Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
A shoot tip sends down units of auxin in an hour. Light from one side makes of it go to the shaded side. How many units reach each side, and how many more does the shaded side get?
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.
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. Shaded side. units.
Step 2. Lit side. units.
Step 3. The difference. units.
Answer: units on the shaded side, on the lit side, more on the shaded side, so the shoot bends towards the light.
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