18 PLANT PHYSIOLOGY 



siderations. The higher the osmotic pressure of the external 

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

 the shrinkage of the protoplasm. Conversely, the smaller the 

 decrease in volume of the protoplasmatic 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 

 solution is found that causes but an incipient shrinking of the 

 protoplasm, 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 

 purpose, a series of solutions of increasing concentration is pre- 

 pared, differing from one another by the same magnitude, e.g., 

 by 0.1 gram molecule. Thin sections of the plant organ under 

 investigation are placed in these solutions. The highest con- 

 centrations induce strong plasmolysis; the weakest, no plasmoly- 

 sis whatever. Somewhere in the middle, a concentration will be 

 found that will induce only the incipient stages of plasmolysis. 

 The concentration of such a solution will only slightly surpass 

 the osmotic pressure of the cell sap and may be assumed to be 

 equal. 



For computing the pressure in atmospheres, there may be 

 utilized the equation mentioned above, according to which a 

 solution containing 1 gram molecule of an undissociated substance 

 per liter of solvent possesses an osmotic pressure of 22.4 atmos- 

 pheres. But since this equation is applicable only to ideal very 

 diluted solutions of such substances only as do not dissociate, 

 it is preferable to use for various plasmolytic substances special 

 tables that may be found in physicochemical reference books 

 and that quote for each concentration the corresponding osmotic 

 pressure, determined by precise experiment. It is customary to 

 use cane sugar for determining osmotic pressure, for the determi- 

 nations stated above are most accurate for this substance. 

 Moreover it is quite harmless to the cell, and in conditions of 

 plasmolytic experiments its penetration into the cell is excluded. 

 In comparative experiments, such computations usually are not 



