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- Define and investigate geotropism (as a response in which a plant grows towards or away from gravity) and phototropism (as a response in which a plant grows towards or away from the direction from which light is coming).
Geotropism is a response in which a plant grows towards or away from gravity.
In positive geotropism, the plant grows toward gravity. In negative geotropism, the plant grows away from gravity. Note that shoots tend to exhibit negative geotropism and that roots tend to exhibit positive geotropism.
Phototropism is a response in which a plant grows towards or away from the direction from which light is coming.
In positive phototropism, the plant grows towards the direction from which light is coming and in negative phototropism, the plant grows away from this direction. Note that shoots tend to exhibit positive phototropism and roots tend to exhibit negative phototropism.
The hormones that regulate plant growth are called auxins.
Note that auxins in the shoot stimulate cell growth (by stimulating the absorption of water, forcing cells to expand) and that auxins in the root slow down cell growth.
Auxins are produced in the shoot and root tips of growing plants. These regions are called ‘meristems’. More specifically, a meristem is a region of cell division in a plant, but I don’t think you’ll have to know this term at the IGCSE level.
I think the image summarises all that you need to know in regard to the investigation pretty well, but if you want a more detailed experiment plan then use this link:
Take two cardboard boxes. In one, cut a small hole (about 2’’ in diameter) on the top and in the other, cut a hole of the same size on the side of the box, about 3cm up from the bottom. Cover both holes with clear plastic.
Take a photo of two small plant shoots growing in their own separate plant pots from the front. Place one in each box, lining them up with the holes. You can grow your own bean plant shoots for this experiment. Take a photo of both plants from above.
Position the boxes in different parts of the same room, and place lamps next to the boxes. Each one should be lined up with the holes on the boxes so that maximum light can get inside from the direction of the hole.
Every morning, turn on both lamps and every night, turn them off. Make sure they are properly watered and fertilized (both should be provided water and fertilizer in the exact same amounts). Do this for one week. Open the boxes and take pictures of the plants from above and in front. You will notice that both plants grew in the direction of the holes.
Explanation: In phototropism, the auxins move away from the light, towards the shaded side, so this side of the plant will grow more, causing the plant to bend towards the light.
- Explain the chemical control of plant growth by auxins including geotropism and phototropism in terms of auxins regulating differential growth.
Looks like I already explained this in the previous point!
Notes submitted by Sarah.
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