DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY
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. Saccoderm desmids have simple shapes, cylindrical or spindle-shaped, and rarely with any isthmus. Their unique shapes are the basis for taxonomic classification. 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.
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