1	"Stem generally velvety to minutely papillate-hairy (puberulent), or if glabrous then perianth pink-magenta, stigma white, seeds +- spheric"	2
1	"Stem glabrous; if perianth pink-magenta then stigma green and seeds +- flat-sided"	6
2	"Tree < 6 m, trunks > 15 cm diam --- sexually mature when young and +- shrubby"	"Opuntia leucotricha"
2	"Shrub generally < 1 m, all or most branches +- touching the ground"	3
3	"Perianth yellow, stigma green; seed 2--3 mm"	"Opuntia microdasys"
3	"Perianth pink-magenta, stigma white; seed 6.5--9 mm"	4
4	"Spines 2--8, rigid, < 2.6 cm --- c Kern Co."	"Opuntia basilaris var. treleasei"
4	"Spines 0, barbed bristles present"	5
5	"Stem segment length < 2 &times; width, width > 5 &times; thickness"	"Opuntia basilaris var. basilaris"
5	"Stem segment length > 2 &times; width, width < 2 &times; thickness"	"Opuntia basilaris var. brachyclada"
6	"Terminal stem segment generally easily detached when fresh, 2--5.5 cm, width +- = thickness; w CaRH (Siskiyou Co.)"	"Opuntia fragilis"
6	"Terminal stem segments firmly attached when fresh, > 4 cm, width > 4 &times; thickness; mostly SW, D"	7
7	"Fruit dry in age, tan, generally spiny; perianth yellow to pink-magenta; style slender, white, base +- thicker"	8
7	"Fruit juicy, generally with some deep purple, spines 0; perianth yellow, orange, or red; style thick, white to red, base much thicker"	9
8	"Spines generally pale, more numerous and hair-like, curling, reflexed near plant base; ovary areoles 20--33; fruit generally a densely spiny bur"	"Opuntia polyacantha var. erinacea"
8	"Spines generally brown to black, spreading to ascending, straight, stiff on all stem segments; ovary areoles 11--21; fruit less spiny, not bur-like"	"Opuntia polyacantha var. hystricina"
9	"Trunk 1, erect; areoles generally > 38 per ovary"	10
9	"Basal branches several, +- decumbent to ascending; areoles <= 36 per ovary"	12
10	"Mature plant > 3 m; stem bristles < 1 mm, inconspicuous; ovary tubercled when fresh"	"Opuntia ficus-indica"
10	"Mature plant < 2.5 m; stem bristles 2--12 mm, conspicuous; ovary smooth"	11
11	"Longest spines generally 25--50 mm; style white when fresh; seed 3 mm; e PR, DMtns"	"Opuntia chlorotica"
11	"Longest spines 19--25(50) mm; style red when fresh; seed 3.5--4 mm; SW"	"Opuntia oricola"
12	"Style and filaments white"	13
12	"Style pink or filaments yellow to orange-yellow (or both)"	15
13	"Stem segment < 14 cm wide; perianth base red; fruit generally green inside"	"Opuntia phaeacantha"
13	"Stem segment > 15 cm wide; perianth base yellow; fruit red inside"	14
14	"Stem segment lightly glaucous, gray-green, generally obovate, < 20 cm wide"	"Opuntia engelmannii var. engelmannii"
14	"Stem segment strongly glaucous, +- silvery-blue, round, > 25 cm wide"	"Opuntia robusta"
15	"Stem segment > 15 cm wide"	"Opuntia X occidentalis"
15	"Stem segment < 15 cm wide"	16
16	"Stem segment oblong-elliptic to narrowly obovate; major spines generally round, 4--11 per areole"	"Opuntia littoralis"
16	"Stem segment generally obovate; major spines generally flat, generally 0--4 per areole"	"Opuntia X vaseyi"