Amount of Water Used by Plants 17 



but just what are the precise factors, and what their quantitative 

 relations, is not yet so definitely known as it needs to be. The 

 problem is manifestly a complex one, and many of the factors 

 are obscure, and will only be made known in their quantitative 

 relations after much patient critical work has been done having 

 for its prime object the solution of this problem. 



It has already been pointed out that there appears to be 

 relatively less water consumed in the production of a pound of 

 dry matter under some of the conditions which exist in arid 

 America than is required in the more humid sections of this 

 country, and that it appears probable that a part of this differ- 

 ence is to be sought, possibly, in adaptive functions in the plant 

 itself and a part in the differences of soil conditions. 



Under the natural conditions of the field, it would be expected 

 that very much will depend upon the character of the season ; 

 that is, whether the season is humid or dry, whether the tempera- 

 tures are high or low, whether the wind velocities are strong or 

 light, and whether the amount of sunshine is more or less. Very 

 much, too, will depend upon the soil and the character of the 

 rainfall, whether the soil is open and the rains are frequent and 

 heavy, so that considerable amounts of water are lost to the crop 

 by percolation and under-drainage, or whether the soil has a 

 retentive texture, and the rainfall is so proportioned that rela- 

 tively small amounts are lost, nearly all being used in the pro- 

 duction of the crop. Then, too, the manner in which the crop is 

 disposed on the field, whether it covers the surface closely, as do 

 the grasses and small grains, or whether considerable areas of 

 the field are exposed to the direct action of wind and sun, as in 

 many of the hoed crops and in orchards, must have a marked 

 influence in determining the actual amount of water which will 

 disappear or will need to be applied during a season, in order 

 to maintain the best moisture conditions for the particular 

 crop. 



Then, again, the treatment of the soil itself will have much 

 to do with the quantity of water which disappears at once from 

 the surface without in any way benefiting the crop, and also the 

 quantity which drops at once entirely through the root zone, con- 



