Closterium Nitzsch ex Ralfs
	From Greek klosterion, "small spindle"
(Above, Right) Note the round nucleus in the center of the cell and the large rounded, lobed pyrenoids.   (Above) Note the ridged chloroplast.   The nucleus occupies the empty space in the center of the cell, but is not in focus in this image.	Closterium cells are crescent-shaped or elongate and lack spines. Some are quite straight and needle-like, while others are much broader with curved ends. The ends of the cell are usually tapered and may be pointed or rounded. Each semicell has a single axial, ridged chloroplast with at least one pyrenoid. Occasionally there are two chloroplasts per semicell. The nucleus is located in the center of the cell between the chloroplasts.   Terminal vacuoles at the cell tips hold vibrating crystals of barium or calcium sulfate, the function of which are unknown. Brownian motion causes these microscopic particles to move erratically due to the impacts of collisions with the surrounding liquid molecules in which they are suspended (indicated by arrow, below image).   Closterium is a placoderm desmid because the cell walls have pores to secrete mucilage, even though the cells are only slightly contricted 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, and 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 moves in a somersaulting motion by secreting mucilage from alternating ends of the cell.