Plants of Vicia faba and Hordeum vulgare were grown in growth boxes with 7 mW cm−2 PAR, 14 h day/10 h night, at 22/20 °C. Stomata of attached leaves were measured under controlled conditions by means of an optical microscope and the distribution functions of the widths of pores were established. For Vicia faba they appeared to be symmetrical bell-shaped functions. In the process of stomatal opening or closure the shape of the distribution remained constant, its maximum sliding left and right along the aperture axis. This result has been interpreted to mean that increments or decrements of apertures were equal for all stomata independent of their individual apertures. The conclusion has been drawn that the ‘driving force’ is evenly distributed, equal for all stomata, and varies within wider limits than is possible for stomatal apertures. Stomatal opening is limited by the closed state from below and by an anatomically possible maximum aperture from above.