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the movement of plant organs, for example, leaves and petals, under the influence of undirected and spatially uniform illumination. Photonastic responses may include the opening and closing of flower corollas and anthodia. Photonasty occurs either as a result of accelerated growth or a change in the turgor of the cells on one side of an organ. It often depends on the combined influence of illumination and temperature; if, in such cases, movements of the organs are associated with the alternation of day and night, the responses are called nyctinastic.