60 DIFFUSION AND OSMOTIC PRESSURE 



under certain conditions ; to the majority of substances it is 

 usually very slightly permeable, but under certain conditions 

 its permeability may increase ; and to some substances it is 

 usually very readily permeable. Further than this, the pro- 

 toplasm of different plants, and even of different cells in the 

 same plant, has different osmotic properties. The condition 

 of things is thus seen to be very complex. It will be of 

 value to pass in review the most important fragments of evi- 

 dence which have been accumulated upon this question of 

 protoplasmic permeability. 



a) Test by the plasmolytic method. There are several 

 ways of testing the permeability of the protoplasmic sac. 

 The one most frequently resorted to is that of plasmolysis. 

 A bit of tissue or a unicellular organism is subjected to the 

 osmotic action of solutions of the substance which is to be 

 tested, these being of several different concentrations. If 

 plasmolysis occurs in a solution of rather high concentration, 

 this fact is taken as evidence that the protoplasm of the 

 given cells is either impermeable to the solute or very 

 slightly permeable. Of course, it is also theoretically pos- 

 sible that in this case the substance used penetrates the 

 protoplast to some extent and causes a polymerization or 

 precipitation of the osmotically active solutes within the sap. 

 There is no evidence for this phenomenon, however, and its 

 general improbability throws it out of the category of seri- 

 ous objections to the plasmolytic method. If, after being 

 left a short time in the plasmolyzing solution, the cells 

 regain their normal condition, it shows either that the pro- 

 toplasm is somewhat slowly penetrated, or else that some 

 osmotic material has been secreted within the cell. If plas- 

 molysis occurs at a very low concentration, it is sufficient 

 proof that the substance enters the protoplasm; for such 

 plasmolysis is due to alteration in the membrane through 

 poisonous action, or to a precipitation or some similar change 



