Flora of North America species comparison

Heterocladium macounii grows almost exclusively on rock throughout its range and is only rarely collected on the other substrates frequented by the other two North American species. In the field, H. macounii can be difficult to separate from H. dimorphum, but the small, distinct stem papillae and multiple laminal cell papillae are good distinguishing traits of the former. There have been observations that H. macounii grows exclusively on rock; though this is generally the case, a few collection labels mention soil over rock or humus. Displaying a classic Arcto-Tertiary disjunct distribution reported for some bryophytes, the species is found in North America in the southeastern and northwestern United States and Alaska. Recently it has also been reported at high elevations in Mexico and El Salvador.

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

The laminal cells of Heterocladium dimorphum are best described as prorate rather than papillose, as the papillae are at the distal ends. The prorae are much more difficult to distinguish than the papillae of H. macounii but are most clearly present and observable along the leaf margins. The stem and branch leaves also exhibit a distinct area of elongate cells in the proximal portion of the leaf around the costa extending to the insertion. These cells are smooth and larger than the quadrate laminal cells toward the margin and in the distal portion of the leaf.

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