SIZE AND DISTRIBUTION OF STOMATA 91 



The actual opening is more or less oblong or elliptical 

 in surface view. There is, again, a great range in the area of 

 the pore of the fully opened stoma in different species. 

 Measurements of the pore made by Renner (19 10) for a 

 number of plants are summarised in Table XXI. 



TABLE XXI 

 Examples of the Size of the Stomatal Pore 



Taking the sunflower as a typical case, we have 330 

 stomata per sq. mm., each with an area when full open of 

 0*0000908 sq. mm. The combined stomatal area per 

 square millimetre is therefore 330 X 0*0000908, or 0*03 

 sq. mm., or one thirty-third of the total area. 



§ 5. Diffusion through the Stomata 



The proper functioning of the leaf requires a restricted 

 loss of water vapour and a sufficiently free passage of carbon 

 dioxide and oxygen. It seems obvious that restriction should 

 be the result of the practical limitation of gaseous diffusion 

 to the stomata, which, though so numerous, are so small, 

 and occupy so small a fraction of the total leaf surface. 

 It might well be thought that the access of carbon dioxide 

 would be unduly impeded by these means, and that, in the 

 case of the sunflower, for example, the flow of gas would be 

 only one thirty-third of what might take place into a free 

 absorbing surface of the same area as the leaf. This is not 

 so. It has been shown by Browne and Escombe (19056) 

 that a perfectly absorbing surface of sodium hydroxide 



