2.6.7Inquiry Process
Determining Stomatal Density- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 69 of 160
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.
Part 1 of 5Preparing and Observing the Sample
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
- 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
- Stomatal density Divide the mean number of stomata per field by the field area to give stomata per square millimetre.
Replicates
- Replicates Counting at least five fields and calculating a mean reduces the effect of uneven distribution of stomata.
Fair comparison
- Fair comparison Compare leaves of similar age and size from the same side of the leaf and view them at the same magnification.
| Quantity | Example value |
|---|---|
| Field diameter | 0.50 mm |
| Field area | 0.196 mm² |
| Mean count per field | 18 |
| Density | about 92 per mm² |
Part 3 of 5Interpreting Differences
Interpreting Differences
Light
- Light Leaves grown in strong light often have more stomata than shade leaves, which supports faster photosynthesis.
Water availability
- Water availability Plants in dry habitats often have fewer or sunken stomata, which reduces water loss.
Leaf surface
- 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
- 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.
- Radius = 0.50 ÷ 2 = 0.25 mm.
- Area = π × 0.25² = π × 0.0625 = 0.196 mm².
- 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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