How a tree works
A tree is a living water pump and food factory. Find out how water travels from the roots up to the leaves and out into the air, why leaves change colour in autumn, and test which branches give off the most water.
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A tree's roots take up water and minerals from the soil. Most of the big roots are near the surface, in the top 30 cm or so, and they often spread wider than the branches.[1]
Inside the trunk, a layer called the xylem (sapwood) carries water up to the leaves. The phloem (inner bark) carries the food made in the leaves to the rest of the tree.[1]
Leaves make food by photosynthesis: they use light energy to turn carbon dioxide and water into sugar, and they give out oxygen. The green pigment chlorophyll captures the light.[3][6]
Leaves have tiny holes called stomata. Carbon dioxide gets in through them, and water vapour escapes. Giving off water vapour like this is called transpiration. A large oak tree can give off about 151,000 litres of water in a year![3][5]
In autumn, a layer at the base of each leaf stalk of a deciduous tree seals off the water, the green chlorophyll fades and the yellow and orange colours that were hidden underneath show. Some leaves also make new red colours. Then the leaf falls.[6][2]
Each year a tree grows a new ring of wood: light wood in spring and early summer, darker wood later. Counting the rings tells you the tree's age, and wide rings show good growing years.[4]
Trees are record-breakers. Coast redwoods can grow to about 112 m tall, and some bristlecone pines are around 5,000 years old.[7][8]
Try it: Fair test: catching transpiration
You need
- Three clear plastic food bags
- Soft ties or string
- Small tree or shrub branches in the school grounds (with permission)
- Paper towels
- A kitchen scale (optional)
Safety
Ask permission before using a tree. Avoid plants you do not know (some are poisonous or irritate skin) and wash hands afterwards. Never put plastic bags over heads. Take all bags and ties away at the end.
What to do
- Choose two leafy branches on the same tree: one in the sun and one in the shade.
- Choose a third twig and gently pinch off its leaves, or use a twig that has none.
- Weigh three dry paper towels, put one in each bag, then slide each bag over a branch so the leaves are inside. Tie loosely.
- Predict which bag will collect the most water.
- After 2–3 hours, look for droplets. Take the bags off and weigh the damp paper towels (or compare how wet they look).
- Remove all bags and string from the tree.
What happens, and why?
Water travels from the roots up through the xylem and leaves the leaves as water vapour through the stomata. Inside the bag the vapour cannot escape, so it condenses into droplets. Leafy branches give off much more water than a bare twig, because the water vapour leaves through the leaves. Comparing a sunny and a shady branch tests whether light and warmth make a difference. Only the leaves or the light should change between bags: that keeps the test fair.[5][1]
Play: In or out?
Does the tree take this in, or give it out? Sort each one.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16