OCR A Level Biology:复习笔记1.2.15 Practical: Investigating the Rate of Transpiration

Practical: Investigating the Rate of Transpiration

Using a potometer

  • Air movement, humidity, temperature and light intensity all have an effect on the rate at which transpiration occurs
  • The table below explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low

Transpiration Rate Factors Table

  • A potometer can be used to investigate the effect of environmental factors on the rate of transpiration


  • Plant shoot
  • Cutting board
  • Scalpel/scissors
  • Paper towels
  • Potometer
  • Volume scale
  • Beaker
  • Capillary tube
  • Stopwatch
  • Vaseline


  • Cut a shoot underwater
    • This is done to prevent air from entering the xylem


  •  Place the shoot in the tube
  • Set up the apparatus as shown in the diagram
  • Make sure it is airtight, using vaseline to seal any gaps
    • If air enters the apparatus the readings will be affected


  • Dry the leaves of the shoot
    • Any moisture present on the leaves will affect the rate of transpiration


  • Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water
  • Set up the environmental factor you are investigating
  • Allow the plant to adapt to the new environment for 5 minutes
  • Record the starting location of the air bubble
  • Leave for a set period of time
  • Record the end location of the air bubble
  • Change the light intensity or wind speed or level of humidity or temperature (only one - whichever factor is being investigated)
  • Reset the bubble by opening the tap below the reservoir
  • Repeat the experiment
  • The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa

Investigating transpiration rates using a potometer


Environmental factors can be investigated in the following ways:

  • Airflow: Set up a fan or hairdryer
  • Humidity: Spray water in a plastic bag and wrap around the plant
  • Light intensity: Change the distance of a light source from the plant
  • Temperature: Temperature of room (cold room or warm room)

Exam Tip

Remember when designing an investigation to ensure a fair test you must keep all factors the same other than the one you are investigating.