IX 



METHODS OF EXTRACTING SAP 



181 



examined. In order to see if the increasing concentration 

 which is characteristic of the sap pressed from untreated 

 leaves occurs in the case of the sap pressed from the leaves 

 exposed to toluene vapour, the sap from this lot was 

 divided into first, second, and third pressings. 



Table 24. 

 Hedera hcl /. : leaves. 



These results show that, with an exposure to toluene 

 vapour of 48 hours, the protoplasm has become permeable, 

 and no longer tends to keep back the dissolved substances 

 of the vacuoles. 



Of course, such prolonged exposure has the objection 

 that during this process enzymes in the cells may consider- 

 ably alter the nature of the dissolved substances, and so 

 lead to a change in the concentration and constitution 

 of the sap. Accordingly, experiments were made to 

 determine if shorter exposures would be sufficient. 



By means of these experiments it was found that shorter 

 exposures, e.g., 1 to 5 hours, caused a marked concentra- 

 tion of the sap expressed when compared with that from 

 the same leaves untreated ; but much longer exposures 

 were needed to render all the cells permeable, and so 

 allow the sap obtained to be a fair sample of that of the 

 uninjured leaf. The prolongation of the exposure makes 

 the method objectionable. Accordingly, it was abandoned 

 as unsatisfactory. 



Protoplasm rendered permeable by intense 

 cold. -The possibility that the protoplasmic membranes 



