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.)