THE VECTORIAL PHOTO‐EXCITATION IN SOLAR‐TRACKING LEAVES OF Lavatera cretica (MALVACEAE)*,†

Abstract

Abstract— The diaphototropic responses of the solar‐tracking leaves of Lavatera cretica were studied under constant levels of vectorial photo‐excitation (negligible variations in angle of incidence and fluence rates). The results showed the following:(a) The photoreceptors, that are associated with the (major) veins, can equally perceive vectorial excitation (by an oblique light‐beam in the plane of symmetry of the vein), directed either towards its tip (tip‐oriented: TO), or towards its base (base‐oriented: BO).(b) The mechanism of photo‐perception is apparently qualitatively the same for TO and BO excitation.(c) The response to TO and BO vectorial excitation, which takes place in a circular sheath of motor‐cell tissue in the pulvinus (a2–3 mm long segment at the top of the petiole), is also highly directional and results in bending in the vertical plane of the oblique beam. Each sector of the motor tissue can expand longitudinally in response to TO excitation, presumably by influx of solutes, and contract longitudinally in response to BO excitation of the same vein, presumably by efflux of solutes.(d) The angular velocity of the vectorial response to a constant level of vectorial excitation remains constant over large angular displacements and long periods, and was similar for TO and BO over a wide range of fluence rates (30‐400 μmol m−2s−1) of white light (400‐700 nm).(e) Reversal of the direction of vectorial excitation (TO to BO, and vice versa) results in reversal of the direction of the vectorial response (laminar reorientation). The change in the direction of laminar reorientation involved a larger overshoot in the original direction when the change was from BO to TO, than in the reverse case.