206 Mineral Nutrition of Plants 



In the immediately preceding discussion, the pronounced effects of 

 the external conditions of aeration and temperature on the over-all 

 process of salt accumulation were considered. Aside from the possible 

 influences of aeration and temperature upon permeability itself, it 

 would be expected that the temperature condition might also modify 

 the rates of diffusion and exchange. Effects of this sort would be mani- 

 fested through increase of kinetic energy of mobile substances in solu- 

 tion. Since the energy intensity for migration of a solute would be 

 proportional to the absolute temperature, this influence, however, would 

 be relatively small. 



Figure 12 A. Effects of temperature and aerobic nature of the culture medium 

 on the rate of inward movement of radioactive potassium. Barley plants 3 weeks 

 old. Sets A and B: 0.5 C, C0 2 -free air passed through the culture solution; 

 sets C and D: 20.5 ° C, CO a -free air passed through the culture solution; sets 

 E and F: 20.5 ° C, CO,-free N 2 gas passed through the culture solution. The ex- 

 perimental points indicate radioactivity measurements of K*C1 culture solutions 

 in which excised roots were immersed. The data are plotted as differences between 

 the initial culture value and the values at subsequent periods of time. 



