Practice Questions

1.Explain why the root of a horizontally placed seedling bends downwards.

When a seedling is placed on its side, gravity causes the plant hormone auxin to move to the lower side of the root. In roots, auxin inhibits cell elongation. The cells on the upper side therefore grow faster than those on the lower side. This unequal growth causes the root to bend downwards, growing in the direction of gravity (positive gravitropism).

2.Explain why the shoot of a horizontally placed seedling bends upwards.

When the seedling is placed horizontally, auxin accumulates on the lower side of the shoot. In shoots, auxin stimulates cell elongation. The cells on the lower side grow faster than those on the upper side, causing the shoot to bend upwards, away from gravity (negative gravitropism).

3.Describe what happens to auxin distribution when a plant is placed on its side.

When a plant is placed on its side, auxin moves and accumulates on the lower side of the root and shoot due to gravity. This uneven distribution of auxin causes different growth rates on the two sides of the plant, which results in bending.

4.Why does a seedling grown on a clinostat show less bending?

A clinostat slowly rotates the plant, constantly changing its orientation relative to gravity. Because the direction of gravity is continually changing, auxin cannot accumulate on one side of the plant. As a result, growth occurs more evenly and the seedling grows straight instead of bending.