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|genus=Oncidium
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Oncidium (Greek, a tubercle; alluding to the crest on the labellum). Orchidaceae. Epiphytal orchids for greenhouse growing.

Pseudobulbs usually present, wanting in a few species, 1-2-lvd., with sheathing Lvs. at the base: Lvs. plane, terete or triangular: petals like the dorsal sepal but often much larger; lateral sepals either free or partially united; labellum variable, but never with its base parallel to the column (Odontoglossum), spreading nearly at right angles to the column; column short, winged.—A broad genus with over 300 species distributed in Mex., Cent, and Trop. Amer., and in the W. Indies. In range of altitude the genus extends from the hot coast regions to elevations of 12,000 ft. in the Andes. The fls. of this genus show a remarkable diversity of form. In O. varicosum, O. tigrinum and related species, the labellum is greatly developed, forming the most conspicuous part of the fl., while in O. serratum and O. macranthum it is inconspicuous. The sepals and petals vary in size in relation to each other and to the rest of the fl. A remarkable example is O. Papilio, in which the petals and dorsal sepal have been transformed into linear-erect segms., recalling, on a large scale, the antennae of some insect. The general habit of the plants is no less variable than the fls. They range in size from small erect forms, scarcely 6 in. in height (O. pumilum), to those resembling 0. altissimum, with immense climbing panicles 9-12 ft. high and covered with numerous medium-sized fls. The prevailing color of the fls. is yellow, spotted and barred with brown. White or rose-colored fls. occur in a few rare examples (O. incurvum, O. ornithorhynchum).

As a class, oncidiums are short-lived under cultivation. Few growers succeed in maintaining them in good condition for any great length of time. The stock is constantly renewed from the tropics.

Cultivation of oncidiums.

The genus Oncidium embraces a great number of species which are found growing under such peculiar and varied conditions in their native homes that imitation of the same is usually impracticable and often quite impossible. A fair degree of success, however, may be obtained by careful observation and distribution of the exceptionally difficult species among the several orchid departments.
The Sarcoptera section, which embraces such species as O. Cavendishianum, O. Lanceanum, O. luridum, 0. pulvinatum and others of similar structure, and the O. Papilio section, with O. ampliatum, may be successfully grown in a bright warm portion of the cattleya department in small baskets suspended from the roof, using for a compost a mixture of clean chopped peat and sphagnum moss, freely interspersed with lumps of broken charcoal. O. cucullatum, O. incurvum, O. macranthum, O. ornithorhynchum, O. Phalaenopsis, O. varicosum, with a few others of like nature, do well under treatment similar to that given for odontoglossums, which see.

When a good collection of species is cultivated, a large number, including many of the above, can be readily grown in one house if it be especially adapted to them. Such a house should be a span-roof structure of east and west exposure, at an angle of about 40°, which will admit the longest possible light. The early morning and late afternoon sun striking the glass at right angles produces and prolongs the natural sun heat for a greater part of the day, while at midday, when the outside temperature is highest, the sun's rays strike the glass obliquely, giving less heat, with little danger of the plants becoming sunburned from lenses in the glass. Oncidiums require more sun and air than most orchids.

The benches may be of either wood or stone, and should be covered an inch or two in depth with sand, ashes or gravel. The benches and paths should be wet down once or twice daily to insure a moist atmosphere. Ventilators should be arranged on both sides of the roof: air may then be freely admitted without causing direct drafts on the plants by using the ventilators on the sheltered side. In winter the temperature should range from 50° to 55° F. at night and 60° to 65° by day, or a few degrees more with sun heat and ventilation. In summer it must be kept as low as outside conditions will permit. From March until October, shading must be applied to the glass sufficiently heavy to keep down the temperature without excluding indirect solar influence. A good shading is made of turpentine and white lead; it stands well and is easily removed. It can be applied rapidly with a whitewash brush on a long pole, and removed with a hard brush in the fall.

Oncidiums may be grown in either pots or baskets, but as many species are of rambling habit, the latter are preferable. The tiny species, such as O. Limminghii, do best on blocks with little compost beneath them. Clean chopped peat fiber and live sphagnum moss, equal parts, make the best general growing material, and this should be liberally interspersed with broken pieces of charcoal. 
The plants in all cases must be securely fastened, and the compost must be pressed in moderately firm, but should be used sparingly. Overhead syringing should be given frequently, about once or twice a day in bright weather, but care must be taken not to keep the compost too wet, for the roots are liable to decay: it is advisable to let them dry out occasionally.

Stock is increased by division or notching the rhizome between the pseudobulbs just before the growing season, allowing about three or four pseudobulbs to each piece and separating the parts after the first growth is matured.
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{{Taxobox
| color = lightgreen
| name = ''Oncidium''
| image = Oncidium incurvum.jpg
| image_caption = ''Oncidium incurvum''
| regnum = [[Plant]]ae
| divisio = [[Flowering plant|Magnoliophyta]]
| classis = [[Liliopsida]]
| ordo = [[Asparagales]]
| familia = [[Orchidaceae]]
| subfamilia = [[Epidendroideae]]
| tribus = Maxillarieae
| subtribus = Oncidiinae
| genus = '''''Oncidium'''''
| genus_authority = [[Sw.]]
| subdivision_ranks = [[Species]]
| subdivision = 
See text.
}}

'''''Oncidium''''' is a [[genus]] that contains about 330 [[species]] of [[orchids]] from the subfamily [[Epidendroideae]] of the orchid family ([[Orchidaceae]]). This is a complex, difficult genus, with many species being reclassified. Calls are made for splitting this genus into multiple genera.