| | 1. Racemes axillary, generally opposite; herbage generally glabrous (except V. chamaedrys); perennial herb |
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| | 3. Leaf lanceolate to ± ovate, obtuse to acute; pedicel 5–10 mm; widespread |
| V. americana |
| | 3' Leaf obovate, rounded; pedicel 3–6 mm; East of Sierra Nevada (near Bridgeport, Mono Co.) |
| V. beccabunga |
| | 2' Leaves sessile (lower rarely short-petioled) |
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| | 4. Racemes alternate; leaf linear-lanceolate (length generally >> 5 × width); fruit deeply notched |
| V. scutellata |
| | 4' Racemes opposite; leaf lanceolate to ovate (length < 5 × width); fruit entire or slightly notched |
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| | 5. Leaf lanceolate (length 3–5 × width); corolla 3–5 mm, pink; fruit notched 0.1–0.3 mm |
| V. catenata |
| | 5' Leaf elliptic to ovate (length 1.5–3 × width); corolla 5–10 mm, lavender to blue; fruit rounded to obcordate |
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| | 6. Plant glabrous; leaf ± entire; fruit rounded, notched < 0.1 mm; corolla pale lavender, violet-lined |
| V. anagallis-aquatica |
| | 6' Plant hairy; leaf crenate to dentate; fruit obcordate; corolla bright blue with white center |
| V. chamaedrys |
| | 1' Most racemes terminal, or flowers solitary in axils; herbage generally hairy; annual or perennial herb |
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| | 7. Rhizomed perennial herb |
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| | 8. Flowers solitary in axils — mat-forming in lawns, San Francisco Bay Area |
| V. filiformis |
| | 8' Flowers in ± distinct racemes |
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| | 9. Style 6–9 mm, > fruit; corolla 8–13 mm |
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| | 10. Leaf hairy; corolla 8–10 mm, pale blue to lavender-rose; Klamath Ranges |
| V. copelandii |
| | 10' Leaf generally glabrous; corolla 10–13 mm, deep blue; Cascade Range, High Sierra Nevada |
| V. cusickii |
| | 9' Style 0.8–3 mm, generally < fruit; corolla 6–10 mm |
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| | 11. Style 2–3 mm, fruit generally < calyx, wider than long, stem decumbent |
| V. serpyllifolia subsp. humifusa |
| | 11' Style 0.8–1.3 mm, fruit > calyx, longer than wide, stem erect nearly from base |
| V. wormskjoldii |
| | 7' Fibrous- or tap-rooted annual |
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| | 12. Pedicel 0.5–2 mm, < calyx; seeds many, flat, smooth |
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| | 13. Leaf triangular to ovate, crenate to serrate; sepals unequal (outer pair longer); corolla blue to violet |
| V. arvensis |
| | 13' Leaf oblong to spoon-shaped, entire to slightly serrate; sepals ± equal; corolla whitish |
| V. peregrina subsp. xalapensis |
| | 12' Pedicel 4–30 mm, generally > calyx; seeds 1–12 per chamber, cup-shaped, outer surface rough |
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| | 14. Pedicel 15–30 mm; leaf crenate to serrate; style 2–3 mm; fruit lobes spreading |
| V. persica |
| | 14' Pedicel 4–15 mm; leaf palmately 3–9 lobed; style 0.6–1.6 mm; fruit lobes ± parallel (or fruit ± spheric) |
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| | 15. Stems prostrate; flowers solitary in axils; fruit ± spheric, barely notched; seeds 1–2 per chamber; style 0.6–0.9 mm |
| V. hederifolia |
| | 15' Stems ± erect; inflorescence a terminal raceme; fruit obcordate; seeds 6–12 per chamber; style 1.2–1.6 mm |
| V. triphyllos
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