Rock Mosses

Black rock-moss Andreaea rupestris, photographed by Sture Hermansson.

Upon first examining the picture above, you might think that you were looking at a patch of moss. Which would not be an unreasonable assumption to make, because that is exactly what you are looking at. But this is not just any moss, but perhaps one of the most interesting mosses out there.

The hundred or so species of the genus Andreaea, found in cooler regions around the world, are commonly known as rock mosses or granite mosses in reference to their preferred growth habitat on acidic rocks. They can often look black or red rather than green (presumably relative to how dry they are), and they are often brittle. Glime (2013) notes that a characteristic feature of granite mosses is that a hand brushed over one will come away with small fragments stuck to it, and suggests that this may act as a method of vegetative dispersal. Usually, granite mosses are autoicous: a single plant has both male and female reproductive structures, but they are borne in separate clusters. What makes Andreaea really interesting, though, is how it produces spores. As explained in the diagram I used in an earlier post, mosses produce spores from a sporophyte (diploid plant) that grows supported by the gametophyte (haploid plant) that comprises the green, vegetative stage of the moss life cycle. In most mosses, the sporophyte holds itself up by means of a long stalk called a seta, and spores are released from the terminal capsule by the ejection of a covering operculum.

Capsules of Andreaea, photographed by David Tng.

Andreaea does things differently. In this moss, the capsule is not supported by its own stalk, but is instead lifted up on an extension of the gametophyte called a pseudopodium. And instead of popping off an operculum, the Andreaea capsule splits open longitudinally into a squashed crown. Andreaea shows its difference after the spores are released as well: while the spores of other mosses germinate into a filamentous protonema (the moss 'seedling', as it were), the protonema of Andreaea bears thalloid appendages.

Such is the distinctive of Andreaea that it has been classified in a separate class from most other mosses, the Andreaeopsida. Phylogenetic analysis has demonstrated that Andreaea is one of the earliest diverging of all mosses, being the next to diverge from the main moss lineage after the Sphagnopsida (the group that includes Sphagnum). In some classifications, the class Andreaeopsida is restricted to Andreaea alone, but there are two other small genera that have been grouped with it in the past.

Andreaeobryum macrosporum, photographed by Masaki Shimamura.

Andreaeobryum macrosporum is a single unusual moss species found in north-west North America. Like Andreaea, it is found growing in rocks, though its preference is for basic rocks such as limestone. It also resembles Andreaea in having a spore capsule that opens through slits, though in the case of Andreaeobryum the apex of the capsule eventually wears off as well and the capsule splays fully open. The biggest difference between Andreaea and Andreaeobryum is that the capsule of the latter is not raised on a gametophytic pseudopodium, but possesses its own supporting seta like that of typical mosses (albeit a particularly short and stubby one). The phylogenetic position of Andreaeobryum remains uncertain: a molecular analysis by Chang & Graham (2011) recovered an Andreaea-Andreaeobryum clade, but with very low support.

Takakia lepidozioides, from here.

The real wild-card in basal moss phylogeny, however, is the little green monster known as Takakia. Takakia is a genus of two species found in western North America and eastern Asia. It was first discovered in the Himalayas and described in 1861—as a liverwort. This is a bit like being presented with a new species of snake, and describing it as a type of eel. But it has to be pointed out that Takakia possesses some very un-moss-like features. It has finely divided leaves, a feature common in liverworts but not known from any other moss. Its leaves contain oil bodies: again, unlike any other moss, but like many liverworts. Matters were not helped by the fact that Takakia was first described from vegetative material only, and it was not until the description of sporophytes in the 1990s that Takakia was conclusively accepted as a moss (Renzaglia et al. 1997).

Even so, its position within the mosses remained uncertain. A relationship with Andreaea has been suggested, as the capsule (though borne on a seta rather than a pseudopodium) opens through a single spiral slit. However, recent phylogenetic analyses have not supported a direct relationship between the two. Chang & Graham (2011) found in the analysis of their data that its position was vulnerable to the analytical model used: it could be placed as the sister taxon to all other mosses, or it could be the sister to the Sphagnopsida (with the two together being the basalmost moss clade). We have not heard the last of little Takakia.

REFERENCES

Chang, Y., & S. W. Graham. 2011. Inferring the higher order phylogeny of mosses (Bryophyta) and relatives using a large, multigene plastid data set. American Journal of Botany 98 (5): 839-849.

Glime, J. M. 2013. Bryophyta - Andreaeopsida, Andreaeobryopsida, Polytrichopsida. Chapt. 2-6. In: Glime, J. M. Bryophyte Ecology. Volume 1. Physiological Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologists. Last updated 29 June 2013 and available at .

Renzaglia, K. S., K. D. McFarland & D. K. Smith. 1997. Anatomy and ultrastructure of the sporophyte of Takakia ceratophylla (Bryophyta). American Journal of Botany 84 (10): 1337-1350.