686 



PLANT GROWTH AND PLANT COMMUNITIES 



The extent of this mechanical diffusion barrier was determined ex- 

 perimentally by inserting it into an agar cokimn and measuring the 

 diflFusion retardation of KCl. As is well known, in dilute agar gels the 

 diffusion coefficient of KCl is identical with that in water ( Moreno, 

 1957). The ratio of KCl's diflFusion in agar in the absence of barrier to 

 its diflFusion in the presence of barrier was 2.56. To correct for the 

 barrier eflFect, which in essence corresponded to a reduction of the cross- 

 section of the boundary to 39 per cent, the quantity of iron diflFused 

 into the root plug was multiplied by 2.56. Probably this was too severe 

 a correction. 



The columns were dismantled after two days in one experiment, 

 after five days in another, and after ten days in a third. The plugs were 

 sliced with a razor blade into discs of approximately 1.5 mm. thickness. 

 The root-plug portion near the contact zone was embedded in wax and 

 sliced into 0.1 mm. segments with the aid of a microtome. 



Diffusion curves. The results of the two-day runs, expressed as 

 parts per million of iron diflFused into the root plug, are plotted in 

 Figure 11. The experimental values (circles) are well described by the 

 ideal diflFusion equation ( Crank, 1956 ) : 



Fe=: (Fe)oerfc 



X 



2VDt 



Fe=(Fe^erfc^ 



Distance X from boundary 



Figure 11. The relative dif- 

 fusion of iron into root piles 

 resulting from contact decom- 

 position and from solubility of 

 Fe203 in a COg-saturated so- 

 lution. 



