Desmids and Spirogyra algae. Polarised light micrograph of a colony of (bottom) and an individual (centre) Desmidium swartzii desmid and Spirogyra sp. filamentous green algae (upper right). Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Spirogyra is named for the spiral arrangement of its chloroplasts. It's cylindrical cells connect end to end in long green filaments. Magnification: x200 when printed 10 centimetres wide.