48 CELLULAR 



If we have ten times as many hydrogen ions 

 inside as outside and if potassium ions are pres- 

 ent inside we should expect ten times as many 

 potassium ions inside as outside providing both 

 these cations are diflFusible. This might seem at 

 first sight to afford a means of explaining the 

 accumulation of potassium when the sap is more 

 acid than the external solution, as is normally 

 the case with Nitella and Valonia. But it also 

 requires that the accumulation of all other dif- 

 fusible cations should take place to the same 

 extent and this is not the case. 



We see, therefore, that we may explain accu- 

 mulation in many cases where a substance exists 

 in two forms whose relative proportions vary 

 with the differences in acidity, and it is possible 

 that this explanation applies to more substances 

 than is at present suspected. It may be objected 

 that this would require that all cells having 

 equally acid sap should behave in the same way 

 in regard to accumulation. This, however, would 

 not necessarily be true, since differences in per- 

 meability may bring about differences in the 

 relative proportions of electrolyte in the sap. 



