Lysimachia has been shown not to be monophyletic by Manns & Anderberg (2009). Rather than split the genus (as was done in some American manuals in the past), they chose to make it monophyletic by uniting Anagallis, Trientalis, and other genera not found in Michigan, with it. We await more deliberation.
1. Leaf blades orbicular or nearly so, on short, distinct, glabrous petioles; plant prostrate, creeping.
1. Leaf blades at least twice as long as broad, sessile or on indistinct, ciliate, or pubescent petioles; plant erect.
2. Flowers in ± dense terminal or axillary racemes or panicles.
3. Leaves alternate, pubescent; flowers white.
3. Leaves opposite or whorled, glabrous or pubescent; flowers yellow.
4. Inflorescence a panicle or flowers in axils of upper leaves; stem and leaves (at least beneath) soft-pubescent; leaves usually at least partly whorled.
5. Flowers on slender pedicels in axils of upper leaves; sepals green throughout; corolla glandular-margined.
5. Flowers in panicles; sepals dark-margined; corolla not glandular.
4. Inflorescence a raceme; stem and leaves glabrous (or with a little loose pubescence, as along midrib beneath); leaves all opposite (often with axillary tufts).
6. Raceme terminal (rarely a subsidiary raceme or even lateral leafy branches with racemes); corolla lobes (4.5–) 5.5–7.5 (–8.5) mm long.
6. Racemes on axillary peduncles; corolla lobes (2.5–) 3–5 (–5.5) mm long.
2. Flowers solitary on axillary peduncles.
7. Leaf blades ± rounded to subcordate at base, on long (ca. 1–3 cm) strongly ciliate petioles; flowers 1.7–2.5 (–3.3) cm broad.
7. Leaf blades ± tapered, sessile or on short glabrous to ciliate petioles; flowers 1–1.8 (–2.5) cm broad.
8. Primary stem leaves in whorls of 3–6 (–7) (very rarely merely in opposite pairs in depauperate plants); calyx, corolla, and leaves ± streaked or dotted with red or black; anthers ca. 1 mm long or less; corolla lobes essentially entire.
8. Primary stem leaves opposite; calyx, corolla, and leaves without streaks or dots; anthers ca. 2–3 mm long; corolla lobes coarsely toothed (erose) and/or mucronate at apex.
9. Primary stem leaves linear, 1.5–6 (–10.5) mm wide, the lateral veins obscure, the margins of the upper leaves smooth and ± revolute, except for sometimes a few cilia or minute teeth near the base and/or at the apex.
9. Primary stem leaves ovate to lance-elliptic, (7–) 9–23 mm wide, mostly with distinct pinnate venation, the margins of especially the upper leaves scabrous, papillose, or ciliate throughout (and often strongly ciliate at base), flat.
10. Sepals apparently nerveless or very weakly nerved; leaf blades lanceolate to lance-elliptic, usually paler beneath; stem ca. 1–2.5 mm in diameter at base.
10. Sepals with 3–5 distinct nerves; leaf blades ovate to lanceolate, green beneath (hardly paler than above); stem ca. 4–7 mm in diameter at base.