Page:Life Movements in Plants.djvu/45

Rh record of movement of the tree is found to follow these fluctuations with astonishing fidelity, the rise of temperature being followed by a fall of the tree and vice versâ. That the movement is determined by the temperature variation is exhibited in a striking manner in Fig. 4, where, between 8 and 9, a common twitch will be noticed in the two curves.

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.

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, 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