What are mosses and liverworts?
Mosses and liverworts, together with the hornwarts, are usually studied together and are collectively called ‘bryophytes’. Along with the algae, lichens and fungi, they form part of a group usually known as the ‘lower plants’ (the fungi are included as honorary members of the plant kingdom). Lichens are sometimes confused with mosses, indeed, one lichen is called ‘reindeer moss’, but lichens are stable associations of a fungus and an alga and have no relationship with bryophytes. There is little change of confusing bryophytes with higher plants, except possibly with the unfortunately named club-mosses, common in Scotland, which are really allied to the ferns.
The three groups which comprise the bryophytes are now thought to be only distantly related but share a similar life cycle and simple structure. They are an extremely old group of plants with possible ancestors dating back some 450 million years. They are fairly easily separated from each other with just a little experience.
Mosses have stems with leaves and, with the exception of the bog-mosses (Sphagnum), have a superficially uniform structure which masks a considerable complexity and variation. Some mosses like the Common haircap Polytrichum commune can have shoots up to 80 centimetres tall and the aquatic Greater water-moss Fontinallis antipyretica can be over a metre long. These giants contrast with tiny ephemeral species like the earth-mosses (Ephemerum) and bladder-mosses (Physcomitrium) which are only a few millimetres high. Both Polytrichum and Physcomitrium form cushions or turfs of upright stems but many mosses creep along the ground, clambering over rocks, trees and other plants. These carpets are a feature of our woods and heaths.
There are two very different forms of liverwort. Like mosses, leafy liverworts have stems and leaves but the leaves are arranged differently on the stem, often with two leaves placed laterally and a row of smaller ‘underleaves’ below. There is a much greater variety of leaf shape and ornamentation than in mosses and a very different fruiting body.
Again there is a huge variation in size and shape amongst the leafy liverworts from large and relatively simple plants like Taylor’s flapworth Mylia taylori to the tiny, but complex, Toothed pouncewort Drepanolejeunea hamatifolia. The other group of liverworts lacks the differentiation into stems and leaves and consists of a strap of green tissue. These species are usually called ‘thalloid’ liverworts. The large size and net-like surface pattern of the Great scented liverwort Conocephalum conicum make it one of our more familiar thalloid species, in contrast to the small rosettes of Crystalwort (Riccia species), frequent but mostly overlooked on bare soil.
Hornworts look very similar to thalloid liverworts but differ in a number of technical ways and the fruiting body, which is green and long-lived, is completely different. The two groups are only distantly related. Hornworts are easily overlooked, usually growing on disturbed soil in ‘weedy’ places in gardens, ditches and arable fields.
Bryophytes reproduce sexually, a process which requires at least a film of water, and produce a ‘sporophyte’, which consists of a stem with a capsule at the top containing the spores. The form of this fruiting body differs in mosses, liverworts and hornworts and provides the principal means of distinguishing them.
Moss sporophytes are very varied and provide a useful means of identifying species. The pictures in this book give some idea of the range of size and shape. The moss capsule is a complex and beautiful structure when seen through a microscope, having evolved various means of controlling spore release. Liverwort sporophytes are short lived and look very different to those in mosses as the picture of Overleaf pellia Pellia epiphylla shows. Hornwort fruiting bodies are different again, being green and horn-shaped, hence the name. They are long-lived and have no capsule but split lengthways to release the spores.
In a large number of mosses and liverworts sexual reproduction seems to be a rare event, and many species have evolved forms of ‘vegetative reproduction’ giving clones of the parent material. Most green tissue of mosses and liverworts can produce new plants and the simple fragmentation of stems or leaves is thought to be an important mechanism for dispersal. For this reason, trying to ‘wire-brush’ mosses off an old wall is doomed to failure! Many species take this dispersal tactic a good deal further. The special mechanisms that have evolved include deciduous or fragile leaves, the production of small particles of plant tissue called ‘gemmae’, either in special organs or attached to the leaf surface, and even the growth of tiny plantlets, called ‘bulbils’ in the angle where the leaves join the stem.