tropizm

Tropizm, or tropism, is a directional growth response of an organism or its part toward or away from a stimulus. The response is typically structural and directional, resulting from uneven growth on different sides of the organ. Tropisms are often described as positive when growth or movement is toward the stimulus and negative when it is away from it.

In plants, tropisms are best known and studied. Major types include phototropism (growth toward light), gravitropism or geotropism (growth influenced by gravity), hydrotropism (toward water), thigmotropism (response to touch or physical contact), and chemotropism (growth toward or away from chemical signals). The underlying mechanism usually involves differential distribution of growth-regulating hormones, especially auxins. Light perception by photoreceptors such as phototropins leads to auxin redistribution to the shaded side of a stem, causing cells on that side to elongate more and the stem to bend toward the light. In roots, gravity often directs growth downward, while light can inhibit it, producing characteristic curvature.

Historically, experiments by Charles and Francis Darwin on coleoptiles and later work by others, including Fritz Went, helped establish the role of chemical signals (auxin) in tropisms. Tropisms also occur in fungi and some microorganisms, where growth is directed toward or away from nutrients, toxins, or other stimuli. In animals, directional growth responses are less standardized under the term tropism; movement toward or away from stimuli is more commonly described as taxis or kinesis.

Tropism is distinct from nastic movements, which are non-directional in relation to the stimulus, and from general locomotion.