Plants 14-80 cm. Leaves 1-4, wide-spreading to ascending, scattered along stem, gradually to abruptly reduced to bracts distally; blade linear-lanceolate, linear-oblong, or oblong-obovate, 7-23 × 0.7-5 cm. Spikes lax to dense. Flowers resupinate, not showy to inconspicuous, whitish green or yellowish green, lip often paler; lateral sepals reflexed; petals linear to narrowly oblong-spatulate, margins entire or apex rarely slightly lacerate; lip descending, without basal thickening, deeply 3-lobed, 10-17 × 13-17 mm, lobes extensively and deeply divided, commonly into filiform divisions, margins rarely merely fringed or middle lobe merely lacerate-fringed, lateral lobes narrowly to broadly cuneate, middle lobe cuneate-flabellate, usually narrow; spur slenderly to markedly clavate, 11-23 mm; rostellum lobes nearly parallel, directed downward, short, rounded; pollinaria nearly straight; pollinia remaining enclosed in anther sacs; viscidia linear; ovary slender to stout, mostly 8-20 mm. 2n = 42. Flowering May--Aug. Sphagnum bogs, alluvial and swamp forests, stream banks, riparian meadows, sand flats, moist and seeping slopes, prairies, roadside banks, ditches, old fields, borrow pits; 0--1200 m; Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que.; Ala., Ark., Conn., Del., D.C., Ill., Ind., Kans., Ky., Maine, Md., Mass., Mich., Minn., Miss., Mo., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tex., Vt., Va., W.Va., Wis. Hybrids with Platanthera grandiflora (P. ×keenanii P. M. Brown; see note under 20. P. grandiflora) and P. psycodes [P. × andrewsii (M. White) Luer] are variably intermediate between the parents, with various purple-green shadings and lobes of the lip generally narrower and more dissected than in the purple-flowered parents. The linear viscidia of P. lacera and the large anther sacs of P. grandiflora provide means for determination. The ratio of viscidium length/width of 1-3.2 distinguishes hybrids and purple-flowered parents from P. lacera, which is characterized by a ratio of 2.7-4.6. The parentage of hybrids can be determined by different sized anther sacs: P. ×andrewsii, 1.4-2.3 mm; P. ×keenanii, 2.5-4.2 mm. Those hybrids can be distinguished from their purple-flowered parents by less intense coloring and the ratio of viscidium length/width: P. psycodes 1-1.5; P. × andrewsii, 1.3-3; P. grandiflora, 1-1.5; P. × keenanii, 1.5-3.2. The plants known as Platanthera lacera var. terrae-novae (Fernald) Luer (Habenaria lacera var. terrae-novae Fernald) apparently represent swarms of P. × andrewsii and introgressed P. lacera (P. M. Catling and V. R. Catling 1994).