THE OSMOTIC CHARACTERS OF THE CELL 



15 



tration than the contents of the vacuole, water will be withdrawn from the vacuole, 

 which in consequence will become smaller. At this point the behaviour of 

 the cell membrane differs from that of the protoplasmic lining, for the pro- 

 toplasm is capable of contracting proportionally to the decrease in size of the 

 vacuole, while the rigid cell-wall, on the other hand, can contract only to 

 the extent of regaining the size it possessed before being elastically extended 

 by the osmotic pressure. When the tension of the cell-wall is relaxed 

 (Fig 4, //) no further decrease in size is possible, and a state of plasmolysis 

 ensues, where the protoplasm becomes separated from the cell- wall. This 

 separation appears first at the angles of the cell (Fig. 4, ///) and proceeds 

 continuously therefrom, until the whole of the protoplasm has assumed the 

 form of an elliptical or spherical mass lying free in the interior of the ceil 

 (Fig. 4, IV). If the solution employed for bringing about plasmolysis be 

 coloured by a suitable dye, e. g. indigo-carmine or aniline-blue, it will be seen 

 that the coloured fluid penetrates the cell-wall and fills the space between the 

 wall and protoplasm. Further, this experiment demonstrates the fact that 

 the membrane is permeable to the dye, but that the protoplasm is not. If we 

 once more immerse the plasmolysed cell in water, plasmolysis disappears and 

 the cell regains its original size and form, without apparently having suffered 

 any injury. On the death of the protoplasm, e. g. as a result of a sufficiently 

 high temperature, its diosmotic characters are completely altered ; dead proto- 

 plasm presents no impediment to the free passage of colouring matters, 

 salts, &c. 



By employing the plasmolytic method of investigation we are able readily 

 to determine that protoplasm is impermeable to a large number of substances 

 even though these be soluble in water. If only the proper degree of concen- 

 tration be ascertained, plasmolysis can be affected by cane sugar as well as by 

 grape sugar, by common salt as well as by potassium nitrate, but the exact 

 concentration must be determined by experiment. With the view of ascer- 

 taining the relative plasmolytic or osmotic activities of different substances 

 it is necessary to determine the precise concentration of each which will bring 

 about the first trace of plasmatic retraction from the wall (Fig. 4, III, lower 

 part of the cell). The degree of concentration which induces this change may 

 be considered as having a slightly higher osmotic value than that of the cell 

 contents, while a concentration exactly similar to that of the cell contents will 

 obviously produce no reaction at all. Thirty years ago, by the plasmolytic 

 method, De Vries studied empirically the degrees of concentration of a variety 

 of substances which were capable of giving equivalent osmotic effects (briefly 

 termed isosmotic concentrations). The material he employed was red beet, 

 and the results he obtained were as follows : — 



1. Cane sugar . . . 27-28% 5. Sodium nitrate . . . 6-7% 



2. Magnesium sulphate . . 26-28% 6. Potassium chloride . . 4-5% 



3. Sodium sulphate , . 17-18% 7. Sodium chloride . . . 3-4% 



4. Potassium nitrate . . 6-7% 



At first sight these numbers appear to follow no law, but in 1884, De Vries, 

 after the examination of a very large number of variously constituted sub- 

 stances, successfully established a definite relation between the different isos- 

 motic solutions. He was able to show, as indeed was only to be expected, that 

 the osmotic effect depends not on the specific gravity of the substance button the 

 number of the molecules dissolved. Now if substances be dissolved in quantrties 

 proportional to their molecular weights, similar numbers of molecules will be 

 obtained in each solution. When as many grams are dissolved in a litre of water 

 as is indicated by the molecular weight of the substance we speak of this unit as 

 a gram-molecule to the litre (briefly ' G.M.^)._Tjius to obtain a G.M. of cane sugar, 



