The Path of Gaseous Exchange. 6 1 



case shown in Fig. 2, but in which there is perfectly still air 

 outside, so that shells are produced outside as well as in. Here the 

 density of the gas will vary from p at a remote distance outside to 

 p, at the perforation to zero at the absorbing surface. 



In the case of the leaf in still air, we are dealing with an 

 approximation to this last case, but as in actual fact there will 

 always be more or less movement of air outside the leaf there will 

 be a corresponding approach to the conditions indicated in Fig. 2. 



If the partial pressure of carbon dioxide in ordinary air is called 

 P, then in the last case, from Larmor's formula, we have the quantity 

 passing through any shell outside = 2k(P — PJD, where D is the 

 diameter of the perforation. Similarly, the quantity passing through 

 any shell inside = 2kP, D. 



Now when a constant flow is established, these two quantities 

 must be the same so that 



2k(P— P,)D = 2kP,D 

 whence P — P, = P, 

 or P = 2P, 

 That is, in still air the pressure of carbon dioxide at the perforation 

 will only be one half of the pressure of the gas there when this is 

 kept constantly renewed. Consequently the diffusion gradients 

 will only be half as steep in the former case and the rate of 

 absorption will correspondingly be reduced to half. 



Similarly, in the case of the leaf, the rate of passage of carbon 

 dioxide will be increased in the same way when the air outside the 

 leaf is constantly renewed, provided that the cells surrounding the 

 space into which the stomata open are perfect absorbers of the 

 gas. Also, other things being equal, the velocity of flow through 

 the stomata will be proportional, not to the areas of the stomata, but 

 to their diameters. 



Since the surface of the leaf is perforated, not by one, but by 

 many stomata, the researches of Brown and Escombe on diffusion 

 through multiperforate septa become of great interest in relation 

 to the intake of carbon dioxide. The multiperforate septa consisted 

 of sheets of celluloid of thickness 0-08 to 01 mm. in which a series 

 of holes at definite distances from one another were punched. The 

 septa were fixed to the open ends of glass tubes containing sodium 

 hydroxide. The following table gives some of Brown and Escombe's 

 results. In each case the area of cross sections of the tubes was 

 measured and the diffusion through the perforate septum compared 

 with that down an open tube. 



