What it shows
Plant roots can bend towards moisture, a response called hydrotropism, even when gravity pulls them straight down. A tray with a damp side and a dry side shows roots growing toward the water.
The science
Roots respond to several stimuli, such as gravity (gravitropism), moisture (hydrotropism) and light. Cells on the side of the root tip that is nearer the moisture grow more slowly, and the root curves toward the wetter side. In nature this helps roots find water in the soil. In this test the damp patch must not make the sawdust wet everywhere, otherwise the root has no difference to sense.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A shallow plastic tray or box lid, about 20 cm by 12 cm
- Dry sawdust or dry sand to fill the tray
- A small damp cotton wool pad or a damp clay pot piece
- Soaked gram or pea seeds, 6
- A glass jar to hold water
- A spoon
- A notebook
Safety
For this project: Wash hands after handling soil and seeds. Keep the sprouts away from small children who may put them in their mouth.
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.
Fill the tray with dry sawdust or dry sand to a depth of about 3 cm and level it.
Place the damp cotton wool pad against one short end of the tray and press it into the sawdust. Only that end should be damp.
Plant six soaked seeds in a row in the middle of the tray, about 8 cm from the damp end, with the tips of the little roots pointing down.
Keep the tray in a shaded place at room temperature and add water to the damp pad only, never to the sawdust.
After 3 to 4 days, gently scrape away the dry sawdust with a spoon to look at the roots, without breaking them.
Draw the direction of each root with an arrow pointing from the seed to the root tip, and note whether they curve toward the damp pad.
Set up a control tray with no damp pad, but water poured evenly through the sawdust, and compare the root directions.
Photograph both trays and write your conclusion in two sentences.
What you should see
Most roots in the first tray curve toward the damp pad while roots in the control tray grow straight down, showing that moisture can bend them.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is hydrotropism?
The growth of a plant part toward or away from water.
2Why is there a control tray?
To show that the roots bend because of the damp pad and not by chance.
3What other tropisms do you know?
Phototropism (light), gravitropism (gravity) and thigmotropism (touch).
4Why is this useful to a plant?
Roots reach water in dry soil and the plant survives.
Take it further
Repeat with a second damp pad on the opposite side and see which side the roots choose.
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.
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