98 THE BIOLOGY OF FLOWERING PLANTS 



allow for these we must simply double the correction x, 

 and the formula for diffusion through the stoma in still air 

 becomes 



L + 2:v ^ . Trr 



Li -f- 



3 



If, however, the leaf is swept by wind, no diffusion shells 

 will form outside, the value /)q will occur and be maintained 

 at the stomatal opening, and the correction is again reduced 

 to X (for the internal shells), the formula being given by 

 equation (4). 



This equation applies equally to diffusion of carbon 

 dioxide into and out of the leaf, and to the diffusion of water 

 vapour. In assimilation, />! represents the density or pressure 

 of carbon dioxide in the atmosphere, and pQ is zero. In 

 transpiration, p^ is the pressure of the vapour in saturated 

 air, and pQ the pressure of the vapour in the atmosphere. 



The total diffusion through all the stomata on one 

 square centimetre leaf surface per hour is given by 

 Browne and Escombe (modified) as 



Q^nxd^H^t-ZM^U^ . . (6) 



2 



where ?i = number of stomata per square centimetre, 

 and 3600 is introduced to bring the value from seconds to 

 hours. Applying this to the actual case of the sunflower, 

 we have : — 



n = 33,000, 



d = o"i45 C.G.S, units, 



pi = 0*0003 atmos., 



p = o, 



r = 0-000535 cm., 



L = 0*0014 cm. 



From v/hich it may be calculated that Q = 2*095 c.c. COo 

 per square centimetre of leaf surface per hour. In wind 

 the value becomes 2*578 c.c. This means that if the m.eso- 

 phyll cells absorb carbon dioxide instantly and completely, 



