The diffusion porometer

Abstract

-A diffusion porometer apparatus is described by means of which changes in the diffusive capacity of stomata can be measured directly and continuously. The H diffusing through the leaf from the porometer cup was constantly replaced by electrolytic H, so that no changes in pressure occurred, and the rate of diffusion could be measured by the rate of supply of H, i.e., by measuring the current used in the electrolysis. The stomata of Pelargonium leaves attached to the plant did not close in the dark in H. Fuchsia stomata closed under similar conditions whether the leaves were attached to the plant or detached. The rate of outward diffusion of H from the porometer was independent of the relation between diffusing area and mass of H, and directly proportional to the concn. gradient of H, thus proving that it is the diffusion rate that is being measured. Records are given of the variation in H diffusion with varying stomatal aperture, and the relation between mass flow of air and diffusion of H for given stomatal apertures was detd. by using on the same leaf the diffusion and resistance porometers. The flow resistance/diffusive capacity curve is a hyperbola, the flow relative to the diffusion increasing rapidly for small stomatal apertures, while the reverse holds for wide open stomata.