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an order of green algae of the subphylum Conjugatophytina. There are 25 genera comprising 2,000 species. The Desmidiales are microscopic unicellular free-living or occasionally filamentous forms. The cells consist of two symmetrical halves with a single nucleus in the center. Each half of the cell contains a discoid chromatophore. Reproduction takes place by division into two cells on the transverse plane; the sexual process is conjugation. The zygote germinates after a period of dormancy, usually producing two new individuals. The Desmidiales live in fresh water, usually in peat marshes and in bodies of water with little lime; certain varieties are found in soil.