7.1.7Practical Skills
Practical: Measurement of Stomatal Density — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 8 of 11
Part 1 of 4Why measure stomatal density?
Definition
- Definition Stomatal density is the number of stomata per unit area of leaf surface.
Uses
- Uses It helps predict how a plant copes with dry spells, wind or damp conditions, and compare species or habitats.
Skills practised
- Skills practised Microscope work and processing data mathematically.
Part 2 of 4Method
1
Cut a leaf from a live plant, such as a geranium or spider plant, and lay it underside up on a flat surface.
2
Paint a patch of clear nail varnish on the underside and leave it for about 5 minutes to dry.
3
Peel off the varnish layer with sticky tape and discard the leaf.
4
Lay the varnish impression flat on a microscope slide; no coverslip or water is needed.
5
Focus on the sample and choose a magnification where about 15 to 100 stomata are visible.
6
Count the stomata in the field of view, including any partly visible at the edge, and use a clicker to avoid losing count.
7
Repeat in at least 3 different fields of view and calculate the mean.
Part 3 of 4Calculating density
1Field diameter
- Field diameter Measure the diameter with a stage micrometer at the same magnification, e.g. 0.46 mm.
2Radius and area
- Radius and area r = 0.46 ÷ 2 = 0.23 mm; area = π r2 = 3.14 × 0.232 = 0.166 mm2.
3Density
- Density Density = mean stomata ÷ area = 16 ÷ 0.1662 = 96.27.
4Round
- Round 96 stomata per mm2, to the nearest whole number.
Part 4 of 4Limitations
Evaluating the method
Weak imprints
- Weak imprints Some species do not give clear impressions.
Varnish type
- Varnish type Solvent-based varnish can damage cells; water-based is safer but dries more slowly.
Variation
- Variation Stomatal density varies between leaves and parts of leaves, so use several fields of view.
Question to explore
- Question to explore Does a plant grow more stomata depending on conditions in its habitat?
Key terms
| Term | Meaning |
|---|---|
| Stomata | Pores on the leaf surface that control gas exchange and water loss. |
| Stomatal density | The number of stomata per square millimetre of leaf. |
| Field of view | The circular area visible through the microscope. |
| Stage micrometer | A slide with a tiny accurate scale for measuring microscopic lengths. |
Avoid these mistakes
1
Using the diameter instead of the radius in the area calculation.
2
Counting stomata in only one field of view.
3
Using a different magnification for counting and for measuring the field of view.
4
Forgetting to give the units as stomata per mm2.
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