Affixes

Words in ‑tropism usually contain the sense of tropism, the growth or movement of an organism in a particular direction in response to an external stimulus. Examples include geotropism (Greek gē, earth), growth in response to gravity; heliotropism (Greek hēlios, sun), directional growth in response to sunlight; phototropism (Greek phōs, phōt‑, light), orientation in response to light; thigmotropism (Greek thigma, touch), the turning or bending of a plant or other organism in response to a touch stimulus; chemotropism, growth or movement in response to a chemical stimulus. One instance that stands aside from this group is neurotropism, the tendency of a virus, toxin, or chemical substance to attack or affect the nervous system preferentially. Associated adjectives are formed using ‑tropic.