The definition of tropism states the turning or bending movement of an organism or part, in this case a plant, toward or away from an external stimulus, including light, heat, and gravity. The plant in the picture below is exhibiting phototropism and geotropism. Phototropism is the growth of a plant in response to sun light. As you can see, the way the plant grows upward and bends allows it to get the most potential sunlight it can. In this case, the roots would exhibit negative tropism and grow downward, while the stems with exhibit positive tropism and grow upward. Geotropism is the growth of a plant in response to gravity. The plant in the picture is growing upward and exhibiting this geotropism.