To answer the question of what substances organic substances are formed from, we will carry out the following experiment.
Place two plants (pelargonium, begonia) in a dark cabinet.
After two or three days, we will rearrange both plants on glass under glass hoods and expose them to the light. Under one cap next to the plant we put an alkali solution, which is able to absorb carbon dioxide from the air, and under the other – a glass with a soda solution to create an excess of carbon dioxide in the air. Lubricate the edges of the caps with petroleum jelly to limit the access of fresh air to them.
Cut a leaf from each plant in a day. Immerse them in boiling water for 2-3 minutes, then in hot alcohol. Rinse the leaves in water, and then fill them with a weak iodine solution in a glass cup. In this case, it will be seen that starch was formed only in the leaves of the plant that was under a hood with carbon dioxide (the leaf will turn blue).
Conclusion: starch is formed in plant leaves only in the presence of carbon dioxide in the air. For the formation of carbohydrates, carbon dioxide is needed, which enters through the leaf stomata from the air, and water, which is absorbed by the roots from the soil.