ACTION OF STIMULUS OF LIGHT. _Experiment 148._--The pulvinus of the leaf was stimulated by a horizontal beam of light thrown in a lateral direction; the areas contiguous to line of junction of the upper and lower halves of the anisotropic organ thus underwent differential excitation. When light struck on the left flank, the responsive torsion was anti-clockwise; the responsive reaction thus made _the upper and the less excitable half of the pulvinus face the stimulus_. Figure 149 gives a record of the torsional movement when light struck the left flank of the organ; on the cessation of stimulus the response is followed by recovery. [Illustration: FIG. 149.--Record of torsional response of pulvinus of _Mimosa pudica_.] DIRECTIVE ACTION OF STIMULUS. _Experiment 149._--If now the direction of stimulus be changed so that light strikes on the right flank instead of the left, the responsive torsion is found to be reversed, the direction of movement being clockwise. Here also the responsive movement is such that it is the less excitable upper half of the organ that is made to face the stimulus. It will thus be seen that the torsion, anti-clockwise or clockwise, depends on two factors, namely the direction of stimulus, and the differential excitability of the organ. EFFECT OF DIFFERENT MODES OF LATERAL STIMULATION. I shall now proceed to show that the torsional response is induced not merely by the action of light, but by all forms of stimulation. _Effect of chemical stimulation: Experiment 150._--Dilute hydrochloric acid was at first applied on the left flank of the pulvinus along the narrow strip of junction of the upper and lower halves. This gave rise to a responsive torsion against the hands of a clock. Chemical stimulation of the right flank induced, on the other hand, a torsional movement with the hands of a clock. Here also the direction of stimulus is found to determine the direction of responsive torsion. _Effect of thermal radiation: Experiment 151._--I next employed thermal radiation as the stimulus; the source of radiation was a length of electrically heated platinum wire. It is advisable to interpose a narrow horizontal slit, so as to localise the stimulus at the junction of the upper and lower halves of the pulvinus. Stimulus applied at the left flank induced left-handed or anti-clockwise torsion; application at the right flank gave rise to right-handed torsion. _Geotropic stimulus._--The stimulus of gravity induces, as I shall show in a subsequent chapter, a similar responsive torsion, the direction of which is determined by the direction of the incident stimulus. EFFECT OF DIFFERENTIAL EXCITABILITY ON THE DIRECTION OF TORSION.