What it shows
Plants grow towards light. A seedling kept by a window bends towards it, and turns back if we turn the pot around.
The science
A plant needs light to make food. Cells on the shaded side of the stem grow longer than cells on the lit side, so the stem bends towards the light. This bending is called phototropism. The plant is not choosing; a growth chemical collects on the dark side.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A small potted seedling or a bean sprouted in a cup (free)
- A window with sunlight
- A shoebox with a hole cut in one side
- A notebook and pencil
- Masking tape
Safety
For this project: An adult must cut the hole in the box. Wash hands after touching soil.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Place the seedling in the shoebox so that the hole faces the window.
Put the box on a table near the window and keep it there.
Draw the plant on day one with the shoot straight up.
Water the plant a little every day, keeping the box in the same place.
After two days, look through the box top and draw the plant again.
Note which way it bends and tape a paper arrow on the box to show it.
Turn the box around by half a turn and look again after two more days.
Draw what has changed and write the reason.
What you should see
The shoot bends towards the hole where the light comes in. When the box is turned, the plant slowly bends the other way.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the plant bend?
To reach the light.
2What does the plant use light for?
To make food.
3What happened when you turned the box?
The plant bent in the new direction of the light.
4Does a plant have eyes?
No, but it senses light.
Take it further
Make a cardboard maze in the box and see whether the plant finds the light.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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