HardBiologyHigh School (9–10)GeneralPlant Biology › Plant Hormones and MovementsHuman Body › Hormones and the Endocrine SystemConceptualapplicationBoard ExamA-LevelIB
In a dark room, Rahul cuts the tip off an oat shoot…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In a dark room, Rahul cuts the tip off an oat shoot. He then puts the tip back, but only on the left half of the cut stump. A day later the shoot has bent. Which way did it bend, and why? There was no light from any side.
Need a reminder? The key ideaShow
Auxin is made at the shoot tip. In light from one side, it moves to the shaded side, where it makes the cells grow longer, so the shoot bends towards the light. Gibberellins help stems grow, cytokinins promote cell division, and abscisic acid slows growth and makes leaves wilt.
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. Where the auxin goes. The tip sits on the left half, so its auxin moves down the left side of the stump.
Step 2. Which side grows more. The cells on the left get auxin and grow longer. The right side gets little and grows less.
Step 3. The bend. The longer left side ends up on the outside of the curve, so the shoot bends to the right.
Answer: it bent to the right, away from the side with the tip, because auxin made the left side grow longer. No light was needed: auxin alone caused the bend.
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