Omphalea (Euphorbiaceae) in Madagascar: A New Species and a New Combination

Abstract

Omphalea ankaranensis, a shrub endemic to limestone karst hills of northern Madagascar, is described and illustrated. The species is closely related to Omphalea palmata Leandri of western Madagascar but differs in its entire to coarsely crenate-dentate leaf blade margins, broader inflorescence bracts, lax cymules, and shorter pistillate flower pedicels. The new combination Omphalea oppositifolia (Willdenow) L. J. Gillespie is made for the widely distributed rainforest species of eastern Madagascar, previously known as Omphalea biglandulosa Persoon. A synopsis including new lectotypifications and a key to the four Madagascan species of Omphalea is provided. Omphalea L. is a genus of ca. 17 species belonging to the Euphorbiaceae. Six species are canopy lianas, while the remaining species range in habit from shrubs to large canopy trees. The genus is known from the Neotropics, southeast Asia to northeast Australia, Tanzania, and Madagascar, with centers of diversity and endemism in the Caribbean (6 species, 5 endemic) and in Madagascar (4 species, all endemic). While several species are widespread, the majority have limited ranges and some are quite rare. Species occupy a diversity of habitats from lowland rainforest, wet montane forest, to dry forest and thicket on limestone and sandy soils. The genus is characterized by an androecium of two or three stamens with connate filaments, styles that are completely fused into a usually massive stylar column, large foliaceous inflorescence bracts, and a complex inflorescence architecture. Inflorescences are technically thyrses and consist of spicate, racemose, or paniculate main axes that bear numerous cymules. These cymose units may be glomerate to lax and each is subtended by a usually foliaceous bract (note that bracteoles are defined as subtending axes within a cymose unit). Other features common in the genus but rarely found elsewhere in the family include red latex, liana habit with tendril-like climbing stems, mushroom-shaped androecia, and large fruits. Omphalea has long been considered a well-defi ed genus and one that is rather isolated in the Euphorbiaceae. The genus has been allied with genera that belong to all three uniovulate subfamilies in Webster's classification (1975, 1994). In the past Omphalea was usually placed in either tribe Hippomaneae (Baillon, 1858, as tribe Excaecarieae; Pax & Hoffmann, 1912), which is included in Webster's subfamily Euphorbioideae, or tribe Gelonieae (Mueller, 1866; Pax & Hoffmann, 1931; Hutchinson, 1969), a heterogeneous grouping of genera belonging to all three uniovulate subfamilies. More recently, Webster (1994) treated the genus as the monogeneric tribe Omphaleae, close to tribe Plukenetieae in subfamily Acalyphoideae. In contrast, Airy Shaw (1980) included the genus in his tribe Jatropheae (which comprises genera placed by Webster in subfamily Crotonoideae) in his tentative scheme for natural grouping of genera. Recent preliminary evidence from molecular and specialist herbivore phylogenetic studies appears to support a relationship with members of subfamily Crotonoideae (Wurdack & Chase, 1996; Lees & Smith, 1991). Its isolated position and unusual combination of plesiomorphic (e.g., extrastaminal disc, imbricate staminate sepals) and derived (e.g., presence of laticifers, foliaceous bracts, few connate stamens, entirely connate styles) character states suggest that Omphalea may represent a relatively early evolutionary branch within the uniovulate euphorbs (Gillespie, 1988b), perhaps near the base of subfamily Crotonoideae. Three species of Omphalea were previously known from Madagascar. The east coast rainforest tree 0. oppositifolia (Willdenow) L. J. Gillespie was first collected, illustrated, and described as the genus Hecatea by Du Petit-Thouars (1804a, 1804b, 1805) and formally named as a species by Willdenow (1805) in the early 1800s. Much later, Leandri (1938) described two dry forest species, 0. occidentalis and 0. palmata, from the west coast of Madagascar. A fourth species was recently discovered by the author on limestone karst in the Reserve Speciale Ankarana in northern Madagascar. A specimen of this new species from Montagne des NovoN 7: 127-136. 1997. This content downloaded from 207.46.13.112 on Mon, 03 Oct 2016 04:55:17 UTC All use subject to http://about.jstor.org/terms