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 plants which are potted. Remember that they should be same type.
  • Give numbers to your potted plants.
  • Keep number 1 potted plant in dark room for couple days. Because keeping plant in dark room will remove all the starch from the leaves.
  • Keep number 2 potted plant in sunlight.
  • After couple days take one leave from each of plants . Then put a few drops of iodine solution on the leaves.
  • Observe the colour of the leaves.
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Blue-black colour is not observed on the leave of  the plant number 1 kept in dark room. This  indicates the absence of starch. 
Leaves of the number 2 plant kept in sunlight turns blue-black colour. This indicates the presence of starch. 
The presence of starch in leaves can also be tested by Iodine test in the laboratory. In the laboratory,
chlorophyll is first removed from the leaf by boiling the leaf in alcohol and then 2 drops of iodine solution is added in the leaf, its colour changes to blue black which indicates the presence of starch.