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Sphacelariales (division Phaeophyta) An order of brown algae in which growth occurs from a large, dense, brown, apical, meristematic cell. This cell divides transversely, and then the daughter cells divide longitudinally. The thallus is generally filamentous and branched. Sphacelaria cirrhosa is a common British species; it forms small, dense tufts, 0.5–2.5 cm tall, on other seaweeds. It is found throughout the year on the lower shore and below low-water mark.