• adaptation;
  • diaheliotropism;
  • lamina;
  • motor tissue;
  • paraheliotropism;
  • photonasty;
  • phototropism;
  • pulvinus;
  • solar tracking;
  • stress evasion

Abstract. Mature leaves of many plants re-orientate their laminae photonastically in response to non-directional light signals, and/or phototropically in response to directional light signals, by flexing of pulvini, most commonly subtending their bases. Physiological and structural specializations of the pulvinus enable it to flex, by rapidly undergoing differential and repeatedly reversible axial volume changes (expansion/contraction) in opposite sectors of its motor tissue. Light-driven leaf movements are adaptations that contribute to the efficiency of the photosynthetic apparatus in the leaf. The phototropic response maximizes the harvesting of photosynthetically radiant energy. The photonastic response to dark-to-light transitions increases the interception of light by unfolding the lamina. Another photonastic response modulates the interception of radiant energy by the lamina, allowing it to evade damage by light in excess of its photosynthetic capacity when the leaf is under stress. The same unidentified blue-absorbing pigment system appears to be involved in all these responses. Non-directional light signals are perceived in the pulvinus. Perception of directional light signals may be localized in other parts of the leaf in different plants: for example, the pulvinus in most leguminous species, and the lamina in malvaceous and at least one leguminous species. The perception of non-directional and directional light signals, their transduction to differential volume changes in the target cells, and their transmission between the two where the sites are separate, are discussed.