The leaves of the so-called "Compass plants" exhibit very complex movements, these being modified according to the intensity of incident light. Thus in compass plants the leaves, under moderate intensity of light in the morning or in the evening, turn themselves so as to expose their surfaces to the incident rays. But under intense sun light, the leaves perform bendings and twistings so that they stand at profile at midday. I have not yet been able to secure "Compass plants" at Calcutta. I shall, however, describe my investigations on the complicated torsional movements exhibited by certain leaflets by the action of vertical light. The results obtained from these will show that torsional movements, even the most complex, are capable of explanation from the general laws that have been established. _Torsional movement of leaflet of_ Cassia alata: _Experiment 152._--These leaflets are closed laterally at night but place themselves in an outspread position at day time. The character of the movement is, however, modified by the intensity of light. With moderate light in the morning the leaflets open out laterally. But under more intense light, the pulvinules of the leaflets exhibit a torsion by which the formerly infolded surfaces of the leaflets are exposed at right angles to light from above (Fig. 150). Such complicated movements, in two directions of space, are also exhibited by other leaflets which are closed at night in a lateral direction. [Illustration: FIG. 150.--Leaflets of _Cassia alata_: open in daytime, and closed in evening.] For obtaining an explanation of these complex movements under different intensities of light, we have first to discover the particular disposition of the two halves of the pulvinule which are unequally excitable; we have next to explain the responsive movements under the directive action of moderate and of intense light. _Determination of differential excitabilities of the organ: Experiment 153._--In the leaflet of _Cassia_ the movement of opening under diffuse stimulation of light can only be brought about by the contraction of the outer half, which must therefore be the more excitable. This is independently demonstrated by the reaction to an electric-shock. On subjecting the half closed leaflets to diffuse electric stimulation, they open outwards in a _lateral_ direction. The disposition of the unequally excitable halves of the pulvinule is thus different from that of the main pulvinus of _Mimosa_. In the latter, the plane that divides the two halves is horizontal, the lower half being the more excitable. Thus in the pulvinule of _Cassia_ the plane that separates the two unequally excitable halves is vertical, the outer half being the more excitable than the inner. By inner half is here meant that half which is inside when the leaflets are closed.