Pireella Cardot, Rev. Bryol. 40: 17. 1913.
[for Louis Piré, 1827--1887, Belgian bryologist]
Plants 1--5 cm, creeping stolons forming erect stems that are simple or irregularly pinnate from a short or long stipe-like region, dull dark green to glossy yellowish green. Stem leaves dense, obscurely seriate; in basal stipe region appressed to erect, triangular-lanceolate, 0.5--1.7 mm; in distal region erect dry, spreading wet; broadly ovate-acuminate, 1.3--2.6 mm; concave, not plicate; base rounded, auriculate or decurrent; margin plane; costa single, short in proximal leaves, percurrent or shortly excurrent in distal leaves; medial laminal cells sinuose, prorate, firm-walled, porose; alar cells rhombic or quadrate, in small groups or extending up margin in a few rows. Branch leaves weakly or strongly seriate in five rows, spreading wet and dry; narrow ovate-acuminate, concave with flattened apex, not plicate; costa percurrent; medial laminal cells sinuose, distinctly prorate. Seta 5.4--12.4 mm. Capsule cylindrical; annulus not differentiated; operculum rostrate from a low rounded or conic base. Calyptra covering the capsule, hairy.
Species 13 (2 in the flora): se United States, Mexico, West Indies, Central America, n and c South America.
Pireella, with 13 species, is the largest genus of the Pterobryaceae in the Neotropics, and is composed of three distinct groups, two of which are not represented in the flora area, and are not covered by the description above. The two species in North America are rather similar and have frequently been misidentified in the past, but differ in the shape of the leaf base, the number, size and distribution of alar cells, the shape and stance of the leaves, and the branching pattern. The leaf-base characters are most useful, but can only be distinguished reliably on the distal stipe or stem leaves. C. B. Arzeni (1954) clarified the distinction between these species, but misidentifications have continued. This may in part be attributed to misidentified material in herbaria, in particular to exsiccati issued by W. Bauer and by A. J. Grout, and material annotated by C. B. Arzeni and by E. Britton. The distributions of the species differ in the flora area, but elsewhere their ranges overlap.
