What it shows
Plants lose water through their leaves, and conditions change how fast. A potometer measures water uptake, and results show how much water crops need on hot or windy days.
The science
Water is pulled up the xylem from root to leaf by transpiration, which is evaporation through the stomata. Wind, heat and dry air speed it up, while high humidity and shade slow it. The movement of an air bubble in the potometer tube shows the water drawn in.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A leafy shoot cut under water
- A bent glass or plastic tube, or a 1 ml pipette with a rubber tube
- A small bowl of water
- A clamp and stand
- A table fan, a lamp (LED) and a plastic bag
- A ruler and a stopwatch
- Vaseline to seal joints
Safety
For this project: An adult must handle glass tubes and cut shoots. Keep electric lamps and fans away from water. Use an LED lamp rather than a hot bulb.
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.
Cut a shoot under water and fit it into the potometer tube, keeping all joints watertight with Vaseline.
Introduce one small air bubble into the tube from the open end.
Mark the bubble position and time how far it moves in five minutes in still air.
Repeat with the fan blowing on the shoot for the same time.
Repeat with the LED lamp near the shoot, and again in shade.
Cover the shoot with a plastic bag to raise humidity and measure again.
Calculate the bubble speed in cm per minute in each condition.
Draw a bar chart and explain each result.
What you should see
The bubble moves fastest in wind and bright light and slowest in humid or shaded air. The bar chart gives a clear order for the conditions.
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 transpiration?
Loss of water vapour from leaves.
2Why does wind increase it?
It removes humid air near the leaf.
3What does the bubble measure?
The rate of water uptake.
4Why does it matter for irrigation?
Crops need more water on hot, windy days.
Take it further
Compare a broad leaf plant with a narrow leaf one and their uptake rates.
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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