_Post-maximum after-effect: Experiment 216._--In this light was continued till there was a complete neutralisation, the curve of response returning to zero line; to all outer seeming the responsive indication of the tissue is the same as before excitation. But stoppage of stimulus at _c_ causes an over-shooting at a rapid rate far _below_ the zero line; and it is after a considerable period that the curve returns to the zero line of equilibrium. The condition at post-maximum _c_ is thus one of dynamic equilibrium where two opposite activities, "A" and "D," balance each other; for had the condition of the 'neutralised' tissue been exactly the same when fresh, cessation of stimulus would have kept the galvanometric spot of light at the zero position. The electric investigation described above shows that the after-effect is modified by duration of stimulation, and that: (1) the after-effect of pre-maximum stimulation is the continuation of response in the original direction (upward, and away from zero line), followed by recovery, (2) the after-effect of the maximum is an electric recovery towards zero position, and (3) the after-effect of post-maximum stimulation is an over-shooting _downward_ below the zero line. TROPIC RESPONSE UNDER LIGHT AND ITS AFTER-EFFECT. I shall now describe the after-effect of light as seen in mechanical response, and the results will be found parallel to those given by the electric response. The specimen employed is the terminal leaflet of _Desmodium gyrans_, the pulvinus of which is very sensitive to light. Pulvinated organs, generally speaking, exhibit a diurnal variation of turgor in consequence of which the position of equilibrium of the leaf or leaflet undergoes a periodic change. But this equilibrium position of the organ remains fairly constant for nearly two hours about midday, the variation of temperature at this period being slight. We may therefore obtain the pure effect of light by carrying out the experiment at this period, and completing it within a short time to avoid complication arising from the autonomous variation of turgor. The period of experiment of the plant may be shortened by a choice of suitable intensity of light; a given tropic effect induced by prolonged feeble light may thus be obtained by short exposure to stronger light. The source of light for the following experiment was a 50 c.p. incandescent lamp. The intensity was increased to a suitable value by focussing light on the upper half of the pulvinus by means of a lens. The intensity was so adjusted that the maximum positive curvature was attained in the course of about 6 minutes, and complete neutralisation after an exposure of 17 minutes.