In total 3525 species, subspecies and varieties placed in 166 families are included on this DVD-ROM. All vascular plants (pteridophytes, gymnosperms and angiosperms) are included, as is traditional in British Floras. These are placed in classes (Lycopodiopsida, etc.), subclasses (Magnoliidae, etc.), families (Lauraceae, etc.), genera, species and subspecies. In addition within the angiosperms superorders (Magnoliiflorae, etc.) and orders (Laurales, etc.) are given. Below the family level, subfamilies or tribes (both in the case of Family Asteraceae and Family Poaceae) are defined only for those families with 20 or more genera. Below the genus level, subgenera or sections are defined only for those genera with at least 20 species. The classification of each taxon can be found in the Classification field of the Species and Higher Taxa modules.
Apomictic microspecies are covered in full in most genera, but not for the three notorious genera: Genus Rubus, Genus Taraxacum and Genus Hieracium, for which specialist accounts already exist or are in preparation by experts. In these genera a separation into relatively easily recognized groups of microspecies (here called sections) is provided instead. A full account of these genera would have greatly exceeded my own abilities and the scope of one volume. In addition to the above 3 problem genera, Ranunculus auricomus is an apomictic complex in which probably over 100 microspecies could be segregated; however, this complex has not been sufficiently researched in the British Isles to permit such a detailed treatment.
The coverage of alien taxa has been as thorough and consistent as possible. Many more aliens are included than in any other British Flora, yet a considerable number of aliens traditionally found in other Floras have beenomitted. To merit inclusion, an alien must be either naturalized (i.e. permanent and competing with other vegetation, or self-perpetuating) or, if a casual, frequently recurrent so that it can be found in most years. All this applies as much to garden escapes or throwouts as to unintentionally introduced plants. Rarity, or the requirement of a highly specialized habitat, has not been taken into consideration (any more than is the case with natives). Cultivated species have been included if they are field crops or forestry crops or, in the case of trees only, ornamentals planted on a large scale. Exclusively garden plants, however abundant, whether crops or ornamentals, have not been covered, but most of the commoner taxa are included anyway because of their occurrence as escapes or throwouts. Also excluded are non-tree ornamentals planted en masse on new roadsides or in parks, etc. The aim of this revamped and expanded set of criteria is to include all taxa that the plant hunter might reasonably be able to find 'in the wild' in any one year. Any such plant, whether native, accidentally introduced or planted, affects wild habitats and is part of the ecosystem, and botanists and others might be expected to need or want to identify it. Ornamental trees (but not shrubs or herbs) have been included because they are long-lived and frequently persist decades after all other signs of planting have disappeared from the area, so that the finder can not be expected to know that they were once planted. Doubtless, some additions to and removals from the list finally adopted are justified, but the selection of taxa is as judicious as it is largely due to the enormous help I have received from many correspondents (but especially Eric Clement, Douglas Kent and David McClintock), who have made alien plants their special study and have generously given me the benefit of their advice.
All interspecific and intergeneric hybrids are included, but the level of treatment varies. Hybrids that have attained distributions no longer tied to those of their parents (i.e. those that occur at least sometimes in the absence of both parents) are treated exactly like species, except that the multiplication sign is inserted between the generic name and the specific epithet (e.g. Salix x rubens) and the parental formula is given (e.g. Salix alba x Salix fragilis). This has been normal procedure for some genera (e.g. Genus Circaea, Genus Mentha) in the past, but a consistent application of the criterion has resulted in many more such taxa being similarly treated. NB: for technical reasons, the x in the name of such hybrids is placed between brackets and at the end of its name on this DVD-ROM: Salix rubens(x).
Other hybrids are placed in their appropriate systematic position, but are not keyed and are not provided with their own number; they always occur with at least one parent and their identity can usually be deduced because of this. They are provided with as much information as their situation appears to warrant. The only exception to the above isthat some highly fertile hybrids that can occur in the absence of both parents (e.g. Genus Geum, Genus Hyacinthoides) are not treated as separate entities, since they form a spectrum of variation linking that of their parents.
All hybrids can be traced via the Hybrids field in the Index module. The hybrids are mentioned twice (or sometimes three times) to be able to be linked to the parents shown between square brackets behind the hybrid name. In the Synonyms field of the parents' entry in the Species module, a full list of possible hybrids is given.