224 Mineral Nutrition of Plants 



ward modification of the hydrogen ion concentration by a moderate 

 alteration of the pH of the external medium is counteracted within the 

 protoplasm by direct buffer action of constituent compounds and, pos- 

 sibly, by a similarly effective modification in the rates of some metabolic 



.7- 

 .6- 



* 5 



a. 



o 





o 



a: 2 



PLANT I -AIR 



Br. IN EXTERNAL SOLUTION. 



PLANT E- N 2 



TIME 



Figure 20. Effects of differential gas treatments on the concentration 

 of bromide ions in exudates from decapitated tomato plants. Data same as 

 in Figure 19. 



reactions. Thus, the pH in cells is maintained within rather narrow 

 limits (•?, y8). If any marked change of internal hydrogen ion concen- 

 tration did occur, no doubt accumulation, respiration, and other proc- 

 esses would be strikingly modified. Since no marked change of in- 

 ternal conditions occurs with the alteration of external hydrogen ion 

 conditions, this factor was not discussed under external influences with 

 relation to inorganic solute absorption. That this latter process is not 

 significantly modified by such an external influence has been shown 



