2 28 Permeability of Cells for Dyes and Alkalies [Dec. 



The composition of the dye Compounds is different with each 

 type of cell stiidied. In Spirogyra neutral red is precipitated by 

 tannin as a tannate ; in Elodea, a soluble red Compound is f ormed in 

 the cell sap, probably with some organic acid; in Paramcecium, 

 colorless granules of unknown composition are stained red, while 

 in marine eggs the granules which stain (as a rule the heaviest 

 of the visible substances present) are possibly lecitho-protein in 

 nature. Before the yellow color appears in alkali the above men- 

 tioned Compounds must be broken up and the free dye base liberated. 



Studies of the dye combinations formed in various plant and 

 animal cells have revealed the interesting fact that neutral red as 

 well as many other basic dyes, fails to enter cells in the presence of 

 very weak acid (too weak to härm the cells), i. e., in the acid or 

 dye Salt condition. Only the free dye base can penetrate the plasma 

 membrane of living cells. A small amount of the base is formed 

 in neutral distilled water, through hydrolytic dissociation, and it 

 is found that cells stain readily in distilled water. Methylene blue, 

 saffranin, methyl violet, Bismark brown, thionin, chrysoidin and 

 toluidin blue behave as does neutral red. On the other band, the 

 acid dyes, eosin, Bordeaux red, säureviolett and aurantia cannot 

 penetrate in neutral or slightly alkaline condition (as the dye salt) 

 but do enter in acid condition (as free color-acid), staining and kill- 

 ing the cell. Salts are highly ionized in Solution whereas weak bases 

 and weak acids (such as the free color-bases and color-acids) are 

 not. The above relation among dye Compounds as well as a similar 

 one discovered by Overton for the alkaloids, together with the ob- 

 servations described below on various alkalies, all point to the con- 

 clusion that cells offer great resistance to the entrance of the ions 

 of a substance, but very little to the undissociated molecule. 



Whether this is a primary relation or depends on the lipoid- 

 solubility of un-ionized and the lipoid-insolubility of ionized sub- 

 stances, as Overton has maintained, is not yet definitely settled. 

 According to Robertson, it is the free color bases and acids and 

 not their salts which dissolve readily in fatty and fat-dissolving 

 substances like ethyl acetate. Such are, in a general way but with 

 many exceptions, the permeability relations of cells for dyes. 



