| 1. Stem segments cylindric to club-like, tubercled; spine epidermis sheath-like, generally deciduous |
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| 2. Major spines distinctly flat, epidermal sheath separating only from tip (subg. Corynopuntia) |
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| 3. Perianth yellow; largest spine with cross-rows of rough papillae; ovary bristles stiff, with reflexed barbs |
| O. parishii |
| 3' Perianth pink-purple; largest spine smooth; ovary bristles hair-like, with ascending barbs |
| O. pulchella |
| 2' Spines not flat, epidermal sheath separating from entire spine (subg. Cylindropuntia) |
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| 5. Stem < 1 cm diam, tubercle < 2 mm high |
| O. ramosissima |
| 5' Stem > 1.5 cm diam, tubercle > 3 mm high |
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| 6. Terminal stem segment generally < 2 dm, tubercle length < 2 × width |
| O. echinocarpa |
| 6' Terminal stem segment generally > 2 dm, tubercle length > 3 × width |
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| 7. Stem generally branched only above; trunk 1 |
| O. acanthocarpa var. coloradensis |
| 7' Stem generally branched near base; trunks severalmany |
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| 8. Stem generally < 2 cm diam, filaments green |
| O. parryi |
| 8' Stem generally > 2.5 cm diam, filaments red- to green-purple |
| O. wolfii |
| 4' Fruit spineless (sometimes with a few deciduous bristles) |
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| 9. Fruit generally bearing flowers, perianth purple-red |
| O. prolifera |
| 9' Fruit not bearing flowers, perianth yellow, greenish, or red-brown |
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| 10. Terminal stem segment > 3 × longer than wide |
| O. parryi |
| 10' Terminal stem segment generally < 3 × longer than wide |
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| 11. Tubercle length ± = width |
| O. bigelovii |
| 11' Tubercle length ± 2 × width |
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| 12. Spines 710 per tubercle; fruit < 2.5 cm; seeds irregularly shaped, < 3 mm |
| O. ×fosbergii |
| 12' Spines 916 per tubercle; fruit 2.53.5 cm; seeds ± spheric, ± 3 mm |
| O. ×munzii |
| 1' Stem segments flat, not tubercled; spine epidermis not separable (subg. Opuntia) |
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| 13. Stem generally minutely papillate-hairy, spines 0 (28 per areole in c Kern Co.); perianth pink to magenta; filaments red; fresh stigma white |
| O. basilaris |
| 14. Spines 28, rigid, < 2.6 cm; rare, c Kern Co. |
| var. treleasei |
| 14' Spines 0 (barbed bristles present) |
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| 15. Stem segment length < 2 × width, width > 5 × thickness |
| var. basilaris |
| 15' Stem segment length > 2 × width, width < 2 × thickness |
| var. brachyclada |
| 13' Stem glabrous, generally bearing spines; perianth generally yellow to dull red (rarely pink); filaments white, yellow, red, or pink; fresh stigma green; not in Kern Co. |
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| 16. Terminal stem segment easily detached, < 3.5 cm, width < 3 × thickness; w High Cascade Range (Siskiyou Co.) |
| O. fragilis |
| 16' Terminal stem segments firmly attached when fresh, > 4 cm, width > 4 × thickness; mostly Southwestern California, Desert |
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| 17. Fruit becoming dry, tan, generally spiny; perianth yellow to pink-magenta; style slender, white, base slightly thicker |
| O. erinacea |
| 18. Ovary spiny; spines generally in all stem areoles; smaller spines at lower edge of stem areole 3 or more, strongly reflexed |
| var. erinacea |
| 18' Ovary spineless; spines in < 50% of stem areoles; smaller spines at lower edge of stem areole 04, ± reflexed |
| var. utahensis |
| 17' Fruit juicy, generally with some deep purple, spines 0; perianth yellow, orange, or red; style thick, white to red, base much thicker |
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| 19. Trunk 1, erect; areoles generally > 38 per ovary |
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| 20. Mature plant > 3 m; stem bristles < 1 mm, inconspicuous; ovary clearly tubercled when fresh |
| O. ficus-indica |
| 20' Mature plant < 2.5 m; stem bristles 212 mm, conspicuous; ovary smooth |
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| 21. Longest spines generally 3247 mm; style white when fresh; seed 3 mm; se California Floristic Province, Desert |
| O. chlorotica |
| 21' Longest spines 1925 mm; style red or pink when fresh; seed 3.54 mm; w California Floristic Province |
| O. oricola |
| 19' Basal branches several, ± decumbent to ascending; areoles < 36 per ovary |
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| 22. Style and filaments white |
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| 23. Stem segment > 15 cm wide; perianth base yellow; fruit red inside |
| O. engelmannii var. engelmannii |
| 23' Stem segment < 14 cm wide; perianth base red; fruit green inside |
| O. phaeacantha |
| 22' Style pink or filaments yellow (or both) |
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| 24. Stem segment > 15 cm wide |
| O. ×occidentalis |
| 24' Stem segment < 15 cm wide |
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| 25. Stem segment oblong-elliptic or narrowly obovate; major spines generally round, 411 per areole |
| O. littoralis |
| 25' Stem segment obovate; major spines generally flat, 14 per areole |
| O. ×vaseyi
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