Diurnal Phototropism in Solar Tracking Leaves of Lavatera cretica.

On a clear day, leaf laminas of Lavatera cretica tracked the solar position throughout the day. The laminar azimuth did not diverge from the solar azimuth by more than 12 degrees from sunrise to sunset. Tracking of the solar elevation started 1 to 2 hours after sunrise and ceased 1 to 2 hours before sunset. On an overcast day, the laminas reoriented horizontally. After sunset, following a clear day, the laminas performed a nocturnal reorientation, with three well defined phases. During the initial phase the laminas relaxed their strained sunset-facing orientation to one perpendicular to their petioles. This equilibrium configuration was maintained throughout the following phase, which was apparently concerned with time-measuring. During the final phase, the laminas reoriented, before sunrise, to a position facing the direction of the anticipated sunrise. This directional information is phototropic and was retained for 3 to 4 diurnal cycles, probably in the pulvinus itself, which is the site of the response. Laminas of plants transferred from sunlight either to darkness, or to a simulated natural photoperiod under overhead illumination, were facing the originally anticipated direction of sunrise at the time of each of the three to four subsequent sunrises (after which they reverted to the dark orientation in darkness, or to the horizontal one with overhead illumination). Cotyledonary laminas required directional information for the nocturnal reorientation during 3 or 4 cycles of simulated sunrise to sunset transitions.