Synopsis of Hexasepalum (Rubiaceae), the Priority Name for Diodella and a New Species from Brazil

Abstract

Hexasepalum is the priority name for Diodella. In the present paper, we provide a synopsis of Hexasepalum. The genus differs from the remaining genera of the tribe Spermacoceae by the combination of infundibuliform flowers, 2-lobed stigmas, schizocarpic fruits, ventral face of the mericarps with two depressions, seeds with T or Y shaped ventral grooves, and pollen grains with two types of supratectal elements. We propose two combinations, i.e. Hexasepalum domingense and H. vaginale , and the formal synonymization of H. angustifolium (including Diodella crassifolia). We also propose two new synonyms: Diodia rosmarinifolia [= Diodella rosmarinifolia] is placed in the synonymy of H. apiculatum, and Diodia incana is now considered a synonym of H. teres. We choose lectotypes for H. vaginale, and for two synonyms of H. apiculatum. In addition, we describe and illustrate a new species endemic from Bahia, Brazil, H. zappiae sp. nov. Detailed descriptions are provided only for the species imperfectly known; otherwise comprehensive taxonomic descriptions are mentioned for each species. The descriptions of the Hexasepalum species are complemented with a key, geographical distributions, and illustrations of key morphological characters. Line drawings are provided for species not illustrated before in literature