Quantitative studies on the wetting of leaves by water

Abstract Quantitative studies on the wetting by water of the exterior surfaces of leaves of Sinapis arvensis, Triticum vulgare, and other plants, have been made, using advancing contact angle as a measure of the extent to which wetting takes place. The magnitude of the contact angle of water on a leaf surface has been found to vary regularly with the position of the leaf on the plant and to show also a characteristic diurnal fluctuation through a range which may be as much as 30°. The diurnal variations in contact angle are shown to be correlated with changes in leaf water content. On detached leaves contact angle varies markedly as wilting proceeds, this change being reversible on recovery of turgor. Evidence is presented to show that these variations in the behaviour of water on leaves are caused by changes in the degree of corrugation of the leaf surface produced by changes in the water content of the tissues. The events in the diurnal cycle are explained on this basis. The significance of the observed phenomena in connexion with the retention of water by leaves, the exchange of water and dissolved substances between leaf and water, and stomatal behaviour, is discussed.