What are mosses and liverworts?

Mosses and liverworts belong to a group of plants known as the BRYOPHYTES. They are usually green, usually small, and are amongst the simplest of land-dwelling plants (a few are aquatic). They lack some of the complex structures seen in other types of plant - for example, they do not produce flowers or seeds, and most have no internal means for transporting water or nutrients.

Most have very little resistance to drying out, and are confined to areas which are damp and sheltered. Some, however, are more resistant to drying out - for instance, certain species of sand-dunes.

Bryophytes have no roots, but they do have thin (one cell thick!) root-like structures which serve for attachment and water absorption. These are known as rhizoids.

 

 

If they produce no flowers, how do mosses and liverworts reproduce?

 

The bryophyte life-cycle is comprised of two separate generations: the gametophyte generation and the sporophyte generation. Each has a characteristic means of reproduction.

Plants in the gametophyte stage are able to re-produce sexually, in a similar way to flowering plants. However, the sexual parts are contained in rather small and inconspicuous structures, usually quite unlike flowers. The male organs (antheridia) produce mobile antherozoids which require a film of water in which to move to the female organs (archegonia). After fertilisation, a new plant develops, which remains attached to (and thus appears to be part of) the parent plant. This is the sporophyte.

The sporophyte consists of little more than a capsule, borne upon a thin stalk known as a seta. The mature capsule contains large numbers of microscopic spores. These are eventually released and dispersed (usually by wind) and each is then capable of germinating to eventually develop into a new gametophyte plant.

A further common means of reproduction in the gametophyte is through shedding fragments of leaves or other parts of the plant body, which can then regenerate into new plants. In some species specialised groups of cells known as gemmae are produced for this purpose.

 

 

What are the differences between a moss and a liverwort?

 

There are certain clear-cut differences between mosses and liverworts. For example:

• Capsule opening mechanisms: In most liverworts, when a spore capsule is ripe, it merely bursts open along four longitudinal axes; moss capsules nearly always have rather more complex opening mechanisms.
• Structure of rhizoids: In liverworts, the rhizoids are nearly always composed of a single, elongated cell; in mosses the rhizoids are multicellular.
• Early development of the gametophyte: In mosses, following spore germination, but preceding development of the mature gametophyte plant, a juvenile stage known as a protonema forms. This is absent or much reduced in liverworts.

Other differences are less clear-cut:

• All mosses consist of a stem (branched or not) bearing leaf-like structures. Many liverworts do not have this type of structure (though most do!).
• Many liverworts have leaves which are deeply lobed or divided. No mosses have leaves like this.
• All liverworts with a leafy structure have their leaves arranged in a two- or three-rowed pattern. Very few mosses have this sort of arrangement of leaves.
• Many mosses have leaves with a thickened nerve. No liverworts have leaves with such nerves (some have a line of differently coloured cells which may give the appearance of a nerve (e.g. Diplophyllum albicans).

Once the main characteristics of each type are known there is usually no danger of confusion. There are a few species which might cause dificulties for the beginner,however.

 

How many species of moss and liverwort are there?

Worldwide the number of species of mosses and liverworts has been estimated at around 14-15,000 (though this is still a matter of some debate,and no-one knows for sure). Just over 1000 occur in Britain, (289 species of liverwort, and 745 species, sub-species and varieties of moss). A large proportion of the British species occur in Wales (76% occur in North Wales alone).