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2.6.7Inquiry Process

Determining Stomatal Density- Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 69 of 160

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Part 1 of 5Preparing and Observing the Sample

2.6.7 Determining Stomatal Density- Skills

The aim is a standardised count that can be compared between leaves.

1
Paint a thin layer of clear nail varnish on the lower surface of a leaf and let it dry completely.
2
Peel the dried layer off with clear sticky tape, which gives a negative impression of the epidermis.
3
Mount the impression on a slide, add a coverslip and view it under a light microscope at a fixed magnification.
4
Measure the diameter of the field of view with a stage micrometer or a transparent ruler, and record it in millimetres.
5
Count every stoma lying inside the field, deciding in advance how to treat stomata touching the edge, and repeat at several random positions.

Part 2 of 5Calculating Density

Field area
  • Treat the field of view as a circle and calculate its area using area = πr², where r is half the measured diameter.
Stomatal density
  • Divide the mean number of stomata per field by the field area to give stomata per square millimetre.
Replicates
  • Counting at least five fields and calculating a mean reduces the effect of uneven distribution of stomata.
Fair comparison
  • Compare leaves of similar age and size from the same side of the leaf and view them at the same magnification.
QuantityExample value
Field diameter0.50 mm
Field area0.196 mm²
Mean count per field18
Densityabout 92 per mm²

Part 3 of 5Interpreting Differences

Interpreting Differences
Light
  • Leaves grown in strong light often have more stomata than shade leaves, which supports faster photosynthesis.
Water availability
  • Plants in dry habitats often have fewer or sunken stomata, which reduces water loss.
Leaf surface
  • In most land plants the lower epidermis has far more stomata than the upper, protecting against overheating and water loss.
Carbon dioxide level
  • Plants grown in higher carbon dioxide concentrations often develop fewer stomata.

Part 4 of 5Worked Example

Question

  • The mean count of stomata is 18 per field.
  • The field of view has a diameter of 0.50 mm.
  • Calculate the stomatal density.

  1. Radius = 0.50 ÷ 2 = 0.25 mm.
  2. Area = π × 0.25² = π × 0.0625 = 0.196 mm².
  3. Density = 18 ÷ 0.196 = 91.7 stomata per mm².

Answer The stomatal density is approximately 92 stomata per mm².

Part 5 of 5Exam Checklist & Tips

2.6.7 – Key Points for the Exam

  • Stomatal density The number of stomata per unit area of leaf surface.
  • Field of view The circular area of a specimen that can be seen through the microscope at one time.
  • Replicate A repeat measurement used to improve reliability.
  • Stage micrometer A slide with a known printed scale used to calibrate a microscope.

Common Mistakes

  • Counting stomata in one field only; results are unreliable without repeats.
  • Using the diameter instead of the radius in the area formula.
  • Reporting density without units, or in the wrong unit such as per mm instead of per mm².
  • Comparing upper and lower surfaces from different leaves, which adds an uncontrolled variable.

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