146 TRANSPIRATION AND ASCENT OF SAP ch. 



This was connected with the high-pressure apparatus by 

 suitable couplings ; and, by gradually 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 raising 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 pressure 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, the latter being 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. 

 of water from the vessel below. 



A similar branch, similarly arranged, and exposed to 

 a pressure between 8 and 9 atmospheres, drew up, in one 



