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Published In: Species Plantarum 1: 433. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library

Project Name Data (Last Modified On 9/1/2017)
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1. Oxalis L. (wood sorrel, lady’s sorrel)

Plants annual or more commonly perennial herbs (occasionally woody elsewhere), sometimes with rhizomes, stolons, or bulbs (sometimes with tubers elsewhere). Aerial stems absent (in species with bulbs) or present and variously erect to prostrate. Leaves all basal or alternate on the aerial stems, then sometimes appearing fasciculate, mostly long-petiolate, the petiole jointed at the base. Stipules absent or, if present, then relatively small, scalelike or herbaceous, fused to the petiole for part or all of the length. Leaf blades palmately trifoliate (sometimes the first leaves simple; also with 4 to several palmate or pinnate leaflets elsewhere). Leaflets narrowly to broadly obtriangular to shallowly obcordate, angled at the base to a minute, thickened stalklike portion (pulvinus), broadly rounded to truncate or shallowly notched at the tip, often reddish- or purplish-tinged. Inflorescences basal (in stemless species, then arising directly from the bulb) or axillary, usually relatively long-stalked, consisting of a simple umbel or small panicles (these with a central flower flanked by a pair of branches bearing 2 or more flowers), occasionally reduced to a solitary flower. Flowers actinomorphic, hypogynous, perfect, the stalk often hinged and with a small, slender bract at the base. Calyces of 5 free sepals, these usually slightly overlapping laterally, persistent but not becoming enlarged at fruiting. Corollas bell-shaped to broadly funnelform, of 5 free petals, these tapered to short stalklike bases and attached to the base of the stamen tube, overlapping laterally, yellow or pink to purple, rarely white, sometimes with reddish or green markings toward the base on the upper side. Stamens 10 in 2 cycles of alternating longer and shorter ones, the filaments fused into a shallow ring toward the base, the anthers not exserted, attached toward their midpoints, yellow. Pistil 1 per flower, of 5 fused carpels. Ovary superior, 5-locular, somewhat 5-angled in cross-section, with several to numerous ovules in a single longitudinal series per locule, the placentation axile. Styles 5, sometimes persistent at fruiting, the stigmas 1 per style, often more or less 2-lobed. Fruits capsules, cylindrical (oblong-ellipsoid in O. violacea), usually tapered to a short beak, somewhat 5-angled in cross-section, dehiscent by a longitudinal slit on each valve. Seeds 2 to several per locule, surrounded by a translucent aril, ejected from the fruit when the aril abruptly turns inside-out, 1–2 mm long, more or less elliptic with pointed ends, somewhat flattened, the surfaces with 5–13, broad, transverse (longitudinal elsewhere) ridges usually connected irregularly to form a raised network (often appearing somewhat wrinkled or only faintly ridged in O. voiolacea), reddish brown to dark brown, the tops of the ridges sometimes grayish or whitened. Five hundred or more species, nearly worldwide.

Species of Oxalis exhibit a high degree of morphological variation in most of their vegetative structures. The groups that do not form bulbs tend to vary in their stem thickness, orientation, degree of branching, and hairiness. The leaves can vary similarly in size, vestiture and degree of anthocyanin (purplish pigments) production. This creates difficulty in species determinations. The leaflets usually exhibit a circadian rhythm: at the end of each day they droop or fold downward from a small thickened basal portion (pulvinus), becoming oriented parallel to the petiole, only to spread again the following morning (Johnsson et al., 2006).

Species of Oxalis tend to accumulate oxalates (oxalic acid, potassium oxalate, calcium oxalate) in their tissues, resulting in a tart or sour flavor. For this reason, fresh leaves are sometimes used as an ingredient in salads and greens, similar to the use of Rumex acetosella (sheep sorrel, Polygonaceae), which has a similar flavor. The leaves of some species traditionally were used medicinally for scurvy. The South American species, O. tuberosa Molina (oca), is cultivated as a root crop for its starchy tubers, particularly in the Andean region. However, readers should note that Oxalis tissue that has not been treated to break down or leach out the oxalates can be toxic to humans and livestock when consumed in large quantities, as a build-up of oxalate crystals can lead to kidney damage and other symptoms (Burrows and Tyrl, 2001).

It is unclear whether the original shamrock of Irish folklore was a species of Trifolium, some other member of the Fabaceae, or a species of Oxalis (Colgan, 1896; Everett, 1971; E. C. Nelson, 1991). Whatever the case, the shamrocks currently in the horticultural trade mostly are cultivars of several Oxalis species, commonly the South American O. regnellii Miq. (with three, strongly and broadly obtriangular leaflets) and the Mexican O. tetraphylla Cav. (O. deppei Lodd.; with four leaflets). A specimen at the Missouri Botanical Garden herbarium collected in 1962 by Frederick Comte (#4633) documents another cultivated species, O. corymbosa DC. (O. martiana Zucc.; O. debilis Kunth var. corymbosa (DC.) Lourteig), as a weed in a greenhouse in Kirkwood (St. Louis County). This species is known as lilac oxalis and pink wood sorrel for its relatively showy, light purple to reddish purple or pink corollas. It is a perennial, bulb-forming native of tropical South America, but is widely cultivated as a houseplant and has long been a widespread weed in tropical and warm-temperate portions of the Old World. Although this species occasionally becomes established as an escape in some southeastern states (K. R. Robertson, 1975), it is not cold-hardy in Missouri’s climate and thus is not considered likely to become a member of the flora.

Various species of Oxalis act as hosts in the complex life cycles of the common rusts of maize, sorghum, and related grasses (Puccinia sorghi Schwein. and related fungi), some of which are commerically important crop pathogens. Several species also can be aggressive weeds of greenhouses, lawns, crop fields, and disturbed ground.

The taxonomy of the yellow-flowered wood sorrels with aerial stems has long been controversial. Differences in interpretation of type specimens (K. R. Robertson, 1975; D. B. Ward, 2004) and thus the application of various names to plants of differing morphologies by the two most recent monographers of the group (Eiten, 1963: Lourteig, 1979) have been complicated by the general morphological variability of the plants. Although the reduced fertility of artificially produced interspecific hybrids has been studied (Lovett Doust et al., 1981), the frequency of natural hybridization is poorly known. Many of the species also are heterostylous (Eiten, 1963), that is, two or more, commonly three, different kinds of flowers are produced, differing in the lengths of the styles relative to the stamens (long and ascending above the relatively short filaments, short and curved outward between the long filaments, and sometimes also intermediate). In spite of this phenomenon, potential inbreeding is high (Ornduff, 1972; Lovett Doust et al., 1981) and facultative apomixis is known to occur in at least O. dillenii (Lovett Doust et al., 1981). At the species level, the present treatment substantially follows that of D. B. Ward (2004), who studied the genus in Florida, and Nesom (2009), who studied the group for a forthcoming treatment in the Flora of North America series. Both of these works differ markedly from the Missouri treatments of Steyermark (1963) and Yatskievych and Turner (1990), and neither agrees entirely with either of the last two taxonomic revisions of the group (Eiten, 1963; Lourteig, 1979).

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