Solar Tracking - Nastic and Epinastic Responses
Many plants such as sun flowers are capable of solar tracking in which the flat blade of leaves or inflorescence will remain at right angles to the sun throughout the day, maximizing the light harvested by the leaf. Darwin studied the phenomenon first. This movement is presently thought to be controlled through a mechanism of maintenance of turgidity levels in petiole (pulvinus). Later studies also indicate a role for active pumping of K' ions across the membrane.
Figure: Solar tracking in a plant with cup-shaped leaves such as some members of the Mal- vacea family (e.g. Malva or Lavatera). The leaves receive directional signals from the sun and swivel around to face it as pulvinus cells at the base of the blade gain or lose water. (a and b) The leaves track the sun during the day, much as a radio telescope tracks a satellite (c) An hour or two after sunset, the blades arc nearly horizontal in the "relaxed" position they maintain during most of the night (d) About an hour before sunrise, the blades face the point on the eastern horizon where the sun came up the day before.