20 THE SYNTHESIS OF CARBOHYDRATES 



the diameter of the opening ; * they visualize zones of equal 

 density above the stomate varying from the atmospheric 

 density, /?, to a lower density, p', at the stomate cut perpen- 

 dicularly by the lines of flow of carbon dioxide converging 

 to the opening of the stomate f (Fig. i). Thus it is that the 

 increased flow of gas through the stomate is possible. 



The second section in the route of the carbon dioxide is 

 through the tube formed by the guard cells. Through this 

 tube the flow is inversely proportional to the length of the 

 tube, but the system of external shells increases the resistance 

 to the flow (Fig. 2). Finally, in the third section the tube 



FIG. 2. 



opens into the air space system of the mesophyll bounded by 

 the absorbing surfaces of the chlorenchyma ; here the con- 

 verse of the first part of the path obtains, diffusion shells over 

 the lower opening of the stomate, where the density of the 

 carbon dioxide is p' y being formed (Fig. 3). Thus in the 

 whole system there is a gradient of density from p to, say, o, 

 with a set of shells at either end of the stomatal tube (Fig. 4). 

 The obstruction to gaseous diffusion inseparable from a 

 multiperforate septum such as the stomatal epidermis of a 

 leaf, varies according to the distance apart of the perforations : 

 if they are placed at distances roughly equal to ten times the 



* Larmor's Law of Diameters : Q = zkpD where Q = amount of CO 2 

 absorbed in a given time ; k = coefficient of CO 2 in air ; p = density of at- 

 mospheric CO 2 at a point far removed from the absorbing disc ; D = diameter 

 of disc. 



fQ = 2k(p-p')D. 



