234 SECTIONAL ADDRESSES 



diffuse into or out of the vacuole according to the difference between the 

 osmotic pressure of the cell sap and the sum of the osmotic pressure 

 of the external solution and the inwardly directed pressure of the stretched 

 and elastic cell wall. Dissolved substances were supposed to enter the 

 vacuole according to the laws of diffusion expounded by Graham and 

 Fick more than eighty years ago. The method of measuring the rate 

 of entry of dissolved substances by observing the rate of deplasmolysis 

 of plasmolysed cells placed in a solution of a penetrating substance assumes 

 that this substance diffuses unchanged through the protoplasm into the 

 vacuole, where it still remains unchanged and so increases the osmotic 

 pressure of the vacuole approximately in proportion to the amount of it 

 which has entered the cell. 



Although Collander's work on the absorption of a number of non- 

 electrolytes indicates that this assumption may, in the case of such 

 substances, be quite justified, it has been known now for thirty years that 

 the entry of electrolytes into cells cannot be explained as the simple 

 diffusion of a substance through a membrane (cell wall and protoplasm) 

 from a region of higher concentration to one of lower concentration. In 

 the first place it was shown that the two ions of a salt could be absorbed 

 at different rates by living cells as long ago as 1909. In that year obser- 

 vations of this kind were published by Meurer and by Ruhland on the 

 absorption of salts by storage tissue (carrot and beetroot) and by Pantanelli, 

 using a great variety of plant material. These observations have since been 

 extended by many others, and it has been established beyond a doubt 

 that, at any rate under certain conditions, the two ions of a salt are 

 absorbed by living cells at different rates. Since the total of positive 

 and negative electrical charges must remain equal in the external solution, 

 it follows that either some other ion must accompany the excess of the 

 more rapidly absorbed ion into the cells, or that some ion of the same sign 

 as the more absorbed one must diffuse out into the external solution to 

 balance the excess of the less absorbed ion remaining. If the former is 

 the case and the external solution is one of a single salt, the solution must 

 become acid or alkaline, since an excess absorption of kation would involve 

 some absorption of the hydroxyl ions of water, leaving some of the anion 

 balanced by hydrogen ions ; similarly, if there is an excess absorption of 

 anion the solution will become alkaline. It was suggested by Pantanelli 

 that this might be the reason why culture solutions sometimes become 

 acid or alkaline. My own experience has been that all plant tissues 

 absorb hydrogen and hydroxyl ions with considerable rapidity, and 

 that solutions containing plant tissue tend to become less, and not more, 

 acid or alkaline. It would be unwise, however, to assume that such is the 

 case under all conditions, and, as far as I am aware, there is no evidence 

 regarding the range of hydrogen-ion and hydroxyl-ion concentrations 

 over which absorption of these ions takes place. W. J. Rees and I found, 

 however, that organic acids of the formic acid series are absorbed until 

 the pH of the external liquid is as high as 6 • 55, a value little removed from 

 that of pure water. 



At any rate, the few experiments I have made myself on this point 

 indicate that the excess of sodium absorbed by carrot tissue from a 



