Tropism

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).^[1] 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 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.

[edit]Types of tropisms

Example of Gravitropism in the remaints of a cellar of a roman villa in the Archeologic Park in Baia, Italy

• 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
• 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
• Host tropism or cell tropism, the host range of pathogens
  • Ecotropism, limited host range (e.g. infects only one species or cell type)
  • Amphotropism, wide host range (e.g. infects many of entry into cells used by a given strain of HIV)
  • Neurotropism, a virus that preferentially infects the host's nervous system.
  • Photoperiodism, is response to seasons