Thigmotropism is a plant's response to what?

• A. Light
• B. Temperature
• C. Gravity
• D. Contact

Answer: (D)

Thigmotropism is a plant growth response to touch or physical contact. When a growing part of a plant encounters a solid object, receptors detect the mechanical stimulus and trigger a hormonal change that alters growth on the side away from the contact. This differential growth causes the plant to bend or coil around the object, helping it climb or secure itself. You can see this in vines and pea tendrils that coil when they touch a support.

This differs from phototropism, where growth is directed by light; gravitropism, where roots grow downward and shoots upward in response to gravity; and thermotropism, where temperature influences growth. The touch-based reaction is what defines thigmotropism.