nastic, -nastic; nasty, -nasty; -nastism +

(Greek: nastos, pressed close, crammed full; firm, solid)

aitionastic (adjective)
1. A change in direction or bending caused by external forces.
2. The curvature of part of a plant stimulated by a widely spread or a scattered stimulus.
Literally, "pressed close above", but applied not only, for example, to leaves which lie flat on the ground, but also to ovules that curve in a downward direction, because the tops grow faster.
Curvature arising from endogenous forces.
chemonasty, chemonastic
A response to a diffuse chemical stimulus; a change in the structure or position of an organ in response to a diffuse chemical stimulus.
An ovule that curves towards the horizontal edge of the carpel.
epinasty, epinastic
1. That phase of vegetable growth in which an organ grows more rapidly on its upper than on its under surface.
2. A downward bending of leaves or other plant parts, resulting from excessive growth of the upper side.

In plant physiology, the state in which more vigorous growth occurs in the upper surface of an organ, such as in an unfolding leaf, causing a downward curvature.

A reference to an ovule with a horizontal curvature in the direction of the upper face of the carpel.
geonasty, geonastic
1. A reference ti curvatures toward the ground.
2. Growth curvature towards the ground.
Curvature towards the earth at night.
haptonastic, haptonasty
The growth movement of a plant in response to a touch or contact stimulus.

The leaf movements of the Venus flytrap Dionaea muscipula following a tactile stimulus, and the rapid collapse of the leaflets of the sensitive plant Mimosa pudica are examples of haptonasty.

hydronasty, hydronastic
Plant movement induced by changes in atmospheric humidity.
An organ that curves upwards because the ventral surface grows more rapidly.
1. The state of growth in a flattened structure in which the under surface grows more vigorously than the upper side.
2. An upward bending of leaves or other plant parts, resulting from growth of the lower side.

In plant physiology, the state in which more vigorous growth occurs in the lower surface of an organ, such as a young fern frond, causing an upward curvature.

nastic (adjective)
1. Plant movements in response to diffuse stimuli and to structural curvatures resulting from differential growth of opposite surfaces.
2. Relating to a response of a plant part; such as, growth or a loss of turgidity; to external stimuli that is independent of the direction of origin of such stimuli.
3. The movement or growth of cellular tissue on one surface more than on another one, as in the opening of petal or young leaves.

Movements are rapid, reversible responses to stimuli; such as, water, temperature, humidity, light, etc. Nastic movements occur as a result of changes in water pressure within specialized cells or differing rates of growth in parts of the plant.

Nastic movements are among a plant's more beautiful motions: a typical example is the opening of a flower. They are the result of differing responses of different parts of the plant structure to the same external stimulus (Scientific American).
The American Heritage Dictionary of Science by Robert K. Barnhart, Houghton Mifflin Company, Boston, 1986.
nastic movement
Movement of a flat plant part, oriented relative to the plant body and produced by a variety of stimuli that cause disproportioinate growth or increased turgor pressure in the tissues of one surface.

The opening and closing movements of many flowers, and the responses of leaves to changes of temperature and light, are externally directed, or paratonic, nastic movements. Specialized plants, such as the insectivorous sundew, move in response to the touch and chemical stimuli of captured insects.

Nastic movements are responses to stimuli that uniformly affect the plant or else elicit a uniform response regardless of the direction they come from, whereas tropisms are movements in response to stimuli coming from one direction; geotropism, for example, is the response to gravity. The distinction between nasticisms and tropisms is sometimes unclear.

—Modified excerpts from The Columbia Electronic Encyclopedia