338 J. DAVIDSON 



This periodical expansion of the water solution of the soaps, 

 offers, according to him, a physical explanation of the inner 

 movements of the plant ceil. The oil membrane, according to 

 him, conditions the diosmotic properties of the cell, — only those 

 substances which are soluble, in oil can enter the cell. Quinke 

 offered very little evidence in favor of his hypothesis and it 

 has not met with serious consideration among physiologists. 



Overton's lipoid theory, which is very similar to Quinke's, 

 has on the contrary, found very many followers among plant 

 and animal physiologists. His views were evidently formed 

 independently of Quinke as he never refers to him in his works. 

 He carried out extended investigations on the penetrability of 

 a large number of compounds into the living cell and studied 

 at the same time the solubility of these compounds and the 

 nature of their solvents. These parallel observations led him 

 to the conclusion that the plasma-membrane stands in its sol- 

 vent capacity very near to the fatty oils. On the basis of fur- 

 ther consideration, however, and on the basis of the fact that 

 algal threads preserve for a long time their diosmotic properties 

 in a 2% solution of sodium carbonate which would be sufficient 

 to saponify the oils if they were present, he concluded that these 

 are not the substances which condition the selective permea- 

 bility of the cell.^^ The lipoids, which are non-saponifiable 

 oil-like substances, he found, were analagous in their solvent 

 properties to the plasma-membrane. He found that all dyes 

 that are capable of passing through the plasma-membrane, 

 as may be observed by the coloring of the cell sap, are very 

 soluble in melted cholesterin and in solutions of cholesterin, 

 lecithin, cerebrin and other lipoids. When these lipoids are 

 suspended in a water solution of anilin dyes to which the plasma- 

 membrane is permeable they rapidly absorb the dye. On the 

 basis of these observations and taking into consideration the 

 fact that lecithin and cholesterin are always found in plant and 

 animal cells, he advanced the hypothesis that it is the impreg- 

 nation of the plasma-membrane with lipoids that accounts for 



^^ Ruhland, I.e., pp. 3-4. 



