Description from Flora of China
Shrubs or trees, unarmed. Raphides absent. Leaves opposite, sometimes with domatia; stipules caducous, interpetiolar, triangular, acute. Inflorescences terminal and sometimes in axils of uppermost leaves giving tripartite appearance, cymose and usually rounded-corymbiform, pedunculate, bracteate with bracts caducous. Flowers pedicellate, bisexual, distylous, generally showy. Calyx limb deeply 5-lobed, lobes well developed to subleaflike, usually deciduous shortly after anthesis and often before corolla. Corolla red to pink or white, salverform; tube prolonged and in long-styled form expanded in throat to enclose anthers, with upper part of throat spreading with lobes at anthesis, glabrous inside; lobes 5, imbricate (and apparently quincuncial) in bud, rounded, margins usually crisped and slightly irregular, sometimes with a lamellate basal appendage on each side. Stamens 5, inserted in upper part of corolla tube, included in long-styled form or partially to completely exserted in short-styled form; filaments short; anthers dorsifixed. Ovary 2-celled, ovules numerous in each cell on axile placentas; stigmas 2, linear, included in short-styled form or exserted in long-styled form. Fruit capsular, obovoid to ellipsoid, smooth, septicidally dehiscent from apex for half or completely, woody, becoming deflexed, with calyx limb caducous; seeds numerous, small, fusiform, flattened, prolonged into narrow wings at each end; endosperm fleshy; embryo minute, slightly clavate; cotyledons obtuse.
The relationships of Luculia have not been clear until recently, when molecular studies (e.g., Bremer et al., Syst. Biol. 48: 413-435. 1999) found it to be basal in the Rubiaceae. Additional references regarding Luculia’s biology and classification, not listed here, were cited by Rydin et al. (Pl. Syst. Evol. 278: 101-120. 2009).
The treatment here summarizes the existing taxonomy, as a basis for future work; it seems possible that there are only two species in China, but evaluation of this is beyond the scope of this study and the available specimens. Hutchinson (in Sargent, Pl. Wilson. 3: 408. 1916) gave taxonomic significance at the species level to the presence vs. absence of "striking flap-like processes upon and continuous between the lobes of the corolla" for separating species of Luculia; these structures have not been much studied or detailed by other authors.
About five species: Bhutan, China, India, Myanmar, Nepal, Thailand, Vietnam; three species (one endemic) in China.
(Authors: Chen Tao (陈涛); Charlotte M. Taylor)