CHAPTER IX 



METHODS OF EXTRACTINO SAP FOR CRYOSCOPIC 



OBSERVATIONS 



In the earlier experiments with the thermo-electric 

 method, the sap employed was obtained by simply crushing 

 the tissues in linen till they yielded the necessary amount 

 of liquid. In a few cases at the beginning of the investi- 

 gation, when a difficulty of obtaining sufficient sap was 

 anticipated, a modification of this method was used. A 

 weighed quantity of leaves was broken up to a nearly 

 uniform pulp in a mortar and a measured quantity of 

 water was added. When thorough mixing was effected, 

 the diluted sap was squeezed from the sludge and its 

 freezing-point determined. The correction for dilution was 

 obtained by a separate determination of the percentage of 

 water in the sample of leaves used. 



Method of simple pressure. The method of 

 extraction by simple pressure had always been adopted 

 hitherto in cryoscopic determinations both for animal 

 and vegetable juices. 



The sap so obtained has been regarded as a fairly average 

 sample of the sap of the organ pressed. This seemed a 

 reasonable view to take, inasmuch as the pressures applied 

 so completely crushed the cells of the tissues that the sap 

 expressed contained large quantities of protoplasmic frag- 

 ments, which in the case of green organs were particularly 



17:. 



