Determination of Rate of Transpiration by Ganong’s Potometer

Aim of the experiment
Determination of rate of transpiration by Ganong’s Potometer

Theory:
Using a potometer, the amount of water absorbed by a leafy shoot can be measured. It is also known that most of the water absorbed by a plant is lost through transpiration. Hence, the amount of water absorbed can be considered approximately to be the amount of water transpired though the result is not very accurate since some amount of water absorbed is retained for use by the leafy shoot for its metabolic activities.
Materials required:
Ganong’s potometer, water, beaker, leafy twig, knife or blade, Vaseline
Procedure:
The apparatus was filled with water. A leafy twig (cut under water to prevent air clogging) was inserted through the cork of the vertical tube of the potometer. Some water was poured into the reservoir funnel, and the stop cock was closed. The apparatus is made airtight by proper sealing. An air bubble was inserted in the horizontal limits of the apparatus. Then, the twig is exposed to sunlight for 1-2 hrs and allowed to transpire.
Observation:
As water was lost by transpiration, the inserted bubble moved in the horizontal graduated tube toward the transpiring twig. Three readings of the time taken by a bubble to cover a distance of 1 cm were taken, and mean value was also calculated.
Inference:
Amount of water transpiried =                                     ml min-1  

where, r = radius of the bore of the horizontal tube of the potometer.
=
=

So, in the experiment, the leafy twig transpired at the rate of ml min-1 on the assumption that rate of absorption and that of transpiration are the same.