OSMOTIC EQUILIBRIUM 95 



affords the readiest and most accurate method of measuring 

 osmotic pressures when sufficient material is available. 



The plasmolytic method is, on the other hand, applicable 

 to the study of individual cells, and for this reason alone 

 has an extended range of usefulness. Contraction of the 

 distended cell before actual plasmolysis ensues may be, 

 however, a very considerable source of error. For if the 

 diameter of a cell, assumed to be spherical, decreases by 

 10 per cent, from this cause, its volume is diminished by 

 27-1 per cent., whilst even a shrinkage of 2 per cent, in 

 diameter occasions a volume change of 5-9 per cent. De 

 Vries (1877) has shown that the linear dimensions of young 

 and turgid cells may be diminished by as much as 20 per 

 cent, before plasmolysis is apparent. It has been pointed 

 out by Schwendener and Krabbe (1898) that scarcely any 

 extension is normally produced in the walls of grown 

 cells. 



In addition to this channel for the entrance of in- 

 accuracies, there is the possibility that the cell may not 

 be completely impermeable to the substance used to pro- 

 duce plasmolysis, which tends to raise the result still more. 

 Also it must be remembered that even the same cell may 

 alter its permeability under various conditions of external 

 medium, of illumination, or at different periods of its Me. 

 The changes induced in the permeability of the cells of the 

 pulvinus in Mimosa pudica by stimulation furnish a striking 

 example of this. 



