Dixon — A Transpiration Model. 121 



there would be a balance of loss and gain through the lower 

 membrane. Hence the upward motion in the tube would come 

 to a standstill. 



These considerations, on the actions of perfect semipermeable 

 membranes, taken in conjunction with the observed facts of 

 transpiration into saturated spaces, led me previously to believe 

 that it was necessary to assume that there was, in transpira- 

 tion, an expenditure of stored energy, and that vital pheno- 

 mena entered into the process. But our model shows that 

 with imperfectly semipermeable membranes, such as the leaf- 

 cells in all probability possess, transpiration into saturated spaces 

 is possible over long periods, and that, if photosynthesis is per- 

 mitted, such transpiration might be indefinitely prolonged. 



Conclusions. 



A consideration of the action of the model described in this 

 note leads to the following conclusions : — 



(1) A state of tension may exist in the water (solvent) of the 

 leaf-cells, while simultaneously the dissolved substances may be 

 exerting an osmotic pressure. This latter is apparent from the 

 fact that these cells remain in a turgid state. 



(2) The tension set up by evaporation at the surfaces of the 

 leaf-cells during transpiration is transmitted, through the solvent 

 in these cells, to the water in the conducting vessels and tracheids 

 of the leaf. 



(3) The simultaneous presence of pressure and tension in 

 these cells, coupled with a slight leakage of the solute through 

 the membrane, is adequate to account for the observed facts of 

 transpiration into a saturated atmosphere. 



(4) There appears no need to invoke the intervention of 

 special vital actions, i.e. the utilization of stored energy in 

 transpiration. 



