Determining Stomatal Density: Skills (DP IB Biology)

Determining Stomatal Density

• 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

  • The layer of varnish now forms a leaf cast

• 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 (you can read about this in our revision note on microscope skills)

• 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

• You can read about using a stage micrometer in our revision notes on microscope skills

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

NOS: Reliability of quantitative data is increased by repeating measurements

• Reliability refers to the level of trust that we can place in numerical measurements

  • These types of measurements are known as quantitative data

• Repeating the stomatal count for the same species of leaf under the same conditions will increase the reliability of the results

  • It is very possible that the data gathered during a single count could contain errors that we may not be aware of

  • Taking repeated measurements will identify anomalous measurements and allow us to calculate a mean

    • Anomalous measurements are those that deviate from the expected measurements

    • Anomalies are omitted when calculating the mean for a data set

• If repeated stomatal counts deliver similar results, the data is said to be reliable

  • We can therefore place a higher level of trust in the data than what would have been possible if we got very different results with every count

• Repeating measurements is a crucial step in gathering data during a scientific investigation