Algae, organisms that lack true roots, stems, and leaves; that usually contain chlorophyll; and whose reproductive organs are single-celled structures. Green algae and the few chlorophyll-containing bacteria were probably the first organisms capable of carrying on photosynthesis (the process of manufacturing their own food).

About 25,000 species of algae have been identified. They range in size from microscopic single-celled forms to multicelled (many-celled) forms more than 150 feet (45 m) long. They abound in both fresh and ocean water and in moist environments on land. Masses of microscopic freshwater algae that are often seen floating on the water surface are called pond scum. The larger marine algae are commonly called seaweed. Certain species of algae contaminate reservoirs and swimming pools, giving the water a fishy taste and a foul odor.

Algae are the major source of the oxygen needed by aquatic animal life. Single-celled algae known as phytoplankton provide the food for larger organisms that, in turn, provide food for yet larger organisms. The interrelationship of these organisms is part of what is called the food web.

EudorinaEudorina is a green algae.