Hill & Preston (1998) carried out an analysis of the bryophytes of the British Isles in terms of world distribution, habitat and distribution in the British Isles. They established a system of elements based on distribution in northern and western Eurasia, dividing the flora up first into latitudinal categories and secondly into longitudinal ones. They recognised four major biomes (large, naturally occurring zones of dominant vegetation adapted to the particular conditions in which they occur) and these are listed below. Many species occur in more than one major biome and five composite categories, also listed below, were recognised. The floristic element/eastern limit category or biome to which each species belongs is given in italics in the distribution data of that species.

Major biomes

1. Arctic-montane. Species with their main distribution either to the north of or (on mountains) above the tree line, or both.

2. Boreal-montane. Species with their main distribution in the coniferous forest zone. They may occur in the Boreal zone and/or in the coniferous forest zone of mountains to the south.

3. Temperate. Species with their main distribution in the cool temperate, broad-leaved deciduous forest zone. May occur on mountains to the south or in cool steppes in continental interiors.

4. Southern. Species with their main distribution in the warm temperate zone south of the broad-leaved deciduous forest zone.

Composite categories

1. Boreo-arctic Montane. Species occurring in both Arctic-montane and Boreal-montane zones.

2. Boreo-temperate. Occurring more or less equally in the Boreal and Temperate zones or, if absent from the Boreal Zonobiome, ascending to the subalpine zone on mountains.

3. Southern-temperate. Species which are found more or less equally in the Temperate and Southern (Mediterranean) zones.

4. […]

Alternation of generations heteromorphic; the sexual generation, the leafy gametophyte dominant (‘the moss plant’), the asexual generation, the sporophyte dependent on the gametophyte for survival. Gametophyte stems without xylem or phloem but sometimes with tubular conducting cells; multicellular rhizoids often present; leaves typically unistratose but sometimes thicker; costa present or not; cells isodiametric to linear. Female gametangia, archegonia, flask-shaped and containing a single female gamete; male gametangia, antheridia, producing numerous biflagellate male gametes. Fusion of gametes resulting in the development of an embryo in the venter of the archegonium. Sporophyte consisting of foot embedded in gametophyte tissue, seta, frequently with tubular conducting cells and capsule; capsules usually with stomata and photosynthetic tissue, usually dehiscing by lid, occasionally cleistocarpous, rarely by longitudinal slits; later development of capsule controlled by the calyptra derived from neck of archegonium; spores produced by meiosis in spore sac of the capsule. About 12 000 species (almost certainly an overestimate). A highly successful group of plants occurring in almost every available natural habitat except the sea.

Traditionally, the division Bryophyta has included three classes, Hepaticopsida or Marchantiopsida (liverworts), Anthocerotopsida (hornworts) and Musci (mosses). Recent evidence, however, suggests that at an early stage in the evolution of terrestrial plants the liverworts diverged from other land plants, then hornworts diverged and finally mosses (see Willis & McElwain, 2002). Thus, the original concept of the division Bryophyta as consisting of three classes cannot be maintained and it is here considered to consist only of the mosses.

Until the beginning of the 1980s the classification of mosses was based upon the morphology and anatomy of the gametophyte and sporophyte and it was sometimes a matter of dispute as to which of the two should be used for a particular group. Over the past 20 years or so new techniques have been developed, especially DNA sequencing, which provide information on the genetic relationships of taxa. This has confirmed some older classifications but others have been shown to be incorrect. The classification of mosses is now in a state of flux as a result of the development of these new techniques, Rather than using a traditional classification, such as that used by Corley et al. (1981), which is clearly very out of date, I have followed the tentative classification of D. H. Vitt (in Shaw & Goffnet, 2000). However, it is likely that further studies will lead to refinements, and even major changes, to this classification.

Since the publication of the first edition in 1978, in which 693 species were described, there have been considerable advances in our knowledge of British and Irish mosses and in the taxonomy of critical groups such as Andreaea and the Hypnum cupressiforme, Racomitrium heterostichum, R. canescens and Schistidium apocarpum complexes. Since 1978, eight species new to science have been described in Britain, 25 species not previously known in the British Isles have been discovered and taxonomic revisions have led to the addition of a further 51 species. Fourteen species have been reduced to varieties of or to synonymy with other species or removed from the British list. This has resulted in an increase of 70 in the number of species making a total of 763.

Additionally, critical studies of generic limits, as in the Amblystegiaceae and Pottiaceae, and DNA studies have led to a considerable increase in the number of genera – from 175 to 214 in the British Isles. There have also been major changes in the overall classification of mosses. Clearly, the first edition of this book was in dire need of updating. Apart from additions and taxonomic changes, the original descriptions of species have been revised and in some instances augmented and footnotes added or increased where thought necessary. The key to genera and keys to species have been revised in the light of experience since they were first published.

Protonemata thalloid, with secondary filaments consisting of ± unbranched rows of cells with oblique dividing walls. Leafy shoots lacking rhizoids, consisting of erect main axis from which sprout fascicles of branches (in a few species there is only one much reduced branch per fascicle). Leaves ecostate, with two main types of cell. Elongate cells, dead at maturity, constitute a border. The main part of the lamina (except sometimes in perichaetial leaves) is composed of a mesh of two types of cell, chlorocysts, which are narrow and green, and hyalocysts, which are hyaline, inflated and dead at maturity of the leaf. Antheridia lateral, in leaf axils. Antherozoids biflagellate, with elongate body coiled in 2 turns of a sinistrorse spiral. Archegonia in groups of 1–5 at apex of a short specialised branch or the main stem. Setae absent, capsules joined directly to foot and exserted from perichaetium by elongation of pseudopodium composed of gametophytic tissue just below the archegonium. Capsules globose, urn-shaped when dry and empty, with a convex lid, lacking a peristome; exothecial cells brown at maturity, with scattered non-functional stomata; spore sac amphithecial in origin, overarching columella; calyptra a thin hyaline membrane, irregularly ruptured at maturity by growth of capsule. Spores arising in tetrads, each with a conspicuous triradiate scar on inner face.

There are two orders, Sphagnales and Ambuchananiales. The latter contains a monotypic genus, Ambuchanania, known only from Tasmania. The ordinal status of the Ambuchananiales is still uncertain.

Acrocarpous or pleurocarpous. Plants minute to large; stems simple to much branched, with or without central strand. Leaves spirally arranged, rarely distichous; costa single or double, well developed or not, or lacking; cells usually unistratose except sometimes at margins or towards apex, isodiametric to linear, thin-walled to strongly incrassate, smooth, papillose or mamillose. Early divisions of zygote resulting in radially symmetrical embryo. Setae short or long; capsules erect to pendulous, globose to cylindrical, dehiscing by lid or more rarely cleistocarpous; mouth with peristome of one (haplolepideous mosses) or two (diplolepideous mosses) concentric rings of articulated teeth, each ring of 16 simple or variously divided teeth, teeth variously ornamented, sometimes reduced or absent; spore sac cylindrical, surrounding but not overtopping columella; spores usually small, to 40 μm, numerous, mostly 5000–50 000 000 per capsule, usually wind-dispersed.

Subclass 1 Diphysciideae

Dioicous or autoicous. Acrocarpous. Protonemata persistent. Stems short, simple. Leaves lingulate or lingulate-spathulate; costa present; cells unistratose at base, 2(−3)-stratose above, rounded-quadrate, mamillose or smooth. Perichaetial leaves larger than stem leaves; margins ciliate above; costa longly excurrent. Setae very short; capsules oblique, ovoid, gibbous, terete, tapering to small mouth; lid conical; outer peristome of 16 very short teeth, inner conical with 16 plicae. One family, genera.

Acrocarpous. Plants frequently large, stems simple or branched, tough, with internal anatomical differentiation. Leaves often with broad costa bearing adaxial longitudinal lamellae. Setae long; capsules dehiscent; peristome nematodontous.

Polytrichales

Plants large, usually rhizomatous. Stems with central strand with differentiated conducting tissue. Leaves usually differentiated into sheathing base with narrow costa, and blade composed largely of expanded costa bearing longitudinal lamellae on adaxial side, lamina usually narrow, 1–2-stratose; margins toothed or entire, sometimes bordered. Capsules erect to horizontal, spherical to cylindrical, terete or 2–6-angled, hypophysis present or not; annulus absent; peristome nematodontous, with 32 or 64 teeth; columella expanded at top into membranous epiphragm joined to tips of peristome teeth; calyptrae cucullate, glabrous or hairy. A probably ancient group of one family and about 23 genera.

Polytrichaceae

Pogonatum P. Beauv., Mag. Encycl., 1804

Dioicous. Shoots arising from decumbent rhizome-like stems or from persistent protonemata. Leaves with broad sheathing basal part abruptly narrowed into lingulate to narrowly lanceolate blade consisting mainly of costa bearing numerous straight lamellae on adaxial side.

It is now more than fifty years ago that the third edition of The Student's Handbook of British Mosses by H. N. Dixon & H. G. Jameson was published in 1924. Since that time the very considerable taxonomic and nomenclatural changes have been such that it is difficult for the non-expert to equate the 1924 taxa with those of today. Taxonomic revisions have resulted in the splitting of genera (and Dixon had an extraordinarily broad concept of the genus in The Handbook) and the recognition of numerous additional species especially in the Bryum capillare, B. erythrocarpum and Plagiothecium denticulatum/nemoreum complexes. As a consequence of the marked increase in interest in field bryology since the end of the Second World War numerous new species have been added to the British and Irish lists. Dixon recognised 115 genera and 625 species (in present day terms) compared with the 175 genera and 692 species described here. There is also an increasing awareness of the extent of morphological variation and the existence of taxonomically difficult groups requiring further study.

It was felt that a new moss flora was long overdue, hence the production of this book. It must be stressed that this is not a revision of Dixon & Jameson but is a completely new moss flora embodying recent ideas and views, some doubtless controversial, on the taxonomy of mosses.