Flora of North America species comparison

Oxalis corniculata in the flora area is recognized by a combination of its small flowers (petals yellow, 4–8 mm); sparsely hairy, herbaceous stems creeping and rooting at nodes; and stipules with free flanges and apical auricles. Peduncles and leaves (one to three) are produced at the nodes, short erect stems less commonly so. Specimens have been seen documenting its distribution in the United States as listed above; it may also occur in intervening areas.

The typical form of Oxalis corniculata is strictly annual with consistently herbaceous, prostrate stems. At least some populations in western Oregon are distinctly more erect, with decumbent-ascending stems, than those of the eastern United States. In contrast, stems of O. dillenii characteristically are initially erect but may become decumbent to prostrate, occasionally rooting at the nodes; they almost always become more or less woody. Stems arising from nodes of laterally oriented stems characteristically are erect. In most of the United States, O. corniculata usually occurs in urban and highly disturbed habitats, but along the Gulf Coast it occasionally grows in less obviously disturbed sites and might be native there. However, assignment of nativity awaits a clearer understanding of patterns of variation within what is recognized as a highly variable species.

Variants of Oxalis corniculata and closely similar forms occur in Mexico, the West Indies, Central America, and South America, as well as in other parts of the world, including the flora area. Plants with bronze-purple to maroon leaves and hairy capsules have been recognized as var. atropurpurea (for example, in Florida, D. B. Ward 2004; in California, L. Abrams and R. S. Ferris 1923–1960, vol. 3). Variety atropurpurea in Malaysia has features of a distinct species, differing from typical O. corniculata in karyotype as well as in floral and vegetative morphology and is isolated by post-pollination reproductive barriers (B. R. Nair and P. Kuriachan 2004). Australasian variants sometimes identified as O. corniculata recently have been treated at specific rank (for example, P. J. de Lange et al. 2005). In view of the significant variation in ploidy level reported for the species, formal recognition of these and probably still other segregates may be justified.

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

Oxalis montana of eastern North America and O. oregana of the Pacific region have sometimes been treated as disjunct geographical taxa of the European (or Eurasian, depending on taxonomic interpretation) O. acetosella Linnaeus. The three are very similar and surely are closely related. Section Acetosellae Reiche also includes the Asian O. griffithii Edgeworth & Hooker f., O. leucolepsis Diels, and O. obtriangulata Maximowicz, as well as O. magellanica G. Forster (South America, New Zealand, Tasmania); the first two of these also have been treated as taxa within O. acetosella (see synonyms in Liu Q. R. and M. F. Watson 2008). Among all these, typical O. acetosella, the two American taxa, and O. magellanica have erect flowers, while those of the strictly Asian taxa are distinctly nodding, perhaps suggesting that the latter are monophyletic.

Oxalis oregana stands apart from O. montana and typical O. acetosella in its larger leaves and flowers and its strigose-villous (versus glabrous) sepals. Each of the three is distinct in petal coloration and this difference between O. acetosella and O. montana is perhaps the only one between them besides the geographical disjunction. In O. montana, the orange-yellow region near the petal base is constricted to a spot (versus a lateral band in typical O. acetosella) and a light and diffuse but distinct rose colored band lies immediately distal to the spot, connecting among the petals to form a circle. Oxalis acetosella in the strict sense occurs from Iceland to southern Europe (and possibly northern Africa), reportedly stretching as a broad band completely across Eurasia to Japan to Korea (J. F. Veldkamp 1971; Liu Q. R. and M. F. Watson 2008). Attributions of O. acetosella to Pakistan and other Himalayan localities apparently are based on plants of O. griffithii.

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