TRANSPIRATION AND ASCENT OF SAP DIXON. 409 



another. This necessitates a certain interval between the openings. 

 Brown and Escombe found that a membrane of 1 square centimeter 

 area, perforated with 100 holes 0.38 millimeter diameter and 1 milli- 

 meter apart, transmitted by diffusion under identical conditions as 

 much vapor as an open tube of the same cross section, although the 

 total area of the holes was only 11.34 per cent of the cross section of 

 the tube. When the distance between these holes is increased their 

 efficiency in diffusion rapidly increases; thus, according to these 

 authors, holes of the same diameter 6 millimeters apart on a mem- 

 brane 1 square centimeter in size transmitted one-fifth as much as the 

 open tube, while the total transmitting area was reduced by the inter- 

 position of the membrane to 0.3 per cent of the whole cross section. 

 Figures like these will enable us to form some idea of the efficiency 

 of a leaf. Brown and Escombe,^ taking as an example a leaf of Heli- 

 anthus in which tlie average area of the stomatal opening is 908X10'^ 

 square millimeters (= a circle 0.0107 millimeter diameter) and the 

 spacing of the apertures 8 to 10 diameters, and allowing for the 

 resistance of the stomatal tube (which leads through the epidermis), 

 found that the amount of diffusion from a square meter could be as 

 much as 1,730 cubic centimeters of water per hour, when the state of 

 saturation of the surrounding space was one-fourth of that of the 

 spaces within the leaf. The greatest amount of transpiration observed 

 in the same time was 276 cubic centimeters. This clearly shows that it 

 is not the resistance offered by the stomata to diffusion which puts 

 the limit on transpiration in still air. * * * 



The considerations just stated show that the stomata when open 

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

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

 the physical conditions under which the water vapor 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 vapor pressure of the water imbibed b}' the cell membranes 

 of the mesophyll cells there exceeds the vapor pressure in the ad- 

 joining 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 os- 

 mosis 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 



1 Brown and Escombe, loc. cit. Pbil. Trans. Roy. Soc. Lend., p. 279. 



' H. H. Dixon, Role of Osmosis in Transpiration. Proc. Roy. Irish Acad., ser. 3, vol. 

 3, 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. 10, N. S., 1903, p. 119, and 

 Notes from the Botanical School, Trin. Coll., Dub., No. »j, p. 222. 



