Morphology of Mosses (Phylum Bryophyta)

With over 12,000 species recognized worldwide (M. R. Crosby et al. 1999), the Bryophyta, or mosses, are the most speciose of the three phyla of bryophytes. The other two phyla are Marchantiophyta or liverworts and Anthocerotophyta or hornworts. The term “bryophytes” is a general, inclusive term for these three groups though they are only superficially related. Mosses are widely distributed from pole to pole and occupy a broad range of habitats. Like liverworts and hornworts, mosses possess a gametophyte-dominated life cycle; i.e., the persistent photosynthetic phase of the life cycle is the haploid, gametophyte generation. Sporophytes are matrotrophic, permanently attached to and at least partially dependent on the female gametophyte for nutrition, and are unbranched, determinate in growth, and monosporangiate. The gametophytes of mosses are small, usually perennial plants, comprising branched or unbranched shoot systems bearing spirally arranged leaves. They rarely are found in nature as single isolated individuals, but instead occur in populations or colonies in characteristic growth forms, such as mats, cushions, turfs, or wefts. While uniform in general life-history features, mosses show extensive morphological and anatomical diversification in both gametophyte and sporophyte organization. Currently, mosses are classified into five morphologically distinct superclasses (B. Goffinet and W. R. Buck 2004) that are circumscribed as follows: Superclass I, comprising only Takakia; Superclass II, for Sphagnum and Ambuchanania Seppelt & H. A. Crum; Superclass III, with Andreaea and Acroschisma Lindley; Superclass IV, comprising only Andreaeobryum; and Superclass V, all the peristomate mosses, comprising most of the diversity of mosses. Although molecular data have been undeniably useful in identifying the phylogenetic relationships among moss lineages, morphological characters continue to provide definition of systematic groupings (D. H. Vitt et al. 1998) and are diagnostic for species identification. This chapter is not intended to be an exhaustive treatise on the complexities of moss morphology, but is aimed at providing the background necessary to use the keys and diagnostic descriptions of this flora.