Photosynthesis in Relation to Stomatal Frequency and Distribution.

The concept that stomates play an all-important role in controlling the diffusion of carbon dioxide into leaves during photosynthesis seems to be quite generally accepted by teachers and authors of botany texts. In a plant physiology text by MAXIMOV (6) may be found the following, "That the carbon dioxide enters the leaf mainly through the stomates may be shown by a simple experiment. If on a certain portion of the leaf the stomata are coated with vaseline, and the leaf is then exposed to light and afterward treated with iodine, the blue color reaction will be observed only in those portions where the stomata remained open." Also in the text by MEYER and ANDERSON (7) one finds "Critical experiments have shown, however, that the proportion of this gas (CO2) entering the leaves by this route (directly through epidermis) is relatively small, and that practically all of the carbon dioxide entering leaves diffuses in through the stomates." Similar statements may be found in many other botanical books. It seems that this concept stems mainly from the research of Blackman and his contemporaries. BLACKMAN (1) in a review of the research regarding this problem concludes that "Under normal conditions, practically the sole pathway for carbon dioxide into or out of the leaf is by the stomata." Further support and impetus were given to this idea by the classical experiments of BROWN and ESCOMBE (2) which demonstrated and partially explained the enormous diffusive capacity of small pores and stomates. More recently MASKELL (5) working with Prunus lauro-cerasus and NUTMAN (9) studying Coffea arabica have reported that photosynthesis in these plants is directly related to stomatal movement. On the other hand considerable evidence has appeared in the literature which indicates that the rate of diffusion of carbon dioxide through epidermal cells of leaves may be quite appreciable and should not be minimized or ignored. Blackman mentions that Boussigault and Barthelmv reported data to the effect that the carbon dioxide exchange during "assimilation" was independent of stomatal distribution. MITCHELL (8) found that the leaves of tomato and Pelargonium absorbed carbon dioxide and accumulated carbohydrates in appreciable quantities although the stomates appeared to be closed. Furthermore, the amount of carbon dioxide absorbed by the leaves in which the stomates appeared to be closed was approximately equal to the amount absorbed by the same leaves when