oxalis

Oxalis
Oxalis griffithii
Scientific classification
Kingdom:	Plantae
Division:	Magnoliophyta
Class:	Eudicotyledoneae
Subclass:	Rosidae
(unranked):	Eurosids I
Order:	Oxalidales
Family:	Oxalidaceae
Genus:	Oxalis L.
Species
About 1000, see text

Oxalis (pronounced /ˈɒksəlɪs/)^[1] is by far the largest genus in the wood-sorrel family Oxalidaceae: of the approximately 900 known species in the Oxalidaceae, 800 belong here. The genus occurs throughout most of the world, except for the polar areas; species diversity is particularly rich in tropical Brazil, Mexico and South Africa.

Many of the species are known as wood-sorrels (in American English typically written "woodsorrels" or "wood sorrels") as they have an acidic taste reminiscent of the unrelated sorrel (Rumex acetosa) proper. Some species are called yellow-sorrels or pink-sorrels after the color of their flowers instead. Other species are colloquially known as false shamrocks, and some are rather misleadingly called "sourgrasses". For the genus as a whole, the term oxalises is also used.

Description and ecology

Floral diagram of Oxalis

These plants are annual or perennial. The leaves are divided into three to ten or more obovate and top notched leaflets, arranged palmately with all the leaflets of roughly equal size. The majority of species have three leaflets; in these species, the leaves are superficially similar to those of some clovers. Some species exhibit rapid changes in leaf angle in response to temporarily high light intensity to decrease photoinhibition.

The flowers have five petals, which are usually fused at the base, and ten stamens. The petal color varies from white to pink, red or yellow; anthocyanins and xanthophylls may be present or absent but are generally not both present together in significant quantities, meaning that few wood-sorrels have bright orange flowers. The fruit is a small capsule containing several seeds. The roots are often tuberous and succulent, and several species also reproduce vegetatively by production of bulbils, which detach to produce new plants.

Common wood sorrel (Oxalis acetosella)

Pale Grass Blue (Pseudozizeeria maha) of the dry-season brood laying eggs on Oxalis

Several Oxalis species dominate the plantlife in local woodland ecosystems, be it in the Coast Range ecoregion of the North American Pacific Northwest, or the Sydney Turpentine-Ironbark Forest in southeastern Australia where least yellow sorrel (O. exilis) is common. In the United Kingdom and neighboring Europe, common wood sorrel (O. acetosella) is the typical woodland member of this genus, forming large swaths in the typical mixed deciduous forests dominated by downy birch (Betula pubescens) and sessile oak (Quercus petraea), by sycamore maple (Acer pseudoplatanus), common bracken (Pteridium aquilinum), pedunculate oak (Q. robur) and blackberries (Rubus fruticosus agg.), or by common ash (Fraxinus excelsior), dog's mercury (Mercurialis perennis) and European rowan (Sorbus aucuparia); it is also common in woods of common juniper (Juniperus communis ssp. communis). Some species – notably Bermuda-buttercup (O. pes-caprae) and creeping woodsorrel (O. corniculata) – are pernicious invasive weeds when escaping from cultivation outside their native ranges; the ability of most wood-sorrels to store reserve energy in their tubers makes them quite resistant to most weed control techniques.

Tuberous woodsorrels provide food for certain small herbivores – such as the Montezuma Quail (Cyrtonyx montezumae) –, though the oxalic acid content probably makes the plants toxic to many mammals. The foliage is eaten by some Lepidoptera, such as the Polyommatini Pale Grass Blue (Pseudozizeeria maha) – which feeds on creeping wood sorrel and others – and Dark Grass Blue (Zizeeria lysimon).

Use by humans

Oxalic acid, the toxin found in many wood-sorrels

Some oca (O. tuberosa) tubers

The edible tubers of the oca (O. tuberosa), somewhat similar to a small potato, have long been cultivated for food in Colombia and elsewhere in the northern Andes mountains of South America. The leaves of scurvy-grass sorrel (O. enneaphylla) were eaten by sailors travelling around Patagonia as a source of vitamin C to avoid scurvy.

A characteristic of many members of this genus is that they contain oxalic acid (whose name references the genus), giving the leaves and flowers a sour taste, refreshing to chew in small amounts^[2]. However, in large amounts these species are toxic, interfering with proper digestion and kidney function. In India, creeping wood sorrel (O. corniculata) is only eaten seasonally, starting December/January. The leaves of common wood sorrel (O. acetosella) are commonly thought^{[citation needed]} to make a very good tea when dried.

Four-leaved pink-sorrel (O. tetraphylla) grown as a pot plant

In the past, it was a practice to extract crystals of calcium oxalate for use in treating diseases and as a salt called sal acetosella or "sorrel salt" (also known as "salt of lemon"). Growing oca tuber root caps are covered in a fluorescent slush rich in harmaline and harmine which apparently suppresses pests; this phenomenon has been studied to some extent at the Colorado State University^[3]. Creeping wood sorrel and perhaps other species are apparently hyperaccumulators of copper. The Ming Dynasty text Precious Secrets of the Realm of the King of Xin from 1421 describes how O. corniculata can be used to locate copper deposits as well as for geobotanical prospecting. It thus ought to have some potential for phytoremediation of contaminated soils.

Several species are grown as pot plants or as ornamental plants in gardens. Species with four regular leaflets – in particular O. tetraphylla (four-leaved pink-sorrel) – are sometimes misleadingly sold as "four-leaf clover", taking advantage of the mystical status of four-leaf clovers.

Selected species

Oxalis arborea

Oxalis articulata ssp. rubra

Oxalis corymbosa

Oxalis dehradunensis

Oxalis gigantea branch

Oxalis luteola

Oxalis magnifica

Purple wood sorrel (O. purpurea)

See also

• Louis Feuillée

Footnotes

References

• Bais, Harsh Pal; Park, Sang-Wook; Stermitz, Frank R.; Halligan, Kathleen M. & Vivanco, Jorge M. (2002): Exudation of fluorescent β-carbolines from Oxalis tuberosa L. roots. Phytochemistry 61(5): 539–543. doi:10.1016/S0031-9422(02)00235-2 PDF fulltext
• Bais, Harsh Pal; Vepachedu, Ramarao & Vivanco, Jorge M. (2003): Root specific elicitation and exudation of fluorescent β-carbolines in transformed root cultures of Oxalis tuberosa. Plant Physiology and Biochemistry 41(4): 345-353. doi:10.1016/S0981-9428(03)00029-9 Preprint PDF fulltext
• Łuczaj, Łukasz (2008): Archival data on wild food plants used in Poland in 1948. Journal of Ethnobiology and Ethnomedicine 4: 4. doi:10.1186/1746-4269-4-4 PDF fulltext

External links

Wikimedia Commons has media related to: Oxalis

• Nature's Secret Larder – Wood sorrel
• Plant Profile for Oxalis – USDA Plant Database
• Glossary of Archaic Chemical Terms: Sorrel
• Oxalis
• Pictures and Information of Oxalis adenophylla in Chile
• Pacific Bulb Society website – with many references to Oxalis species