Description from Flora of China
Ricardia Adanson; Richardsonia Kunth.
Herbs, annual or perennial, unarmed. Raphides present. Leaves opposite, without domatia; stipules persistent, interpetiolar and fused to petioles or leaf bases, truncate to rounded, setose. Inflorescences terminal, capitate, several to many flowered, pedunculate and enclosed by paired leaflike bracts (or sessile with involucral leaves in other morphological interpretations), bracteate. Flowers sessile, bisexual, monomorphic. Calyx with ovary portion turbinate to globose, limb deeply 4-8-lobed. Corolla white or pink, funnelform, inside glabrous or pubescent at throat; lobes 4-6, valvate in bud. Stamens 3, 4, or 6, inserted in corolla throat, exserted; filaments developed; anthers dorsifixed near middle. Ovary 3- or 4-celled, ovules 1 in each cell, axile and attached at middle of septum; stigmas 3 or 4, linear or spatulate, exserted. Fruit schizocarpous, subglobose to obovoid or tricoccous, dry, bony, with calyx limb deciduous; mericarps 3 or 4, indehiscent, 1-celled with 1 seed, ellipsoid to angled, usually papillose to muricate on dorsal surface (i.e., abaxially) and with 1 or more grooves and sometimes papillose to muricate on ventral surface (i.e., adaxially); seeds medium-sized, ellipsoid to plano-convex; endosperm corneous; cotyledon leaflike; radicle cylindrical, hypogeous.
As noted by Chaw and Peng (J. Taiwan Mus. 40: 71-83. 1987), Asian collections of Richardia have long been confused in herb. with various other weedy Rubiaceae. Richardia was studied in detail by Lewis and Oliver (Brittonia 26: 271-301. 1974). The synonymous name Richardsonia has frequently been used for this genus, including in older references about invasive weeds. H. S. Lo (in FRPS 71(2): 203. 1999) described the flowers as sometimes polygamo-dioecious, but the origin of this description is unknown. Lewis and Oliver did not report this condition, although they did mention that the plants frequently have both chasmogamous and cleistogamous flowers. H. S. Lo also described the anthers as dorsifixed near the base, but other authors have all considered them to be dorsifixed near the middle, which agrees with specimens studied.
Richardia stellaris (Chamisso & Schlechtendal) Steudel is naturalized in Australia and perhaps may be expected in China; it can be recognized by its narrowly triangular to narrowly elliptic, sharply acute leaves.
Fifteen species: widespread in the Antilles and North and South America, three species naturalized in the Old World tropics; two species (both introduced) in China.
(Authors: Chen Tao (陈涛); Charlotte M. Taylor)