vii OSMOTIC PRESSURES OF LEAF CELLS 145 



the pump at my disposal, air-pressures above 8 to 10 atmos- 

 pheres. Higher pressures were obtained by means of liquid 

 carbon dioxide. At the time there seemed a priori no reason 

 to believe that the presence of carbon dioxide would falsify 

 the results of experiments, which were not continued for 

 a long duration. However, subsequent experimental work 

 showed that the presence of this gas profoundly modified 

 the behaviour of the leaves when exposed to high pressures, 

 and consequently rendered the experiments made with 

 carbon dioxide of little value in estimating the actual 

 osmotic pressures obtaining in the leaves under normal 

 conditions. 



In the first experiment, a short branch of Acer macro- 

 phyllum was sealed into the apparatus, and the pressure 

 raised by means of an air-pump, and maintained for 

 fifteen minutes between 8 and 10 atmospheres. During 

 this time gas was continually bubbling out from the lower 

 end of the branch, showing that the pressure had been 

 transmitted to the inner tissues. No loss of turgescence, 

 however, of the leaves could be observed. 



In a second experiment, a similar branch was exposed 

 to a pressure of 8, or nearly 8, atmospheres, for fifteen 

 minutes, and during this time showed no loss of turgescence. 



From these two preliminary experiments, it appears 

 that the pressure within the cells of the leaves of Acer 

 macrophyllum, which confers rigidity on the leaves, was 

 greater than 8 atmospheres. The osmotic attraction 

 which would give rise to this pressure would be capable of 

 drawing up a column of water 240 feet high. 



In a similar experiment, a branch of Cratcegus oxy- 

 acantha was exposed to a pressure of about 8 atmospheres 

 for fifteen minutes without showing signs of loss of 

 turgidity. 



As the pump I had at my disposal was unable to compress 

 air above a pressure of about 10 atmospheres, I discarded 

 it in favour of a bottle containing liquid carbon dioxide. 



L 



