Description from Flora of China
Shrubs or perennial herbs, with cystoliths. Leaf blade margin entire or obscurely crenate. Inflorescences thyrses or rarely axillary cymes or racemes; bracteoles 2, small. Calyx deeply 5-lobed; lobes usually covered with gland-tipped trichomes. Corolla orange or pink, funnel-shaped, usually bent ca. 90º and abruptly inflated near middle; limb 2-lipped; lower lip 3-lobed, lobes orbicular or ovate and equal; upper lip 2-lobed; lobes ascending cochlear in bud. Stamens 2, included; anthers 2-thecous; thecae parallel, equal, hirsute, muticous at base; staminodes 2. Ovary ovoid, long hirsute; style filiform; stigma 2-cleft. Capsule cylindric, 8-12-seeded; retinacula present. Seeds ovate in outline, compressed, tomentose.
Five of the Chinese species treated here in Cystacanthus have also been treated in Phlogacanthus. In addition to the macromorphological characteristics (geniculate corolla with an inflated throat in Cystacanthus) distinguishing Cystacanthus and Phlogacanthus, R. W. Scotland (Rev. Palaeobot. Palynol. 72: 229-243. 1992) discussed palynological differences between them in ornamentation of the aperture margins (i.e., aperture margins with spines ± restricted to a single row in Phlogacanthus and with the spines throughout the aperture margin in Cystacanthus). A thorough reevaluation of taxa treated in these two genera will be necessary to determine whether both genera are worthy of recognition and if so, to which genus each species pertains.
About 15 species: mainland Asia; eight species (three endemic) in China.
(Authors: Hu Jiaqi (胡嘉琪 Hu Chia-chi), Deng Yunfei (邓云飞); Thomas F. Daniel)