IS THE SYNTHESIS OF CARBOHYDRATES 



is hardly surprising that earlier physiologists 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 owing to a falling in the partial pressure of carbon dioxide 

 from the " respiratory chamber " of the stomatal apparatus 

 to the chlorenchyma. 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, p, 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 absorp- 

 tion where the pressure of carbon dioxide is, ideally, zero. 

 The interposition of the stomata exercises a profound influence 

 on this diffusion. Brown and Escombe * demonstrated that 

 the rate of diffusion through such an absorbing disc as is 

 represented by a stomate is proportional to the diameter of 

 the opening ; f they visualize zones of equal density above the 



* Loc. cit. 



f Larmor's Law of Diameters : — Q = 2kpT> where Q = amount of 

 C0 2 absorbed in a given time ; k = coefficient of COj, in air ; /) = density 

 of atmospheric C0 2 at a point far removed from the absorbing disc ; D ■= 

 diameter of disc. 



