Silver hair moss (Fabronia pusilla) COSEWIC assessment and status report: chapter 3
Species Information
Name and classification
Fabronia pusilla Raddi is a member of the moss family Fabroniaceae. It is the only species of Fabronia in Canada (Ireland et al. 1987). Other North American species of Fabronia include F. ciliaris (Brid.) Brid., found from the north-eastern to south-western portions of the United States, and F. ravenelii Sull. found from Maryland to Florida (Anderson et al. 1990, Crum and Anderson 1981). Grout (1928-1940) considered F. pusilla to be a subspecies of F. ciliaris, but most other authors do not agree. Fabronia pusilla has long, often cilia-like teeth along the leaf margins and the teeth are often comprised of more than one cell. The marginal teeth of F. ciliaris are less distinct, never cilia-like, and are comprised of one cell (Crum and Anderson 1981, Lawton 1971).
Description
Fabronia pusilla is a tiny, pleurocarpous or creeping moss that grows in thin, flat mats, often admixed with other bryophyte species, in particular Homalothecium spp. Its narrow stems have an irregular branching pattern, with the leaves appressed along its length except for the leaf ends that tend to stick outwards towards the end of the stem.
Its stem leaves range in size from 0.4 to 0.75 (0.85) mm long and from 0.2 to 0.35 mm wide. They are ovate-lanceolate and awned, with each awn, or leaf tip, ending in a single, generally 50 to 80 µm long, cell. The awns are hyaline or colourless. The upper two-thirds of the leaf margins are bordered by sharp, often multicellular and cilia-like teeth, with the terminal cell usually much longer than the adjacent lower cells. Together, the awns and the teeth contribute to giving the plant a silvery to whitish-green cast. The narrow leaf costa is relatively short and often less than half the length of the leaf. Median and upper leaf cells range in size from 30 to 45 µm × 10 to 12 µm, and the basal cells are quadrate to short-rectangular and about the same width as the upper cells. Distinct alar regions are lacking.
Fabronia pusilla is autoicous, with male and female organs on the same stem. The perigonial bracts are ovate and lack distinct teeth and costae. The perichaetial bracts are longer, and more leaf-like and have small teeth along their margins, but also lack costae. The erect to somewhat curved seta is about 3 mm long. Its capsule is erect, ovate to obovate, with a somewhat wrinkled base when mature. The operculum is low-conical, and the peristome teeth are more or less fused in eight pairs. Lawton (1971) reports its spores to be finely papillose and from 9-14 µm in diameter, whereas Grout (1928 -1940) reports them as rough and from 12 to 17 µm in size. Buck (1994) describes the spores as papillose.
Although the marginal teeth are not visible in the field under the low magnification of a hand lens, its diminutive size, obvious leaf tips, and the general whitish green cast help to distinguish this species. Some small members of the Leskeaceae that have a similar habitat may be confused with it at first. Its small size and habit of growing with other larger pleurocarpous mosses may result in F. pusilla being overlooked in field surveys.
Taxonomic keys are found in Grout (1928 – 1940), Lawton (1971), and Sharp et al. (1994), with Lawton and Sharp et al. providing illustrations. Grout’s illustrations of F. gymnostoma Sull. & Lesq. appear to be of Fabronia pusilla, because of the multicellular teeth along the leaf margins. Crum and Anderson (1990) consider F. gymnostoma to be a variety of F. ciliaris (var. ciliaris). Crum and Anderson (1981) provide a discussion about the differences between F. pusilla and F. ciliaris. Figure 1 provides illustrations of F. pusilla modified from Lawton and Sharp et al (1994).
Figure 1. Illustrations of Fabronia pusilla: 1 and 2 are outlines of typical leaves (redrawn from Lawton1971), 3 is a capsule and upper portion of the seta (redrawn from Lawton 1971), and 4 is detail of the leaf margin and apex (redrawn from Lawton 1971 and Sharp et al. 1994).
Nationally significant populations
To date, the only extant populations of Fabronia pusilla are at the south-west end of Sumas Mountain near Abbotsford, British Columbia, where the species was collected twice by W. B. Schofield in 1968. Ryan (1996) reported these collections along with a third collection made on a trunk of black cottonwood, Populus trichocarpa. Investigation of this collection shows that this specimen is not F. pusilla, but a different species in the Leskeaceae family. The only other collections of this species were made by J. Macoun in 1890 in the Arrow Lake area in the Kootenay Valley of south-eastern British Columbia, but these locations are now submerged behind a dam (Tan 1980). If still extant, the Abbotsford site should be considered a nationally significant population.