2.6.7Additional lessons
Determining Stomatal Density: Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 33 of 142
Section 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.
Section 2 of 5Calculations
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
- Density = mean number of stomata in the field ÷ area of the field, for example in stomata per mm².
Calibration
- Measure the diameter of the field with a stage micrometer or a transparent ruler at the same magnification.
Sample size
- More fields of view reduce the effect of random variation across the leaf.
Units
- Convert micrometres to millimetres before calculating, as 1000 µm equals 1 mm.
Section 3 of 5Interpreting Results
Interpreting Results
Upper and lower surfaces
- Many plants have more stomata on the lower surface, which reduces water loss from the surface exposed to sunshine.
Habitat
- Plants from dry habitats often have fewer stomata or stomata in pits, to reduce water loss.
Variation
- Compare means and spread of data, and use a statistical test if a difference between groups is claimed.
Limitations
- Counts depend on the choice of field of view, so random selection avoids bias.
Section 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.
- Mean number of stomata = (11 + 13 + 12 + 14 + 10) ÷ 5 = 12.
- Radius = 0.25 mm, so area = π × 0.25² = 0.196 mm².
- Density = 12 ÷ 0.196 = 61 stomata per mm².
Answer The stomatal density is about 61 stomata per mm².
Section 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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