The segregate genera of Potentilla (Rosaceae)
Plant biologists have long recognized that the Potentilla–Fragaria complex contained many diverse elements that could be recognized as separate genera. Several distinctive genera were recognized from the complex, including Comarum (the marsh cinquefoils, described by Linnaeus in 1753) and Argentina (the silverweeds, described by Lamark in 1778). Unfortunately, several confusing issues arose with unnecessary subdivision and incorrect placement of new species (e.g., plants that should have been included in Potentilla were described as members of Ivesia). These problems caused Bentham and Hooker to disregard many of the generic segregates of Potentilla and, instead, recognize a large, variable genus of cinquefoils (Rydberg 1898). This taxonomic system has been followed by most American floristicians and has concealed the true evolutionary history of the group.
Rydberg (1898) produced a detailed treatment of the Potentilla–Fragaria complex (which he referred to as the Potentilleae). He recognized six different genera within what is commonly referred to as Potentilla in New England—Argentina, Comarum, Dasiphora, Drymocallis, Potentilla, and Sibbaldiopsis. Rydberg utilized style origination (Figures 1 and 2) and ovule type, in part, to discriminate major groups of genera. As these feature are not macroscopic (compared to features like leaf arrangement, fruit type, and presence/absence of an epicalyx), it is likely botanists disregarded these characters as being taxonomically useful for generic level separation. Erikkson et al. (1998) used Internal Transcribed Spacers of nuclear ribosomal DNA to reconstruct the phylogeny of Potentilla sensu lato and relatives. Their work supported many of the segregate genera recognized Rydberg (all of those listed above for New England). Their research suggests that character states such as trifoliate leaves, presence of stolons, and pubescent achenes—morphological features that were considered to be of taxonomic importance by some biologists—have been independently gained or lost more than once in this group. This implies that strict reliance on such morphological characters will lead to an erroneous classification system (i.e., placing distantly related species within the same genus). As an interesting note, the data of Erikkson et al. (1998) also shows that the genus Duchesnia is best included in Potentilla.
Figures 1 (left) and 2 (right). Style origin in two species of cinquefoil. 1—terminal style of Potentilla. 2—lateral style of Comarum.
Following is a key to the New England segregates of Potentilla.
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1a. Plants with evident woody stems; leaves with 5–7 pinnately-arranged entire leaflets with +/- revolute margins ......................................................................................... Dasiphora |
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1b. Plants herbaceous or woody only at the very base; leaves pinnately or palmately compound, the leaflets toothed or lobed, only rarely with revolute margins |
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2a. Petals white; surface of ovary and achenes pubescent; leaves with 3 leaflets that are entire for most of the margin and are provided with 3 (rarely 5) teeth at the very apex .............................................................................................................. Sibbaldiopsis |
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2b. Petals yellow, yellow-white, or red; ovary and achenes glabrous; leaves with 3–31 leaflets, the teeth or lobes not restricted to the very apex |
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3a. Flowers solitary at the tips of leafless peduncles produces from the nodes of an eventually prostrate to spreading stem |
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4a. Leaves pinnately compound with 5–31 principal leaflets, conspicuously pubescent with gray-white tomentum on the abaxial surface (this sometimes partially concealed by lustrous sericeous hairs); style lateral, emerging from the side of the carpel ............................................................................. Argentina |
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4b. Leaves palmately compound with 3–5 leaflets, glabrous or moderately pubescent with villous or sericeous hairs on the abaxial surface; style terminal, emerging from near the apex of the carpel .............................(in part) Potentilla |
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3b. Flowers few to numerous, arranged in cymes borne at the summit of a leafy stem |
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5a. Petals yellow-white; style subbasal, emerging from near the base of the carpel; plants viscid pubescent, especially above, the hairs with brown septa; stamens mostly 25–30 per flower ................................................. Drymocallis |
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5b. Petals red or yellow; style terminal or lateral; plants not viscid pubescent; stamens mostly 20 per flower |
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6a. Petals red; style lateral, emerging from the side of the carpel; principal leaves with 5–7 pinnately arranged leaflets; wetland species ......... Comarum |
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6b. Petals yellow; style terminal, emerging from near the apex of the carpel; leaves with 3–7 palmately or subpalmately arranged leaflets; primarily upland plants ............................................................................. (in part) Potentilla |
Checklist of New England Potentilla segragates:
Argentina anserina (L.) Rydb.
Argentina egedii (Wormsk.) Rydb.
Comarum palustre L.
Dasiphora floribunda Raf. (synonym: Pentaphylloides floribunda)
Drymocallis agrimonioides (Pursh) Rydb. (synonym: Potentilla arguta)
Sibbaldiopsis tridentata (Ait.) Rydb.
Literature Cited
Erikkson, T., M.J. Donoghue, and M.S. Hibbs. 1998. Phylogenetic analysis of Potentilla using DNA sequences of nuclear ribosomal internal transcribed spaces (ITS), and implications for the classification of Rosoideae (Rosaceae). Plant Systematics and Evolution 211: 155–179.
Rydberg, P.A. 1898. A monograph of the North American Potentilleae. Memoirs from the Department of Botany of Columbia University, volume II.