To show that water vapour is given off by plants.
Take a potted plant and cover the pot and base of stem with cellophane or rubber. Place the potted plant on a Vaselined glass plate, invert a bell-jar over the pot and plant, and leave them outside the laboratory. Set up a control experiment with no plant.
After an hour, drops of colourless liquid are seen inside the bell-jar with the plant. To show that these drops are water, touch them with anhydrous copper sulphate (white) and its colour changes to blue. No drops of water are found in the control experiment where there was no plant.
The presence of stomata makes the leaves suitable for exchange of gases which goes on during photosynthesis and respiration, and this makes loss of water inevitable. Stomata are tiny pores in the epidermis of leaves, each pore being surrounded by a pair of special cells called guard-cells which control the size of the stomatal pores. The behaviour of the stomata and the rate of transpiration are determined by environmental factors. Transpiration exerts a force against that of gravity, and this is known as transpiration pull, enables water to be absorbed.