THE OSMOTIC CHARACTERS OF THE CELL 21 



times more extensive. As an example of an elaborate alteration we may take 

 the formation of the insoluble, and therefore osmotically inactive, starch from 

 sugar absorbed by the cell, while the precipitates by ammonium carbonate 

 already alluded to, removable by mere washing in water, may be quoted as a 

 simple illustration of the same phenomenon. In most cases we are unacquainted 

 with the mode in which storage in the cell-sap is effected. For instance, when a 

 nitrate or other inorganic salt accumulates in the cell-sap and reaches a higher 

 concentration there than in the surrounding fluid, it is not impossible that a loose 

 union occurs between that body and some other, but such unions are not very 

 probable. When unilateral accumulation occurs without alteration of the sub- 

 stance, purely physical diffusion conditions, which we have hitherto accepted 

 as essential, cannot he operative, or, at least, not entirely so. On this question 

 we must await the results of further inquiry. Nathansohn (1902) has made 

 an attempt in this direction, but his results cannot be said to be above criticism. 

 He experimented with Codium tomentosum, the cell-sap of which he studied 

 by quantitative chemical methods, but he overlooked the very large inter- 

 cellular space system in that plant, which, in addition, communicates freely 

 with the exterior, an omission which vitiates his researches. [Nathansohn 

 has recently extended his osmotic studies (1904, Jahrb. f. wiss. Bot. 39, 607 

 and 40, 403). He finds that the permeability of protoplasm for any substance 

 is not constant in degree but varies with external conditions. Protoplasm 

 becomes, for example, impermeable to a substance if it be present in the 

 vacuole in an amount bearing a certain quantitative ratio to the concentration 

 of the same substance outside. This relation between the internal and external 

 concentration may be again restored if. the solution outside the vacuole be 

 diluted, when a diffusion of the more dilute into the more concentrated solution 

 takes place. In opposition to H. Fischer (1904, Ber. d. bot. Gesell. 22, 485) 

 who explains this phenomenon by the Law of Distribution, Nathansohn (1904, 

 Ber. d. bot. Gesell. 22, 556) holds that it is due to variations in protoplasmic 

 permeability, which cannot be accounted for in a purely physical manner.] 



As a result of ingenious arguments, into which we cannot enter at 

 present, Pfeffer has shown that the permeability of protoplasm does not 

 depend on the protoplasm as a whole, but only on a very thin, microscopically 

 indistinguishable layer, which may be termed the plasmatic membrane. An 

 outer plasmatic membrane determines what substances shall enter the proto- 

 plasm, while an inner plasmatic membrane determines what shall enter the 

 vacuole. These two membranes would appear to have different properties, 

 since substances may enter the protoplasm in considerable quantities and yet 

 bring about plasmolysis, owing to the fact that they are unable to penetrate 

 the inner plasmatic membrane. Sugar, as we shall see later, behaves in this 

 way in many cases. 



Further, a single cell not infrequently contains several vacuoles whose con- 

 tents differ from each other, and the plasmatic membranes of these vacuoles 

 have in all probability different permeabilities. The cell's organization, as 

 HoFMEiSTER (1901) has shown, must operate so as to keep the varied chemical 

 products apart, and here the plasmatic membranes must be of service ; 

 should these membranes alter in character, the previously separated products 

 may come in contact with and react upon each other, and hence the variations 

 in the plasmatic membranes must be of fundamental importance in the life of 

 the cell. 



Finally, we must inquire into the causes of varying permeability of proto- 

 plasm to different substances. So far as purely physical conditions determine 

 permeability, a survey of diosmotic substances first of all may possibly give 

 us a clue. Overton's (1899) comprehensive observations, made with the aid 

 of various methods, enable us to present the following summary : — 



