Plants, like other living things, have biological clocks that allow them to respond to changes in time. Among the more obvious clock responses are "sleep movements" such as the closing of flowers and changes in leaf position that many plants display at night. The growth rate of most plants also differs according to time of the day in a clock-dependent fashion. In addition, changes in expression of genes and many biochemical activities are under control of the biological clock. The circadian clock that controls these and other time-dependent responses requires environmental cues to stay synchronized with the 24 hours of the day. Light and temperature are two of the most important environmental stimuli since they usually change between night and day.
An interesting aspect of the many clock-dependent responses is that they typically persist even in the absence of a daily change in environmental cues. Indeed, the clock-like rhythms can continue for several days in the absence of external stimuli. However, in most plants the time between peaks (period) of a particular response will drift until the clock is reset (entrained) by an appropriate external stimulus.
The most easily observed circadian responses are leaf movements but other responses can be seen with the right equipment and patience.