Leaves are exquisitely adapted to power the world by harvesting the sun’s energy and withstanding multifaceted physical demands. The flexibility of leaves allows both active and passive movements. Some leaves are heliotropic, meaning that they actively reorient in response to the direction of the sun. This movement is accomplished by changes in turgor pressure in the plant’s motor cells. The movement of water due to an osmotic gradient generates turgor pressures. These localized pressures cause the motor cells on one side of the petiole, or leafstalk, to elongate. In addition to active movements, passive movements, such as those caused by wind, can increase cooling and gas exchange. The broad leaves of trees such as aspen and tulip poplars are thought to quake or flutter for such purposes. The wind can also facilitate the shedding of leaves in the fall. During strong winds and floods, wind induced motion can be destructive. Many broad leaves reconfigure into stable shapes that reduce drag and oscillations in such extreme events.
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