tropism

thumb|300px|Daisies (Bellis perennis) facing the Sun after opening in the morning showing [[heliotropism]] thumb|Phycomyces, a fungus, exhibiting phototropism

In biology, a tropism is a phenomenon indicating the growth or turning movement of an organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus (as opposed to nastic movements, which are non-directional responses). Tropisms are usually named for the stimulus involved; for example, a phototropism is a movement to the light source, and an anemotropism is the response and adaptation of plants to the wind.