160 



IRRIGATION PRACTICE 



added to the acre. Such large gains, could not, of course, 

 continue for any length of time without resulting in total 

 yields far above the maximum for the crops in question. 



Many of the effects of irrigation are more clearly 

 understood if it is kept in mind that the crop responds 

 readily to the application of water. 



FlG. 30. Effect of little, medium and much water on wheat. 



106. Proportion of roots. The vigor and general 

 condition of the plant depend largely upon the develop- 

 ment of the root-system. In the early stages of growth, 

 the plant uses most of the materials gathered from the 

 air and soil for the development of large and numerous 

 roots, which, radiating through the soil, may readily 

 absorb water and plant-food. When the roots are well 

 developed, carbon-assimilation by the leaves is hastened, 

 and growth is rapid. Later in the life of the plant, root- 

 growth becomes slower and slower, and the energies of the 

 plant are more largely directed to the development of 

 the parts above ground. When at last the stems are well 



