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 seedlings

• Investigating the effect of gravity on germinated seedlings involves:

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

  • using a clinostat (which rotates slowly) so that gravity acts equally on all sides of the seedlings, cancelling out the one-sided effect of gravity

  • recording the direction of growth of shoots and roots

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 (root grows from here) facing downwards

   • B: radicle facing horizontally

   • C: radicle 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

   • This prevents light affecting the direction of growth, so only gravity affects the results

Analysis of results:

Use this image

• In the first petri dish:

  • all roots have grown downwards (positive gravitropic response) regardless of which way they were initially facing (horizontal, up or down)

  • all shoots have grown upwards (negative gravitropic response)

• In the second petri dish:

  • all roots and all shoots have grown straight outwards in whichever direction they were placed (no gravitropic response) as the effect of gravity has been cancelled out by the revolving of the clinostat

Investigating the effect of light on seedlings

• It is also possible to investigate the effect of light on seedling growth as follows

  1. Add damp cotton wool to two Petri dishes

  2. Set up equal numbers of germinated seeds in each Petri dish

  3. Place one Petri dishes in each light condition, e.g.:

     • light from one side (e.g. a box with a slit, with a lamp shining through it) vs complete darkness

     • unidirectional light vs light from all directions

  4. Measure the height of each seedling at the start; leave the seedlings for two days, then measure again

     • Use a ruler to measure the height of straight seedlings

     • Use a piece of string and a ruler to determine the height of curved seedlings

     • Record their appearance (e.g. which way they have bent) as labelled biological drawings

  5. Calculate the mean height increase for each Petri dish

• Variables that would need to be controlled include:

  • temperature

  • volume of water

  • plant species

  • age of seedlings