Un tropismo (del griego τροπή tropḗ 'giro, vuelta, fuga, punto de retorno') es un fenómeno biológico que indica el crecimiento o cambio direccional de un organismo, normalmente una planta, como respuesta a un estímulo medioambiental. Los tropismos difieren de las nastias en que estas no son respuestas direccionales. Si el órgano se mueve en la misma dirección que el estímulo se denomina tropismo positivo (+), pero si lo hace inclinado (crecimiento con dirección horizontal o en ángulo) alejándose del estímulo es un tropismo negativo (-). Ambas son respuestas realizadas para acercarse o alejarse del estímulo recibido.

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A tropism (from Greek τρόπος, tropos, "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", "tissue tropism", or "cell tropism", or in which case tropism refers to the way in which different viruses/pathogens have evolved to preferentially target specific host species, specific tissue, or specific cell types within those species. Tropisms are usually named for the stimulus involved (for example, a phototropism is a reaction to sunlight) 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). 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 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 towards a certain stimulus. 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.

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