426 



Scientific Proceedings, Royal Dublin Society. 



After 48 hours the freezing-point and the electrical conductivity of the sap 

 pressed from the two lots were examined. In order to see if the increasing 

 concentration, whicli 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. 



Hedera Helix : 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 considerably alter the nature of the dissolved 

 Substances, and so lead to a change in the concentration and constitution of J;he 

 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., 1-5 hours, caused a marked concentration of the sap expressed when 

 compared with that from the same leaves untreated ; but mueli 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 prolonga- 

 tion of the exposure makes the method objectionable. Accordingly, it was 

 abandoned as unsatisfactory. 



It occurred to us then that the protoplasmic membranes might be rendered 

 permeable by exposure to low temperature ; while, at the same time, the low 

 temperature would have the effect of arresting changes taking place in the 

 tissues experimented upon. Through the kindness of Prof. W. H. Thompson 

 we have been able to obtain ample supplies of liquid air from the Physiological 

 Department, Trinity College, for this purpose. Tissues immersed in liquid 

 air immediately become frozen hard. From the liquid air they were 

 without delay transferred to, and enclosed in, a stoppered vessel to prevent 

 the condensation of moisture on them from the atmosphere owing to 

 their extreme cold. When they had assumed the temperature of the 

 surroundings, they were pressed in the usual manner. 



It is generally found that after this treatment comparatively small 



