Dixon — On the Osmotic Pressure in the Cells of Leaves. 65 



In a similar experiment, a branch of Cratcegus oxyacantha 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 using a bottle containing liquid CO2. This was connected with 

 the high-pressure apparatus by suitable couplings ; and, by carefully 

 opening the valve at the mouth of the bottle, the pressure could be 

 adjusted at will to any pressure up to 60 atmospheres. This has the 

 additional advantage that careful observations are possible while rais- 

 ing the pressure, which cannot be done while using the pump unless 

 an assistant is employed. 



By means of this arrangement, the pressure was raised round the 

 same branch as was used in the last experiment, to 16 atmospheres, 

 and was maintained at this for fifteen minutes. But even at this 

 pressure the leaves showed no loss of turgescence. When the pres- 

 sure reached 10 atmospheres, the bubbling of gas through the stem 

 became very marked. 



As it appeared possible that a certain amount of collapse of the 

 osmotic cells of the leaves might take place without making itself 

 noticeable by the flagging of the leaves, a number of experiments were 

 made in which the branch dipped into a vessel beneath, which latter 

 was weighed before and after the experiment. Any increase in weight 

 of this vessel would be due to the forcing backwards by the external 

 pressure of the cell-sap contained in the cells of the leaves, which 

 would in turn displace a certain amount of water from the conduits 

 of the branch into the vessel. A decrease, on the other hand, of the 

 weight of the vessel would show that the external pressure had not 

 crushed the osmotic cells, and that they had, in spite of its action, 

 drawn up water from the vessel. 



The first experiment of this kind was made on a branch of Acer 

 macrophyllum, which bore 14 well- grown leaves. This branch was 

 sealed into the high-pressure apparatus, and kept at a pressure of 

 8 atmospheres ; during one hour of intermittent sunlight this branch, 

 drew up 0*1 gr. from the vessel below. 



A similar branch, similarly arranged, and exposed to a pressure 

 between 8 and 9 atmospheres, drew up, in one and a-half hour's 

 sunshine, 0'342 gr. of water from the weighed vessel. 



From these experiments, it follows, that the osmotic cells of the 

 leaves of Acer macrophyllum were able to remain turgescent and draw 

 up water against a pressure of 8 atmospheres. Consequently, the 

 osmotic solution in the cells must be capable of generating a tension 



K.I. A. PEOC, SEK. m., VOL. IV. P 



