taxis

n. A hereditary reflexive movement toward or away from a particular class of stimuli which differs from a tropism in that the organism has bodily motility. Thus some flagellate protozoa move toward light in taxis, while some plants have a tropism in which their leaves turn toward light but lack the ability to move their entire being through space. Positive taxis is movement toward a stimulus, while negative taxis is movement away from a stimulus.