Source : Webster's Revised Unabridged Dictionary (1913)

Nyctitropism \Nyc*tit"ro*pism\, n. [From Gr. ?, ?, 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.