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).