Description from Flora of China
Herbs, pubescent, without cystoliths. Leaves opposite, shortly petiolate; leaf blade pinnately veined, margin entire. Inflorescences axillary or terminal spikes; bracts overlapping. Flowers ± spirally arranged, sessile, subtended by a bract; bracteoles absent. Calyx 4-lobed to base; lobes unequal, anterior lobe apically 2-cleft. Corolla tube slender, sometimes widening into a throat; limb 2-liped; lower lip 3-lobed, upper lip 2-lobed; lobes ± equal, descending cochlear in bud. Stamens 2, included or slightly exserted; filaments basally pubescent; anthers 2-thecous; thecae subspherical, muticous, opening at base by a pore that has a small flap; staminodes absent. Ovary with 8-28 ovules in 2-4 rows per locule; stigma usually unequally 2-lobed. Capsule conical; retinacula absent. Seeds subspherical, small, flattened on 1 face, lacking trichomes.
A single species, Nelsonia canescens, is sometimes recognized (e.g., A. B. M. E. Hossain, Willdenowia 14: 397-403. 1984), but Vollesen (e.g., K. Vollesen in J. H. Seyani & A. C. Chikuni, eds., Proceedings of the XIII Plenary Meeting of AETFAT, Zomba, Malawi, 2-11 Apr 1991, 315-325. 1994) presented arguments for recognizing up to five species.
Five species or fewer: widely occurring in tropical Africa, Asia, Australia, and South America (introduced elsewhere); one species in China.
(Authors: Hu Jiaqi (胡嘉琪 Hu Chia-chi), Deng Yunfei (邓云飞); Thomas F. Daniel)