Evergreen terrestrials with thin fibrous sparsely branched stems with a few relatively broad, thin-textured, pleated leaves. Congested unbranched inflorescences with crowded, white, greenish or brown flowers arise at the stem apex. The flowers are upside-down with the bases of the lateral sepals and petals fused together and the basal margins of the labellum fused with the column. The labellum is unlobed, broadest at the base and with a deep basal spur.
Significant Generic Characters
Evergreen terrestrial orchids; roots thin, wiry, unbranched, with small fleshy tubercles; stems thin, wiry, fibrous, unbranched or sparsely branched; leaves narrow to relatively broad, plicate, sessile, non-articulate, sheathing at the base; inflorescence a condensed terminal raceme; flowers non-resupinate, crowded; bases of lateral sepals and petals fused together, subsimilar; labellum fixed to the base and proximal margins of the column; lamina unlobed, with a broad basal spur and short apex; callus consisting of small callosities; column short, narrow.
Size and Distribution
A genus of about 22 species distributed in North America, Central America, India, Malaysia, Philippines, Indonesia, New Guinea and Australia where there is a single endemic species, Tropidia territorialis, known from a few sites in northern parts of the Northern Territory, including Melville Island, and another, Tropidia viridifusca, on Norfolk Island that also occurs in New Caledonia and Vanuatu. State occurrence: Northern Territory, New South Wales (Norfolk Island).
Tropidia territorialis grows in deep shade in evergreen monsoon forests developed on surface springs or where there is subsoil moisture.
Pollination: The flowers of both Tropidia territorialis and Tropidia viridifusca are insect-pollinated but the vector is unknown.
Flowering: Both species flower in summer (wet season).
Hybrids: Natural hybrids involving Tropidia territorialis are unknown.
Fire: In some sitesTropidia territorialis occurs in fire-prone habitats and it is believed that after fires the plants reshoot from subterranean organs. Fire does not play a role in Tropidia viridifusca.
The name Tropidia is derived from the Greek tropideion, keel; reference to the boat-shaped labellum found in some species.
Perennial geophytic herbs, evergreen, sympodial. Plants glabrous. Flowering and non-flowering plants monomorphic. Roots filamentous, wiry, unbranched, with fleshy mycorrhizal tubercles. Rhizome superficial. Stems erect, emergent, unbranched, wiry, fibrous, multinoded, the basal nodes with closely sheathing scale-like bracts, distal nodes each bearing a leaf. Trichomes absent. Leaves few per shoot, more or less distichous, plicate, non-articulate, lasting many years, thin-textured, coriaceous, sheathing at the base. Venation simple, parallel. Inflorescence a condensed terminal raceme. Peduncle much shorter than the rhachis, thin, wiry, sparsely bracteate. Rhachis longer than the peduncle, thin, wiry, curved. Floral bracts relatively large, foliaceous. Pedicel short, merging with the ovary. Ovary straight, not twisted, narrow. Flowers non-resupinate, crowded, small to relatively large, white, greenish or brown, lasting a few days, opening sequentially in a spiral sequence. Perianth segments narrow to broad, subsimilar. Dorsal sepal free, similar to the lateral sepals. Lateral sepals connate proximally, with a basal gibbosity that encloses the labellum base, similar to the lateral sepals. Petals connate proximally with the bases of the lateral sepals. Labellum fixed to the base and proximal margins of the column, the basal part parallel to and closely embracing the column, markedly dissimilar in size and shape to the sepals and petals, calcarate. Labellum lamina unlobed; base broad and saccate, with a bilobed spur; apex short, narrow, entire or emarginate. Spur present (see labellum lamina). Callus consisting of proximal swellings. Nectar unknown. Column at an angle from the end of the ovary, short, narrow, straight, lacking free filament and style, fleshy. Column wings erect. Column foot absent. Pseudospur absent. Anther dorsal, contiguous with the column, 2-celled, persistent, with a long rostrum. Pollinarium present. Pollinia 2, narrow, grooved, sectile, attached by short caudicles to a common caudicle. Viscidium small, narrow. Rostellum erect, elongate, bifid. Stigma terminal, broad. Capsules dehiscent, glabrous, porrect; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.
Tropidia was originally recorded from Australia as Tropidia curculigoides Lindl. (Jones and Dunlop 1988) but examination of the types shows significant departure from the Australian material. The type collection of Tropidia curculigoides (Lindley 1840) consists of 2syntypes in the Lindley herbarium, one from mountains in Sylhet [now in northern Bangladesh] and the other from Zeylona (now Sri Lanka). The former specimen consists of a single stem with an immature terminal inflorescence. The second specimen consists of 2 flowering stems and associated sketches of the flowers and floral parts. Two obvious features from these specimens are the large leaves (to 20 x 2.5cm) with very long-acuminate to attenuate apices (Lindley records them as " acuminatissima" ) and the peduncles covered with imbricate bracts. By contrast the Australian plants have shorter leaves (to 12 x 2cm) with very short, subacute apices and the peduncles are bractless. Additionally the plants of Tropidia curculigoides exhibit lateral inflorescences whereas the racemes are exclusively terminal in the Australian plants. Comber (1990) illustrates a species from Java as Tropidia curculigoides which is clearly incorrect as the ovaries are covered with scurfy brown scales (Lindley when describing Tropidia curculigoides clearly states " all parts glabrous" and there is no indication of scales on the ovaries he sketched). Three further similar Javan species described by Blume (1859), viz. Tropidia graminea, T. pedunculata and T. squamata, are also distinct from the Australian material, the former having narrow, 3-veined, ovate-lanceolate leaves with long-acuminate apices, T. pedunculata having broad, 7-veined, elliptic leaves with acuminate apices and T. squamata having elliptic leaves, papillate ovaries and different floral characters. Significantly no species illustrated by Schlechter (1982) is a match for the Australian plants. It seems apparent from examination of the literature and herbarium specimens that Tropidia is a specialised group that has been used as a botanical dumping ground and the genus is well overdue for a comprehensive revisionary treatment.
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.
Jones, D.L. and Dunlop, C.R. (1988). The genus Tropidia Lindley recorded from Australia. Orchadian 9(6): 129-131.