Effect of light or gravity on seedling growth
GCSE Biology (8461) · Required practical 8 — method, variables, the marks examiners report students losing.
Investigate the effect of light (phototropism) or gravity (gravitropism) on the growth of newly germinated seedlings.
Apparatus
- Newly germinated seedlings (e.g. cress or mustard) on damp cotton wool in Petri dishes
- A light-proof box or foil with a small window for a one-directional light source (for phototropism)
- A clinostat or a way to lay dishes on their side (for gravitropism)
- Ruler and a protractor to measure growth and angle of bending
Method
- 1Phototropism: set up several dishes of seedlings; give one all-round light, one light from one side only, and one no light (dark control).
- 2Gravitropism: lay some dishes on their side so gravity acts sideways on the shoots and roots.
- 3Keep all dishes at the same temperature with the same water and the same seedlings.
- 4Leave for several days, then observe and measure the direction and angle of growth.
- 5Record whether the shoots and roots grew towards or away from the light or the pull of gravity.
Variables
Independent
Direction of light (or of gravity)
Dependent
Direction and angle of seedling growth
Control
- Type and age of seedlings
- Temperature and water supply
- Time left to grow
Results & processing
- Shoots grow towards light (positive phototropism) and away from gravity (negative gravitropism).
- Roots grow away from light (negative phototropism) and towards gravity (positive gravitropism).
- In one-sided light the shoots bend towards the light; the all-round-light and dark dishes act as controls.
Where students lose marks
No control set-up.
Fix: Include all-round light (and a dark dish) so any bending can be linked to the direction of light, not another factor.
Saying the shoot 'wants' to reach the light.
Fix: Describe it in terms of unequal auxin distribution causing faster growth on the shaded side, so the shoot bends towards the light.
Different water or temperature between dishes.
Fix: Keep water, temperature and seedling type the same so only light/gravity direction differs.
Improve the method
- Use many seedlings per dish and describe the overall response, not one plant.
- Use a clinostat to rotate seedlings and cancel out one-sided gravity as a further control.
- Measure the angle of bending with a protractor for quantitative results.
Try it — exam-style
A seedling is given light from one side only. Its shoot bends towards the light. Name this response and explain it in terms of auxin.
State how a root responds to gravity and name this response.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on effect of light or gravity on seedling growth?
The auxin explanation is a classic higher-mark question — I drill it until it is automatic, and your first lesson is free.