310 PHOTOSYNTHESIS 



tion of H2O : CO2 is 108 : 264 on the basis of the hypothetical equation ; 

 moreover, from what we know of the products of photosynthesis prob- 

 ably only a small amount is present in the form of hexoses with a molecu- 

 lar weight of 180 so that corrections would have to be applied for 

 disaccharides and starch if these are present. Undoubtedly, however, 

 carefully executed experiments on this problem would be a contribution 

 of considerable value to the whole problem of photosynthesis. 



6. The Absorption of Carbon Dioxide by the Unilluminated Leaf 



Through the researches of Brown and T^scombe there was made a 

 very considerable advance in our knowledge of the manner in which car- 

 bon dioxide gains access to the centers of photosynthetic activity in the 

 leaf. These investigations demonstrate that the absorption of atmospheric 

 carbon dioxide by an illuminated leaf proceeds at about one half the 

 rate which the same surface of the leaf would possess if it were covered 

 by a constantly renewed film of a solution of caustic alkali. Or if only 

 the area of the stomatal openings is considered, the absorption by the 

 leaf per unit area would be 43 to 64 times as fast as the absorption of a 

 unit area of a freely exposed solution of caustic alkali. The explanation 

 for this remarkable phenomenon as given by Brown and Escombe has 

 been discussed in Chapter 2. They dealt especially with the purely physi- 

 cal process of the diffusion of carbon dioxide through stomata. They 

 did not consider the agent or leaf constituents which effectuate the absorp- 

 tion of carbon dioxide. 



In considering the absorption of carbon dioxide by the leaf it must 

 be realized that the leaf surface cannot strictly speaking be comi>ared 

 to an equal area of a caustic alkali solution. The stomata open into inter- 

 cellular spaces in the parenchyma which possess a very much larger sur- 

 face than that of the leaf. This large'^ surface of cell membranes, im- 

 pregnated with water, must greatly aid in the absorption of carbon 

 dioxide. On the other hand, the leaf is normally exposed to partial 

 pressures of carbon dioxide which are very small and the diffusion of 

 dissolved carbon dioxide is so very slow as compared with its gaseous 

 diffusion, that it is exceedingly difficult to explain the relatively rapid 

 rate of the photosynthetic activity. At 0.03 volume per cent, 1,000 cc. 

 of water dissolves at 25° 0.033 cc. carbon dioxide. 1,000 grams of 

 Helianthus leaves, with an area of 3.3 square meters, contain 850 cc. 

 water and would dissolve only 0.027 cc. carbon dioxide under normal 

 conditions. During photosynthesis there is absorbed about 1,500 cc. car- 

 bon dioxide by these leaves per hour. There is much to favor the opinions, 

 variously expressed by Willstatter and Stoll, Warburg and others that 

 the first step in photosynthesis is not a simple splitting of carbon dioxide 

 under the influence of light, but that the carbon dioxide undergoes a 

 primary change through absorption by some constituent of the leaf. 



It has been known for some time that vegetable tissue is capable of 

 absorbing carbon dioxide in quantities considerably above that accounted 



