PLANT STRUCTURE 9 



DIFFUSION THROUGH THE STOMATA 



At the end of the nineteenth century it seemed difficuh to 

 believe that the stomata provided the main path for the ex- 

 change of gases between the external air and the internal 

 atmosphere of the land leaf. Many plants have stomata con- 

 fined to the lower epidermis of the leaf. Using such a plant, 

 Nerium oleander^ F. F. Blackman in 1895 showed that the 

 application of petroleum jelly to the lower surface alone was 

 sufficient to markedly decrease the diffusion of carbon di- 

 oxide. When the external concentration of carbon dioxide 

 was as high as 6% the absorption by the treated leaf was only 

 one-seventh that by the untreated leaf, demonstrating that 

 the magnitude of cuticular diffusion is small. These obser- 

 vations were extended by Blackman and later by Brown and 

 Escombe to show that in many plants when either only the 

 upper or the lower surface of a leaf was supplied with carbon 

 dioxide the rate of photosynthesis was roughly proportional 

 to the stomatal frequency of the surface supplied. This was 

 not true in all cases, possibly because the stomata were not 

 open to the same extent on both sides. Whilst the lack of 

 precise information as to the degree of stomatal opening in 

 these experiments renders a simple interpretation impos- 

 sible, nevertheless the results taken as a whole indicate that 

 the main pathway for gaseous diffusion is via the stomata and 

 that diffusion through the cuticle is small by comparison. 



TABLE 2.1 



THE RATE OF PHOTOSYNTHESIS BY THE TWO SURFACES OF A LEAF 



{from Brown and Escombe (1900)) 



2 



