ABSORPTION OF CARBON DIOXIDE 19 



carbon dioxide must pass through the openings at the rate of 

 777 c.c. per sq. cm. per hour, an amount so considerable 

 when regard is had to the stomatal area and to the fact that 

 this rate of absorption is about fifty times greater than the 

 absorption of atmospheric carbon dioxide by a normal solution 

 of caustic potash, that it is hardly surprising that earlier physi- 

 ologists laid much stress on cuticular gaseous interchange. 



The conditions obtaining in an active green leaf are briefly 

 these : the active chlorenchyma is absorbing carbon dioxide, 

 which must enter the cell in a state of solution, from the air- 

 space system on which it abuts ; hence the pressure of carbon 

 dioxide in the immediate neighbourhood of these surfaces of 

 absorption will have a very low value, possibly approximating 

 to zero under ideal conditions. Diffusion currents are thus 

 set up, a falling gradient of carbon dioxide density from the 

 " respiratory chamber " of the stomatal apparatus obtaining. 

 Renewal of carbon dioxide is from the external air through 

 the pore of the stomate, a cylinder of a certain length and, 

 in the simplest cases, of approximately uniform diameter 

 when the guard cells are fully turgid. Clearly various tensions 

 of carbon dioxide occur : the maximum in the atmosphere, 

 theoretically at an infinite distance from the leaf but practically 

 at a distance equivalent to five or six times the diameter of 

 the stomate, and the minimum at the absorbing surfaces of 

 chlorenchyma ; thus a gradient of density of carbon dioxide 

 is formed so that a drift of this gas from the outer atmosphere 

 to the chlorenchyma obtains. The path followed by the 

 carbon dioxide is obstructed by the stomates and may be 

 divided into three sections : from the remote atmosphere 

 where the pressure of carbon dioxide is greatest, /o, to the 

 outer opening of the stomate; through the shaft of the 

 stomate when the pressure of carbon dioxide is less, p ; and 

 from the inner opening of the stomatal shaft to the surfaces 

 of absorption where the pressure of carbon dioxide is, ideally, 

 zero. The interposition of the stomata exercises a pro- 

 found influence on this diffusion. Brown and Escombe* 

 demonstrated that the rate of diffusion through such an ab- 

 sorbing disc as is represented by a stomate is proportional to 



* Loc. cit. 



