SELECTIVE PERMEABILITY 339 



the diosmotic properties of the eelL^** Since, however, the cell 

 is permeable to water, he abandoned the possibility that cho- 

 lestrin is the material that constitutes the plasma-membrane 

 (since it is absolutely impermeable to water) and. limited the 

 lipoids of the plasma-membrane to compounds of cholesterin, 

 as lecithin, lanolin, etc., which on swelling become permeable 

 to water. 1^ 



Examining the chemical nature of the dyes experimented upon, 

 Overton^o found that it is the basic dyes that are soluble in 

 lipoids and are capable of passing through the plasma-membrane, 

 while the sulfonic acid dyes are neither soluble in lipoids nor 

 capable of entering the cell. He explained this phenomenon by 

 the fact that the basic dyes being salts of weak acids undergo 

 hydrolysis and it is the free dye base that passes through the 

 plasma-membrane and appears in the vacuole. Later, however, 

 he abandoned this explanation, as he found that the acetate of 

 rosanilin which is a salt of weak acid and a weak base does not 

 enter the cell any faster than the nitrate or the sulphate. He 

 therefore arrived at a new conclusion, viz., that it is the salt of 

 the basic dye that passes through the plasma-membrane and 

 not the free base. This eliminates the possibility of a chemical 

 reaction and emphasizes more his view that permeability really 

 means physical solubility in the plasma-membrane. 



But in reality the fact that the acetate of rosanilin does not 

 seem to pass through the plasma-membrane any faster than the 

 nitrate or hj^drochloride does not conflict with his original 

 hj-pothesis that it is the free base to which the cell is really 

 permeable. His experiments on permeability were not by any 

 means quantitative. He would measure the time required for 

 the appearance of the dye in the vacuole but not the intensity 

 of color which perhaps, could not be measured. One hundred 

 drops of phenolphthalein, for instance, would not produce more 

 coloration in an alkaline solution than ten drops. The work of 



'* Overton, E., Studien iiber die Aufnahme der Anilinfarben durch die leben- 

 den Zellen. Jahrb. f. Wiss. Bot. 34: 669-701. 1899-1900. 

 '^ Overton, i.e. 

 20L.C. 



