What experiment can be done to prove that sowing is necessary for the formation of starch in the leaves?

To prove that light is needed for the formation of starch in the leaves, the following experiment can be performed. Take some kind of houseplant, for example primrose or geranium (pelargonium), place it in a dark cabinet for three days so that the nutrients out of the leaves take place. Cut out a shape or word on a black paper envelope. After three days, remove the plant from the cabinet and place one of the leaves in an envelope. Then place the plant in sunlight or under a light bulb. Cut off the leaf after 8-10 hours. Remove paper. Immerse the leaf in boiling water, and then for a few minutes in hot alcohol, in which chlorophyll dissolves well. When the alcohol turns green and the leaf becomes discolored, rinse it with water, spread it on a plate and pour it over with a weak iodine solution. Blue letters appear on the discolored sheet. It is known that starch turns blue from iodine. The letters will appear in the part of the sheet on which the light fell. This means that starch was formed only in the illuminated part of the leaf.