Flora of North America species comparison

Plants of Sciuro-hypnum plumosum from drier habitats have narrower, straight leaves, while northern populations from semipermanently wet rocks sometimes have strongly falcate leaves. The transition between these extremes does not allow for satisfactory segregation of these forms. Although S. plumosum is extremely variable, it is usually easy to recognize with stereomicroscope or hand lens. The sometimes variegated color patterning with ferruginous or even reddish spots is characteristic. However, in evenly colored plants, the leaves are very strict and very broadly channeled; the narrowing of the costa in the proximal third of leaf and the opaque cells across the leaf base are also characteristic. Brachythecium acuminatum may sometimes be confused with S. plumosum, as both have more or less isodiametric cells across the leaf base; however, the former species has larger and more transparent basal cells and its leaves usually have distinct submarginal plicae. Sciuro-hypnum oedipodium also may have an extensive opaque area in leaf corners, but never across the whole leaf base. In some cases, especially in wetter habitats, cells in the leaf corners of S. plumosum can be enlarged and pellucid, but if so, small opaque cells remain between them and laminal cells.

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

Sciuro-hypnum latifolium is distinguished by medium-sized plants, 3–6 cm stems, pale color, and acute and strict distal leaves with more concave leaves in older parts of the stem, which are more or less rounded distally and often suddenly contracted to a short, narrow, usually reflexed acumen. The distal leaves are usually imbricate, but when growing among tall herbs in wet meadows and in minerotrophic fens, these leaves tend to be erectopatent. Other diagnostic characters for S. latifolium include subentire leaf margins, narrow laminal cells forming a dense areolation, and abruptly differentiated pellucid alar cells. The type of the synonym Brachythecium nelsonii was described from La Plata Mines, Wyoming. A. J. Grout (1928–1940, vol. 3) thought that S. nelsonii was related to B. rivulare and did not compare it with the S. glaciale group, where S. latifolium belongs. The latter position was suggested by E. Nyholm (1954–1969) and W. C. Steere (1978), and was recently confirmed by molecular phylogenetic analysis by S. Huttunen and M. S. Ignatov (2004). The leaf costae are occasionally double or almost absent (in individual leaves of plants with otherwise single costate leaves); the alar region often has a delimiting pit at the leaf border.

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