IX 



METHODS OK EXTRACTING SAP 



187 



paper, enclosed in a metal box, and placed for ten minutes 

 in a water-oven at 95. The half-leaves were then 

 cooled on ice and pressed, the sap flowing out easily. 

 Sample B was immersed in liquid air, and then pressed. 

 The results of two pairs of comparative experiments on 

 different sets of leaves were as follows : 



Table 28. 

 Ilex aquifolium : leaves. 



From these numbers it is evident that ten minutes' 

 exposure to 95 is not sufficient to render the membranes 

 permeable with certainty. Owing to the likelihood of 

 serious changes taking place in the sap, it would not be 

 feasible to expose the leaves for longer to so high a 

 temperature. 



A similar objection was found to apply to the use of 

 chloroform. For this test, samples A and B were prepared 

 in the same way as in the foregoing experiment. A was 

 then exposed to the vapour of chloroform for thirty 

 minutes, pressed, and to the sap obtained a few drops of 

 chloroform were added to ensure saturation. The freezing- 

 point was then determined in the usual way, except that 

 the control-tube of the apparatus which usually contained 

 pure water was charged with distilled water, saturated 

 with chloroform. This change was, of course, not made 

 when examining sample B, which before pressing had 

 been immersed in liquid air. The conductivities were 

 determined in the usual manner. 



