Section 2

Plant Movements

• Tropisms and nastic movements are plant responses to environmental stimuli. Tropisms occur slowly; nastic movements happen more quickly.

• A tropism is a response in which a plant grows either toward or away from an environmental stimulus. Tropisms can occur in response to light, touch, gravity, and chemicals.

• Phototropism is thought to occur in some plants when auxin moves to the shaded side of a plant and causes cells there to elongate more than the cells on the lighted side.

• Gravitropism is thought to occur when auxin accumulates on the lower sides of a horizontal root and stem. This accumulation causes cell elongation on the lower side of the stem and inhibits cell elongation on the lower side of the root.

• Nastic movements are responses to environmental stimuli but are independent of the direction of the stimuli. Thigmonastic movements occur in response to touch, and nyctinastic movements occur in response to the daily cycle of light and dark.

Vocabulary tropism (p. 636) phototropism (p. 636) solar tracking (p. 637)

thigmotropism (p. 637) gravitropism (p. 638) chemotropism (p. 638)

nastic movement (p. 638) thigmonastic movement (p. 638)

nyctinastic movement (p. 639)

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