Engaging questions:

  • What are the mechanisms of the nutrition of plants?
  • What is the reserve material of plants?
  • Do the plants nourish only during the daytime or also at night?
  • How can we detect the presence of starch in the solution?
  • What is the starch?

Equipment and chemicals:

  • plant – Geranium (stored in a darkness for 48-72 hours before the Activity ),
  • solution of baking soda or NaHCO3 (10g/200 ml), CAS: 144-55-8
  • ethanol 95%, (CAS number: 64-17-5) ,
  • I2 solution in KI (1 g I2/25 KI/500 ml of water – prepare the day before, store in a dark bottle) CAS: 7553-56-2 the solution is not classified as dangerous, but is the cause of stains difficult to remove.
  • paper towels,
  • tweezers/forceps,
  • 3 crystallisers.

Description of the Activity :

  1. Place 2 layers of paper towels on top of the acrylic glass plate.
  2. Drip the towel with the baking soda solution.
  3. Place 1 Geranium leaf on the paper towel with the green side (top side) up. The leaf should be “starved”. To get this, store the plant in darkness for at least 48 hours before carrying out the Activity . The leaf should be removed from the plant just before the Activity .
  4. Place the small object made of the material impermeable to light (e.g. a coin) on top of the leaf.
  5. Place the clear plexiglass sheet on top. This way, a sandwich with the Geranium leaf inside would be formed.
  6. Hold the sandwich together with rubber or paper clips..
  7. Place the sandwich in direct sunlight, or in the light of the projector.
  8. Let the leaf to be exposed for 45 minutes.
  9. After 30 minutes of exposure, heat about 100 ml of ethanol in the water bath or heating mantle.
  10. After exposure, take the sandwich apart. Using the forceps (tweezers) pick up the leaf by the stem (petiole) and place it in the beaker of hot ethanol.
  11. After about 5 minutes, the leaf should be nearly white.
  12. Fill two crystallising dishes  halfway with water. Place the leaf in the first crystallising dish and swirl it under  water to remove ethanol.
  13. Put the I2/KI solution to the second crystallising dish.
  14. Immerse the leaf in the I2/KI solution. Watch the changes.
  15. Transfer the leaf into the second crystallising dish with water to wash off excess iodine.
  16. Dry the leaf on the paper towel.
  17. Look at the leaf and record your observations.

Experiment  developed by:



  • Explain the observed change in colour.
  • Where does the light pass through the leaf and where it was blocked?
  • Where is the coloured stain greatest?
  • Which areas contain the highest concentration of starch?
  • Why is the starch accumulation not uniform?
  • How do plants synthesise starch?
  • When do plants consume starch?
  • What is a role of starch in the human diet?
  • How much of starch do we consume every day?