Xanthidium desmid. Polarised light micrograph of a Xanthidium sp. desmid (centre) with Spirogyra sp. algae. Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Spirogyra is a filamentous green algae named for the spiral arrangement of its chloroplasts. Its cylindrical cells connect end to end in long green filaments. Magnification: x400 when printed 10 centimetres wide.