94 THE BIOLOGY OF FLOWERING PLANTS 



a series of shells, elliptical in section quite near the pore, but 

 soon becoming practically hemispherical : over the surface 

 of each shell the density of the gas is equal (Fig. 8). 



FiG. 8. — Diffusion Shells outside and inside a pore in a septum, 

 arrows show the direction of flow of the gas. 



The 



The amount of gas entering the pore in unit time may 

 be calculated from a formula deduced by Larmoor : 



Q = d{p,-p)^r . . . . (i) 



where Q is quantity of gas ; 



d is a. physical constant (the diffusion constant) 

 depending on the nature of the gas, and the 

 atmospheric temperature and pressure, and 

 expressing the amount which diffuses across 

 unit area, in unit time, under unit pressure ; 

 p^ is the pressure of the gas in the atmosphere ; 

 p is the pressure of the gas at the pore ; 

 r is the radius of the pore. 

 For a gas leaving the pore, p^ and p are reversed. As 

 may be seen, in this formula it is the radius and not the area 



