This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Orchid Family. Perennial herbs, with corms, bulbs or tuberous roots, sheathing entire leaves, sometimes reduced to scales, the flowers perfect, irregular, bracted, solitary, spiked or racemed. Perianth superior, of 6 segments, the 3 outer (sepals) similar or nearly so, 2 of the inner ones (petals) lateral, alike; the third inner one (lip) dissimilar, often markedly so, usually larger, often spurred, sometimes inferior by torsion of the ovary or pedicel. Stamens variously united with the style into an unsvmmetrical column; anther 1, or in a few genera 2, 2-celled; pollen in 2-8 pear-shaped usually stalked masses (pollinia), united by elastic threads, the masses waxy or powdery, attached at the base to a viscid disk (gland). Style often terminating in a beak (rostellum) at the base of the anther or between its sacs. Stigma a viscid surface, facing the lip beneath the rostellum, or in a cavity between the anther-sacs (clinandrium). Ovary inferior, usually long and twisted, 3-angled, i-celled; ovules numerous, anatropous, on 3 parietal placentae. Capsule 3-valved. Seeds very numerous, minute, mostly spindle-shaped, the loose coat hvaline, reticulated; endosperm none; embryo fleshy.
About 430 genera and over 5000 species, of wide distribution, most abundant in the tropics, many of those of warm regions epiphytes.
Anthers 2; lip a large inflated sac. (Cypripedieae.)
Lip not fissured; stems leafy.
Lip fissured in front; leaves 2, basal.
Pollinia with a caudicle, which is attached at the base to a viscid disk or gland. (Orchideae.)
Gland enclosed in a pouch.
Sepals free; lip 3-lobed.
Sepals united above into a hood; lip entire.
Gland not enclosed in a pouch.
Lip not fringed nor cut-toothed.
Stem leafy; anther sacs mostly parallel.
Valves of the anthers dilated at the base enclosing the gland below. 5. Perularia.
Valves not dilated at the base.
Gland surrounded by a thin membrane.
Beak of the stigma with 2 or 3 appendages.
Beak of the stigma not appendaged.
Lateral sepals free; anther-sacs opening in front.
Bases of the lateral sepals adnate to the claw of the lip; anther-sacs
Stem scapiform; leaves 1-2, basal; anther-sacs divergent.
Basal leaves 2; ovary straight.
Basal leaf 1; ovary arcuate.
Lip fringed or parted and cut-toothed.
Pollinia not produced into a caudicle (except apparently in No. 25).
Pollinia granulose or powdery. (Neottieae.)
Flowers comparatively large, solitary or few; anthers incumbent on a long column.
Leaves not grass-like; lip free.
Flowers terminal; lip crested.
Flowers axillary; lip not crested.
Flower solitary; lip adherent to the base of the column.
Flowers racemose; lip free.
Flowers numerous, in spikes or racemes; anthers erect, jointed to a short column.
Anther operculate; leaves broad, alternate.
Anther not operculate.
Leaves green, borne on the stem.
Leaves alternate; spike mostly twisted.
Leaves 2, opposite; spike not twisted.
Leaves white-reticulated, basal.
Pollinia smooth and waxy. (Epidendreae.)
Plants with corms or solid bulbs, rarely with coralloid roots; leaves basal or cauline.
Leaves unfolding before or with the flowers.
Leaf cauline; lip ovate, or auricled at the base.
Leaf or leaves basal.
Leaves 2; lip flat; flowers racemed.
Leaf 1; lip saccate; flower solitary.
Leaf 1, basal, unfolding after the flowering time.
Flowers long-spurred; lip 3-lobed.
Flowers not spurred; lip 3-ridged.
Plants with coralloid roots, bulbless; the leaves reduced to scales.
Pollinia 4, in 2 pairs; flowers gibbous or spurred.
Pollinia 8, united; flowers not gibbous nor spurred.