Plant Hormones - Experiments on Plant Responses (GCSE Biology)

Experiments on Plant Responses

Plant Hormone Experiments

Seeing the Effect of Auxins

Charles Darwin himself did the first experiments on auxins. He took the tip of plants, known as the coleoptile. This is the area in which auxins are produced.

He then placed three plants, one control, with the plant untouched, one with the top of the coleoptile removed and another with the top of the coleoptile covered, to shield from light. He directed light towards them all and found that the coleoptiles without the tip and those with the covered tip grow upwards, unlike the coleoptile that was left alone,
which grew towards the light.

This showed that in the auxins were produced in the tip of the coleoptile and they were used to cause the plant to grow towards in the light, in its presence.

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The Effect of Light

You can see the results above in experiments in the lab:

1. Take four coleoptiles:
   • Cut the tip off one coleoptile
   • Cover the tip of one coleoptile with black paper
   • Remove the tip of one coleoptile then place it on an agar jelly block for 8 hours in the dark. After eight hours, place the agar jelly block on the rest of the coleoptile.
   • One is a control, leave it as it is.
2. Place a lamp to the right of each coleoptile then leave it for a few days
3. You should see that the coleoptile without the tip and the coleoptile covered in dark paper will grow upwards, whereas the coleoptile with the agar block and the control coleoptile will grow towards the light.
4. The agar block coleoptile is used to prove that the auxins are made in the tip of the coleoptile, so in the ten hour period, the auxins are transferred from the tip to the agar jelly block.

The Effect of Gravity

You can also see the effect of gravity by placing seedlings on a damp cotton wool in a petri-dish. Place the seedlings with their roots in different directions. After leaving them for a week in the dark, you will notice all the roots have grown downwards towards gravity.