leaves are com- 

 pound. 



FIG. 316. Cabbage and lettuce are clusters of leaves. 



