The big idea: Stomatal density is the number of stomata per unit area of leaf: count the stomata in a field of view of known area, then divide by that area.
The points to remember
- Make a leaf cast (clear nail varnish on the lower surface, peeled off with tape) or use a micrograph.
- Count the stomata in the field of view at high power: one stoma = one pore with its two guard cells.
- Area of a round field of view = πr², using the radius in mm.
- Density = number of stomata ÷ area, in stomata mm⁻².
Remember it as: Count, then divide by the area.
Two traps: Count stomata, not guard cells: two guard cells make one stoma.
A count alone is not a density: divide by the area and give the unit.
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The big idea: Biological material varies, so one count can mislead. Repeat the count and use the mean.
The points to remember
- Count several fields of view on each leaf, and use several leaves.
- Calculate the mean: repeated counts make the result more reliable.
- The spread of the counts shows how much the material varies.
- Means shown with error bars (or ± standard error): if the bars of two means do not overlap, the difference is likely to be significant.
Trap: 'The means are different' is not enough to call a difference significant: compare the size of the error bars with the gap between the means.
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The big idea: Stomatal density is a trade-off: more stomata let in more carbon dioxide and cool the leaf by transpiration, but lose more water.
| Condition | Stomatal density | Why it helps the plant |
|---|---|---|
| Hot habitat | Often higher | More transpiration cools the leaf; more carbon dioxide taken in |
| Dry habitat | Often lower | Less water lost |
| Raised carbon dioxide | Often lower | Enough carbon dioxide still enters, with less water lost |
The points to remember
- Other factors also matter: water supply, humidity, wind and light.
- Describe a scatter graph with the word correlation: 'a positive correlation between temperature and stomatal density'.
- Read values with their unit: 175 stomata mm⁻².
- A hypothesis is supported only where the data show the predicted change; one species may fit and another may not.
Trap: Say 'positive correlation', not 'positive relationship' or 'directly proportional'.
Real example: leaves kept in herbaria. Dried leaves kept in herbarium collections for about 200 years have more stomata per square millimetre than leaves of the same species grown today, as carbon dioxide in the air has risen.
How it is asked: Expect an image of a leaf cast, or tables and graphs of stomatal density to describe, compare and use as evidence.
IB-style questionOutline[1 mark]
Outline how the stomatal density of a leaf can be estimated within a known field of view.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.