Revision of the Madagascan endemic Homollea (Rubiaceae - Pavetteae), with description of two new species

Petra De Block

doi:10.5852/ejt.2018.423

Abstract

Homollea Arènes (Rubiaceae, subfamily Ixoroideae, tribe Pavetteae) is a genus of shrubs and small trees endemic to western and northern Madagascar. The genus comprises five species occurring in dry deciduous forest, often in limestone areas. The five species are narrow endemics and their conservation status is either Endangered (4 species) or Critically Endangered (1 species). Homollea is characterized by few-flowered, pseudo-axillary, pedunculate inflorescences, well-developed calyces with the lobes much longer than the tube, laterally flattened seeds with a shallow, elongated to linear hilum and entire endosperm, ovules arising from the upper margin of the placenta, and, pollen grains with supratectal elements in the shape of microgemmae. Until now, three species were known and their descriptions are amended. Two further species, H. furtiva De Block sp. nov. and H. septentrionalis De Block sp. nov., are described as new for science. The five species are dealt with in detail: descriptions, distribution maps, conservation assessments, illustrations, lists of exsiccatae and an identification key are given.

Highlights

Madagascar boasts an extremely rich and unusually diverse as well as a highly endemic flora
At the time of writing, 91 described Rubiaceae genera (Ferm et al 2016; Govaerts et al 2016) are present in Madagascar, of which 30 are endemic to the island and five further ones endemic to Madagascar and the Comoros (Govaerts et al 2016; Ferm et al 2016), which is equivalent to 33% and 38.5% of endemicity at genus level, respectively
Homollea is characterized by sheathing stipules, coriaceous or subcoriaceous leaves, few-flowered, pseudo-axillary, pedunculate inflorescences, well-developed calyces with the lobes much longer than the tube, laterally flattened seeds with a shallow, elongated to linear hilum and entire endosperm, ovules arising from the upper margin of the placenta, and, pollen grains with supratectal elements in the shape of microgemmae (De Block & Robbrecht 1998)

Summary

Introduction

Madagascar boasts an extremely rich and unusually diverse as well as a highly endemic flora. At the time of writing (end 2016), 91 described Rubiaceae genera (Ferm et al 2016; Govaerts et al 2016) are present in Madagascar, of which 30 are endemic to the island and five further ones endemic to Madagascar and the Comoros (Govaerts et al 2016; Ferm et al 2016), which is equivalent to 33% and 38.5% of endemicity at genus level, respectively (vs 19% for the vascular flora as a whole) These rates are bound to become higher still as new genera continue to be described

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Conclusion