The geographical relationships of British and Irish bryophytes

The bryophyte flora of the British Isles comprises four native hornworts, 2 84 liverworts and 716 mosses. These species are about two thirds of the Eur opean total. Past phytogeographical studies have concentrated particularly on Atlantic bryophytes, especially liverworts. The theory that these specie s can be divided into two distinct categories, one of Holarctic origin and the other of tropical and Southern Hemisphere origin, is confirmed for the British Isles. A system of elements established by us for British and Irish vascular plants is based on distribution in northern and western Eurasia, dividing the flora up first into latitudinal categories and secondly into l ongitudinal ones. For bryophytes, two extra elements have been added, Hyper oceanic Temperate and Hyperoceanic Southern-temperate. About 40% of the flo ra belongs to arctic and boreal elements and 20% to southern elements. The remaining 40% belongs to elements of the temperate broadleaved forest zone including those that extend into the boreal zone. The composition of each e lement is considered in terms of world distribution, habitats and distribut ion in the British Isles. Most species have enormous world ranges; only abo ut 5% are endemic to Europe. Very few species are convincingly continental in that they are commoner in eastern Europe than the west. Several southern species, however, are more common in semi-arid continental interiors than in the relatively humid regions of northwest Europe. Only six liverworts an d 13 mosses are known in the British Isles as established introductions; al l except four originate from temperate or subtropical regions outside Europ e.