Measuring transpiration

The rate of transpiration can be investigated by measuring the volume of water absorbed.

Volume of water absorbed

A potometer can be used to measure the volume of water absorbed. Usually a leafy shoot is used instead of a single leaf.

A simple potometer comprises:

  • a container of water
  • a capillary tube
  • rubber tubing to connect the capillary tube to the stem of the leafy shoot
  • a scale to measure the distance travelled by a bubble in the capillary tube
A leafy branch placed in capillary tube and sealed with rubber tubing. Other end is in water. An air bubble is approximately half way up tube. A ruler is used to measure volume of water absorbed.

The faster the bubble moves, the greater the rate of water uptake - and so the greater the rate of transpiration. You can vary the conditions to investigate the effect of:

  • changing temperature
  • humidity
  • wind speed (for example, with a hair dryer)
  • light intensity