THE NATURE OE PHOTOSYNTHESIS 67 



reasonable to conclude that the path of gaseous exchange under these con- 

 ditions is primarily through the stomata. If, for instance, in the case 

 of leaves with stomata confined to the lower surface, it were assumed that 

 the carbon dioxide passed through the cuticle this would have to possess 

 a permeability for carbon dioxide 50 to 100 times that of the cuticle on 

 the upper surface. This seems impossible in view of the fact that leaves 

 with thin cuticles gave the same results as those with very heavy cuticles. 



Leaves of Nuphar advena and Catalpa bignonioidcs which have sto- 

 mata respectively only on the upper and lower surface show an absorption 

 of carbon dioxide during photosynthesis only on the surfaces containing 

 the stomata. 



In the absorption of carbon dioxide during photosynthesis the relations 

 are slightly different. Brown and Escombe found that the intake of carbon 

 dioxide by the lower surface of leaves illuminated on the upper surface is 

 always less than might be expected from the stomatic ratio. The quantity 

 of carbon dioxide taken up by the lower surface is sometimes half the 

 amount expected. Such a difference in the ratios of gaseous exchange 

 to stomatic openings in egress and ingress of carbon dioxide is not 

 surprising. With a constant rate of carbon dioxide formation within 

 the leaf during respiration, the rate at which the gas diffuses out is inde- 

 pendent of the degree of o^jening of the stomata. If the stomata should 

 partially close during the diffusion, the partial pressure of the carbon 

 dioxide in the leaf will increase. This increase in carbon dioxide-pressure 

 will be inversely proportional to the changed linear dimension of the 

 stomata. The increased carbon dioxide-pressure will tend to counter- 

 balance the eft'ect of the diminished aperture, and the quantity of carbon 

 dioxide emitted from the leaf will be only very slightly affected by the 

 changes in size of the stomata. The increase in friction through the smaller 

 openings is probably so slight that it could not be detected by the ordinary 

 methods. 



The conditions of absorption of carbon dioxide during photosynthesis 

 are, however, quite dift'erent. Given now a constant rate of carbon diox- 

 ide fixation within the leaf, the partial pressure of the carbon dioxide 

 in the atmosphere surrounding the leaf is constant. The ingress of car- 

 bon dioxide must therefore vary directly with the linear dimensions of 

 the stomata, and the relative rate of carbon dioxide-absorption during 

 photosynthesis will be proportional to the number of stomata per equal 

 area as well as to the degree to which the stomata are opened. In view 

 of the fact that Brown and Escombe in their exi>eriments illuminated the 

 upper surface of the leaves, it is not surprising that they should have found 

 an apparent excess of photosynthesis on this surface. The one sided 

 illumination in all probability brought about the partial oi>ening of the 

 stomata on the illuminated surface and, moreover, the light active in photo- 

 synthesis would be largely absorbed by the chloroplasts of the parenchyma 

 into which the stomata of the illuminated surface open. This would 

 result in a steeper diffusion gradient between the atmosphere and the 



