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Description of Closterium

Closterium is a crescent-shaped or elongate desmid. Some species are straight and needle-like, but most are broader with curved ends €“ rather like a smile. The ends of the cell are usually tapered and may be pointed or rounded. The desmid is formed of two hemicells that are also mirror images of each other €“ but there is only a single central nucleus. Each semicell has one or two axial, ridged chloroplasts with at least one pyrenoid. Terminal vacuoles at the cell tips contain crystals of barium or calcium sulfate that vibrate with Brownian motion. Closterium cell walls have pores to secrete mucilage, even though the cells are only slightly constricted in the middle compared to other placoderms. The cell wall may be smooth or lined by thin longitudinal striae or large pores that are visible with high-resolution microscopy. The wall is sometimes yellow or brown in color. Some species have extra sections in the cell wall called girdle bands. Polymers in the cell wall may help protect the cell from drying out and allow them to survive for months in environments such as the dried mud at the edges of lakes. Like some desmids, Closterium can move in a somersaulting motion by secreting mucilage from the ends of the cell.

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