CONDUCTIVITY AND PERMEABILITY 199 



heating the plants to 100 C in a tightly stoppered bottle 

 (without addition of sea water) the conductivity equals 

 that of sea water. 



This is confirmed by some observations on Valonia. 

 This marine alga forms a large, multinucleate cell consist- 

 ing of a cell wall, within which is a delicate layer of 

 protoplasm, forming a sac which encloses a very large 

 central vauole. By drying the exterior and pricking the 

 cell, the sap can be made to spurt out and may then be 

 collected for examination. It was found by Wodehouse 

 (1917) that the sap of uninjured cells gives little or no 

 test for S0 4 : the contents of each cell were accordingly 

 tested by this method by Dr. Crozier, who kindly collected 

 the sap for these experiments, and rejected those which 

 contained more than a minimum amount of S0 4 . Deter- 

 minations of the electrical conductivity of the sap, by the 

 writer, showed that it was not much higher (in no case 

 more than 20%) than that of the surrounding sea water. 



Since the cell sap of Laminaria has about the same 

 electrical resistance as the solution which bathes the cell, 

 it is evident that if the electrical resistance of the cell 

 increases when it is transferred from sea water to another 

 solution of the same conductivity, the change must be due 

 to an increase in the resistance of the thin layer of proto- 

 plasm which bounds the cell. This has led the writer to 

 assume that the resistance is proportional to a substance, 

 7lf, at the surface of the cell ; if M forms a layer at the sur- 

 face, it is obvious that an increase in the thickness of this 

 layer will increase the resistance, and vice versa. It is 

 therefore assumed that the resistance depends upon the 

 amount of M which is present in the surface. 3 



8 See page 57. 



