CARBON DIOXIDE 17 



distribution of the stomates, results which Brown and Escombe 

 confirmed in respect to plants with stomates on but one sur- 

 face of the leaf, but in plants 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 distribution of the sto- 

 mates on the two sides ; in light of a lesser intensity, however, 

 there is a closer, but not very close, correspondence between 

 the intake of carbon dioxide and the proportional distribution 

 of the stomates. It is suggested * 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 illumination 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 10,000. Thus there are set up diffusion 

 currents the gradients of which vary according to the con- 

 ditions, rate of use and degree of atmospheric motion. 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 Es- 

 combe, 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 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 



* Brown and Escombe : " Proc. Roy. Soc," B, 1905, 76, 29. 

 VOL. II. — 2 



