Flora of North America species comparison

Dicranum scoparium is undoubtedly the most polymorphic species of the genus in North America. It is commonly called the broom moss because the leaves are “swept” or turned in the same direction. However, the habit of the leaves varies from strongly falcate-secund, i.e., the swept state, to straight and erect, especially prevalent in northern plants. The leaf shape varies from the typical lanceolate and long-acuminate to the odd ovate-lanceolate and short-acuminate. The leaves are typically smooth, in sharp contrast to the strongly undulate leaves of D. polysetum, but on rare occasion they can be somewhat undulate-rugose. The leaf margins are usually distinctly toothed in the distal 1/3 but at times plants in some populations have extremely weak serrations, appearing nearly entire. The costae vary in extent from subpercurrent to shortly excurrent. The abaxial surface of the costae have two to four toothed ridges, compared to the two in D. polysetum, but in some forms they are almost nonexistent. The distal areolation of the leaves seems to remain constant in the myriad forms of D. scoparium, the cells being short-sinuose with pitted walls, generally containing conspicuous oil-drops.

Plants growing in arctic North America and those in extremely wet habitats, such as bogs, often have a different superficial appearance. The leaves are straight, erect, ovate-lanceolate, short-acuminate with weak serrations on the margins, and the abaxial ridges on the costae are poorly developed. Sometimes the plants have distal shoots with abnormal appearing leaves that are shorter and broader than those below. Some of these collections have been referred to Dicranum latifolium, which I consider to be a synonym of D. scoparium. Further field and laboratory studies are required to clarify the taxonomic status of those plants. In the sterile state, large-leaved plants of D. scoparium can be confused with D. majus. The latter is immediately distinguished by a double row of guide cells, instead of the single row in the former, thicker stereid bands, distal cells that are narrower and more elongate and costae that have small teeth or serrulations distally on the abaxial surface instead of the characteristic 2–4 toothed ridges of D. scoparium. When fruiting, the solitary, rarely paired, sporophytes of D. scoparium distinguish it from D. majus, which has multiple sporophytes, 2–5 per perichaetium.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Genera 50–52, species ca. 900 (17 genera, 90 species in the flora).

Distinctive characters of this large acrocarpous family include the erect, often tomentose stems; mostly narrow, lanceolate, occasionally falcate or falcate-secund leaves, with a single, narrow to broad costa, with or without rhizoids at the base, sometimes ending in a hyaline, occasionally toothed apex, costa in cross section with or without stereid bands, leaf cells usually smooth, sometimes mammillose, or rarely with a single papilla on one or both sides, papillae rarely forked, asexual propagation by specialized deciduous branches, deciduous leaves or leaf apices, rarely rhizoidal tubers; sporophytes usually solitary or rarely clustered, setae mostly elongate, straight or rarely flexuose or cygneous, capsule cylindric to ovoid, erect to horizontal, smooth or ribbed, sometimes strumose, operculum usually obliquely rostrate, peristome single, with 16 teeth often divided 1/2 way to the base, usually striolate or pitted-striolate proximally, papillose distally.

(Discussion copyrighted by Flora of North America; reprinted with permission.)