# Explain why the roots of a seedling grow downwards and the shoot grows upwards when the seedling is placed horizontally on damp soil.

> Cambridge International IGCSE Biology (0610) — 14.5 Tropic responses · Explain · 4 marks

## Mark scheme (4 marks)

1. Identifies that this growth response is gravitropism
2. States that auxins accumulate on the lower side of the horizontal root and shoot
3. States that in the root, the higher concentration of auxins on the lower side inhibits growth, so the lower side grows more slowly, causing the root to bend / grow downwards (positive gravitropism)
4. States that in the shoot, the higher concentration of auxins on the lower side stimulates growth, so the lower side grows more quickly, causing the shoot to bend / grow upwards (negative gravitropism)

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