The Zygnematales ( (zygos) + νήμα (nēma) (nom.), νήματος (nēmatos) (gen.)), also called the Conjugales, are an order of green algae, comprising several thousand different species in genera such as the well-known Zygnema and Spirogyra. Most members of this group develop into unbranched filaments, one cell thick, which grow longer through normal cell division. Most live in freshwater, and form an important component of the algal scum that grows on or near plants, rocks, and various debris.