The diffusion porometer

Abstract

Various devices have from time to time been described for investigating the movement of stomata. These depend for their action on the rate of flow of air under uniform pressure drawn or pushed through the stomata; the measure of stomatal aperture is thus some function of the rate of flow. This principle was first used by Dutrochet (1832), and by Müller, in 1872, and an apparatus, using a cup attached to a leaf, was constructed by Darwin and Pertz in 1912. Self-recording porometers on the principle of Darwin’s porometer were perfected by Balls (1912), Knight (1915, 1922), Laidlaw and Knight (1916), and Neilson-Jones (1914). For use with amphistomatous leaves a double porometer has been described by Leick (1928). All the instruments mentioned measure the rate of flow of the gas. Recently a porometer has been described by Gregory and Pearse (1934) in which the fall in pressure of the air on passing through the leaf is the basis of measurement and nearly instantaneous readings of stomatal aperture are thus possible. The conversion of these measures of stomatal aperture into terms of change of diffusive capacity presents a problem which has not yet been completely resolved by experimental methods. It would therefore appear that a method of direct estimation of the diffusive capacity of the stomata is very desirable. The present paper describes an apparatus which makes possible a direct and continuous measure of the change in the diffusive capacity of stomata during their opening and closing.