Transpiration and stomatal densitySpec B3.1.9, B3.1.10
In short
Transpiration is the loss of water vapour from the leaves and stems of plants. It is a consequence of gas exchange because stomata must open to let carbon dioxide in, and water vapour then diffuses out. Its rate rises with light, temperature and wind, and falls with humidity. Stomatal density is stomata per unit area.
Water evaporates from the moist walls of mesophyll cells into the air spaces, then water vapour diffuses out through open stomata. Because stomata must be open for CO₂ to enter, transpiration is a consequence of gas exchange.
| Factor | Effect on rate | Reason |
|---|---|---|
| Light intensity | Increases | Stomata open in the light for photosynthesis. |
| Temperature | Increases | More evaporation from cell walls and faster diffusion of water vapour. |
| Humidity | Decreases | Moist air outside lowers the water vapour concentration gradient. |
| Wind speed | Increases | Moving air removes water vapour from around the stomata, keeping the gradient steep. |
| Soil water availability | Decreases if water is short | Guard cells lose turgor and stomata close. |
The rate of transpiration can be estimated with a potometer, which measures water uptake by a cut shoot.
Stomatal density
To make a leaf cast, paint clear nail varnish on the lower epidermis, let it dry, lift it off with clear tape and stick it to a slide. Count stomata at high power in several fields of view, or count them on a micrograph with a known scale.
Calculating stomatal density
At high power the field of view has a diameter of 0.40 mm. Counts of stomata in five fields of view are 24, 27, 22, 26 and 26. Calculate the mean stomatal density. (Practice data.)
- Mean count = (24 + 27 + 22 + 26 + 26) ÷ 5 = 125 ÷ 5 = 25
- Radius = 0.40 ÷ 2 = 0.20 mm
- Area = πr² = 3.14 × 0.20² = 0.126 mm²
- Density = 25 ÷ 0.126 = 198.4
Answer: about 200 stomata per mm² (2.0 × 10² mm⁻²)
Repeated counts vary because biological material is variable. Replicate counts and a mean increase the reliability of the data; the range or standard deviation shows the spread.
Quick check
What happens to the surface area-to-volume ratio as an organism grows larger?
Show answer
It decreases.
What is the function of surfactant in the alveoli?
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It reduces surface tension so the alveoli do not stick together and collapse when air is breathed out.
Which three volumes add up to the vital capacity?
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Tidal volume, inspiratory reserve volume and expiratory reserve volume.
Why does increased humidity reduce the rate of transpiration?
Show answer
It reduces the concentration gradient of water vapour between the air spaces and the outside air.
Written and checked against the IB Biology SL specification · Updated October 2026