Phylum bryophyta (liverworts and mosses)


The bryophytes

Bryophytes are the simplest land plants. They are thought to have evolved from green algae. The phylum contains two main classes, the Hepaticae, or liverworts, and the Musci, or mosses.

Neither of the two groups is particularly well adapted to life on land and they are mainly confined to damp, shady places.

Bryophytes are small simple plants, with strengthening and conducting tissues absent or poorly developed. There is no true vascular tissue (xylem or phloem).

They lack true roots, being anchored by thin filaments called rhizoids which grow from the stem.

Water and mineral salts can be absorbed by the whole surface of the plant, including the rhizoids, so that the latter are mainly for anchorage, unlike true roots. (True roots also possess vascular tissue, as do true stems and leaves.)

The plant surface lacks a cuticle, or has only a delicate one, and so there is no barrier against loss (or entry) of water. Nevertheless, most bryophytes have adapted to survive periods of dryness using mechanisms that are not fully understood. For example, it has been shown that the well-known xerophytic moss Grimmia pulvinata can survive total dryness for longer than a year at 20oC. Recovery is rapid as soon as water becomes available.

Alternation of generation in bryophytes

In common with all plants and some advanced algae, bryophytes show alternation of generations. Two types of organism, a haploid gametophyte generation and a diploid sporophyte generation, alternate in the life cycle.

The haploid generation is called the gametophyte (gameto, gamete; phyton, plant) because it undergoes sexual reproduction to produce gametes are also haploid. The gametes fuse to form a diploid zygote which grows into the next generation, the diploid sporophyte generation. It is called the sporophyte because it undergoes asexual reproduction to produce spores. Production of spores involves meiosis, so that there is a return to the haploid condition. The haploid spores give rise to the gametophyte generation. One of the two generations is always more conspicuous and occupies a greater proportion of the life cycle; this is said to be the dominant generation.

In the bryophytes, the gametophyte generation is dominant. In all other plants the sporophyte generation is dominant. It is customary to place the dominant generation in the top half of the lifecycle diagram.

One point that must be remembered is that gamete production involves mitosis, not meiosis as in animals; meiosis occurs in the production of spores.

Classification and characteristics of the phylum bryophyta

General characteristics Alternation of generation in which the gametophyte generation is dominantNo vascular tissue, that is no xylem or phloemBody is a thallus, or differentiated into simple ‘leaves’ and ‘stems’No true roots, stems or leaves: the gametophyte is anchored by filamentous rhizoidsSporophyte is attached to, and is dependent upon, the gametophyte for its nutritionSpores are produced by the sporophyte in a spore capsule on the end of a slender stalk above the gametophyteLive mainly in damp, shady places
Class Hepaticae (liverworts) Class Musci (mosses)
Gametophyte is a flattened structure that varies from being a thallus (rare) to ‘leaf’ with a stem (majority), with intermediate lobed types Gametophyte ‘leafy’ with a stem
‘Leaves’ (of leafy types) in three ranks along the stem ‘leaves’ spirally arranged
Rhizoids unicellular Rhizoids multicellular
Capsule of sporophyte splits into four valves for spore dispersal; elaters aid dispersal Capsule of sporophyte has an elaborate mechanism of spore dispersal, dependent on dry conditions and involving teeth or spores
e.g. pellia, a thalose liverwort marchantia, a thalose liverwort, with antheridia and archegonia on stalked structures above the thallus lophocolea, a leafy liverwort, common on rotting wood e.g. funaria Mnium, a common woodland moss similar in appearance to funaria Sphagnum, bog moss: forms peat in wet acid habitats (bog)

Class Hepaticae – liverworts

They are more simple in structure than mosses and, on the whole, more confined to damp and shady habitats. They are found on the banks of streams, on damp rocks and in wet vegetation. Most liverworts show regular lobes, or definite stems with small, simple leaves. The simplest of all though are the thalloid liverworts where the body is flat thallus with no stem or leaves.

An example is Pellia, a liverwort that is common throughout Britain. The plant is a dull green with flat branches about 1cm wide.

Class musci – mosses

They have a more differentiated structure than liverworts but, like liverworts, are small and found mainly in damp habitats. They often form damp cushions.

Funaria is a common moss of fields, open woodland and disturbed ground, being one of the early colonisers of such ground. It is especially associated with freshly burned areas, for example after heath fires. It is also a common weed in greenhouses and gardens.

As with liverworts, water is essential for fertilisation. When the surface of the plant is wet, mature antheridia absorb water and burst, releasing the male gametes (sperm) onto the surface. The sperm each have two flagella. They swim toward the archegonia, each of which contains one female gamete or ovum.

Fertilisation, that is fusion of sperm nucleus with the ovum nucleus, takes place in the archegonium. The product is a diploid zygote which grows out of the archegonium to become a new sporophyte.