Geotropism (or gravitropism) is the growth movement by a plant due to gravity. Plant roots usually show positive geotropism and stems show negative geotropism. In this experiment a kidney bean is placed on a rotating turntable, known as a clinstat, and the roots which grow over a period of time are observed. Students should see that the roots do not grow straight downwards as would be expected but towards the outside edge of the turntable. The root growth has been affected by the rotation of the clinostat.
Equipment typically required (per set):
- Kidney bean(s)
- Paper towel
- Aluminium foil
- Sticky tape
- 250ml beaker
- Marker pen
The kidney bean is soaked in a beaker of water for 24 hours and then wrapped in the paper towel. The towel should be rolled loosely around the bean and then moistened with water.
A square of foil should be folded around the paper towel so that it is covered completely. Mark the centre point of the foil square and place onto the clinstat so that the centre mark faces the centre of the turntable.
Start the clinstat up on a very low rotational speed. Every day or two, open the foil and ensure the paper towel is moist. Try not to disturb the bean during these checks and make sure they go back in exactly the same position each time.
After 5 days or so, open the packets and observe the direction of the root which should have grown.
The clinostat exterts centripetal force on the bean, creating a false gravity. Instead of the root growing in response to the Earth's gravity, it grows in response to this centripetal force and outwards from the centre of the clinostat.