taxis

(tactic movement, pl. taxes) A free directional locomotor movement exhibited by whole organisms that change their physical position in response to external stimuli. In the plant kingdom tactic movements are confined mainly to small, generally unicellular, aquatic plants, such as Euglena, which moves using a flagellum. In addition, certain reproductive cells and bacteria also show tactic movements. Compare tropism , nastic movements. See also phototaxis , chemotaxis.