Bryophytes are the oldest land plants on earth, and have been around for 400 million years or more. Although small, they can be very conspicuous growing as extensive mats in woodland, as cushions on walls, rocks and tree trunks, and as pioneer colonists of disturbed habitats. They comprise three main taxonomic groups: mosses (Bryophyta), liverworts (Marchantiophyta) and hornworts (Anthocerotophyta) which have evolved quite separately.Worldwide there are possibly 10,000 species of mosses, 7000 liverworts and 200 hornworts. Most bryophytes have erect or creeping stems and tiny leaves, but hornworts and some liverworts have only a flat thallus and no leaves.

Bryophytes have a distinct lifecycle from the other major land plant groups; ferns and flowering plants. Bryophytes as often referred to as 'upside-down' plants; the green and often leafy part underneath that we would think of as the moss or liverwort itself, is equivalent to tiny parts within a flower, or to a small, rarely seen part of the fern (the gametophyte). The part that is equivalent to all of the flowering plant or fern that we normally see is the fruit of the moss or liverwort (the sporophyte). As in all plants, these two parts, or generations, alternate with each other in the life-cycle. The spores produced by the moss fruit will germinate into green leafy plants. These plants produce gametes, and the resulting embryos grow up into new fruits. But what makes bryophytes different from all other plants, is that the fruit or spore-bearing generation remains semi-parasitically attached to the green gamete-bearing generation, and never becomes independent.

Bryophytes play an important role in ecosystems, particularly as they tend to be some of the first plants to pioneer bare or disturbed habitats, contributing to soil development. In bogs and forest (especially in the montane tropics) they absorb huge quantities of water, thereby acting as a sponge and maintaining humidity over dry periods and preventing rapid run-off and flooding (excessive flooding in India is thought to be partly due to loss of bryophyte cover in Himalayan forests). They act as a home to many animals, particularly invertebrates, and provide a moist foothold for many plants such as ferns and orchids in forests, as in the oceanic woods of western Scotland or the mossy forests of the Andes and Himalayas.

Bryophytes are often used as indicators of environmental conditions of an area. Certain species can indicate the underlying geology of a habitat; for example Tortella tortuosa on limestone, Andreaea and Racomitrium on acidic or granitic rocks. Some species are found associated with specific mineral or metal deposits eg. Ditrichum plumbicola and Grimmia atrata. Bryophytes can therefore assist in geobotanical prospecting and are very useful ecological indicators for botanical survey work, capable of revealing subtle changes in substrate.

They are also be used regularly as pollution indicators. Bryophytes lack a protective layer or cuticle and so are highly sensitive to pollutants in the immediate area. Generally bryophytes are considered just as sensitive as lichens to air pollution. Indicator species, such as Antitrichia curtipendula, are only found in areas with low air pollution, whereas Dicranoweisia cirrata thrives on trees in areas of high pollution. Heavy metals such as copper, chromium, lead, vanadium, nickel and cadmium accumalate in cell walls of bryophytes. They are therefore widely used in monitoring heavy metal air pollution in large cities and areas surrounding power stations or metallurgical works. Bryophytes can also be used as bio-indicators in a similar way to monitor water pollution.

To get started in bryology (the study of mosses and liverworts), you need relatively few bits of equipment. A good hand lens is essential kit; 10x magnification is fine for scanning overall features but a 20x will be needed to see finer identification details. A double lens incorporating both low and high magnification can be highly useful for studying bryophytes in the field. Many species cannot be identified without a high power (compound) microscope and a stereo microscope is desirable, though not essential for examining whole specimens without preparation. A basic dissection kit is handy for preparing samples for study under microscopes. A decent idenfication guide is essential for beginners; choose one with clear keys and written at the level of your botanical terminology understanding. The British Bryological Society www.britishbryologicalsociety.org.uk is a great resource providing species checklists, details of local groups, courses and more detailed literature on bryophytes.