114 TEXTBOOK OF PLANT PHYSIOLOGY 



external solution is higher than the total molecular concentration 

 of the cell sap, the pressure of the external solution directed inward 

 exceeds the pressure of the cell sap directed outward. Conse- 

 quently, turgor pressure disappears, and the cell decreases in vol- 

 ume. Moreover, the protoplasmic sac withdraws somewhat from 

 the cell wall, which, with lessened internal pressure, contracts 

 slightly. 



The phenomenon of plasmolysis is of importance in many ways. 

 In the first place, it shows a difference in osmotic properties of the 

 protoplasm and the cell wall. Second, it may be used frequently 

 to indicate whether the cell under investigation is alive or not, 

 because impermeability is peculiar only to living cells. Finally, 

 plasmolysis enables one to determine with great accuracy the mag- 

 nitude of the osmotic pressure within every living cell. 



A method for determining the osmotic pressure in cells was 

 devised by De Vries as early as 1884. It is based on the following 

 considerations: The higher the osmotic pressure of the external 

 solution above the osmotic pressure of the cell sap, the greater is 

 the shrinkage of the protoplasm. On the other hand, the smaller 

 the decrease in volume of the protoplasmic sac, the less it will 

 withdraw from the cell wall, thus showing a smaller difference 

 between the two pressures. If a concentration of the outer solu- 

 tion is found which causes but an incipient shrinking of the proto- 

 plasm, which is usually observed in some corner of the cell, one then 

 may assume that this concentration of the outer solution balances 

 the concentration of the cell sap within. Since the concentration 

 of the external solution is known, it is easy to calculate its osmotic 

 pressure and from this the almost equal osmotic pressure of the 

 cell sap. 



The problem then is to find a concentration of the external 

 solution that produces only incipient plasmolysis. For this pur- 

 pose a series of solutions of increasing concentration is prepared, 

 differing from one another by the same magnitude, for instance, 

 by 0.1 mol. Thin sections of the plant organ under investigation 

 are placed in these solutions. The highest concentrations induce 

 strong plasmolysis; the weakest, no plasmolysis whatever. Some- 

 where in the middle, a concentration will be found which will 

 induce only the incipient stages of plasmolysis. The concentration 

 of this solution corresponds to the molecular concentration of the 

 cell sap. Hence it has been called the osmotic value. 



