CHAPTER V 

 OSMOTIC PRESSURE IN PLANTS 



BETWEEN a pure solvent and a solution there is a differ- 

 ence in free energy. This is rendered apparent in several 

 ways, by their disparity in vapour pressure, boiling-point, 

 freezing-point, and osmotic pressure. As these values vary 

 with the molecular concentration of the solution e.g., the 

 cell sap in the present discussion their measurement 

 affords a ready means of studying, in the gross, changes 

 which occur in the metabolism of vegetable cells. 



The connection between the above-mentioned properties 

 of solutions becomes apparent when one considers the 

 work necessary to separate a given quantity of the pure 

 solvent from a solution in a reversible manner and iso- 

 thermally. It is evident that there must be a quantitative 

 relationship between the various properties, since when 

 the process is completed the same amount of work will 

 have been done in each case, irrespective of the method 

 adopted. Hence it is immaterial whether the removal of 

 solvent be effected by evaporation, by crystallization or 

 by the movement of a semi-permeable membrane through 

 the liquid. 



As is well known, the pioneer measurements in this 

 domain were carried out on plant cells by the plasmolytic 

 method, in which the osmotic pressure of the cell sap is 

 balanced against that of an external solution of known 

 concentration. Direct measurements of the osmotic pres- 



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