Required Practical: Plant Growth (AQA GCSE Biology): Revision Note

Effect of light or gravity

• Aim: To investigate the effect of light or gravity on the growth of newly germinated seedlings

• Investigating the effect of gravity on germinated seedlings involves:

  • setting up petri dishes with seeds and allow them to germinate

  • using a clinostat to alter the direction of gravity

  • recording the direction of growth of shoots and roots

Method for Investigating the gravitropic response

Use this image

1. Add some damp cotton wool to two petri dishes

2. Place 3 bean seedlings in the cotton wool in each petri dish

   • A: radicle facing downwards

   • B: radicle facing horizontally

   • C: radicle (root grows from here) facing upwards

3. Cover each dish with a lid

4. Attach one petri dish to a support so that it’s on its side

5. Attach the second petri dish to a clinostat (as shown in the diagram above)

6. Place both in a light-proof box (so that the seedlings are in complete darkness), leave for two days and then observe growth of the seedlings

Analysis of results

Use this image

• In the first petri dish all radicles (roots) have grown downwards (positive gravitropic response) regardless of which way they were initially facing (horizontal, up or down) and all plumules (shoots) have grown upwards (negative gravitropic response)

• In the second petri dish, all radicles and all plumules have all grown neither up nor down but straight outwards in whichever direction they were placed as the effect of gravity has been cancelled out by the revolving of the clinostat—they have shown no gravitropic response at all

• The experiment needs to be done in a light-proof box in order to control the effect of light on the growth of the seedlings

  • Light is a control variable when investigating gravity