An evergreen shrub or tree, up to 40 ft high, with a pinkish, flaking bark. Leaves leathery, oblanceolate to obovate, rounded to obtuse at the apex, narrowed to a roundish or slightly cordate base, 3 to 7 in. long, 1 to 2 in. wide, glabrous on both sides; petiole up to 1 in. long. Flowers in a terminal cluster of ten to twelve; pedicels 3⁄8 to 5⁄8 in. long, eglandular. Calyx cup-like, undulate, up to 5⁄8 in. long. Corolla tubular-campanulate, five-lobed, fleshy, up to 13⁄4 in. long, pink, spotted, with five nectar-pouches at the base. Ovary eglandular (but see below); style glabrous. (s. and ss. Thomsonii)
R. hylaeum was discovered by Farrer in 1920 on the Chawchi pass, on the border between Burma and Yunnan, and was introduced by Forrest and by Kingdon Ward in 1924. Forrest's seed was collected near the type-locality (F.24660), but the seed sent home by Kingdon Ward (KW 6401) was taken from a fruiting branch collected by one of his bearers near Pemakochung at the head of the Tsangpo gorge. It is also in cultivation from seeds collected in the Seinghku valley, north-west Burma (KW 6833).
R. hylaeum is an uncommon species, but is quite hardy south of London in a sheltered place. It is handsome in foliage and bark, but otherwise of no ornamental value. Although the ovary in R. hylaeum is said by Cowan and Davidian to be always eglandular, it is glandular in some cultivated plants, which would therefore come out in their key as R. eclecteum, some forms of which resemble R. hylaeum in flower, though the typical variety of R. eclecteum is very distinct from it in foliage.
From the Supplement (Vol. V)
It was remarked in the third paragraph on page 686 that some plants grown as R. hylaeum differ from the description in having glandular ovaries. These belong to the newly described species:
† R. faucium Chamberlain – Near to R. hylaeum but the leaves smaller, tapered at the base, petioles shorter and ovary glandular. The type is Ludlow, Sherriff and Elliot 12289, collected in Pome province, south-east Tibet (Rev. 2., pp. 421-2).
This was introduced from the type-area by Kingdon Ward (KW 6401). But the other number mentioned on page 686 (KW 6833) is R. hylaeum. Other introductions of R. faucium are L.S. & E. 12019, 12045 and 12208. (Notes Roy. Bot. Gard. Edin., Vol. 36, pp. 124-5 (1978)).