to be an important regulator to the thigmotropic response in Arabidopsis thaliana roots. Under normal circumstances, high ethylene concentrations in the roots promote straight growth. When the root encounters a rigid object, the thigmotropic response is activated and ethylene production is down-regulated, leading to the root to bend while growing rather than growing straight. Like phototropism, a thigmotropic response in stems requires light. Plant biologist Mark Jaffe performed a simple preliminary experiment using pea plants that led to this conclusion. He found that when he snipped a tendril off of a pea plant and placed it in the light, then repeatedly touched one side of it, the tendril would begin to curl. However, when performing this same experiment in the dark, the tendril would not curl. In roots 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. Research suggests that this active obstacle avoidance by roots is driven by polar auxin transport. 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. Misconception 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. Nastic movements are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. References External links Thigmotropism in Tendrils Tropism Plant physiology Auxin action