Transpiration describes the evaporation of water from plants through the leaves, roots, stem and flowers. Most of the water is lost through the surface openings on the leaves called stomata.
This practical involves taking a small cutting from a geranium and weighing it before and after the cutting has been placed in a tube of water. You should find that over time, the mass (of the cutting, tube and water) has decreased and that the volume of water has also decreased.
Equipment typically required (per set)
- Geranium cutting with stem and leaves (at least 10cm long)
- Measuring cylinder (graduated) 50cm3
- Accurate balance (to two decimal places)
- 10ml vegetable oil
- 10ml Pipette
The cutting is placed into the measuring cylinder and enough water is added so that the level on the cylinder reads 25cm3. A small amount of oil is added using a pipette so that it sits on top of the water ensuring that no water can evaporate directly from the cylinder.
The equipment is then usually weighed by placing onto a pan-top balance.
Over 2-3 days the mass and volume of the water may be measured. Throughout the experiment, both the mass and volume should decrease steadily.
Ensure that the cutting is healthy and freshly cut for this practical to work best. Cuttings left in a draughty or hot atmosphere may lose mass and volume at a greater rate than those left in colder conditions. When taking the cutting, make sure that the cut is clean and that the leaves are secure and alive.
Some factors may affect the outcome of this practical. If the cutting continues to grow, the mass loss may not be as high. Conversely, if leaves die during the experiment, further loss in mass may be recorded.
Class sets of plants in cylinders may need to be stored between lessons. These should be placed in a dry room where the temperature is consistent. The cuttings should not be moved around unnecessarily and care should be taken so that leaves are not damage or removed.
Geraniums work well in this practical although other plant types may be used.