The appliance here described was the result of an attempt to devise a self-recording modification of Mr. Francis Darwin’s “Porometer.” This modification was designated the “ Stomatograph,” since it auto matically records the changes which take place in the mean stomatal aperture of a given leaf, by measuring the velocity at which air under constant pressure can be passed through the tissues. Certain special mechanical difficulties had to be overcome in order to employ it for researches on stomatal movement in the open field, with particular regard to the cotton plant in Egypt.