Heliotropism, in vegetable physiology, is the response of the plant to the directive influence of light. Light has in general a retarding influence upon growth, so that illuminated parts grow more slowly than those in shadow. From the unequal growth thus caused curvature results, which is more marked when the light is more intense. The effect is produced mainly by light between the violet and ultra-violet of the spectrum, but even the dark ultra-red rays have some effect. Among radially symmetrical organs, such as axes, stems are generally positively heliotropic - bending, that is, towards the light, as seen in plants glowing in a window, owing to the more rapid growth on the shaded side - while roots are mostly negativelg lteliotropie. A reversal of direction in one organ is exemplified in the flower-stalk of Linaria Cymbalaria, the ivy-leaved toad-flax, which in the flower stage is positively heliotropic and so droops, and in fruit becomes negatively so and erect. Vertical or bilateral organs, such as the leaves of Iris, are positively heliotropic. Dorsiventral organs, such as most leaves, exhibit what is termed dia-heliotropism, placing their dorsal surfaces at right angles to the direction of incidence of the light.