Putting the Moss in its Box

Moss classification is tough. To the casual observer there is very little to differentiate one patch of green in the undergrowth from another. Closer inspection reveals that, while there are certainly distinct groups, identification to the species level is an arduous task in some cases. Mosses' fruiting bodies provide something of an answer. The eminent Finnish bryologist Viktor Ferdinand Brotherus (1849 - 1929) was most influential in introducing classification based on moss peristomes, the structures in the fruiting bodies that regulate spore release (1). This poses problems however as these structures, by their very nature, are exposed to the vagaries of the elements and may lead to incorrect classification. Molecular characters are becoming more and more popular for moss taxonomy, however single-gene classification systems are just as likely to be faulty as any other type of single character classification, and therefore caution is needed (1). A combination of techniques is often needed.

Hart's Tongue Thyme Moss, Plagiomnium undulatum

One of these is the use of the karyotype (number and appearance of chromosomes in the cell) of the moss species. Bowers (2) used this cytological approach to re-classify some members of the family Mniacea. Species of one genus in this family, Plagiomnium, are especially difficult to separate morphologically. In once species, P. undulatum (Hart's Tongue Thyme Moss), cytology revealed variability in karyotype formula and chromosome set length, but a high uniformity of chromosome number (3). P. undulatum is common in damp, shaded places and is notable for its long stems, which may grow to 10 cm in length (4). Male and female structures are borne on different plants with the male forming rosette like structures as seen in the example pictured.

References:

1. Buck and Goffinet 2000, Bryphyte Biology (eds. Shaw and Goffinet) pp. 71-123
2. Bowers 1980, Lindbergia 6 pp. 22-31
3. Przywara et al. 2003, Acta Biologia Cracoviensia, Series Botanica 45 pp. 105-110
4. Phillips 1980, Grasses, Ferns, Mosses and Lichens of Great Britain and Ireland p. 133