Green algae are microscopic protists; found in all aquatic environments, including marine, freshwater and brackish water. The green algae (singular: green alga) are the large group of algae from which the embryophytes (higher plants) emerged. As such, they form a paraphyletic group, variously included among the Plantae or with the Protista. The green algae include unicellular and colonial flagellates, usually but not always with two flagella per cell, as well as various colonial, coccoid, and filamentous forms. In the Charales, the closest relatives of higher plants, full differentiation of tissues occurs. There are about 6000 species of green algae. Many species live most of their lives as single-cells, other species form colonies or long filaments.