Moss - Classification

Classification

Traditionally, mosses were grouped with the liverworts and hornworts in the Division Bryophyta (bryophytes), within which the mosses made up the class Musci. This definition of Bryophyta, however, is paraphyletic and now tends to be split up into three divisions. In such a system, the Division Bryophyta contains exclusively mosses.

The mosses are grouped as a single division, now named Bryophyta, and divided into eight classes:

division Bryophyta
class Takakiopsida
class Sphagnopsida
class Andreaeopsida
class Andreaeobryopsida
class Oedipodiopsida
class Polytrichopsida
class Tetraphidopsida
class Bryopsida


liverworts






hornworts



vascular plants



Bryophyta

Takakiopsida



Sphagnopsida




Andreaeopsida



Andreaeobryopsida




Oedipodiopsida




Tetraphidopsida



Polytrichopsida



Bryopsida









The current phylogeny and composition of the Bryophyta.


Six of the eight classes contain only one or two genera each. Polytrichopsida includes 23 genera, and Bryopsida includes the majority of moss diversity with over 95% of moss species belonging to this class.

The Sphagnopsida, the peat-mosses, comprise the two living genera Ambuchanania and Sphagnum, as well as fossil taxa. However, the genus Sphagnum is a diverse, widespread, and economically important one. These large mosses form extensive acidic bogs in peat swamps. The leaves of Sphagnum have large dead cells alternating with living photosynthetic cells. The dead cells help to store water. Aside from this character, the unique branching, thallose (flat and expanded) protonema, and explosively rupturing sporangium place it apart from other mosses.

Andreaeopsida and Andreaeobryopsida are distinguished by the biseriate (two rows of cells) rhizoids, multiseriate (many rows of cells) protonema, and sporangium that splits along longitudinal lines. Most mosses have capsules that open at the top.

Polytrichopsida have leaves with sets of parallel lamellae, flaps of chloroplast-containing cells that look like the fins on a heat sink. These carry out photosynthesis and may help to conserve moisture by partially enclosing the gas exchange surfaces. The Polytrichopsida differ from other mosses in other details of their development and anatomy too, and can also become larger than most other mosses, with e.g. Polytrichum commune forming cushions up to 40 cm (16 in) high. The tallest land moss, a member of the Polytrichidae is probably Dawsonia superba, a native to New Zealand and other parts of Australasia.

They appear to be the closest living relatives of the vascular plants.

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