Phil's Orchid Site - Genus review

This is the start of a series on the genus oncidium. It covers a discussion of the sections of the genus, with the particular cultural requirements applicable to each section. The individual pages will be added tot he site over a peirod of time as they become available.

This series was originally designed to provide information on habitats and to detail cultural information for this extensive and complex group of plants. Oncidiums were viewed on a wide basis as one group with the discussion in cultural groupings. While mention is made of the taxonomic changes and new genera created within the oncidium complex, with the emphasis on culture, it has been thought relevant to keep the wide grouping with the changes in genera etc referred to in the text. Culture issues are often the issues raised, and it is believed the discussion on the sectional groupings is the most informative way the requirements of the various sections can be best understood.

The material was originally written some years ago, but is still basically correct. Some updatign has occurred, and further updating will take place. The basic information presented is, however, considered to be still valid. Most of the changes invovle the separating of individual oncidium secitons into gneera of their own. Where this has occurred, the relevant pages will be noted. The overall affinity to the genus when viewed in its general form is, however, still relevant.

The series has been extensively researched from a considerable number of sources. The text is fully annotated, and significant further information can be gained from the references most lof wh ich sh ould still be available. The full list of references is shown on the references page.

I trust you find the information interesting and useful, and perhaps even encourage you to grow members from this interesting aggregation of plants. There are many that produce large spectacular flowers, many that are of easy culture, and many that can find a successful home in viertually any collection of orchids. Enjoy.

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THE GENUS ONCIDIUM includes plants producing brightly coloured attractive flowers from plants which generally are highly adaptable to culture under a wide range of conditions. They were one of the very first of the tropical epiphytic orchids introduced into European cultivation. They remain a popular and keenly sought after genus of orchids. They are desirable and suitable for not only those who are just starting to diversify their collection of orchids from the cymbidiums which most people commence their orchid growing career with in New Zealand, but also for those who already have a more diversified collection. Oncidiums are not 'fashionable' orchids today, but they still are plants that attract attnetion when grown well, and generally they are not difficult platns to grow. Flowers range from small to large in size, as does the size of the plants, and all collecitons of orchidaceous plants have room for at least some oncidiums, which are sure to please.

Oncidiums contain a wide variation of plant form, size, shape, colour and size of flowers, which makes them an extremely interesting group of orchids to grow. While yellow and brown colouration predominates, red and white flowers are also found amongst those plants being popularly grown. Growers, and even the botanists who have studied this aggregation of plants, have had difficulty in specifically delineating the genus in a way they all could agree. The same species at various times have been shifted around within the Oncidiinae subtribe, to and from such genera as miltonia, odontoglossum and brassia. Alex Hawkes, in his Encyclopaedia of Cultivated Orchids ¹ notes that the distinctions between these genera are slim, often the specific identification "involving rather tenuous floral characteristics". This affinity does, however, allow them to be widely used in Oncidiinae intergeneric breeding.

In reviewing a genus, botanists compile a general descriptI6n, and it is interesting to look at such a description for oncidiums as it provides information on its main characteristics, plus some measure of its diversity.

Oncidiums are primarily epiphytic plants with stem pseudobulbs of one internode, enclosed by sheaths, either as bracts or bearing leaves, in two rows on opposite sides of the pseudobulbs. The pseudobulbs are commonly terminated by one to three well developed leaves, although occasionally the pseudobulbs may be reduced in size, rarely almost aborted, then with a terminal appendage in lieu of a leaf. Leaves are commonly with a flat yellowish-green blade, rarely terete or triangular. The inflorescence is a raceme or panicle, rarely one flowered. Flowers vary in size from small to conspicuous, all perfect or rarely intermixed with aborted ones on the same inflorescence. All open simultaneously, rarely in succession. The flowers have spreading sepals and petals, commonly contracted at their base, sometimes wavy, of similar size, or with petals larger in size, the lateral sepals variously joined. The lip may be entire or lobed, commonly fiddle-shaped, firmly joined without articulation to the column, at an arching right angle. The disc base has small conical projections, crisped. The column is short, stout, often ovoid, commonly without distinct auricles on the sides of the stigma, and with a fleshy plate over the stigma.

The rostellum is either short or proboscus like. The stigma is more or less large. There are two pollinia which are waxy, which more or less show longitudinal grooves on the distant linear stipe, with a small viscidium. The anther is terminal, which falls away during pollination.

The genus oncidium is an extensive one. Nearly 400 species are listed by Garay and Stacy ⁽²⁾ with up to 750 odd species mentioned by some authors. Whatever is the true number, it is a large genus even by orchid standards. The species mainly come from Central America and northern South America, from a wide range of natural habitats. The wide environmental diversity of this region has allowed and encouraged the development of many species, many of which are distinctive and attractive. Some individual species grow over an extensive geographical range, others are limited to a specialised habitat of a few square kilometres only.

Oncidiums belong to the Vandoideae sub family, orchids which are considered in evolutionary terms the most highly evolved. They appear to be mostly bee pollinated, mimicry with pseudoantagonism playing an important part in the reproductive process. Some oncidiums, for example, mimic the flowers found on a vine, from which certain bees obtain oil. These bees have been seen to seize some oncidium flowers for an instant, leaving the oncidium flower when they find no oil, but pollinate other flowers when they make the same mistake again. In another example of pollination systems, some females of certain bee species are known to gather around certain flower clusters. The males tend to pick such flower clusters for their territories, defending this against other insect invaders. When the wind moves the inflorescence, the oncidium flowers apparently look enough like an insect in flight to arouse the aggression of the male bee, which attacks the flower. He strikes it and receives pollinia on his face; pollination takes place when this is repeated on another flower. This has been called "pseudoantagonism" or "pseudotrespassing". Oncidium hyphaematicum, planilabre and stipitatum are pollinated in this way. ²⁶ Further pollination/pollinator information is published by Van der Pijl and Dodson.²⁶

Most oncidiums have yellow and brown coloured flowers, with some white, and infrequently red. These colours are consistent with hymanoptera (bee and wasp) pollinated flowers. Clear red colouration is infrequent as most bees are blind to red, the few flowers of red colour often reflecting ultraviolet rays the bees can perceive.²⁶

The genus contains plants with wide variation in their 2n chromosome numbers, this playing an important part in cross breeding fertility, and may, when more details become known, be an important aspect in a re-evaluation of the whole genus. Many oncidiums are largely, or completely, self infertile, although artificial crossing with related genera such as odontoglossum, brassia, miltonia, etc. is possible. Naturally, individual species retain their purity by growing over a distinct geographical or altitudinal range, or by differing flower colour, form, callosities or scent, attracting different, and often very specific, insect pollinators.

It has been suggested that the genus oncidium should be broken up into a number of different smaller genera. However, while it is diverse, viewed overall breaking it up may be no more than convenience. Garay and Stacy² have suggested possible candidates for separation are the Sections Stellata, Waluewa, Rhinocerotes, Paucituberculata and Concoloria. The Oncidium (Variegata) Section, Plurituberculata and Cebolletae have also been suggested as candidates for separation by Williams and Dresler². The different physiological forms and chromosome numbers of some Sections have given rise to these suggestions. The completion of further detailed chromosome and other studies may confirm some division of the genus. The Hawaiian breeder W.W.G Moir who has completed much breeding work including oncidiums, also states that "it should be separated into many genera". Robert Dresler3 notes that the Plurituberculata and Cebolletae, with 2n chromosome counts of 28, 30, 32, 34 and 36, seem to be more closely related to Trichocentrum (2n 24, 28), and may be separated as the genus 'Lophiaris'. Also, closely allied to Lophiaris and to Rossioglossum (2n 44), is the Section Glanduligera (2n 38) for which the generic name 'Psychopsis' is available. Most species of "oncidium" have 2n 56. The Section Oncidium (variegata) from the West Indies have 2n 40, 42 and cross easily with loniopsis (46) and other members of the Comparettia complex comprising the genera Comparettia (42), Diadenium, Plectrophora, Aeokowhleria, Scelochilus and Rodriguezia (28, 42). Hybrids between the Section Oncidium species and the main 56 2n chromosome plants are usually sterile, and this has confirmed to some that the Oncidium Section plants should be removed from the known genus. Unfortunately the name "oncidium" is fixed to the West Indian Section Oncidium Species variegatum. The removal of the Section Oncidium, taking the generic name "oncidium" would require the renaming of the bulk of the species with 56 chromosomes left, although the name could possibly be conserved for the bulk of the genus. Already some species have been removed. The Kranzlin Iridifolia Section containing the short lived Oncidium pusillum has now been transferred to a new genus Psygmorchis (as P. pusilla). This Section Iridifolia was excluded because the leaves lack articulation (ie. abscission cells in the leaves forming a join), in view that the closely allied lockhartia with a similar lack of articulation had been excluded from oncidium for this reason earlier. Sweet⁴³ has also suggested that certain miltonia species should be transferred to the Oncidium Section Stellata, and indicated that the Section should also be given separate generic rank under the name 'Gymizodon'. Obviously from further studies botanists will either confirm or reject some of these currently held opinions, but whatever happens, we can be assured of further developments (and confusion) in the classification of those plants we now call "oncidium". In recent years the splitting of the original oncidium genus into separate genera has occurred and is continuing. For the purposes of this series the oncidium genus is considered in its widest form, and readers should note that some sections have been separated into their own genus, as will be noted in the particular sections.

It is to be emphasised that the information has been compiled from a number of sources which are listed on the reference page. When I became interested in oncidiums I found little information which related to the different types of plants included in the genus. A look at the vegetative forms indicated that there must be a considerable variation in the cultural requirements for some of the different groups, and my researches led to the original studies and eventually to this series.

The information has been related to New Zealand conditions, including my own growing experience but has wider application. Many of the Section plants are poorly represented in local collections, and it is hoped this work will stimulate more growers to diversify their oncidium collections. In this way a real expansion of growing knowledge may occur, with the information gained disseminated in due course. Because of the nature of the work, it is inevitable some errors will occur. Nevertheless it should provide a wide range of basic data which does not seem to be readily available except in many books and articles, many of which will not be readily available to most growers.