Potentilla sect. Potentilla
Perennials, openly matted or ± tufted, often stoloniferous; taproots usually replaced by thick rootstocks; vestiture mostly of long hairs, glands absent or sparse, rarely common, sometimes reddish. Stems usually becoming ± prostrate, sometimes ascending to erect, often flagelliform, often rooting at nodes, lateral or central to persistent or ephemeral basal rosettes, 0.3–12+ dm, lengths (1–)2–10+ times basal leaves. Leaves: basal and cauline not in ranks; cauline (proximal to flowering and/or branching nodes) 0–3(–7); primary leaves ternate or palmate, 2–20(–30) cm; petiole: long hairs appressed to spreading, weak to stiff, glands absent or sparse, sometimes common; leaflets 3–5(–7), at tip of leaf axis, separate to slightly overlapping, obovate to narrowly elliptic, cuneate, or oblanceolate, margins flat or slightly revolute, distal 1/2–3/4 evenly incised 1/4–1/2 to midvein, teeth 2–13 per side, surfaces similar to ± dissimilar, abaxial usually green, sometimes silvery white, cottony hairs absent, adaxial green, not glaucous, long hairs usually ± stiff, sometimes weak or absent. Inflorescences solitary flowers at stolon nodes or 3–30-flowered, cymose, open. Pedicels straight or slightly curved in fruit, (1–)2–12(–17) cm, proximal not longer than distal. Flowers 4–5(–10)-merous; hypanthium 1.5–5(–7) mm diam.; petals usually bright yellow, rarely cream, ± obcordate or obovate to round, (2–)4–9(–12) mm, usually longer than sepals, apex rounded to retuse; stamens 15–20; styles subapical, columnar-clavate to ± filiform, not papillate-swollen proximally, 0.6–1.5 mm. Achenes smooth or rugose.
e North America, Eurasia, n Africa, Atlantic Islands, introduced in w North America, Mexico, West Indies, Bermuda, Central America, South America, c Africa (Ethiopia), Pacific Islands (New Zealand), Australia.
Species 7 or 8 (5 in the flora).
The species of sect. Potentilla comprise a monophyletic clade that includes Duchesnea, diverging basally to the species placed here in Horkelia, Horkeliella, and Ivesia (C. Dobeš and J. Paule 2010; M. H. Töpel et al. 2011). There are at least some morphological features by which this and other clades basal to the core Potentilla could be treated as separate genera. However, since the type of the genus (P. reptans) is in this section, doing so would require either a massive renaming of most Potentilla, or else conserving the type of the genus on a different species. If the latter course were taken, the species in this section would comprise Tormentilla.
Among the distinctive features of sect. Potentilla are the high percentage of stoloniferous species (shared with Duchesnea), a tuberous rootstock in many species, and the presence of tetramerous-flowered species (P. anglica, P. erecta). The section is also distinctive in having an amphi-Atlantic distribution, with native species in both eastern North America and Europe. The native material has sometimes been treated as a single species, or with confused nomenclature (M. L. Fernald 1931), leading to much unreliability in older herbarium annotations.
When leaves are palmate, the lateral leaflet pairs are usually more or less fused at the base, suggesting a ternate origin. Distal cauline leaves and inflorescence bracts are sometimes opposite. For comparison with other sections, counts of cauline leaves are restricted to nodes proximal to the first flowering stolon node, but descriptions of cauline leaves otherwise include all well-developed foliar structures at stolon nodes (until such time as these root and form new basal rosettes).
|1||Flowers 4(–5)-merous; stems usually openly branched; leaflets 3–5; hypanthia 1.5–4 mm diam||> 2|
|1||Flowers 5(–10)-merous; stems not branched; leaflets (3–)5(–7); hypanthia 2.5–7 mm diam||> 3|
|2||Stems soon becoming prostrate, ± flagelliform, eventually rooting at some nodes; inflorescences mostly solitary flowers at stolon nodes; cauline leaves: petioles 0.3–4(–8) cm; basal leaves ± persistent.||Potentilla anglica|
|2||Stems mostly ascending to erect, not flagelliform, not rooting at nodes; inflorescences cymose, 3–30-flowered; cauline leaves: petioles usually 0 cm; basal leaves ephemeral.||Potentilla erecta|
|3||Petals 7–9(–12) × 6–9(–11) mm; anthers (1–)1.3–2 mm; epicalyx bractlets elliptic or oblong to ovate, 4–10 × 1.5–3.5 mm, often much larger than sepals (especially in fruit); petiole hairs tightly to loosely appressed, 0.5–1.5 mm, leaflet surfaces similar, abaxial green, sparsely to moderately hairy; cauline leaves 2–3(–4) proximal to 1st flowering node; achenes ± rugose; introduced.||Potentilla reptans|
|3||Petals 4–7(–8) × 3–6.5(–8) mm; anthers 0.4–1 mm; epicalyx bractlets linear to narrowly lanceolate or lanceolate-elliptic, 2–5(–6) × 0.8–1.5(–2) mm, slightly smaller than to larger than sepals (especially in bud); petiole hairs appressed to spreading, (0.5–)1–3 mm, leaflet surfaces similar to ± dissimilar, abaxial green to silvery white, sparsely to densely hairy; cauline leaves 0–1 proximal to 1st flowering node; achenes smooth or faintly rugose; native||> 4|
|4||Cauline leaves not fully expanded at anthesis, 0–1 proximal to 1st flowering node, leaflets ± resembling those of basal leaves, apex rounded to obtuse, teeth 2–7 per side on distal 1/2 of leaflet; rootstocks erect, stout, 0.5–2 cm.||Potentilla canadensis|
|4||Cauline leaves well expanded at anthesis, (0–)1 proximal to 1st flowering node, leaflets often more elongate than those of basal leaves, apex acute to obtuse, teeth 4–8(–13) per side on distal 1/2–3/4 of leaflet; rootstocks horizontal, irregularly thickened or moniliform, 1–8 cm.||Potentilla simplex|