Plants and algae

Oncidium

Oncidium (dancing ladies, butterfly orchid) is a neotropical orchid genus of about 339 accepted species with pseudobulbs and showy sprays; DNA studies merged Odontoglossum and other genera into it, and it is widely grown as a cut-flower and pot orchid.

Onigiri (おにぎり)

Onigiri (おにぎり), also omusubi, is the Japanese hand-pressed rice ball, typically triangular, salted, nori-wrapped, and filled with umeboshi, salmon, or tuna mayo; a staple of bento and convenience stores with a history from Yayoi-era carbonized rice to the 1978 film-packaging era.

Onion

Onion (Allium cepa), bulb crop of the amaryllis family from central Asia, cultivated over 4,700 years; bulbing is day-length controlled; second only to tomato in global production.

Onobrychis

Onobrychis (sainfoins), Fabaceae genus published by Miller in 1754, native Europe to Mongolia, Arabia, Ethiopia; about 130–208 species; includes forage legume sainfoin O. viciifolia, non-bloating, tannin-rich.

Onobrychis viciifolia

Onobrychis viciifolia (common sainfoin) is a perennial Fabaceae forage legume native to Central and SE Europe, non-bloating due to condensed tannins, drought tolerant via a deep taproot, and under study as a protein-rich perennial pulse crop.

Ononis

Ononis (restharrow), Fabaceae genus of about 88 annual/perennial herbs and shrubs, Mediterranean-centered, monadelphous stamens, dimorphic anthers, used in medicine and as forage.

Onychium

Onychium (claw ferns) is a genus of 12 accepted terrestrial fern species in Pteridaceae subfamily Pteridoideae, native from NE tropical Africa through tropical and subtropical Asia to N Australia, sister to Actiniopteris, with submarginal continuous sori protected by a reflexed false indusium.

Operculina

Operculina Silva Manso is a pantropical genus of climbing vines in Convolvulaceae, type species O. turpethum, defined by an operculate capsule; includes medicinal jalapa and turpeth species.

Ophioglossaceae

Ophioglossaceae, the adder's tongue fern family (Martinov, 1820), are eusporangiate ferns with trophophore and sporophore frond segments, subterranean mycorrhizal gametophytes, and record chromosome counts up to 1440 in Ophioglossum reticulatum.

Ophioglossum

Ophioglossum, the adder's-tongue ferns (adder's-tongue), a cosmopolitan genus of about 50-64 simple-leaved ferns in Ophioglossaceae, known for the highest chromosome counts of any organism, 2n up to 1,260.

Ophrys

Ophrys (bee orchids) is a Mediterranean-centered orchid genus pollinated by sexual deception: flowers mimic female insects' looks and sex pheromones, inducing pseudocopulation by males; species counts range from nine to 400+.

Ophrys apifera

Ophrys apifera, known as the bee orchid, is a European orchid whose flower mimics a female bee; unlike most of its genus, it pollinates itself by wind.

Opuntia

Opuntia (prickly pear, cactus pear) is a genus of about 150 American cacti with jointed cladodes, glochids, and edible fruits, used for food, forage, and cochineal dye, and invasive in many regions.

Opuntia decumbens

Opuntia decumbens Salm-Dyck is a prickly pear cactus in the family Cactaceae, native from Mexico to Central America, first published in 1834 in the Hortus Dyckensis.

Opuntia leucotricha

Opuntia leucotricha, called nopal duraznillo in Spanish and Aaron's-beard prickly-pear in English, is a prickly pear cactus from the Altiplano of central Mexico that grows 3 to 4 m tall with white hair-like spines.

Orange (fruit)

Sweet orange (Citrus sinensis), a mandarin and pummelo hybrid fruit whose cultivars all descend from one ancestor via somatic mutation. Main varieties Valencia, navel, Hamlin, and blood orange; grown for fresh fruit and juice; citrus greening (HLB) cutting supply.

Orbea

Orbea Haw. (1812) is a genus of about 60 stem-succulent stapeliads in Apocynaceae, native to Africa and the Arabian Peninsula, with carrion-scented fly-pollinated flowers; type species Orbea variegata.

Orchid

Orchid (Orchidaceae): one of the largest flowering-plant families, over 700 genera and roughly 30,000 species on all continents except Antarctica; defined by bilateral flowers, dust-like mycorrhiza-dependent seeds, deceptive pollination, CITES protection, and major horticultural trade.

Orchid cultivation

Orchid cultivation is the horticultural practice of growing orchids, most of which are epiphytes or terrestrials, so they need chunky bark or moss instead of soil.

Orchid ecology and biogeography

Orchid ecology and biogeography is the study of where the orchid family grows and what threatens it; most of its 30,000 species are tropical epiphytes.

Orchid fleck dichorhavirus

Orchid fleck dichorhavirus (orchid fleck virus, OFV, Dichorhavirus orchidaceae) is a bipartite negative-sense RNA rhabdovirus causing chlorotic or necrotic spots in orchids worldwide, transmitted and replicated in Brevipalpus false spider mites.

Orchid mycorrhiza

Orchid mycorrhiza is the symbiosis between orchids and orchid mycorrhizal fungi (OMF), mainly rhizoctonias such as Tulasnella and Ceratobasidium, required by all orchids for seed germination, forming pelotons, and supporting autotrophic, mycoheterotrophic, and mixotrophic adult nutrition.

Orchid nothogenera and intergeneric hybrid genera

Orchid nothogenera are single-word names for hybrids between two or more orchid genera, such as × Doritaenopsis, governed by the ICN and registered with the RHS.

Orchid phytochemistry

Orchid phytochemistry is the study of the chemical compounds produced by orchids, including phenanthrenes, alkaloids, and vanillin; orchids are the plant kingdom's richest source of phenanthrenes.

Orchid pollination

Orchid pollination is the study of how the more than 29,000 orchid species transfer pollen in packaged pollinia, often deceiving pollinators instead of offering nectar rewards.

Orchid propagation

Orchid propagation is the human-managed reproduction of orchids by sterile seed culture, clonal micropropagation, and vegetative division, back-bulbs, and keikis, replacing natural germination that depends on mycorrhizal fungi.

Orchidaceae

Orchidaceae (orchids), one of the two largest flowering plant families with about 760 genera and 26,000 species, cosmopolitan except Antarctica, over 75% epiphytic, defined by resupinate zygomorphic flowers, a labellum, fused column, pollinia, and fungus-dependent microseeds.