(i) Leaves of chhui-mui plant begin to fold up and droop in response to a stimulus. Name the stimulus and write the cause for such a rapid movement. Is there any growth involved in the movement ?
(ii) Define geotropism in plants. What is meant by positive and negative geotropism ? Give one example of each type.
Generated by claude-sonnet-4-6 · 2026-06-15 06:49 · grounding rag
Model Answer
(i)
Stimulus: Touch
Cause: The leaves of chhui-mui (Mimosa) move because plant cells change their shape by changing the amount of water in them. When touched, information is transmitted from cell to cell by electrical-chemical means, causing cells to lose water and shrink. This results in the leaves folding up and drooping.
Growth involved: No. This movement is independent of growth; it is an immediate response to the stimulus.
(ii)
Geotropism is the directional growth movement of plant parts in response to the pull of gravity (earth).
- Positive geotropism: Growth in the direction of gravity (downward). Example: Roots of a plant grow downwards into the soil.
- Negative geotropism: Growth against the direction of gravity (upward). Example: Shoots/stems of a plant grow upwards, away from the earth.
Source: Chapter 6, Sections 6.2, 6.2.1, 6.2.2
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Explanation
- Part (i) carries ~2–3 marks: name the stimulus (1 mark), explain the cause using water movement in cells (1 mark), and state no growth is involved (1 mark).
- Part (ii) carries ~2 marks: define geotropism, then define positive and negative with one example each. Examiners expect the word "gravity" in the definition.
- Key phrase to remember: plant cells change shape by changing the amount of water in them — this explains chhui-mui movement without growth.
- Don't confuse phototropism (light) with geotropism (gravity).