Weissia Hedwig, 1801.

Weissia is readily recognized in the field by the combination of the small size, opaque leaves (a product of the dense papillae), plane to incurved leaf margins and generally mineral soil habitat. The most common species, W. controversa, is one of the most widespread mosses on disturbed soil of cultivations or of bare patches of soil in lawns. We have decided not to recognize the genus Hymenostomum. This has been separated from Weissia on the basis of a peristome reduced to a simple membrane. Members of this segregate (see W. inoperculata) may even have the operculum separating tardily, if at all, from the urn. Many species of Weissia in California reside in the deserts.

Key to Weissia Etc.

Mosses included in this section are acrocarpous mosses with crispate leaves and densely papillose median laminal cells. The leaves have plane to incurved leaf margins and those margins are entire or nearly so.

Species included in this key are all in Pottiaceae:
Oxystegus tenuirostris (W. J. Hooker & Taylor) A. J. E. Smith
Plaubelia sprengelii (Schwägrichen) Zander, not known from CA
Trichostomum brachydontium Bruch in F. A. Mueller
Trichostomum crispulum Bruch in F. A. Mueller
Trichostomum sweetii (E. B. Bartram) Stark
Weissia andersoniana Zander
Weissia andrewsii E. B. Bartram
Weissia condensa (Voit in J. W. Sturm) Lindberg
Weissia controversa Hedwig
Weissia inoperculata (H. Crum) H. Crum, W. C. Steere & L. E. Anderson
Weissia ligulaefolia (E. B. Bartram) Grout
Weissia planifolia Dixon, not known from CA
Weissia sinaloensis Bartram, not known from CA

A. Leaf margins plane to erect when moist .....B
A. Leaf margins incurved when moist .....F

B. Hyaline, rectangular cells of the leaf base extending to shoulder, and thus reaching somewhat farther along the margins than along the costa; upper portion of leaf lamina tending to be quite fragile and separating from the leaf in small pieces .....Oxystegus: O. tenuirostris
B. Hyaline, rectangular cells of the leaf base not extending to shoulder; upper portion of leaf lamina not fragile .....C

C. Leaves minutely mucronate to rounded at apex, mostly less than 2 mm long, typically with costa only 3-4 cells wide at mid-leaf .....D
C. Leaves cucullate or mucronate-recurved at apex, mostly more than 2 mm long, typically with costa 5-6 cells wide at mid-leaf .....E

D. Leaves broadest near the middle .....Weissia sinaloensis not known from CA
D. Leaves ligulate with parallel sides .....Weissia planifolia not known from CA

E. Leaf apices with a short and reflexed mucro; leaf base with the rectangular cells only gradually demarcated from the quadrate cells of the mid-leaf .....Trichostomum: T. brachydontium
E. Leaf apices channeled and somewhat cucullate; leaf base with the rectangular cells abruptly demarcated from the quadrate cells of the median leaf lamina .....Trichostomum: T. crispulum

F. Cells with high and branched papillae that arise from a mammillose cell lumen; leaves with limb constricted above junction with differentiated basal cells .....Weissia andersoniana
F. Papillae low and unbranched, mostly not arising from a mammillose cell lumen; leaves not strongly constricted at base of limb .....G

G. Median leaf cells bulging mammillose on adaxial surface, smooth and flat abaxially; papillae low to absent; leaf incurvature loose and broad, not extending to leaf apex .....Plaubelia: P. sprengelii not known from CA
G. Median leaf cells not bulging mammillose, mostly strongly papillose; leaf incurvature tight and narrrow, extending to near leaf apex .....H

H. Costa mostly less than 50 µm wide at base .....I
H. Costa 60-90 µm wide at the base .....K

I. Leaves broadest below the middle; apex acute to narrowly obtuse or mucronate .....Weissia controversa
I. Leaf base ligulate with parallel sides, mostly with the more elongate cells extending farther along costa than along margin .....J

J. Leaf apex acute above the inrolled margin; capsule cleistocarpous .....Weissia inoperculata
J. Leaf apex broadly acute to rounded; capsule stegocarpous .....Weissia ligulaefolia

K. Margin at mid-leaf inrolled in nearly a complete spiral .....Weissia andrewsii
K. Margin at mid-leaf incurved but not so much inrolled .....L

L. Leaves ligulate, with essentially parallel margins .....Trichostomum: T. sweetii
L. Leaves narrowly ovate-lanceolate, broadest near the base .....Weissia condensa