270 DIFFUSION, OSMOSIS, AND FILTRATION. 



Physiological methods of estimating osmotic pressure have also been 

 devised. The method of de Vries l is based upon the plasmolysis of the 

 protoplasts of vegetable cells. The cells filled with coloured sap from 

 the middle nervure of the leaf of Tradescantia discolor are useful 

 for the purpose, sections of this part being allowed to soak for three to 

 five hours in the solutions whose osmotic pressures are to be determined. 

 If the cells are plasmolysed, i.e. if the protoplasts are found on 

 examination to have shrunk from the cell walls, the osmotic pressure 

 of the solution producing this effect is above that of the cell sap, for 

 water has passed from the latter to the former, as evidenced by the 

 diminution in volume. By investigating a series of solutions with 

 sections from the same leaf, it is of course possible to find two of 

 slightly differing concentration of the substance under investigation, 

 one of which just causes plasmolysis, while the other (weaker) does 

 not. A solution of concentration equal to the mean of these two is 

 said to be isotonic with the cell sap. 



De Vries, on preparing a number of solutions of different substances, 

 all isotonic with the same batch of cells, and expressing their 

 concentrations in gramme-molecules to the litre, found that it required 

 a lower gramme -molecular concentration of some substances than of 

 others to obtain isotony. The term " water extracting power " 

 ( Wasseranzieliungsvermogen) was used to express this peculiarity which 

 is obviously related to what has above been termed dissociation. 

 Taking O'l grin, molecule to the litre of saltpetre as a standard, and 

 giving it a magnitude of 3, the relative value (as regards plasmolysis 

 of vegetable cells) of a molecule of a number of substances was 

 expressed in terms of that of a molecule of saltpetre, and the numbers 

 expressing this ratio called isotonic coefficients of the substances. 



Thus, to barium chloride (BaCL, + 2Aq = 244) is given the 

 isotonic coefficient 4, which means that f'244 parts by weight of 

 barium chloride in aqueous solution exert the same plasmolysing 

 action as 101(KN0 3 = 101) parts by weight of saltpetre i.e. a T83 

 per cent, solution of crystallised barium chloride is isotonic by the 

 method, with a I'Ol per cent, solution of potassium nitrate. 



Since cane sugar on this system is given the value of 2 for its 

 isotonic coefficient, and since, being a non-electrolyte, it is not 

 dissociated in solution, it is merely necessary to divide the isotonic 

 coefficients of de Vries by 2, in order to obtain ordinary dissociation 

 coefficients. 



It is obvious that the substances in solution must exert no 

 deleterious action on the protoplast of the cell, and must, moreover, 

 be quite unable to diffuse through it, if the method is to be exact. 



Here, again, we are met with the difficulty, that the protoplast is 

 not a strictly semipermeable membrane. It must let certain substances 

 pass, otherwise the cell sap could not have any other constituent than 

 water ; and it is only because the permeability to certain substances is 

 so far below that to water, that it is possible to obtain fairly approximate 

 measures of osmotic pressure by this method. With other substances 

 the permeability is so great that the values are far too low. 



Thus with sodium chloride, by this method, the dissociation coefficient 

 is reckoned as 1*5 (de Vries' isotonic coefficient 3), but by lowering of 



1 Jahrb f. wis*. Botanik, 1884, Bd. xiv. S. 427; Ztschr. f. physikal. Chcm., Leipzig, 

 1888, Bd. ii'. S. 415. 



