8d. Potentilla Linnaeus sect. Potentilla
Barbara Ertter, James L. Reveal
Potentilla sect. Tormentilla (Linnaeus) Tausch; Tormentilla Linnaeus
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.
Species 7 or 8 (5 in the flora): 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.
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).
SELECTED REFERENCE Fernald, M. L. 1931. Potentilla canadensis and P. simplex. Rhodora 33: 180–191.