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A tropism (from Greek τροπή, trope, "a turning") is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus (as opposed to nastic movements which are non-directional responses). Viruses and other pathogens also affect what is called "host tropism" or "cell tropism" in which case tropism refers to the way in which different viruses/pathogens have evolved to preferentially target specific host species, or specific cell types within those species. Tropisms are usually named for the stimulus involved (for example, a phototropism is a reaction to light) and may be either positive (towards the stimulus) or negative (away from the stimulus).
Tropisms are typically associated with plants (although not necessarily restricted to them).[a] Where an organism is capable of directed physical movement (motility), movement or activity in response to a specific stimulus is more likely to be regarded by behaviorists as a taxis (directional response) or a kinesis (non-directional response).
In English, the word tropism is used, in a sometimes derisive way, to indicate an action done without cognitive thought: However, "tropism" in this sense has a proper, although non-scientific, meaning as an innate tendency, natural inclination, or propensity to act in a certain manner.
In botany, the Cholodny-Went model, proposed in 1927, is an early model describing tropism in emerging shoots of monocotyledons, including the tendencies for the stalk to grow towards light (phototropism) and the roots to grow downward (gravitropism). In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone.
- Chemotropism, movement or growth in response to chemicals
- Geotropism (or gravitropism), movement or growth in response to gravity
- Heliotropism, movement or growth in response to sunlight
- Sonotropism, movement or growth in response to sound.
- Hydrotropism, movement or growth in response to water
- Phototropism, movement or growth in response to lights or colors of light (e.g.- the sunflower)
- Thermotropism, movement or growth in response to temperature
- Electrotropism, movement or growth in response to an electric field
- Thigmotropism, movement or growth in response to touch or contact
- Exotropism, continuation of growth "outward," i.e. in the previously established direction
See also 
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- Haga, Ken; Takano, Makoto; Neumann, Ralf; Iino, Moritoshi (January 1, 2005). "The Rice COLEOPTILE PHOTOTROPISM1 Gene Encoding an Ortholog of Arabidopsis NPH3 Is Required for Phototropism of Coleoptiles and Lateral Translocation of Auxin(W)". Plant Cell. Retrieved 2012-06-22. (subscription required)