of ten minutes, the geotropic torsion underwent a continuous diminution till a new state of equilibrium was reached at 34°C. This took place shortly after the stoppage of the heating current at (H). On return to normal temperature the torsional balance was restored to its original position of equilibrium. Application of cold at C, is seen to bring about a new state of balance with an increase of geotropic torsion. [Illustration: FIG. 186.--Effect of variation of temperature on geotropic torsion. Application of warmth at H diminishes the geotropic torsion; return to normal temperature (H) restores the original torsion; cooling at C, increases the geotropic torsion.] The position of geotropic equilibrium is thus seen to be modified by variation of temperature, the tropic effect being diminished with the rise, and enhanced with the fall of temperature. It may be thought that the phenomenon just described may not be different from ordinary thermonasty, exhibited by the perianth leaves of _Crocus_ and _Tulip_ in which a rise of temperature induces a movement of unfolding, and a fall of temperature brings about the opposite movement of closure. In these cases the movement is determined solely by the natural anisotropy of the organ, and not by the paratonic action of a directive external force. Thus the inner side of the perianth leaves undergoes an expansion with rise of temperature attended by the opening of the flower; this movement of opening does not undergo any change on holding the flower in an inverted position. But the torsional movement of the leaf of _Mimosa_, and the induced variation of torsion under change of temperature are not solely determined by the natural anisotropy of the organ; it is, on the contrary, regulated by the directive action of the stimulus of gravity. The pulvinus in normal position does not exhibit any geotropic torsion and in the absence of an antecedent torsion change of temperature cannot induce any variation in it. It is only after the pulvinus had become torsioned under the lateral action of geotropic stimulus that a responsive variation is induced in it by the action of changing temperature. The change in torsion is, moreover, determined in reference to the paratonic action of incident geotropic stimulus. This will be clearly understood from the tabular statement given below. TABLE XLVI.--SHOWING THE EFFECT OF RISE OF TEMPERATURE ON GEOTROPIC TORSION.