Explain the experiments of Darwin and Went that led to the discovery of auxins.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define. Auxins are plant hormones made at the shoot tip that cause cell elongation.
2 · Darwin's experiment.
Canary grass coleoptiles lit from one side bent towards the light.
Tip removed or covered with an opaque cap: no bending.
Conclusion: the tip senses light and sends an influence down.
3 · Went's experiment.
Oat coleoptile tips were kept on agar blocks.
Agar block placed on one side of a cut stump: the stump bent away from it.
Plain agar: no bending.
Conclusion: a chemical (auxin) from the tip causes growth.
4 · Draw the experiments. [Diagram: Darwin's seedlings with light, tip cut and tip capped; Went's tip on agar, agar on one side of a stump, bent stump, plain agar control]
5 · Uses of auxins. Rooting of cuttings, seedless fruits, preventing fruit drop, and 2,4-D as a weedicide.
6 · Link to daily life. Rooting powder for rose cuttings and 2,4-D sprayed in paddy fields are auxins.
7 · Close. So these experiments proved that a chemical from the tip controls growth and bending.
Scoring tip: Write the conclusion of each experiment in one clear line. That line carries a mark on its own.
