- Tropism
A tropism (from Greek, "tropos", to turn) is a biological
phenomenon , indicating growth or turning movement of a biologicalorganism , usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus (as opposed tonastic movement s 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. The word tropism comes from the Greek "trope" ("to turn" or "to change"). 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
plant s (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 ataxis (directional response) or akinesis (non-directional response).In English, the word "tropism" is used in 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.
Types of tropisms
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Chemotropism , movement or growth in response tochemical s
*Gravitropism (or geotropism), movement or growth in response togravity
*Hydrotropism , movement or growth in response tomoisture orwater
*Heliotropism , movement or growth in response tosunlight
*Phototropism , movement or growth in response tolights or colors of light
*Thermotropism , movement or growth in response totemperature
*Thigmotropism , movement or growth in response to touch or
*Host tropism orcell tropism , thehost range ofpathogen s
**Ecotropism , limited host range (e.g. infects only one species or cell type)
**Amphotropism , wide host range (e.g. infects many species or cell types)
**HIV tropism , the means of entry into cells used by a given strain of HIV
** Neurotropism, avirus that preferentially infects the host's nervous system.ee also
*
Rapid plant movement
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