The test tube funnel experiment demonstrates that oxygen is evolved during photosynthesis. A few branches of Hydrilla are kept in a beaker containing pond water in which a small amount of sodium bicarbonate is dissolved. The branches are covered with a glass funnel and a test tube full of water is kept inverted over the stem of the funnel as shown in the figure.
Now the apparatus is kept in sunlight for 4 to 6 hours. The gas bubbles may be observed from the ends of hydrilla branches kept within the glass funnel. These gas bubbles are collected in the test tube by the downward displacement of water. The gas is tested for oxygen. When a burnt splinter is taken near the mouth of the tube, it glows brightly and proves that the gas is oxygen.The test tube and funnel experiment demonstrates that oxygen evolves during photosynthesis.
Ganong's light screen experiment demonstrates that light is essential for photosynthesis. When a pot plant is kept for 48 hours in dark room, the leaves become free from starch. Thus dark treated plant is called destarched plant. Ganong's light screen is a clip like instrument with a tin foil disc having a star shaped opening through which light can enter. This closes the lower hollow cylindrical box like structure. The advantage of light screen is to allow free ventilation and at the same time it cuts off light. The light screen is fixed to a leaf of the destarched potted plant as shown in the figure. The entire experimental setup is placed in sunlight for 4 to 6 hours.
The leaf subjected for experiment is tested for starch. Only the star shaped part of the leaf exposed to the sunlight turns blue. The Ganong's light screen experiment demonstrates that light is essential for photosynthesis.