While trying to obtain some clue to the mysterious movement of the tree, my attention was strongly attracted by certain striking similarities which the record of the movement of the tree showed to the curve of the diurnal variation of moto-excitability, of the pulvinus of Mimosa pudica, an account of which will be found in a subsequent Paper of the series.[A]
PERIODIC MOVEMENT OF TREES AND DIURNAL VARIATION OF MOTO-EXCITABILITY IN MIMOSA PUDICA.
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.
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.
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.
As regards the possibility of light exerting any marked influence on the movement of the Palm tree, I have shown from study of time-relations of the movement, that this could not be the case. Moreover, it is impossible for light to reach the living tissue through the thick layer of bark that surrounds the tree. That the effect of light is negligible will appear from the accounts of following experiments, where the possibility of the effect of changing intensity of light is excluded by maintaining the plant in constant darkness, or in constant light.