1	Flower pedicellate; bract blades usually not mottled.		(2)
+	Flower sessile, bract blades usually mottled (except in T. pusillum).		(19)
2 (1)	Stigmas ± connate at least basally, or closely grouped and uniform, of uniform thickness throughout.		(3)
+	Stigmas distinct, often basally thickened or subulate.		(11)
3 (2)	Scapes usually 0.4–2 dm at anthesis.		(4)
+	Scapes usually 2.0–4.5 dm at anthesis.		(7)
4 (3)	Bract blade bluish green, apex round-obtuse; scapes 6-gonal in cross section.		6 Trillium nivale
+	Bract blade green, often with maroon undertones early, apex acuminate, blunt-acute, or rounded; scapes round in cross section.		(5)
5 (4)	Pedicel recurved below bracts at anthesis, or rarely leaning to ± horizontal distal to bracts; petals strongly arcuate-recurved from base, white, pink, or rose.		1 Trillium catesbaei
+	Pedicel ± erect at anthesis; petals erect-spreading to widely spreading, not recurved or recurved only in distal 1/2, white or white with madder purple flecks.		(6)
6 (5)	Bracts strongly petiolate, blade apex long-acuminate, weakly to strongly cordate basally, glossy, bluish green, sometimes with lighter silvery green pattern on or near main veins; sepals shorter and narrower than petals, apex rounded-apiculate; petals erect-spreading, ovate-cordate to ± orbicular, widest at or beyond middle, round-tapered distal to middle to apiculate-acuminate tip, margins mostly without undulations, white, usually sparsely to very heavily flecked with madder-purple, color not changing with age; Siskiyou Mountains of n California and Oregon.		10 Trillium rivale
+	Bracts very short-petiolate, subsessile, or sessile, blade apex obtuse, not glossy, green with maroon undertones; sepals about as large and prominent as petals, apex round to blunt; petals spreading-ascending, weakly recurved in distal 1/2, oblong to narrowly lanceolate, widest in proximal 1/3, margins strongly undulate, white, aging to deep rose abaxially; e U.S.		9 Trillium pusillum
7 (3)	Stigmas connate into style 2–6 mm; bract blades elliptic-ovate, ovate-lanceolate to lanceolate, usually 1.5–8 cm wide.		(8)
+	Stigmas barely connate basally, rarely completely distinct; bract blade ovate to rhombic, usually 8–20 cm wide.		(9)
8 (7)	Pedicel erect or only slightly leaning; petals erect to widely spreading, linear to narrowly elliptic, white at anthesis, darkening with age to light to medium deep pink, with inverted V-shaped, white base; bracts horizontal to drooping distally, blade lanceolate to lanceolate-ovate; stamens straight.		8 Trillium persistens
+	Pedicel leaning to almost horizontal or clearly recurved and declined; petals arcuate-recurved, oblong-lanceolate, white, pink, or rose, darkening to pink or nonfading with age; bracts and their margins often slightly raised, exposing flowers at anthesis, blade elliptic-ovate; stamens recurved.		1 Trillium catesbaei
9 (7)	Petals with distinct, dark red, inverted V-shaped mark basally; bracts strongly petiolate; ovary 3-lobed or -angled		14 Trillium undulatum
+	Petals uniformly white or pink, or with faint pink blush, no distinct, V-shaped marking; bracts subsessile, sessile, or short-petiolate; ovary 6-lobed or -angled.		(10)
10 (9)	Petals erect basally, flaring and recurving above middle, margins undulate-wavy, overlapping basally, rolled, forming tube that conceals ovary when viewed from above, obovate to lanceolate, rarely suborbicular, 4–7.5 × 2–4 cm, white or rarely pink, aging to pink-purple; e North America.		5 Trillium grandiflorum
+	Petals erect-ascending, widely spreading in distal 1/2, distal margins usually flat to weakly undulate to undulate, scarcely touching basally to overlapping and forming tube, not concealing ovary when viewed from above, linear to obovate, 1.5–7 × 1–4 cm, white, often with pink or blush markings, aging to deep rosy pink, purple, or dark red; w North America.		7 Trillium ovatum
11 (2)	Stigmas short, 2–4 mm, uniformly thick, recurved and appearing ± lobed adaxially, at least in dried specimens; bracts strongly petiolate, blade ovate-lanceolate, mostly less than 6 cm wide, weakly to strongly cordate basally.		10 Trillium rivale
+	Stigmas of various lengths, basally thickened, gradually tapered, erect distally, reflexed, or coiled, not lobed adaxially; bracts sessile to subsessile, blade elliptic, obovate, or rhombic, mostly more than 6 cm wide, gradually tapered, rounded or weakly cuneate basally.		(12)
12 (11)	Anthers ± equaling filaments, pale lavender-pink or -gray; petals oblong-lanceolate, recurved-reflexed from just above base, extending behind plane of sepal bases for more than 1/2 their length, white or rarely pale pink, thin-textured, adaxial veins inconspicuous.		2 Trillium cernuum
+	Anthers longer or shorter than, rarely equaling filaments, white, creamy white, yellow, purple, or maroon; petals ovate to obovate, spreading, flat, or, if recurved, in distal 1/2 only, red, maroon purple, rose, cream, or white, more rarely pale yellow, rarely 2-colored, heavy- or fleshy-textured, adaxial veins conspicuous, appearing engraved.		(13)
13 (12)	Ovary greenish white or white, occasionally with reddish purple stains at apex.		(14)
+	Ovary dark purple or maroon.		(15)
14 (13)	Pedicel strongly recurved beneath bracts; anthers thin, dark purple; petals ovate-elliptic, white, rarely 2-colored rose and white, apex acuminate; stigmas shorter than ovary; rhizome tapered to point distally.		11 Trillium rugelii
+	Pedicel erect, declined, or recurved; anthers thick, creamy white or yellow; petals ovate-lanceolate to broadly ovate, creamy white, not 2-colored, apex acute; stigmas ± equaling ovary; rhizome praemorse.		4 Trillium flexipes
15 (13)	Pedicel recurved or declined; flower held proximal to or below bracts.		(16)
+	Pedicel erect, or leaning from base, not recurved or declined; flower erect or dangling but not distinctly below bracts.		(17)
16 (15)	Pedicel declined, ± straight; stamens 1.5–2 times taller than small, ± globose to conical-pyramidal ovary; anthers maroon to grayish purple; petals 3–6.5 × 4–6 cm, usually maroon to brownish red, ovate-suborbicular, spreading to weakly recurved, longer than sepals.		15 Trillium vaseyi
+	Pedicel recurved beneath bracts; stamens ± equaling or not more than 1/3 longer than prominent, flask-shaped ovary; anthers dark purple; petals 2.5–5 × 0.8–3.5 cm, usually white or 2-colored, distally white, ovate to broadly ovate, recurved, ± equaling sepals		11 Trillium rugelii
17 (15)	Perianth open, usually flat; petals carried in ± same plane as sepals, or ascending slightly, spreading, not recurved, lanceolate to ovate, to 2+ times width of sepals; sepals flat to somewhat sulcate apically; flower odor fetid, of wet dog.		3 Trillium erectum
+	Perianth gaping, strongly 3-dimensional; petals erect-spreading or recurved in distal 1/2, ovate to ovate-orbicular, wider than sepals; sepal apex sulcate; flower odor musty or fruity.		(18)
18 (17)	Petals creamy white, 1.5 times sepal length, 4–7 × 1.5–4 cm, not recurved to weakly so at tip; pedicel not strongly reflexed, flowers facing ± upward, odor sweet, applelike; sepal apex mildly sulcate.		12 Trillium simile
+	Petals dark reddish maroon to purplish, less than 1.5 times sepal length, 1.8–5 × 1–3 cm, often recurved in distal 1/2; pedicel reflexed at tip; flowers facing outward at ± right angle to pedicel, odor funguslike; sepal apex strongly sulcate.		13 Trillium sulcatum
19 (1)	Stigmas connate basally into short style, threadlike, tips widely spreading; petals spreading from base with distal 1/2 flat or weakly ascending, exposing stamens and ovary, of short duration, margins undulate		9 Trillium pusillum
+	Stigmas distinct, erect, subulate with tips divergent-recurved, or linear and usually slightly thickened basally with tips recoiled; petals mostly erect from base, ± connivent, often at least partially concealing stamens and ovary, or erect-spreading, tips incurving slightly, long-lasting, sometimes spiral-twisted, margins entire.		(20)
20 (19)	Scapes decumbent or semidecumbent, or subterranean; bracts resting near or on ground, or with petioles arising from scape apex at or near ground surface.		(21)
+	Scapes erect, straight; bracts held well above ground, or bract tips touching ground only in early anthesis.		(23)
21 (20)	Scapes ± erect, largely subterranean; bracts long-petiolate, blade not mottled; individual bracts Plantago-like; w United States.		31 Trillium petiolatum
+	Scapes decumbent or semidecumbent, S-shaped, aerial; bracts sessile or only short-petiolate, blade heavily mottled; se United States.		(22)
22 (21)	Anther dehiscence extrorse; scapes distinctly puberulent especially just below bracts; petals linear-lanceolate or oblanceolate, 4–8 cm or more; filaments 2–5 mm.		21 Trillium decumbens
+	Anther dehiscence introrse; scapes glabrous; petals narrowly elliptic-oblanceolate, 2.5–5.5 cm; filaments 1–2 mm.		33 Trillium reliquum
23 (20)	Sepals strongly reflexed at bases, often below bracts and parallel to scape, or adpressed to bracts; anther connectives strongly curved inward; rhizomes horizontal, elongated, slender, very brittle, white.		(24)
+	Sepals not strongly reflexed, spreading or somewhat erect; anther connectives straight; rhizomes horizontal or erect, thick or somewhat compressed-thickened and superficially bulblike, not brittle, brownish.		(25)
24 (23)	Petals lanceolate-ovate, base attenuate to weakly clawed, ca. 2 times longer than wide; bracts petiolate.		32 Trillium recurvatum
+	Petals linear to narrowly spatulate, clawed, at least 4 times longer than wide; bracts sessile.		26 Trillium lancifolium
25 (23)	Petals white, rarely tinged pale pink or purplish near bases.		(26)
+	Petals maroon, reddish brown, bronze, yellow, or green.		(28)
26 (25)	Anther dehiscence introrse; ovary purple.		18 Trillium chloropetalum
+	Anther dehiscence latrorse; ovary green or greenish white, rarely purple at base.		(27)
27 (26)	Scapes usually 2.2–5.8 dm; petals oblanceolate to obovate, 4.8–8 cm, white to creamy white, rarely very pale pink or pinkish basally.		16 Trillium albidum
+	Scapes usually 1.7–3 dm; petals linear to linear-lanceolate, 2.2–4.5 cm, white, rarely purplish basally.		30 Trillium parviflorum
28 (25)	Petals yellow, pale yellow, greenish, or bicolored green with purplish base.		(29)
+	Petals usually maroon, reddish brown, pink, greenish maroon, purplish bronze, white, or brown throughout (except in pigment-free mutant individuals with petals pale green or yellowish green); occasionally yellow in T. cuneatum, T. chloropetalum, T. gracile, T. maculatum, and T. foetidissimum, occasionally 2-colored in T. ludovicianum.		(33)
29 (28)	Petal bases not clawed.		(30)
+	Petal bases weakly to strongly clawed.		(31)
30 (29)	Petals oblanceolate to lanceolate, apex long-acuminate; stamens 11–18 mm; anther connective extending 0.5 mm or less beyond sacs; stamens slightly longer than or ± equaling pistil; flower odor strongly of lemon; se United States.		28 Trillium luteum
+	Petals oblanceolate to obovate, apex variably acute to almost truncate; stamens 17–26 mm; anther connective extending 1–1.5 mm beyond sacs; stamens ca. 2 times as long as pistil; flower odor spicy, of rose; California.		18 Trillium chloropetalum
31 (29)	Petals widely spatulate, pale sulfur-yellow fading to ± cream, with broadly cuneate basal claw, claw green, sometimes obscurely marked with purple; apex apiculate, nipplelike in at least 1 petal.		22 Trillium discolor
+	Petals narrowly spatulate to linear-spatulate, green, bicolored, green with purplish base, rarely all purple, obscurely to clearly narrow-clawed basally, claw purple or green; apex round-acute to obtuse, lacking nipple.		(32)
32 (31)	Bract (under magnification) with numerous stomates across entire adaxial surface; pistil 2/3 stamens; anther dehiscence introrse-latrorse; petals green or yellowish green distal to weak purplish claw (very rarely purplish green throughout); flower odor of rotten fruit.		37 Trillium viride
+	Bract adaxially without stomates or with stomates near apex only; pistil 1/2 stamens; anther dehiscence latrorse; petals green or yellowish green distal to distinct purplish claw (rarely, petal purple throughout); flower odor spicy or musty.		38 Trillium viridescens
33 (28)	Anther dehiscence extrorse; stamens not obscured by petals; petals horizontal, with 1–2 spiral twists.		35 Trillium stamineum
+	Anther dehiscence introrse or latrorse; stamens at least partially obscured by petals; petals erect to divergent and slightly spreading with tips ± incurved, not spirally twisted.		(34)
34 (33)	Anther dehiscence latrorse, occasionally introrse basally with age.		(35)
+	Anther dehiscence introrse.		(38)
35 (34)	Anther connectives extended 1–2 mm beyond sacs; margins of distal 1/3 of bract blade straight to apex.		(36)
+	Anther connectives extended scarcely if at all to 0.5 mm beyond sacs; margins of distal 1/3 of bract blade convex-curved to apex.		(37)
36 (35)	Scape 1.5 times longer than bracts; bract tips at anthesis often touching ground; petals 3–4 times longer than wide.		36 Trillium underwoodii
+	Scape 2.5–3 times longer than bracts; bract tips at anthesis held well away from ground; petals ca. 2 times longer than wide.		20 Trillium decipiens
37 (35)	Petals 0.9–2.7 cm wide, oblanceolate to elliptic-obovate, narrowed to cuneate but not clawed base, erect.		19 Trillium cuneatum
+	Petals 0.4–0.8 cm wide, linear-oblanceolate, weakly clawed basally, erect-spreading.		27 Trillium ludovicianum
38 (34)	Stamens ca. 2 times as long as pistil.		(39)
+	Stamens equaling, less than, or very slightly longer than pistil.		(40)
39 (38)	Petals oblanceolate to obovate, 6.5–10 × 1.5–2.5 cm; anther connective extended 1–1.5 mm beyond sacs; California.		18 Trillium chloropetalum
+	Petals linear-elliptic to oblanceolate, 2–4 × 0.3–0.8 cm; anther connective extended 0.1–1 mm beyond sacs; e Texas and w Louisiana.		24 Trillium gracile
40 (38)	Anther connective not extended beyond sac, or, if extended, rarely to 0.5–1 mm.		(41)
+	Anther connective extended 1 mm or more beyond sac.		(42)
41 (40)	Scape 1.4–4 dm; bracts 7–15 × 4.6–6.7 cm; petals narrowly oblanceolate-spatulate to linear-spatulate, spreading-erect, becoming connivent-erect with age, 4–7 × 0.7–1.7 cm, clear dark red-maroon, reddish purple, or rarely yellow above purplish base or clear sulfur yellow, distal margins upraised from about widest point to near petal base; stigmas 2–4 mm; se United States.		29 Trillium maculatum
+	Scape 2.5–5.5 dm; bracts 11–18 × 12–17 cm; petals oblanceolate, spreading-erect, 5.5–11 × 2–3.5 cm, glossy dark maroon-red or purple, margins ± flat, widest at or below middle, cuneate basally; stigmas 6–8 mm; n California and extreme sw Oregon.		25 Trillium kurabayashii
42 (40)	Anther connective extended 2–5+ mm beyond sac.		34 Trillium sessile
+	Anther connective extended ca. 1–1.5 mm beyond sac.		(43)
43 (42)	Scapes 2.5–6 dm; petals linear, erect, 5–10 cm, 8–10 times longer than wide, glossy, dark purple to red-purple, inrolling with age; bract blades 10–22 × 8.7–15 cm, obscurely and sparsely or not at all mottled or spotted darker greenish brown, subsessile; c, s California.		17 Trillium angustipetalum
+	Scapes 0.8–2.8 dm; petals narrowly elliptic to linear-lanceolate, weakly incurved, 2–5 × 0.3–0.5 cm, not glossy, pinkish purple, light to dark reddish or brownish purple, rarely yellow, not inrolling with age; bract blades 6.7–12 × 3.8–6 cm, strongly mottled, sessile; e Louisiana, sw Mississippi.		23 Trillium foetidissimum