A student claims: 'Tropic movements in plants are really the same as the movement of the leaves of a touch-me-not plant — both are just responses to an external stimulus.' Evaluate this claim. In what important way are the two types of movement fundamentally different?
Generated by claude-sonnet-4-6 · 2026-06-26 01:02 · grounding rag
Model Answer
The student's claim is partially correct but fundamentally flawed.
Both movements are indeed responses to external stimuli. However, they differ in a crucial way:
- The touch-me-not (Mimosa) movement is independent of growth. Cells change shape by losing or gaining water (changing turgor pressure), causing rapid folding of leaves.
- Tropic movements (e.g., phototropism, geotropism) are dependent on growth. A hormone like auxin causes unequal growth on two sides of the shoot, making it bend directionally. If growth is stopped, tropic movement does not occur.
Thus, the fundamental difference is: touch-me-not movement involves no growth, while tropic movement is caused by growth.
Source: Chapter 6, Section 6.2 – Coordination in Plants; Section 6.2.2 – Movement Due to Growth
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Explanation
Examiners expect you to:
- Acknowledge what is correct in the claim (both are stimulus responses) — don't dismiss it entirely.
- Clearly contrast the two types: growth-independent (turgor/water change) vs. growth-dependent (auxin-driven differential growth).
- Use textbook terms: turgor pressure / water content change for touch-me-not; auxin / differential growth for tropisms.
The key sentence the examiner wants: "If growth is prevented, tropic movement will not occur, but the touch-me-not movement will still happen." This shows you understand the fundamental distinction.