nastic movement

noun

: movement of a flat plant part (as a leaf or bud scale) that is oriented in respect to the plant rather than an external source of stimulation, is brought about by disproportionate growth or increase of turgor in the tissues of one surface of the part, and typically involves a curling or bending outward or inward of the whole part in a direction away from the more active surface

the opening and closing of four-o'clocks involves nastic movements

compare tropism

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