Leaves simple, usually alternate, often distichous, rarely opposite, well-developed and green or sometimes all reduced to scales, often fleshy, usually sheathing at base, sheath usually closed and encircling stem, usually not petiolate.
Inflorescences terminal or lateral; spicate, racemose or paniculate or flowers solitary; flowers bracteate and often resupinate (with the ovary twisted to present the median petal lowermost in the flower). Flowers zygomorphic, bisexual or rarely unisexual and then plants polygamous or monoecious. Perianth of 6 ± petaloid segments in 2 whorls or the outer whorl sepaloid, free or variously fused in each whorl; outer segments ('sepals') often ± similar, imbricate or ± valvate; inner segments ('petals') usually unequal with laterals similar to each other and often to the sepals with middle petal (lower petal if flower resupinate) modified in structure and called the labellum. Stamens 1 [or 2], usually fused partly or completely to the 2 lateral staminodes and the style to form the 'column'; anthers 2-locular, introrse, dehiscing by longitudinal slits; pollen granular or agglutinated into mealy, waxy or bony pollinia; pollinium sometimes extended intoa sterile stalk (caudicle), pollinia free or loosely united. Ovary inferior, 1-locular with 3 parietal placentas [or rarely 3-locular with axile placentas]; stigma 3-lobed, part of the median lobe forming a sterile outgrowth (rostellum). Part of rostellum sometimes modified into viscid disc (viscidium) to which pollinia are attached; ovules numerous.
Fruit usually a capsule, mostly opening laterally by 3 or 6 longitudinal slits or rarely indehiscent; seeds numerous, minute, usually dust-like, without endosperm.
Orchids are important horticulturally. Many species and cultivars are widely grown for their bizarre beauty and for the cut-flower industry. Numerous nurseries specialize in the production of species, either by meristem culture or raising plants from seed, or specialize in producing artificial hybrids in order to achieve 'improvements' in flower size, shape and colour and the presentation of the flowers on the raceme. Many orchid species are potentially infertile as shown by the number of natural hybrids and the range of hybrids produced artificially. The hybrids are generally intermediate between the parents. Intergeneric hybrids are known to occur naturally between Caladenia and Glossodia.
| | Key to the genera | |
| 1 | Anther easily removed, cap-like, incumbent, bent downwards through 90–120° (except in Oberonia); plants mostly epiphytes or epiliths, or less commonly climbers or terrestrial herbs; tuberoids absent although roots or rhizomes fleshy in some terrestrial taxa; pseudobulbs often present; inflorescences lateral or terminal | 2 |
| Anther persistent, not cap-like, erect, parallel to the axis of the column or bent downwards through less than 90° ; plants growing in soil or very rarely humus-epiphytes or humus-epiliths; tuberoids often present; pseudobulbs absent; inflorescences terminal | 24 |
| 2 | Climbing plants with stems arising from the ground; seeds winged | 3 |
| Plants not climbing (sometimes creeping epiphytes appear to be climbing); seeds not winged
Back to 1 | 4 |
| 3 | Leaves ovate, 3–6 cm long; labellum with numerous transverse, parallel ridges on both sides of median ridge, not warty; column with a short, descending foot, tapering into the median ridge of the labellum | Erythrorchis |
| Leaves triangular, less than 2.5 cm long; labellum without ridges but with most of its surface densely warty; column footless
Back to 2 | Pseudovanilla |
| 4 | Plants with leaves reduced to scales | 5 |
| Plants with well-developed green leaves
Back to 2 | 9 |
| 5 | Plants epiphytic or epilithic; roots creeping over surface of host; flowers less than 5 mm long | 6 |
| Plants terrestrial; roots subterranean; flowers more than 5 mm long
Back to 4 | 7 |
| 6 | Stems creeping, consisting of globose pseudobulbs, connected by filiform rhizomes | Bulbophyllum |
| Stems almost vestigial, not creeping, not differentiated into pseudobulbs and rhizomes
Back to 5 | Taeniophyllum |
| 7 | Sepals and lateral petals fused into a tube; flowers not resupinate, the labellum being the uppermost perianth segment | Gastrodia |
| Sepals and lateral petals free; flowers resupinate
Back to 5 | 8 |
| 8 | Labellum with a prominent basal spur, free, glabrous | Epipogium |
| Labellum without a spur, fused to the base of the column, hairy on the upper surface
Back to 7 | Dipodium |
| 9 | Leaves pleated, thin; plant growing in soil | 10 |
| Leaves conduplicate or laterally flattened, thin to leathery or succulent; plant epiphytic, epilithic or rarely growing in soil
Back to 4 | 14 |
| 10 | Dorsal sepal 3–6 cm long; sepals and lateral petals white on the outside, brown on the inside | Phaius |
| Dorsal sepal 0.5–1.9 cm long; sepals and lateral petals not as above
Back to 9 | 11 |
| 11 | Labellum with a hollow spur protruding backwards, with a deeply 4-lobed lamina | Calanthe |
| Labellum lacking a hollow spur, with unlobed or obscurely 3-lobed or deeply 2-lobed lamina
Back to 10 | 12 |
| 12 | Inflorescence axis erect basally but abruptly curved over towards the tip at flowering time, terete; flowers pink; labellum shallowly saccate | Geodorum |
| Inflorescence axis completely erect, square in cross-section; flowers yellow-green to reddish purple; labellum not saccate
Back to 11 | 13 |
| 13 | Fleshy stems held above ground; lateral sepals free | Diteilis |
| Fleshy stems subterranean; lateral sepals partly connate along adjacent margins
Back to 12 | Empusa |
| 14 | Sepals and petals less than 1 mm long; leaves laterally flattened; flowers arranged in loose whorls in the inflorescence | Oberonia |
| Sepals and petals more than 1 mm long; leaves dorsiventrally flattened to terete; flowers not whorled
Back to 9 | 15 |
| 15 | Labellum not saccate, lacking a hollow spur, attached to the base of the footless column | 16 |
| Labellum saccate and/or possessing a hollow spur and/or attached to the tip of a prominent column foot
Back to 14 | 17 |
| 16 | Inflorescences lateral; sepals 4–10 mm wide | Cymbidium |
| Inflorescences terminal; sepals 1–3 mm wide
Back to 15 | Cestichis |
| 17 | Growth sympodial; stems often swollen to form pseudobulbs; lateral sepals basally fused to the prominent column foot to form a mentum | 18 |
| Growth monopodial; stems never swollen; lateral sepals free; column foot present or absent
Back to 15 | 19 |
| 18 | Stems either not differentiated into rhizome and pseudobulb, or if so differentiated then pseudobulb composed of more than just the most apical internode; each stem with 1–6 leaves at its apex | Dendrobium |
| Stems differentiated into rhizome and pseudobulb; pseudobulb composed of only the most apical internode; each pseudobulb with 1 leaf at apex
Back to 17 | Bulbophyllum |
| 19 | Roots strongly tuberculate, rough; sepals and lateral petals more than 10 times longer than broad, filiform; flowers lasting less than 2 days | Rhinerrhiza |
| Roots smooth; sepals and lateral petals less than 10 times longer than broad, not filiform; flowers lasting more than 2 days
Back to 17 | 20 |
| 20 | Labellum shallowly saccate, and sometimes with a solid, forward-pointing spur | 21 |
| Labellum with a prominent, hollow, backward-pointing spur
Back to 19 | 22 |
| 21 | Labellum with a hollow pouch at its front; inflorescence 1–25-flowered | Sarcochilus |
| Labellum with a hollow pouch at its rear; inflorescence 25–75-flowered
Back to 20 | Peristeranthus |
| 22 | Labellum spur lacking calli; labellum midlobe papillose | Papillilabium |
| Labellum spur with calli arising inside or overhanging its mouth; labellum midlobe not papillose
Back to 20 | 23 |
| 23 | Labellum spur without internal calli but thickened base of midlobe extended part way across the orifice of the spur in the form of a broad, 2-fid callus | Schistotylus |
| Labellum spur with a finger-like callus arising from the upper anterior wall and directed towards its apex
Back to 22 | Plectorrhiza |
| 24 | Inflorescence a dense capitulum with only the flowers and involucral bracts emerging above the soil surface, the peduncle and the rest of the plant entirely subterranean | Rhizanthella |
| Inflorescence racemose or spicate or 1-flowered, mostly or wholly above the soil surface
Back to 1 | 25 |
| 25 | Flowers not resupinate, the labellum positioned above the column | 26 |
| Flowers resupinate, the labellum positioned below the column
Back to 24 | 31 |
| 26 | Column almost as long as dorsal sepal | 27 |
| Column much shorter than dorsal sepal
Back to 25 | 29 |
| 27 | Column wings discontinuous, 4-lobed; labellum passively motile on basal hinge; leaves absent or 2—6 on a lateral shoot | Arthrochilus |
| Column wings continuous, the whole column deeply concave; labellum actively motile, elastic, irritable, upcurved when set, recurved when sprung; leaf solitary, basal
Back to 26 | 28 |
| 28 | Surface of labellum lamina smooth; column foot absent | Caleana |
| Surface of labellum lamina warty; column foot present, separating the points of insertion of the labellum and lateral sepals from those of the dorsal sepal and lateral petals
Back to 27 | Paracaleana |
| 29 | Leaves narrow-elliptic to narrow-ovate, lamina flat, petiolate, connected to rhizome separately from the inflorescence or absent; labellum several times broader than sepals and lateral petals; plant lacking tuberoids, with fleshy roots | Cryptostylis |
| Leaf linear, terete, tubular, enclosing inflorescence basally or rarely absent; labellum 1–2 times as broad as sepals and lateral petals; plant with paired tuberoids
Back to 26 | 30 |
| 30 | Labellum articulate on a conspicuous claw, mobile; free apical part of leaf flattened | Genoplesium |
| Labellum sessile at the base of the column, immobile; free apical part of leaf terete
Back to 29 | Prasophyllum |
| 31 | Stem differentiated into the erect, flowering shoot and succulent, creeping rhizome which is either rootless or which bears wiry roots at its nodes; tuberoids absent; stigma in 2 lateral parts, at the base of the deeply cleft rostellum | 32 |
| Stem not differentiated into erect and creeping parts (except in Adenochilus), not succulent; plants with tuberoids or less frequently with fleshy roots or lacking fleshy subterranean structures; stigma entire
Back to 25 | 33 |
| 32 | Petals fused to dorsal sepal, forming a hood over the column; lateral sepals free; roots present; rhizome not constricted | Zeuxine |
| Petals free; sepals fused for about half their length to form a tube; roots absent; rhizome conspicuously constricted at the nodes
Back to 31 | Cheirostylis |
| 33 | Lateral petals linear, 5–10 times narrower than lateral sepals; stigma sunk in a deep cavity that is partially covered by 2 flaps of the anther lid and a fleshy flap that is the lower margin of the stigma | Eriochilus |
| Lateral petals slightly broader to slightly narrower than lateral sepals; stigma not as above
Back to 31 | 34 |
| 34 | Flowering shoot leafless, but may have short acute leaf-like bracts | 35 |
| Flowering shoot with 1 or more prominently developed leaves
Back to 33 | 37 |
| 35 | Dorsal sepal and lateral petals adherent, forming a hood | Pterostylis |
| Dorsal sepal and lateral petals free
Back to 34 | 36 |
| 36 | Labellum densely covered by a beard of long, hair-like calli; column about a fifth as long as labellum | Calochilus |
| Labellum with knob-like calli; column about half as long as labellum
Back to 35 | Burnettia |
| 37 | Flowering shoot with a single, well-developed leaf that is much larger than the other, scale-like, foliar organs | 38 |
| Flowering shoot with 2 or more well-developed leaves of similar size
Back to 34 | 52 |
| 38 | Leaf cordate, heart-shaped or reniform, held horizontally | 39 |
| Leaf not cordate, ovate to linear
Back to 37 | 44 |
| 39 | Inflorescence consisting of a solitary, sessile flower; flower dominated by the labellum and hood-like dorsal sepal; lateral petals and sepals minute | Corybas |
| Inflorescence pedunculate, 1–many-flowered
Back to 38 | 40 |
| 40 | Leaf basal, ground-hugging | 41 |
| Leaf cauline, held horizontally above ground level
Back to 39 | 42 |
| 41 | Labellum unlobed, almost flat, pinkish brown; plant not turning black on drying | Cyrtostylis |
| Labellum shallowly 3-lobed, the lateral lobes erect, white with prominent red stripes; plant turning black on drying
Back to 40 | Pyrorchis |
| 42 | Flower predominantly white; labellum 3-lobed, with a dense, central band of numerous, yellow, clubbed calli; plant rhizomatous, lacking tubers | Adenochilus |
| Flower predominantly green to brown or purplish; labellum unlobed (but sometimes coarsely toothed), with a central plate-like or channelled callus and sometimes with 2 basal, gland-like calli; plant tuberous
Back to 40 | 43 |
| 43 | Leaf green on undersurface, often deeply lobed; labellum coarsely toothed, lacking distinct, basal, gland-like calli | Acianthella |
| 43 | Leaf with purple pigmentation on undersurface, entire; labellum entire, with two basal, gland-like calli | Acianthus |
| 44 | Leaf at least sparsely hairy | 45 |
| Leaf glabrous
Back to 38 | 48 |
| 45 | Labellum with only 1 or 2 basal calli | Glossodia |
| Labellum with more than 2 calli
Back to 44 | 46 |
| 46 | Tubers partially encased in a few-layered tunic; flowers predominantly white, pink, yellow, red, brown or green | Caladenia |
| Tubers entirely encased in a multilayered tunic; flowers predominantly blue
Back to 45 | 47 |
| 47 | Labellum prominently 3-lobed, with 2 central rows of white or yellow calli | Cyanicula |
| Labellum scarcely 3-lobed, with 4–6 rows of calli that are blue on the midlobe but white more basally
Back to 46 | Pheladenia |
| 48 | Leaf linear, terete, tubular, enclosing inflorescence basally; dorsal sepal less than 3 mm long; flowers uniformly green | Microtis |
| Leaf linear to ovate, flat or channelled; dorsal sepal more than 3 mm long; flowers variously coloured but rarely uniformly green
Back to 44 | 49 |
| 49 | Column not hooded nor collared; labellum 3-lobed, concave | 50 |
| Column hooded or collared (with a mitra); labellum not lobed, flat
Back to 48 | 51 |
| 50 | Inflorescence 1-flowered; column wings prominent, extending beyond the anther; plant lacking tubers | Adenochilus |
| Inflorescence usually 2–8-flowered; column wings reduced, not exceeding anther; plant tuberous
Back to 49 | Lyperanthus |
| 51 | Labellum usually densely covered by a beard of long, hair-like calli, or very rarely lacking calli but then column with 2 basal, lateral, dark glands; base of column with prominent ridged collar | Calochilus |
| Labellum lacking calli, similar to lateral petals; column lacking basal, lateral glands; top of column ornamented by prominent hood or collar
Back to 49 | Thelymitra |
| 52 | Lateral sepals much longer and narrower than dorsal sepal; leaves linear, basal | 53 |
| Lateral sepals similar in length and breadth to dorsal sepal; leaves mostly not linear, basal or cauline
Back to 37 | 54 |
| 53 | Lateral petals oblong and concealed under dorsal sepal | Orthoceras |
| Lateral petals consisting of a prominent claw ending in a rounded to ovate lamina, exposed
Back to 52 | Diuris |
| 54 | Sepals and lateral petals usually bright pink or rarely white, 4–5 mm long; column about as long as labellum; inflorescence many-flowered with flowers arranged in a spiral | Spiranthes |
| Sepals and lateral petals green or brown to reddish, sometimes striped with white, usually more than 5 mm long; column at least half as long as labellum; inflorescence 1–18-flowered, flowers not arranged in a conspicuous spiral
Back to 52 | 55 |
| 55 | Dorsal sepal and lateral petals adherent, forming a hood; lateral sepals basally fused; labellum usually irritable | Pterostylis |
| Sepals and petals free; labellum not irritable
Back to 54 | 56 |
| 56 | Inflorescence 6–18-flowered; leaves several, basal or cauline or both; labellum calli obscure or absent | Rimacola |
| Inflorescence 1-flowered; leaves 2, basal; labellum calli conspicuous
Back to 55 | Chiloglottis |
