Crum, Howard Herbarium, University of Michigan, Ann Arbor, Michigan.
An order of the true mosses (subclass Bryidae) which consist of two families and about five genera and generally grow in dry, exposed places, especially on rock. They are often dark, even blackish, with erect-ascending and simple or forked stems, or prostrate and freely branched stems. The leaves, in many rows, are often hair-pointed (see illustration) with a single and well-developed costa. The cells are generally smooth but commonly have side walls that are nodulose-thickened. The capsules are immersed to exserted (sometimes on curved setae), and are usually erect, symmetric, and sometimes ribbed. The operculum is differentiated, and the single peristome (rarely lacking) is composed of 16 teeth which are commonly perforated or variously cleft. The calyptrae are cucullate or mitrate. The chromosome numbers are generally 12 and 13.
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