Spirogyra (division Chlorophyta) A genus of green freshwater algae, in which the thallus consists of unbranched filaments. It is capable of a spiral gliding motility when illuminated. The chloroplast forms a spiral, ribbon-like band which extends the length of the cell. Conjugation occurs by the formation of a tube connecting 2 cells; the contents of 1 cell pass through the tube and fuse with those of the other cell. Spirogyra is very common in standing fresh water, especially in spring.

spi·ro·gy·ra / ˌspīrəˈjīrə/ • n. Bot. a filamentous freshwater green alga (genus Spirogyra, division Chlorophyta) containing spiral bands of chloroplasts.