Description from Flora of China
Shrubs or small trees, unarmed; branches usually longitudinally ridged on each side and surrounded at base by persistent leafless stipules, with bark often yellowed and hardened. Raphides present. Leaves opposite, decussate on ascending branches and distichous on horizontal branches, without domatia; stipules persistent at least on younger nodes, interpetiolar, bilobed, often becoming hardened with age. Inflorescences terminal on principal branches, or terminal on axillary short shoots and apparently axillary, umbelliform to fasciculate and several flowered or reduced to 1 flower, sessile to pedunculate, bracteate with bracts usually reduced. Flowers pedicellate or sessile, bisexual, usually distylous [occasionally fused by their ovaries]. Calyx limb truncate or 4 or 5(or 6)-lobed, with lobes infrequently unequal with 1 larger than others. Corolla white, salverform, inside glabrous; lobes (4 or)5(or 6), valvate in bud. Stamens (4 or)5(or 6), inserted at or above middle of corolla tube, included in long-styled flowers or partially exserted in short-styled flowers; filaments short; anthers dorsifixed. Ovary 2-celled, ovules 1(or 2) in each cell, on axile placentas attached near or above middle of septum; stigma 2-lobed, exserted in long-styled flowers, included in short-styled flowers. Infructescences occasionally displaced to pseudoaxillary by subsequent branch growth. Fruit simple [or infrequently multiple], purple-black to blue-black, drupaceous, fleshy, subglobose or globose, with calyx limb persistent; pyrenes 1 or 2, 1-celled, subglobose when solitary to plano-convex when paired, thin-walled, with membranous preformed germination slits; seeds medium-sized, subglobose to plano-convex, on ventral face with deeply concave hilum; testa membranous; endosperm corneous; embryo small, with hypocotyl; radicle hypogynous.
This genus was revised in a broad treatment by Johansson (Opera Bot. 94: 1-62. 1987), who reported that the flowers are strongly fragrant. This genus was also treated for China almost simultaneously and apparently independently by Ruan (Acta Phytotax. Sin. 26: 443-449. 1988), whose conclusions differed markedly from those of Johansson.
Two species have been recognized in China, Prismatomeris tetrandra and P. connata, with two subspecies recognized for P. connata (Y. Z. Ruan in FRPS 71(2): 178-179. 1999); P. tetrandra subsp. multiflora included only plants from Yunnan, while plants of the rest of China were treated in P. connata, with plants from the mainland in subsp. connata and plants from Hainan in subsp. hainanensis. However, wide and continuous morphological variation was documented by Johansson (loc. cit.) and Puff et al. (Rubiaceae of Thailand, 118. 2005) within P. tetrandra in adjacent countries, both across the region and in local populations; and Johansson (in herb.) recognized one species and no infraspecific taxa in China. Prismatomeris connata was distinguished originally by its connate rather than separate stigmas; however, this character was later said by Y. Z. Ruan to vary within this species. Its subspecies were distinguished by the degree of lobing of the calyx limbs and the shape and degree of swelling of the stigmas, but as there appears on the specimens studied to be continuous variation in these characters and some plants from the mainland match the characters given for the Hainan subspecies these plants are not separated here.
Fifteen species: Bangladesh, Borneo, Cambodia, China, India (including Andaman Islands), Indonesia, Laos, Malaysia, Myanmar, Philippines, Singapore, Sri Lanka, Thailand, Vietnam; one variable species in China.
(Authors: Chen Tao (陈涛); Charlotte M. Taylor)