| 1. Flowers unisexual (dioecious); heads generally 11.5 cm |
| C. arvense |
| 1' Flowers bisexual; heads generally > 1.5 cm |
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| 2. Leaf surfaces harshly bristly above; stems spiny-winged |
| C. vulgare |
| 2' Leaf surfaces glabrous to tomentose above; stems wingless or ± short-winged |
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| 3. Heads nodding; leaf surface ± glandular-puberulent above |
| C. fontinale |
| 4. Fruit 45 mm; phyllary spines 12 mm; sw San Francisco Bay Area |
| var. fontinale |
| 4' Fruit ± 3.54 mm; phyllary spines 26 mm; eastern San Francisco Bay Area, c Outer South Coast Ranges |
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| 5. Cauline leaves with larger spines 1018 mm; most outer phyllaries with 0 marginal spines; eastern San Francisco Bay Area |
| var. campylon |
| 5' Cauline leaves with larger spines 47 mm; many outer phyllaries with marginal spines; c Outer South Coast Ranges |
| var. obispoense |
| 3' Heads generally erect; leaf surface not glandular above |
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| 6. Outer phyllary margins spiny-fringed or with expanded, scarious appendages |
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| 7. Corollas 3045 mm, generally red to rose-purple; inner phyllaries long, straight, entire, purple-tipped |
| 3 C. andersonii |
| 7' Corollas 1630 mm, white to purple; inner phyllaries often ± crinkled, sometimes with expanded, scarious appendages, generally not purple-tipped |
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| 8. Heads generally not closely subtended by clustered leafy bracts, each often subtended by 1 leaf |
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| 9. Phyllary midribs conspicuously sticky-glandular; outer phyllary margins minutely spiny-ciliate |
| 2 C. hydrophilum |
| 10. Heads generally 2.53 cm; fruit ± 5 mm; deltaic Great Central Valley (Suisun Marsh) |
| var. hydrophilum |
| 10' Heads generally 33.5 cm; fruit 45 mm; n San Francisco Bay Area (Mount Tamalpais) |
| var. vaseyi |
| 9' Phyllary midribs not glandular (or inner inconspicuously glandular); outer phyllary margins generally ± conspicuously fringed or spiny-ciliate |
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| 11. Biennial, stout, generally 1030 dm; stem base 210 cm diam, hollow; middle and inner phyllaries entire; San Joaquin Valley |
| 2 C. crassicaule |
| 11' Perennial, slender, generally 111 dm; stem base generally < 2 cm diam, not hollow; middle and inner phyllaries often with fringed appendages; Northwestern California, n Central Western California |
| 4 C. remotifolium |
| 8' Heads generally closely subtended by clustered, ± leafy bracts |
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| 12. Leaves closely gray-white-tomentose (felt-like) on both surfaces, very prominently wavy-margined; corollas white or pale yellow dunes, headlands, s Central Coast |
| C. rhothophilum |
| 12' Leaves glabrous to cobwebby-tomentose above, sometimes white-tomentose below, flat to wavy-margined; corollas white to purple |
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| 13. Phyllaries conspicuously cobwebby-tomentose |
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| 14. Upper leaves ± stiff, strongly spiny, spines stout, often 1015 mm; most outer phyllaries spiny-margined; s North Coast, n Central Coast |
| C. andrewsii |
| 14' Upper leaves thin, ± weakly spiny, spines slender, generally < 8 mm; few outer phyllaries spiny-margined; North Coast, Outer North Coast Ranges, CC, San Francisco Bay Area, South Coast |
| 2 C. brevistylum |
| 13' Phyllaries ± glabrous or very thinly cobwebby |
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| 15. Plant 1030 dm; most outer phyllaries spiny-ciliate, also minutely rough-ciliate; San Joaquin Valley |
| 2 C. crassicaule |
| 15' Plant generally 110 dm; most outer phyllaries entire or minutely rough-ciliate; s Central Coast |
| 4 C. loncholepis |
| 6' Outer phyllary margins generally entire (bracts subtending head generally spiny) |
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| 16. Middle and outer phyllaries generally spreading to reflexed |
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| 17. Main phyllary spine tips generally 720+ mm |
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| 18. Perennial; heads sessile or on short leafy peduncles, sometimes in ± congested cyme-like clusters |
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| 19. Phyllaries densely, persistently cobwebby; Central Coast |
| 3 C. occidentale var. compactum |
| 19' Phyllaries thinly cobwebby or becoming glabrous |
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| 20. Corollas bright pink to red; corolla tube < throat; s High Sierra Nevada, White and Inyo Mountains |
| 2 C. arizonicum |
| 20' Corollas rose-purple; corolla tube > 2 × throat; Desert Mountains |
| 2 C. nidulum |
| 18' Biennial; heads generally long-peduncled, often in ± open cyme-like clusters |
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| 21. Corolla tube abruptly expanded to throat; corollas generally white to pale purple (sometimes darker purple to reddish) |
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| 22. Cauline leaves with main spines generally 815 mm; eastern Mojave Desert |
| C. neomexicanum |
| 22' Cauline leaves with main spines generally < 8 mm; Sierra Nevada, South Coast Ranges, Southwestern California |
| 2 C. occidentale var. californicum |
| 21' Corolla tube gradually expanded to throat; corollas generally rich purple to bright pink or red (sometimes white to pale purple) |
| 2 C. occidentale |
| 23. Plants compact, rounded, mound-like, generally < 5 dm Central Coast |
| var. compactum |
| 23' Plants erect, often >> 5 dm |
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| 24. Head length generally ± = width; phyllaries generally very narrow, needle-like, radiating; corollas generally purple; mostly coastal, Central Western California, Southwestern California |
| var. occidentale |
| 24' Heads generally longer than wide; phyllaries generally clearly flattened, widened toward base, spreading to reflexed; corollas generally red or bright pink, sometimes purple; generally interior mtns, n&c California vars. candidissimum, venustum (see 33., 33') |
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| 17' Main phyllary spine tips generally 16 mm |
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| 25. Corollas white to pale purple |
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| 26. Biennial; heads generally in open cyme-like clusters; Sierra Nevada, c&s South Coast Ranges, Southwestern California |
| 2 C. occidentale var. californicum |
| 26' Perennial; heads generally in ± congested flat-topped or raceme-like clusters; generally n California |
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| 27. Some phyllaries generally fringed or spiny-toothed |
| 4 C. remotifolium |
| 27' All phyllaries entire |
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| 28. Stems, leaves tomentose; jointed-multicellular hairs 0; Klamath Ranges |
| 2 C. ciliolatum |
| 28' Stems, leaves with hairs of 2 kinds, tomentose and jointed, multicellular hairs, especially along midveins (leaf lower surface); widespread |
| 3 C. cymosum |
| 25' Corollas deep purple to bright pink or red |
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| 29. Corolla tube abruptly expanded to throat; perennial herb |
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| 30. Corollas 3045 mm, generally red to rose-purple; inner phyllaries long, straight, entire |
| 3 C. andersonii |
| 30' Corollas 1625 mm, white to purple; inner phyllaries generally ± crinkled, sometimes with expanded, scarious appendages |
| 4 C. remotifolium |
| 29' Corolla tube gradually expanded to throat; biennial |
| 2 C. occidentale |
| 31. Plants compact, rounded, mound-like, generally < 5 dm Central Coast |
| var. compactum |
| 31' Plants erect, often >> 5 dm |
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| 32. Head length generally ± = width; corollas generally purple; mostly coastal, Central Western California, Southwestern California |
| var. occidentale |
| 32' Heads generally longer than wide; corollas generally red or bright pink; generally interior mtns, n&c California |
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| 33. Plant densely white-tomentose; phyllaries persistently white-tomentose (except spine tip); outer phyllaries generally very long, spreading to reflexed; Klamath Ranges, North Coast Ranges, Cascade Range, Modoc Plateau, n Sierra Nevada |
| var. candidissimum |
| 33' Plant gray-tomentose to ± green (rarely densely white-tomentose); phyllaries ± cobwebby to tomentose or becoming ± glabrous; outer phyllaries short to long, ascending to spreading or reflexed; North Coast Ranges, s Sierra Nevada, and White and Inyo Mountains southward |
| var. venustum |
| 16' Middle and generally outer phyllaries appressed or stiffly ascending (spine tip sometimes abruptly spreading) |
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| 34. Leaves all basal or crowded on very short, densely leafy stem; heads ± sessile, subtended by rosette leaves |
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| 35. Perennial from creeping rootstock; generally ± dry habitats North Coast, Outer North Coast Ranges, Central Coast, San Francisco Bay Area, n Outer South Coast Ranges |
| 3 C. quercetorum |
| 35' Biennial or short-lived perennial herb that flowers once, taprooted; generally ± wet habitats |
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| 36. Plants ± glabrous; main leaf spines generally 710 mm; s Central Coast |
| 4 C. loncholepis |
| 36' Plants generally ± tomentose (especially lower surface of leaves); main leaf spines generally 27 mm; generally interior mtns |
| 3 C. scariosum |
| 34' Leaves basal and cauline; heads sessile or peduncled, clearly raised above rosette leaves |
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| 37. Phyllary spine tips 1020+ mm |
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| 38. Corolla rose-purple to red, lobes = or > tube |
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| 39. Corolla bright pink to red; corolla tube < throat; s High Sierra Nevada, White and Inyo Mountains |
| 2 C. arizonicum |
| 39' Corolla rose-purple; corolla tube > 2 × throat; Desert Mountains |
| 2 C. nidulum |
| 38' Corolla dull white to pale purple, lobes < tube |
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| 40. Plant sparsely cobwebby-tomentose, becoming glabrous; heads closely subtended by well developed leaves s Central Coast |
| 4 C. loncholepis |
| 40' Plant persistently tomentose; heads generally not closely subtended by leaves |
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| 41. Corollas 2027 mm; biennial from taproot |
| 3 C. canovirens |
| 41' Corollas 2937 mm; perennial herb from creeping rootstock |
| 2 C. ochrocentrum |
| 37' Phyllary spine tips generally < 10 mm |
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| 42. Corollas generally 2550 mm |
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| 43. Corollas bright purple to red |
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| 44. Phyllaries in 34 series, outer linear-lanceolate, inner much longer, straight, red- or purple-tipped; corollas red or rose-purple |
| 3 C. andersonii |
| 44' Phyllaries strongly graduated in 510 series, outer lanceolate to ovate, inner often twisted or crinkled at tip; corollas purple |
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| 45. Biennial or short-lived perennial herb, taprooted generally interior mtns |
| 3 C. scariosum |
| 45' Perennial from creeping rootstock |
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| 46. Phyllaries generally ovate, midribs not glandular; North Coast, Outer North Coast Ranges, Central Coast, San Francisco Bay Area, Outer South Coast Ranges |
| 3 C. quercetorum |
| 46' Phyllaries generally lanceolate, midribs with sticky-glandular zone; weed, s Cascade Range, San Francisco Bay Area, Outer South Coast Ranges, South Coast, Modoc Plateau |
| 2 C. undulatum |
| 43' Corollas white to pink or lavender |
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| 47. Biennial or short-lived perennial herb, taprooted generally ± wet soil of meadows |
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| 48. Plants ± glabrous; main leaf spines generally 710 mm; s Central Coast |
| 4 C. loncholepis |
| 48' Plants generally ± tomentose (especially lower surface of leaves); main leaf spines generally 27 mm; generally interior mtns |
| 3 C. scariosum |
| 47' Perennial from creeping rootstock |
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| 49. Phyllary spine tips generally 510 mm; leaves generally < 3 cm wide, deeply divided into many narrow, spiny lobes |
| 2 C. ochrocentrum |
| 49' Phyllary spine tips generally 15 mm; leaves often > 3 cm wide, variously divided, variously spiny |
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| 50. Phyllaries ± persistently tomentose |
| 3 C. cymosum |
| 50' Phyllaries generally becoming glabrous |
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| 51. Phyllaries generally ovate, midribs not glandular; North Coast, Outer North Coast Ranges, Central Coast, San Francisco Bay Area, Outer South Coast Ranges |
| 3 C. quercetorum |
| 51' Phyllaries generally lanceolate, midribs with sticky-glandular zone; weed, s Cascade Range, San Francisco Bay Area, Outer South Coast Ranges, South Coast, Modoc Plateau |
| 2 C. undulatum |
| 42' Corollas generally 1624 mm |
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| 52. Phyllary midribs generally without a sticky-glandular area |
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| 53. Heads generally conspicuously peduncled |
| 4 C. remotifolium |
| 53' Heads generally ± sessile or short-peduncled |
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| 54. Phyllaries connected by tangled cobwebby hairs; involucres 2.53.5 cm; heads closely subtended by ± leafy bracts; North Coast, Outer North Coast Ranges, Central Coast, San Francisco Bay Area, South Coast |
| 2 C. brevistylum |
| 54' Phyllaries ± tomentose or becoming glabrous; involucres 1.52.5 cm; heads often not closely subtended by leafy bracts; High Sierra Nevada (eastern slope), n Great Basin Floristic Province |
| 3 C. canovirens |
| 52' Some phyllary midribs with a ± sticky-glandular area or raised translucent gland |
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| 55. Phyllary bodies (at least outer) ascending or loosely appressed, spine tip generally erect or ascending; heads commonly medium to large, in cyme- or raceme-like clusters |
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| 56. Hairs all cobwebby, often dense and persistent on leaves |
| 2 C. ciliolatum |
| 56' Hairs of 2 kinds, cobwebby and jointed, multicellular, generally ± loose and irregularly deciduous from leaves in age |
| 3 C. cymosum |
| 55' Phyllary bodies tightly appressed, spine tip ascending to abruptly spreading; heads commonly small, ± ovoid, in panicle-like clusters |
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| 57. Leaves (both surfaces) thinly tomentose, becoming ± glabrous above |
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| 58. Hairs of 2 kinds, cobwebby and jointed, multicellular, generally ± loose and irregularly deciduous from leaves in age; High Sierra Nevada (e slope), n Great Basin Floristic Province |
| 3 C. canovirens |
| 58' Hairs all cobwebby, often dense and persistent on leaves; deltaic Great Central Valley, n San Francisco Bay Area |
| 2 C. hydrophilum vars. (see 10., 10') |
| 57' Leaves (both surfaces) densely tomentose, especially below |
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| 59. Phyllaries greenish or straw-colored; corollas white to pale purple; East of Sierra Nevada, Mojave Desert |
| C. mohavense |
| 59' Phyllaries generally blackish near tips; corollas generally dark purple (rarely white) |
| C. douglasii |
| 60. Upper cauline leaves with larger spines mostly < 7 mm; Klamath Ranges, n North Coast Ranges, Cascade Range, n Sierra Nevada, Modoc Plateau |
| var. breweri |
| 60' Upper cauline leaves with larger spines generally 720 mm; North Coast, North Coast Ranges, San Francisco Bay Area, n Central Coast |
| var. douglasii
|