The response by a plant protoplast to an inducing or to an inhibitory stimulus is preceded by a period of non-responsiveness, and the withdrawal of the stimulus is succeeded by a period of continued response or inhibition as the case may be. During these intervals, the familiar latent period and period of after-effect, or temps de memoir of Massart, the protoplast manifests the property of functional inertia (Harris, Brit. Med. Assoc., 1900), and in the case of the inducing stimulus the two phases follow in the order of (1) the Anabolic and (2) the Katabolic, while in inhibition the order is reversed. This property is exhibited by the protoplasm of growing and of adult organs, as well as by that of isolated and excised organs; further, it varies in amount and appears under different aspects.