Spirogyra (common names include water silk, mermaid's tressses, and blanket weed) is a genus of filamentous charophyte green algae of the order Zygnematales, named for the helical or spiral arrangement of the chloroplasts that is diagnostic of the genus. It is commonly found in freshwater areas, and there are more than 400 species of Spirogyra in the world. Spirogyra measures approximately 10 to 100 µm in width and may stretch centimeters long.