A potometer measures the rate of water uptake by a plant cutting. The cutting is fixed into a tube joined to a thin capillary tube that contains a small air bubble. As the plant takes up water, the bubble moves along the capillary tube. Most of the water taken up is lost by transpiration, so water uptake gives a good estimate of the transpiration rate.
To set it up, cut the stem under water so that no air enters the xylem, and make sure all the joints are airtight. Dry the leaves. Then record how far the bubble moves in a set time. Rate of water uptake = volume of water taken up ÷ time. If the bubble moves 20 mm in 5 minutes, the rate is 4 mm per minute.
To investigate a factor, change only one variable, such as light intensity, air movement or temperature, and keep the others the same. For example, use a fan to increase air movement or a lamp at different distances. Repeat the readings and calculate a mean.
You can also measure the mass of a plant cutting before and after. Percentage change in mass = change in mass ÷ starting mass × 100. A cutting that starts at 50 g and ends at 45 g has lost 5 g, which is a 10% loss.
