IB Biology HLGas Exchange SystemsPaper 1 & 2~10 min read
Determining Stomatal Density
Stomatal density is just the number of stomata in a known area. Getting it involves clear nail varnish, a piece of tape, a microscope and one small calculation – and the calculation is where most of the marks live.
📚 What you need to know
Stomatal density is the number of stomata per unit area, usually per mm².
It tells biologists how a plant is likely to cope with 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 it off with tape to give a leaf cast, and view it on a slide.
Adjust the magnification until a countable number of stomata is in view; 15 to 100 works well. Count partly visible ones as one.
Use a stage micrometer at the same magnification to find the diameter of the field of view.
Area of the field of view = πr²; density = mean count ÷ area.
Count at least three fields of view and take a mean – repeats increase the reliability of quantitative data.
Why anyone measures this
Stomata are the plant’s doors. How many doors a leaf has says a lot about the conditions it grew in and how it will cope somewhere new.
A plant with a high density opens a lot of pores, so it can photosynthesise fast but is more vulnerable to a dry spell.
Comparing species tells growers which plants will do well in a windy or wet climate before they move them there.
The same species grown in different habitats can end up with different densities, which is an interesting question in its own right.
The method
You are not looking at the leaf itself under the microscope. You are looking at a cast – a thin film of dried varnish that has taken the shape of the leaf surface, including the little dents where the stomata are.
Geraniums and spider plants work well – their stomata leave a clear imprint and the film peels off in one piece.
🧩 Full method
Take a leaf from a living plant and place it underside up on a flat surface such as a tile.
Paint a patch of clear nail varnish onto the lower surface.
Leave it to dry for about five minutes.
Press a piece of tape onto the dry varnish and peel it off. The film that comes away is your leaf cast; the leaf itself can be discarded.
Lay the cast flat on a microscope slide. No coverslip and no water are needed – it is an impression, not living tissue.
Focus as usual and adjust the magnification until a countable number of stomata is in view.
Count the stomata in that field of view, using a clicker or an app so you do not lose track. Count a stoma on the edge as one.
Move to a different area and repeat, for at least three fields of view, then take a mean.
Use a stage micrometer at the same magnification to measure the diameter of the field of view.
The “same magnification” line is the one people skip. If you count at one power and measure the field of view at another, your area is wrong and every number after it is wrong too. Write down the magnification next to every count.
Turning a count into a density
The field of view is a circle, so its area comes from the circle formula. Two steps, then a divide.
The three lines you always write
r = d ÷ 2 • area = πr² • density = mean count ÷ area
Fourteen stomata are shown here. A real field of view at high power usually holds a few dozen, which is why you scale everything to a square millimetre.
Reliability: why three counts, not one
Reliability is how much you can trust a measurement. Numerical measurements like these are quantitative data, and one number on its own tells you very little.
A single count may include mistakes you never notice – a missed stoma, a bubble in the varnish, an unusually thick patch.
Repeating in different areas lets you spot anomalous results, values that sit well away from the rest. Anomalies are left out of the mean.
If repeats give similar values, the data is reliable and you can trust the mean. If they are wildly different, you cannot.
Reliable is not the same as accurate. Three counts that agree closely are reliable. If your stage micrometer was misread, they can all be reliably wrong. Repeats fix random error, not a fault in the method.
Limitations of this method
Not all species give a clean imprint – hairy or waxy leaves are difficult.
Solvent-based varnish can damage some of the cell structure it touches. Water-based varnish is gentler but takes much longer to dry.
Density varies across a single leaf, and between leaves on the same plant, so where you sample matters.
Stomatal density can change with the conditions the plant grew in, so two plants of the same species are not guaranteed to match.
Worked examples
WE 1
Calculating stomatal density
Three fields of view give counts of 22, 25 and 25 stomata. The diameter of the field of view at that magnification is 0.40 mm. Calculate the stomatal density in stomata per mm². Use π = 3.14 and give your answer to the nearest whole number. (4 marks)
Step 1: mean count
(22 + 25 + 25) ÷ 3 = 24 stomataStep 2: radius
r = 0.40 ÷ 2 = 0.20 mm
Step 3: area of the field of view
area = πr² = 3.14 × 0.20² = 0.1256 mm²Step 4: divide
24 ÷ 0.1256 = 191.08…
191 stomata per mm²square the radius, never the diameter – using 0.40 here would make your answer four times too small
WE 2
A different magnification
At a lower magnification the field of view has a diameter of 0.50 mm and the mean count is 30 stomata. Calculate the density and comment on how it compares with a value of 191 stomata per mm² from the same leaf. (3 marks)
Step 1: radius and area
r = 0.25 mm, so area = 3.14 × 0.25² = 0.19625 mm²Step 2: density
30 ÷ 0.19625 = 152.87…
Step 3: comment
153 is well below 191, which suggests the sample area was different or a number of stomata were missed at the lower magnification.
153 stomata per mm²density is per unit area, so it should not change with magnification – a big difference means an error somewhere
WE 3
Explaining the repeats
Explain why the student counted three fields of view rather than one. (3 marks)
Point 1: the problem with one
A single count could contain errors that are not obvious, and one small patch may not represent the whole leaf.
Point 2: what repeats give you
Several counts let anomalous results be identified and left out, and allow a mean to be calculated.
Point 3: the conclusion
If the repeats are similar, the data is reliable, so more trust can be placed in the mean value.
Repeats identify anomalies and increase reliabilityuse the words anomalous, mean and reliability – these are the terms the mark scheme uses
💡 Exam tips
Halve the diameter before you square it. This is the single most common slip in this calculation.
Watch the units. If a diameter is given in micrometres, convert to mm first (divide by 1000) or your density will be out by a factor of a million.
Show every line of working: mean, radius, area, density. Method marks are given even if the final number is wrong.
Round only at the end, and to the precision the question asks for.
Say stomata per mm² in the answer, not just a number.
For “improve this method” questions, reach for more fields of view, more leaves, and the same magnification throughout.
⚠ Common mistakes
Using the diameter in πr². Always convert to the radius first.
Forgetting to take a mean when several counts are given.
Measuring the field of view at a different magnification from the one used for counting.
Ignoring stomata on the edge. Count them as one each, using a consistent rule.
Saying repeats make the result accurate. They improve reliability; accuracy depends on the method itself.
Painting the upper surface. Most stomata are on the lower surface of a typical leaf.
Up next: Haemoglobin & Oxygen – back to animals, and the protein that makes carrying oxygen around a large body possible at all.
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