PLANT ROOT-SOIL INTERACTIONS 



681 



slight modifications— was repeated during various seasons, always pro- 

 ducing the same pattern of results. 



Growth and iron occunnilation. As seen from Table III and Figure 

 7, the plants from the sand check (7) were yellow-white, tiny dwarfs, 

 and so were all other plants in the lower columns. Moreover, all plant 

 sextets had the same iron content, as determined by the bathophenan- 

 throline method (Smith et al., 1952). Evidently the specimen in the 

 lower columns (except W) did not receive iron from the Fe-grains 

 above. This want casts serious doubt on the efficacy of iron solubility 

 and of chelate excretion, as it operated in this experiment. In confirma- 

 tion, whenever iron chelate Fe-EDTA was added to the yellow-white 

 dwarfs they turned green and developed into healthy plants. 



In the CO--set the plants were higher in iron— 68 against 48 p. p.m. 

 —but this gain was nullified by the poor growth in the C02-enriched 

 percolating solution. 



The plants in the upper columns improved in growth and color as 

 the proportion of iron sand increased. As seen in Figure 8, the relation- 

 ship between the mean iron contents of the entire plants (Fc)— as sex- 

 tets—and the percentage of Fe-sand ( i ) is exponential 



Figure 7. Double-column ex- 

 periment showing the growth 

 of alfalfa plants as influenced 

 by the number of contact sites 

 on the roots (in 0, 4, 10, and 

 , 20 per cent FcoOg-sand) . 



