Why do you need to use water plants, rather than land plants, to investigate the rate of photosynthesis? 

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I suspect you are referring to a specific experiment, which measures the rate of photosynthesis at differing intensities of light. This experiment could certainly be done with land plants, but using a water plant makes it easier to have a simple way to measure the rate of photosynthesis--the counting of bubbles which come off a specific area of the plant. A water plant such as elodea (the type you commonly see in aquariums) does the process of photosynthesis just as plants that grow in the air do, but you can actually see the bubbles that indicate photosynthesis is occurring--they form on the leaves before they float to the surface, enabling an easy measurement of bubbles per minute.

Plants have chloroplasts, which in addition to giving plants their green color, perform photosynthesis, the process on which life on earth is dependent. They use the energy of the sun to use carbon dioxide and water to produce sugars and oxygen. When you are counting the bubbles on the underwater leaf, you are counting bubbles of oxygen formed during photosynthesis.

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