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Stomata of Zea mays L. respond to changes in hydrostatic pressure in the water supply of the leaves almost instantaneously and in all leaf parts simultaneously. Therefore, the leaf is a hydraulic unit. The stomata are part of it and their aperture is controlled by the water potential in the water-conducting system. Stomatal aperture is not uniquely related to the relative water content of a leaf. The relation depends also on the humidity in the air and is different for the upper and the lower epidermis.
Klaus Raschke (1970) studied this question.