Phototropism (photo means light). The orientation of a plant in response to light, either growing toward the source of light (positive phototropism) or away from it (negative phototropism). Seeds grow straight up through the ground to reach the sunlight above. Plants that have a light on all sides will continue to grow straight. If the sunlight is uneven, the plant bends toward the light because the growth of cells on the dark side is faster than the cells on the light side.