nyctinasty

noun

nyc·​ti·​nas·​ty 'nik-tə-ˌna-stē 

plural nyctinasties

: plant movement (such as the closing of a flower's petals or the reorientation of a leaf's position) that occurs in response to changes in light intensity (such as the onset of darkness) and that typically involves changes in cellular turgor : the nastic movement of the parts of some plants that occurs in the absence of a directional stimulus and that is associated with the plant's circadian cycle

In leguminous plants, nyctinasty is caused by volume change of cells in the pulvini, a special organ located at the base of the leaflet.

—Minoru Ueda et al.

The prayer plant family consists of roughly 550 species. They all share the quirk of folding up their pretty leaves for the night—like praying hands—then lowering them again to a flat position when the sun returns in the morning; this circadian movement is called nyctinasty.

—Gynelle Leon

Portulaca … is a member of a large group of plants that close their flowers as the sunlight decreases such as at dusk, or when clouds are present, or when it rains. This response, known as nyctinasty, is a biological rhythmic event in which the leaves or petals close in response to darkness.

—Anne Ditmer

nyctinastic

ˌnik-tə-ˈna-stik 

adjective

nyctinastic movement

Word History

Etymology

nyctinastic (borrowed from German nyctinastisch —later nyktinastisch— from Greek nykti-, combining form of nykt-, nýx "night" + German -nastisch, as in epinastisch "displaying epinasty") + -y entry 2 — more at night entry 1, epinasty

Note: The term was introduced by the German botanist Wilhelm Pfeffer (1845-1920) in Pflanzenphysiologie: ein Handbuch, 2. Ausgabe (Leipzig, 1904), p. 476, as a replacement for nyctitropisch.

First Known Use

1912, in the meaning defined above