Lecture 3 — 53 — Absorption 



there is continued movement of ions into the vacuole, 

 until the concentration there seems eventually to be- 

 come higher than in the protoplasm and certainly 

 higher than that of the external dilute solution.* 



The accumulation of bromide in the vacuolar sap 

 takes place when the cells are in the light and so 

 evolving oxygen; also in the dark, at least for a 

 limited period, if aeration is provided. In the experi- 

 ments so far performed the Nitella cells did not ac- 

 cumulate in the vacuolar sap either bromide or rubi- 

 dium to a concentration higher than that of the ex- 

 ternal solution, except when the cells were in an 

 aerobic environment. There is an indication from 

 these experiments of the operation of a process akin 

 to secretion and this point will be of continued interest 

 as our discussion proceeds. I may add here that Dr. 

 Blinks of Stanford University conducted experiments 

 with radioactive rubidium on large marine Halicystis 

 cells. The general conclusions are similar to those 

 based on work with Nitella cells. 



Use of Radioactive Isotopes : — This may be the 

 place to interrupt the discussion in order to make a 

 general observation on the use of radioactive isotopes, 

 since I shall report experiments in other lectures in 

 which these isotopes were found useful. It might first 

 be asked whether or not the radiation of the exploding 

 atoms injures the cells. Although this might happen 

 we are satisfied that this factor does not invalidate 

 evidence from experiments of the kind reported. The 

 radiation given off is too weak. Barley plants have 



*In these experiments measurements were actually made 

 on the cell vacuolar sap and on the residue of the cells after 

 removing the sap. Brooks had observed that very little of the 

 radioactivity of the Nitella cells could be assigned to the cell 

 wall. In the present experiments values were computed in 

 terms of the total moisture of the residues and the computed 

 concentrations for protoplasm may therefore be low. Never- 

 theless, it is significant that with the elapse of time relative 

 concentrations of the test ions in the sap increase more than 

 those in the residue. 



