Light comes from the right. In coleoptiles (a) and (b), a thin sheet of mica is pushed into the…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Light comes from the right. In coleoptiles (a) and (b), a thin sheet of mica is pushed into the stem just below the tip. No chemical can pass through mica.
- The mica sheet is on the left (shaded) side.
- The mica sheet is on the right (lit) side.
- In a third coleoptile, the cut tip is put back on the left half of the stump, in the dark.
What does each coleoptile do? Explain what each result shows about the signal.
Need a reminder? The key ideaShow
Auxins are indole compounds made at growing tips. IAA and IBA are natural; NAA and 2,4-D are synthetic. They help cuttings to root, keep apical dominance, cause parthenocarpy, prevent early fruit and leaf drop, and 2,4-D kills dicot weeds. In Went's Avena test a coleoptile bends away from an agar block that holds auxin, and a bigger angle means more auxin.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The mica blocks the shaded side, where the signal travels. No bending.
Step 2. (b) The shaded side is open, so the signal passes. The coleoptile bends towards the light.
Step 3. (c) The left half gets the signal and grows faster. The coleoptile bends to the right, away from the tip.
Step 4. What it shows. The signal is a chemical, because mica stops it. It moves down the shaded side, and it makes cells grow longer.
Answer: (a) no bending; (b) bends towards light; (c) bends away from the side of the tip. The signal is a chemical that travels down the shaded side.
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