96 THE BIOI>OGY OF FLOWERING PLANTS 



rate of diffusion if no interference were present, and it may 

 be seen that the observed value is as great as the theoretical 

 when the pores are lo diameters apart ; that is to say, 

 there is no mutual interference at this distance ; the vapour 

 shell with a radius 5 diameters that of the pore has the 

 same tension as the vapour of the atmosphere. 



The epidermis may be looked on as such a multiperforate 

 septum, of which the stomata are the pores. Very generally 

 they are not less than 10 diameters apart ; in the sun- 

 flower, with a large number of stomata, they lie at an 

 average distance of 8 diameters apart. But the results so 

 far considered require some modification before they can 

 be applied to the leaf. 



(i) The stoma is not a circle but an ellipse. It has 

 been found possible, however, to treat it as a circle of equal 

 area, and to use the radius of such a circle in calculation. 



(2) The stoma is not a simple pore in an indefinitely 

 thin septum. It is a little tube the depth of which is, as we 

 have seen, often greater than its diameter. 



The rate of diffusion through a tube depends on the 

 area of the cross-section, the drop in pressure between the 

 two ends, and the length of the tube. It is given by the 

 equation 



where L is length of tube, 



r is its radius, 



p^ and p are the pressures at the two ends. 

 Let us consider the case of water vapour leaving the 

 stoma, and let us simplify it for the moment by supposing 

 that at the inner opening the air is saturated and the pore 

 kept supplied by vapour at maximum pressure, p^^. The 

 rate of diffusion through the stomatal tube is then given 

 by equation (2). But we cannot use this equation to 

 calculate the amount of vapour passing through the tube, 

 because it contains the quantity p (pressure of vapour at 

 outer opening), which we cannot determine. The amount 

 of vapour passing through the stomatal tube is, of course. 



