72 



very small 1 , but it is much more likely to mean that 

 molar gas exchange resulting from water movement is 

 relatively more important than is diffusion. Such 

 molar gas exchange, as noted, will he greatly lessened 

 in heavy soils because of the smallness of their indi- 

 vidual pore spaces. 



In addition to the oxygen and carbon dioxide 

 in gaseous form in the soil both of these gases are 

 normally present in the soil solution and the oxygen so 

 supplied is probably quite important to the respiration 



1. This might occur, for instance, because 

 of the effect of adsorbed films of gas held more or less 

 permanently on the surface of soil particles or water 

 films. The interference of water vapor is also a 

 possibility. On adsorbed gas films in soils see Patten 

 and Gallagher,- U. 3. Bur. of Soils, Bull. 51, 50 pp. 

 (1908); Ehrenberg,- Bodenkolloide, pp. 223-227 (1915). 

 On adsorption in general see Ereundlich,- Kapillarchemie 

 pp. 50-290 (1909). It is doubtful whether adsorbed gas 

 films can have any direct action in supplying oxygen 

 for root respiration, although this has been suggested 

 by Ehrenberg (loo. cit .) and by Czapek,- Biocheraie der 

 Pflanzen, vol. 2, p. 378 (1905 J. If such adsorbed air 

 is available to organisms it would so ton be exhausted 

 under anaerobic conditions. If it is not available, 

 it is without influence. 



