Desmids and spirogyra. Polarised light micrograph of two Micrasterias rotata desmids (round) and a single filament of Spirogyra sp. green alga (lower right). Desmids are a common group of freshwater single-celled algae that have intricate cell walls. Micrasterias is highly adapted to nutrient poor, acidic water. Spirogyra are filamentous green algae, named for the spiral arrangement of their chloroplasts (green). Their cylindrical cells connect end to end in long green filaments. Magnification: x200, when printed 10 centimetres wide.