Tropism


A tropism 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. 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 and may be either positive or negative.
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. Where an organism is capable of directed physical movement, movement or activity in response to a specific stimulus is more likely to be regarded by behaviorists as a taxis or a kinesis.
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 and the roots to grow downward.
In both cases the directional growth is considered to be due to asymmetrical distribution of auxin, a plant growth hormone.

Types

in plants
in viruses