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an order of green algae. The thallus is lamellate (consisting of one or two layers of cells), tubular or saclike (consisting of a single layer of cells), or filamentous (consisting of two rows of cells). The cells are mononuclear, and the chloroplasts are lamellate and have pyrenoids. Vegetative reproduction is achieved with parts of the thallus; asexual reproduction is by means of zoospores having two or four flagella. The sexual process is iso-gamy or heterogamy. The zygote appears without a resting period, and meiosis occurs with the formation of the zoospores. The sporophyte and gametophyte have the same structure, or one may be multicellular or unicellular but considerably smaller than the other. Sometimes one of the forms is omitted from the developmental cycle.
The algae grow in seas and, less commonly, in brackish and fresh waters. Some species of the genera Viva, Monostroma, and Enteromorpha are used as food. Monostroma are cultivated in Japan.
The Russian common name for the order Ulvales is also used to designate a family of that order, Ulvaceae, which comprises the above-mentioned genera.