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
Apparatus
- 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
Method
- 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
Limitations
- 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
