lO PHOTOSYNTHESIS 



At first it was found difficult to believe that the stomata 

 which occupy such a small proportion of the total surface 

 area of the leaf represent the main gaseous diffusion path. 

 Brown and Escombe (1905), in a classical investigation, 

 studied the diffusion of gases through multiperforate septa. 

 The septa were made from thin sheets of celluloid and 

 covered the end of a glass tube containing at a fixed distance 

 from the septa sodium hydroxide solution. The septum 

 represented the epidermis of the leaf and the sodium hy- 

 droxide the assimilatory mechanism, the whole thus making 

 a simplified physical model of a leaf. With septa 0-09 mm. 

 thick and pores of 0-38 mm. diameter uniformly spaced but 

 varying in number in different septa, they found that dif- 

 fusion per pore increased as the number of pores dimin- 

 ished, so that it was not until the total pore area had been 

 reduced to a small fraction of the area of the whole septum 

 that the total diffusion was appreciably reduced. For ex- 

 ample, with a total pore area of only 1-25% of the septum 

 area, diffusion was still 40% that for the open tube with no 

 septum, whilst a total pore area of 0*31% permitted 14% of 

 the maximum diffusion. These numerical results cannot be 

 directly applied to a consideration of the stomata on a leaf. 

 Their importance lies in the experimental demonstration of 

 the result, independently deduced by Larmor from the laws 

 of diffusion, that provided individual pores are small in 

 diameter and the septum thin a relatively small total pore 

 area will permit a rate of diffusion not far below that of a 

 free surface with no septum. 



For a septum of area A, radius R, and thickness /, having 

 n pores equally spaced, each of area a and radius r, Larmor's 

 formula for still air gives the total equivalent diffusion path 



as — + — ( /+ — ). This is to be compared with an equiva- 

 4 na\ 2 / 



lent diffusion path for the open tube of nR/^. Even when 



the pores occupy i/iooth of the total area, i.e. A/na=ioo, 



provided / and r are small the contribution of the second 



term is small and the extra resistance due to the septum will 



be relatively small. For a leaf the term /+— is much smaller 



