What it shows
The lower surface of many leaves is dotted with tiny pores called stomata, each between two bean-shaped guard cells. Counting them under a microscope shows where a leaf exchanges gases and water vapour.
The science
Stomata are pores in the epidermis that let carbon dioxide enter for photosynthesis and let oxygen and water vapour leave. Each pore is surrounded by two guard cells which swell with water to open it and lose water to close it. Many plants have more stomata on the lower surface, where it is cooler and out of the direct sun, so less water is lost. Counting stomata per field of view lets you compare the two surfaces.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A fresh Rhoeo (boat lily) or Tradescantia leaf
- A school compound microscope or a phone with a clip-on magnifier lens
- Glass slides and cover slips (or clear tape and a microscope slide)
- A dropper and water
- A fine paintbrush
- Tweezers and a blotting paper
- A notebook for sketches
Safety
For this project: Glass slides and cover slips break easily and an adult must supervise their use. An adult must help with the blade, if one is used, to tear or cut the leaf. Wash your hands after handling plants.
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.
Fold the leaf with its lower (purplish) side outwards and tear it across slowly so a thin, almost clear skin sticks out at the torn edge.
With tweezers pull off a piece of this thin skin and place it flat in a drop of water on a slide.
Lower a cover slip slowly from one side with a needle or a paintbrush to avoid trapping air bubbles.
Soak up extra water at the edge of the cover slip with blotting paper.
Look under low power first, then high power, and find the pairs of bean-shaped guard cells with a gap between them.
Count the stomata in one field of view and repeat for three different fields, then calculate the average.
Repeat with a peel from the upper surface of the leaf, which usually gives far fewer stomata, and compare.
Draw one stoma and label the guard cells, the pore and the other epidermal cells.
What you should see
The lower surface shows many more stomata per field of view than the upper surface. Your sketch shows guard cells with the pore between 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 a stoma?
A tiny pore in the leaf surface between two guard cells.
2What do guard cells do?
They open and close the pore as they swell with water or lose it.
3Why are there more stomata on the lower surface?
It is shaded and cooler, so less water is lost while gases still enter.
4Which gas enters through stomata for photosynthesis?
Carbon dioxide.
Take it further
Compare the stomata density of a Rhoeo leaf with that of a thick succulent such as a jade leaf, which usually has fewer.
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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