16 PLANT PHYSIOLOGY 



tendency of the solution to increase its volume by the absorption 

 of water. The height of the mercury column indicates the 

 magnitude of osmotic pressure that may be determined as the 

 equivalent of the external pressure to which the solution must 

 be subjected to prevent the penetration of the solvent through 

 the membrane, i.e., to stop the increase in the volume of the solu- 

 tion occurring at the expense of the water particles penetrating 

 into the osmometer. It is this external pressure that is usually 

 measured in determining the osmotic pressure of a solution. 



5. Impermeability of Plasma and the Phenomenon of Plas- 

 molysis. Osmotic Pressure of the Cell Sap and Methods of Its 

 Determination. — The easiest way of convincing oneself that 

 protoplasm is but slightly permeable, or is impermeable, to the 

 substances dissolved in water is by observing cells having colored 

 cell sap, such as those of a red beet root or the leaves of red 

 cabbage. While alive, these cells may be kept in water for many 

 days without losing their pigment. But as soon as the structure 

 of the protoplasmic membrane is destroyed, e.g., by boiling or 

 even by heating at 60 to 70°C. or by the addition of some poison, 

 the colored sap diffuses into the surrounding water through the 

 cell walls. More exact analytical methods have shown that 

 other substances dissolved in water, e.g., the salts and sugars, 

 are not lost to any extent from the living cells. Otherwise the 

 very existence of submerged plants would be impossible. 



The phenomenon of impermeability of the protoplasm to 

 substances dissolved in water may be detected in another way, 

 also. If a cell, preferably having colored sap, is placed in a suffi- 

 ciently strong solution of some harmless substance, such as sugar 

 or potassium nitrate, the following results may be observed 

 under the microscope. First, the cell decreases somewhat in 

 volume, after which the protoplasm begins to pull away from the 

 cell wall. If the solution is very strong, the protoplasm will 

 shrivel -up into a ball-like mass containing the highly concen- 

 trated cell sap (Fig. 6). The space between the cell wall and 

 the protoplasmic sac will be filled with the external solution, 

 which has penetrated through the wall. This phenomenon of 

 shrinkage of the protoplasm is called ^'plasmolysis." 



The different stages observed in watching the cell in water or 

 in a solution may be explained in the following way. The 

 protoplasm is readily permeable to water and impermeable to 



