114 DIFFUSION AND OSMOTIC PRESSURE 



water of the substratum in which the organism is growing, 

 are to be regarded as one continuous mass of liquid. Thus, 

 if the diffusion tension of water in any part of the plant 

 becomes less than it is at any other point, diffusion takes 

 place and equilibrium is restored. In the same way, if the 

 diffusion tension within the plant falls below that of the 

 substratum, diffusion of water into the plant must imme- 

 diately occur. 



This process of simple diffusion is sufficient to account 

 for absorption and for transport in the simple plant bodies 

 and in any small portion of larger bodies. But in the more 

 complex bodies of higher plants this is not sufficient. Just 

 how the sap is raised in trees is not surely known. There 

 are at present two main theories to account for it: (1) It is 

 supposed to be raised by periodic pumping action of living 

 cells in the trunk. (2) It is supposed that evaporation and 

 the resulting osmotic concentration in the leaves will draw 

 it up from the roots, the cohesion of the minute water 

 columns being supposed to be of sufficient magnitude to 

 prevent their being broken by the strain. 



