ABSORPTION OF MATERIALS IN GENERAL I 29 



pheres, but it may be much higher, as much as 157 atmospheres having been reported 

 for moulds. The actual osmotic pressure in a cell is generally much lower than the 

 osmotic value of its sap. 



The rate of diffusion of water and solutes through protoplasm is influenced by 

 temperature, by the condition of the protoplasm, etc., as well as by the concentration 

 difference (or gradient) between the interior and exterior of the cell. Carbon dioxide 

 continually diffuses out of roots into the soil solution (excepting when the transpira- 

 tion rate is so high that the flow of water into the roots is more rapid than the diffusion 

 rate of carbon dioxide). This substance (forming carbonic acid when dissolved in 

 water) acts as a solvent on many solid soil constituents. Organic acids appear to 

 diffuse out of roots when the latter are poorly supplied with oxygen, and these acids 

 may have a similar action on solid materials in the soil. Some fungi and algae 

 normally give off organic acids, as do many bacteria also. 



