Robbrechtia, a New Rubiaceae Genus from Madagascar

Abstract

Two new Rubiaceae species, Robbrechtia grandifolia and R. milleri from Madagascar are described. Clearly belonging to the tribe Pavetteae, the species possess an unusual combination of characters (sheathing stipules with intra- petiolar lobes, young shoots and inflorescence parts covered with copious colleter exudate, long-tubed flowers with a well- developed calyx, a bilobed stigma and unilocular fruits with a single, deeply ruminate seed) that makes the establishment of a new genus, Robbrechtia, necessary. Detailed descriptions, illustrations and distribution data of the new taxa are given. The characters of the genus are evaluated and compared to those of the other representatives of the tribe Pavetteae. During field work for a survey of the Pavetteae from Madagascar a tree bearing fruit was collected (Davis 2210) in the Moramanga area, ca. 75 km east of An- tananarivo. With its large leaves and stipules, its size- able fruits with persistent large calyx and the pubes- cence of the vegetative parts, the plant's habit closely resembled that of the genus Homolliella (Rubiaceae, tribe Pavetteae), represented in that area by two spe- cies. However, upon dissection the fruit proved to be unilocular with one deeply ruminate seed, very differ- ent from the fruit of Homolliella but not unlike that of several other genera of the Pavetteae. A subsequent survey of herbaria uncovered a group of undetermined specimens sharing a striking character combination, unique in the Pavetteae: sheathing stipules with intra- petiolar lobes, young shoots and inflorescence parts covered with copious colleter exudate, long-tubed flowers with a well-developed calyx, a bilobed stigma, unilocular, one-seeded fruits, and seeds with deeply ruminate endosperm. Moreover, this character combi- nation occurred in two previously undescribed species. They are here accommodated in a new genus, Robbrech- tia, named in honour of Prof. Dr. Elmar Robbrecht (Na- tional Botanic Garden of Belgium), who has dedicated his professional life to the study of the Rubiaceae. In Madagascar the Rubiaceae are the second largest family of flowering plants, represented by ca. 100 gen- era and 800-1,000 species. Despite the size of the fam- ily and its strong representation in all vegetation types on the island, the Malagasy Rubiaceae are very poorly known. Recent treatments exist only for a small num- ber of genera and even at the generic level identifica- tion remains difficult in certain tribes, e.g. the Octotro- pideae, Vanguerieae and the Pavetteae. The tribe Pavetteae A.Rich. ex Dumortier was re- vived by Robbrecht (1984) to accommodate all repre- sentatives of the subfamily Ixoroideae formerly placed in Coffeeae s.l. and Gardenieae possessing terminal in- florescences, 3- or 4-colporate tectate pollen grains, drupaceous fruits, seeds with an adaxial excavation surrounded by an annulus and exotestal cells either parenchymatic or with thickenings along the outer tan- gential wall. A detailed study stressing the importance of fruit, seed and seed-coat characters was later exe- cuted by Bridson and Robbrecht (1985). These authors proposed an informal classification of the tribe into two groups, notably genera related to Ixora and genera related to Tarenna. Recently, Andreasen & Bremer (1997; 2000) resurrected the tribe Ixoreae to accom- modate Ixora and its closest allies. The newly delimited tribes Ixoreae and Pavetteae share the following char- acters: woody habit, absence of raphides, entire stip- ules (but fimbriate in Rutidea), terminal inflorescences, contorted aestivation, drupaceous fruits and secondary pollen presentation. They differ in characters such as number of ovules per locule, structure of the stigma, shape of the thickenings of the exotestal cells, etc. An overview of these differences is given in Table 1. In the following text, the tribe Pavetteae is used as delimited by Andreasen & Bremer (1997; 2000). Only ca. seventy species of Pavetteae occur in Mad- agascar but endemism at the specific level is 98%. Of the six genera currently recognized in Madagascar, three are endemic: Homollea (three species; Arenes 1960), Homolliella (five species; De Block, unpubl. data) and Schizenterospermum (four species; Arenes 1960). The paleotropical genus Tarenna is represented in Madagascar with ca. 10 species. Other representatives of the Pavetteae are the Afro-Malagasy genera Coptos- perma (ca. 37 species in Madagascar: De Block, unpubl. data) and Paracephaelis (ca. seven species; De Block, un- publ. data). Coptosperma was recently newly delimited to include Enterospermum (De Block et al. 2002). Generic delimitation is still under discussion for the Malagasy representatives of the Pavetteae. One of the main discussion points is the taxonomic importance of ruminate endosperm, a character present in ca. 60% of the Malagasy Pavetteae against 17% for the continental African species. Botanists studying Malagasy Pavet- teae recognized two main genera, notably Tarenna (en- tire endosperm) and Enterospermum (ruminate endo- sperm) (Homolle 1938; Capuron 1973). Botanists fo-