The participation of two rhythms in the leaf movements of xanthium plants given various light-dark cycles.

We have shown Xanthium strumarium exhibit two distinct leaf movement rhythms with one occurring in continuous light and presumably related to an endogenous rhythm initiated by the "light-on" signal and the other occurring in continuous dark and presumably related to an endogenous rhythm initiated by the "light-off" signal. Characteristic of the light-on rhythm is a sudden and rapid downward movement of the leaf occurring about 16 hours after the light-on signal. Characteristic of the light-off rhythm is an immediate and sudden upward movement following the light-off signal. Under certain photoperiodic treatments, the two movements seem to be in conflict.The characteristic light-on downward movement was predominant in the light-dark 14: 10 and 16: 8 treatments while the light-off upward movement was predominant in light-dark 8: 16 and 10: 14 treatments. In the light-dark 12: 12 treatment, one movement was predominant in about half of the cases and vice versa. Thus leaf movements of X. strumarium plants given these light-dark treatments were found to fit a hypothesis based on the participation of both a light-on and a light-off rhythm. The over-all leaf movement rhythm may be controlled by what might be called two clocks. Both of these clocks may have essentially the same basic mechanism with one important distinction. One is rephased by the light-on signal and the other by the light-off signal. Furthermore, the leaf movements related to each clock are distinctive.The hypothesis of the participation of a light-on and a light-off rhythm was also found to be applicable to published data (M. Holdsworth, 1959. Effects of day length on the movements of pulvinate leaves. New Phytol. 58: 29-45). Interpretation of the Bauhinia leaf movements along this line is also presented.