Dixon — A Transpiration Model. 117 



highly refracting liquid, which are seen falling down from the 

 lower membrane into the funnel, and also by testing the outer 

 surface of the upper membrane. Taking the leakage into account 

 the action of the model seems to be as follows : — The osmotic 

 pressure in the cell, due to the dissolved sugar, draws water in from 

 below. At the same time a small amount of sugar and water 

 passes through both the lower and upper membranes by leakage. 

 On the outer side of the upper membrane this solution is concen- 

 trated by ' evaporation ; and when it becomes more concentrated 

 than the solution within the cell, it will act osmotically on the 

 liquid there, and draw more water to the surface. The solution 

 leaking back into the funnel, on the other hand, is quickly diluted, 

 and is powerless to act on the liquid in the cell. The upward flow 

 is in this case also maintained, owing to the fact that the vapour- 

 tension of water in the space above the cell is less than that of the 

 liquid in the funnel. This tension is communicated through the 

 liquid, on the outside of the upper membrane, and through the 

 liquid in the cell. The former has a higher vapour-pressure than 

 the space above, but lower than the solution in the cell, while the 

 solution in the cell has a higher vapour-pressure than the liquid 

 above, and a lower vapour-pressure than the liquid in the funnel. 

 Furthermore, as the cell remains turgid, these vapour-pressures 

 must be so related to each other that the inflow of water into 

 the cell at least balances the loss of water by evaporation and of 

 solution by leakage. 



In the case of the model, the water-column which is raised in 

 the tube is only a few centimetres in height, and consequently is 

 urged up under atmospheric pressure into the space left for it by 

 the evaporative and osmotic actions taking place above. But in 

 the case of high trees, the water in the tracheidal tubes of the 

 leaves is drawn into the osmotic cells in a state of tension, and 

 consequently the water in these turgid cells must be in a tensile 

 state. To render the working of the model in this respect comparable 

 to the transpiratory process taking place in high trees, it would be 

 necessary to remove the atmospheric pressure below, and allow the 

 water to be drawn up in a tensile state into the cell distended by 

 osmotic pressure. 



The simultaneous presence of pressure and tension within the 

 cell, at first sight, appears paradoxical; but a little consideration will 

 show that it is quite possible for the solvent, water, to be in a state 



SCIENT. PE.OC. K.D.S., VOL. X., PART I. L 



