tropism

Tropism is a biological phenomenon in which growth or movement is directed toward or away from a directional stimulus. In plants, tropisms involve directional growth of a part of the organism in response to stimuli such as light, gravity, or touch, with the stimulus acting as an orientation cue. Because the response occurs through growth and is oriented to the stimulus, tropisms are distinguished from nastic movements, which are non-directional and driven by internal cues or rapid turgor changes. Well-established plant tropisms include phototropism, gravitropism (geotropism), thigmotropism, hydrotropism, and chemotropism, typically caused by uneven growth on opposite sides of the growing organ.

In phototropism, shoots usually exhibit positive phototropism, bending toward light, while roots tend to show negative phototropism. Gravitropism describes growth responses to gravity, with shoots typically displaying negative gravitropism (growing upward) and roots positive gravitropism (growing downward). Thigmotropism is growth in response to touch, common in climbing plants that coil around supports. Hydrotropism directs root growth toward moisture, and chemotropism refers to responses to chemical stimuli, such as pollen tube guidance toward ovules in some plants or chemical cues guiding fungal or other growth. The underlying mechanism often involves the perception of the stimulus by specialized receptors and the hormone-mediated redistribution of growth-promoting substances, notably auxin, leading to differential cell elongation on the sides away from or toward the stimulus.

In animals, tropism is less commonly used; directional behavioral responses are more often described as taxis or other forms of movement. The term is mainly associated with plant biology.