To demonstrate that starch is formed in photosynthesis.
Identify a healthy green leaf on a plant and cover a portion of it on both sides with two uniform pieces of black paper, either in the morning before the sun rises or the previous evening so that the experiment is performed with a starch-free leaf. Or, keep a healthy, green pot plant in a dark room for 1 or 2 days so that its leaves become starch-free, and then cover a portion of a leaf of this plant as above. Fix the pieces of paper properly with clips. To make sure that there is no starch, collect a few neighbouring leaves in the morning, decolourize them with alcohol and dip them into iodine solution. Note that they do not turn black, thus all the leaves are starch-free. Now let the plant be exposed to light for some time, preferably till the evening and collect the leaf and decolourize it with alcohol. Place the blanched leaf in iodine solution for a minute.
The exposed portion turns blue or black, showing the presence of starch in it. The screened portion, on the other hand, turns yellowish brown as no starch formed in it, the yellow color is due to the action of iodine solution on protoplasm and cellulose. It is better to place the iodine-treated leaf in benzol for a few minutes as benzol removes the brownish colour from the protoplasm and the cell-walls, and then the bluish-black colour of starch becomes clear.
The experiment show that photosynthesis takes place only in the presence of sunlight and that starch is formed in photosynthesis.