Dixon — A Transpiration Model. 119 



hours, without being able to detect any diminution of curva- 

 ture. 



In order to expose the water surrounding the piece of tissue 

 to a greater tension, the lower part of the water column may be 

 replaced by mercury. Working in this way I have submitted the 

 osmotic cells of the peduncle of Doronieum to a tension of 75 cm. 

 of mercury for one hour. Daring this time the turgor of the 

 cells remained unaltered. 



These experiments show the possibility of realizing experi- 

 mentally the conditions we have assumed of pressure and tension 

 in the transpiring cells of the leaves. 



In another place it has been shown 1 that the leaves of plants 

 continue to draw up water, even when surrounded with a saturated 

 space. Contrary to my anticipations, our Transpiration Model 

 may be made to imitate this phenomenon. The experiment may 

 be fitted up by enclosing the thistle-head funnel (furnished with 

 the double membrane, containing sugar solution as described above) 

 in a small, wide-necked bottle, which contains a little water. 

 When the funnel is set up in position, this water lies round its 

 stem, and the space over the membranes becomes saturated with 

 water-vapour. With this arrangement the rise of water may be 

 observed, as before, by the motion of the suspended particles in 

 the capillary supply-tube. 



The rise of water in this form of the experiment is made possible 

 by the distension of the cell and by the leakage of sugar solution 

 through the membranes. Water is drawn up into the cell by 

 osmosis, while sugar solution passes through both the upper and 

 lower membranes. It is evident that, as long as the solution 

 above the lower membrane is more concentrated than that in the 

 funnel, water will pass up the supply tube. This equalization of 

 the concentrations will require a long time, as the rate of leakage 

 and that of diffusion through the lower membrane are very slow. 

 In one experiment the motion of particles in the supply tube was 

 observed for seven consecutive days, and even then the rate of 

 motion showed no appreciable falling off. During the experiment 

 sugar solution accumulated on the outside of the upper membrane. 



1 Transpiration into a Saturated Atmosphere. Proc. Koy. Irish Acad., 1898, p. 627. 



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