biology Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, History of life, origin, evolution, and ...

, a tropism is a

phenomenon A phenomenon ( phenomena), sometimes spelled phaenomenon, is an observable Event (philosophy), event. The term came into its modern Philosophy, philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be ...

indicating the growth or turning movement of an

organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...

, usually a

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

, 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). Tropisms are usually named for the stimulus involved; for example, a phototropism is a movement to the light source, and an anemotropism is the response and

adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...

of plants to the wind. Tropisms occur in three sequential steps. First, there is a sensation to a stimulus. Next,

signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a biochemical cascade, series of molecular events. Proteins responsible for detecting stimuli are generally termed receptor (biology), rece ...

occurs. And finally, the directional growth response occurs. Tropisms can be regarded by

ethologists Ethology is a branch of zoology that studies the behaviour of non-human animals. It has its scientific roots in the work of Charles Darwin and of American and German ornithologists of the late 19th and early 20th century, including Charle ...

as ''

taxis A taxis (; : taxes ) is the motility, movement of an organism in response to a Stimulus (physiology), stimulus such as light or the presence of food. Taxes are innate behavioural responses. A taxis differs from a tropism (turning response, often ...

'' (directional response) or '' kinesis'' (non-directional response). The Cholodny–Went model, proposed in 1927, is an early model describing tropism in emerging shoots of

monocotyledon Monocotyledons (), commonly referred to as monocots, ( Lilianae '' sensu'' Chase & Reveal) are flowering plants whose seeds contain only one embryonic leaf, or cotyledon. A monocot taxon has been in use for several decades, but with various ranks ...

s, including the tendencies for the stalk to grow towards light (phototropism) and the roots to grow downward (

gravitropism Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feat ...

). In both cases, the directional growth is considered to be due to asymmetrical distribution of

auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essent ...

, a plant growth

hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

. The term "tropism" () is also used in unrelated contexts.

Virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

es and other pathogens 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. In English, the word ''tropism'' is also 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.

Types

Tropisms can be distinguished according to the orientation with respect to the direction of the stimulus. They can commonly be either ''positive'' (towards the stimulus) or ''negative'' (away from it). Both of these are ''orthotropic'', and can be contrasted with tropisms that are ''diatropic'' (perpendicular to the stimulus) or ''plagiotropic'' (at an oblique angle). According to the type of stimulus, tropisms can be: Upsidedown-tree

* Aerotropism: the growth of plants towards or away from a source of

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

* Chemotropism: the movement or growth in response to

chemical A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...

s *

Electrotropism In biology, electrotropism, also known as galvanotropism, is a kind of tropism which results in growth or migration of an organism, usually a cell, in response to an exogenous electric field. Several types of cells such as nerve cells, muscle cel ...

, or galvanotropism: the movement or growth in response to an

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

* Exotropism: continuation of growth "outward," i.e. in the previously established direction *

Gravitropism Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general feat ...

(sometimes referred to as geotropism): is movement or growth in response to

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

** Apogeotropism: negative geotropism *

Heliotropism Heliotropism, a form of tropism, is the diurnal or seasonal motion of plant parts (flowers or leaves) in response to the direction of the Sun. The habit of some plants to move in the direction of the Sun, a form of tropism, was already known by th ...

: the diurnal motion or seasonal motion of plant parts in response to the direction of the Sun, (e.g. the

sunflower The common sunflower (''Helianthus annuus'') is a species of large annual forb of the daisy family Asteraceae. The common sunflower is harvested for its edible oily seeds, which are often eaten as a snack food. They are also used in the pr ...

) ** Apheliotropism: negative heliotropism * Hydrotropism: movement or growth in response to

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

; in plants, the root cap senses differences in water moisture in the soil, and signals cellular changes that cause the root to curve towards the area of higher moisture ** Prohydrotropism: positive hydrotropism * Hygrotropism: movement or growth in response to moisture or humidity * Inotropism: muscular contraction in response to drugs *

Magnetotropism Magnetotropism is the movement or plant growth in response to the stimulus provided by the magnetic field in plants (specifically agricultural plants) around the world. As a natural environmental factor in the Earth, variations of magnetic field lev ...

: movement or growth in response to magnetic fields * Phototropism: movement or growth in response to

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

s or colors of light ** Aphototropism: negative phototropism ** Skototropism: negative phototropism of vines * Selenotropism: motion of plant parts in response to the direction of the Moon * Thermotropism: movement or growth in response to

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

* Thigmotropism: movement or growth in response to

touch The somatosensory system, or somatic sensory system is a subset of the sensory nervous system. The main functions of the somatosensory system are the perception of external stimuli, the perception of internal stimuli, and the regulation of bo ...

or contact * Traumatotropism: orientation deviation after suffering a wounding

References

External links

* * {{Authority control

Types

See also

References

External links