Tropism, movement of a plant in response to external directional stimuli. If a plant is laid on its side, the stem will soon start to bend upward again; this movement (geotropism) is a response to the force of gravity. The stem is said to be negatively geotropic. Roots are generally positive geotropic and grow downward. Phototropisms are bending movements in response to the direction of light. Stems are generally positive phototropic (bend toward the light). Most roots are negatively phototropic. Some roots exhibit positive hydrotropism: They bend toward moisture. This response is more powerful than the response to gravity. Tropisms are controlled by differences in concentration of growth hormones.