ABSTRACT
The orientation of plant organs perpendicular to the plain of the sun’s rays, which is called ortho- or dia-heliotropism, maximizes light absorbance, whereas the orientation parallel to the sun’s rays, which is called para-heliotropism, minimizes light absorbance. Heliotropic movements occur by asymmetric volume changes at both sides of the moving organ via reversible turgor-mediated alterations in the pulvinus shape or cell elongation. The pulvinus found at the base of the leaf or the leaflet petiole is a vascular tissue surrounded by a cylinder-like cortex of thin-walled elastic parenchyma cells. The daily alterations in sun position and light intensity and on the other hand the absolute dependence of plants on light led to the evolution of heliotropic responses in plants to optimize sunlight utilization under unfavorable conditions. Heliotropic movements also occur in plants and organs lacking pulvini by a differential and irreversible cell growth on two opposite sides of the organ or Anemone rivularis..
