Biological Clocks

Bean plants regularly extend their leaves in the daytime so as to take in the maximum amount of light. At night, they lift their leaves parallel to the side of the stem in what may be described as a “sleeping” position. The leaf movements occur day after day throughout the entire life of the plant. It might be inferred that these responses are triggered by the coming of daylight in the morning and the coming of darkness at night. However, when bean plants are placed in a controlled situation wherein the light is constant twenty-four hours a day, the same responses occur at the same times noted earlier (that is, the plants extend their leaves at morning and lift them up parallel to their stems at night). Likewise, when the plants are placed in darkness for twenty-four hours a day, the responses are again the same. How does this happen? Why does it happen? The answers to both questions are unknown (although the temptation exists to say that they extend their leaves because they need light).