Dendrobium williamsianum Rchb.f., Gard. Chron. (1878, II) 652.
Type: Williams cult. (Goldie) s.n. (New Guinea).
Roots very many from rhizome and stem bases, to 50 cm long, 2-3 mm thick, white, smooth, occasionally branching. Rhizome creeping, 8-10 mm thick. Stems 1-2 cm apart, erect, few-leaved, 25-120 (-200) cm long, fusiform, slender at base, broadening to 8-10 mm thick in centre then tapering gradually to apex; internodes 1.5-6 cm long. Sheaths overlapping on lower stem, shorter than internode on upper stem, persistent. Leaves spreading, usually decurved towards leaf apex, oblong to elliptical, lower leaves largest, 7-12 by 2-3 cm, reducing to 2-3.5 by 1.2-1.5 cm at stem apex, moderately thick, tough, leathery; apex acute, bilobulate, the rounded lobules unequal. Inflorescences 1-several, from nodes near apex, suberect to arching, 15-35 cm long, with up to 10 flowers held horizontal in a moderately-spaced raceme on the upper third. Peduncle with 5 papery sheathing sterile bracts on lower half. Floral bract triangular, 5-7 mm long. Pedicel and ovary 2.4-3 cm long. Flower opening widely, to 4-7.5 cm across, petals and sepals held more or less flat in a horizontal plane over lip. Dorsal sepal oblong to narrowly elliptic, 2.3-3.2 cm by 11-13 mm, 7-veined, obtuse, apiculate. Lateral sepals oblong, 3.5-3.9 cm by 16-18 mm, 7-9 veined, obtuse, apiculate to mucronate. Mentum 9-16 mm long, straight, almost flat. Petals broadly obovate, 3.2-3.4 cm by 21-23 mm, rounded in front, tip often notched. Lip porrect, apex decurved, 2.6-3.2 cm by 26-33 mm, obovate or broadly elliptic in outline, ventrally channelled, not distinctly 3-lobed, sides erect, forward margins erose, apex rounded to apiculate; disc with low oblique ridges leading away from callus; 3 keels extend from a raised plate at the lip base and each terminate before the lip apex in a tall freestanding erose-margined oblong flap. Column 5 mm, apex laterally serrate with an awl-like upper tooth and a broad acute lower arm. Fruit ellipsoid, 2.5 cm by 18 mm, with 3 large protruding juga.
(after O'Byrne, 1994).
Colours: Sepals and petals white suffused with pale lilac to mauve with purple veins; lip purple with deep purple patches on the disc, base white, margins white or pale purple, keels lilac. Column and anther white with lilac streaks, the white column foot has deep purple patches.
Habitat: Epiphyte on smaller branches high in the canopy of the rainforest, also in small trees in forest patches in savannah areas. Altitude 50 to 300 m (all fide O'Byrne).
Flowering time in the wild: Flowering throughout the year. Flowers last up to 10 weeks.
Distribution: New Guinea (endemic).
Distribution in New Guinea: Papua New Guinea (Central Province only).
Notes: Dendrobium williamsianum is a very distinctive relative of the famous Dendrobium bigibbum, and is easily recognised by the high lamellae on the lip. The flowers are held in an almost horizontal position.
Cultivation: Requires light shade and high temperatures as well a strict resting period of several months.