British ferns (Filicopsida)
This data set is generated from a DELTA database (Dallwitz 1980; Dallwitz, Paine, and Zurcher 1993). It comprises descriptions of the species, compiled from readily accessible sources and comprehensively illustrated mainly from Sowerby and Johnson (1859 and 1863); facilities for interactive identification and information retrieval using the program Intkey, with hints on using this to best effect; and source references.
The descriptive data are mainly from Bentham and Hooker (1945 edition), Clapham, Tutin and Warburg (1962) and Stace (1997). The nomenclature and generic interpretations follow the latter work, with minor adjustments via later sources listed in the References. Comparing the Clapham et al. generic interpretations of British ferns with those in recent lists shows that these have remained relatively stable since the mid-twentieth Century; the main exception being Thelypteris, with the 5 species formerly included now presented under 4 genera, one of which (Gymnocarpium) is now associated with Woodsia in a different family. The species concepts also remain extensively the same, though this fact is sometimes obscured by nomenclatural changes; but unlike the genera, which are for the most part convincingly defined and fairly readily identifiable, treatments of species are inevitably complicated by the frequent occurrence of hybrids, which can render precise assignments of Dryopteris and Asplenium specimens in particular difficult or impossible. Attempts at interactive identification using this package thus cannot be expected always to lead to a single species; but lists of taxa remaining unseparated should usually be confined to a single genus, and the existence of hybrids can be ascertained by reading the descriptions. Hybrids are frequently more or less intermediate between the parent species in form, are often but not always found growing in close proximity to one or both of them, and often (but again, not always) produce only abortive spores.
Comparisons of the family assignments of Clapham et al., Stace and Hassler and Swale, which are all included in the present generic descriptions, reveal that the major taxonomic problem posed by the Polypodiaceae sensu lato (i.e., the majority of ferns) of older treatments remains unsatisfactorily resolved after fifty years of taxonomic endeavour; cf. Stace’s remarks on the inadequately defined Adiantaceae and Woodsiaceae. An interesting modern development, however, is the association (cf. Hassler and Swale) of the Ophioglossaceae with Psilotum, and by implication with some of the earliest vascular plant fossils, in the Order Psilotales.
The database contains no original observations; the descriptions were merely compiled from readily accessible sources. British ferns have been studied so extensively that it should be possible to construct a useful database entirely from the literature, and we hope this one meets that expectation. However, standard taxonomic publications are poor sources of reliable comparative data, and practical testing against living and/or herbarium material is sure to reveal errors and other inadequacies in the descriptions, and hence in the key. We would appreciate having these pointed out, so that they can be corrected without delay; and we would particularly like to hear from people interested in contributing to further development of the package. In addition to improving the character list and adding character and taxon illustrations, this could include extending the geographical coverage, or preparing separate accounts for other regions.