Phylum bryophyta (liverworts and mosses)

v

The bryophytes

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

Neither of the two groups is particularlywell adapted to life on land and they are mainly confined to damp, shadyplaces.

Bryophytes are small simple plants, withstrengthening and conducting tissues absent or poorly developed. There is notrue vascular tissue (xylem or phloem).

They lack true roots, being anchored bythin filaments called rhizoids whichgrow from the stem.

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

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

Alternation of generation in bryophytes

In common with all plants and some advancedalgae, bryophytes show alternation of generations. Two types of organism, ahaploid gametophyte generation and a diploid sporophyte generation, alternatein 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 adiploid zygote which grows into the next generation, the diploid sporophytegeneration. It is called the sporophytebecause it undergoes asexual reproduction to produce spores. Production ofspores involves meiosis, so that there is a return to the haploid condition.The haploid spores give rise to the gametophyte generation. One of the twogenerations is always more conspicuous and occupies a greater proportion of thelife cycle; this is said to be the dominantgeneration.

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

One point that must be remembered is thatgamete production involves mitosis, not meiosis as in animals; meiosis occursin 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 thanmosses and, on the whole, more confined to damp and shady habitats. They arefound on the banks of streams, on damp rocks and in wet vegetation. Mostliverworts show regular lobes, or definite stems with small, simple leaves. Thesimplest of all though are the thalloid liverworts where the body is flatthallus with no stem or leaves.

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

Class musci – mosses

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

Funariais a common moss of fields, open woodland anddisturbed ground, being one of the early colonisers of such ground. It isespecially 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 forfertilisation. When the surface of the plant is wet, mature antheridia absorbwater and burst, releasing the male gametes (sperm) onto the surface. The spermeach have two flagella. They swim toward the archegonia, each of which containsone female gamete or ovum.

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