Dendrobium geotropum T.M.Reeve, Orchadian 7 (1983) 183, fig. 1, 2, 3.
Type: Reeve 1044 (holo LAE; iso AMES, CBG, E, K, L, NSW).
A medium to large epiphytic herb growing downwards (geotropic), up to 60 cm long. Roots thick, flexuous, glabrous, up to 4.5 mm diameter. Rhizomes extremely short. Pseudobulbs thick, clavate with slender basal section somewhat moniliform, 5-27 by 0.8-3.0 cm, dilated apical section becoming shallowly, grooved with age, with approximately 8 nodes, when young covered with white scarious sheaths, 2-to 3-leaved at the apex (small plants may be 1-leaved). Leaves large relative to the stems, pointing downwards, occasionally spreading at the apices, ovate-elliptic to lanceolate, 6-24 by 1.8-5.5 cm, acute, glabrous, leathery. Inflorescences terminal, pendulous, often partly enclosed by the leaves, 3-12 flowered; peduncle 3-18 cm long; rachis 2-14 cm long. Bracts obovate-spathulate, concave, subacute c. 6-8 by 5 mm. Ovary with pedicel 10-18 mm long, ovary 6-grooved. Flowers c. 15-25 mm across, fleshy, glabrous, not usually widely opening, quite variable in size even on the same inflorescence, lasting several weeks at least, not scented. Dorsal sepal oblong-ovate, 1.4-1.9 cm by 8-9.5 mm, obtuse. Lateral sepals obliquely triangular, 1.5-1.9 cm by 10-12 mm, acute, at the base forming a blunt mentum 6-8 mm long. Petals rhombic-elliptic, 15-18 by 7-9 mm, acute. Lip hinged, slightly trilobate, oval in shape with the small sidelobes and midlobe margins incurved (subcordate when expanded), 1.3-1.4 cm by 7-9 mm, (up to 14 mm wide when expanded), with prominent median callus of 3 parallel ridges c. 7-7.5 by 3 mm. Column typical of section Latouria, except stigmatic area which is proportionately larger than most species, 4-4.5 by 4 mm; column foot 5.5-7 mm long; anther cucullate 2-2.5 by 2-2.5 mm; pollinia 4 in 2 pairs, c. 1.25 mm long. Fruit commonly seen in nature, c. 5-6 by 2.5 cm.
(after Reeve, 1983).
Colours: Roots white with greenish tips. Pseudobulbs yellowish brown to dark brown, shiny. Leaves mid- to dark-green sometimes suffused with purple. Sepals and petals pale green with outside of sepals sometimes spotted/suffused purple, lip dark maroon except for large white callus and the underneath midline which is green near the apex and whitish near the base. Column whitish with slight purplish colouration below stigma, foot green at base. Anther greenish-white; pollinia golden-yellow. Seeds orange-yellow.
Habitat: Epiphyte on tree trunks, growing upside down. Altitude 2000 to 2850 m.
Flowering time in the wild: February, March, October, November.
Distribution: New Guinea (endemic).
Distribution in New Guinea: Papua New Guinea (Southern Highlands, Enga, Western Highlands, and Morobe Provinces).
Map: GEOTRMAP.JPG [Dendrobium geotropum T.M.Reeve, map of distribution (original).
Notes: Dendrobium geotropum is similar in appearance to Dendrobium mayandyi but differs in the geotropic habit of the plant, in the larger non-spreading leaves, in the wider petals, the subcordate shape of the expanded lip and its 3-ridged callus. The shape of the flowers is similar to those of Dendrobium pachystele but D. geotropum differs in the much thicker stems which bear 2-3 leaves at the apex, in the longer terminal inflorescences, in the colour of the flowers and the shape of the lip.
(after Reeve, 1983).
Cultivation: Cool growing epiphyte. Requires shaded position and perfect drainage. Because of its downwards growing stems it should be mounted on a slab of bark or tree fern, or grown in a basket.