If microscope views show different areas of a leaf, compare stomata per unit area. Divide the number of stomata in each view by the actual leaf area it represents. The view with fewer stomata can still have a higher stomatal density.
The 2027 SEC G3 Biology K325 syllabus covers stomata, leaf structure and microscope observation. This practice applies those ideas to interpreting two fields of view with different sizes.
- Count each stoma once.
- Find the actual leaf area represented by each view.
- Use the same area units for both views.
- Compare densities and consider how the fields were selected.
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Decide what the denominator represents
Use the actual area of leaf in the field of view. If you have a calibrated diameter for a circular field, halve the diameter to find the radius and calculate the area with πr². An image printed larger on the page still represents the same leaf area.
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Worked example | A smaller field has the higher density
View A contains 24 stomata in a circular field 0.50 mm across. Its area is π × 0.25² ≈ 0.196 mm². This gives about 122 stomata per mm².
View B contains eight stomata in a field 0.25 mm across. Its area is π × 0.125² ≈ 0.0491 mm². This gives about 163 stomata per mm². View B contains one third as many stomata as A but represents one quarter of the area. Its density is therefore higher.
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Make a comparison about the leaf surface
A single field describes the part of the leaf observed. For a comparison between upper and lower surfaces, sample several fields on each side and keep the selection and counting rules consistent. If field areas vary, divide the total count by the total area when estimating the density across the sampled area.
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Frequently asked questions
What should I do with a stoma on the boundary?
Set a boundary-counting rule and apply it to all images. For adjacent areas, a rule that includes one boundary and excludes the opposite one helps avoid double-counting.
Does higher density prove faster transpiration?
Density gives the number of stomata per unit area. Stomatal opening and environmental conditions also affect water loss.