Rate of transpiration can be measured by an instrument called potometer, Gannon's potometer and Farmers potometer are used commonly for the measurement of rate of transpiration.
Here the movement of water bubble in the direction of the absorbing cut end of the twig is considered as the rate of transpiration.
Relation between transpiration to absorption can be studied by T/A apparatus (Fig.2.14). The Plant twig fitted in air-tight manner is allowed to transpire. As a result a transpiration pull is set up.
Then water is drawn from the side tube to the cut end of the twig. The initial water level in the side tube decreases. The difference in the water level in the side tube is considered as the amount of water absorbed.
The amount of water transpired is determined by taking the difference in weight of the apparatus before and after the experiment. Then the transpiration and absorption ratio is determined which is almost equal to one under normal condition.