680 PLANT GROWTH AND PLANT COMMUNITIES 



TABLE III 

 Quantitative Contact Experiment 



* y = yellow; w = white; g = green; o-b =: olive brown. 

 *" The iron content of the plants is the means of four replicate sextets. Individual 

 sextets are plotted in Figure 8. 



The influence of carbon dioxide excretion is more difficult to eval- 

 uate. Average soil air contains 0.30 volume per cent of CO2, and at 

 that partial pressure the pH of a CaCOa solution is 7.8, still too high 

 for solubilizing iron. Inside the root, carbon-dioxide pressures are high 

 ( Lundegardh and Burstrom, 1933 ) , and conceivably at the root surface 

 there might be pockets and cavities where the liquid phase is com- 

 pletely saturated with carbon dioxide. Under such conditions, and in 

 the presence of solid CaCOs, a pH of 6.3 obtains, and iron solubility 

 would be in the realm of practical significance, as suggested by Chap- 

 man (1939) and Milad (1939). To assess the role of CO2, Treatment 

 VI ( see Table III ) was included. The upper column contained Fe-sand 

 but no plants. Instead, CO2 gas was continuously bubbled into it 

 through a glass tube reaching to the bottom of the Fe-sand layer. The 

 lower column was filled with white sand and was given to plants. The 

 percolating solution was maintained at pH 6.3. 



Three treatments measured contact effects proper. The upper 

 column in Treatment II contained 4 per cent iron sand, in Treatment 

 III, 10 per cent, and in Treatment IV, 20 per cent. There were six plants 

 in each vessel. All the lower columns carried white sand with plants. 



Each treatment was replicated four times, and the entire set— with 



