green algae. Unicellular, colonial, multiuncleate, or multicellular, characterized by chlorophylls a and b and various carotenoids. The carbohydrate food reserve is starch. The cell walls consist of polysaccharides, including cellulose in some forms. Motile cells have two lateral or apical flagella. True multicellular genera do not exhibit complex patterns of differentiation. Multicellularity has arisen at least three times, and quite possibly more often. There are about 9,000 known species and possibly many more.