Plant
Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom. The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the brown, red and green algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, and they are no longer classified as plants as defined here.
Phycology
Algae are important as primary producers in aquatic ecosystems. Most algae are eukaryotic, photosynthetic organisms that live in a wet environment. They are distinguished from the higher plants by a lack of true roots, stems or leaves. They do not flower. Many species are single-celled and microscopic (including phytoplankton and other microalgae); many others are multicellular to one degree or another, some of these growing to large size (for example, seaweeds such as kelp and "Sargassum").