Thigmotropism is a movement in which a plant moves or grows in response to touch or contact stimuli. The prefix thigmo- comes from the Greek for "touch" (θιγμός). 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 exhibit triple response, caused by pulses of ethylene which cause the stem to thicken (grow slower and stronger) and curve to start growing horizontally.
Roots also rely on touch to navigate their way through the soil. Generally, roots have a negative touch response, meaning when they feel an object, they would grow away from the object. This allows the roots to go through the soil with minimum resistance. Because of this behavior, roots are said to be negatively thigmotropic. Thigmotropism seems to be able to override the strong gravitropic response of even primary roots. Charles Darwin performed experiments where he found that in a vertical bean root, a contact stimulus could divert the root away from the vertical.
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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