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1. the growth response of plant parts to the stimulus of light, producing a bending towards the light source
2. the response of animals to light: sometimes used as another word for phototaxis



change in the direction of growth of plant organs under the influence of illumination from one side. In positive phototropism, the stem bends toward the light source. In plagiotropism, or diatropism, the leaf blades move at an angle toward the falling light. In negative phototropism the plant organs bend in a direction opposite to the light source (for example, ivy stems and the apices of certain roots). The same organ may be positively phototropic to weak light, negatively phototropic to intense light, and not at all phototropic to light of medium intensity.

Plant species differ in their capacity for phototropism. Phototropic responses may change even in plants of the same species: in young individuals the responses are always stronger, given the same conditions, than in older plants. Phototropism may manifest itself only in the young organs of a plant. In stems and leaves the phenomenon leads to uniform leaf distribution, so that the leaves shade each other only slightly. Positive phototropism and negative geotropism enable the apices of sprouts to emerge to the soil surface even when the seeds have been planted very deeply.

The process of phototropism includes a series of successive reactions: perception of a light stimulus, excitation of cells and tissues, transmission of excitation to cells and tissues of the growth zone of the organ, and intensification or diminution of cell and tissue growth in the growth zone. Perception of the light stimulus is effected by a specific photoactive complex that includes carotenoids and flavin. Transmission of the excitation through the plant occurs with the participation of bioelectric currents and plant hormones known as auxins. (For a discussion of the mechanisms of these processes, see.)

The manifestations of phototropism depends on the spectral composition of the illumination. Maximum phototropic sensitivity in plants has been found in the absorption spectra of yellow and orange pigments—carotenoids and flavins. It is conjectured that photosensitive proteins containing the pigments perceive the light stimulus. Carotenoid “eyes” have also been found in certain unicellular algae displaying phototaxis and in the sporangiophores of fungi capable of phototropism.


Darwin, C. “Sposobnosf k dvizheniiu u rastenii.” Soch., vol. 8. Moscow-Leningrad, 1941.
Thimann, K. V., and G. M. Curry. “Phototropism.” In Symposium on Light and Life. Baltimore, 1961. Pages 646–70.


A growth-mediated response of a plant to stimulation by visible light.
(solid-state physics)
A reversible change in the structure of a solid exposed to light or other radiant energy, accompanied by a change in color. Also known as phototropy.
References in periodicals archive ?
Their urban projects (they already have at least four distinguished houses under their belt) tend to exploit specifics of orientation, perhaps most flamboyantly expressed in Mosaic House (AR August 2007) that mimicked phototropism to exploit high-level daylight.
European Space Agency Flight Engineer Thomas Reiter began the first of three runs of the Analysis of a Novel Sensory Mechanism in Root Phototropism experiment in the European Modular Cultivation System.
Hatching rate was calculated by counting the number of nauplii per spawn sample through collection by positive phototropism.
varia showed a clear tendency to negative phototropism, with preference for surfaces not exposed to the light in horizontally-positioned opaque collectors.
Second, the behaviors of geoduck larvae in relation to currents, temperature, photoperiod, and phototropism are not well understood.