DESMID

any of the beautiful, single-celled (sometimes filamentous or colonial), microscopic green algae of the order Zygnematales, class Chlorophyta, characterized by extensive variation in cell shape. Typically the cell is divided symmetrically into semicells connected at a central point. The three-layered cell wall is impregnated with openings or pores and pectin spicules; irregular desmid movement is caused by the flow of a gelatinous substance through these pores. Conjugation (temporary union for the exchange of nuclear material) is the usual method of sexual generation. When a conjugation tube is not formed, the two protoplasts unite in a gelatinous sheath that surrounds the cells. Usually cell division occurs across the contact point, each half develops another semicell, and two complete desmids are formed. Spores are rare. The distribution of desmids is worldwide, usually in acid bogs or lakes. Since most species have a limited range, the presence of specific desmids is helpful in characterizing water samples. One of the more common desmid genera, the sickle-shaped Closterium, often contains gypsum crystals.