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2.6.7Additional lessons

Determining Stomatal Density: Skills — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 33 of 142

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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Method

Stomatal density is the number of stomata per unit area of leaf surface.

1
Paint a patch of clear nail varnish on the surface of the leaf and allow it to dry.
2
Peel off the dried varnish with clear tape, which gives an impression of the epidermis on the tape.
3
Place the tape on a microscope slide and view it under the microscope at a fixed magnification.
4
Count the stomata in the field of view, and repeat in several different random fields of view.
5
Calculate the mean count and divide by the area of the field of view to give stomatal density.

Part 2 of 5Calculations

Field of view area
  • Field of view area Treat the field as a circle and use area = πr², where r is half the measured diameter of the field.
Stomatal density
  • Stomatal density Density = mean number of stomata in the field ÷ area of the field, for example in stomata per mm².
Calibration
  • Calibration Measure the diameter of the field with a stage micrometer or a transparent ruler at the same magnification.
Sample size
  • Sample size More fields of view reduce the effect of random variation across the leaf.
Units
  • Units Convert micrometres to millimetres before calculating, as 1000 µm equals 1 mm.

Part 3 of 5Interpreting Results

Interpreting Results
Upper and lower surfaces
  • Upper and lower surfaces Many plants have more stomata on the lower surface, which reduces water loss from the surface exposed to sunshine.
Habitat
  • Habitat Plants from dry habitats often have fewer stomata or stomata in pits, to reduce water loss.
Variation
  • Variation Compare means and spread of data, and use a statistical test if a difference between groups is claimed.
Limitations
  • Limitations Counts depend on the choice of field of view, so random selection avoids bias.

Part 4 of 5Worked Example

Question

  • The diameter of the field of view is 0.50 mm.
  • Five fields on a leaf contain 11, 13, 12, 14 and 10 stomata.
  • Calculate the stomatal density.

  1. Mean number of stomata = (11 + 13 + 12 + 14 + 10) ÷ 5 = 12.
  2. Radius = 0.25 mm, so area = π × 0.25² = 0.196 mm².
  3. Density = 12 ÷ 0.196 = 61 stomata per mm².

Answer The stomatal density is about 61 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.
  • Field of view The circular area visible through the microscope at a given magnification.
  • Epidermis The outer layer of cells of the leaf.
  • Random sampling Choosing fields of view without bias so that the results represent the whole leaf.

Common Mistakes

  • Counting stomata in only one field of view and treating this as representative.
  • Using the diameter instead of the radius when calculating the area of the field.
  • Forgetting to convert the units before calculating density.
  • Assuming that a lower density means fewer stomata in the whole leaf, rather than fewer per unit area.

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