Photosynthesis is the process by which green plants convert carbon dioxide and water, in the presence of sunlight, to glucose and oxygen. This process can be expressed by the following chemical reaction:
`6CO_2 + 6 H_2O + sunlight -> C_6H_12O_6 + 6O_2`
The glucose that is produced is used as an energy source or converted to starch for storage.
Thus, the presence of starch in a leaf is direct evidence that photosynthesis has taken place. Starch can be detected by the iodine test. In this test, a leaf is boiled in water and then boiled in ethanol until it decolorizes. The decolorized leaf is then washed with water and a few drops of iodine solution are placed on it. If the leaf turns blue-black, starch is present in it.
Thus, the iodine test can be conducted to see if starch is present in the leaf, which is direct evidence of photosynthesis occurring in the leaf.
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