114 Diffusion and Osmotic Pkessueb 



water of the substratum in which the organism is growin 

 are to be regarded as one continuous mass of liquid. Thu 

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

 becomes less than it is at any other point, diffusion tak( 

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

 diffusion tension within the plant falls below that of tl 

 substratum, diffusion of water into the plant must immi 

 diately occur. 



This process of simple diffusion is sufficient to accoui 

 for absorption and for transport in the simple plant bodi( 

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

 complex bodies of higher plants this is not sufficient. Jus 

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

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

 supposed to be raised by periodic pumping action of livin 

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

 the resulting osmotic concentration in the leaves will dra^ 

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

 columns being supposed to be of sufficient magnitude t 

 prevent their being broken by the strain. 



