the periodic closing and opening of plant organs, predominantly petals, during the day and night. Sleep movements are among the nyctinastic movements produced by the alteration of day and night. They are caused by change in illumination (photonasty) and change of temperature (thermonasty); such changes cause the flowers and inflorescences of many plants to open and close at certain hours. For example, the flowers of dandelion, chicory, flax, water lilies, oxalis, and many cacti open during the day and close in the evening or, in overcast weather, during the day. In scented tobacco (Nicotiana alata var. grandiflora), dame’s violet, evening primrose, queen of the night, and many Caryophyllaceae, the flowers open in the evening hours and close during the day. The flowers of salsify, lettuce, and some other plants open in the early morning hours and begin closing by noon. By selecting plants according to these characters, Linnaeus created a “flower clock.”

In many plants the position of the leaves also changes as a result of changes in the intensity of illumination. As a result of uneven growth of cells on different sides of the leafstalk, the leaf blade may be directed downward at night and upward during the day, or vice versa. In leaves that have completed their growth, movement occurs owing to changes in the turgor pressure of cells of the pulvini of the leaf articulations (for example, in Leguminosae, Oxalidaceae, and Amaranthaceae).

Sleep movements have great biological significance. In closed flowers the internal organs are protected from chilling and excessive moisture (rain, dew); in open flowers pollination occurs under the most favorable conditions.