H2 Biology Stomatal Density Practical: Leaf Peel, Graticule and t-Test Workflow

2 | Objective lens choice and counting rule

Start with the $10\times$ objective for orientation. Count with the $40\times$ objective only if the field still contains enough stomata for a reliable count. If one field has too few stomata, increase the number of fields instead of changing the method halfway through the investigation.

Use a consistent edge rule:

• Count stomata fully inside the field.
• Count stomata touching the top and left boundary.
• Do not count stomata touching the bottom and right boundary.
• Record damaged or unclear stomata as excluded, not as zero.

This avoids double counting when fields overlap.

3 | Density calculation workflow

Record at least five non-overlapping fields per leaf surface or treatment if time allows.

Mean count:

$$\text{mean count} = \frac{18 + 21 + 19 + 24 + 20}{5} = 20.4$$

If the field radius is $0.25 \text{ mm}$, the field area is:

$$A = \pi r^2 = \pi(0.25)^2 = 0.196 \text{ mm}^2$$

Stomatal density:

$$\text{density} = \frac{20.4}{0.196} = 104 \text{ stomata mm}^{-2}$$

State the objective lens, total magnification, field radius, number of fields, and whether counts are from the upper or lower epidermis.

4 | Graticule calibration and guard-cell length

A common extension is to measure guard-cell length with an eyepiece graticule.

Calibration example:

If one guard cell is 4.5 graticule divisions long:

$$\text{guard-cell length} = 4.5 \times 8.0 = 36 \text{ micrometres}$$

For a biological drawing, draw guard cells with clean single lines, show the pore if visible, label subsidiary cells only when observed, and add a scale bar calculated from the same calibration. A scale bar labelled 20 micrometres is better than a vague magnification statement when the question asks for measurement.

5 | t-test interpretation

If the practical compares two samples, such as shaded leaves and sun-exposed leaves, the t-test helps decide whether the difference in mean stomatal density is larger than expected from sampling variation.

Do not write that $p < 0.05$ proves the causal explanation. The result supports a difference between the sampled groups; the biological cause still depends on variable control.

6 | Evaluation points

Strong ACE answers connect the limitation to the density value.

Pair this investigation with the hydrogencarbonate indicator photosynthesis practical when revising how leaf structure links to gas exchange and photosynthetic rate.

7 | Related guides

• H2 Biology microscope practical guide
• H2 Biology practical 2026 lab mastery guide
• H2 Biology hydrogencarbonate indicator practical

Sources

1. SEAB - H2 Biology (Syllabus 9477) GCE A-Level 2026
2. SEAB - H2 Biology Specimen Paper 4 (9477/04), first exam 2026

Contact us to learn more about our tailored packages and available pricing options.