potometer practical

• Pedometers can be used to measure the uptake of water as a way to measure the rate of transpiration
• Transpiration


  Water vapour evaporating out of open stomata, mainly found on leaves, which draws up a continuous water column through the xylem of the plant
• Key factors affecting the rate of transpiration

  • Temperature
  • Humidity
  • Air movement or wind
  • Light intensity
• Potometer


  Measures how much water a particular piece of plant is taking up in a period of time
• The amount of water taken up is almost the same as the amount of water which evaporates by transpiration, so the uptake of water is proportional to the rate of transpiration
• Setting up a potometer

  1. Cut plant sample underwater to avoid air entering xylem

  2. Fill potometer equipment with water to remove air bubbles

  3. Place plant sample in potometer through rubber seal

  4. Cover joints with petroleum jelly to make airtight

  5. Introduce one air bubble deliberately
• Using a potometer


  • Measure distance air bubble moves in a set time period
  • Use volume of cylinder and time to calculate volume of water taken up, representing rate of transpiration
• Petroleum jelly is used to cover joints to prevent air entering and water leaking, which would affect accuracy
• The reservoir and tap allow resetting the apparatus and repeating measurements
• Cutting plant sample underwater

  Prevents air entering xylem and breaking continuous water column
• Introducing one air bubble


  Allows measuring distance it moves to calculate water uptake
• To compare transpiration rates between plant species, the leaf surface area must be controlled