A potometer is used in measuring the rate of transpiration of a plant


  1. A leafy shoot is cut whilst underwater to ensure that water does not touch the leaves
  2. The potometer is filled with water with no air bubbles inside
  3. Using a rubber tube, the leafy shoots are fitted to the potometer whilst underwater
  4. The potometer is removed from the water and all the joints are sealed with waterproof jelly
  5. An air bubble is introduced into the capillary tube
  6. The distance the bubble travels is then measured over a set time
  7. Using the mean result, the volume of water lost is calculated
  8. Once the air bubbler reaches the end of the rube, the reservoir tap can be opened to reset the apparatus
  9. The volume of water taken can be calculated using:

The potometer is only an estimation for the transpiration as it does not account for water used through photosynthesis, however only 1% of water used by a plant is the result of photosynthesis

To maintain fair conditions, ensure that either the same leave, or similar sized leave in reference to surface area or used.

e.g. In an experiment the distance the bubble moved was 15.28mm2 during 1mminute in a 0.5mm radius capillary tube. Calculate your answer in cm3 h-1

π x 0.52 x 15.28 = 12.001mm3

12.001 x 60 (minutes) = 720.01 mm3h-1

720.01 / 1000 = 0.72cm3 h-1