Science 7–10 · Year 8

Stomatal density: counting pores on the two faces of a leaf

Science understanding: Biological sciences

Practical, model not builtLow risk

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The idea

Stomata are the pores through which a leaf exchanges gases and loses water, and their number per square millimetre can be measured from an impression and a calibrated field of view.

Safety card

Low riskAn adult supervises

Hazards

Nail varnish solvent vapour

Controls

Ventilated room; caps on when not in use; no naked flames

Note

Chemicals: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package (section 1, general information for all staff) and complete a RiskAssess risk assessment before the lesson.

What you need

Leaves of Tradescantia zebrina (the SAPS resource views its leaf discs directly, without an impression), ivy, a broad-leaved garden plant and a eucalypt
Clear nail varnish, clear sticky tape, slides, scissors
Compound microscope with a calibrated 40x objective field (0.45 mm diameter with an 18 mm field number)
Tally counter or a results grid

How to do it

Paint a 1 cm square of clear nail varnish on the lower surface of a leaf and let it dry for ten minutes; repeat on the upper surface of a second leaf of the same plant.
Press sticky tape onto the dried varnish, peel it off with the impression, and stick it flat on a slide.
At 400x count every stoma (the paired guard cells with a slit) in the field; do this for five different fields on each impression.
Calculate density: mean count divided by the field area (0.159 mm2 for a 0.45 mm field).
Repeat for each species, upper and lower surfaces.
Compare lower with upper surfaces, and compare a shade plant with the eucalypt.

What you should see

Broad-leaved garden plants show more stomata on the lower face than the upper, the pattern the SAPS leaf temperature notes rely on. Compare the two faces of the eucalypt leaf with those of the broad-leaved plant and relate any difference to how each leaf is held. A mean of 30 stomata in a 0.159 mm2 field is 189 per mm2. The learner knows it worked when guard-cell pairs are clearly visible in the impression and the five field counts are similar.

What changes

What you change: Leaf surface (upper or lower) and plant species
What you measure: Stomatal density (per mm2)
What you keep the same: Field area
Number of fields counted (5)
Leaf age and position on the plant
Same observer counting rule

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Stomata are only on the underside of every leaf.
Stomata are holes that stay open all the time.
More stomata always means more photosynthesis.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-LIV-01 SC4-WS-01 SC4-WS-05
Australian Curriculum v9AC9S8U02AC9S8I03AC9S8I04

Sources

The pages the author read to write this activity.

All Concept Studio activities