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Treated here in the traditional sense, as a monospecific genus of circumboreal distribution. See discussion of alternative applications below; for the botanical description see L. borealis.
In the traditional sense, Linnaea is a genus of a single diminutive species with a circumboreal distribution in the north northern hemisphere (Cullen et al. 2011). In has traditionally been placed in the Honeysuckle family, the Caprifoliaceae. This family is home to several large and important genera including Abelia, Lonicera, Symphoricarpos, and Weigela; consequently it has been much studied and subject to considerable (proposed) reorganisation.
In recent years molecular phylogenetic studies have shown the well-known genus Abelia to be polyphyletic. In order to achieve monophyly – a tenet of modern botany – authors have resurrected older names to accommodate sections of Abelia sensu lato, including Vesalea for those species distributed in Mexico, and Zabelia for a number of the Asiatic species; several more including the type, A. chinensis, remain in a much-reduced Abelia, and a new genus, Diabelia, was described in 2010 to accommodate the rest (Christenhusz 2013; Landrein 2010). Each of these genera contains only a small number of species. Christenhusz (2013) pointed out this wasn’t concordant with approaches to other subfamilies within the Caprifoliaceae, and expanded Linnaea in order to provide an alternative approach whilst still maintaining monophyly. In so doing he sinks the horticulturally significant Abelia (excluding Zabelia), Dipelta, and Kolkwitzia into a much-enlarged Linnaea. The nomenclatural ramifications of these changes are not excessive; the revised Linnaea amounts to only 16 species (15 new combinations; with 9 Abelia species previously treated in Section Zabelia now treated as species of Zabelia, itself restored to generic rank) (Christenhusz 2013).
As Christenhusz points out ‘…the delimitation of genera is arbitrary, and depends on tradition and preference of the user. Here, I am merely making the names available to provide a choice’ (Christenhusz 2013). This choice now available to authors is reflected in a range of approaches in major works. Plants of the World Online (plantsoftheworldonline.org) has adopted Christenhusz’s system, whereas others, for example the Catalogue of Life (catalogueoflife.org) have not. The Flora of China, where many of the groups in question have a significant distribution, has opted to retain separate genera, necessarily invoking the segregation of Abelia, and goes further by placing them into the family Linnaeaceae (Yang & Landrein 2011).
One of the principal arguments against accepting an enlarged Linnaea is the loss or partial loss of several genera well known in horticulture (Abelia, Dipelta, Kolkwitzia) and the ensuing nomenclatural turmoil that so often pits horticulturist against botanist. In this case, however, the status quo has been shown to be untenable due to the polyphyly of Abelia (sensu lato). The near-universal agreement that genera must be monophyletic demands either the broadening of Linnaea, or else the splitting of Abelia; either way, nomenclatural turmoil is unavoidable. For Trees and Shrubs Online we have opted to split Abelia. It seems preferable to retain the familiar names Dipelta and Kolkwitzia, and Abelia in a reduced sense, rather than lump all these together in a single genus, hitherto well-known as a monospecific, circumboreal subshrub of understated charm.