What Parts of the Plant Can Respond to Touch?
The clearest example of thigmotropism is the coiling that occurs in some tendrils. However, roots also depend on touch sensitivity to navigate their way through the soil. The general touch response in roots is negative. That is, when a root “feels” an object, the root grows away from the object. In comparison, most tendrils grow toward the touch stimulus, allowing for the tendril to wrap around the object which it is in contact with. Therefore, roots are said to be “negatively thigmotropic”. This allows the roots to follow the line of least resistance through the soil. In addition to thigmotropic responses, roots (as well as other organs) are known to grow in response to gravity. This “gravitopism” allows the roots to grow in the direction of gravity, which is down into the earth. Interestingly, thigmotropism seems capable of overriding the strong graviptropic responses of even primary roots. Darwin himself performed experiments which compared these two responses. He found that in a ve
