Feather Orchids
Epiphytes with very short almost indiscernible rhizomes anchored by roots which arise from nodes at the base of the pseudobulbs. The cylindrical, hard pseudobulbs appear to be cut-off square at the apex and the short, broad, thick, fleshy leaves are confined to the pseudobulb apex and arise more or less at the same plane. The flowers are borne on short thin racemes that mainly arise from the apical node of a pseudobulb, but are also sometimes produced from leafless lateral nodes. Each flower, which lasts several days, is relatively large, thin-textured and scented. The perianth segments are flat and the bases of the lateral sepals are fused with the column foot. The petals are generally narrower than the sepals. The labellum, which is distinctly 3-lobed, has large lateral lobes and a small midlobe. Its callus consists of 3 ridges, the central one strongly sinuate/convolute and crested on the midlobe.
Similar Genera
Significant Generic Characters
Epiphytic/lithophytic orchids; rhizomes almost indiscernible; pseudobulbs cylindrical, hard, multinoded, with an abruptly truncate apex; leaves arising more or less at the same plane on the apex, thick, fleshy, not much longer than wide, flat, not basally sheathing; racemes mainly from the apical node, sometimes from leafless lateral nodes, 1-12-flowered; flowers lasting several days, relatively large, thin-textured, scented; perianth segments narrow, flat; lateral sepal bases fused with the column foot; petals narrower than the sepals; labellum stiffly hinged to the apex of the column foot, 3-lobed; lateral lobes large, pointed; callus of 3 ridges the median one strongly sinuate/convolute and crested on the midlobe.
Size and Distribution
A genus of about 8 species, 5 endemic in Australia and 3 in New Caledonia. The Australian species are restricted to the east and are found between Mt Finnigan (15º49' S) near Cooktown in northeastern Queensland and Moruya (35º55’ S) in southern New South Wales. State occurrence: Queensland, New South Wales.
Ecology
The Australian species of Tropilis range from lowland situations to more than 1500 m alt. and occur in tropical, subtropical and temperate regions. They grow mainly on trees, rarely rocks, in situations ranging from shade to bright light, fluctuating humidity and free air movement. Habitats include rainforest, woodland and open forest.
Biology
Pollination: The flowers of the native species of Tropilis last several days and are pollinated by native bees.
Reproduction: Reproduction in Tropilis is solely from seed. Seed dispersal takes 4-6 months after pollination and the capsules develop in a pendulous position. Apomixis is unknown in the genus.
Seasonal Growth: Tropilis plants grow actively during the spring and summer months and are relatively quiescent for the remainder of the year.
Flowering: Flowering occurs in late winter-spring.
Hybrids: Natural hybrids involving species of Tropilis are unknown.
Derivation
Tropilis, which is derived from the Greek tropis, keel of a ship, apparently refers to the prominent keels on the labellum.
Botanical Description
Perennial, evergreen, epiphytic or lithophytic herbs, sympodial. Plants glabrous. Roots elongate, branched, produced from nodes on the base of the pseudobulb. Rhizome superficial, branched. Pseudobulbs well-developed, multinoded, crowded, hard, elongate, base narrow, main part mostly cylindrical, apex truncate, when young covered by scarious bracts. Trichomes absent. Aerial growths absent. Leaves few (1-4 per shoot), distichous, apical, arising more or less in the same plane, sessile, nearly as wide as long, thick, fleshy, coriaceous, smooth, flat, not grooved or channelled; base not sheathing the pseudobulb; margins entire; apex entire or unequally emarginate. Inflorescence relatively short, racemose, erect, arising mainly from the apical node, sometimes from leafless lateral nodes on the pseudobulb, 1-12-flowered. Peduncle shorter than the rhachis, thin, sometimes much reduced, with scattered scarious bracts. Floral bracts small, scarious, sheathing the base of the pedicel. Pedicel relatively long, thin, merging with the ovary. Ovary short, straight, porrect from the pedicel. Flowers resupinate, stalked, lasting many days, pale green, cream, white or apricot, sometimes ageing pink, scented; labellum with distinct purplish striae. Perianth segments narrow, thin, widely spreading, entire, flat, straight. Dorsal sepal free, subsimilar to the lateral sepals, straight; apex entire, flat. Lateral sepals subsimilar to the dorsal sepal, falcate, attached by their bases to the column foot; apex entire. Petals free, subsimilar to the sepals, narrower; apex entire. Labellum stiffly hinged to the apex of the column foot, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina more or less elliptical, relatively thin-textured, strongly three-lobed; lateral lobes large, erect, flanking the column, entire, pointed; mid-lobe short, porrect to recurved; apex entire or apiculate. Callus consisting of 3 narrow median parallel ridges, the central one becoming strongly sinuate/convolute and crested on the midlobe. Nectar absent. Spur absent. Column at an angle to the ovary, lacking free filament and style, fleshy, shorter than the perianth segments, nearly straight. Column foot longer than the column, at right angles to the column, curved. Pseudospur absent. Column wings reduced, ventral, with short tooth-like apical stelidia. Anther terminal, incumbent, 2-celled, persistent, attached dorsally by a ligulate claw, smooth, erostrate; apex smooth. Pollinarium absent. Pollinia 4 in 2 unequal pairs, straight or falcate, orange, hard, waxy. Viscidium absent. Rostellum ventral, small, transverse. Stigma entire, vertical, concave. Capsules dehiscent, glabrous, pendulous; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Tropilis has been included in Dendrobium but can be immediately distinguished by the very hard pseudobulbs, apical non-sheathing leaves lasting many seasons, inflorescences mainly arising from apical nodes and the labellum distinctly 3-lobed. Tropilis has also been confused with Thelychiton from which it can be distinguished by the truncate apex on its pseudobulbs, apical leaves arising more or less in the same plane, leaf base not sheathing the pseudobulb and callus being strongly sinuate/convolute and crested on the labellum midlobe.
Notes
Species of Tropilis often exhibit a high degree of host specificity. For example Tropilis aemula usually grows on ironbarked eucalypts, an unnamed species is confined to Brush Box (Lophostemon confertus) and another species to Forest Oak (Allocasuarina torulosa). At one site in the Numinbah Valley, Queensland, Tropilis aemula was observed growing in abundance on an old ironbark right beside a large Brush Box on which were growing numerous plants of the undescribed species. Seedlings and adult plants of both taxa were readily distinguishable morphologically but the host specificity of each species was strictly maintained.
Nomenclature
Tropilis Rafin., Fl. Tell. 2: 95 (1837). Type species: Tropilis aemula (R.Br.) Rafin.
Infrageneric taxa: No infrageneric taxa are currently recognised.
References
Clements, M.A. and Jones, D.L. (2002). Nomenclatural changes in the Dendrobieae (Orchidaceae) 1: the Australasian region. Orchadian 13(11): 485-497.
Dockrill, A.W. (1969). Australian Indigenous Orchids. Volume 1. The Society for Growing Australian Plants, Halstead Press, Sydney.
Dockrill, A.W. (1992). Australian Indigenous Orchids. Volume 1 & 2. Surrey Beatty & Sons in association with The Society for Growing Australian Plants, Chipping Norton, NSW.