STOMATA AND GASEOUS EXCHANGE 89 



that surface. It may be noted that the thin cuticle of the 

 leaves of Platanus, Ampelopsis, and Polygonum is as 

 effective in stopping diffusion of carbon dioxide as is the 

 thick cuticle of Hedera, Nerium, and Prunus laurocerasus. 

 Where stomata are present on both surfaces the relation is 

 not quite so close, but here we must take into account the 

 different conditions of the stomata on the two surfaces, 



Browne and Escombe found that in assimilation the 

 carbon dioxide absorbed by the upper surface, when 

 stomata are present there, was always considerably greater 

 than the stomatal ratio would account for. Thus for 

 Rumex alpimis, with a stomatal ratio of 37/100, the gas 

 exchange ratio was 73/100. They believe that this is due 

 not to excessive diffusion through the cuticle but to wider 

 opening of the stomata on the more strongly illuminated 

 upper surface, and perhaps to more rapid utihsation of the 

 carbon dioxide by the pahsade tissue. As regards respiration 

 their results confirm those of Blackman. The diffusion of 

 oxygen has not been measured directly ; it may be taken 

 to agree with that of carbon dioxide, the determination of 

 which in gaseous mixtures is much more convenient to 

 carry out. 



We may conclude that the stomata are the main path of 

 the gaseous exchanges of the leaf. As regards carbon dioxide 

 and oxygen, the cuticle offers an almost complete barrier 

 to diffusion ; the stomata are also the chief exits for the 

 water vapour, though appreciable amounts pass through 

 the cuticle. 



In submerged water plants, with a fine cuticle, diffusion 

 of carbon dioxide and oxygen in solution takes place directly 

 through the saturated epidermal walls over the whole plant, 

 except, perhaps, where the leaves have the special 

 permeable hydropotes described by Mayr (19 15). 



§ 4. Distribution and Dimensions of Stomata 



It is a well-known fact that stomatal numbers vary 

 greatly in different plants ; that their relative numbers are 



