I have shown (Fig. 130) the relation between the stimulus and the resulting excitation, the latter being determined from the diminution of the rate of growth. Under unilateral action of light, the excitatory contraction gives rise to tropic curvature. We may thus obtain the characteristic excitation curve, by making the plant organ record its tropic movement under continuous action of light applied on one side of the organ. [Illustration: FIG. 131.--Characteristic curve of iron under increasing magnetising force. (After Ewing).] [Illustration: FIG. 132.--Simple characteristic curve of phototropic reaction. Excitation increases slowly in the first part and rapidly in the second; it is uniform in the third, and undergoes decline in the fourth part (_Erythrina indica_).] _Experiment 134._--I give below the characteristic curve of excitation (Fig. 132) of the pulvinus of _Erythrina indica_, traced by the plant itself, and exactly reproduced by photomechanical process. A parallel beam of light from a Nernst lamp was thrown on the upper leaf of the pulvinus, and the increasing positive curvature was recorded on a smoked glass plate which was moved at an uniform rate. The successive dots are at intervals of 20 seconds; the horizontal distances between successive dots are equal, and represent equal increments of stimulus; the vertical distances between successive dots represent the corresponding increments of excitation. The gradient at any point of the curve--increment of excitation divided by increment of stimulus--gives the susceptibility for excitation at that point. The following table will show how the susceptibility for excitation undergoes variation through the entire range of stimulus. The average susceptibility for each point has been calculated from the data furnished by the curve. TABLE XXX.--SHOWING THE VARIATION OF SUSCEPTIBILITY FOR EXCITATION AT DIFFERENT POINTS OF THE TROPIC CURVE.