Crum, Howard Formerly, Herbarium, University of Michigan, Ann Arbor, Michigan.
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An order of the true mosses (subclass Bryidae) consisting of a single family, the Fissidentaceae, and about 800 species in the genus Fissidens plus a few others distributed in four or five segregate genera. Fissidens is the largest genus of mosses. The Fissidentales are unique in leaf form and arrangement. The plants, erect and simple or forked, grow (except at the earliest stages) from a two-sided apical cell. The leaves, arranged edgewise to the stem in two rows, have two sheathing blades which clasp the stem and the base of the next leaf above (see illustration) Beyond the sheathing blade and toward the stem is a ventral blade; on the side away from the stem is a dorsal blade extending to the leaf base or nearly so. The costa is single and well developed or rarely lacking. The cells are isodiametric and often papillose, though leaf margins are often bordered by linear cells. The archegonia are usually terminal. The capsules are generally exserted, erect, and symmetric, with a peristome that consists of 16 teeth that are characteristically forked and spirally ridged at the tips. The calyptra is cucullate and the chromosome numbers are 5, 6, 8, and 10. See also: Bryidae; Bryophyta; Bryopsida
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