A gardener grows two identical seedlings in separate pots. Seedling A is placed near a window with light coming from one side, while Seedling B is kept in complete darkness. After a week, Seedling A bends toward the light but Seedling B grows straight up.
(i) Explain the role of auxin in causing Seedling A to bend toward the light, clearly describing how the unequal distribution of auxin produces directional growth.
(ii) The movement of Seedling A toward light is described as growth-dependent. How is this fundamentally different from the way the leaves of a touch-me-not plant (Mimosa) move when touched? In your answer, refer to the cellular mechanism responsible for each type of movement.
Generated by claude-sonnet-4-6 · 2026-06-26 01:01 · grounding rag
Model Answer
(i) Role of Auxin in Phototropism:
When light falls from one side on Seedling A, auxin (synthesised at the shoot tip) diffuses towards the shady side of the shoot. This causes a higher concentration of auxin on the side away from light. Auxin stimulates the cells on the shady side to elongate more than the cells on the lit side. Since one side grows longer than the other, the shoot bends towards the light. This directional growth movement is called phototropism.
(ii) Growth-dependent vs. Growth-independent Movement:
The bending of Seedling A is growth-dependent — it occurs because cells on one side elongate permanently due to auxin. If growth is prevented, no movement occurs.
The leaves of the touch-me-not (Mimosa) move independently of growth. When touched, information is conveyed cell to cell by electrical-chemical signals. Cells at the base of the leaf change their water content — they lose water and shrink, causing the leaf to fold and droop rapidly. No cell elongation or growth is involved; shape changes through osmotic swelling or shrinking of cells.
Source: Chapter 6, Sections 6.2, 6.2.1, 6.2.2
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Explanation
- For part (i), the key chain is: light from one side → auxin shifts to shady side → greater cell elongation on shady side → bending toward light. Examiners want all three steps clearly stated.
- For part (ii), the contrast must be explicit: growth (cell elongation, permanent) vs. no growth (change in water content of cells, reversible). The word "osmotic" or "water content change" is expected for the Mimosa mechanism.
- Do not confuse the two types of movement — the textbook explicitly states Mimosa movement is growth-independent, while phototropism is growth-dependent.