| | 1. Upper cauline leaves clasping or surrounding stem |
| |
| | 2. Upper cauline leaves surrounding stem, ovate to round; basal leaves 2–3-pinnately lobed or -divided; pedicel in fruit glabrous; petals yellowish; seed winged |
| L. perfoliatum |
| | 2' Upper cauline leaves clasping stem, linear to oblong; basal leaves rarely lobed; pedicel in fruit hairy; petals white; seed wing 0 |
| |
| | 3. Annual, biennial; stem generally 1; fruit with scale- or sac-like hairs; style 0.2–0.6 mm, < to ± > notch; anthers yellow |
| L. campestre |
| | 3' Perennial; stems several; fruit with 0 or few scale- or sac-like hairs; style ± 1 mm, generally > notch; anthers purple |
| L. heterophyllum |
| | 1' Upper cauline leaves not clasping, not surrounding stem |
| |
| | 4. Stigma generally exceeding fruit (except in L. pinnatifidum); fruit tip notched to not |
| |
| | 5. Stem prostrate to ascending; fruit valves with prominent, ascending-spreading tips; style < to > 1/2 fruit; petals yellow |
| L. flavum |
| | 6. Fruit 3–4.5 mm, 3–3.5 mm wide; style < 1/2 fruit |
| var. felipense |
| | 6' Fruit 2–3 mm, 1.5–2.2 mm wide; style > 1/2 fruit |
| var. flavum |
| | 5' Stem generally erect, outer decumbent or not; fruit valves without prominent tips; style < 1/2 fruit; petals white, rarely pale cream-colored (yellow fading whitish in L. jaredii) |
| |
| | 7. Plants rhizomed, individuals often in contact, in colonies; basal leaves < 3 dm, toothed |
| L. latifolium |
| | 7' Plants not rhizomed, individuals not in contact, not in colonies; basal leaves generally < 1 dm, generally lobed or compound |
| |
| | 8. Style ± = fruit tip notch, < 0.2 mm — fruit widely elliptic to round, ± 2 mm, hairs sparse, weak, simple; alien |
| L. pinnatifidum |
| | 8' Style > fruit tip notch (or notch 0), > 0.3 mm |
| |
| | 9. Annual; petals yellow, fading whitish; lower pedicels 4–5 × fruit |
| L. jaredii |
| | 9' Perennial, rarely biennial or annual; petals white, rarely pale cream-colored; lower pedicels generally 3 × fruit |
| |
| | 10. Plant glabrous, grayish, ± woody; fruit obovate to round |
| L. fremontii |
| | 11. Fruit widely ovate to ± round or obovate, stalk above receptacle ± 0 |
| var. fremontii |
| | 11' Fruit widely obovate to ± round, stalk above receptacle ± 1 mm |
| var. stipitatum |
| | 10' Plant with at least some hairs, not grayish, base rarely woody; fruit elliptic to round |
| |
| | 12. Stem hairs a mixture of long, flat and short, club-shaped; upper cauline leaves generally pinnately lobed; winter annual, biennial |
| L. thurberi |
| | 12' Stem hairs all long, flat or short, club-shaped or scale-like, rarely ± 0; upper cauline leaves generally entire; biennial to perennial herb |
| L. montanum |
| | 13. Hairs of stems, pedicels linear, long (several–many × longer than wide) |
| var. canescens |
| | 13' Hairs of stems, pedicels oblong, club-shaped, or scale-like, short (generally not > 2 × longer than wide), or ± 0 |
| |
| | 14. Hairs of stems, pedicels ± dense, oblong; perennial herb, rarely biennial |
| var. cinereum |
| | 14' Hairs of stems, pedicels sparse to ± dense, club-shaped or scale-like, or ± 0; perennial herb, ± woody at base or not |
| var. montanum |
| | 4' Stigma exceeded by fruit; fruit tip notched |
| |
| | 15. Pedicel in fruit very flat, > (2)3 × wider than thick, generally = or < fruit |
| |
| | 16. Fruit tip notch deep, generally very narrow, > 1/3 seed pouch; fruit valves persistent; raceme in fruit 1–1.5 cm wide, < or > basal leaves, or basal leaves 0 or shed early |
| L. latipes |
| | 17. Stem branched above base, nodes spaced; raceme in fruit > basal leaves; basal leaves 0 or shed early |
| var. heckardii |
| | 17' Stem branched at or near base, nodes not spaced; raceme in fruit generally < basal leaves; basal leaves generally present |
| var. latipes |
| | 16' Fruit tip notch shallow, often wide, generally < 1/5 × seed pouch; fruit valves readily shed; raceme in fruit generally < 1 cm wide, generally > basal leaves |
| |
| | 18. Leaf oblanceolate to oblong, toothed or with obovate to oblong lobes; fruit margin not upturned |
| L. lasiocarpum var. lasiocarpum |
| | 18' Leaf linear or with linear lobes; fruit margin upturned or not |
| |
| | 19. Fruit hairs ± throughout, rarely 0; pedicel suberect to ascending, rarely ± reflexed |
| L. dictyotum |
| | 20. Fruit valve tips > 1 mm, acuminate, ± ascending, winged; fruit tip notch U- or V-shaped |
| var. acutidens |
| | 20' Fruit valve tips < 1 mm, rounded to acute, erect, unwinged; fruit tip notch ± closed or narrowly U- or V-shaped |
| var. dictyotum |
| | 19' Fruit hairs 0, rarely few, marginal; pedicel ± recurved to ± ascending |
| L. nitidum |
| | 21. Fruit valve tips ascending, ± beak-like |
| var. oreganum |
| | 21' Fruit valve tips erect or ± ascending, not beak-like |
| |
| | 22. Fruit generally fringed with hairs; stem hairs dense |
| var. howellii |
| | 22' Fruit glabrous; stem hairs 0 to ± dense |
| var. nitidum |
| | 15' Pedicel in fruit cylindric to flat, generally < or ± 2 × wider than thick, = or > fruit (< fruit in L. strictum) |
| |
| | 23. Leaf linear or with linear segments; fruit valve tips generally acute, notch generally ± 90–160° — ephemeral annual |
| L. oxycarpum |
| | 23' Leaf sometimes narrow but not linear, generally without linear segments; fruit valve tips generally rounded, notch < 45° (± 90° in L. strictum) |
| |
| | 24. Leaves lobed or divided below, generally above; stem generally prostrate to ascending, branched, 0.5–2(3) dm |
| |
| | 25. Fruit ± ovate, veins prominent, valve tips ascending, generally winged |
| L. strictum |
| | 25' Fruit ± elliptic, round, or obovate, veins 0 or obscure, valve tips erect, unwinged |
| L. oblongum |
| | 26. Fruit round to obovate, hairy on margin; pedicel hairs generally on both sides |
| var. insulare |
| | 26' Fruit ± elliptic to obovate-elliptic, hairs 0 on margin; pedicel hairs 0 or on upper surface |
| var. oblongum |
| | 24' Leaves entire to lobed below, entire to toothed, rarely ± lobed or dissected above; stem erect, branched or not, generally > 2 dm |
| |
| | 27. Petals = to 2(3) × sepals; fruit hairs 0 |
| L. virginicum |
| | 28. Pedicel ± cylindric, hairs sparse to ± dense; embryonic root at edges of both cotyledons |
| var. virginicum |
| | 28' Pedicel ± flat, generally ± winged, hairs 0 to ± dense; embryonic root at back of 1 cotyledon |
| |
| | 29. Inflorescence and upper stem glabrous |
| var. medium |
| | 29' Inflorescence and upper stem hairy |
| |
| | 30. Cauline leaves not divided, not lobed, entire to shallowly dissected; stem 2–7 dm |
| var. pubescens |
| | 30' Cauline leaves divided or lobed; stem generally 1–2 dm |
| var. robinsonii |
| | 27' Petals < sepals, vestigial, or 0; fruit hairs 0 or minute |
| |
| | 31. Seed not margined; fruit valve tips ± acute; axillary racemes in fruit many |
| L. ramosissimum var. bourgeauanum |
| | 31' Seed generally margined or winged; fruit valve tips rounded; axillary racemes in fruit 0 |
| L. densiflorum |
| | 32. Fruit ± 2.5 mm, glabrous; pedicels very slightly flat, > 9 per cm |
| var. densiflorum |
| | 32' Fruit ± = or > 3 mm, hairy at least on margin or glabrous; pedicels flat at least on 1 side, < 9 per cm |
| |
| | 33. Pedicel flat on upper, lower sides, ± 2 × wider than thick; fruit glabrous |
| var. ramosum |
| | 33' Pedicel flat generally on lower side, < ± 2 × wider than thick; fruit hairy at least on margin or glabrous |
| |
| var. macrocarpum |
| | 34' Fruit hairy at least on margin |
| |
| | 35. Fruit hairy on margin |
| var. elongatum |
| | 35' Fruit hairy ± throughout |
| var. pubicarpum
|