Plants in low cushions or turfs or loosely cespitose, green distally, brown to tan or yellow proximally. Stems to 1 cm, hyalodermis weakly differentiated to distinct, seldom absent, sclerodermis weakly differentiated in 1-2 layers, central strand present; axillary hairs with proximal 1-2 cells somewhat thicker walled. Leaves incurved, commonly tubulose and often contorted or spiraled when dry, spreading when moist; long-ligulate, oblong or triangular to long-lanceolate, 1.5-2.5(-4) mm; adaxial surface broadly channeled across leaf; base scarcely differentiated to ovate or rectangular, occasionally half-sheathing; distal margins usually sharply incurved, occasionally merely erect, seldom plane, entire; apex sharply acute to subulate, occasionally broadly acute, obtuse, or weakly cucullate; costa shortly and sharply mucronate, adaxial outgrowths absent, adaxial cells quadrate to short-rectangular except occasionally elongate near apex, in 4-8 rows; transverse section ovate, occasionally circular or semicircular, adaxial epidermis present, adaxial stereid band present, guide cells 4-6(-8) in 1 layer, hydroid strand absent, abaxial stereid band present, crescent in sectional shape, abaxial epidermis present or occasionally absent; basal cells differentiated across leaf, rarely rising higher along margins in a V shape, rectangular, occasionally rhomboid, 2-5:1; distal medial cells subquadrate to hexagonal, 7-13 µm, papillae 2-fid, 2-6 per lumen, occasionally fused into a large multiplex papilla covering the lumen, occasionally spiculiform and branching. Specialized asexual reproduction rare, as rhizoidal gemmae. Sexual condition monoicous (usually autoicous), occasionally dioicous; perichaetia terminal, interior leaves occasionally sheathing seta, little different from cauline leaves or somewhat larger. Seta 0.1-1.3 cm. Capsule stegocarpic or cleistocarpic; theca elliptical to short-ovate or cylindric, 1-2.2 mm, annulus in ca. 2 rows of persistent, vesiculose cells or not differentiated; operculum when present conic to rostrate; peristome teeth 16 or rudimentary or absent, occasionally removed with operculum, oblong-truncate to long-triangular, often irregularly cleft, straight or twisted weakly counterclockwise, mouth occasionally closed by a remnant of the spore sac. Calyptra cucullate. Spores 14-28 µm. KOH laminal color reaction yellow.
Species ca. 97 (10 in the flora): worldwide except Antarctica; found mostly on soil.
A world evaluation of Weissia and Trichostomum (R. H. Zander 1993) at the generic level suggested that species of the two genera might be shuffled into a more natural arrangement if taxa with identical or nearly identical gametophytes were grouped, with differences in sporophytes easily and simply explained by reduction. It is insufficient, however, to simply split Weissia and Trichostomum into two genera, as was implemented by Zander, based on leaf margins sharply incurved or erect to nearly plane, respectively. Weissia jamaicensis and T. crispulum have similar gametophytes but differ markedly in the leaves of the former being sharply incurved marginally and of the latter nearly plane to merely erect. No major changes were made, however, for this treatment given the need for a full revision. Certain Weissia species with erect leaf margins may have a somewhat cucullate leaf apex similar to that of T. crispulum, but the lamina of the latter is generally broader, 16-20 cells across on one side of the costa just above mid leaf (except in strongly reduced specimens with short-ovate leaves), while Weissia species with which it may be confused have laminae usually 10-15 cells across a lateral lamina. The present species concepts are largely identical with those of A. Stoneburner (1985) with exceptions as noted.
All species of Weissia with sharply inflexed distal leaf margins have a tendency to show adaxially bulging cells in leaf section, possibly because the inflexion crowds the cells. This differential bulging of cell superfices is probably homoplastic to adaxially bulging distal laminal cells in other pottiaceous genera without sharply inflexed distal margins, such as Hyophila. Thus, the close relationship of Weissia with Hyophila and relatives cladistically demonstrated by R. H. Zander (1993) is probably an artifact. Regarding the hymenium that may be seen closing the newly deoperculate capsules of the eperistomate segregate genus Hymenostomum, such a membrane is present in all species of the genus, being the top of the spore sac, and it is simply more evident in the eperistomate species.
Weissia condensa (Voit) Lindberg
This name replaces Weissia tortilis (Schwägrichen) Müller Hal., an illegitimate homonym according to M. F. V. Corley et al. (1981). Synonyms of this European species have been applied to specimens of W. controversa from the flora area include Hymenostomum tortile (Schwägrichen) Bruch & Schimper; W. euteiches R. H. Zander, superfluous name; and W. tortilis (Schwägrichen) Müller Hal., illegitimate homonym. European specimens of W. condensa usually have cucullate leaf apices, thick costae to 100 µm wide basally, distal laminal cells 8-10 µm wide, and capsules usually eperistomate; the leaves commonly have rather stout and long mucros of (4-)7-10 cells. Weissia sharpii is similar but has large distal laminal cells, 10-13 µm wide (2-)1:1. The combination of a strong mucro, wide costa and generally eperistomate but stegocarpic capsule is not found in the flora area and thus this species is excluded. Although A. Stoneburner (1985) attributed an eperistomate condition to W. condensa, R. H. Zander (1994h) pointed out that W. condensa may occasionally have a very rudimentary peristome, one or two segments high, hidden below the capsule stomal rim (e.g., Slovakia, Pilous 139, MO). The thickness of the capsule wall in numbers of cell layers was used by Zander to distinguish W. condensa from W. controversa in Mexico, but this is apparently not a good feature because all Weissia capsules examined in the present study are three to four cell layers thick although W. controversa occasionally has 2-3 layers, with the innermost layer difficult to demonstrate or possibly eroded or resorbed.
Weissia flavescens. E. Britton (= Trichostomum brittonianum R. H. Zander)
This species was reported from Florida by W. D. Reese (1991), but those sterile plants proved to be indistinguishable from Trichostomum crispulum.
Weissia wimmeriana (Sendtner) Bruch & Schimper
This is a European species whose report from Minnesota, according to A. L. Andrews (1933) was based on material of Weissia controversa.