CONDUCTIVITY AND PERMEABILITY 203 



the original osmotic pressure, plus the increased pres- 

 sure due to the withdrawal of water (thus increasing the 

 concentration) plus the osmotic pressure due to the pene- 

 tration of alcohol. The total osmotic pressure will there- 

 fore be greater than that of the external solution and 

 water will accordingly be absorbed by the cell. The result 

 will be that the plasmolyzed cell will recover, and return 

 to its original expanded condition. 6 The time required for 

 recovery is usually regarded as approximately propor- 

 tional to the rate of penetration of the alcohol. 



In this way we may obtain a rough measure 

 of permeability. 



This method; was employed by Overton (1895) in 

 the well-known studies on permeability which gave rise to 

 the lipoid theory. He came to the conclusion that there 

 is rapid penetration of alcohol, and of many other organic 

 substances, but that inorganic salts do not penetrate. He 

 attributed this to their insolubility in the lipoid layer, by 

 which he supposed the cell to be surrounded. The writer 

 repeated his experiments with salts, and came to the 

 opposite conclusion. 7 It was found in experiments with 

 salts of NH 4 , Cs, Rb, Na, K, Li, Mg, Ca, Sr and Al, that 

 the protoplast which is plasmolyzed and left in the solu- 

 tion expands again to its normal size, showing that all 

 these salts penetrate the protoplasm. 



The following experiment will serve to illustrate the 

 procedure. Filaments of Spirogyra were placed in 0.4 M 

 NaCl solution. Within two minutes the protoplasts of 

 the cells were so far plasmolyzed that they no longer 

 touched the end walls of the cells. Several of these were 



'Since the effect of the alcohol outside the cell is exactly balanced 

 by that within the cell the final effect is the same as that of placing the 

 cell in pure water. 



7 Cf. Osterhout (1911 ). 



