Practical - Measurement of Stomatal Density (OCR Gateway GCSE Biology: Combined Science)

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Biology Project Lead

Stomatal Density

Measurement of stomatal density on the surface of a leaf

  • This is not a required practical but is excellent practice for developing microscope skills and use of mathematical processing of data
  • The density of stomata (the number of stomata per unit of area) can be a useful measurement to biologists
    • To assess the plant's likely response to a dry spell of weather
    • To predict its behaviour in windy or wet climates if the plant was being moved for agricultural / horticultural reasons
  • This technique can be used to assess how stomatal density varies from species to species


  • A plant to sample a leaf from
  • Clear nail varnish (ideally solvent based)
  • Sellotape
  • Microscope
  • Microscope slides
  • Stage micrometer
  • Counting device (clicker/ phone app etc.)
  • Calculator


  • Select a leaf from a live plant and cut it off the plant
    • Geraniums and spider plants make good subjects for this experiment
  • Place the leaf upside down on a flat surface such as a tile or worktop
  • Paint clear nail varnish onto the underside of the leaf
  • Wait for the nail varnish to dry (approx. 5 minutes)
  • Peel off the layer of varnish using sellotape
    • Discard the leaf
  • Place the dried varnish impression on a microscope slide
    • A coverslip is not required as this isn't a biological sample, just an impression of one
    • A drop of water is not required either, so long as the sample is laid flat on the slide
  • Use the usual steps to focus on the sample
  • Adjust the zoom such that a countable number of stomata are visible in the field of view
    • Between 15 and 100 is ideal
    • Even if a stoma is partially visible at the edge, still count it as 1
  • Count the stomata in that field of view
    • You may wish to use a clicker or phone app so you don't lose count!
  • Move the field of view to another area of the nail varnish layer and repeat
  • Count at least 3 separate fields of view and take a mean value
    • Repeat readings allow you to eliminate anomalous results and calculate a reliable mean

Measurements to take

  • Use a stage micrometer to measure the diameter of the field of view
    • This has to be at the same magnification power that you used when counting the stomata
  • The stage micrometer will be calibrated in micrometers
    • A typical microscope allows the scientist to look at a field of view of about 0.5 mm diameter when on full power (× 400)
  • You will have calculated a mean number of stomata per field of view from the previous stage

Worked example

A study reveals a mean count of 16 stomata per field of view at a magnification of × 400. The stage micrometer calculates the diameter of the field of view at a magnification of × 400 to be 0.46mm

Calculate the stomatal density based on these data. Give units in stomata per mm2

Use a value of π = 3.14 and give your answer to the nearest whole number of stomata.


Step 1: Calculate the radius of the field of view

Radius = Diameter ÷ 2

Radius = 0.46 mm ÷ 2 = 0.23 mm

Step 2:  Calculate the area of the field of view

Area = πr2 = π × 0.232

Area = 0.1662 mm2

Step 3: Divide the mean number of stomata by the area of the field of view to calculate density

Density = 16 ÷ 0.1662 = 96.27 stomata per mm2

Step 4: Round to the required precision (nearest whole number)

Density = 96 stomata per mm2


  • Not all plant species have easily accessible stomata that create a strong imprint
  • Solvent-based nail varnish can destroy some of the cell structure it comes into contact with
  • Does the plant grow more stomata (guard cells) according to the conditions in each individual habitat?
  • Water-based nail varnish is safer to use but dries more slowly

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.