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.
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