202 INJURY, RECOVERY, AND DEATH 



solution. We therefore obtain in this manner an approxi- 

 mate measure of the osmotic pressure of the cell.^ 



Strictly speaking, plasmolysis measures the osmotic 

 pressure within the cell without telling us anything 

 regarding the penetration of substances into the proto- 

 plasm. We may, nevertheless, learn something about 

 permeability by this method. When, for example, we 

 find that a substance (such as alcohol) fails to produce 

 as much plasmolysis as is expected, we infer that this 

 substance penetrates the cell so rapidly, as to partially 

 offset its own osmotic effect. Let us suppose that the 

 osmotic pressure within the cell is 10 atmospheres. On 

 placing the cell in a solution of alcohol, whose osmotic 

 pressure is 11 atmospheres, plasmolysis might be iex- 

 pected to occur, but if there is an immediate penetration 

 of alcohol, which raises the osmotic pressure within the 

 cell to 11 atmospheres, no plasmolysis will take place. 



On placing the cell in a solution of alcohol strong 

 enough to cause plasmolysis we should expect the alcohol 

 to penetrate until the osmotic pressure of alcohol is the 

 same inside and outside of the cell. When this has 

 occurred, the osmotic pressure in the cell will be equal to 



° There are a number of disturbing factors which interfere with such 

 measurements. Among these may be mentioned: 



1. The plasmolyzing agent may have a toxic action and may 

 alter the permeability of the cell. 



2. Plasmolysis may produce mechanical injury due to the 

 tearing of the outer layer of protoplasm. 



3. Exosmosis of dissolved substances may occur during ex- 

 posure to the reagent. 



4. The time necessary to produce plasmolysis is an important 

 factor which is frequently overlooked. 



5. The shrinkage of the protoplasm away from the cell wall 

 is preceded by a diminution of volume of the entire cell.. In some 

 cases, where the cellulose wall is considerably stretched by the 

 internal osmotic pressure, this may be of considerable importance. 



