138 THE OSMOTIC QUANTITIES OF PLANT CELLS 



conditions within them, which may in turn affect their osmotic pressures. 

 (5) The time required for an equilibrium to be obtained between a cell and 

 a solution which just causes incipient plasmolysis is often so great that various 

 types of changes may take place in the cell during this period and render the 

 determination of dubious accuracy (Ernest, 1935)- 



When other plasmolyzing agents than sucrose are used other sources of 

 error are often introduced into the determination in addition to those listed 



above. 



It is doubtful if measurements of osmotic pressure by the plasmolytic 

 method are ever very accurate except perhaps for types of cells which are 

 inherently well adapted to the method. The principal advantage of this 

 method is, that with rare exceptions, it is the only feasible one available for 

 measuring directly the osmotic pressures of living plant cells. 



The principle of the cryoscopic method for determining osmotic pressures 

 has already been described in Chap. VIII. Briefly, as usually applied to 

 plants, the method consists in expressing the juice from a sample of plant 

 tissue and determining its freezing point depression with a suitable thermom- 

 eter ^ or a thermocouple system (Chap. XIV). The osmotic pressure can 

 then be calculated from the freezing point depression by the usual equation. 



It has been found to be impossible to obtain homogeneous samples of sap 

 from plant tissues unless they are first treated in such a way as to kill the cells. 

 Consequently the tissue is usually first subjected to freezing, heating, grinding, 

 or exposure to toxic vapors before the sap is expressed. 



Doubtless the expression under high pressure of the sap from tissues which 

 have previously been subjected to such treatments has a modifying effect upon 

 the properties of the expressed sap of most tissues as compared with the 

 original cell sap. Furthermore the sap obtained represents a mixture of con- 

 tributions from all of the cells — living and dead — in the tissues. At the 

 best, determinations of osmotic pressure made on such saps can represent no 

 more than an average of the osmotic pressures of all the cells in the tissue. 

 In spite of the possibility of a considerable modification in the properties of 

 the sap inherent in this method, there is probably a consistent correlation 

 between values obtained in this way and the mean osmotic pressure of the cell 

 sap of the cells in a tissue. 



Magnitude of the Osmotic Pressures in Plant Cells. — In many dis- 

 cussions of the osmotic pressure of plant cells no distinction is made between 

 the "osmotic pressure of the cells at incipient plasmolysis" and their osmotic 

 pressure under more or less turgid conditions. When the term osmotic pres- 

 sure is employed without qualification it can be assumed to refer to the osmo- 



2 A Haidenhain or Beckmann thermometer is generally used. 



