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.
To find the rate of water uptake of a plant.
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.
|Time (min)||Distance moved by bubble (mm)|
Transpiration is the loss of water vapour from the stomata of the leaves and so cannot be measured directly. Instead we assume that the water taken up by the plant is equal to the water lost and, the rate of the movement of the air bubble gives an indication of the amount of water being lost from the plant.
= 3.2 mm/min
As this method is only an indication of the rate of transpiration, it is most useful when comparing transpiration in different environments.
It could be used to measure the rate of transpiration: