THE PERMEABILITY OF PROTOPLASM 101 



three to six hours this was succeeded by a progressive 

 increase in weight (except in the case of Arum, where 

 there was a loss). Ilex aquifolium and Cavendishia acu- 

 minata showed a progressive increase in weight from the 

 time of first immersion to the end of the experiment. It 

 must, however, be noted that the first weighing was not 

 made till three hours had elapsed, so the initial loss may 

 have been missed. An increase in sodium chloride was 

 found in all the treated leaves. The subsequent recovery 

 of the initially plasmolyzed cells was due, as previously 

 pointed out, to gradual penetration of the salt. 



Of surpassing interest in this connection is the work of 

 Overton, Osterhout, and Czapek, by the method of plas- 

 molysis and by electrical conductivity and surface tension 

 measurements. These researches have imparted to such 

 investigations a degree of exactness hitherto unknown. 



OVERTON'S THEORY OF THE LIPOID SURFACE FILM. 



It was shown by Overton (1895) that the simple alcohols, 

 aldehydes, ketones, esters of fatty acids, and alkaloids, do 

 not as a rule produce plasmolysis. Accordingly, it is 

 evident that to them the protoplasmic surface is perme- 

 able. With glycols plasmolysis is more easily effected, 

 whilst glycerol and erythrite act even still more readily, 

 On the other hand, all sugars, amino-acids, and salts of 

 organic and inorganic acids, bring about plasmolysis with 

 rapidity. 



As pointed out by Overton, his results as to permeability 

 may be summarized in the statement that substances 

 which are soluble in fat pass through the protoplasmic 

 membrane with readiness. From this he concluded that 

 a thin film of lipoid substances constitute the t)uter layer 

 of the protoplasm. The term lipoid has been used by 

 some authors to denote any fat-like substance soluble in 



