76 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



sures of plant juices with artificial semi -permeable mem- 

 branes have never been carried out, owing to manipulative 

 difficulties and to the fact that by calculation from cryo- 

 scopic data the desired values may be more readily 

 obtained. However, by balancing an external gas pres- 

 sure against the internal osmotic pressure of leaves on a 

 severed branch, and noting when wilting supervened, Dixon 

 (1896) has been able to obtain approximate values. The 

 method is fully described in his book on " Transpiration 

 and the Ascent of Sap," so will not be given here. 



As in other branches of physiology, cryoscopic measure- 

 ments have recently been extensively employed, to the 

 almost complete exclusion of the plasmolytic. The work 

 of the Neapolitan physiologists Cavara (1905), Trinchieri 

 (1909), and Nicolosi-Roncati (1907), was carried out with 

 the Beckmann apparatus, which as usually constructed 

 requires about 15 c.c. of liquid, though tubes of smaller 

 capacity are also in use. The introduction of thermo- 

 couples for these determinations by Dixon and Atkins 

 (1910) has effected a considerable saving in the volume of 

 sap necessary, 2-5 to 3 c.c. being sufficient for each experi- 

 ment. A description of the modification would be super- 

 fluous here, as it may be found in " Transpiration and the 

 Ascent of Sap." 



Owing to the liability of all liquids of animal or vegetable 

 origin to undergo changes when heated, the determination 

 of boiling-points has never assumed any importance in 

 physiology. 



An elegant though tedious method based upon vapour 

 pressure differences has been worked out by Barger (1906) 

 in Errera's laboratory. In it drops of a solution of un- 

 known concentration are drawn up in a capillary-tube 

 between drops of various solutions of known strength. 

 Isothermal distillation or condensation takes place from 



