Description from Flora of China
Lianas, climbing shrubs, erect shrubs, or small trees, rarely dioecious, sometimes with spines; branching sometimes sylleptic with growth continued from an axillary or subapical node, with lateral branches sometimes surrounded at base by persistent leafless stipules. Raphides present. Leaves opposite, rarely ternate, or sometimes anisophyllous and apparently 1 at flowering nodes, sometimes with domatia in axils of secondary and occasionally tertiary veins, margins rarely sinuate-undulate or lyrate; stipules persistent or infrequently caducous, interpetiolar, united around stem, or fused to petioles, triangular, entire. Inflorescences terminal, axillary, or leaf-opposed, capitate with 1 to several hemispherical to subglobose heads, these fasciculate or cymose, few to many flowered, pedunculate or sessile, bracteate or bracts reduced. Flowers sessile, shortly to fully fused by their ovaries [to free or nearly so], bisexual and distylous, rarely bisexual and monomorphic, or rarely dioecious. Calyx limb truncate to sinuate or rarely in 1-3 flowers of an inflorescence with 1 petaloid calycophyll (Morinda citrifolia). Corolla white or pink, funnelform, salverform, or campanulate, inside glabrous or pubescent in throat, [tube sometimes fenestrate]; lobes 3-7, valvate in bud. Stamens 3-7, inserted in corolla throat or tube, exserted or included; filaments short; anthers dorsifixed, sometimes with connective prolonged into an apical appendage. Ovary 2-celled with ovules 2 in each cell, or incompletely to completely 4-celled due to secondarily formed false septa with ovules 1 in each cell, ovules attached to septum near base; stigmas 2, linear, exserted or included. Fruit multiple with entire fruiting heads comprising one fruit (i.e., drupecetum) [sometimes fruit simple]; individual fruit drupaceous, fleshy, generally obovoid, blue to black, with calyx limb persistent; pyrenes 2-4, 1-locular, with 1 seed, cartilaginous or bony, subtrigonous to plano-convex, adaxially (i.e., ventrally) flat or sulcate; seeds medium-sized, subtrigonous or ellipsoid; endosperm abundant, corneous; embryo small; cotyledons oblong; radicle inferior.
Morinda includes a notable range of breeding systems (Johansson, Opera Bot. 122: 1-167. 1994), but most of the species are apparently distylous, with the anthers and stigmas separated and their positions reciprocal between the short-styled and long-styled form of the same species; however, this biology has been sometimes overlooked. Also, as noted by Johansson (loc. cit.), the position of the inflorescences, in particular terminal vs. leaf-opposed, deserves careful observation and aids identification of species. In particular, the leaf-opposed inflorescences often are produced on the terminal node, then later displaced by subsequent growth from the axil of that leaf, and can be confused with true terminal inflorescences that have two subtending leaves. The inflorescences with "fasciculate to umbellate" peduncles actually appear to be condensed cymes or racemes, with the peduncles arising from a very shortly prolonged structure at the stem apex that also bears several stipuliform bracts, usually one above the other. The twining Asian species apparently share having their lateral branches surrounded at the base (i.e., at the divergence from main stem) by persistent leafless stipules. Some species of Morinda have petaloid bracts or possibly calyx lobes; this character appears to vary within some individual species. Y. Z. Ruan’s (in FRPS 71(2): 179-202. 1999) taxonomy of Morinda distinguished species based on different characters, in particular pubescence, leaf shape, peduncle length compared across developmental stages, drying color and texture, pattern of tertiary leaf veins on dried specimens, and degree of fusion of flowers, than used by many other authors (e.g., Johansson, loc. cit.; Springate et al., Fl. Bhutan 2(2): 804. 1999).
About 80-100 species: widespread in tropical and subtropical regions worldwide; 27 species (18 endemic) in China.
(Authors: Chen Tao (陈涛); Charlotte M. Taylor)