732 PLANT GROWTH AND PLANT COMMUNITIES 



vided that two conditions were fulfilled: ( 1) the complexing of cations 

 by roots should not involve the formation of diflFusible molecules that 

 could pass into the external medium, and (2) the rate processes op- 

 erating should be so slow that simple difiFusion could maintain an 

 approximate equilibrium state. 



In the case of rapidly growing plants, it would seem that this 

 simple Donnan equilibrium model would be unrealistic and that some 

 consideration should be given to diffusion processes in the medium 

 surrounding the roots. 



Picture a linear root as occupying the center of a long cylindrical 

 space in a colloidal medium. The first question that arises is whether 

 the root surfaces are actually in contact with the colloidal system or 

 whether a film of water, free from colloid, separates the two. On a 

 micro scale the existence of such a film would not be unreasonable; 

 indeed, in a soil system under a given moisture tension, it might be 

 assumed to have roughly the same thickness as the water films between 

 soil grains. Under a tension of one-third atmosphere, such a film might 

 be about five microns thick. At 15 atmospheres the value would be 0.1 

 micron if under that pressure the surface tension of water has its nor- 

 mal value, which is doubtful. Thus the final step in the diffusion path 

 of cations to the root surface would be through this attenuated water 

 film. Under equilibrium conditions the root could be thought of as 

 bathed by a true solution, which would be the external phase common 

 to two Donnan systems— the soil colloids on the one side and the root 

 surfaces as a colloidal system on the other. All the cationic relationships 

 discussed above would hold. The ionic composition of the exchange 

 sites concerned directly in uptake would be governed by the ionic 

 activity ratios. If there are several kinds of such sites, endowed with 

 greater or less specificity for given cations, these general activity rela- 

 tionships would still hold, but the ionic compositions on the different 

 groups of sites would be different. 



Thus under true equilibrium conditions such ratios as slu/Q-k, 

 an/V^ca, aK/V^ca, etc., will govern the relationships between variations 

 in the soil system and variations in the composition of exchange sites 

 of all kinds on the root. 



Now consider the root as a more active participant in such a sys- 

 tem. For the moment, assume that some unspecified reaction within the 

 root produces hydrogen ions, which immediately change the ionic 

 composition of all exchange sites on its surface. Hydrogen ions then 

 exchange between the root sites and the solution immediately adjacent. 

 The accession of fresh metallic cations to the root surface will be de- 

 termined now by their speed of diffusion across this layer of changed 

 composition. As long as the layer of changed composition remains 



