222 Mineral Nutrition of Plants 



example, results in effects which suggest an induced anaerobiosis. Here, 

 a reduced influx of inorganic solute occurs, similar to the effect of 

 nitrogen in replacing the usual oxygen contents of external bathing 

 media. This may be due either to a reduced permeability of protoplasm 

 or to a decreased, inwardly directed, gradient of energy intensity of the 

 inorganic solute. Passage of carbon dioxide gas through the root bath- 

 ins medium likewise results in restricted influx. Here, the effect is 

 probably twofold. Oxygen concentrations in the medium and within 

 the roots are probably reduced to adverse levels, and a tendency toward 

 reversal or inhibition of the usual respiratory reactions is created. 

 Deleterious effects of carbon dioxide are evident not only on the ac- 

 cumulation of inorganic solute from dilute solutions (3/) but also on 

 influx, where movement is with the direction in which the concentra- 

 tion of the solute decreases. Relatively prolonged treatment with this 

 gas is quite destructive to the organization of protoplasm, and an early 

 reduced influx is followed by an increase indicative of permanent in- 

 jury. Thus, while the effects of low temperature, nitrogen, and cyanide 

 are usually quite reversible, treatments with carbon dioxide above ap- 

 proximately 20 per cent in air frequently lead to irreparable damage to 

 the cells. In one experiment (Figures 19 and 20) with decapitated plants, 

 the stumps were subjected to suction. The rate of exudation was sup- 

 pressed with either nitrogen or carbon dioxide applications to the 

 roots. This indicates a reduced permeability to water or some secondary 

 effect, possibly associated with a modified concomitant flux of solute. 

 At the same time, the rate of bromide movement into and through the 

 system was decreased. Metabolic accumulation of inorganic salt was 

 simultaneously arrested (32). Where carbon dioxide was used, the 

 later rise in this curve suggests a destruction of the usual differential 

 permeability of the tissues, with the drawing of solution through the 

 roots as though through an open conduit. 



Hydrogen ion concentration 



Studies have shown an effect of the change of hydrogen ion con- 

 centration on the structure of proteinaceous colloidal systems. Any 

 marked shift of pH in protoplasm would probably lead to a deranged 

 cytoplasmic organization. However, in living tissues, a tendency to- 





