Outline. The mass lost by an uprooted plant in a flask of water is compared with the mass lost from an identical flask with no plant.
Prior knowledge. Evaporation takes place from an exposed water surface
Advance preparation and materials
Flask or bottle. Any pair of small, narrow-necked vessels will do, e.g. 100 cm3 conical flasks,
small fruit-juice bottle, small ‘medicine’ bottles. Allow one matched pair per group.
Plants. Any small plants which can easily be dug up. Wash the soil off the roots and store the plants in a container of water from which the student can collect them. One plant per group.
Balance. One or two per class.
(a) Label each bottle or flask with your initials.
(b) Fill each vessel with water to within 1 cm of the rim.
(c) Place the uprooted plant in one of the vessels.
(d) Weigh the vessel with the plant and record its mass. Weigh the other vessel
(with no plant) and record its mass.
(e) Leave both vessels in a (potentially) sunny position in the laboratory.
(f) After one day, weigh both vessels again and record their masses.
(g) If there has been little change of mass in the vessel with the plant, the experiment can be
continued for several more days.
(h) At the end of the experiment, work out the mass lost by each vessel. Subtract the mass lost by the vessel with water only, from the mass lost by the vessel with the plant. This will give the loss in mass resulting from the plant’s transpiration.
(i) Calculate the rate of transpiration in grams per day.
1 The vessel with the plant will be losing water (a) from the shoot and (b) directly from the exposed water surface. The vessel lacking a plant enables us to calculate the amount of mass loss resulting from (b).
2 The mass loss in the vessel with the plant is made up of these two components. By deducting the mass loss resulting from direct evaporation, we are left with the mass loss caused by transpiration from the plant.
3 (a) The plant will interfere with the direct evaporation from the vessel, depending on the shape and size of the stem and the position of the lower leaves.
(b) In the vessel with the plant, the water level may drop below the neck, exposing a greater surface area of water for evaporation.