4 Henry H. Dixon. 



surface establishes a steepness of gradient of vapour pressure depending 

 on the rapidit}^ of the motion ; for it constantly sweeps before it the 

 air crowded with water-molecules escaping from the surface and re- 

 places it with air in which thej' are less densely distributed. The 

 rate of evaporation will depend on this gradient, and consequently 

 evaporation from an open surface is liable to be almost indefinitely 

 increased under the action of a strong wind. The same wind blowing 

 across a surface protected with a perforated covering like the cuti- 

 cularized epidermis of plants will cause a much smaller increase in 

 evaporation.') It is able directly to disturb only the gradient of 

 density of water vapour on the outside of the epidermis; while the 

 layers of equal density within the stomata ai-e only displaced somewhat 

 by the reduction of vapour pressure at the opening. Thus, quite 

 apart from the closing of the stomata, which is often occasioned by 

 the excessive loss of water, the possession of a perforated covering is 

 of advantage to plants in controlling, though not harmfully limiting, 

 evaporation. 



The considerations just stated show that the stomata when open 

 provide ample means for the exit of water vapour from the inter- 

 cellular spaces of the leaves. We will now proceed to inquire into 

 the physical conditions under which the water vapour enters these 

 spaces. 



As long as the spaces are not saturated there will be a flux of 

 water molecules from the adjoining moist surfaces into the spaces, 

 since the vapour pressure of the water imbibed by the cell-membranes 

 of the mesophyll cells there exceeds the vapour pressure in the 

 adjoining intercellular spaces. How is this loss made good? On first 

 thoughts, it might appear impossible for pure water to pass easily 

 from the cells which possess a considerable osmotic pressure within 

 their more or less perfect semipermeable membranes, and we know 

 experimentally it is not possible to extract water from them by osmosis 

 unless the pressure of their solutions is balanced by an equal external 

 osmotic pressure. This balancing pressure may amount to several 

 atmospheres.-) While this is true in the case of abstracting water 

 from the cell so as to diminish its total water content, quite a small 

 dilference of pressure will cause water to move across the cell when 

 it is distended to its maximum with water. The osmotic pressure of 



^) Brown and Escorabe. On the Physiological Processes of Green Leaves. 

 Proc. Roy. Soc. B., Vol. 76, 1905, p. 79. 



^) H. H. Dixon, Rôle of Osmosis in Transpiration. Proc. Roy. Irish Acad., 

 Ser. 3 Vol. Ill, 1896, p. 774, and Notes from the Botanical School, Trinity College, 

 Dublin, No. 2 p. 42 ; Idem, A Transpiration Model. Proc. Roy. Dub. Soc, Vol. X, 

 N. S., 1903, p. 119, and Notes from the Botanical School, Trin. Coll. Dub., No. 6 p. 222. 



