The Intake of Carbon 209 



mirable device to permit rapid diffusion with a minimum 

 direct exposure of delicate cells. Uncutinized surfaces are 

 moist and may absorb CO 2 directly, but the epidermis is 

 usually cutinized, and therefore it is through the stomata 

 largely or entirely that a constant gaseous diffusion takes 

 place between the air spaces of the leaf and the external 

 atmosphere. The epidermis is an effective multiperforate 

 septum, which means that, with a difference of gradient 

 within and without, the relatively small stomatal areas 

 are far more efficient in diffusion than would be suggested 

 by their actual area. They are in fact sufficient to pro- 

 vide for the maximum diffusion of CO 2 which may take 

 place from a natural atmosphere into the plant. 



The CO 2 which enters the air chambers of the leaf is 

 rapidly absorbed by the moist cell-walls within. These 

 cell-walls absorb the carbon dioxid just as would any mem- 

 brane moistened with water. The above capacity for 

 absorption is so great that there is during photosynthesis 

 practically no tension of CO 2 in the air spaces. The carbon 

 dioxid in solution is presented by the cell-sap to the chloro- 

 plast, and there is, of course, continuous absorption and 

 migration through diffusion in solution, so long as photo- 

 synthetic action proceeds. The CO 2 absorbed does not 

 immigrate to any considerable distance before it is used. 

 This is easily demonstrated by the fact that in small 

 darkened areas no starch would be produced. It must be 

 transferred, in some leaves, however, as far as the upper 

 palisade layers, for in these there is usually abundant 

 starch-making. It is apparent that in general the sphere 

 of each stoma is more or less local. The intake of carbon 

 dioxid is greatest, usually, over the lower surface of the 



