Experimental Studies of the Factors Controlling Transpiration

Abstract

Data are presented which show, when stomatal control is eliminated, that wheat leaves may lose 5–6 per cent. and Pelargonium leaves 10–12 per cent. of their water without any reduction in the transpiration rate. Experiments in which Pelargonium and wheat leaves, with stomatal control present, were submitted to cycles of changing water content also failed to establish any direct relation between transpiration rate and leaf water content. It is concluded that leaf water content over the range of 70–100 per cent. of that present in the turgid state has no significant effect in determining the rate of water loss from leaves. A repetition of Knight's experiment showed that stomata opened in still air and closed in moving air. This was not recorded by Knight, who used a porometer cup permanently attached to the leaf. It is concluded that the higher transpiration rate recorded by Knight after a period of still air was due to wider stomatal aperture and not to the higher leaf water content as suggested by him.