4 

 Barnes (1) advances the theory that the amount of 

 salts absorbed is dependent upon the living cortex of the 

 rootlets and the mesopjiyll of the leaves. Ke states* ■ "if - 

 the cortex be freely permeable equilibrium in the distri- 

 bution of any given salt will occur, assuming for a time 

 no evaporation from the aerial parts. If then evaporation 

 concentrates the solution the higher diffusion tension of 

 thfltsalt will tend to drive it to those regions ^here the 

 diffusion tension is lower. This tendency, therefore, 

 wou.ld operate against the further supply of that raaterial 

 in the leaves. If the cortical layers be not freely per^ 

 meable, the amount absorbed is regulated wholly by proto- 

 plasmic activity and cannot be affected directly by outside 

 supply. The phenomena of selective absorption sho^r that 

 transpiration does not determine in those cases the amoimt 

 of salts absorbed." 



The experiments conducted by certain investigators 

 serve to indicate that an increase or decrease in trans- 

 piration, as influenced by the hviraidity of the atmosphere, 

 does not necessarily produce a corresponding increase or 

 decrease in growth. 



