Description from Flora of China
Shrubs, subshrubs, or perennial herbs, not rarely clambering or climbing vines or rarely lianas, unarmed; stems often prickly and/or longitudinally ribbed or winged. Raphides present. Leaves opposite and with interpetiolar, triangular or ovate, persistent to caducous (Rubia siamensis) or reduced (R. tibetica) stipules or with leaflike stipules in whorls of 4, 6, to many in middle stem regions; domatia none; main veins single or 3-5(or more) and then palmate, secondary veins lateral. Inflorescences thyrsoid, with terminal and/or axillary cymes, usually paniculiform and often expanding from new axes developing with age; individual cymes few to many flowered, pedunculate, bracteate. Flowers pedicellate or sessile, rather small, usually bisexual and monomorphic, rarely polygamo-dioecious (R. cordifolia). Ovary inferior (hypanthium), ellipsoid, subglobose, 2-celled, ovules 1 in each cell, erect, axile. Calyx limb reduced and obsolete. Corolla white to cream, yellow, greenish or red to purplish, often turning black when dried, mostly rotate, but rarely also campanulate to funnel-shaped, inside glabrous or infrequently papillose; lobes predominantly 5 (rarely also less or more), valvate in bud, often long acuminate. Stamens usually 5, inserted at corolla base (or tube), exserted; filaments developed to reduced; anthers dorsifixed. Stigmas 2-lobed, included or exserted. Fruit developing into 2 separate or (by reduction) into only 1 subglobose, baccate, berrylike mericarp with fleshy meso- and endocarp, dark red, purple, black, or infrequently orange (R. cordifolia), glabrous or somewhat hairy; seeds ("pyrenes") 2, ellipsoid, subglobose, or plano-convex, with membranous testa; endosperm corneous; embryo subincurved; cotyledons leaflike; radicle prolonged, basiscopic.
As already mentioned in the present volume under Galium, Rubia is the type genus of the family, the tribe Rubieae, and the subtribe Rubiinae. As an Old World clade, Rubia is related to the Mesoamerican genus Didymaea and occupies a relative basal position within Rubiinae: its 5-lobed corollas, fleshy fruit, and always perennial growth form apparently are plesiomorphic features. This and its clear separation from the somewhat more apomorphic Sherardia-Asperula-Galium group is well documented by DNA data (Natali et al., Opera Bot. Belg. 7: 193-203. 1996; Soza & Olmstead, Taxon 59: 755-771. 2010). Rubia is keyed out from among the other Chinese taxa of Rubieae under Galium on p. 107. Its best differential characters are the dominantly 5-merous flowers combined with baccate, berrylike mericarps. The latter also occur independently among New World taxa of Galium (and Relbunium).
Among the Rubieae tribe Rubia (after Galium and Asperula in their present circumscription) is the third largest and obviously monophyletic genus. Nearly half of its recognized species occur in China. Because of excessive variability, the occurrence of hybridization and polyploidy as well as the lack of detailed studies (particularly on material in the major herbaria of China and elsewhere), our knowledge of Rubia is limited and the present treatment of the genus still quite provisional.
More recent taxonomic surveys of Rubia are available for the former Soviet Union (Pojarkova, Fl. URSS 23: 382-417. 1958), India (Deb & Malick, Bull. Bot. Surv. India 10(1): 1-16. 1968), Iran (Ehrendorfer et al., Fl. Iranica 176: 48-72. 2005), Bhutan (Long, Edinburgh J. Bot. 53: 108-110. 1996; Long, Fl. Bhutan 2(2): 823-825. 1999), and Taiwan (T. Y. A. Yang, Fl. Taiwan, ed. 2, 4: 321-324. 1998). Particularly in the first of these contributions, but also in the following two, the infrageneric taxonomy of Rubia is briefly considered. The majority of the Chinese species are characterized by 3-5(-11) palmate veins in their relatively broad leaves. These taxa correspond to R. sect. Oligoneura Pojarkova (= R. [unranked] Cordifoliae Candolle; R. sect. Cordifoliae (Candolle) K. Schumann ex Deb & Malick). Within this section Pojarkova (loc. cit.) has recognized two series: one predominantly climbing vines with long leaf petioles as R. ser. Cordifoliae (Candolle) Pojarkova (with 16 species in China; see under R. cordifolia), the other mostly erect perennial herbs with very short leaf petioles as R. ser. Chinenses (with three species in China; see under R. chinensis). The latter is close to the informal R. mandersii group, where the leaves are sessile (three species in China; see under R. mandersii). Finally, there are two species groups with vines. One is the R. sikkimensis group with sessile leaves and leaflike stipules (two Chinese species; see under R. tenuis), in the other, the R. siamensis group, typical (not leaflike) interpetiolar stipules appear between the opposite leaves (four species in China; see under R. siamensis).
The remaining ten Chinese species have leaves with only 1(-3) main vein(s) and predominantly pinnate vein branching. In the present treatment this refers to the species Rubia chitralensis, R. deserticola, R. dolichophylla, R. rezniczenkoana, R. schugnanica, R. tibetica, and R. tinctorum, here provisionally accommodated in R. sect. Rubia s.l. (including R. sect. Meganthera Pojarkova (= R. sect. Rubia s.s.), R. sect. Campylanthera Pojarkova, and R. sect. Chonanthe Pojarkova). The placement of the E Asiatic species R. haematantha (the R. haematantha group; see there) as well as R. pseudogalium and R. truppeliana (the R. truppeliana group; see there) is doubtful; they may belong to R. sect. Oligoneura in spite of their very narrow leaves and only 1 main vein. The more detailed infrageneric subdivisions of Rubia by Pojarkova listed above have been based on size and form of anthers and other flower details and are in need of more general and detailed study.
Relevant characters for the separation of Rubia taxa on the species level are growth habit, indumentum, number, shape and consistency of leaves and stipules, presence or absence of petioles, inflorescence structures, color and morphology of corollas from rotate to funnel-shaped, fruit color, etc. Particularly, leaf characters often vary excessively under different environmental and developmental conditions (e.g., in more widespread taxa as Rubia cordifolia and its allies; see further comments there). These facts are difficult to evaluate, at least on dried specimens.
The ground-up rhizomes and roots of Rubia tinctorum, the type species of the genus, have long been the source of important red textile dyes (madder red, alizarin, rose madder, alizarin crimson). This use was of course much more important before the invention of aniline dyes (e.g., madder colored the red coats of the 18th-century British army). Nevertheless, R. tinctorum is still widely cultivated at a local scale and used, in particular, to color wool for handmade oriental rugs in C and SW Asia (Murphy, Root of Wild Madder, 1-297. 2005) but also in fine art painting. The worldwide occurrence, cultivation, chemistry, and cultural role of this species was discussed in detail by Chenciner (Madder Red. 2000). The stems of R. manjith are also used to produce a red dye (fide Long, loc. cit. 1999).
The key here generally follows that of H. S. Lo (in FRPS 71(2): 287-290. 1999), with the measurements updated from the descriptions and species added as appropriate.
About 80 species: extending from tropical and temperate Asia to Japan and Indonesia, through the Himalaya to SW Asia, E to S Africa, through the Mediterranean to W Europe, Macaronesia, and the Azores; locally introduced and persisting from cultivation in Mexico, Chile, and elsewhere; 38 species (20 endemic) in China.
(Authors: Chen Tao (陈涛); Friedrich Ehrendorfer)