Elodea is a plant that lives in water. Like all plants, elodea can make its own food through a process called photosynthesis. Oxygen gas is also produced during photosynthesis. Photosynthesis can only happen in the presence of light. To measure the rate of photosynthesis, a piece of elodea can be placed in an inverted test tube filled with a sodium bicarbonate solution. Oxygen gas produced by photosynthesis will collect at the top of the test tube.