Description from Flora of China
Rubia membranacea var. caudata Z. Ying Zhang; R. membranacea var. incurvata Z. Ying Zhang.
Vines or climbing herbs; stems to 2 m, quadrangular, glabrous or hirtellous at nodes, scaberulous, retrorsely aculeolate, or sometimes subsmooth. Leaves in whorls of 4; petiole 0.5-2.5(-4) cm; blade drying membranous to papery, lanceolate to subovate, 1-6(-8) × 0.5-2(-4) cm, base rounded to cordate, margins usually aculeolate, apex acuminate or shortly acuminate; principal veins 3 or 5, palmate. Inflorescences thyrsoid, paniculate, with terminal and axillary, few- to many-flowered cymes, 2-3 cm; axes glabrous and smooth; pedicels 2-5 mm; bracts narrowly lanceolate, 1-5 mm. Ovary ca. 1.8 mm, glabrous. Corolla purplish red, rotate, fused basal part 0.2-0.6 mm; lobes spreading, ovate-lanceolate, 2-3(-4) mm, caudate. Mericarp berry dark blue or black, 5-9 mm in diam. Fl. May-Jun, fr. Aug-Oct.
We have seen no authentic material of Rubia membranacea, which was well illustrated in H. S. Lo (in FRPS 71(2): 293, t. 62, f. 7-12. 1999). The short cymes indicated together with small leaves and relatively large rotate purplish flowers might be distinctive. We have seen only one ± corresponding collection (Sichuan: Mianning Xian, Lamagetou Nature Reserve, D. E. Boufford et al. 32941), but it deviates in habit and more loose cymes.
The two varieties of this species listed above were described and figured by Z. Ying Zhang (Fl. Tsinling. 1(5): 17, 421. 1985) but not cited by H. S. Lo (loc. cit.: 314-315). They were distinguished from var. membranacea by the orientation of the corolla lobes, said to be "incurved" in var. incurvata and long caudate and glabrous in var. caudata, differences of doubtful taxonomic relevance.
● Sparse forests, forest margins, thickets, grasslands; 1100-3000 m. Hubei, Hunan, Sichuan, Yunnan.