37^ NORMAN 



Table 3. Cation-retention capacity of plant roots 



m.e. per 100 g. m.e. per 100 g. 



Species dry roots Species dry roots 



Soybean 58.9 » Rye 15.1 » 



Red clover 47.9 ^ Barley 12.3 ^ 



Ragweed 58.9 « Alfalfa 41.4 " 



Potato 38.1 a Soybean 41.1 ^ 



Tomato 34.6 ^ Redtop 14.1 ^ 



Corn 26.0 ^ Reed canary grass . . 11.8 '^ 



Orchard grass 25.6 =i Buckwheat 38.7 <= 



Timothy 22.6 •-» Oats 18.8 « 



^ By electrodialysis and titration. Drake, M., J. Vengris, and W. G. Colby. Soil 

 Sci. 72:139-147. 1951. 



^ By electrodialysis and titration. McLean, E. 0., and F. E. Baker. Soil Sci. Soc. 

 Amer. Proc. 17:100-102. 1953. 



'^ By electrodialysis and titration. McLean, E. 0., and D. Adams. Soil Sci. Soc. 

 Amer. Proc. 18:273-275. 1954. 



to point out here, however, that the methods used for determining the ex- 

 change capacity of roots will include sites throughout the free space, in addi- 

 tion to those on the external surfaces of the roots. Direct-contact exchange 

 of bases on the soil colloids would presumably only be possible with the ex- 

 ternal surfaces. Root surfaces and clay colloids in contact would come to an 

 equilibrium which would depend on the relative ease of replacement of cations 

 on the two insoluble exchange systems. 



The processes of diffusion and cation exchanges are essentially physical 

 and involve no expenditure of energy by the cell. Both will proceed at low 

 temperature or in an inert gas. No ion specificities characteristic of the special 

 requirements of the plant are evident in these steps. 



The key process of ion accumulation, which is partially selective and which 

 does involve a metabolic mechanism, has been a recalcitrant problem for the 

 plant physiologist. Its full solution yet awaits further clarification of the 

 biochemical events within plant cells. However, the view which is gaining 

 wide acceptance is that the essential feature in uptake is a binding of the ion 

 with a carrier, the carrier being a compound, perhaps analogous to an 

 enzyme, produced by the cell. Carriers are presumed to exist for both anions 

 and cations and to possess specificity or selectivity for particular ions, but 

 this specificity is not absolute, and ions of other elements chemically related 

 in the periodic table may also be bound. The carrier-ion complex is then pre- 

 sumed to be able to pass through some form of barrier or membrane not per- 

 meable to the ion alone. Once in this "inner" space, the ion is again released 

 in an irreversible step. The barrier is considered to be quite impermeable to 

 the free ions because, if an ion is once absorbed, it does not then exchange 



