Oberonia is a highly characteristic genus, easily recognised by the laterally flattened leaves and the terminal rat tail-like inflorescences with generally tiny flowers. In New Guinea the only other genus with this combination of characters is Hippeophyllum. This differs in that the column is relatively slender and the lip is 3-lobed with the mid-lobe abruptly bent downward. Moreover, Hippeophyllum has distinct creeping rhizomes, which is exceptional in Oberonia, being found only in some species of sect. Otoglossum. Species of Octarrhena are sometimes mistakenly identified as Oberonia spp., since they are superficially similar, but Octarrhena is easily distinguished by the lateral inflorescences, apart from floral details.
Most species of Oberonia are easily cultivated, but their ornamental value is limited since the flowers, apart from being tiny, are usualy rather dull coloured. As with many small-flowered orchids, however, the flowers are often truly exquisite when viewed in close-up.
Oberonia is badly in need of revision, many species are hard to identify and the sectional classification should be re-evaluated in the light of molecular evidence. While attempting to identify material of this genus we very often failed to make an unequivocal match with any described species. This may indicate that some species should be considered to be rather variable, but it undoubtedly also hints at the presence of a good number of undescribed species. It is likely that some widespread species will eventually be found in New Guinea, such as Oberonia equitans (G.Forst.) Mutel, O. imbricata (Blume) Lindl., O. monstruosa (Blume) Lindl. , and Oberonia padangensis Schltr., but at present we have not seen reliable records or material of these species from our area. The Fijian Oberonia heliophila Rchb.f. is very similar to several New Guinea species of section Scytoxiphium, and may well occur in New Guinea (it is listed as such in our checklist on vol. I). However, having compared material from Fiji with specimens from New Guinea we have not found an exact match. The species in this section, which contains the most robust members of the genus, are exceptionally difficult to identify, even by Oberonia standards.
Prior to Schlechter (1911), Oberonia was usually divided into two main groups: those with short stems (sect. Acaules) and those with elongated stems (sect. Caulescentes).
Schlechter proposed a more complicated, and perhaps more realistic system for the New Guinea species, with 2 subgenera: Menophyllum with leaves without articulation, and Apotemnophyllum with leaves with articulation, which classification is followed here. Almost all Oberonia species in New Guinea belong to subgenus Menophyllum, only 1 New Guinean species are of subgenus Apotemnophyllum: Oberonia cryptantha Schltr.
Subgenus Menophyllum was subdivided in 9 sections:
Subgenus Apotemnophyllum contained 1 section:
Schlechter did not provide a key and was rather vague about the diagnostic characters involved, which to some extent we had to surmise by examining the species he assigned to the various sections.
Key to the New Guinea sections of Oberonia
1a. Leaf blades not articulated with the sheaths. Stems very short to much elongated == Subgenus Menophyllum 2
1b. Leaf blades articulated with the sheaths. Stems very short == Subgenus Apotemnophyllum 11
2a. Stems elongated. Lip pandurate. Rachis, ovary and bracts pubescent == Adenorhachis
2b. Not this combination of characters == 3
3a. Stem almost absent. Lip at the base with long narrow teeth == Arachnochilus
3b. Not this combination of characters == 4
4a. Stem almost absent. Sepals and petals reflexed. Lip very fleshy, about rectangular in outline, often with emarginate apex == Scytoxiphium
4b. Not this combination of characters == 5
5a. Lip at the base relatively narrow, much widening towards the apex and there with two very broad lobes. Sepals and petals not reflexed == Platyacron
5b. Not this combination of characters == 6
6a. Stems rather short. Inflorescence with distinct elongated peduncle == Podostachys
6b. Stems rather short to much elongated. Peduncle very short == 7
7a. Stems much elongated. Flowers curved inwards towards the apex of the rachis. Basal auricles of the lip short == 8
7b. Stems rather short to much elongated. Flowers at right angles to the stem. Basal auricles of the lip short to elongated == 9
8a. Stems erect or patent. Leaves thick-leathery == Hymenobractea
8b. Stems pendulous. Leaves soft and rather thin-textured == Platystreptus
9a. Stems much elongated, with many relatively very short leaves. Lip with large basal auricles clasping the column, front margin denticulate. Petals narrow, like the sepals reflexed or revolute == Labidous
9b. Not this combination of characters == 10
10a. Lip at the apex with two tails or two narrow pointed teeth == Diurium
10b. Lip otherwise == Otoglossum
11a. Floral bracts relatively large, longer than the ovary, almost hiding the flowers == sect. Aphananthos
11b. Floral bracts inconspicuous, not longer than the ovary == sect. Oberonia
Oberonia contains c. 300 species, in New Guinea 87 species, 2 varieties and 1 hybrid:
Oberonia bifida var. brachyloba
Oberonia linearis var. brevipetala
Oberonia x hybrida