132 



IRRIGATION PRACTICE 



is taken up by the roots, passed through the plant and 

 evaporated at the leaves throughout the season. The 

 pounds of water thus actually passing through the plant 

 for each pound of dry matter produced, give the trans- 

 piration ratio. Under agricultural conditions, however, 

 as water passes through the plant, some water is also 

 evaporated from the soil surrounding the plant. This 



direct loss from 

 the soil surface, 

 if completely 

 checked, would 

 seriously hinder 

 plant-growth. The 

 pounds of water 

 passing through 

 the plant and 

 evaporating from 

 the soil belonging 

 to the plant, for 

 each pound of dry 



matter produced, 



FIG. 23. Stomatal apparatus in carnation leaf 

 through which transpiration occurs. 



giye 



transpiration ratio. Students of this subject have not 

 always carefully distinguished between these ratios; con- 

 sequently, in modern agricultural books, the two ratios 

 are found in the same tables as meaning the same thing. 

 Of the two ratios, the evapo-transpiration ratio is more 

 nearly the measure of the true agricultural needs of the 

 plant. 



In the earlier investigations of the water-cost of dry 

 matter it was dimly thought that, possibly, under all 

 conditions, an invariable relationship existed between 

 the quantities of water transpired and of dry matter pro- 



