Transpiration and the Ascent of Sap. 7 



pressure of water vapour in the funnel than in the cell. There will 

 be then a continuous fall in the pressures of aqueous vapour as we 

 pass from the funnel to the cell from the cell to the membrane and 

 from the membrane to the surrounding space. This pressure g-radient 

 will, of course, lead to a flow of water across the cell, which may 

 be observed in the stream of water in the capillary tube marked by 

 the passage of suspended particles across the field of the microscope. 



The amount of water passing off will depend on the difference 

 between the vapour-pressure (oj) at the water-surface in the mem- 

 brane and the vapour pressure (q) in the surrounding space. When 

 (Q^ — Q) is large, evaporation is great when (ê>i— (>) is small, evaporation 

 will be insignificant. The supply to make good this loss depends 

 upon the difference of vapour pressure Qo in the cell and q^ ; and 

 again the flow into the cell is dependent on the difference between 

 the vapour pressure q.^ in the funnel and (>.,. 



Supposing ((), — q) is much greater than ((>2 — d) then the mem- 

 brane will lose more water than it receives and the water-surface 

 will retreat into it, until the capillary forces developed in the mem- 

 brane diminish ^i so far that the flow due to the difference (Q2 — Q1) 

 will equal that produced by (ç, — q). If again the flow due to the 

 difference {q^ — (»J is greater than that produced hy (^^ — ç.,) the loss 

 of water will be greater than the supply and the cell will tend to 

 collapse. The converse will lead to a distension of the cell. It is 

 obvious that all the actions tend to produce a steady state. As we 

 have seen, reduction of water in the membrane diminishes the vapour- 

 pressure in it by the development of steeper curvature in its menisci \). 

 and reduction of water in the cell concentrates the solution and con- 

 sequently lowers its vapour pressure. Both actions favour an influx of 

 water to the impoverished region and, conversely, also oppose a loss. It 

 is to be noticed that the water-vapour pressure ç., in the cell depends 

 not only on the concentration of the solution but also on the cell's state 

 of turgor. When completely distended part of the osmotic pressure 

 of the dissolved substances is borne b}^ the stressed cell wall and the 

 vapour pressure of the solvent is increased above that corresponding 

 to the concentration by the amount of this stress. 



Thus we see that the gradient necessary to the flow of water 

 across the evaporating cells may be maintained within wide limits 

 irrespective of the osmotic pressure in the cell; but, if the cell is to 

 remain turgescent, the flow determined by ((»3 — Q.2) ii^^^st be as great 

 as, or gi-eater than, that determined by (q.2~Qi)- Furthermore, during 



M Cf. Lord Kelvin. Proc. Eoy. Soc. Edin. 1870. H. H. Dixon, On the 

 Phj'sics of the Transpiration Current. Notes from the Botanical School, Trinity 

 College, Dublin, No. 2, 1897, p. 19. 



