A tropism is a biological

phenomenon A phenomenon ( : phenomena) is an observable event. The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be directly observed. Kant was heavily influenced by Gottfrie ...

, indicating growth or turning movement of a biological

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...

, 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 Nastic movements are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. The movement can be due to changes in turgor. Decrease in turgor pressure causes shrinkage while ...

which are non-directional responses). Tropisms are usually named for the stimulus involved (for example, a

phototropism Phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hor ...

is a reaction to sunlight). Tropisms occur in three sequential steps. First, there is a sensation to a stimulus. Next, signal transduction occurs. And finally, the directional growth response occurs. Tropisms are typically associated with

plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...

s (although not necessarily restricted to them). Where an organism is capable of directed physical movement (

motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...

), 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). 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 grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one ...

s, 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 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 essenti ...

, a plant growth

hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...

. The term "tropism" () is also used in unrelated contexts. Viruses 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, growth of plants towards or away from a source of oxygen * Chemotropism, movement or growth in response to

chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., w ...

s * Electrotropism, or galvanotropism, movement or growth in response to an

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field ...

* Exotropism, continuation of growth "outward," i.e. in the previously established direction * Gravitropism, sometimes referred to as geotropism, movement or growth in response to

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

** 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 ...

, 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 large annual forb of the genus ''Helianthus'' grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), ...

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

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

; in plants, the root cap senses differences in water moisture in the soil, and signals cellular changes that causes the root to curve towards the area of higher moisture ** Prohydrotropism, positive hydrotropism * Hygrotropism, movement or growth in response to moisture or humidity * Magnetotropism, movement or growth in response to magnetic fields *

Phototropism Phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hor ...

, movement or growth in response to

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...

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 Thermotropism or thermotropic movement is the movement of an organism or a part of an organism in response to heat or changes from the environment's temperature. A common example is the curling of ''Rhododendron'' leaves in response to cold temper ...

, movement or growth in response to

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

* Thigmotropism, movement or growth in response to

touch In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch ( haptic perception), as well as temperature ( thermoception), body position (proprioception), and pain. It ...

or contact * Traumatotropism, orientation deviation after suffering a wounding

References

{{Tropism

Types

See also

References