Theory:

The iodine test is used to test for the presence of starch. Starch turns into an intense "blue-black" colour upon addition of aqueous solutions of the tri-iodide anion. To prove this, first, let's have two healthy potted plants. Remember that they should be the same type.
  • Give numbers to your potted plants.
  • Keep number 1 potted plant in the darkroom for couple days. Keeping plant in the darkroom will remove all the starch from the leaves.
  • Keep a number 2 potted plant in sunlight.
  • After couple days take one leave from each of plants, and, put a few drops of iodine solution on the leaves.
  • Observe the colour of the leaves.
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Observation:
Blue-black colour is not observed on the leave of  the plant number 1 (that kept in dark room). It  indicates the absence of starch. 
Leaves of the number 2 plant (that kept in sunlight) turns blue-black colour. It indicates the presence of starch. 
Important!
We can also perform an iodine test to know the presence of starch in the leave. In the laboratory, first chlorophyll is removed from the leaf by boiling the leaf in alcohol. And, then two drops of iodine solution is added in the leaf. We can notice that its color changes to blue-black, which indicates the presence of the starch.