AGRICULTURAL BOTANY 141 



Functions of Leaves. --This elaborate and peculiar 

 arrangement of the cells of a leaf is an important feature 

 of plant life. 



Leaves do three kinds of work : 



1. They furnish means for the evaporation of water 

 brought up from the soil. 



2. They take in air, utilizing the carbon dioxide dur- 

 ing the process of food making and throwing off oxy- 

 gen. 



3. They change the substances received from the 

 soil and from the air into plant food in a form capable 

 of forming plant tissue. 



Transpiration. All parts of the plant that are ex- 

 posed to the air evaporate water more or less rapidly 

 in the form of vapor. This process is called transpira- 

 tion. 



Leaves, presenting as they do so much surface to the 

 air, are particularly well adapted to promote trans- 

 piration. Under the influence of sunlight the stomata 

 remain open during the day and thus present numerous 

 openings for the passage of vapor into the air. A proper 

 amount of transpiration is beneficial to the plant as 

 it aids in the circulation of water to the extremities 

 of the plant. On the other hand, when the air is very 

 dry, transpiration goes on so rapidly that it retards 

 the growth of the plant, and the water pressure of the 

 cells is so reduced that the plant may shrink, or wilt. 



It is an interesting fact that the stomata aid materi- 

 ally in regulating the amount of moisture transpired. 

 When the cells of the plant are full of moisture, the 

 guard cells about the stomata enlarge the openings into 

 the intercellular spaces and transpiration is increased ; 

 on the other hand, when the cells have transpired more 



