PLANT ROOT-SOIL INTERACTIONS 687 



* Obtained by extrapolating the diffusion curve to x = o, and in the case of solu- 

 tion piles by multiplying the extrapolated value by 2.56 to correct for the diffusion 

 barrier of disc + cellulose membrane. 



(Fe)o is the concentration of iron in p.p.m. in the root at the iron 

 oxide-root boundary, which may be considered as invariant; x is the 

 distance from the boundary in centimeters; D is the diflFusion coefR- 

 cient; t is the time in seconds, and crfc is the error function comple- 

 ment. For the contact and solution pairs and for the three time periods 

 —six root piles in all— the crucial parameters are listed in Table IV. 



The most striking observation is the inefficiency of the CO2- 

 mechanism, as seen from the curves in Figure 11 and from the (Fe)o 

 concentrations at the plug boundary. Whereas the contact-decompo- 

 sition of Fe^Os furnished a large, steady supply of iron— 1,800, 1,900, 

 and even 3,000 p.p.m. at the root boundary— the COo-saturated solu- 

 tion charged the root plane with only 12 to 38 p.p.m. of iron. Since, in 

 the growth-contact trials with Fe-sand, the roots containing 110 to 125 

 p.p.m. of iron still carried strongly chlorotic tops, the COo-mechanism, 

 in the light of the root-plug test, appears incapable of delivering suffi- 

 cient iron to the root. But contact transfer of iron is highly efficient. 

 Near the contact zone the iron concentration in the root reaches over 

 1,500 p.p.m. 



Judging from the diffusion coefficients in the solution piles ( Table 

 IV), the dissolved Fe ions, presumably as Fe++ or Fe+ + + bicarbon- 

 ates, appear to diffuse somewhat faster than the adsorbed Fe ions. 

 Since the dissolved Fe ions are accompanied by high concentrations of 

 CO2, they are expected to react but mildly with the COOH groups and 

 therefore to diffuse to a considerable extent in the larger pores. The 

 contact experiments, on the other hand, are at a disadvantage as far as 

 COo-supply is concerned, for no correction could be made for the CO2- 

 diffusion barrier consisting of the Visking membrane and the Incite 

 disc with only five holes. Nevertheless, contact proves a hundredfold 

 superior in acquisition of iron. 



