The Path of Gaseous Exchange. 



Tablb V. 



Assimilation by the two Surfaces of various Leaves 

 illuminated oh Upper Surface. 



55 



Brown and Escombe's results confirm Blackman's in regard to 

 leaves with the stomata confined to one surface. With leaves 

 bearing stomata on bojh surfaces, when illuminated on the upper 

 surface there is always less intake of carbon dioxide by the lower 

 surface than might be expected from the relative distribution of the 

 stomata over the two surfaces, whereas the ratio of carbon dioxide 

 respired from the two surfaces follows very closely the ratio of 

 stomatal distribution. 



Brown and Escombe explain this result in the following way. 

 During respiration, if there is a steady evolution of carbon dioxide, 

 the rate at which this will escape from the leaf will be independent 

 of the degree of opening of the stomata, for should the stomatal 

 aperture decrease, the partial pressure of carbon dioxide inside 

 the leaf will correspondingly increase and the rise in ' diffusion 

 potential ' will counterbalance the effect of diminished stomatal 

 aperture. 



In the case of assimilation, on the other hand, the • diffusion 

 potential ' will remain constant, for the partial pressure of the 

 carbon dioxide diffusing inwards varies constantly from 0-0003 

 atmosphere outside the leaf to zero where there is complete 

 absorption of the carbon dioxide. Hence, if the stomatal opening 



