What it shows
A clear bag tied over a leafy branch fills with droplets within a few hours, while a bag over a bare branch stays dry. Leaves lose water vapour through tiny pores (stomata), a process called transpiration.
The science
Water travels up from the roots through thin tubes in the stem to the leaves. Most of it escapes as vapour through thousands of tiny openings, the stomata, mostly on the underside of leaves. Inside the sealed bag the air becomes saturated, so the vapour condenses into droplets on the cool plastic. The flow of water upwards, pulled by this evaporation, also carries minerals from the soil to every leaf.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 2 clear polythene bags
- 2 pieces of string or rubber bands
- A living plant in the sun with leafy and bare branches (or a potted plant)
- A small cup to pour the collected water into
- A spoon or a syringe without a needle to measure it
Safety
For this project: Never put a bag over your own or anyone else’s head. Do not pull or damage the branches.
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.
Step 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.
In the morning choose a healthy branch with 8 to 10 leaves and a bare branch of similar size on the same plant.
Slip a bag over each branch and tie it tightly at the stem so no air gets in or out.
Make sure both bags get the same sunshine.
Check both bags every hour and note what you see inside.
After 4 to 6 hours, untie the leafy bag carefully over a cup and pour out the water that has collected.
Measure the water with the spoon or syringe and write it down.
Repeat on a cloudy day or in the evening and compare the amounts.
Count the leaves in the bag and work out roughly how much water each leaf gave off.
What you should see
The leafy bag has mist and droplets inside and gives a few millilitres of water; the bare branch bag stays dry. More water collects on a hot sunny day than on a cloudy one.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Where did the water in the bag come from?
From the soil, carried up the stem and released by the leaves as vapour.
02Why did the bag over the bare branch stay dry?
Transpiration happens through leaves, and that branch had none.
03Why is there more water on a sunny day?
Heat and light open the stomata and speed up evaporation.
04Is transpiration a waste of water?
No, it pulls water and minerals up the plant and cools the leaves.
Take it further
Compare a branch of a thick-leaved plant such as aloe with a thin-leaved plant such as hibiscus.
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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