THE WORK OF LEAVES 



287 



from the leaf. In leaves that take a horizontal posi- 

 tion the stomata are on the under side. Sometimes 

 there are as many as 100,000 to 

 each square inch. 



310. The Work of Leaves. The 

 leaf has several very important du- 

 ties to perform. For one thing, it 

 controls the rate at which water 

 shall evaporate from the plant (cf. 

 309). Then it also serves as the 

 "breathing apparatus" of the plant, 

 taking in air, and giving off carbon 

 dioxide, etc. Its third great duty 



"laboratory" 

 Here carbon 



fep Guard Cell 



Open.ng 



FIG. 234. Breathing Pore 

 (Stoma) of a Leaf. 



to serve as the 

 of the plant, 

 dioxide and 



water are brought together, 

 and are combined chemically 

 in compounds that the plant 

 uses for food and for the 

 building of its tissue. These 

 compounds are sugar, starch, 

 etc. ; they belong to the class 

 known as carbohydrates 

 (cf. 184). 



There is more oxygen in 

 the carbon dioxide and 

 water, taken together, than 

 is needed to form the carbohydrate; so the excess of oxy- 

 gen is given off into the air (Fig. 235). As a result, the 

 green plant takes up carbon dioxide from the air, and 



Fio. 235. The leaves act upon the carbon 

 dioxide, and produce oxygen. 



