SOIL CONDITIONS AFFECTING PLANT GROWTH 47 



expression is convenient and it gives useful information to the 

 practical grower in irrigated districts, but from the strictly 

 scientific point of view it suffers from the disadvantage that 

 it implies a causal relationship between transpiration and as- 

 similation when in reality there is none. 



The transpiration coefficient is not constant but varies 

 with the plant and the conditions, increasing with the tempera- 

 ture and to some extent with the water supply, but decreasing 

 as the food supply increases. 



Table IX. — Transpiration Coefficients, i.e. Amount of Water Trans- 

 pired During the Production of One Part of Dry Matter. Briggs 

 and Shantz (55c). 



With the plant variations we are not concerned. It is 

 demonstrated, however, that a difference exists between 

 different varieties of the same crop and that there are con- 

 siderable prospects for breeding or selecting varieties specially 

 suited for dry conditions. This work is already in hand 

 with good results in Australia, the Western States and else- 

 where. Unfortunately no correlation has been traced between 

 water requirements and plant structure, so that the breeder 

 has no guide in his selection except actual and tedious trials. 



The effect of soil conditions has been studied at the Be- 

 sentchuk Agricultural Experiment Station, Samara, Russia, 

 situated in a district which suffers from prolonged summer 

 drought and excessive variations in crops. The transpiration 



