Description from Flora of China
Herbs or subshrubs, unbranched or little branched, unarmed. Raphides absent. Leaves opposite and often clustered at ends of stems, without domatia; stipules persistent or deciduous, interpetiolar, triangular to oblong. Inflorescences terminal or sometimes displaced to pseudoaxillary, borne on principal stems or sometimes short shoots at stem apex or in 1 or both leaf axils, 1-flowered [or cymose to fasciculate and several flowered], bracteate [or bracts reduced]. Flowers pedicellate or subsessile, bisexual, apparently monomorphic. Calyx with ovary portion usually relatively prolonged; limb lobed essentially to base; lobes 4 or 5, often with 1 well-developed colleter in each sinus. Corolla pale green or white to blue or purple, funnelform to campanulate, glabrous inside; lobes 4 or 5, valvate or reduplicate-valvate in bud. Stamens 5, inserted near base of corolla tube, included or exserted; filaments short to well developed, glabrous, free or fused in basal portion; anthers linear, at apex with sharply acute or spurred appendage, these connate into a tube surrounding stigma and united with it at tops of both structures. Ovary 1-celled, ovules many per cell on 2 T-shaped parietal placentas; stigma 1, clavate, relatively large, 10-ridged, sometimes fused at top to anther appendages. Fruit baccate or perhaps occasionally tardily capsular with irregular dehiscence, fleshy, ovoid to cylindrical or turbinate, color not reported, with calyx limb persistent; seeds numerous, reddish brown or nearly black, small, compressed or lenticular; endosperm fleshy; embryo small, straight.
This genus was monographed by Bremekamp (J. Arnold Arbor. 28: 261-308. 1947), who named a number of subgenera and series. C. Y. Wu’s new species did not fit into Bremekamp’s classification; consequently, he described a new monotypic subgenus for it, Acranthera subg. Sinacranthera C. Y. Wu, distinguished from A. subg. Acranthera and A. subg. Androtropis Bremekamp by its funnelform corollas, inflorescences borne on opposite brachyblasts bearing two rudimentary leaves, and red to purple corolla color (vs. basal part of corolla cylindrical, inflorescences borne at stem apices, and blue corollas in A. subg. Acranthera and inflorescences borne at stem apices and pale green corollas in A. subg. Androtropis) and from the other subgenera by its well-developed, subglobose disk (vs. inconspicuous and presumably flattened). Acranthera is considered based on recent molecular and some morphological data to be a rather isolated basal lineage in Rubiaceae: its closest relative is probably Coptosapelta, and these two genera are probably most closely related to Luculia (Alejandro et al., Amer. J. Bot. 92: 544-557. 2005; Rydin et al., Pl. Syst. Evol. 278: 101-123. 2009).
Puff et al. (Ann. Missouri Bot. Gard. 82: 357-382. 1995) concluded that the flowers have an "anther-style and stigma complex" that is unique in Rubiaceae and suggested that at least some Acranthera species may be buzz-pollinated. Bremekamp described the ovaries as 2-celled with laminar axile placentas, but Puff et al. showed that the ovaries are actually 1-celled, with parietally inserted placentas that are T-shaped and meet closely enough in the middle to appear superficially to be borne on a median septum, though there is none.
About 40 species: China, India, Indonesia, Malaysia, Sri Lanka, Thailand, Vietnam, with most species apparently in Borneo; one species (endemic) in China.
(Authors: Chen Tao (陈涛); Charlotte M. Taylor, Christian Puff)