Flora of North America species comparison

Species 18–21 (4 in the flora).

The North American representatives of sect. Rivales are morphologically distinctive in being annuals, biennials, or short-lived perennials (especially in cold regions) with very short, conic-tapered styles. Plants grow primarily in wet habitats, especially where periodically inundated. These distinctions led E. L. Greene (1906c) to adopt Tridophyllum. However, the gross morphological cohesiveness is not matched by molecular monophyly. Instead, chloroplast data scatter Potentilla norvegica, P. rivalis, and P. supina among the core Potentilla, while P. biennis is sister species to the monophyletic Ivesia-Horkelia-Horkeliella clade (C. Dobeš and J. Paule 2010; M. H. Töpel et al. 2011). Nuclear markers, on the other hand, place P. norvegica as sister to the Ivesia-Horkelia-Horkeliella clade (Töpel et al.), suggesting a possible hybrid origin and/or multiple origins of the biennial habit.

Existing herbarium annotations of Potentilla biennis, P. norvegica, and P. rivalis are not reliable, but the three species can be readily distinguished by vestiture of proximal petioles and stems. Only P. biennis has prominently septate gland-tipped trichomes; P. norvegica is characterized by relatively stiff, spreading, tubercle-based hairs to 3 mm, resembling those of P. recta. In addition, the tiny, smooth, pale achenes of P. biennis and P. rivalis contrast sharply with the larger, darker, strongly ridged ones of P. norvegica.

Basal leaves usually wither by anthesis, and cauline leaves (that is, those proximal to the first flowering and/or branching node) often wither by mid-anthesis as well. Unless otherwise indicated, leaves include both basal and cauline leaves. If leaves are ephemeral, and/or where stems branch at the proximal nodes, proximal inflorescence bracts can be used as leaf-equivalents in the keys and descriptions.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Potentilla rivalis is most abundant in central and western North America; reports of occurrences in more eastern states (including those listed here) need confirming, as P. rivalis and P. norvegica often have been confused. Potentilla leucocarpa Rydberg was provided as a superfluous replacement name for P. millegrana; specimens annotated by Rydberg with this name include both P. biennis and P. rivalis.

Potentilla rivalis is sometimes divided into three species or varieties (for example, H. J. Scoggan 1978–1979). In a strict sense, var. rivalis has subpalmately compound (5-foliate) leaves. Variety milligrana, the most common phase, has 3-foliate leaves. Plants with both 3- and 5-foliolate leaves are var. pentandra, which also tends to have five stamens, though this latter character is not correlated with the leaf features. On the Great Plains, where var. pentandra is confined, all three expressions are often found in a single population (R. L. McGregor 1986b).

(Discussion copyrighted by Flora of North America; reprinted with permission.)