64 Proceedings of the Royal Irish Academy. 



the inside, are also attached three hooks, from which are suspended a 

 "wire hasket, caiTving drying materials, and a manometer. The latter 

 consists of a simple, straight glass-tube, closed at one end ; the other 

 end dips into a small vessel containing mercury. This tube is marked 

 off with \, ^, \, i, &c., of its length from its closed end, and the posi- 

 tion of the mercury index tells directly the pressures in atmospheres. 

 When the upper end of the glass-cylinder is in position, the drying 

 materials and manometer hang in the cylinder. The connection be- 

 tween the glass-cylinder and pump or bottle of CO2 is made by means 

 of a flexible lead tube with screw couplings. 



The results described in this Paper are necessarily only preliminary, 

 as I was unable to procure, by the pump at my disposal, air-pressures 

 above 8-10 atmospheres. Higher pressures were obtained by means 

 of liquid CO,, as there seemed a priori no reason to believe that the 

 presence of CO2 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 

 beha-viour of the leaves when exposed to high pressures, and conse- 

 quently rendered the experiments made with CO2 of little value in 

 estimating the actual osmotic pressures obtaining in the leaves under 

 normal conditions, although they have an important bearing on the 

 question as to whether the tension is established in the sap directly by 

 evaporative or osmotic actions in the leaf. 



I hope immediately to proceed with the investigation of this ques- 

 tion {i.e. the actual osmotic pressures obtaining in the cells of leaves), 

 as I have been, through the kindness of ilr. S. Geoghegan, C.E., put 

 into a position of dealing with high air-pressures. 



In the first experiment, a short branch of Acer macropliyllum was 

 sealed into the high-pressure apparatus, and the pressure raised by 

 means of an air-pump, and maintained for fiiteen minutes at a pressure 

 between 8 and 10 atmospheres. During this time gas was continually 

 bubbling out from the lower end of the branch, showing that the pres- 

 sure had been transmitted to the inner tissues. ITo 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 duiing 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 

 internal pressure confers rigidity on the leaves, was greater than 8 atmo- 

 spheres. The osmotic attraction which would give rise to this pres- 

 sure would be capable of drawing up a column of water 240 feet high. 



