OSMOTIC PRESSURES AND ELECTRICAL CONDUCTIVITIES 149 



however, is merely a matter of the form in which the results 

 are expressed. 



Mention has already been made of the fact that, owing 

 to the method by which the sap was expressed, all but the 

 most recent cryoscopic determinations are minor limits, 

 for in these only were the cells rendered permeable. 



Electrical conductivity determinations show the varia- 

 tions in the organic and inorganic salts and acids of the 

 sap. It has been pointed out by Dixon and Atkins 

 (1915, 1) that by finding the freezing-point of solutions of 

 some standard substance, such as potassium chloride, 

 which have conductivities equal to those of the plant 

 juices under examination, and by subtracting these values 

 respectively from the freezing-points of the saps as actually 

 observed, it is possible to get a very approximate measure- 

 ment of the proportion of the osmotic pressure due to non- 

 electrolytes. The vajues thus obtained for the conduc- 

 tivities are probably somewhat too low, owing to the 

 increased viscosity of the sap occasioned by the sugars in 

 solution. 



LEAVES AND OTHER AERIAL ORGANS. 



In Chapter X. of " Transpiration and the Ascent of 

 Sap," Dixon analyzes the most recent cryoscopically deter- 

 mined data for the osmotic pressures of leaves, and draws 

 attention to the following points : 



1 . The concordance of the results obtainable by examin- 

 ing leaves from different branches of the same tree growing 

 under similar conditions. This agreement is even found 

 with leaves not rendered permeable, but pressed to the 

 same degree, as far as was feasible. 



2. The chance of slightly low values being afforded by 

 leaves plucked from a tree in the sap of which considerable 

 tension had existed. For if the cells are not fully dis- 



