THE OSMOTIC PRESSURES OF PLANT CELLS 137 



tion of ]\Ii, then the volume molar concentration of sucrose equivalent to 

 that of the cell sap at its original volume M {i.e. before the shrinkage due to 

 plasmolysis), is related to Mi by the following equation: 



M,V, 



M = 



V 



in which V and Vi represent the volume of the cell in its original condition, 

 and at incipient plasmolysis respectively. Having determined Mj the corre- 

 sponding molarity for the cell sap of the "normal" cell can be readily 

 calculated. 



The practical difficulty of all such methods is in measuring accurately 

 the volume changes which occur in cells during plasmolysis. The usual prac- 

 tice is to measure the change in the length (sometimes also in the breadth) 

 and to assume that such measurements are proportional to the change in 

 volume of the cell. For some t>'pes of cells this assumption is probably valid, 

 but with others it may result in serious errors. 



Formerly solutions of a number of different compounds, including certain 

 electrolytes, were used in plasmolytic determinations, but at the present time 

 sucrose solutions are almost universally employed. They are preferred be- 

 cause: (i) sucrose is apparently non-toxic to the protoplasm, (2) it does not 

 penetrate into plant cells at a very appreciable rate (Holier, 1926), (3) it 

 apparently has little or no influence on membrane permeability, and (4) the 

 exact osmotic pressures of different molar concentrations of sucrose are known 

 with greater accuracy than for most other solutions. 



Even when sucrose is used as a plasmolyzing agent the method is subject 

 to a number of errors, of which the following are the most serious : ( i ) An 

 outward diffusion of solutes into the plasmolyzing solution may occur during 

 the determination thus resulting in an apparent osmotic pressure which is less 

 than the true one. (2) In many plant cells it is difficult to observe the initial 

 stages in the separation of the protoplasm from the cell wall during plasmol- 

 ysis. This is most easily observed in cells with a colored sap or in which 

 the cytoplasm contains chloroplasts, hence they are generally used to demon- 

 strate plasmolysis. (3) The cytoplasm sometimes adheres so strongly to the 

 cell wall that a solution of considerably greater concentration than one just 

 barely hypertonic to the cell sap may be required to bring about plasmolysis. 

 As a result of this source of error the apparent osmotic pressure is greater 

 than the true one. (4) If the osmotic pressure of the cells of organs of the 

 higher plants is to be measured by this method, strips of tissue thin enough 

 to permit its examination under a microscope must be cut out of the organ. 

 The resultant mechanical shock to the cells may have profound influences on 



