Desmidium desmid. Scanning electron micrograph (SEM) of a Desmidium species desmid green alga. Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Desmidium is one of the filamentous desmid genera characterized by rather firm intercellular connections. Cells connect to adjoining cells at the flat, apical face of the cells. Magnification: 1000 when printed at 10 centimetres wide.