PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 269 



observation of the entrance of the substance, e.g., a dye; the 

 osmotic method, in which the rate of plasmolysis and ease with 

 which a cell regains its original distended form are criteria of 

 the rate of entrance of external salts; the chemical method, 

 through analysis of the cell contents; and the electric method, 

 in which the conductivity of the cell sap is measured in relation 

 to the external solution. The last two methods are the most 

 accurate, though they offer difficulties enough. Chemical 

 analyses can be satisfactorily made of cell sap only; the proto- 

 plasm itself undergoes many changes at death which make it 

 impossible to know what was there when the cell was alive. 

 Cells which have very large central vacuoles, as has the coenocy- 

 tic alga Halicystis, are excellent material for chemical analyses 

 of the sap. 



One of the first facts definitely known about the permeability 

 of the cell was that substances, such as ether, alcohol, chloroform, 



ABC 



Fig. 141. — Diagrammatic sketches indicating those regions of an Elodea leaf 

 which first succumb to the toxic effect of ethyl alcohol: A, after brief treatment; 

 B, after longer treatment; C, after one hour. 



methane, and xylene, which dissolve fats, enter with extreme 

 rapidity, while salts enter very slowly. The German botanist 

 Overton reasoned that if fat solvents enter readily while salts 

 do not, then the protoplasmic membrane must be fatty or 

 lipoid in nature. That the outer layer can not be pure lipoid, 

 particularly cholesterin, is obvious, otherwise water-soluble sub- 

 stances such as salts, so necessary to organisms, could not enter. 

 (Water-soluble substances would pass through a hydrophilic 

 lipoid such as lecithin much more readily than through a hydro- 

 phobic substance such as cholesterin.) 



As organisms require salts, these must of course enter cells, but 

 it was early learned that they enter very slowly and not all at the 

 same rate. This latter statement is also true of the fat solvents, 

 which ordinarily enter so rapidly; thus, cells within the same 



