The following paper, by B. E. Livingston, was read : 



Evaporation as a Climatic Factor Influencing 

 Vegetation. 



By Burton Edward Livingston, 

 Desert Botanical Laboratory, Tucson, Arizona. 



All plants, excepting aquatics, are influenced in their growth 

 and behavior to a marked degree by fluctuations in the exter- 

 nal water supply. This is almost wholly due to the continual 

 loss of water occurring from all aerial surfaces of the plant 

 body, especially from those surfaces which, by the presentation 

 of wet membranes to the air, are possible channels for the en- 

 trance of the food material, carbon dioxide, and for the exit of 

 the waste product, oxygen. By far the greater portion of the 

 water that the plant appears to use is not used at all in the true 

 sense, but makes up for the loss by transpiration, and is itself 

 soon lost by transpiration. This water must, of course, be 

 drawn from the soil by the roots and transmitted to the tran- 

 spiring organs by the conducting system. 



The rate of water loss by any plant, while it does not strictly 

 follow the rate of evaporation in its hourly fluctuations, is deter- 

 mined for the entire day or for longer periods by the evapora- 

 tion rate, i. e., by the evaporating power of the air around it. 

 This is of course true only so long as the water supply of the 

 soil is adequate to keep the tissues at their normal condition of 

 saturation. 



Every plant has a certain maximum rate of water intake 

 through its roots, this maximum being highest for any stage of 

 growth with the optimum amount of water in the soil. This 

 rate increases as the root system becomes more extensive, and 

 decreases when for any reason the root system is injured or par- 

 tially destroyed. Of course this maximum also decreases with 

 a lowering of the moisture content of the soil. 



There must likewise be a maximum rate at which water can 



