A Taxonomic Revision of the Mascarene Hibiscus sect. Lilibiscus (Malvaceae), with Morphometric Analysis and Description of Two New Species

Hibiscus L. sect. Lilibiscus Hochr. comprises 31 species, the majority of which occur on volcanic islands in the Indian and Pacific Oceans. Four species are currently described in the Mascarene Archipelago: H. boryanus DC., H. fragilis DC., H. genevei Bojer, and H. liliiflorus Cav. Though they are a small group, the Mascarene species in section Lilibiscus have a complex taxonomic history. While type collections and species descriptions are sufficiently detailed to apply the existing names H. liliiflorus and H. genevei to species, the brief species descriptions and insubstantial type collections of H. boryanus and H. fragilis have complicated the application of these two names to the remaining two species. In addition, morphological variation within H. boryanus s.l. as it is currently delimited suggests that the name comprises three distinct species. Thus, the names H. boryanus and H. fragilis could each be applied to one of four species, leaving two species to be described. To determine the proper application of the names H. boryanus and H. fragilis, we conducted a thorough study of herbarium specimens and species descriptions combined with morphometric analyses of seven floral characters obtainable from the 220-year-old type specimens of H. boryanus and H. fragilis and 70 other flowering specimens. Although morphometric analyses were sufficient to determine that the name H. fragilis is best applied to the red-flowered species in Mauritius, additional lines of evidence inferred from the original description of H. boryanus were necessary to apply this name to the yellow-flowered species in Réunion. Consequently, we describe two new species formerly treated under H. boryanus s.l.: a magenta-flowered species endemic to Mauritius (H. dargentii Mashburn) and a red-and-yellow–flowered species endemic to Réunion (H. igneus Mashburn). We provide an identification key to differentiate the Mascarene Hibiscus sect. Lilibiscus species and new and updated species descriptions for all six species.