Tropism

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"; 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 occur in three sequential steps. First, there is a sensation to a stimulus, which is usually beneficial to the plant. Next, signal transduction occurs. And finally, the directional growth response occurs. 
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.
Types.
in plants (and bacteria)
in viruses
External links.
 == Welcome! ==
Hi Tropism! I noticed and wanted to welcome you to the Wikipedia community. I hope you like it here and decide to stay.
As you get started, you may find this short tutorial helpful:
Alternatively, the page covers the same topics.
If you have any questions, we have a friendly space where experienced editors can help you here:
If you are not sure where to help out, you can find a task here:
Please remember to sign your messages on s by typing four tildes (~~~~); this will automatically insert your username and the date.
Happy editing! 