_Torsional response in a- and b-positions: Experiment 193._--When the leaf is in _a-position_, the geotropic torsion is found to be with the movement of the hands of a clock. In the _b-position_, on the other hand, the torsion is against the hands of a clock. In both these cases the _geotropic torsion makes the less excitable upper half of the pulvinus face the vertical lines of gravity_. The incident stimulus is vertical, and it is the upper flank, consisting of the upper and lower halves of the pulvinus (on which the vertical lines of gravity impinge) that undergoes effective stimulation. _Algebraical summation of geotropic and phototropic effects: Experiment 194._--We are, however, able to adduce further tests in confirmation of the above. If the direction of the incident geotropic stimulus is vertical, and should it act more effectively on the upper flank, it follows that stimulus of light acting from above would enhance the previous torsional response due to geotropism. In the above case, the lines of gravity and the rays of light coincide. The effect of rays of light acting from below should, on the other hand, oppose the geotropic torsion. The additive effect of stimulus of light and gravity is seen illustrated in figure 180. The first part of the curve is the record of pure geotropic torsional movement. Light from above is applied at L; the rate of movement is seen to become greatly enhanced. Light is next cut off, and the enhanced rate induced by it is also found to disappear, the response-curve being now due solely to geotropic action. The effect of geotropism in opposition to phototropism will be found in the following experiments, where the opposing action of light of different intensities is seen to give rise to a partial, to an exact, or to an over-balance. [Illustration: FIG. 180.--Additive effect of stimulus of gravity G, and of light L. Application of light at--L increases torsional response. Removal of light restores original geotropic torsion.] [Illustration: FIG. 181.--Algebraical summation of geotropic and phototropic actions. Light applied below at--L, opposes geotropic action. Cessation of light restores geotropic torsion. Cessation of light is indicated by L within a circle.] BALANCE OF GEOTROPIC BY PHOTOTROPIC ACTION.