Stomatal Density Virtual Lab: Count Stomata on a Leaf Cast

This stomatal density virtual lab follows the nail-varnish peel practical used in IB Biology and Cambridge IGCSE courses. Students make a cast of a bean leaf, view it at a known field diameter, count the stomata in three fields on each surface and calculate density = count ÷ field area. They practice a counting rule, πr² and the mean of several fields, and the result leads into gas exchange and water loss. Every value below was read from the simulation.

Curriculum links

• IB Diploma Biology, B3.1 Gas exchange: leaf adaptations for gas exchange; stomatal density from leaf casts or micrographs.
• Cambridge IGCSE Biology (0610), 6.2 Leaf structure: the tissues of a dicot leaf and how they suit it for photosynthesis.
• AQA GCSE Biology 4.2.3.1 (plant tissues): epidermis, mesophyll and guard cells and their functions.

Simulic is not affiliated with or endorsed by the International Baccalaureate, Cambridge International or AQA.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"You count the stomata on the upper and lower surfaces of a bean leaf. Which surface has more stomata per mm²?"

Some students expect the upper surface, "because it faces the sun", or no difference.

Method in the simulation

1. The page opens on Leaf cross-section. Click the Stomatal count tab. Keep Plant species Bean, Leaf surface Lower surface and Objective lens ×40 – field diameter 0.45 mm.
2. Press Make a nail-varnish cast: varnish, dry, peel with tape, mount.
3. Tick Show stage micrometer scale to see the field diameter. Field area A = πr², with r = 0.45 ÷ 2 = 0.225 mm.
4. Click each stoma to mark it. Count rule: count a stoma only if its center lies inside the field; the simulation refuses the others. Then press Record count.
5. Press Next field and repeat until you have three fields. The table adds a mean row.
6. Change Leaf surface to Upper surface, make a new cast and count three fields.
7. Density = count ÷ field area, in stomata per mm².

The cast comes from the starting value Random seed (same number, same cast), which is 1 unless the teacher changes it.

Surface	Field	Count	Density (per mm²)
Lower	1
Lower	2
Lower	3
Upper	1
Upper	2
Upper	3

Expected results

Field area at ×40: π × 0.225² = 0.159 mm². With seed 1:

Surface	Field 1	Field 2	Field 3	Mean density
Lower	45 (283/mm²)	48 (302/mm²)	47 (296/mm²)	293 per mm²
Upper	3 (19/mm²)	3 (19/mm²)	6 (38/mm²)	25 per mm²

• The lower surface has about 12 times as many stomata per mm² as the upper surface.
• Upper counts are small: one stoma changes the density by 6 per mm². At ×10 (field area 2.545 mm²), upper field 1 holds 110 stomata: 43 per mm².

Questions for students

1. (Prediction, asked again after the lab) Which surface of a bean leaf has more stomata per mm²?
2. Which variable must stay the same when you compare the two surfaces?
3. What is the area of the ×40 field of view?
4. What is the mean stomatal density of the lower surface over fields 1–3?
5. Explain why the bean leaf has most stomata on its lower surface, and why you count several fields.

Answers for teachers: (1) The lower surface, by far. (2) The objective lens, and so the field area (also the species). (3) 0.159 mm² (accept 0.157–0.161). (4) About 293 per mm² (accept 275–312). (5) The lower surface is shaded and cooler, so less water evaporates through its stomata while CO₂ still enters. Stomata are spread unevenly, so one field can mislead; a mean of several is more representative.

Common misconceptions

• "Count every stoma you can see." Stomata cut by the edge count only if their center lies inside the field, so every student gets the same count.
• "Area = πd²." Using the diameter instead of the radius gives π × 0.45² = 0.636 mm², four times too big, so the density comes out four times too small.
• "One field is enough." On the upper surface, field 3 gave twice the density of field 1 (38 against 19 per mm²).
• "Every leaf has more stomata underneath." A floating water lily leaf has them only on its upper surface.

Extension

• Compare species: on the Compare species tab, press Add class data (3 fields per surface for five species). After our six bean fields, the table read: bean 40 upper / 292 lower, sunflower 86 / 172, oak 0 / 432, maize 50 / 59, water lily 469 / 0 per mm². Explain each pattern from where the plant lives.
• Bigger sample: switch the upper surface to ×10 and press Count for me. Why is one ×10 field (110 stomata) a better estimate than three ×40 fields (12 stomata in total)?

FAQ

Why do all my students get the same counts?

The cast is generated from Random seed (same number, same cast), which is 1 on a new link. Pin a different seed in the class link's starting values to give a group new fields. The answer range for question 4 then needs editing: across all 9,999 seeds, the three-field mean stayed between 214 and 335 per mm².

Can the link open on the counting screen?

Yes. In the class link's starting values, set Starting screen to Stomatal count. You can also pin Plant species, Leaf surface and Objective lens (×10 eyepiece).

Does this replace a real leaf peel?

No. Use it to teach the counting rule and calculation before real peels. To calibrate a microscope first, see the microscopy virtual lab.

Related simulations and guides

Transpiration and stomatal control Microscopy lab – slides, calibration, cell size and electron microscopes

For what open stomata do, see the transpiration virtual lab or the plant transport virtual lab.