i THE NATURE Oh TRANSPIRATION 3 



The flow into the shell = the gradient x area = x p x A 



R r K 'k 



Therefore the flow from the aperture is proportional to 

 its radius and not to its area. 



Under the " static " conditions to which these calcula- 

 tions apply, the shape of the stomata also contributes to 

 their efficiency. It will easily be understood that separated 

 as they are from one another by distances relatively great 

 compared with their diameters, the diffusion of water 

 vapour from adjacent stomata will not interfere. Conse- 

 quently the rate of diffusion at the margins will be greater 

 than over the middle of the apertures. Therefore, an 

 opening having the longest margin relatively to its area 

 will be the most efficient ; and the slit-like form of the 

 stomata is seen to be particularly advantageous. 



The consideration that the margins are the most effec- 

 tive parts of the stomata in diffusion suggests another way 

 of looking at the " diameter law." It is evident that for 

 very small holes the marginal region bears a very large 

 relation to the whole opening. For circular apertures the 

 area decreases as the square of the radius while the margin 

 is reduced only as the radius. In the case of slit-like 

 apertures the whole opening may be regarded as marginal. 

 But it is this marginal region which is most effective ; 

 and therefore we should expect the amount of water vapour 

 diffusing through such an aperture to be approximately 

 proportional to the margin (and therefore to the diameter) 

 rather than to the area of the aperture. 



The diffusion of water vapour from the intercellular 

 spaces of the leaves through the stomata has been thus 

 explained on simple physical principles. It remains to be 

 seen how the supply of water to these spaces is to be 

 accounted for. 



Functions of evaporation, osmosis, and im- 



R 2 



