It is stated in every textbook of botany that the direction of growth of stems, roots and leaves is influenced by light as well as by gravity. In this phenomenon of plant growth alterations by unilateral light, referred to as phototropism, the stems are generally positively phototropic, roots generally negatively phototropic (if they respond at all), and leaves plagiotropic (Fig. 214). It is further stated that “in both photo- and geotropism the curvature is the result of differential growth on the two sides of the plant axis. The side towards which the curvature occurs grows less rapidly than the side opposite and the resultant of this differential growth is the curvature” [76].