in botany: A genus of the natural order of algae, belonging to the cryptogamia class of plants. The male flower is pedunculated, and naked; the anthera quadrivalved; the female flower is sessile, naked, with roundish seeds. There are 29 species, all natives of Britain, growing in woods, shady places, by the sides of ditches, &c. Many of them are beautiful objects for the microscope.