In the normal position of the plant, the leaf of _Mimosa_ assumes, under geotropic action, an equilibrium position which is approximately horizontal. I shall proceed to show that this position of equilibrium also undergoes appropriate variation under changing temperature, the leaf undergoing a fall during rise, and an erection during fall of temperature. I stated that the torsional response is one of the means of recording geotropic effect and its variations. In the ordinary position of the plant, the geotropic variation will be indicated by the responsive up or down movement of the leaf in a vertical plane. Taking the leaf of _Mimosa_, we have thus the means of studying the effect of variation of temperature by two independent means of inquiry, namely, by record of ordinary responsive movement in a vertical plane, and also by record of torsional response. The variation of temperature which induces these movements may be simultaneously recorded by means of a differential metallic thermometer. The Multiplex Recorder employed for this research consists of three recording levers. A photographic reproduction of the apparatus will be found in a subsequent chapter (see Fig. 190). The first lever is attached to the leaf of _Mimosa_ placed in the normal position; the second lever records the torsional response of _Mimosa_ leaf, the plant being placed on its side; the third lever attached to the differential metallic thermometer gives a continuous record of variation of temperature. [Illustration: FIG. 187.--Simultaneous record (_a_) of variation of temperature, (_b_) of up or down movement of leaf of _Mimosa_, and (_c_) of variation of torsion. Rise of temperature is attended by fall of leaf and diminution of torsion, fall of temperature inducing the opposite effect.]