THE PERMEABILITY OF PROTOPLASM 109 



2. It is important to make frequent observations, keep- 

 ing the same individual cell under observation throughout. 



3. A distinction must be drawn between true and false 

 plasmolysis, and where this is impossible the results must 

 be rejected. 



4. In experiments in which the recovery of the cell after 

 plasmolysis is the proof of permeability, care should be 

 taken to maintain a constant temperature, for the process 

 takes place more rapidly with increase of temperature. 

 Controls in sea-water should be examined at various 

 temperatures where constancy cannot be secured. Plas- 

 molysis should be carried to the same degree in each case, 

 as the rate of recovery is reduced when the action has 

 proceeded to any considerable extent. 



Uniformity of material is also necessary. Osterhout 

 employed long filaments of Spirogyra, of Chsetomorpha, 

 or portions of the leaf of Elodea. 



RATE OF PENETRATION OF SALTS AND IONS. 

 Since the more rapidly a plasmolyzing agent penetrates 

 a cell the shorter will be the time required by the latter 

 to recover from plasmolysis, it is clear that a measurement 

 of these times furnishes a comparison of the relative rates 

 of penetration of various substances. This may also be 

 determined by the following method: When two com- 

 pounds are used to produce the same degree of plasmolysis, 

 it is evident that the molecular concentration, and conse- 

 quently the osmotic pressure, must be the same in each 

 case, provided both fail to penetrate or do so to the same 

 degree. If, however, one enters the cell more readily than 

 the other, it will require a higher concentration to produce 

 plasmolysis than does the more slowly penetrating body. 

 Accordingly, by obtaining the ratios of the osmotic pressures 

 of the solutions with respect to that of a balanced solution 



