22^ SECTIONAL ADDRESSES 



increase in concentration of the absorbed ion in the vacuole. The 

 adsorption would then have to be followed by elution of the salt at the 

 surface of the vacuole. In this connection it is interesting to note that 

 S. C. Brooks has obtained some evidence that Valonia, immersed in 

 sea-water containing rubidium chloride, accumulates rubidium in the 

 protoplasm for two days, after which this kation passes from the proto- 

 plasm to both vacuole and external solution. The same worker has also 

 found that when cells of Nitella are placed in o-oiM. solutions of radio- 

 active potassium chloride there is an accumulation of potassium in the 

 protoplasm after 6 hours before any appreciable amount of potassium 

 appears in the vacuole. Previously M. M. Brooks had found that when 

 Valonia is immersed in a solution of methylene blue the cell wall and 

 protoplasm become deeply stained by the dye before any appreciable 

 coloration of the vacuole is observable. 



If adsorption is indeed operative in the absorption of salts, one would 

 expect it to be partly mechanical and partly electrical, and the unequal 

 absorption of the two ions of a salt could be related to the electrical 

 charges on adsorbents in the protoplasm. Further, the occurrence of 

 electrical adsorption would render the conformity of salt absorption with 

 the equation for mechanical adsorption only approximate. 



Another mechanism which has been suggested as possibly operative 

 in the absorption of salts is one of interchange between ions within and 

 without the cell under conditions which give rise to the ionic distribution 

 between the cell interior and exterior characteristic of what is called 

 Donnan equilibrium. If the solution exterior to the cell contains a salt 

 both ions of which can penetrate the cell membranes, while the interior 

 of the cell contains an electrolyte one ion of which can penetrate the 

 membrane while the other is immobile, then at equilibrium there will 

 generally be inequality of concentration of any ion on the two sides of 

 the cell membrane. There are probably in the protoplasm protein salts 

 which provide the necessary conditions for Donnan equilibrium. A 

 difficulty is that in a condition of Donnan equilibrium the products of 

 the concentration of any pair of oppositely charged ions should be the 

 same on the two sides of a membrane, so that if one ion of a salt is 

 absorbed to such an extent that its concentration is higher inside the 

 cell than outside, the other ion can only be absorbed to a concentration 

 inside the cell which is lower than its concentration outside. But actually 

 this is not necessarily the case. Thus I showed in 1924 that storage tissue 

 can absorb both ions of sodium chloride until the concentration of both 

 is higher than that of the same ion outside the tissue, it being assumed 

 that the ion remains active inside, an assumption for which there is good 

 evidence. Briggs has shown that this does not present an insuperable 

 difficulty to the view that absorption may be conditioned by Donnan 

 equilibrium if the two ions are absorbed by different phases in the cell, 

 and he shows that actual observations of salt absorption can be so 

 explained if the kation is absorbed by the protoplasm and the anion by 

 the vacuole. And in this connection it must be emphasised that just as 

 adsorption must take place if the cell contains adsorbents of ions capable 

 of reaching the adsorbent, so, if the cell system involves the conditions 



