The excitability of the main pulvinus of _Mimosa pudica_ I find does not remain constant during the 24 hours, but undergoes a striking periodic change. At certain hours of the day, the excitability is at its maximum; at a different period it practically disappears. The period of insensibility is about 7 A.M., which, strangely enough, is also the time when the palm tree attains its maximum height. At about 3 in the afternoon the excitability of _Mimosa_ reaches its climax, and this is the time when the head of the palm tree bends down to its lowest position. For the determination of the periodic variation of excitability of _Mimosa_ I devised a special apparatus by which an electric stimulus of constant intensity was automatically applied to the plant every hour of the day and night, the responsive moment being recorded at the same time. The amplitude of responsive fall of leaf under uniform stimulus gave a measure of excitability of the leaf at any particular moment. In the lower curve of Fig. 5 is given the record of diurnal variation of excitability of _Mimosa_. Comparison of this figure with Figs. 3 and 4, will show the remarkable resemblance between the curves of diurnal movement of the Palm tree, and of diurnal variation of moto-excitability of _Mimosa_. The excitability of _Mimosa_ reached its maximum at about 3 in the afternoon, when the Palm was at its lowest position. After this hour excitability fell continuously till 7 or 8 next morning. Corresponding to this is the continuous erection of the Palm from its lowest position at 3 P.M. to the highest between 7 and 8 A.M. Still more remarkable is the modifying influence of variation of temperature on the diurnal curve of excitability in _Mimosa_, and the diurnal curve of movement of the Palm. This will be quite evident from the inspection of the temperature curves in Figs. 4 and 5. [Illustration: Fig. 5. Curve of variation of moto-excitability of _Mimosa pudica_. The upper curve gives variation of temperature and the lower, the corresponding variation of excitability.] I have shown elsewhere[B] that the variation of moto-excitability of the pulvinus of _Mimosa_ is a physiological function of temperature. The remarkable similarity between the diurnal variation of moto-excitability of _Mimosa_ and diurnal movement of the Palm is due to the fact that both are determined by the physiological action of temperature. I shall presently describe experiments, which will establish the physiological character of the movement of the tree in response to changes of temperature. [B] Bose--“Irritability of Plants,” d. 60. The records that have been given show that it is the diurnal variation of temperature, and not of light that is effective in inducing the periodic movement of the tree. Further experiments will be given in support of this conclusion. RELATIVE EFFECTS OF LIGHT AND TEMPERATURE.