The Absorption of Carbon Dioxide by Unilluminated Leaves.

In the process of photosynthesis as carried out by the higher plants, the leaf absorbs carbon dioxide from the air and transforms it into organic matter. This assimilation of carbon dioxide is accomplished through a number of intermediate steps and anly complete analysis of the mechanism of this process requires that the carboni be traced through these consecutive steps. Obviously the first step is the absorption of carbon dioxide from the air. The critical experiments of BLACKMAN (4) demonstrated that the carboni dioxide of the air enters the leaf throuoh the stomata. At first this view was difficult to accept because it seemed impossible that the volume of carbon dioxide necessary to maintain active photosynthesis could diffuse through such a small area. The stomatal area in the sunflower leaf is only 3.75 per cent. of the total. This difficulty was removed, however, when BROWN and ESCOMBE (5, 6) showed that the diffusion of a gas through a perforated septum is almost unobstructed if the apertures have the proper dimensions and distribution. Examination showed that the required coniditions are fulfilled by the stomata of the sunflower leaf, and that the amouint of carbon dioxide necessary to maintain the maximum rate of photosynthesis so far observed can easily diffuse into the leaf provided "the interior of the leaf were a perfect 'sinlk' of atmospheric carbon dioxide." These small openings have the power of "drinking in atmospheric carbon dioxide" about fifty times faster than apertures of like dimension-s filled with "a conistantly renewed solution of alkali hydroxide." This exceptional ability to "drink in" carboni dioxide led WILLSTXTTER and STOLL (33a) to search for the carboni dioxide absorptive agent in leaves. Their researches (33b) showed that some leaves, particularly sunflower and nettle leaves, even when unilluminated, absorb reversibly considerable quantities of carbon dioxide. This absorption is not a function of the life process of the leaves. Probably the green pigments of the leaf have nothing to do with this reaction because yellow varieties of certain species absorb carbon dioxide as well as tlle green varieties. WILLSTXTTER and STOLL (33c) isolated no substances from leaves wlhich could accounit for the carbon dioxide combining capacity. They suggested that alkali and alkaline earth bicarbonates, especially magnlesium bicarboiiate, might be the source of the 1X:3