Co-ordination and Response » Geotropism and Phototropism

Plants respond too

Plants do not have muscles, so they cannot run away or reach for things. But they do respond to changes around them. A houseplant on a windowsill slowly turns its leaves towards the glass. A seed that lands upside down still sends its shoot up and its root down. Plants respond to three main stimuli: light, gravity and water.

Plants respond by growing. A response in which a plant grows towards or away from a stimulus is called a tropism. Plant responses are much slower than animal responses, because the plant has to grow to make them happen.

Key terms:

• Tropism: a growth response of part of a plant towards or away from a stimulus.
• Positive tropism: growth towards the stimulus.
• Negative tropism: growth away from the stimulus.
• Phototropism: a growth response to light.
• Geotropism: a growth response to gravity.

Phototropism: responding to light

Leaves make food by photosynthesis, so they need light. A shoot that is lit from one side grows towards the light. This is positive phototropism. Shoots (stems) are positively phototropic.

Roots do the opposite. If a root is lit from one side, it grows away from the light. Roots are negatively phototropic. This response is weak, because roots are normally buried in dark soil anyway.

Geotropism: responding to gravity

Gravity pulls downwards, and plants respond to it. Roots grow downwards, in the same direction as gravity. Roots are positively geotropic. Shoots grow upwards, against gravity. Shoots are negatively geotropic.

This works in the dark too. A seed germinating under the soil cannot see any light, but its root still grows down and its shoot still grows up. If you turn a growing seedling on its side, the root curves back downwards and the shoot curves back upwards.

Why these responses help the plant

Each response has a job to do. If you can say why a response helps, you can answer almost any exam question on this topic.

• Shoots grow towards light: the leaves get more light, so there is more photosynthesis and the plant makes more food.
• Shoots grow upwards: a shoot that starts underground grows up and reaches the surface, where there is light.
• Roots grow downwards: roots reach water and mineral ions in the soil, and hold the plant firmly in place.
• Roots grow away from light: they stay in the soil rather than growing out into the air, where they could not take in water.

Plants also respond to water. Roots grow towards a source of water in the soil, which is a positive response. This helps the plant to find the water it needs.

Investigating tropisms with seedlings

You can test these responses with seedlings. Here is a simple experiment on light.

1. Grow several identical seedlings in pots, all in the same soil and watered equally.
2. Put one pot in a box with a hole cut in one side, so light only comes from one direction.
3. Put a second pot in an open space with light from all around, or in a box with no hole. These are the controls.
4. Leave them for a few days and compare the direction of growth.

The shoot in the box with one hole bends towards the hole. The shoot with light from all sides, or in total darkness, grows straight up.

To test gravity, fix a seedling on damp cotton wool inside a sealed dish and lay it on its side. Leave it in the dark, so light cannot affect the result. After a few days, the root has curved downwards and the shoot has curved upwards.

Exam-style question

A student lays a germinating bean seedling on its side in a dark cupboard for three days.

(a) Describe what happens to the root and the shoot. [2 marks]

(b) Name the type of response shown by the root. [1 mark]

(c) Explain why this response helps the plant. [2 marks]