DIFFUSION AND TRANSPIRATION loi 



discrepancy in Browne and Escombe's experiment is, 

 therefore, not completely explained ; and it is probable that 

 their leaves, which were cut, were badly supplied with water. 

 Thoday has emphasised the difficulty of keeping sunflower 

 leaves fully turgid. 



Renner's results for wind do not agree well with his 

 calculated values. He himself observes that this may be 

 due to the " quite primitive " method by which the wind 

 was produced, i.e. by a fan. It is of course clear that wind 

 of different strengths would affect transpiration differently, 

 and very exact methods would be required for a satisfactory 

 investigation. The ratios obtained, however, do approach 

 the theoretical ratios obtained by using equations (7) and 

 (4), and are far greater than the ratios (i'2 for the sunflower) 

 given by equations (4) and (5). 



We have said that Renner's theory of the formation of a 

 " vapour dome " over the leaf as a whole is not quite 

 satisfactory. An alternative explanation of the depression 

 of the transpiration below the values given by Brown and 

 Escombe's formula, may be based on the viscosity of a gas 

 which causes a thin layer to " stick " to a solid or liquid 

 surface. The presence of such a layer, perhaps one-tenth 

 of a millimetre thick, on the surface of the leaf would of 

 course slow down diffusion. The layer would not be 

 removed, though it would be diminished, in wind. This 

 would agree with the fact that observed wind values are 

 almost always too low. 



Even apart from such difficulties, these equations cannot 

 be taken as absolutely precise expressions, because the stoma 

 is not a true cylindrical tube, and because the air space below 

 the stoma does not allow of the formation of regular diffusion 

 shells. They do, however, express the fundamental laws 

 of gas exchange between the leaf and the atmosphere, and 

 in practice they give a good approximation to the actual 

 rates of exchange in transpiration. The important thing is 

 that the work of Blackman, Brovt^ne and Escombe, and 

 Renner, has firmly established the facts : (i) that the 

 stomata are practically alone concerned in exchange of 



