Desmids. Coloured scanning electron micrograph (SEM) of placoderm desmids (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: x250 when printed at 10cm