Cystopteris dickieana R.Sim is a rare fern protected in Britain under the 1981 Wildlife and Countryside Act. Most current floras treat it as a distinct species but ever since it was first discovered in Scotland in the 1830s, there has been considerable debate about its taxonomic status within the C. fragilis complex. This debate centres on the relative importance of two characters, the architecture of the fronds and the surface sculpturing of the spores, in delimiting C. dickieana from other taxa in the C. fragilis complex. The type specimens of C. dickieana have distinctive fronds. Plants with similar frond architecture have, to date, been recorded growing naturally only at the site in Scotland from which the type specimens were collected and at one other site nearby. The type specimens of C. dickieana also have mature spores with surface sculpturing often described as ‘rugose’. These are distinctive and unusual in the genus Cystopteris, in which most taxa have ‘echinate’ spores. However, rugose-spored plants have been recorded not only at, and near, the type locality in Scotland but also at many other sites in the northern hemisphere in populations of plants defined largely on the basis of frond architecture as C. fragilis or C. baenitzii. This indicates that spore sculpturing should not be used alone to delimit C. dickieana from other taxa within the C. fragilis complex but, despite this, the literature on ‘C. dickieana’ contains many reports of studies on material identified as C. dickieana solely on the basis of spore sculpturing. This, combined with the fact that most comparative studies have also failed to include material known to have come from the type locality, has resulted in considerable and continuing uncertainty over the taxonomic status and distribution of C. dickieana.

Cystopteris dickieana R. Sim is a fern species that currently receives legal protection in Britain on the basis that it is endemic to Scotland and extremely rare. However, ever since it was first discovered in the 1830s, there has been considerable debate about its taxonomic status within the Cystopteris fragilis complex. This debate centres on the relative importance of two characters, the architecture of the fronds and the surface sculpturing of the spores, in delimiting C. dickieana from other taxa in the C. fragilis complex and is complicated by the fact that most comparative studies reputed to have included ‘C. dickieana’ have failed to include either the type material of C. dickieana or material from the ’type population‘ (i.e. the population of plants, of uniform and distinctive morphology, that still grows at the type locality for C. dickieana). The principal aim of this investigation was to generate some of the critical data and information currently missing from the debate concerning the taxonomic status of C. dickieana by including material from the ’type population‘ in an allozyme study of several Scottish Cystopteris populations. The frond architecture, spore sculpturing, and allozyme banding patterns were investigated in five natural and two cultivated populations of the C. fragilis complex. All plants from the ‘type population’ of C. dickieana had the distinctive frond architecture and rugose spores of the type material. Two populations comprised plants that had the frond architecture and echinate spores typical of C. fragilis. Also examined were two populations containing plants that had frond architecture typical of C. fragilis but rugose spores similar to those of the type material of C. dickieana. These plants occurred with typical C. fragilis in one of the populations and with typical C. dickieana in the other. In the latter population, situated close to the ‘type population’ for C. dickieana, a third, ‘smooth’ spore type occurred in nine out of 55 fertile plants. Consistent and analysable allozyme results were obtained for seven enzymes at ten loci: eight loci were polymorphic and all but the smallest population showed variation at one or more loci. In total, 22 allozyme phenotypes were recorded in 226 individuals. No consistent correlation was found between frond architecture, spore sculpturing and allozyme banding pattern. Our results provide no support for the recognition of C. dickieana as a distinct species related to C. fragilis.