To show that plants cannot photosynthesize unless carbon dioxide is available.
A glass bottle is fitted with a split cork and a branch with long green leaves is cut, and its cut end dipped into water contained in a beaker. A single leaf will also do. Pour a small quantity of caustic potash solution into the bottle and lay it on its side. The leaf is placed between the two halves of the cork in such a way that one half lies inside the bottle and the other half outside it with the petiole dipped into water contained in a beaker or a dish. The edges of the split cork should be smeared with vaseline and the bottle made air-tight. The fitting up of the apparatus is to be done in the early morning so that the experiment is done with a starch-free leaf. The whole apparatus is then exposed to light for a few hours, preferably till the evening. The leaf is then removed, decolourized with alcohol, and tested with iodine solution for starch grains.
The portion of the leaf lying outside the bottle turns black, while the portion inside turns yellowish.
This evidently shows that plants do not form starch when carbon dioxide is not at their disposal, all the carbon dioxide contained in the bottle having been absorbed by the caustic potash solution.