transpiration. Immerse the whole of the potometer into the sink. Move it about until all the air bubbles come out. Recut the shoot's stem underwater. Put it into the bung; grease the bung with plenty of petroleum jelly if it doesn't stay and then put the bung into the potometer. Make sure the tap is closed, then lift the whole assembly out of the water. Leave the end of the capillary tube out of the water until an air bubble forms, then put the end into a beaker of water. Types of potometers Bubble potometer Mass potometer Ganong's potometer Darwin's potometer Use Set up the conditions of the experiment, such as alterations to lighting (placing the plant in bright light or shadow), wind (directing a fan at the plant), and humidity. Precautions When a twig is cut from a plant, it should be immediately put under water (only the cut portion). Then, a small part is cut under water. This prevents entry of air into the xylem vessels. The conditions of the potometer, other than the alteration that is being tested, should not be changed during a test, as outside conditions (such as temperature) determine water uptake. Everything must be completely watertight so that no leakage occurs. Limitations The potometer does not measure the rate of transpiration accurately because not all of the water that is taken by the plant is used for transpiration (water taken might be used for photosynthesis or by the cells to maintain turgidity). The potometer measures the rate of uptake of water. To measure transpiration rate directly, rather than the rate of water uptake, utilize a scientific instrument which quantifies water transfer at the leaves. The water retained by the plant is so minute that it can be neglected. Introducing an air bubble may not be easy. A twig may not be active for a long time. References Plant physiology Measuring instruments