Required Practical: Plant Growth (AQA GCSE Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
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Effect of Light or Gravity
Aim: To investigate the effect of light or gravity on the growth of newly germinated seedlings
In order to investigate the effect of gravity on germinated seedlings you will:
Set up two petri dishes with 3 mustard seeds and allow them to germinate
Place one dish on a clinostat and the other to a support on its side
Record the direction of growth of both the shoots and the roots for each seed
Method
Investigating the gravitropic response (set-up)
Add some damp cotton wool to two petri dishes
Place 3 bean seedlings in the cotton wool in each petri dish
A - radicle facing downwards
B - horizontally
C - radicle (root grows from here) facing upwards
Cover each dish with a lid
Attach one petri dish to a support so that it’s on its side
Attach the second petri dish to a clinostat (as shown in the diagrams above).
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
Investigating the gravitropic response (results)
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 lightproof box in order to control the effect of light on the growth of the seedlings
Light is a control variable when investigating gravity
Examiner Tip
Make sure you know what a clinostat is and what it does (it cancels out the effect of gravity).
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