What it shows
Plants pull water up through thin tubes in their stems. When the water is coloured, the colour travels up inside the stalk and shows exactly where the tubes run, all the way to the leaves.
The science
Stems contain xylem, a set of narrow tubes that carry water and minerals from the roots to the leaves. Water evaporates from the leaves (transpiration), and this pulls more water up the tubes behind it, helped by water molecules sticking to each other and to the tube walls. The dye is carried along in the water, so the xylem tubes show up as coloured dots in a cut stalk.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 2 fresh celery stalks with leaves (or a leafy stem of balsam), from the vegetable shop
- Red or blue food colour or washable ink (a few drops)
- A tall glass or jar
- Water
- A kitchen knife for the adult only
- A notebook and coloured pencils
Safety
For this project: An adult must cut the stalks and the cross-section. Do not eat celery that has stood in ink; food colour is fine to touch but keep it off clothes.
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 glass about one third with water and mix in ten drops of food colour until it is strongly coloured.
Ask an adult to cut about 2 cm off the bottom of each celery stalk with a sharp knife so fresh tubes are open.
Stand one stalk in the coloured glass and the second stalk in a glass of plain water as your control.
Place both glasses in a bright spot out of direct hot sun and mark the water level with a pen.
After two hours look at the leaves and the stalk surface and note any colour you can see, then look again after one night.
Measure the highest point the colour has reached on the stalk and write it in your notebook.
Ask an adult to cut the coloured stalk across at the middle and look at the cut face for small coloured dots in a ring.
Draw the cross-section and label the dots as xylem tubes, then compare the leaves of the two stalks.
What you should see
The coloured stalk shows colour in its veins and leaf tips after a few hours, and the cut face shows a ring of coloured dots, while the control stalk stays pale green. The water level in the glass has dropped.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which tubes carry water up a stem?
The xylem tubes.
2Why did you keep a stalk in plain water?
As a control, to show the colour comes from the dye and not from the celery itself.
3What pulls the water up?
Water evaporating from the leaves pulls more water up the xylem, and the water molecules stick together as a continuous column.
4Would the colour rise faster in a warm, windy place?
Yes, because the leaves lose water faster, so water is pulled up faster.
Take it further
Measure how high the colour climbs after one hour at the same spot but in a shady room versus in front of a fan, and compare.
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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