Transpiration and the Ascent of Sap. 3 



As the size of an aperture is reduced the relation of its margin to 

 its area is increased; for the value of 27tr does not decrease as fast 

 as 7rr- when r is reduced. So, for a very small aperture like a 

 stoma, the marginal diffusion is verj^ large compared to that over 

 its cross-section and hence the diffusion from a stoma is exceptionally 

 efficient. 



These conditions of marginal diffusion which we have considered 

 as obtaining on the outside of the stoma must also occur within the 

 intercellular space on the inside, perhaps even to an increased extent 

 since the evaporating cell-surfaces approach the margins of the stomata 

 more closely as a rule than the middle regions. 



It will be readily seen that in order to maintain the efficiency 

 of the marginal diffusion on the outside, it is necessary that the 

 diffusion streams from adjoining stomata should not interfere with one 

 another. This necessitates a certain interval between the openiogs. 

 Brown and Es com be found that a membrane of 1 sq. cm area 

 perforated with 100 holes 0'38 mm diam. and 1 mm apart transmitted 

 by diffusion under identical conditions as much vapour as an open 

 tube of the same cross section, although the total area of the holes 

 was only 11-34 ^j^^ 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 mm apart on a membrane 1 sq. cm in size transmitted ^/^ as much 

 as the open tube, while the total transmitting area was reduced by 

 the interposition of the membrane to 0"3 "q of the whole cross-section. 

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

 of a leaf. Brown and E s c m b e ^) , taking as an example a leaf of 

 Helianthus in which the average area of the stomatal opening is 

 908X10~^sq. mm (= a circle O'OIO? mm 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 metre could be as much as 

 1730 CCS of water per hour, when the state of saturation of the 

 surrounding space was ^^ of that of the spaces within the leaf The 

 greatest amount of transpiration observed in the same time was 276 ces. 

 This clearly shows that it is not the resistance offered by the stomata 

 to diffusion which puts the limit on transpiration in still air. 



Wliile the covering of the evaporating surfaces in the leaf by 

 the perforated cuticle, owing to the facts just pointed out, does not 

 limit transpiration in a still atmosphere, it acts as a most beneficial 

 check to excessive evaporation when the leaves are exposed to wind. 

 It is readily seen that a constant stream of air over an evaporating 



^) Brown and Escombe, loc. cit. Phil. Trans. Roy. See. Lond., p. 279. 



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