Oxalis is the largest genus in the wood sorrel family Oxalidaceae. Of the approximately 900 known species in the Oxalidaceae, 800 belong to Oxalis. Many of the species are known as Wood Sorrel or Woodsorrel. The genus occurs throughout most of the world, except for the polar areas; species diversity is particularly rich in tropical Brazil and Mexico and in South Africa.
These plants are annual or perennial. The leaves are divided into three to ten or more round, heart-shaped or lanceolate leaflets, arranged in a whorl 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, though clovers differ in having the leaflets not in a whorl, and of unequal size with two smaller side leaflets and one larger central leaflet. Some species exhibit rapid changes in leaf angle in response to temporarily high light intensity. The flowers have five petals, usually fused at the base, and ten stamens; the petal colour varies from white to pink, red or yellow. The fruit is a small capsule containing several seeds. The roots are often tuberous, and several species also reproduce vegetatively by production of bulbils, which detach to produce new plants.
Also called the Sour Flower.
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 edible leaves of Scurvy-grass Sorrel (O. enneaphylla) were eaten by sailors in southern South America as a source of vitamin C to avoid scurvy.
A characteristic of many members of this genus is that they contain oxalic acid, giving the leaves and flowers a sour taste, refreshing to chew in small amounts. However, in large amounts, these species are toxic, and interfere with proper digestion. 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"). Nevertheless, the leaves of common wood sorrel are commonly thought to make a very good tea when dried.
Several species are grown as ornamental plants in gardens, while others, notably O. pes-caprae and O. corniculata, are pernicious invasive weeds in cultivation away from their native ranges. A species which regularly has leaves with four leaflets, O. tetraphylla, is sometimes misleadingly sold as "four-leaf clover", taking advantage of the mystical status of four-leaf clovers.