174 DRY-FARMING 



is drawn from the soil by the roots through the stems 

 is evaporated into the air. There is some evapora- 

 tion of water from the stems and branches of plants, 

 but it is seldom more than a thirtieth or a fortieth of 

 the total transpiration. The evaporation of water 

 from the leaves through the breathing-pores is the 

 so-called transpiration, which is the greatest cause 

 of the loss of soil-water under dr}'-farm conditions. 

 It is to the prevention of this transpil-ation that 

 much investigation must be given by future students 

 of dry-farming. 



Transpiration 



As water evaporates through the breathing-pores 

 from the leaves it necessarily follows that a demand 

 is made upon the lower portions of the plant for 

 more water. The effect of the loss of water is felt 

 throughout the whole plant and is, undoubtedly, one 

 of the chief causes of the absorption of water from 

 the soil. As evaporation is diminished the amount 

 of water that enters the plants is also diminished. 

 Yet transpiration aj^pears to be a process wholly 

 necessary for plant life. The question is, simply, 

 to what extent it may be diminished without inj uring 

 plant growth. Mam' students lielieve that the car- 

 bon assimilation of the plant, which is fundamentally 

 important in plant growth, cannot be continued un- 

 less there is a steady stream of water passing through 

 the plant and then evaporating from the leaves. 



