Spirogyra cells join together to form filaments. Spirogyra is a green algae that forms long, unbranched filaments. The filaments are made up of many cylindrical cells that are connected end-to-end. Spirogyra are unicellular organisms that connect to form multicellular filaments. To become long filaments, single spirogyra cells link together, end-to-end, forming strands up to several centimeters in length.