68 PHOTOSYNTHESIS 



intercellular spaces of the upper .surface, a condition which would favor 

 a more rapid absorption of carbon dioxide by the upper surface. 



Brown and Escombe ^* then made a very thorough study of the purely 

 physical process by which the carbon dioxide of the atmosphere enters 

 the leaf, and presented the results of their laborious investigations in a 

 masterful fashion. The difficulties in accepting simple diffusion as an 

 explanation of the way in which carbon dioxide gets into the leaf are: 

 (1) the relatively large amount of carbon dioxide absorbed by a leaf 

 during active photosynthesis, about 0.1 cc. CO2 per sq. cm. of leaf sur- 

 face per hour; (2) the low partial pressure of carbon dioxide in the 

 atmosphere, 0.031 per cent by volume; (3) the very small portion of the 

 surface of a leaf which represents stomatic openings, 1 to 3 per cent. 



Thus, Brown and Escombe state: "As a concrete example we will 

 take the case of a leaf with which we have done a considerable amount of 

 work, that of Catalpa bijnomoides, in which we have carefully determined 

 both the number of stomata (which here occur only on the lower sur- 

 face) and the area of the stomatal slits when fully opened. This leaf 

 when placed under favorable conditions for assimilation, can abstract 

 from ordinary air containing three parts COo per 10,000, about .07 cc. 

 of carbon dioxide (measured at 0° and 760 milHms. bar.) per sq. cm. of 

 leaf surface per hour. The stomatal slits when fully open have an area 

 of .0000618 square millim., and since there are 145 of them on each 

 square mm. of leaf, the area of the stomatal openings only represents 

 .9 per cent of the total surface of the leaf on which they occur. It 

 follows from this that if we regard the whole of the carbon dioxide as 

 entering the leaf through these openings, diffusion must take place through 



100 X .07 „ ^^ , ^T • 



them at the rate of = / .77 cc. per hour. Now it will be seen 



later on that the surface of a strong solution of caustic soda, when freely 

 exposed to moderately still air containing the normal amount of carbon 

 dioxide (three parts per lO.OCX) by volume), absorbs that gas at ordinary 

 temperatures at the rate of only about .120 cc. per sq. cm. per hour, 

 and when the rate of the air current passing over the surface is in- 

 creased the maximum absorption is found to be .177 cc. per sq. cm. per 

 hour. It follows, therefore, that the absorption of atmospheric carbon 

 dioxide by the whole of the under surface of an assimilating leaf like 

 that of the Catalpa, must proceed at about one-half the rate which the 

 same absorptive surface of leaf would possess if it were covered with a 

 constantly renewed film of a solution of caustic alkali; we may say, in 

 fact, that the coefficient of absorption of the leaf surface under these 

 conditions is about half that of the surface of the alkaline solution. If, 

 however, we assume that the absorption of carbon dioxide in the leaf 

 takes place only through the stomatal openings — which occupy at the 



"Brown and Escombe, Phil. Trans. Roy. Soc, B., 193, 223 (1900). Brown, 

 H. T., Jour. Chem. Soc, 113, 559 (1918). 



