Reading a Potometer: Wind, Vaseline and a Backward Bubble
A student sets up a simple potometer. A freshly cut leafy twig with six similar leaves is fixed through a bored cork into a water-filled glass tube connected to a capillary carrying an air bubble. The cut was made under water. Four readings are taken at 26 degrees Celsius, each over 5 minutes: (i) with a table fan 1 m away, the bubble moves 32 mm; (ii) fan switched off, 11 mm; (iii) fan on, the lower surface of every leaf smeared thinly with Vaseline, 4 mm; (iv) fan on, the upper surface of every leaf smeared instead, 27 mm. Later, when the twig is left on the bench for an hour, the student finds the bubble has moved backwards towards the shoot. The teacher asks for an explanation of each reading and warns that the apparatus cannot be used to say the plant 'lost 32 mm of water' in five minutes.
1. Interpret readings (i) to (iv). Estimate the share of water loss through the lower and upper leaf surfaces and explain why the leaf is built that way.
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The movement of the bubble measures water drawn into the shoot, which closely follows transpiration.
(i) vs (ii): moving air removes the thin layer of humid air clinging to the leaf, so the water-vapour gradient between the leaf interior and the air steepens. The rate rises from 11 to 32 mm, roughly threefold, for the same leaves and the same temperature.
(iii) With the lower surface blocked, only 4 mm is drawn: this is the part that normally escapes through the upper surface (plus a little through the stem and the cut end).
(iv) With the upper surface blocked, 27 mm is drawn: this is the part that escapes through the lower surface.
The two parts add to 31 mm, close to the 32 mm of the intact twig, so almost all stomatal loss is through the lower surface: about 27/32, roughly 84%, from the lower epidermis and about 4/32, roughly 12%, from the upper. This fits the usual dicot leaf, where most stomata lie in the lower epidermis, shaded from direct sun; that arrangement reduces heating and water loss while still allowing gas exchange.
2. Explain why the bubble can move backwards towards the shoot after the twig has stood on the bench for an hour, and state two precautions this experiment demands.
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Forward movement means water is being pulled towards the shoot. Backward movement means the water column is retreating from the shoot, which happens for one of three reasons. First, a leak: air enters at the cork, the joint or the cut end, and the pressure inside the capillary no longer reflects the pull of the leaves; a single air bubble at the cut end can break the continuous water column. Second, warming: if the apparatus sits in sunlight or is handled, the air trapped in the capillary expands and pushes the index bubble back even with no change in the plant. Third, the shoot wilts: stomata close, transpiration nearly stops, and any small back-pressure or leakage then dominates.
Precautions: cut the shoot under water and assemble it under water so no air enters the xylem; smear Vaseline around the cork and joints and check for leaks; shade the apparatus and avoid handling the tube so the temperature stays constant; use a healthy, turgid shoot.
3. The teacher says the reading cannot be reported as '32 mm of water lost in 5 minutes'. Justify this and name two environmental factors that would change the reading.
Read the worked answer
Three separate problems make the claim wrong. First, the bubble travels a distance in a capillary, not a volume; to convert millimetres to cubic millimetres you must know the internal radius of the capillary and use volume = pi x r^2 x length. Second, a potometer measures water uptake by a detached shoot, not water lost by a plant. The two are close in a well-watered shoot, but not identical: some water is used in photosynthesis and retained in cells, the shoot has no roots and no whole-plant architecture, and it gradually wilts, so its stomata may close during the run. Third, the twig is a cut shoot in a laboratory, not a plant in a field, so the value is a relative rate under those conditions and cannot be generalised.
Environmental factors that would change the rate: light intensity (stomatal opening and leaf temperature), relative humidity of the air, wind speed, and temperature. Carbon dioxide concentration also affects stomatal aperture. Any comparison between two shoots must therefore hold these factors constant, which is why the fan-on and fan-off runs were done at the same temperature and with the same twig.