Background and aims – Further studies of Madagascan Memecylon have revealed that much of the herbarium material collected in the last 25 years is undescribed. As a prelude to a comprehensive taxonomic revision, the current paper presents an overview and analysis of the remarkable diversity of this genus in Madagascar. Key results – Within the paleotropical distribution of Memecylon, Madagascar contains by far the highest concentration of species proportionate to land area. All of the Madagascan Memecylon species are endemic to the island, with the majority being localized endemics known from just one or two sites. As a result of recent field- and collections-based studies, about fifty new species will be proposed. When this is done the total number of Madagascan Memecylon species will increase to 138 (representing a 70% increase from our state of knowledge in 1985). Memecylon is clearly one of the plant genera that has radiated extensively on the island. Floral morphology is strongly conserved, but leaf morphology and inflorescence position are quite variable and often diagnostic at the species level. In several cases, different species have converged on similar vegetative morphologies, leading to taxonomic confusion. Conclusions – When making species determinations in Madagascan Memecylon, both morphological features and ecogeographic factors should be taken into account. Comprehensive taxonomic revisions in species-rich groups like Memecylon are a prerequisite for further study of the mechanisms of species diversification on the island.

DIVERSIFICATION; ENDEMISM; MADAGASCAR; MELASTOMATACEAE; MEMECYLON; PLANT TAXONOMY