Materials Required:
l Black construction paper
l Scissors
l coleus or geranium plant
l Paper clips
l Iodine
Procedure:
- From the black construction paper, cut two squares that are larger than one of the plant’s leaves. Place the leaf (still on the plant) between the two squares of paper and fasten the papers together with paper clips.
- Set the plant in a sunny window and allow it to grow normally for several days. Then take the papers off and put 3 drops of iodine on top of the leaf. Also put iodine on a leaf that was not covered. The covered leaf shows no reaction, but the normal leaf turns bluish-black. What causes this difference?
This Is What Happens:
When the sun is shining, plant leaves manufacture sugar. Some of this sugar is converted into starch and stored in the plant tissue. When iodine reacts with starch, it causes a bluish-black colour change. The colour change that you saw on the normal leaf shows that starch is present. However, the leaf that was covered—protected from the sun and kept dark—could not manufacture starch, and so did not react to the iodine.
