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phototropism pho·tot·ro·pism (fō-tŏt'rə-pĭz'əm)
Growth or movement of a sessile organism toward or away from a source of light.
The growth or movement of a fixed organism toward or away from light. In plants, phototropism is a response to blue wavelengths of light and is caused by a redistribution of auxin from the illuminated side to the darker side of the shoot, resulting in quicker growth on the darker side and bending of the shoot toward the source of light. Certain sessile invertebrates also exhibit phototropism.