Paint a leaf with clear nail varnish, peel it off, and you have a perfect cast of its surface — every stoma preserved in plastic. Count them under a microscope, work out the area you were looking at, and you have a number you can compare between species or between habitats.
📚 What you need to know
Stomatal density is the number of stomata per unit area of leaf surface.
It tells biologists how a plant is likely to respond to dry, windy or wet conditions, and how density varies between species.
Method: paint clear nail varnish on the underside of a leaf, let it dry, peel the cast off with tape, mount it on a slide and count.
Adjust the magnification so a countable number of stomata is visible — between 15 and 100 is ideal. Count a partially visible stoma at the edge as 1.
Count at least 3 fields of view and take a mean.
Measure the diameter of the field of view with a stage micrometer, at the same magnification used for counting.
Density = mean count ÷ area of field of view, where area = πr².
NOS: repeating measurements identifies anomalies and increases the reliability of quantitative data.
The method
Geraniums and spider plants work well because their leaves take a clean imprint. Some species have surfaces too hairy or too waxy to cast properly, which is a genuine limitation of the technique.
Apparatus
A plant to sample a leaf from, clear nail varnish (ideally solvent based) and sticky tape
A microscope, microscope slides and a stage micrometer
A counting device — a clicker or a phone app — and a calculator
Counting rules that matter
Adjust the zoom until you can see a countable number. Between 15 and 100 is the sweet spot: too few and one miscount skews everything, too many and you lose track.
If a stoma is only partly visible at the edge, still count it as 1. Apply the same rule every time so the counts stay comparable.
Move to a different area of the cast and repeat. Count at least 3 fields of view and take a mean.
A quiet detail worth getting right: the stage micrometer must be used at the same magnification you counted at. Change the objective lens and the field of view changes size, which changes the area, which changes your answer completely.
Turning a count into a density
Note how small the field of view is. At high magnification you are typically looking at an area well under a square millimetre, which is why the density number comes out so much larger than the raw count.
The calculation
stomatal density = mean count ÷ (π × r²) where r = diameter ÷ 2
Limitations of the technique
Not all species work. Some leaves do not have easily accessible stomata, or their surface does not take a strong imprint.
Solvent-based varnish can damage some of the cell structure it touches.
Water-based varnish is safer to use but dries much more slowly.
Density is not fixed. A plant may grow more or fewer stomata depending on the conditions of its habitat, so a single reading describes that leaf, not the whole species.
NOS: repeats are what make the number trustworthy
Reliability is the level of trust you can place in a numerical measurement. Stomatal counts are quantitative data, so reliability matters.
A single count could easily contain errors you would never notice — a missed stoma, an unusually dense patch of leaf.
Repeating the count under the same conditions lets you spot anomalous measurements: values that deviate noticeably from the rest. Anomalies are omitted before calculating the mean.
If repeated counts give similar results, the data is described as reliable and you can place more trust in it than in one reading.
Terminology: anomalous results are also called outliers. Either word is fine, but be clear that you omit them from the mean rather than deleting them from the raw data.
Worked examples
WORKED EXAMPLE
A student counts 14 stomata in a field of view whose diameter is 0.42 mm. Calculate the stomatal density in stomata per mm², using π = 3.14 and giving your answer to the nearest whole stoma. [3]
Step 1: radius of the field of viewr = 0.42 ÷ 2 = 0.21 mmStep 2: area of the field of viewA = πr² = 3.14 × 0.21² = 3.14 × 0.0441A = 0.1385 mm²Step 3: divide the count by the area14 ÷ 0.1385 = 101.1Stomatal density = 101 stomata per mm²Do not round the area too early — rounding to 0.14 would give 100, a different answer.
WORKED EXAMPLE
Four fields of view give counts of 22, 25, 24 and 51. Calculate the mean count to use, and justify your choice. [3]
Step 1: identify the anomaly51 deviates far from the other three values, which cluster around 22 to 25. It is an anomalous result, or outlier.Step 2: omit it and calculate the mean(22 + 25 + 24) ÷ 3 = 71 ÷ 3 = 23.67Mean count = 23.7 stomata per field of viewStep 3: justifyAnomalies are omitted from a mean because including them distorts it. Including 51 would give 30.5, which represents none of the readings well.
WORKED EXAMPLE
Two leaves from the same species are compared. Leaf A, grown in a dry sunny spot, has a density of 180 per mm². Leaf B, grown in shade, has 95 per mm². Suggest why a researcher should be cautious about concluding that dry conditions cause higher stomatal density. [3]
Point 1: only two leavesA sample of one leaf per condition is far too small to be reliable. Many leaves from each condition would be needed.Point 2: more than one variable differsThe sunny spot differs in light intensity and temperature as well as water availability, so the cause cannot be isolated.Point 3: what would be neededRepeat counts across many leaves, with only water availability varied and other conditions controlled
💡 Exam tip
Write the working in the order radius → area → divide. Marks are given for the steps, not just the final number.
Halve the diameter first. Using the diameter as the radius is the single most common calculation error here.
Keep everything in mm so the units come out as stomata per mm².
Do not round intermediate values. Carry the full area through to the final division.
Always mention the 3 or more fields of view and the mean when describing the method.
For reliability questions, use the words anomalous, omitted and mean.
⚠ Common mix-up
Using the diameter in πr². That gives an area four times too big and a density four times too small.
Measuring the field of view at a different magnification. The area must match the magnification you counted at.
Ignoring stomata at the edge. Count a partly visible one as 1, consistently.
Including an anomaly in the mean. Identify it, omit it, then calculate.
Confusing reliability with accuracy. Repeats improve reliability; they do not fix a systematic error such as a mis-calibrated micrometer.
Painting the upper surface. Most stomata are on the underside of the leaf.
That completes Gas Exchange Systems. Up next: Transport Systems — once gases have crossed the exchange surface, something has to carry them to the cells that need them.
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