Auxin is a plant hormone. Explain how auxin causes the results in apparatus A.

Auxin is a plant growth hormone produced in the tips of roots and shoots. It plays a key role in tropisms. In apparatus A, the seedling is placed horizontally. 1. **Distribution:** Gravity causes the auxin produced in the root tip to settle and accumulate on the lower side of the root. 2. **Effect in Roots:** Unlike in shoots, where auxin promotes cell elongation, high concentrations of auxin *inhibit* cell elongation in roots. 3. **Differential Growth:** Because there is a higher concentration of auxin on the lower side, the cells there elongate less. The cells on the upper side, with a lower auxin concentration, continue to elongate at a faster rate. 4. **Bending:** This difference in elongation rates (faster on top, slower on the bottom) causes the root to bend downwards, in the direction of gravity. This is positive gravitropism.