Coordination in Plants

15.1 Coordination and response in plants

Plants respond to stimuli by changing the direction in which they grow.

These responses help the plant: shoots grow towards the light for photosynthesis, and roots grow down towards water and mineral ions and to anchor the plant.

The role of auxin

Auxin is unequally distributed in response to light and gravity.

Phototropism in a shoot.

1. Light shines from one side.
2. Auxin moves to the shaded side of the shoot.
3. The cells on the shaded side elongate more than those on the lit side.
4. The shoot bends towards the light.

Gravitropism in a shoot. If a shoot is placed on its side, auxin collects on the lower side. The cells on the lower side elongate more, so the shoot bends upwards, away from gravity.

(In roots, a high concentration of auxin on the lower side inhibits growth, so the roots bend downwards. The details are not required.)

Investigations

• Phototropism: grow seedlings in a box with a hole on one side (or with a light on one side). The shoots bend towards the light. Seedlings with the tip removed or covered with foil do not bend, which shows that the tip detects the light and makes the auxin. Controls: seedlings lit from above, or in the dark.
• Gravitropism: place germinating seeds on damp cotton wool on a vertical board, with the roots pointing in different directions, or turn the seedlings on their side. After a few days the roots always bend downwards and the shoots upwards. Using a clinostat (which slowly rotates the seedlings so that gravity acts evenly) as a control, there is no bending.

Auxin made in the tip moves to the shaded side. The cells on the shaded side elongate more, so the shoot bends towards the light.