I have shown that the pulvinus of _Mimosa_, subjected laterally to the action of stimulus of gravity, exhibits a torsional response. When the _Mimosa_ plant is laid sideways, so that the plane of separation of the upper and lower halves of the pulvinus is vertical, geotropic stimulus acts laterally on the two halves of the differentially excitable pulvinus. When the less excitable upper half is to the left of the observer (see Fig. 179), the responsive torsion under geotropic stimulus will be clock-wise, the less excitable upper half of the pulvinus being thereby made to face the vertical lines of gravity. When the plant is turned over to the other side (the less excitable upper half being now to the right of the observer) the induced torsion will be counter clock-wise. The response is therefore determined by the directive action of stimulus of gravity. Light has also been shown to give rise to torsion (p. 400). Light acting in the same direction as the stimulus of gravity, _i.e._, from above, enhances the rate of torsion, the curve of response being due to the joint effects of light and gravity. [Illustration: FIG. 211.--Record of diurnal variation of torsion in _Mimosa_ leaf. Up-curve represents increase and down-curve decrease of geotropic torsion.] _Experiment 221._--I obtained 24 hours' record of variation of torsional response of _Mimosa_, commencing with thermal-noon at 2 p.m. It is to be borne in mind that increase of torsion indicates increase of geotropic action, just as the erectile movement of the leaf in the normal position indicates the enhanced geotropic effect. Inspection of figure 211 shows that the fall of temperature after thermal-noon was attended by increase of torsion. The curve went up till about 5 p.m., as in the ordinary record of _Mimosa_. The torsion suddenly decreased with the rapid diminution of light after 5 p.m. The torsion then increased with falling temperature from 9 p.m. till thermal-dawn next morning. After 6 a.m. there is a continuous diminution of torsion till 5 p.m. We may now summarise the diurnal variation of torsion exhibited by _Mimosa_. The torsion undergoes a periodic increase during the fall of temperature from afternoon till next morning, and a diminution during rising temperature from morning till afternoon. A sudden diminution of torsion occurs at about 5 p.m. due to the disappearance of light. The torsional record is, to all intents and purposes, a replica of the record of periodic up and down movements of the leaf.