PLANT ROOT-SOIL INTERACTIONS 



689 



Calcium and iron migrate— Fe++ + perhaps slower than Fe++ be- 

 cause of stronger adsorption— by exchange diffusion, hopping, so to 

 speak, from carboxyl group to carboxyl group against a current of 

 hydrogen ions. In the moist root plug the mean distance between two 

 exchange sites is 16.5 Angstrom units. 



Although the acquisition and diffusion of iron were accomplished 

 in a non-living root pile, in single roots metabolic activity appears 

 essential to transporting iron continuously from the surface through 

 the cell wall to the cytoplasm, because a gradient of exchangeable 

 hydrogen ions, and possibly an outward electron flow, must be main- 

 tained. The hydrogen ion might originate in the organic-acid pool, as 

 suggested by Overstreet et al. ( 1942 ) , rather than from calcium dioxide 

 diffusing out. 



A picture of the root surface, based on the experiments 



It appears expedient to distinguish at least two portals for entry 

 of substances into a root: large, 5?:;c-discriminating pores, and small, 

 c/iarge-discriminating pores, the latter acting as ion sieves ( Figure 12 ) . 



The walls of the large pores, with diameters of 50 or 100 Angstrom 

 units and maybe larger, have charge densities which vary according to 

 the abundance of carboxyl groups. Organic molecules and ion pairs 

 {e.g., KCl) readily diffuse through the large channels or are carried 

 through it by the transpiration stream. (In the aforementioned root 

 piles the diffusion coefficient for the chloride ion is 3.4 X 10"^ cm.- per 

 second. ) The size limitation is primarily physical. During their pas- 

 sage, the cations of ion pairs ( KCl ) may interchange with the counter- 

 ions of the electric double layer along the wall, and thus affect per- 



Figure 12. Hypothetical dia- 

 gram of channels in the cell 

 wall for entry of substances 

 into a root. The narrow chan- 

 nel admits cations, largely un- 

 accompanied by dissolved an- 

 ions (the black circles are Fe- 

 cations). The wide channel 

 allows migration of ion pairs 

 {e.g., KCl) and organic mole- 

 cules by solute diffusion and 

 .in the transpiration stream. 





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