General features of the bryophytes
The mosses and liverworts, although morphologically somewhat dissimilar, are classified together as the Bryophyta. Because of their distinctive features they are treated as a division of the plant kingdom, of a rank equal to that of the algal groups and the Tracheophyta (Table 1.2). There are about 25000 species of bryophytes in all. Three classes are recognized, namely Marchantiopsida, Anthocerotopsida and Bryopsida. They represent the simplest of the archegoniate plants and form a single division with the following characteristics:
Habitat Mainly terrestrial.
Plastid pigments Chlorophylls a, b;β-carotene; xanthophyll (lutein).
Food reserves Starch, to a lesser extent fats and oils.
Cell wall components Cellulose, hemicelluloses.
Reproduction Heteromorphic life cycle, the gametophytic phase normally the more conspicuous, and the sporophytic determinate and partly dependent upon it. Sex organs with a jacket of sterile cells, the egg cells enclosed singly in flask-shaped archegonia. Zooidogamous, spermatozoids with two whiplash flagella. Embryogeny exoscopic. Sporophyte producing non-motile, cutinized spores, in some species with heavily thickened and sculptured walls, usually all of one size (homospory). Vegetative propagation of the gametophyte by fragmentation or specialized gemmae.
Growth forms of gametophyte Thallus flattened, with some internal gametophyte differentiation, or consisting of a main axis with leafy appendages.
Although the simplest terrestrial plants, the bryophytes in some parts of the world form a conspicuous component of the vegetation.