716 PLANT GROWTH AND PLANT COMMUNITIES 



analysis the appropriate heat-flow processes. The flow of water is con- 

 trolled in large measure by the potential differences created by evapo- 

 ration at the surface of the soil or at the surface of the mesophyll cells 

 in the leaf. The 580 calories per gram required for this change of state 

 must move to the site of evaporation at a sufficient rate to maintain the 

 process; hence the flows of heat and of water in the soil-plant atmos- 

 phere are inextricably intertwined. 



Flow of gases in soil 



The exchange of oxygen and carbon dioxide between the air-filled 

 voids in the soil and the atmosphere takes place primarily by the 

 process of molecular diffusion in response to the gradients of the partial 

 pressures of the individual gases. Except at very low values of air- 

 filled pores, the diffusion coefficient is proportional to the volume- 

 fraction of gas-filled pores. The diffusion of oxygen through water is 

 only about 1 X 10"^ as rapid as through air at atmospheric pressure. 

 Therefore the movement of oxygen through the soil to the site of its 

 utilization in aerobic respiration in the root is largely determined by 

 the thickness of the water films through which it must diffuse. Gaseous 

 diffusion in both soils and plants is complicated by the solubility of the 

 gases in the liquid phase and by the effects of temperature on such 

 solubilities. 



The diffusion of oxygen to the respiring root may be analyzed by 

 applying Pick's law to the radial diffusion to a unit length of a root. The 

 concentration of oxygen (Cr) at the root surface is given by the follow- 

 ing equation : 



qR2 R 

 C„ = C,+ — In- 



where Cp is the concentration of oxygen in the surrounding soil solu- 

 tion, q is the oxygen consumed per unit volume of respiring tissue, Dg is 

 the diffusion coefficient of oxygen through the water films, and R and fg 

 are the radii of the root and of the root plus the water films, respec- 

 tively. From measurements of oxygen-diffusion rates to a platinum 

 microelectrode (Lemon and Erickson, 1952) having a radius of 0.05 

 cm. in a clay soil with a moisture potential of —0.2 atmosphere, Wie- 

 gand (1956) calculated an effective diffusion path length of .23 cm. 

 when the air-filled pores in the soil contained 19.8 per cent oxygen by 

 volume. Wiegand also has calculated the critical oxygen concentration 

 of 4.45 X 10'^ gm. per c.c. at the root surface for a root having a radius 

 of .037 cm. and consuming oxygen at the rate of 1.12 X 10"^ gm. per 

 c.c. per sec. This value of Cr is equal to the oxygen concentration of 



