Nastic Movements

Plant movements that occur in response to environmental stimuli but that are independent of the direction of stimuli are called nastic movements. These movements are regulated by changes in the water pressure against the cell wall (turgor pressure) of certain plant cells.

Thigmonastic Movements

Thigmonastic (THlG-mah-nas-tik) movements are nastic movements that occur in response to touching a plant. Many thigmonastic movements are rapid, such as the closing of the leaf trap of a Venus' flytrap around an insect. Figure 31-8 shows how the leaves of a sensitive plant fold within a few seconds after being touched.

figure 31-8

(a) Mimosapudica, also called the sensitive plant, is a small shrub that has leaflets. (b) When a leaflet is touched, it folds together. This rapid movement is a thigmonastic movement.

Nastic Movement Tree Com
(a) (b)

figure 31-9

This movement is caused by the rapid loss of turgor pressure in certain cells, a process similar to that which occurs in guard cells. Physical stimulation of the plant leaf causes potassium ions to be pumped out of the cells at the base of leaflets and petioles. Water then moves out of the cells by osmosis. As the cells shrink, the plant's leaves move.

The folding of a plant's leaves in response to touch is thought to discourage insect feeding. In addition, thigmonastic movements may help prevent water loss in plants. When the wind blows on a plant, the rate of transpiration is increased. So if the leaves of a plant fold in response to the "touch" of the wind, water loss is reduced. This could be an important adaptive advantage to a plant.

Nyctinastic Movements

Nyctinastic (NIK-tuh-NAS-tik) movements are responses to the daily cycle of light and dark. These movements involve the same type of osmotic mechanism as thigmonastic movements, but the changes in turgor pressure are more gradual. Nyctinastic movements occur in many plants, including bean plants, honeylocust trees, and silk trees. The prayer plant, shown in Figure 31-9, gets its name from the fact that its leaf blades are vertical at night, resembling praying hands. During the day, its leaf blades are horizontal. The botanist Linnaeus planted many plant species with nyctinastic movements in a big circle to make a "flower clock." The nyctinastic movements of each species occurred at a specific time of day.

figure 31-9

A common houseplant is the prayer plant, Maranta leuconeura. (a) During the day, the leaf blades of the prayer plant are oriented horizontally in response to light. (b) During the night, the leaf blades are oriented vertically. This movement is called a nyctinastic movement.

www.scilinks.org Topic: Nastic Movements Keyword: HM61012

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Responses

  • James Banks
    Why the leaf blades of maranta leuconeura are oriented vertically during night time?
    1 year ago

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