Plant clocks
Many plants open and close their flowers or raise and lower their leaves at roughly the same times each day. For a long time it was assumed that these movements were simply responses to light and darkness. In the eighteenth century, however, the French scientist Jean-Jacques d’Ortous de Mairan noticed that a plant kept in darkness continued to open and close its leaves on a daily schedule.
His observation suggested that plants have an internal timekeeping system. Today this is known as a circadian clock, from Latin words meaning ‘about a day’. The clock runs on a cycle of roughly twenty-four hours, but it is not perfectly precise. In constant conditions it drifts slightly, which is why it must be reset every day by signals from the environment, especially dawn.
An internal clock allows a plant to anticipate regular changes rather than merely react to them. For example, some plants prepare the machinery needed for photosynthesis before sunrise, so they can begin using light as soon as it appears. Others time the release of scent to match the hours when their pollinators are most active.
Experiments have shown that plants whose clocks match the length of the day around them tend to grow better than plants whose clocks do not match. This suggests that the ability to keep time is not a curiosity but a real advantage, which may explain why similar clocks are found in organisms from bacteria to humans.