nyctitropism

from The Century Dictionary.

• noun In botany, the habit of certain plants or parts of plants whereby they assume at nightfall, or just before, certain positions unlike those which they have maintained during the day; the “sleep” of plants.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany Alternative term for nyctinasty