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movements of leaves, petals, and other plant organs in response to stimulation, for example light and temperature. Such movements lack a definite direction. They result from the unequal growth of cells on the upper and lower sides of an organ or from uneven changes in the turgor of the cells. More intensive growth of the upper side of an organ —as in bud-unfolding or blossoming—is known as epinasty. Such accelerated growth on the underside—as in the closing of flower integuments after blossoming—is called hyponasty.
Different stimulants cause various other types of nastic movements. Photonasty, the movement of leaves and petals caused by changes in light intensity, characterizes snowdrops, water lilies, dandelions, and other plants. Thermonasty is caused by variations in temperature and is observed in crocuses and tulips. Nyctinasty, which is associated with diurnal changes, causes leaf and petal movement in many plants. Nyctinastic movements are combinations of photonastic and thermonastic movements. Seismonasty is movement of plant organs is response to mechanical stimuli or vibrations (for example, the movement of the stigmata and the anther stalks in certain insect-pollinated plants and the movement of leaves in insectivorous plants and in such sensitive plants as Mimosa pudica). Chemonasty, which is caused by small amounts of such gas-forming substances as ethylene, acetylene, and CO, effects leaf movement and the movement of other organs in sweet peas, tomatoes, and other plants.
The biological importance of nastic movements varies. In many plants, they are associated with adaptations for cross-pollination by insects and serve to protect the flowers from unfavorable conditions. In insectivorous plants, the movements aid in the trapping of insects. Some nastic movements are dependent on the presence of myosin-type contractile proteins in the cytoplasm.