PERMEABILITY AND THE PROTOPLASMIC MEMBRANE 275 



medium; this could be tested out apart from the plant. The 

 explanation is speculative, but it rests on experiments with 

 30 dyes, in all of which the sodium ion entered rapidly at a pH of 

 3, less rapidly at pH 6, very slowly at pH 8, and not at all at 

 pH 10; while calcium or potassium entered rapidly at pH 10, less 

 so at pH 8, very slowly at pH 6, and not at all at pH 3. The 

 hypothesis fits the case of Valonia well but fails when applied to 

 both Valonia and Halicystis growing in the same waters. 



F. C. Steward points out that certain other neglected factors 

 materially affect permeability; thus, aeration of the aqueous 

 medium containing the tissue with the carbon dioxide-free air 

 causes increased salt absorption. 



Theories of cell permeability rest mostly upon a postulate of 

 membrane structure, under which subject they are best 

 considered. 



THE PROTOPLASMIC MEMBRANE 



Introduction. — The surface layer of protoplasm, the so-called 

 plasma membrane, is the first barrier that outside material must 

 pass on entering the cell and the last that included material must 

 pass on leaving. The membrane is, therefore, a very important 

 part of the cell. The cellulose wall in plants is also an obstacle to 

 diffusing substances but not so severe a one and not in the same 

 sense as is the plasma membrane. Water, salts, and most organic 

 substances pass through the cellulose wall freely, as through 

 porous clay, while the plasma membrane has selective powers 

 which are capable of change and adjustment. The cellulose wall 

 may, however, play a more prominent part in permeability than 

 heretofore suspected, but its role is probably always secondary to 

 that of the membrane. There is considerable evidence, both 

 direct and indirect, to support the presence of a plasma membrane 

 around cells. If we recognize that the law of Willard Gibbs is, 

 at least to a degree, applicable to protoplasm, then those sub- 

 stances which lower surface tension will become concentrated at 

 the surface. Fatlike and other substances in protoplasm which 

 thus affect surface tension will tend to collect at the surface and 

 give to the outer layer a chemical constitution differing from that 

 of the inner protoplasm. In addition to Gibbs' phase rule, there 

 is other evidence that the surface of protoplasm is substantially 

 different from the interior. All fluid surfaces differ from the 



