The members of this cosmopolitan phylum generally live in more or less shady, perpetually moist places, where the danger of drying out is minimized. Some bryophytes inhabit bogs and swamps; others grow in the cold regions of the world, in the tundra and high on mountains.
Still others grow in deserts, near hot springs, and in the tropics. In tropical rain forests bryophytes occur abundantly as epiphytes on the leaves, branches, and trunks of trees. Several species of bryophytes live in fresh water; none is marine.
Bryophytes frequently are important soil formers and soil protectors. They can settle where lichens have begun to covert bare rock surfaces to small patches of soil, and by their metabolism and decay bryophytes then contribute to a further transformation of rock to soil.
Moreover, many bryophytes form dense, soil-covering carpets that minimize soil erosion and retain water between the closely spaced plants. Many bryophytes also absorb water directly through their leaves, which spares the water supplies of soil.
Bryophytes are distinguished from tracheophytes by a life cycle in which the gametophyte generation is always dominant. This generation lasts longer, is physically larger and more conspicuous, is nutritionally independent, and in general represent the “main” plant. Casual reference to a “moss”, for example, is a referent to the gametophyte. By contrast, the saprophyte is small, short-lived, and nutritionally dependent on the gametophyte.
Mosses are believed to be the most primitive members of the phylum. The gametophyte body of a mass typically consists of a branched network of green filaments spread flat over the ground. From such a peritonea grow small no green extensions into soil, and these rhizoids absorb water and minerals. The peritonea also give rise to one or more green shoots that project in the air.
Each shoot consists of an upright stem to which are attached radically arranged leaves. At maturity a shoot also bears terminal sex organs. The stem is composed of an outer epidermis without stomata, a parenchymatous cortex underneath, and a core of water and nutrient-transporting cells (in which the cytoplasm degenerates during development, leaving only the walls).
The leaves of some mosses are formed from a single layer of green cells, but in other cases several differently specialized layers are present. They usually leave air spaces near the upper leaf surface that permit even deep-lying green cells to exchange gases directly with the external atmosphere.
Among mosses of more than small-scale ecological significance are the most peat mosses (Sphagnum). These plant live in swamps and bogs all over the world, but particularly, in the tundra and in colder regions generally.
Metabolic activities make the plants and the water in which they grow too acid for most decay bacterial. As a result, old plants pile up in beds of considerable thickness, filling in swamp areas and forming the main organic component of peat. The body of these plants is specialized for water storage.
The protonema is a flat, broad sheet bearing a highly branched system of long shoots. In the cortex of the mature shoots are thick-walled water-storing hulls of disintegrated cells. Leaves too contain large water-filled cells, of which only the walls remain in the mature plant. This spongy, water-holding property has made peat mosses useful as, for example, packing material for moisture- requiring goods.
Whereas mosses typically have vertical, radically, symmetrical parts, liverworts are horizontal plants. Some are leaf like moss shoots, and in others the body is a flat sheet, or thallus, along the ground. An example of the leafy (foliose) types is Porella, a liverwort with a horizontal stem bearing usually three rows of leaves. The plant is often epiphytic on trees. Sheetlike (thallose) types are exemplified by Marchantia.
The thallus of this plant is ribbon-shaped and lobed, with a median furrow along the upper surface. From the underside project numerous rhizoids. At the forward margin of a thallus is a growing point consisting of a cluster of cells.
As these produce new cells on either side two new lobes are formed, and the growing point thus comes to be located in a notch between the lobes. Later the growing point usually becomes divided into two cells groups, each of which can then produce a thallus branch. A growth pattern of this sort therefor^ leads successively to the formation of two equal branches from one main branch.
Internally a thallus contains three distinct zones, each composed of one or more tissues. The bottom zone is largely absorptive and consists mainly of the no green rhizoids. The middle zone is formed from nongreen parenchyma and functions in storage and conduction of nutrients.
The upper zone is photosynthetic and includes elaborate air chambers and air ports. In types like Marchantia the location of the internal air chambers is marked externally by fine diamond-shaped lines on the surface of the thallus. Some thallus liverworts are secondarily simplified. In Sphaerocarpos, for example, the thallus is only one cell layer thick.
In hornworts the body is likewise a thallus, irregularly scalloped along the margins but without notches, surface furrows, or midribs. Rhizoids are present on the underside. Internally the tissue of the thallus is parenchymatous, and each cell contains a single large chloroplast with a conspicuous pyrenoid (as in green algae).
In this respect hornworts differ from the other biyophyte classes, in which each cell contains numerous chloroplasts without pyrenoids. The central portions of a hornwort thallus are usually several cell layers thick, and air chambers or air ports are often absent. The best known hornwort is probably Anthoceros, fairly common on damp soils.