140 PHYSIOLOGY 



which, would be exerted by 1 per cent, sugar in solution at a given 

 temperature. 



This calculation is carried out as follows : A gramme molecule of any gas at 

 C. and 760 mm. Hg has a volume of 22 -4 litres, therefore 342 grammes of cane 

 sugar (the molecular weight of C^H^Ou = 342), if it could be converted into 

 a gas at C. and 760 mm. Hg, would have a volume of 22-41itres. One gramme 

 of sugar therefore at the same temperature and pressure would have a volume of 



22-4 



-^r litres = 65-5 c.c. In Pfeffer's experiment the gramme of sugar was dissolved 



o4J 



in 100 grammes of water, making a total volume at C. of 100-6 c.c. The 

 gaseous pressure of the sugar molecules in this solution will therefore amount to 



= 0-651 atmosphere. At a temperature of 6-8 the pressure would be 



J. v/\J*O 



0-667 atmosphere, as against the observed 0-664 atmosphere. 



Pfeffer's method is difficult to carry out and is not applicable to 

 all dissolved substances, since the cupric ferrocyanide membrane is 

 permeable for many substances, such as potassium nitrate or hydro- 

 chloric acid. Other indirect methods have therefore been applied to the 

 comparison of the osmotic pressures of different solutions. 



DETERMINATION OF THE OSMOTIC PRESSURE BY PLASMO- 

 LYSIS. Solutions which have the same osmotic pressure are spoken 

 of as isosmotic or isotonic. The method of plasmolysis, which we owe 

 to the botanist De Vries, consists essentially in the comparison of the 

 osmotic pressure of solutions with that of the cell sap of certain plant 

 cells, and depends on the fact that the primordial ' utricle,' the layer 

 of protoplasm enclosing the cell sap, while freely permeable to water, 

 is impermeable to a large number of salts and other crystalloids, such 

 as sugar. It is therefore, so far as concerns these substances, ' semi- 

 permeable.' The cells which have been most used for this purpose 

 are the cuticular cells on the mid-rib of the lower surface of the leaves 

 of tradescantia discolor. If some of these cells are brought into a 

 concentrated salt solution, which is * hypertonic ' as compared with 

 the cell sap, water passes out of the cell into the salt solution, until 

 the contents of the cell attain a molecular concentration equal to 

 that of the surrounding medium. The protoplasmic layer therefore 

 shrinks, leaving a space between it and the cell wall (Fig. 7, p. 24). If, 

 the outer solution has a smaller molecular concentration than the cell 

 sap, water passes into the cell and causes here a rise of pressure which 

 simply presses the protoplasm still more closely against the cell wall. 

 If we determine the concentration of the salt solution at which the 

 shrinkage of the protoplasm, the plasmolysis, just occurs, and another 

 smaller concentration at which plasmolysis is absent, we know that 

 the concentration of the cell sap lies between those of the two salt 

 solutions. Thus, if plasmolysis occurs in a solution containing 0'60 per 

 cent, sodium chloride and is absent in a solution containing 0*59 per 



