Coloured scanning electron micrograph (SEM) of placoderm desmid (Cosmarium botrytis). Desmids are freshwater, unicellular green algae that have a symmetrical appearance. There are two groups of desmids: the Placoderm and the Saccoderm desmids. The placoderm desmid cell is divided in two symmetrical halves (semicells) that show a remarkable variation in shape and surface ornamentation. The placoderm desmid is separated by a median constriction called the isthmus. At cell division the cell splits in two at the isthmus, with each half generating a replica of itself to restore the original shape. Because desmids thrive in clear and nutrient poor freshwater, they are important to ecologists in assessing and monitoring wetland areas. Magnification: x400 when shortest axis printed at 25 millimetres.
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