THE PERMEABILITY OF NON-STOMATE LEAF EPIDERMIS TO CARBON DIOXIDE

It has been generally assumed that the exchange of gases between the intercellular spaces of leaves and the external atmosphere occurs to an appreciable extent only when the stomates of the leaves are open. However, attempts to correlate the observed rate of CO2 absorption either with the number of stomates per unit area or with stomate area have met with only limited success. VERDUIN (16) believed that the lack of correlation between diffusion rates and degree of stomate aperture was attributable to the interference of diffusion shells. MITCHELL (8) has reported that stomates on leaves of Cineraria apparently were closed before wilting occurred, yet these plants in some instances absorbed 70% as much CO2 as the control plants. In a study of the effect of soil moisture on photosynthesis of apple leaves, SCHNEIDER and CHILDERS (12) found on several occasions high rates of photosynthesis when the stomates appeared to be completely closed. VERDUIN and LooMIs (15) found that wilted maize plants absorbed less CO2 than turgid plants. However, the decrease in CO2 absorption was not proportional to the decrease in stomate size as measured by a porometer. The average absorption of CO2 by wilted leaves was 37%o of the controls. Considerable variation in the absorption of CO2 through the epidermal cells of leaves during photosynthesis was observed by FREELAND (4). In all of his experiments, Freeland found that the rate of apparent photosynthesis was greater when CO2 entered through the stomate epidermis than when it entered through the non-stomate epidermis; and it appears from Freeland's work that the amount of CO2 entering through the epidermal cells is a function of the thickness of the cuticle. In plants with thin cuticle the amnount of CO2 that entered the non-stomate surface was approximately equal to the amount which diffused through the surface with stomates. In plants with thick cuticle little or no apparent photosynthesis could be detected when CO2 contacted only the non-stomate epidermis. Cutin is not an impervious barrier. Water loss is known to take place through the cutinized epidermal walls of leaves and stems. Furthermore, CO2 is known to be soluble in cutin so it seems very possible that molecules of the gas may penetrate the cutinized wall of the epidermal cells and reach the internal cells of the leaf. ROBERTS et al. (11) used microchemical methods to show that the cutinized epidermal layer of McIntosh apple leaves contains a large amount of pectinaceous material. This material appears in the outer walls of epidermal cells as parallel and vertical layers, whereas