Measuring water uptake - Potometers

Investigating transpiration

The uptake of water can be measured using a potometer. Under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration.

A simple potometer is a piece of capillary tubing to which a plant has been connected. The water uptake is measured by recording the time taken for a bubble in the tube to move a set distance.

Aim of the experiment

To find the rate of water uptake of a plant.

The potometer is filled with water.

A shoot is cut from a plant.

The end of the shoot is cut under water to ensure the xylem remains water-filled and prevents air locks.

The shoot is inserted into the rubber tubing at the end of the potometer.

The potometer is raised so that a bubble of air is taken up.

The potometer is lowered into the water. The distance travelled by the air bubble is recorded over a period of time.

Variables

• independent variable - time
• dependent variable - the distance moved by the bubble along the capillary tube
• control variables - temperature, air flow or draughts, adequate supply of water

Risks

Care must be taken when inserting the plant shoot into the rubber tubing attached to the capillary tube and with the scissors used to cut the plant shoot.

Example results

Analysis of results

Plot a line graph of the data.