ix METHODS OF EXTRACTING SAP 183 



It is generally found that after liquid air treatment com- 

 paratively small pressure is needed to obtain the sap, 

 which flows easily from the tissues without requiring the 

 disruption of the cells. At the same time the sap is much 

 freer from the debris of broken cells than that from an 

 untreated leaf. This sap has always given a greater 

 depression of freezing-point and usually a higher con- 

 ductivity than that from the same tissues untreated. 

 Furthermore, these determinations differed from similar 

 measurements made on sap of the same tissues exposed 

 to toluene vapour. The results are tabulated on page 1 82. 



The numbers in that table show conclusively that the 

 concentration of the sap pressed from the untreated tissues 

 seldom approximates to the concentration of that obtained 

 from the same tissues after freezing. It is hard to see how 

 freezing could be supposed to alter the concentration of 

 the sap, whereas, as we have already seen, it is certain 

 that the sap pressed from living tissues may be consider- 

 ably less concentrated than that which remains behind, 

 and consequently less concentrated than that which was 

 originally in the cells of the tissue before the pressure 

 was applied. 



Sap unaltered by liquid air. It is well known 

 that chemical changes are arrested at such low tempera- 

 tures as that of liquid air ; however, it seemed just 

 possible that changes might take place in the proteids or 

 in the protoplasm just as the cold was being applied, and 

 that these changes might lead to an increase in the 

 quantities of dissolved substances in the sap. 



To set this doubt at rest, determinations were made 

 of the freezing-points of the sap pressed from the untreated 

 roots of Beta vulgaris and from the leaves (also untreated) 

 of Chamaerops humilis before and after freezing in liquid 

 air ; also of the fluids of an egg and of bull's blood under 

 the same conditions. These liquids were not cleared in 

 any way of the matter suspended in them, so it is certain 



