To test the effect of air movement on transpiration, compare a leafy shoot in still air with a comparable shoot in moving air. Keep temperature, light intensity and the humidity of the surrounding air as similar as possible so the comparison can isolate the effect of air movement.
A bubble potometer estimates water uptake by a shoot. Use the uptake rate as an indicator of transpiration and label the measurement accurately before explaining what the result suggests.
- Change the air movement around the leaves.
- Keep the measurement time, leaf area and other conditions comparable.
- Separate the reading, the explanation and the strength of the conclusion.
Worked example | Check what changed with the fan
A class records bubble movement of 6 mm in 3 minutes in still air and 15 mm in 3 minutes with a fan, using the same capillary. The rates are 2 mm/min and 5 mm/min. The shoot took up water faster in the fan condition, assuming the apparatus was airtight and worked properly.
The humidity of the surrounding air was 75% during the still-air trial and 50% during the fan trial, at the same temperature. Both air movement and surrounding humidity changed. The results show faster uptake under the combined conditions. They leave open how much of the difference came from each change.
Explain what moving air does near a leaf
Water evaporates from moist surfaces inside the leaf and water vapour diffuses out mainly through the stomata. In still air, water vapour can build up near the leaf surface. Moving air carries some of that vapour away, helping maintain a steeper concentration gradient for diffusion.
In a fair test, hold the humidity of the surrounding air steady. The fan can still change the local layer of air immediately next to the leaf. That local change is part of the mechanism being tested.
Improve the comparison
The class repeats the investigation but places the fan trial closer to a bright lamp. Which controls should the class restore?
Answer. Keep light intensity and temperature comparable in both conditions. Check the surrounding humidity too. Use comparable shoots or the same shoot in repeated trials, allow time for readings to stabilise, and compare repeat measurements taken over equal intervals.
Frequently asked questions
Why keep leaf area comparable?
A larger total leaf area can change the rate of water loss and therefore uptake. Comparable leaf area helps isolate the effect of air movement.
Can I turn mm/min directly into water volume per minute?
Multiply the bubble’s distance per minute by the capillary’s cross-sectional area. With distance in millimetres and area in square millimetres, the result is in cubic millimetres per minute.
Official information
SEAB, 2027 SEC G3 Biology syllabus (K325), Topic 10, Nutrition and Transport in Flowering Plants. Syllabus checked 11 November 2026. The worked examples are original.