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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.

Systematically they fall within the division Charophyta, which includes the groups of algae that are most closely related to the higher plants. Charophyta are included with land plants ( Embryophyta) in the clade Streptophyta.

Sexual reproduction in Zygnematales takes place through a process called conjugation. Here filaments of opposite gender line up, and tubes form between corresponding cells. The male cells then become amoeboid and crawl across, or sometimes both cells crawl into the tube. The cells then meet and fuse to form a zygote, which later undergoes meiosis to produce new filaments. As in plants, only the female passes chloroplasts on to the offspring.

The only other group of conjugating algae are the desmids, which live as individual cells often with a striking symmetrical appearance. The desmids are sometimes placed in a separate order Desmidiales, which is closely related to Zygnematales, and the two are placed together in the class Zygnematophyceae.