Nodding Orchids, Shepherd’s Crook Orchids
Deciduous terrestrials with flattish, subterranean or partly emergent pseudobulbs and broad pleated leaves that are distinctly stalked. All of the leaf stalks on a pseudobulb are enclosed together by several tubular bracts. Inflorescences are unbranched and characteristically nod in bud and flower, straightening and elongating as the capsules develop. The flowers are relatively small, crowded and in the native species remain semi-tubular with the segments not spreading widely. The labellum, which is stiffly but flexibly attached to the apex of the column foot, is 3-lobed with large lateral lobes, a short midlobe and a ridged or keeled callus. The column is short and broad with a short column foot.
Similar Genera
Significant Generic Characters
Deciduous terrestrial orchids; stems pseudobulbous, emergent or subterranean, multinoded; leaves, stalked, plicate, the petioles on a pseudobulb all enclosed together in a series of tubular bracts; inflorescence from a basal node on a new growth, racemose, nodding in bud and flower, straightening and elongating in fruit; flowers small, lasting several days; sepals and petals free, similar; labellum stiffly but flexibly attached to the apex of the column foot; lamina 3-lobed; lateral lobes large, erect; midlobe short, broad; callus ridged or keeled; column short, with a short foot; pollinia 2, sessile on a short stipe.
Size and Distribution
A genus of about 20 species widely distributed in tropical and subtropical areas with a single non-endemic species, Geodorum densiflorum, occurring in tropical and subtropical parts of Australia, in New South Wales extending south to the Macleay River (about 31º S). State occurrence: Western Australia, Northern Territory, Queensland, New South Wales.
Ecology
Geodorum densiflorum is widespread in a range of habitats including rainforest, especially monsoonal vine thickets, rainforest margins, open forest, heathland and grassland, usually in well-drained soil, sometimes in sites that are seasonally moist.
Biology
Pollination: Pollination in Geodorum densiflorum is variable. In most areas the flowers are pollinated by native bees but in parts of the Northern Territory autogamous flowers are the norm.
Reproduction: Reproduction in Geodorum is solely from seed. Seed dispersal takes 2-4 months from pollination and after fertilisation the peduncle straightens and the capsules develop in a pendant position. Apomixis is unknown in the genus.
Seasonal Growth: Plants of Geodorum densiflorum are deciduous and grow during the spring and summer months, becoming deciduous in autumn and winter.
Flowering: Geodorum densiflorum flowers in summer (wet season).
Hybrids: Natural hybrids involving Geodorum densiflorum are unknown.
Fire: Geodorum densiflorum often grows in fire-prone habitats. Plants are generally deciduous during the fire season and mostly survive fires, although some damage can result to exposed pseudobulbs
Derivation
The name Geodorum is derived from the Greek ge, earth and doron, gift, apparently in reference to the terrestrial growth habit.
Botanical Description
Perennial geophytic herbs, deciduous, sympodial. Plants glabrous. Roots filamentous. Stems pseudobulbous, subterranean or partially emergent, multinoded. Trichomes absent. Leaves 3-5 per shoot, deciduous annually, plicate, stalked, the petioles of all leaves forming a pseudostem and enclosed together in 2-4 common sheathing bracts. Venation unknown. Flowering and non-flowering plants monomorphic. Inflorescence racemose, arising from a node on a developing new shoot, multiflowered. Peduncle longer than the rhachis, the distal part nodding in flower, straightening in fruit, with few-several tubular or semi-tubular sterile bracts. Rhachis shorter than the peduncle. Floral bracts narrow, scarious, partially sheathing the base of the pedicel. Pedicel short, merging with the ovary. Ovary short, straight. Flowers resupinate, small, crowded, lasting 2-several days, opening sequentially, white, pink or reddish, pedicellate, scentless. Perianth segments free, thin to fleshy, porrect to spreading. Dorsal sepal free, similar to the lateral sepals. Lateral sepals free, similar to the dorsal sepal. Petals subsimilar to the sepals, often asymmetric. Labellum free, stiffly but flexibly attached to the apex of the column foot, markedly dissimilar in size and shape to the sepals and petals, ecalcarate. Labellum lamina 3-lobed, deeply saccate at the base (see pseudospur); lateral lobes large, erect; midlobe short, porrect. Spur absent. Callus consisting of a broad plate or flat keel. Nectar unknown. Column porrect from the end of the ovary, short and broad, straight, lacking free filament and style, fleshy. Column wings vestigial, terminal. Column foot short, at a steep angle to the column base. Pseudospur formed by the column, column foot and large lateral lobes. Anther terminal, incumbent, 2-celled, persistent, smooth, with a short blunt rostrum. Pollinarium present. Pollinia 2, grooved, hard, waxy, sessile. Stipe very short. Viscidium large. Rostellum small, entire. Stigma entire, large, concave. Capsules dehiscent, glabrous, pendant; peduncle elongated in fruit; pedicel not elongated in fruit. Seeds numerous, light coloured, winged.
Taxonomy
Geodorum shares many morphological features with Eulophia but is readily distinguished by the nodding inflorescence. The development of a pseudostem and the common enclosure by tubular bracts of all the petioles on a growth is also a notable feature.
Notes
Several authors have noted the need for a revision of Geodorum and the identity of the Australian species has proved to be controversial. Here the native taxon is treated as Geodorum densiflorum (Lam.) Schltr., but it is by no means certain that this application is correct. This taxon was collected from the Malabar Coast of India but there is no type specimen in existence and it is represented by a plate prepared by the collector and now treated as the type (for a reproduction of this plate see Seidenfaden 1983). This plate is a reasonable match for Australian material except that the depicted flowers show spreading segments, perhaps due to artistic licence (see also plate 723, Mathew 1982 in which the labellum is quite different from Australian material). There is no doubt that the Australian material is a close match to specimens from New Guinea and New Caledonia and appears also to match material from the Solomon Islands and Polynesia. The earliest available name for plants from this region is Geodorum pacificum Rolfe but more research is needed to determine whether this name should be applied to the Australian taxon. Studies of specimens have generally proved to be difficult (see Garay & Sweet 1974, Seidenfaden 1983, Kores 1989) and largely inconclusive and it is apparent that a detailed molecular study of material of known provenance is required.
Nomenclature
Geodorum Jackson, Bot. Repos. 10: t.626 (1811). Type species: Geodorum citrinum Jackson.
Infrageneric Taxa: No infrageneric treatment of Geodorum is available.
References
Dockrill, A.W. (1967). Australasian Sarcanthinae. The Australasian Native Orchid Society, Sydney.
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.
Garay, L. A. & Sweet, H. R. (1974). Orchids of the Southern Ryukyu Islands. Cambridge, Massachussetts.
Kores, P.J. (1989). A precursory study of Fijian orchids. Allertonia 5(1): 157-159.
Matthew, K.M. (1982). Illustrations on the Flora of the Tamilnadu Carnatic, vol. 2. The Rapinat Herbarium, Tiruchirapalli, India.
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.
Seidenfaden, G. (1983). Geodorum, in Orchid Genera in Thailand. Opera Botanica 72: 47-64.