DIFFUSION OF GASES THROUGH STOMA 97 



equal to the amount leaving the opening which is given 

 by Q = d{p — po)^r, where pQ is the vapour pressure in 

 the atmosphere ; but here again we have the unknown 

 quantity p. What happens is that the mode of diffusion 

 from the opening prevents the immediate fall of the vapour 

 pressure at the outer end of the tube to the lowest value, pQ, 

 and therefore adds a certain resistance to diffusion through 

 the tube. We can get over the difficulty by supposing that 

 diffusion takes place through a longer tube than the stoma 

 {=lu -\- x), at the outer end of which the vapour pressure 

 is pQ, and such that the extra length x has the same effect 

 as, in fact, the external diffusion shells produce. The rate 

 of diffusion through this system is, of course, equal to the 

 rate through the stomatal tube or through the diffusion 

 shells, and we therefore have 



and ^ ^'■'(Px-P) ^ j -^r'-iPi-Po) 



L L + X 



From the first pair we can find a value for L, and substituting 

 this in the second pair we find that 



TTr 

 x=^— (3) 



4 



The rate of diffusion through the stoma is therefore 



Q=^!Z2£LZio) .... (4) 



4 



where all the quantities may be determined. 



We assumed that at the inner end of the stoma the air 

 was fully saturated with water vapour. This is not the case. 

 Full saturation will occur at the surface of the evaporating 

 mesophyll cells, and from these a series of diffusion shells 

 of diminishing density will run towards the internal opening 

 of the stoma, resembling those occurring outside the more 

 strongly the larger the air space below the stoma. To 



H 



