THE WATER-COST OF DRY MATTER 



135 



crops varied in very much the same manner. Whether 

 countries or various fields of the same crop in the same 

 country are compared, the water-cost of dry matter will 

 vary widely. 



Meanwhile, under conditions of normal fertility and 

 a favorable growing season, the transpiration ratios fall 



FIG. 24. Determining the transpiration ratio. 



within rather definite limits. In fact, under normal con- 

 ditions, the evapo-transpiration ratio varies from 250 to 

 1,000 pounds of water for each pound of dry matter. This 

 gives a basis for an estimate of the quantity of water 

 required for the production of a good crop of wheat or 

 other standard crops. For example, a crop of wheat 



