Page:Life Movements in Plants.djvu/47

Rh 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 to the highest between 7 and 8  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.

I have shown elsewhere 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.

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