336. Rhododendron arboreum Smith, Exot. Bot. 1: 9. 1805.
树形杜鹃 shu xing du juan
Trees, usually evergreen, (1–)5–20(–30) m tall; trunk well-defined; bark gray-brownish, exfoliating into thin and small irregular flakes; young shoots stout, with distinct leaf traces, densely grayish tomentose, glabrescent. Winter buds ovoid, 10–12 × 5–6 mm, puberulent. Petiole 10–25 mm with dense fawn indumentum intermixed with glands, sometimes glabrescent; leaf blade leathery, oblong-lanceolate or oblong-oblanceolate, 6–15 × 2–4.5 cm; base cuneate or ± rounded; margin revolute; apex acuminate or acute; abaxial surface with indumentum 1- or 2-layered, densely compacted, white to fawn tomentose, sometimes with loosely floccose brown upper layer, hairs dendroid, glabrescent; adaxial surface reticulate to bullate, glabrous; midrib deeply impressed adaxially; lateral veins 15–26-paired. Inflorescence dense, ca. 20-flowered, rachis 10–15 mm, tomentose. Pedicel ca. 0.9 cm, pilose, glandular; calyx lobes 5, 1–2 mm, small, triangular, sparsely glandular and hairy; corolla tubular-campanulate, pink to deeply crimson, rarely white, 3.5–4 cm, with 5 black-blotched basal nectar pouches and dark flecks; lobes 5, 1.2–1.7 cm, apex emarginate; stamens 10, unequal, 1.7–2.7 cm, filaments glabrous; ovary conoid, 4–6 mm, white-tomentose, sometimes also glandular; style ca. 3.3 cm, glabrous. Capsule cylindric, ca. 30 × 6 mm. Fl. May, fr. Aug.
Three varieties occur in China. The name used in Xizang, “da ma” (打马), refers to the medicinal use of the flowers in treating coughs.
Forests, thickets, slopes; 1500–3800 m. ?W Guizhou, S Xizang [Bhutan, N India, Kashmir, Nepal, Sikkim, Sri Lanka, N Thailand, N Vietnam].
