ix METHODS OF EXTRACTING SAP 185 



course rapidly produce plasmolysis. This clearly shows 

 that no appreciable concentration has been effected by 

 the treatment, and that the sap pressed from the untreated 

 organ is not isotonic with that in the vacuoles of its cells. 



Of course the application of liquid air cannot stop 

 changes taking place while the sap is being pressed, as is 

 evidenced by the production of colour in the sap of many 

 tissues during the process. 



The cells treated with liquid air seem to be rendered 

 completely permeable. This appears from the fact that 

 the sap is so easily pressed from the tissues after the 

 exposure, often without any disruption of the cells. Also 

 the concentration of successive pressings from these frozen 

 tissues remains sensibly the same, as is shown in Table 27 : 



Table 27. 

 Hedera helix: leaves. 



Hence we may assume that the sap so obtained is a fair 

 sample of that of the uninjured tissues. 



It will be noticed that in most instances the difference 

 in conductivity between the sap of organs treated with 

 liquid ah* and that of those untreated is not so marked as 

 the difference in freezing-point. Comparison of the ratio 



for the pairs of experiments will make this clear. 



This, perhaps, may be largely attributed to the greater 

 permeability of the protoplasm to electrolytes, so that 

 the sap pressed from the untreated organs is relatively 

 richer in them. 



