Description from Flora of China
Shrubs or trees [or rarely vines], unarmed. Raphides absent. Leaves opposite, isophyllous or at nodes with inflorescences often markedly anisophyllous with smaller leaf reduced to a scale and/or deciduous to give an appearance of alternate leaf arrangement, usually with domatia; stipules persistent or usually caducous, interpetiolar or shortly united around stem, triangular. Inflorescences pseudoaxillary, usually produced at nodes with markedly anisophyllous leaves thus appearing "leaf-opposed," fasciculate to usually cymose, few to many flowered, sessile to pedunculate, bracteate. Flowers sessile or pedicellate, bisexual, monomorphic. Calyx limb cupular or campanulate, 4- or 5-lobed or -denticulate. Corolla white, yellow, or green sometimes flushed with pink or red, salverform, pilosulous to villosulous in throat; lobes 4 or 5, convolute in bud, usually strongly reflexed at anthesis. Stamens 4 or 5, inserted in corolla throat, exserted; filaments very short; anthers dorsifixed, exserted. Ovary 2(or 3)-celled, ovules several to many in each cell on axile placentas; stigma clavate, fusiform, or 2-lobed with lobes sometimes coherent or free. Fruit red to orange, baccate, fleshy, globose and smooth or ridged, with calyx limb deciduous; seeds several to numerous, medium-sized, angled to compressed, embedded in pulp.
The Asian and Malesian portion of this genus was reviewed in detail by Ridsdale (Blumea 41: 135-179. 1996), who recognized five sections; all of our species fall in his Aidia sect. Aidia. An additional species, A. shweliensis, was included in Aidia by W. C. Chen (in FRPS 71(1): 350. 1999) but is here treated as Fosbergia shweliensis. The morphology of Aidia was studied in some detail by Tirvengadum and Sastre (Bull. Mus. Natl. Hist. Nat., B, Adansonia 8: 257-296. 1986), who also provided an overview of the taxonomy. Ridsdale (Reinwardtia 12: 289. 2008) treated A. canthioides in the genus Benkara, but it seems anomalous in that genus and is here retained in Aidia. The species circumscriptions of Ridsdale differ from those of W. C. Chen (loc. cit.: 348-356); neither author is completely followed here. Following Ridsdale, here plants with 4-merous flowers are distinguished from those with 5-merous flowers, which results in the separation of A. racemosa and A. cochinchinensis, both 5-merous representatives of primarily Malesian and Vietnamese species, from the 4-merous, primarily Chinese species A. henryi. Following W. C. Chen, A. henryi and A. merrillii are here considered conspecific. Following Ridsdale, A. densiflora (Wallich) Masamune is not treated here as a species found in China, and the names Merrill (Lingnan Sci. J. 14: 61. 1935) considered synonyms of that species (Randia densiflora (Wallich) Bentham, R. oppositifolia Koorders, and Webera oppositifolia Roxburgh) are here considered misapplied names that do not correspond to any species in the Chinese flora.
About 50 species: tropical Africa, S and SE Asia, Oceania; eight species (one endemic) in China.
(Authors: Chen Tao (陈涛); Charlotte M. Taylor)