Thigmotropism
Thigmotropism is a movement in which an organism moves or grows in response to touch or contact stimuli. The prefix thigmo- θιγμος comes from the Greek for "touch". Usually thigmotropism occurs when plants grow around a surface, such as a wall, pot, or trellis. Climbing plants, such as vines, develop tendrils that coil around supporting objects. Touched cells produce auxin and transport it to untouched cells. Some untouched cells will then elongate faster so cell growth bends around the object. Some seedlings also inhibit triple response, caused by pulses of ethylene which cause the stem to thicken (grow slower and stronger) and curve to start growing horizontally.
Mimosa pudica is well known for its rapid plant movement. The leaves close up and droop when touched. However, this is not a form of tropism but a nastic movement, a similar phenomenon. The difference is that tropisms are influenced by the direction of their stimulus, while nastic movements are not.
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