B. THE SOIL AS A RESERVOIR FOR WATER 



I. FUNCTIONS IN PLANT GROWTH 



When plants grow, they use water. It circulates 

 through their vessels, is built into their tissues, and is 

 evaporated by the leaves. In these capacities it per- 

 forms three important and vital functions for the plant. 

 It is (a) a direct food of the plant, and becomes a part 

 of its tissues either directly as water, or it is broken 

 up and its elements are used in new compounds, (b) 

 It is a carrier of food to the plant, and serves as the 

 medium of transfer for the mineral elements from the 

 soil and the gaseous elements from the air to their 

 appropriate points of assimilation and use in the growth 

 of the plant mechanism, (c) In addition to the last two 

 functions, water serves as a regulator of the physical 

 condition of the plant. It equalizes the temperature 

 of the plant and modifies its stability. 



From 60 to more than 95 per cent of the green weight 

 of the staple crops is due to water. 



In the ordinary processes of growth, the amount of 

 water transpired is many times greater than that used 

 directly as food. Investigations in different parts of 

 the world have shown that for the production of each 

 pound of dry matter ordinary crops transpire from 

 200 to 500 pounds of water. 



Warington has compiled the following figures, show- 

 ing the amount of water used by different crops in the 

 production of organic matter. 



(133) 



