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From The Collaborative International Dictionary of English v.0.48:
Nyctitropism \Nyc*tit"ro*pism\, n. [From Gr. ny`x, nykto`s, night + ? to turn.] (Plant Physiol.) The tendency of certain plant organs, as leaves, to assume special "sleeping" positions or make curvatures under the influence of darkness. It is well illustrated in the leaflets of clover and other leguminous plants.