An evergreen, diffusely branched shrub up to 20 ft high in the wild, with slender downy twigs becoming grey, the down persisting at least two years. Leaves thin, oval-lanceolate, rounded at the base, tapering at the apex to a short mucro, 11⁄2 to 3 in. long, 5⁄8 to 1 in. wide, at first a pleasing purplish red then dark green and glabrous except for the midrib, which is downy above and below; stalk 1⁄4 to 5⁄8 in. long, covered with grey down. Flowers solitary from the clustered terminal leaf-axils, opening in May; pedicels slender, 5⁄8 in. long, downy, rosy purple. Calyx with five ovate or oblong lobes 3⁄8 in. long, ciliate. Corolla deep magenta rose with crimson spots, about 13⁄4 in. wide, five-lobed, the lobes oblong and as long or longer than the tube, which is downy inside. Stamens five, the lower half downy. Ovary covered with sticky glands; style glabrous. Bot. Mag., n.s., t. 502. (s. Ovatum)
Native of W. Yunnan, China; introduced by Forrest in 1914 from the border between Yunnan and Burma (western flank of the Shweli-Salween divide), but described from specimens collected by him in 1918 on the Mekong-Yangtse divide. A closely related species occurs in Burma. R. leptothrium is closely allied to R. ovatum, differing in having the leaves mainly elliptic, in its deeper coloured corollas with oblong (not orbicular) lobes, and in having the calyx-lobes edged with short, gland-tipped bristles. It is a tender species – much less hardy than the cultivated form of R. ovatum. It is botanically interesting, but not of much ornamental value, though the young growths are attractively tinted.