Transpiration is the loss of water vapour from leaves. Water evaporates from moist mesophyll cell surfaces and diffuses out through stomata. This loss draws water up the xylem from the roots. The moving dot is a simplified illustration of the water pathway.
| Increase in… | Usual effect | Reason |
|---|---|---|
| Temperature | Faster transpiration | Faster evaporation and diffusion. |
| Air movement | Faster | Removes moist air near the leaf, maintaining a gradient. |
| Humidity | Slower | Smaller water-vapour concentration difference between leaf and air. |
| Light intensity | Usually faster | Stomata open for gas exchange during photosynthesis. |
These comparisons assume other conditions stay the same and water is available. Severe water shortage can cause stomata to close.
A potometer measures water uptake, which estimates transpiration. Not all absorbed water is transpired: some is used in photosynthesis or to maintain cells. Assemble the shoot under water, make connections airtight, dry the leaves and allow the plant to adjust. Track an air bubble moving along a calibrated capillary tube.
Uptake rate = distance moved ÷ time when comparing bubble movement in the same tube. If the bubble moves 24 mm in 4 min, rate = 6 mm/min. For volume rate, volume = tube cross-sectional area × distance. Compare equal periods, repeat measurements and calculate means.
| Trial | Bubble distance in 4 min |
|---|---|
| 1 | 20 mm |
| 2 | 24 mm |
| 3 | 28 mm |
Mean distance = (20 + 24 + 28) ÷ 3 = 24 mm. Mean movement rate = 24 ÷ 4 = 6 mm/min. These are example data.
Change one factor at a time. For a fan experiment, keep temperature, light, humidity and the plant/leaf area as constant as possible. A lamp can also heat the plant, so control temperature when testing light intensity. A leak can move the bubble without representing plant uptake.
Water evaporates inside leaves, then diffuses through stomata. Potometers estimate loss by measuring uptake.
Build confidence with pictures, clues and instant feedback.
One point for each answer you get right first time.
No timer. Take your time and use a clue whenever you need one.