Mosses and Liverworts

terrestrial or, less frequently, freshwater autotrophic plants united in the division Bryophyta, which includes the most primitive higher plants. The Bryophyta are divided into three classes: Anthocerotales (horned liverworts), Hepaticae (liverworts), and Musci (mosses). Mosses and liverworts are multicellular perennials with relatively simple organization; less frequently, they are annuals. The plants are from 1 to 50 cm tall and are marked by the partial separation of assimilatory, water-storage, and mechanical tissues.

Mosses and liverworts have a single cycle of development; the cycle varies to some degree in each class of plants. The life cycle is characterized by separation of a diploid sporophyte (a modified sporangium) into a haploid gametophyte. As a result, the sexual and asexual generations develop together on the same plant. Thus, the alternation of generations is somewhat conventional in character.

A branched, threadlike or scalelike multicellular protonema capable of photosynthesis grows out of the spore. As a result of the formation of numerous buds on the protonema, growths resembling mats or cushions appear, which promote vegetative reproduction and help the plants tolerate unfavorable conditions. In some mosses and liverworts, scalelike thalli or radially or bilaterally symmetrical shoots grow out of the buds. The thalli and shoots may be bisexual or unisexual; correspondingly there are monoecious and dioecious plants.

The organs of sexual reproduction—the antheridia and archegonia—are most often located in groups among multicellular sterile filaments, or paraphyses, and are surrounded by special leaflike growths. On the antheridia (elongated sacs on a stalk with a single-layer membrane), biflagellate sperms develop, and these need water in order to migrate to the egg. The egg is located in the enlarged base of the archegonium. Fertilization and zygote development occur in the female organ. The sporogonium, the organ of asexual reproduction (spore-bearing), which to a significant degree has lost its independence, develops from the zygote after a few months. The green stemlike body of the young sporogonium subsequently turns yellow, brown, or dark red and differentiates into a superior spore-bearing part, the capsule, and into a lower part, consisting of a stalk and a foot. The foot grows into the tissue of the mother plant. When spores are formed in the archespore (sporogenic tissue), meiosis occurs.

The zygote and the sporogonium are diploid and constitute the asexual generation—the sporophyte. The protonema, the thalloid gametophore, and the leafy gametophore are haploid and represent the sexual generation—the gametophyte. The principal function fulfilled by the gametophytic generation is autotrophic nutrition. Because of this role and the gametophyte’s capacity for vegetative reproduction, the developmental cycle can occur for a long time without formation of a sporophyte (in some species sporogonia are unknown). The gametophytes of mosses and liverworts are morphologically diverse; the single-layer leaflike scales on the stem (phylloids) are marked by great variation. The sporogonia, on the other hand, reveal considerable similarity in many groups.

The stems of leafy mosses have chlorophyll-bearing leaflike and filamentous growths (paraphyllia), club-shaped hairs, a secondary protonema, and filamentous rhizoids. The rhizoids may envelope the aboveground or underground parts of the plants as if with felt. The thalli and stems attach themselves to the substrate or unite firmly among themselves by means of the rhizoids. Apospory, apogamy, hybridization, and polyploidy have been established in mosses and liverworts, proving that they participate in processes of species formation. Along with this, the formation of clones is widespread, fostering preservation of local adaptive changes.

The Anthocerotales are a unique group with two families, comprising six genera and more than 300 species, which are distributed predominantly in the tropics. In the USSR there are four or five species. The thalli are usually in the form of lobes or rosettes and have antheridia and archegonia embedded in them. The sporogonium, which, as a rule, is elongated and siliquose, is capable of elongation owing to the location of the meristem below, the columella in the center, and the stomata in the epidermis. The base of the sporogonium is surrounded by a sheath and opens vertically by means of two valves. Besides spores, short, sterile unicellular or multicellular filaments, or elaters, are formed, which foster spore distribution. The cells of the thallus and sporogonia contain one or more pyrenoid-containing chloroplasts.

The Hepaticae are divided into the two subclasses Marchantiidae and Jungermanniidae. In contrast to mosses and other liverworts, the hepatics have a poorly developed, short-lived protonema, which usually develops only one thallus or cormophyte of dorsoventral structure. The cells of the gametophyte contain one or more fat bodies of constant shape; the rhizoids are unicellular. The sporogonium usually has no chloroplasts or columellae, and its walls lack stomata.

Because some Marchantiidae resemble Anthocerotae, the Anthocerotales were once regarded as a subclass of Hepaticae. The gametophyte is marked by great diversity, but the sporogonia are monotypic. The Marchantiidae are thalloid plants, usually in the form of dichotomously branching small rosettes or large flattened ribbons. The thalli attach themselves to the soil by means of rhizoids. The male and female gametangia are embedded in the tissue of the thallus or are raised on special podetia. The plants grow predominantly in soil; a few species grow on rocks on rocks or in water. Distributed throughout the world, they are especially widespread in the tropics. The subclass has 16 known families, embracing 35 genera and about 420 species, including the fossil Naiadita from the Mesozoic.

The most primitive hepatic species are from the order Sphaerocarpales; they have a simply constructed thallus, and sometimes their fat bodies are in special cells. The sporogonium of these primitive species has a wall one cell in thickness, and there are nutrient cells instead of elaters. Liverworts of the order Marchantiales are characterized by the complex structure of their thalli, which are divided into basal and assimilatory tissue and have fat bodies in special cells and sclerenchymatous fibers (in some species). The thallus often has air chambers and air pores; its underside has ventral scales and smooth, ligulate unicellular rhizoids. The sporogonium usually has a more complex structure. Secondary simplification is characteristic of the Ricciales, whose gametangia and sporogonium develop within small, most often rosette-shaped thalli.

The Jungermanniidae include a group of thalloid and leafy plants in whose vegetative cells there are, as a rule, several fat bodies; the rhizoids are smooth and unicellular. Epiphytic, soil, and rock plants are particularly widespread in the subtropics and tropics. The subclass has 45 families, embracing more than 240 genera and approximately 9,000 species. In representatives of the order Metzgeriales, the thallus has one or many layers and is undifferentiated (some have leaflike ventral or dorsal scales and stems with leaflike lobes). The capsule has two or many layers. Liverworts of the order Haplomitriales are erect, with three-row leaf arrangement and capsules with single-layer lateral walls. The most numerous and diverse liverworts are those of the order Jungermanniales; they are characterized by two rows of entire or lobed veinless lateral leaves and often by smaller ventral leaves, or amphigastria. The walls of the capsule are multilayer.

The Musci are divided into three subclasses: Sphagnales (bog, or peat, mosses), Andreaeales, and Bryales. The plants have radial or, less frequently, bilateral, symmetrical, or spiral leaf arrangement. The leaves, which are entire, serrate, or toothed along the margins, have a single layer or, less frequently, two or many layers; they are veined or veinless and have branched multicellular rhizoids. The superior or lateral sporogonia are diverse in shape, structure, and coloring. There are about 15,000 species (according to other data, up to 25,000 species), constituting 660 genera. Bog mosses have large, erect stems with clustered branches, gathered into a head at the apex; they are whitish green, yellow, brown, or reddish. The single-layer leaves are borne on branches or stems; they have no venation and consist of alternating chlorophyll-bearing and water-bearing cells. The sporogonium consists of a spherical capsule with an operculum and underdeveloped stomata; it lacks a stalk, and its foot is embedded in a leafless, elongated archegonial branch of the stem. A dome-shaped spore sac covers the upper part of the columella. Bog mosses are distributed throughout the world: in swamps, the tundra, and rain forests.

Mosses of the subclass Andreaeidae are small and red-brown to black and have leaves that are one cell in thickness; they grow on rocks in the form of cushions. The elongate-ovate capsule is on a pseudopodium and opens in four to eight valves; it has no stomata, and its dome-shaped spore sac covers the upper part of the columella. The mosses are distributed high in the mountains and in northern latitudes.

Mosses of the subclass Bryidae are the most widespread and diverse. The capsule is usually on a true stalk and has stomata; after the operculum falls off, the capsule usually has a wide opening—the mouth—on top. In many species one or two rows of processes, or teeth, form along the margin (called the simple or double peristome), which serve to distribute the spores. The columella, as a rule, penetrates the pore sac.

Mosses are found throughout the world except in seas, saline soils, and habitats that are covered with glaciers or have undergone severe erosion. The Anthocerotales and Hepaticae are distributed mainly in countries with humid tropical or temperate climates; only a few species have adapted to arid places. The Musci grow almost everywhere; however, they flourish in moist places, forests, and tundra. In swamps, mosses and liverworts are the principal components of peat bogs. When the plants develop intensively, they foster the formation of swamps and lower the quality of meadows and other agricultural lands. Because some mosses and liverworts have antibiotic properties, they are sometimes used in bandages. They yield material used as bedding for livestock (mainly sphagnum mosses) and in construction for the manufacture of flagstone.