Leaves can move in response to a biological clock
Plants are able to respond to changes in time. Among the more obvious of the clock-like responses are "sleep movements" such as the closing of flowers and changes in leaf position that many plants display at night. This short movie shows the movement of bean leaves over a 48 hr period but note that the movie will replay itself unless you pause it. The plant was grown under a 12 hr photoperiod for a week so that it was entrained to the 12 hr diurnal cycle. The plant was then moved into continuous dim light juat after dawn so the movie starts shortly after the plant "woke up". The circadian (biological) clock that controls the response persists even in the absence of a daily change in environmental light cues. The rhythm can continue for several days in the absence of changes in light but the leaf movements eventually dampen and become less obvious. After several days the time between peaks (period) of movement increases slightly.