Take a variegated leaf (that is, a leaf that is partly green and partly white) of a croton plant that was kept in sunlight for a few hours. The green parts of the leaf contain chlorophyll, whereas the white portion does not. Immerse the leaf into boiling water and let it boil for a few minutes, such that the cells in the leaf are killed. Now, to decolourise the leaf, boil it in rectified spirit. Wash the decolourised leaf in hot water so as to remove any chlorophyll sticking to it. Place the leaf in dilute iodine solution.
We observe the parts of the leaf which were green turn blue-black in colour, showing the presence of starch in those areas. The parts of the leaf which were white does not change colour, showing that no starch is present in these portions of the leaf.
This shows that chlorophyll is essential for photosynthesis, as only the green portion of the leaf contained starch.