1 8 THE SYNTHESIS OF CARBOHYDRATES 



main route, but the work of F. F. Blackman * and of Brown 

 and Escombe f has shown that the stomates are the important 

 paths. F. F. Blackman found by direct measurement that the 

 degree of gaseous interchange was proportionate to the distri- 

 bution of the stomates, results which Brown and Escombe con- 

 firmed in respect to plants with stomates on but one surface 

 of the leaf, but with regard to instances in which stomates 

 occur on both surfaces of the leaf, they found that in bright 

 sunlight the intake of carbon dioxide into the upper surface 

 is greater than would be expected from the ratio of distribu- 

 tion of the stomates on the two sides ; in light of a lesser 

 intensity, however, there is a closer, but not very close, corre- 

 spondence between the intake of carbon dioxide and the pro- 

 portional distribution of the stomates. It is suggested J that 

 the greater infusion found to obtain into the upper side of 

 amphistomatous leaves may be accounted for in that partial 

 opening of the stomates is likely when the incidence of illumi- 

 nation is on that side, and that since the palisade parenchyma 

 is the more active part of the mesophyll, there will be a steeper 

 diffusion gradient in the upper side which will promote a more 

 rapid flow of carbon dioxide through the stomates of the upper 

 surface. 



The movements of the carbon dioxide are in accordance 

 with the laws of gaseous diffusion; the pressure of carbon 

 dioxide in the active chlorenchyma will be very low, whilst in 

 the atmosphere surrounding the leaf it will correspond to, say, 

 three parts in 1 0,000. Thus there are set up diffusion cur- 

 rents the gradients of which vary according to the conditions, 

 rate of use and degree of atmospheric motion for example, 

 obtaining. The problem of interchange between the gases 

 contained in the leaf and in the surrounding atmosphere is 

 not, however, so simple as may appear from this statement. 

 Brown and Escombe, experimenting with leaves of Catalpa 

 bignonioides^ found that the rate of absorption of carbon 

 dioxide at normal temperature and pressure was about 0*07 

 c.c. per sq. cm. per hour; since the total area of the stomates 

 was but 0*9 per cent of the total leaf surface, it follows that 



* Blackman : " Phil. Trans. Roy. Soc.," Lond., B. 1895, 186, 485, 503. 



f Brown and Escombe : Id., 1900, 193, 223. 



II bid. : " Proc. Roy. Soc.," Lond., B. 1905, 76, 29. 



