What does phototactic mean?
Phototactic describes a movement or orientation in response to light. The term is most often applied to motile organisms, including microscopic organisms, plant spores, and microorganisms, whose movement changes according to the direction, intensity, or position of a light source. Positive phototaxis means movement toward light, as seen in many moth larvae; negative phototaxis means movement away from it, as may occur in certain aquatic organisms. Phototactic behavior is distinct from phototropism, which concerns growth rather than movement. Although the word is primarily used in scientific writing, its core idea has a familiar parallel in ordinary experience: humans generally turn toward a bright light, while avoiding a beam shining directly into the eyes. Phototactic is also used more broadly to describe any light-directed response involving movement.
adjectiveRelating to, or exhibiting, movement in response to light. Phototactic organisms may move toward light, as in positive phototaxis, or away from it, as in negative phototaxis.
- Describing movement or orientation in response to light, especially in motile organisms or cells.
"Many moth larvae display positive phototaxis, moving toward the light source."
"The single-celled organism altered its direction in a phototactic response to the light."
"The researchers classified the algae as showing positive phototaxis under laboratory conditions."
Some creatures walk toward light, while others flee it: both contrasting journeys are forms of phototaxis.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does phototactic come from?
The word was formed in scientific English from the combining form photo-, ultimately from Greek phōs, meaning “light,” and taxis, meaning “arrangement” or “ordering,” a term also underlying the modern word “taxis.” Its development follows the older biological distinction between taxis, directed movement, and tropism, directed growth. Modern related formations include phototaxis and the adverb phototactically.
Related word forms
How phototactic is actually used
This is primarily a scientific term, especially used in biology. It is most often encountered in discussions of organisms, cells, or microorganisms and in the established terms positive phototaxis and negative phototaxis.
Easily confused with phototactic
Phototaxis describes movement in response to light, whereas phototropism describes the directional growth of an organism, especially a plant, in response to light.