Experiment 1

Apparatus to demonstrate the loss of water through transpiration

Precautions

  1. Use dry bell jars to prevent the cobalt paper from changing colour prematurely.
  2. Water the plants well to ensure a good supply of soil water.
  3. Use bell jars to prevent any loss of water vapour.

Observation
Drops of a colourless liquid collect on the inner surface of the bell jar in the experiment. The liquid that condenses must have come from the leaves in the form of water vapour. The liquid changes dry cobalt chloride paper from blue to pink. This is a positive test for water that was given off during transpiration. The control remains unchanged due to no transpiration taking place.

Conclusion
Transpiration occurs mainly through the leaves.

Experiment 2

Experiment 3

Experiment 4

A potometer can be used to measure the rate of transpiration.

Potometer

Certain environmental conditions can be simulated. These conditions could involve:

Teacher's tip

When conducting this type of experiment, a number of precautions should be taken if an accurate result is desired.

  • Test only one factor, in order to compare it with normal conditions.
  • The branch of the plant must be cut under water to prevent air bubbles forming in the xylem elements.
  • All apparatus must be sealed to prevent air bubbles being trapped.


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