Green algae are microscopic protists. One can find them in all sorts of natural water: salt water, freshwater and brackish water. The group is less used in classification today. Although most are descended from a common ancestor, some are not. Some of their descendants, the land plants are not included in the group, so the group is not monophyletic.
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.
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A growth of the green seaweed Ulva on rock substratum at the ocean shore. Some green seaweeds like Ulva are quick to utilize inorganic nutrients from land runoff, and thus can be indicators of nutrient pollution.
Green algae Facts for Kids. Kiddle Encyclopedia.