CHAPTER IX 



THE MAGNITUDES OF OSMOTIC PRESSURES AND 



ELECTRICAL CONDUCTIVITIES IN PLANTS, AND THE 



FACTORS WHICH INFLUENCE THEM 



OWING to the many inaccuracies to which the method of 

 plasmolysis has been open, reference will be made in this 

 chapter only to determinations by the cryoscopic method, 

 except in a few cases where such data are unobtainable. 



Renner (1912) has pointed out that the discrepancies 

 between the results of cryoscopic and of plasmolytic in- 

 vestigations may be in part due to the fact that, whereas 

 cryoscopic determinations used for calculating osmotic 

 pressures are made with solutions containing weight - 

 normal quantities of solute viz., a certain weight in 1,000 

 grammes of water it has always been customary to 

 employ volume-normal solutions in plasmolytic investiga- 

 tions. Morse advocates the use of the former mode of 

 making up solutions, as with it a better agreement is ob- 

 tained between the direct measurements of osmotic pressure 

 and those calculated by the simple Van't Hoff equation. 

 Thus in this respect cryoscopic determinations are really 

 of greater accuracy than are plasmolytic. Renner counsels 

 the adoption of weight - normal solutions for the latter 

 method also, as it leads to a better agreement. It is to be 

 remembered that cryoscopically determined osmotic pres- 

 sures relate to 0, whereas those measured by the plasmo- 

 lytic method are at air temperatures 10 to 20. This, 



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