PLANTS 



327 



has slightly thicker walls for protection, while on the 

 under side it is pierced by numerous pores called stomata. 

 Many thousand of these openings may occur in a square 



"K; 



1. A Maple Leaf. 



4. A Rose Leaf. 



2. A Wild Lily-of-the- Valley Leaf. 



FIG. 288. 



3. A Birch Leaf. 



inch of leaf surface. The stomata are the lungs of the 

 plant. They take in air and expel gases which the plant 

 does not need. Around each stoma there are two guard 



