Phototropism

Phototropism is the orientation of an organism in response to asymmetric illumination. Phototropism is commonly observed in the stems of higher plants, which grow bent toward a light source. Phototropism can be positive (bending toward a light source) or negative (bending away from a light source), depending on the organism and nature of the illumination. Phototropism and other tropisms are different from nastic movements, which are also common in plants. A tropism is the orientation of an organism in response to an external stimulus in which the stimulus determines the orientation of the movement. A nastic movement is a growth movement in which the stimulus does not determine the orientation of the movement.