What is the external stimulus that triggers thigmotropism in plants?

• A. Gravity
• B. Light
• C. Touch or physical contact
• D. Water

Answer: (C)

Verified step by step guidance

1. Understand the concept of thigmotropism: Thigmotropism is a directional growth response in plants that occurs as a result of physical contact or touch with a solid object.
2. Identify the external stimuli that can affect plant growth: Common stimuli include gravity (gravitropism), light (phototropism), water (hydrotropism), and touch (thigmotropism).
3. Analyze the options provided: Gravity, light, and water are not related to thigmotropism. They are associated with other types of tropisms.
4. Focus on the correct stimulus for thigmotropism: The key external stimulus for thigmotropism is touch or physical contact, which causes the plant to grow in response to the object it touches.
5. Conclude that the correct external stimulus triggering thigmotropism is touch or physical contact, as it directly influences the plant's growth direction in response to the stimulus.