1	Fruits dry at maturity, sometimes tardily so, tan to gray, usually bearing spines and apical flanges; plants low growing, less than 0.5 m (except O. aureispina)		(2)
+	Fruits fleshy or juicy (bleeding) at maturity, green, yellow, orange, pink, red, or purple; flowers and fruits usually spineless, rarely bearing spines and without apical flanges; plants low growing to treelike, to 8 m		(10)
2 (1)	Shrubs or trees with short trunks, erect, to 1(-1.5) m; fruits and stem segments glabrous; spines well distributed on stem segments, usually 4-12 per areole, bright yellow to orange, aging tan to blackish, not gray; inner tepals yellow with orange to red bases; Brewster County, Texas		28 Opuntia aureispina
+	Shrubs low, forming clumps or mats, only 1-3 stem segments tall, trunkless; fruits and stem segments glabrous or minutely papillose (use hand lens); spines variously distributed on stem segments, 0-20 per areole, of various colors including gray; inner tepals yellow, yellow with red bases, apricot, pink, to magenta; widespread		(3)
3 (2)	Seeds subspheric, 6.5-7 mm thick, girdle projecting 0.5-1 mm; inner tepals pink to magenta (rarely white); filaments red-magenta (rarely pale); stigma lobes white to cream; stem segments and fruits minutely puberulent (rarely glabrous), spineless except var. treleasei (Kern County, California), with areoles bearing 0-3(-8) spines, stout, to 10 mm, pale yellow		29 Opuntia basilaris
+	Seeds flattened, 1.5-3.5 mm thick; girdle projecting 1-3.5 mm; inner tepals yellow to red-magenta; filaments white, yellow, or red-magenta; stigma lobes green; stem segments and fruits glabrous or minutely puberulent, usually spiny		(4)
4 (3)	Areoles in distal 1/3 of each stem segment bearing 0-4 spines, spines absent from basal 50-100% of areoles of segment; flowers and fruits spineless or nearly so; stem segments often minutely papillose (use hand lens); Arizona-Utah boundary region		(5)
+	Areoles in distal 1/3 of each stem segment bearing 6-20 spines, spines absent from 0-40% of stem segment; flowers and fruits very spiny to nearly spineless; fruits and stems glabrous; widespread		(7)
5 (4)	Areoles in distal 1/3 of stem segment bearing 0(-1) spines, spines absent from basal 95-100% of stem segment; fruits spineless		30 Opuntia aurea
+	Areoles in distal 1/3 of stem segment bearing 1-4 spines (spines absent from basal 50-80% of stem segment); fruits spiny		(6)
6 (5)	Areoles (4-)7-8 per diagonal row across midstem segment; glochids conspicuous; longest spines per areole (35-)50-70 mm; fruit areoles 16-34.		31 Opuntia pinkavae
+	Areoles 3-5 per diagonal row across midstem segment; glochids inconspicuous; longest spines per areole 8-24 mm; fruit areoles 12-22.		32 Opuntia fragilis (in part)
7 (4)	Stem segments 6-27 × 5-18 cm, obovate to circular (or, if elliptic, then stem segments longer than 9 cm), firmly attached		34 Opuntia polyacantha (in part)
+	Stem segments (1.5-)2-9 × 5 cm, elliptic to narrowly obovate, flattened to sometimes nearly cylindric, firmly attached or readily detached		(8)
8 (7)	Depressed spines at base of areoles 0-3, 1-3 mm; longest spines 8-24 mm; stem segments easily detached; widespread		32 Opuntia fragilis (in part)
+	Depressed spines at base of areoles 5-7, 4-20 mm; longest spines 20-60 mm; stem segments sometimes easily detached (in O. ×columbiana)		(9)
9 (8)	Spines in most areoles; fruit top-shaped to barrel-shaped, usually less than 2 times longer than wide; lower Snake and Columbia River basins		33 Opuntia ×columbiana
+	Spines in all or only distal areoles of stem segment; fruit cylindric, often 2 times longer than wide; e New Mexico and w Texas		34 Opuntia polyacantha (in part)
10 (1)	Plants with stem segments spineless or bearing only vestigial spines (sometimes taxa have spineless individuals that cannot be safely identified without flowers and/or fruits---avoid sterile specimens or those from unhealthy or infested plants; see also spineless plants of dry-fruited species above in key)		(11)
+	Plants with at least some stem segments bearing spines		(25)
11 (10)	Fruits and stem segments puberulent (use hand lens) or velvety; trees or shrubs		(12)
+	Fruits and stem segments glabrous or glaucous; trees, shrubs, or forming low clumps		(13)
12 (11)	Stem segments bright green; areoles (9-)11-16 per diagonal row across midstem segment; usually low shrubs to l m, with many small stem segments; escapes from cultivation		24 Opuntia microdasys
+	Stem segments grayish green; areoles 8-13 per diagonal row across stem segments; low to medium-sized shrubs to 1.5 m, with fewer, larger stem segments; Texas		25 Opuntia rufida
13 (11)	Glochids inconspicuous on midstem segment areoles (use hand lens), short in crescent at adaxial edge of areole or covered by white cottony hairs in subapical region; plants known from cultivation or occasionally as escapes		(14)
+	Glochids usually conspicuous on midstem segment areoles, 3-5+ mm, densely to sparsely arranged; native taxa		(15)
14 (13)	Plants trees, 3-6 m, with well-developed trunks, to 30-45 cm diam.; stem segments broadly oblong to ovate to narrowly elliptic, usually (20-)40-60 × 2-3+ cm; glochids along adaxial margin of areoles		26 Opuntia ficus-indica (in part)
+	Plants low, spreading shrubs, to 2 m, trunkless; stem segments obovate to ovate or circular, 15-25 × 10-20 cm; glochids scattered, usually covered by conspicuous white cottony hairs		27 Opuntia ellisiana
15 (13)	Plants forming low clumps (in Florida specimens may be erect shrubs, sometimes with short trunks); fruits narrowly cylindric with only slightly tapering bases, tardily ripening, fleshy, surfaces red with greenish to reddish pulp		(16)
+	Plants small trees or shrubs; fruits barrel-shaped, sometimes stipitate, fleshy or juicy, surfaces reddish to purple with pulp of various colors		(18)
16 (15)	Stem segments lustrous; stigma lobes white; inner tepals yellow through- out; e, c United States		4 Opuntia humifusa (in part)
+	Stem segments dull; stigma lobes cream to yellowish; inner tepals yellow with red bases or red throughout; c United States (some overlapping ranges with above)		(17)
17 (16)	Inner tepals yellow, with red basal portions; spines ± stout (0.5 mm diam. near base); stem segments commonly subcircular to obovate, flabby and cross wrinkled when stressed		5 Opuntia macrorhiza (in part)
+	Inner tepals red throughout; spines slender (± 0.2 mm diam. near base); stem segments cuneate-obovate to commonly rhombic, ± firm		6 Opuntia pottsii (in part)
18 (15)	Stem segments ± tuberculate, margins scalloped between marginal areoles; fruits juicy, barrel-shaped, tapering below, purplish throughout; primarily se United States coast		8 Opuntia stricta (in part)
+	Stem segments with margins entire, not scalloped; fruits juicy or fleshy, various in shape and color; widespread		(19)
19 (18)	Glochids widely and evenly spaced, radiating like a star or pin cushion; Webb County, s Texas, and cultivated		11 Opuntia aciculata (in part)
+	Glochids not evenly spaced radially, in crescent at adaxial margin and also sometimes in subapical tuft; Texas to California, native		(20)
20 (19)	Areoles with subapical tuft of glochids poorly developed or absent, in addition to crescent at adaxial margin		(21)
+	Areoles with subapical tuft of glochids well developed, in addition to crescent at adaxial margin		(22)
21 (20)	Glochids moderately dense in adaxial crescent, of ± even lengths along margins of areole; style pink to red		22 Opuntia ×vaseyi (in part)
+	Glochids crowded in adaxial crescent, increasing in length abaxially along margin of areole; style white		9 Opuntia chlorotica (in part)
22 (20)	Glochids sparse, of irregular lengths, encircling (or nearly so) areole margins, also in subapical region, these often embedded in wool; large shrubs (sometimes with short trunk); fruits juicy		12 Opuntia engelmannii (in part)
+	Glochids dense, subequal or increasing in length toward adaxial margin of crescent, also dense in subapical tuft; small trees or shrubs; fruits fleshy or juicy		(23)
23 (22)	Inner tepals yellow throughout; erect shrubs or small trees to 2 m; stem segments lavender to red-purple when under stress, at least around areoles		15 Opuntia santa-rita (in part)
+	Inner tepals yellow with red bases; erect to trailing shrubs to 1.5 m; stem segments green, reddish, or purplish, particularly when under stress		(24)
24 (23)	Shrubs erect, erect to decumbent; stem segments purplish when under stress		16 Opuntia macrocentra (in part)
+	Shrubs low, with decumbent to trailing branches; stem segments green, sometimes reddish when under stress		17 Opuntia phaeacantha (in part)
25 (10)	Fruits and stem segment surfaces puberulent (use hand lens); inner perianth all yellow to orange-red, sometimes apically tinged red; spiny to spineless trees, bearing trunks; escapes from cultivation [Opuntia tomentosa, O. leucotricha; see discussion under genus]
+	Fruits and stem segments glabrous or glaucous, not puberulent (except for pubescence of areoles); inner perianth yellow, orange, pink red, magenta (rarely white), sometimes red basally; natives and escapes from cultivation		(26)
26 (25)	Spines (0-)1-3(-5) per areole, strongly barbed; stem segments 2.5-18 cm, easily detached; plants creeping, forming mats or clumps, or dense shrubs to 1 m; inner tepals yellow throughout; se United States, coastal (or slightly inland) and island plants, West Indies		(27)
+	Spines 1-18+, not strongly barbed; stem segments of various lengths, not easily detached; habit various; inner tepals yellow throughout or with bases of a different color; widespread		(28)
27 (26)	Plants creeping or forming mats; stem segments narrowly elliptic to linear, sometimes somewhat cylindric, 2.5-5(-8) × 1.2-2.5 cm; spines 1-2 in distal 3/4 of stem segment areoles; Texas to North Carolina		1 Opuntia pusilla
+	Plants forming clumps or low shrubs; stem segments narrowly elliptic, 5-18 × 3-7 cm; spines 1-3 in most areoles; Florida, West Indies.		2 Opuntia triacantha
28 (26)	Large trees, 3-6 m; trunks well developed, to 45 cm diam.; stem segments 25-60 cm; spines, when present, subulate, white, sometimes basally yellow to brown, erect to spreading; glochids inconspicuous, less than 2 mm; stem segments very thick, obovate, elliptic to oblong; known only from cultivation and escapes		26 Opuntia ficus-indica (in part)
+	Small trees or shrubs, erect to prostrate, to 2.5 m, usually much shorter; trunk, if present, short, to 25 cm diam.; stem segments rarely to 20-30 cm; spines, if present, subulate or acicular; glochids conspicuous, more than 2 mm; stem segments variable in thickness and shape		(29)
29 (28)	Shrubs prostrate, trailing or forming clumps (sometimes erect, even rarely short trunked in O. humifusa in Florida) sometimes with enlarged rootstocks; stem segments to 5-11(-17) cm		(30)
+	Shrubs or trees, erect to sprawling, several segments tall; stem segments 10+ cm		(33)
30 (29)	Fruits subspheric or short ovoid; stigma lobes yellow-green to green; flowers broadly spatulate; stem segments fleshy, green; spines usually distributed over distal 30-85% of stem segments; major spines erect to reflexed; minor spines at base of areole 2-8, deflexed		7 Opuntia tortispina
+	Fruits elongate, subterete, becoming stipitate-obconic with seed development; stigma lobes white to yellow-cream (rarely tinted green); flowers broadly cupped to rotate; stem segments fleshy to flabby, shiny to dull green; spines absent or only in distal 10-30(-50)% of stem segment; major spines erect to spreading; minor spines 0-2(-3), spreading to deflexed		(31)
31 (30)	Inner tepals yellow throughout; stigma lobes white; stem segments lustrous, green; spines, if present, 1 or 2(-3) on marginal areoles, dark; major spine porrect, stout, usually terete		4 Opuntia humifusa (in part)
+	Inner tepals yellow with red bases (occasionally only blushed) or red throughout; stigma lobes cream (or faint tinge of green); stem segments dull dark green; major spines (0-)1-3(-6) in distal areoles, rather stout to slender and flexible, terete or 1 central flattened, white to gray		(32)
32 (31)	Inner tepals yellow, with red basal portions; spines ± stout (± 0.5 mm diam. near base); stem segments commonly subcircular to obovate, of- ten flabby and cross wrinkled when stressed		5 Opuntia macrorhiza (in part)
+	Inner tepals yellow, with red basal portions; spines slender (± 0.2 mm diam. near base); stem segments cureate-obovate to commonly rhom- bic, ± firm		6 Opuntia pottsii (in part)
33 (29)	Stem segments somewhat tuberculate, margins scalloped between marginal areoles; fruits juicy, barrel-shaped, tapering below, red-purple to purplish through- out; se United States, primarily coastal		8 Opuntia stricta (in part)
+	Stem segments not or only slightly tuberculate, margins entire, not noticeably scalloped; fruits juicy or fleshy, various in shape and color; primarily w United States		(35)
34	Shrubs, densely branched, to 1 m; stem segments (4-)10-17.5 × (3-)5-7 cm; spines 2-3(-5) per areole, straight, commonly spirally twisted; fruits reddish and fleshy at maturity, fruit areoles 17-21, bearing few spines to ±9 mm; Florida Keys.		3 Opuntia cubensis
+	Shrubs, sparsely branched, to 2 m; stem segments 10-25(-40) × 7.5-16 cm; spines absent or 0-11 per areole, straight or curved (not spirally twisted); fruits purplish and juicy at maturity, fruit areoles 6-10, spineless; coastal se United States		8 Opuntia stricta (in part)
35 (33)	Major spines acicular, flexible, 1-8+ per areole, all strongly deflexed (sometimes some spines divergent along edges of stem segments); midstem segment areoles bearing marginal glochids that increase in length toward base of areole; fruits subspheric or barrel-shaped		(36)
+	Major spines acicular or subulate, 1-15+ per areole, not all deflexed; midstem segment areoles bearing various other glochid patterns; fruits ellipsoid to spheric, sometimes long tapering at base		(37)
36 (35)	Erect shrubs to small trees, to 2.5 m; spines all yellow, aging blackish; fruits large, barrel-shaped, 30-60 × 18-40 mm		9 Opuntia chlorotica (in part)
+	Erect to sprawling shrubs, to 1 m; spines reddish brown with yellow tips, aging blackish; fruits small, subspheric, 15-28 × 12-23 mm		10 Opuntia strigil
37 (35)	Glochids widely spaced (not dense), at least in most areoles, usually irregular in length; inner tepals of uniform color		(38)
+	Glochids densely clustered, in brushy apical tuft of unequal lengths or, more commonly, in adaxial crescent, partially encircling areoles, of equal length or decreasing in length from apex; inner tepals of uniform color or with bases of different color		(40)
38 (37)	Glochids prominent, radiating in all directions from areoles (like star or pincushion), very widely and evenly spaced; spines 1-3(-8), brown, tipped yellow or mottled brown, basal in areole, usually deflexed; flowers yellow, orange, or red throughout		11 Opuntia aciculata (in part)
+	Glochids obvious, in adaxial crescent, partially encircling areole margins and usually also in subapical clusters above major spines; spines 1-6(-10) per areole; major spines yellow, darkening with age, sometimes with chalky white coating (yellow spines show through when wetted), and sometimes with red-brown bases; tepals yellow throughout (sometimes pink, buff, or red, rarely white)		(39)
39 (38)	Shrubs (occasionally with short trunk) spreading to decumbent, to 3 m; major spines subulate, commonly flattened to angular at base, in some varieties chalky white-coated; tepals yellow (sometimes orange, pink, or red, rarely white); fruits juicy, purple, barrel-shaped, often with long tapering bases; widespread		12 Opuntia engelmannii (in part)
+	Shrubs erect, to 1 m; major spines terete to flattened near base, often curved, yellow, aging red or red-brown, never chalky; tepals yellow to buff; fruits juicy, reddish purple, barrel-shaped, about as wide as long; local in Chisos Moun- tains, w Texas		13 Opuntia chisosensis
40 (37)	Midstem segment areoles strongly obovate, each bearing many large glochids in brushy adaxial crescent; inner tepals yellow to orange throughout without red centers; divergent spines, dark red to black with strongly differentiated yellow tips; deflexed spines ± chalky white		14 Opuntia atrispina
+	Midstem segment areoles subcircular to elliptic to narrowly obovate, each bearing tidy (not brushy) glochids of equal lengths or of decreasing length from apex; inner tepals of 1 color or with differently colored bases; spines divergent and or deflexed, if bearing yellow tips, not in sharp contrast to basal color		(41)
41 (40)	Areoles with subapical tuft of glochids poorly developed or absent, in addition to adaxial crescent; inner tepals yellow, orange, or red, often bearing red to salmon bases; filaments yellow or orange to pink; style usually pink to red (rarely whitish); stigma lobes green to yellow-green; sw California		(42)
+	Areoles with dense subapical tuft (above major spines) of glochids, in addition to adaxial crescent; inner tepals yellow throughout or yellow with red centers (or rarely all reddish); filaments white, cream, or yellowish; style white; stigma lobes whitish or yellowish, sometimes tinged pale green; widespread		(45)
42 (41)	Shrubs or small trees; fruits red-purple with paler interiors; areoles 8-10 per diagonal row across midstem segment; spines 5-13 per areole, well distributed on stem segments, reflexed, usually curved, yellow; inner tepals yellow throughout		20 Opuntia oricola
+	Shrubs (sometimes with short trunks); fruits red-purple throughout; areoles 5-7(-8) per diagonal row across midstem segment; spines 0-11 per areole, well distributed on stem segments to spineless, spreading and reflexed, terete or flattened, straight to slightly curved, yellow ± white coated, often yellow to brown based; inner tepals of various colors, some with different colored bases		(43)
43 (42)	Spines 4-11 per areole, well distributed on stem segments (rarely spines restricted to apical and basal areoles only); longest spines spreading, generally terete, others reflexed; inner tepals yellow to dull red		21 Opuntia littoralis
+	Spines 0-6 per areole, well distributed to absent on stem segments; longest spines spreading to reflexed, generally flattened; others reflexed or very short; inner tepals yellow, orange, deep pink, or dull red, often darker basally		(44)
44 (43)	Spines 0 or 1-6, distributed over distal 0-70% of stem segments; shrubs sprawling to spreading; stem segments 7-22 cm; inner tepals yellow, orange, or dull red; filaments orange-yellow; style usually pink		22 Opuntia ×vaseyi (in part)
+	Spines 3-6, distributed over distal 90-100% of stem segment; shrubs sprawling to erect; stem segments 18-35 cm; inner tepals yellow to deep pink; filaments yellow or white; style pink or white		23 Opuntia ×occidentalis
45 (41)	Inner tepals yellow throughout or yellow with faint reddish blush at bases; spines, if present, yellow, horn colored, or red-brown; spreading, deflexed, or erect, to 60 mm; small trees with erect, short trunks		(46)
+	Inner tepals yellow with red bases or reddish throughout; spines, if present, of various colors, placements, and sizes; shrubs erect to sprawling		(47)
46 (45)	Spines, if present, 1-2 in few distal or marginal areoles per stem segment, deflexed to erect, yellow to horn colored, aging reddish brown, terete, to 40 mm; stem segments thin, often purplish (particularly under stress), at least around areoles; s Ari- zona		15 Opuntia santa-rita (in part)
+	Spines in most areoles, 4-8+ per areole, spreading to deflexed, reddish yellow to red-brown, subterete to flattened or channeled adaxially, sometimes twisted and curved, to 40-60 mm; stem segments thick, gray-green to yellow-green; nw Ari- zona, California, Nevada		18 Opuntia ×curvispina
47 (45)	Fruits sparingly spiny in distal areoles, yellow to red, slightly tuberculate, often sterile, shriveling and drying tan; spines 1-5(-8) per areole, well distributed on stem segment, white with red base, becoming dark red-brown with pale tips, often curved and twisted		19 Opuntia ×spinosibacca
+	Fruits not spiny, red to purplish, not tuberculate, fleshy to juicy; spines on stem segments, if present, 0-10(-20+) per areole, variously colored and distributed on stem segment		(48)
48 (47)	Spines 0-15+ per areole, few and at distal areoles or many and evenly distributed on stem segments; major spines usually reddish brown to blackish (or rarely yellow in w Texas), often partly to wholly chalky white, usually terete except at base, 30-170 mm, erect to upward-spreading (appearing to "need a haircut"), rarely also with small, deflexed spines; stem segments often purplish (particularly under stress), at least around areoles		16 Opuntia macrocentra (in part)
+	Spines (0-)2-8 per areole, in distal 1/3-2/3 areoles of stem segments; major spines gray to tan with red-brown bases or usually red-brown, sometimes blackish, usually flattened, 30-80 mm, spreading or deflexed, also with 1-3+ small, deflexed gray to tan spines; stem segments sometimes reddish under stress		17 Opuntia phaeacantha (in part)