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Nyctitropism, nik′ti-trō-pizm, n. the so-called sleep of plants, the habit of taking at night certain positions unlike those during the day.—adj.

Nyctitropism and nyctitropic, i.e. night-turning, may be applied both to leaves and flowers, and will be occasionally used by us; but it would be best to confine the term to leaves.

The sleep or nyctitropism of leaves is a large subject, and we think that the most convenient plan will be first to give a brief account of the position which leaves assume at night, and of the advantages apparently thus gained.

On this view of the origin of nyctitropism we can understand how it is that a few plants, widely distributed throughout the Vascular series, have been able to acquire the habit of placing the blades of their leaves vertically at night, that is, of sleeping,—a fact otherwise inexplicable.