Buttercup Orchid
Epiphytes or lithophytes with very distinct rhizomes separating the pseudobulbs, anchored by roots that arise from nodes on the rhizomes. The pseudobulbs are hard, cane-like and elongated, usually thickened in the middle. The leaves, which occur on nodes in the upper two-thirds, are longer than wide, flat, without any channel or groove, basally sheathing and with an unequally notched apex. The flowers are borne on short racemes that arise from the upper nodes of a pseudobulb. Each flower lasts a few days, is moderately small and firm-textured. The perianth segments are flat and the bases of the lateral sepals are fused with the column foot. The petals are much narrower than the sepals. The labellum, which is stiffly attached to the apex of the column foot, is distinctly 3-lobed, with a broad shallowly notched midlobe, prominent central ridges and lacks a spur.
Similar Genera
Significant Generic Characters
Epiphytic/lithophytic orchids; plants appressed; rhizomes creeping; roots arising from rhizome nodes; pseudobulbs hard, cane-like, elongated, usually thickened in the middle, growing over two seasons; leaves lasting several seasons, occurring on nodes in the upper two-thirds of the pseudobulb, longer than wide, flat, with an unequally notched apex; racemes short, from the upper nodes; flowers moderately small, firm-textured; perianth segments flat; lateral sepal bases fused with the column foot; petals much narrower than the sepals; labellum stiffly attached to the apex of the column foot; labellum lamina three-lobed; midlobe broad, shallowly notched; callus with prominent central ridges.
Size and Distribution
A genus of about 12 species distributed in Indonesia, New Guinea, Solomon Islands and northeastern Queensland where there is a single endemic species. The Australian species, Trachyrhizum agrostophyllum, is restricted to the ranges and tablelands between Mt Finnigan (15º50' S) and Paluma (19º S) near Townsville. State occurrence: Queensland.
Ecology
Trachyrhizum agrostophyllum occurs on ridges and slopes growing on rocks and the trunks and branches of rough-barked or flaky-barked trees in rainforest, wet sclerophyll forest and beside streams in open forest, mostly above about 700 m alt. Plants usually occur in situations of bright light and with free air movement. The climate is tropical and the majority of rain falls during the summer wet season (December to March), with the remaining months much drier and having sporadic or intermittent rain, particularly localised coastal showers.
Biology
Pollination: The flowers of Trachyrhizum agrostophyllum, which last several days, are insect-pollinated but the vector is unknown.
Reproduction: Reproduction in Trachyrhizum agrostophyllum 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: The plants grow actively during the spring and summer months and are relatively quiescent for the remainder of the year. Each growth takes about 2 years to mature.
Flowering: Flowering occurs in spring.
Hybrids: Natural hybrids involving Trachyrhizum agrostophyllum in Australia are unknown.
Derivation
Trachyrhizum, which is derived from the Greek trachys, rough and rhizos, root, refers to the rough or warty surface of the roots of some species.
Botanical Description
Perennial, evergreen, epiphytic or lithophytic herbs, sympodial. Plants glabrous. Roots elongate, produced from nodes on the rhizome. Rhizome well developed, appressed to the host, branched. Pseudobulbs well-developed, widely spaced, hard, elongate, thickened in the middle, when young covered by scarious bracts. Trichomes absent. Aerial growths produced from the upper nodes. Leaves lasting several seasons, distichous, occurring on the distal two-thirds of the pseudobulb, sessile, much longer than wide, moderately thick, coriaceous, smooth, not grooved or channelled; base sheathing the pseudobulb; margins entire; apex unequally emarginate. Inflorescence racemose, short, porrect to arcuate, arising from an apical node on a mature pseudobulb, few-flowered. Peduncle about as long as the rhachis, the base with imbricate scarious bracts. Floral bracts scarious, small, subtending the base of the pedicel. Pedicel relatively long, thin, merging with the ovary. Ovary short, straight or curved. Flowers resupinate, stalked, lasting a few days, yellow. Perianth segments fleshy, waxy, widely spreading, entire. Dorsal sepal free, subsimilar to the lateral sepals; apex entire, flat. Lateral sepals subsimilar to the dorsal sepal, attached by their bases to the column foot; apex entire. Petals free, smaller than the sepals; apex entire. Labellum 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 quadrangular, fleshy, strongly three-lobed; lateral lobes moderately large, erect, flanking the column, entire; midlobe broad, porrect to upcurved; apex deeply notched, the lobes spreading or erect. Callus consisting of narrow parallel ridges. Nectar absent. Column lacking free filament and style, fleshy, much shorter than the perianth segments, nearly straight. Column wings present, reduced, ventral and with short tooth-like apical stelidia. Column foot well developed, as long as the column, straight or curved. Spur absent. Anther terminal, incumbent, 2-celled, persistent, attached dorsally by a ligulate claw, smooth, erostrate; apex smooth. Pollinarium absent. Pollinia 4 in 2 pairs, straight or falcate, yellow or orange, hard, waxy. Viscidium absent. Rostellum ventral, swollen, transverse. Stigma entire, transverse, concave. Capsules dehiscent, large, glabrous, pendulous; peduncle not elongated in fruit; pedicel not elongated in fruit. Seeds numerous, relatively large, light coloured, winged.
Taxonomy
Trachyrhizum, originally described as a section within Dendrobium by Schlechter in 1912, was raised to generic rank by Brieger in 1981. The Australian species was transferred from Dendrobium to Trachyrhizum by Rauschert in 1983.
Nomenclature
Trachyrhizum (Schltr.) F.G.Brieger in Schltr., Orchideen 1(11-12): 687 (1981).
Type species: Dendrobium schlechteri Rauschert [Dendrobium trachyrhizum Schltr.].
References
Brieger, F.G. (1981). Subtribus Dendrobiinae. In F.G. Brieger, R. Maatsch and K. Senghas (eds), Rudolph Schlechter, Die Orchideen: ihre Bescreibung, Kultur und Züchtung, 3rd edn, Band 1, Teil A, Lieferung 11-12 (Paul Parey: Berlin and Hamburg).
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.
Schlechter, R. (1982). The Orchidaceae of German New Guinea (English translation by R.S. Rogers, H.J. Katz and J.T. Simmons). Australian Orchid Foundation, Melbourne.