Investigating tropismsSpec 14.5.3
In short
Tropisms are investigated by growing young seedlings, such as cress or beans, on damp cotton wool in clear dishes. For phototropism, light reaches them from one side through a slit in a box. For gravitropism, a dish is turned on its side in the dark. Shoots bend towards light and upwards; roots bend away from light and downwards.
You can investigate gravitropism and phototropism in the shoots and roots of young seedlings, such as cress or bean seedlings. Grow them on damp cotton wool or filter paper in clear dishes so the roots can be seen.
Phototropism
- Grow seedlings of the same species and age in identical clear dishes on damp cotton wool or filter paper, with equal water.
- Put one set in a box with a slit or hole so light reaches them from one side only.
- Put a control set in the same conditions but with light from all sides, or in the dark.
- Leave for several days, then record the direction of growth of the shoots and roots, with a sketch or by measuring the angle.
Gravitropism
- Place germinating seeds on damp cotton wool or paper in a dish, and fix the dish upright so the seeds are held in place.
- Keep one dish upright and turn another on its side. Keep both in the dark so light does not affect the result.
- Leave for several days, then record the direction of growth of the shoots and roots.
| Plant part | Light from one side | Dish turned on its side (dark) |
|---|---|---|
| Shoot | Bends towards the light | Bends upwards |
| Root | Bends away from the light | Bends downwards |
Change only one variable. For gravitropism keep the seedlings in the dark, and for phototropism keep their position fixed. Keep temperature and water the same, and use several seedlings so one odd result does not mislead you.
Quick check
Define phototropism.
Show answer
A response in which parts of a plant grow towards or away from the direction of the light source.
In which direction does a root grow in response to gravity?
Show answer
Towards gravity (downwards).
Supplement Where is auxin made in a shoot?
Show answer
In the shoot tip.
Supplement What does auxin do to shoot cells?
Show answer
It stimulates cell elongation.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026