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Herbs or rarely vines, perennial, rarely annual, strongly mycotrophic, epiphytic, terrestrial, lithophytic, or rarely aquatic or subterranean, usually green and photosynthetic, some without chlorophyll and saprophytic. Roots subterranean or aerial, tuberoid or stolonoid, usually with spongy, multilayered velamen. Stems erect or pendent or modified into creeping rhizomes, simple or sympodially or monopodially branched, delicate to stout, or thickened as corms or pseudobulbs, or greatly reduced, sometimes proliferous (especially diverse in sympodial orchids). Leaves solitary, several, or reduced to scales, basal or cauline, alternate, distichous, or sometimes opposite or whorled, either convolute or duplicate, simple, sessile or petiolate; stipules absent; blade articulate or not, plicate or conduplicate, cylindric, triangular, or laterally flattened, margins entire. Inflorescences terminal or lateral, racemes, spikes, panicles, or rarely cymose, erect or variously pendent, 1–many-flowered, lax or dense, flowering successively or simultaneously. Flowers bisexual [rarely unisexual], epigynous, resupinate or not, pedicellate or sessile, 3-merous, usually bilaterally symmetric [rarely nearly radially symmetric], with abscission layer between pedicel and peduncle, rarely between ovary and perianth or ovary and pedicel; perianth of 6 tepals in 2 whorls, all petaloid or sepals sometimes greener and more foliaceous in texture; sepals alike or not, lateral sepals often connate (forming synsepal), or all 3 sepals variously connate and/or adnate or distinct and/or free; petals 3, median petal modified as lip, commonly larger or differing in form and color, lateral petals commonly but not always similar to sepals; nectaries of various sorts; extrafloral nectaries sometimes present on pedicels, bracts, or leaf sheaths; stamens usually 1–2(–3, if 3 the 3d modified into sterile staminode), all on side opposite lip, fully or partially adnate to style, forming column; pollen grains in monads or tetrads, usually in 2–8 pollinia, sometimes subdivided into small packets, rarely granular, sometimes pollinia with caudicles and/or stipes; gynoecium 3-carpellate, connate, forming compound, inferior, 1-or 3-locular ovary; style variously adnate to filaments; stigmas usually 3-lobed, concave to convex, part of median stigma lobe modified into rostellum, often separating anther from fertile portions of stigma, commonly preventing or in some cases facilitating self-pollination; ovules numerous, anatropous, minute. Fruits capsules, opening (dehiscing) by longitudinal slits, rarely fleshy and indehiscent berries. Seeds numerous (millions in some species), minute; endosperm absent.
Perennial, but sometimes short-lived, terrestrial, epiphytic, or lithophytic, autotrophic or rarely mycotrophic herbs (or rarely scrambling vines), with rhizomes, tubers, or rootstocks with mycorrhizal fungi in roots. Stems either sympodial or monopodial, usually leafy, but leaves sometimes reduced to bractlike scales, 1 or more internodes at base often swollen to form a "pseudobulb"; epiphytic species with aerial, photosynthesizing adventitious roots, often bearing 1 or more layers of dead cells (velamen). Leaves 1 to many, alternate or occasionally opposite, often distichous, sometimes terete or canaliculate, glabrous or very rarely hairy, frequently fleshy or leathery, base almost always sheathing, sometimes articulated, sometimes forming a false petiole, margin entire, apex often emarginate. Inflorescence basal, lateral, or terminal, erect to pendulous, racemose, spicate, subumbellate, or paniculate, 1-to many flowered, flowers rarely secund or distichously arranged. Flowers small to large, often quite showy, usually zygomorphic, very rarely ± actinomorphic, bisexual [very rarely monoecious and polymorphic], sessile or pedicellate, most often resupinate with pedicel and ovary twisted through 180°, occasionally not twisted or twisted through 360°. Ovary inferior, 1-locular, placentation parietal (or rarely 3-locular and placentation axile). Sepals usually free but sometimes variously adnate, median (dorsal) one often dissimilar to laterals, laterals sometimes adnate to a column foot to form a saccate, conic, or spurlike mentum. Petals free or rarely partly adnate to sepals, similar to sepals or not, often showy; lip entire, variously lobed or 2-or 3-partite, ornamented or not with calli, ridges, hair cushions, or crests, with or without a basal spur or nectary, margins entire to laciniate. Column short to long, with or without a basal foot, occasionally winged or with lobes or arms at apex or ventrally; anther mostly 1, less often 2 or 3, terminal or ventral on column, caplike or opening by longitudinal slits; pollen usually forming distinct pollinia, less often loose, pollinia 2, 4, 6, or 8, mealy, waxy, or horny, sectile or not, sessile or attached by stalks (caudicles or stipes) to 1 or 2 sticky viscidia; stigma 3-lobed, mid-lobe often modified to form a rostellum, other lobes either sunken on ventral surface of column behind anther or with 2 lobes porrect. Fruit a capsule, rarely berrylike, usually opening laterally by 3 or 6 slits. Seeds very numerous, dustlike, lacking endosperm, rarely winged.
Terrestrial, epiphytic, epilithic or climbing, sometimes mycoheterotrophic, perennial herbs; arising from a creeping rhizome, annually renewed tuber or thickened rootstocks or rhizome; roots often with a spongy layer of velamen. Growth monopodial or more commonly sympodial. Stems sometimes swollen into pseudobulbs, sometimes bearing adventitious or aerial roots. Leaves simple, glabrous or occasionally hairy, often fleshy, usually alternate, often distichous, rarely opposite, well-developed and green or sometimes reduced to scales, usually sheathing at the base (sheath usually closed and encircling the stem), usually not petiolate. Inflorescence terminal or lateral, spicate, racemose, paniculate or flowers solitary; flowers bracteate and often resupinate. Flowers zygomorphic, usually bisexual, or rarely unisexual in which case plants polygamous or monoecious. Perianth of 6 segments in 2 whorls, free or variously fused; outer segments often similar, imbricate or valvate; inner segments usually unequal, median petal modified to form the labellum which often differs greatly in size and shape and may bear calli, hairs, fringes or other appendages. Nectary sometimes contained within a spur or sac from the labellum, or rarely from the dorsal sepal. Stamens 1 or 2, usually fused partly or completely to the 2 lateral staminodes and the style to form the 'column'; anthers 2-locular; pollen usually aggregated into mealy or waxy pollinia. Ovary inferior, 1-celled with 3 parietal placentae, rarely 3-celled. Fruit mostly a dehiscent capsule usually dehiscing by 3 or 6 longitudinal slits, or rarely fleshy and indehiscent. Seeds numerous and minute, without endosperm, embryo not differentiated, requiring infection by suitable mycorrhizal fungus to germinate and/or grow. See also Du Puy et al. (1993: 516) and Green (1994: 529).
Perianth epigynous, composed of 6 petaloid segments (tepals) in 2 whorls, or the outer whorl calyx-like and the inner corolla-like, or the outer rarely corolla-like and the inner minute, free or variously connate in each whorl; outer segments (sepals) imbricate or subvalvate, the middle segments of each whorl generally different in size and colour from the lateral ones, especially the middle petal which is often extremely complicated in structure and is termed the lip or labellum; the basal part of the labellum, the hypochile, is often articulated to the base of the column or is much constricted, when it is termed the claw; the middle part, the mesochile and the apical part, the epichile, may be variously lobed and often bear outgrowths. On account of the twisting of the ovary through 180°, the labellum is often placed in an abaxial position; frequently the labellum or more rarely the odd sepal is prolonged into a sac or spur, sometimes very long
Stylar and filamentous tissue forming a long or short column, with or without a basal foot, occasionally winged or with lobes or arms at apex or ventrally; anther one (or rarely two or three in extra African taxa), terminal or ventral on column, with a concave anther cap or opening by longitudinal slits; pollen in tetrads, agglutinated into discrete masses called pollinia; pollinia mealy, waxy or horny, sectile or not, 2, 4, 6 or 8, sessile or attached by caudicles, a stipes or stipites to one or two sticky viscidia; stigma 3-lobed, the mid-lobe often modified to form a rostellum, the other lobes either sunken on the ventral surface of the column behind the anther or with two lobes porrect
Perianth epigynous, of two whorls of three segments; outer perianth whorl (sepals) usually free but sometimes variously adnate, the median (dorsal) often dissimilar to the laterals, the laterals sometimes adnate to the column foot to form a saccate, conical or spur-like mentum; inner whorl comprising two lateral petals and a median lip; petals free or rarely partly adnate to sepals, similar to sepals or not, often showy; lip entire, variously lobed or two or three-partite, ornamented or not with calli, ridges, hair cushions or crests, with or without a basal spur or nectary, margins entire to laciniate
Stamens 2 or 1; stamens and style united to form a special structure (column), the apex of which may be produced vertically into stelidia or laterally into wings, and the base of which may be produced downwards to form a foot; anther or anthers 2-locular, introrse, opening by a slit lengthwise; often operculate, i$e$ can be lifted like a little cap; pollen granular or generally agglutinated into mealy, waxy or bony masses (pollinia); at one end the pollinium may be extended into a sterile portion (caudicle); the pollinia may be free in the anther-loculi or more or less loosely united
Ovary inferior, 1-locular with 3 parietal placentas or very rarely 3-locular with axile placentas, usually produced at the apex to form the column; stigmas 3 fertile, or more frequently the lateral 2 fertile, the other sterile and transformed into a small outgrowth (rostellum) which lies between the anther and the stigmas; a portion of the rostellum is sometimes modified into a viscid disk or disks (viscidia) to which the pollinia are attached, often by a stalk or stipes
Leaves glabrous or occasionally hairy, entire except at the apex in some cases, alternate or occasionally opposite, often distichous, frequently fleshy and often terete or canaliculate, almost always with a basal sheath which frequently sheaths the stem, sometimes articulated at the base of the lamina and sometimes with a false petiole
Stems usually leafy, but leaves often reduced to bract-like scales, one or more internodes at the base often swollen to form a pseudobulb; aerial, assimilating adventitious roots, often bearing one or more layers of dead cells called a velamen, are borne in epiphytic species
Flowers small to large, often quite showy, hermaphrodite or rarely monoecious and polymorphic, sessile or variously pedicellate, most often twisted through 180 degrees, occasionally not twisted or twisted through 360 degrees
Perennial, terrestrial, epiphytic or saprophytic herbs with rhizomes or tuberous roots or rootstock; stem leafy or scapose, frequently thickened at the base into pseudobulbs and bearing aerial assimilating roots
Perennial, terrestrial, saprophytic or epiphytic herbs or rarely scrambling climbers, with rhizomes, root-stem tuberoids or rootstocks with mycorrhizal fungi in the roots and often elsewhere
Inflorescences erect to pendent, spicate, racemose or paniculate, one to many-flowered, basal, lateral or terminal, the flowers rarely secund or distichously arranged
Flowers bracteate, hermaphrodite or very rarely polygamous or monoecious, zygomorphic; inflorescence spicate, racemose or paniculate, or flowers solitary
Leaves undivided, alternate and often distichous, rarely opposite, sometimes all reduced to scales, often fleshy, sheathing at the base
Fruit a capsule, opening laterally by 3 or 6 slits; seeds numerous, dust-like, lacking endosperm, sometimes markedly winged
Seeds very numerous, minute, often drawn out at each end, or rarely winged, without endosperm; embryo not differentiated
Ovary inferior, unilocular and the placentation parietal, or rarely trilocular and the placentation axile
Fruit usually a capsule, mostly opening laterally by 3 or 6 longitudinal slits
Growth either sympodial or less commonly monopodial
Ovules very numerous and minute
