Flora
WIDESPREAD COMMON
This is the largest group comprising about half of the British flora and consisting mainly of very adaptable species found in a wide range of situations. Many of these adaptable species are emergent or floating-attached plants such as Caltha palustris, Phragmites communis, Eleocharis palustris, Potamogeton natans and Nymphaea alba which also have a very wide world distribution, often cosmopolitan.
Also included in this group are species which are more restricted in their habitat requirements but which are such successful colonists that they have reached almost every suitable locality. They include Rorippa nasturtium-aquatica, Hippuris vulgaris, Myriophyllum spicatum, Veronica ana-gallis-aquatica and Potamogeton pectinatus, which are more or less confined to eutrophic waters and are found throughout the British Isles but are absent from many upland areas where suitable habitats do not exist. Eleogiton fluitans and Potamogeton polygonifolius and other species of mainly oligotrophic or dystrophic waters are abundant in the uplands of the north and west, but more scattered yet by no means rare in the south and east. A few brackish water species are sufficiently widespread to belong to this category; they include Ruppia maritima and Scirpus maritimus.
WIDESPREAD LOCAL
This group comprises 15 species which, although widely distributed in the British Isles and not ascribable to any particular region, are recorded from relatively few rather scattered localities. Some have rather precise habitat requirements and the wide dispersion of suitable sites may explain their distribution. These include brackish water species (Ranunculus baudotii and Ruppia spiralis), species which are more or less restricted to mesotrophic conditions (Eleocharis uniglumis, Pilularia globulifera and Elatine hex-andra], and Utricularia minor which is confined to peat pools. Limosella aquatica, Eleocharis acicularis and Baldellia ranunculoides may have become restricted during the period of extensive forest growth by their requirements for rather open habitats, but it is not possible to explain the present scattered distribution of the other members of this group.
NORTHERN SPECIES
Thirteen species are more or less confined to the north and north-west of Britain, where they may be abundant, and are either absent or found in a few scattered localities in the south. With the exception of Potamogeton filiformis, they are all species of oligotrophic or dystrophic waters and their absence from the south may be largely explained by the lack of suitable habitats. Many have been recorded from Late-glacial deposits in the south of England so that temperature may also have been instrumental in causing their retreat northwards. A few (e.g. Subularia aquatica) are nowadays confined to cold-water lakes or to the cooler, deeper waters of lakes (e.g. Isoetes lacustris). Most of this group are submerged species, particularly rosette-leaved plants such as Isoetes spp., Lobelia dortmanna and Subularia aquatica, but it also includes four species with floating leaves, Nuphar pumila, Nymphaea alba ssp. occidentalis, Sparganium angustifolium and S. minimum, and one emergent, Myosotis secunda, but there are no free-floating species.
Potamogeton filiformis is restricted to eutrophic lakes and, although recorded from Late-glacial deposits in southern Britain, all present-day records are north or west of the 14.4 °C July isotherm. Exposure may also be a factor controlling its present distribution since it is a plant of the wave-washed sandy littoral zone.
Most of this group are also of northerly world distribution, five belong to J. R. Matthew's (1937) continental northern element and two each to the northern montane and oceanic northern elements. Three of the remainder are of circumpolar distribution and one is found throughout most of Europe.
SOUTHERN SPECIES    .      -     •
Twenty-seven species are more or less confined to a line south of the Humber and Mersey and occur only in very few scattered localities farther north, although some (e.g. Lemna trisulca and Sagittaria sagittifolia) are plentiful in central Ireland. All these species are restricted to eutrophic waters and their northerly spread has evidently been halted by the scarcity of this habitat type in northern Britain. Their presence in Ireland indicates the calcareous nature of the central lowlands of that country. Most of these species are of very wide world distribution and many of them including Ceratophyllum demersum, Myriophyllum verticillatum, Typha angustifolia and Lemna minor are almost cosmopolitan. There are also two introduced North American species in this group, namely Acorus calamus and Azolla filiculoides, and a few species of fairly restricted European distribution of which Oenanthe fluviatilis is fairly common and widespread in the chalk streams of southern England, but is otherwise only known from a few localities in Denmark and Germany. A number of other species characteristic of chalk streams, e.g. Groenlandia densa, Apium nodiflorum, Ranunculus circinatus, R. fluitans, R. tripartitus, Berula erecta and S. sagittifolia also belong to this group. Free-floating species such as L. trisulca, L. polyrrhiza, L. gibba, Wolffia arrhiza and Hydrocharis morsus-ranae, although mostly of worldwide distribution, are in Britain confined to the south. The free-floating community is essentially tropical in distribution (e.g. the sudd communities of African lakes), and is restricted at high latitudes by the disruptive effects of ice action.
RARE SPECIES
A number of aquatic plants of apparently very restricted distribution in Britain are recorded from less than 15 lo-km squares in the Atlas of the British Flora. Many of these species belong to very distinct phytogeographical groups.
One group comprises species which are very restricted in their European distribution but which are widely distributed in North America. These' Irish—American' species are all confined to the west of Britain and are also found along the Atlantic seaboard of continental Europe. In Britain, Elodea nuttallii is known only from Esthwaite Water, while Eriocaulon septangulare is confined to the Ardnamurchan peninsula and Coll and Skye in the Hebrides. Potamogeton epihydrus is confined to South Uist and Limosella subulata to three coastal sites in Wrales. Najas flexilis and Lobelia dortmanna also have some claims to belong to this group since they are both far more widespread in America thar they are now in Europe, but they have contracted in distribution in Britain since Late-glacial times.
Another group consists of species which are widely distributed throughout the warmer parts of the world bu; which are on the edge of their range in Britain and northwest Europe. These are found only in the south or south-east of Britain. They comprise Ludzoigia palustris, confined to one pond in the New Forest, Najas marina, restricted to the Norfolk Broads, Potamogeton nodosus in a few large rivers in central southern England (including the Thames, Loddon, Stour and Somerset Avon), and Wolffia arrhiza, found in ponds mostly south of the Thames.
A number of European species reach their most northerly or north-westerly location in Britain. Alisma gramineum occurs in Lincolnshire and Worcestershire, Callitriche truncata in a number of sites south from Lincolnshire (including Dungeness), Stratiotes abides is a native in East Anglia and the north Midlands, but widely introduced elsewhere, and Luronium natans has a range extending from north Wales through the west Midlands to central northern England. Some of these species may be recent arrivals in Britain which are now in the process of extending their range.
Nuphar pumila which is fairly widely distributed north of the Highland Boundary Fault also occurs in Crose Mere in Shropshire where it is a glacial relict. Its hybrid N. spen-nerana (= N. luteax-pumila) also occurs as a relict in northern England outside the present range of N. pumila.
Potamogeton rutilus which is found in Shetland and in the Outer Hebrides is fairly widely distributed in north-west Europe including Scandinavia and Russia, as is Rumex aquaticus which in Britain is confined to Loch Lomond. The minute aquatic plant Elatine hydropiper is recorded from only 21 stations in Britain, but is probably widely overlooked as it has been recorded from a number of new localities during the survey for the Review.
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