Oxalis (Gr., sour, the foliage containing an acid, watery juice), a genus of plants of which the common wood sorrel is a familiar representative. This and a few other genera formerly composed the family oxalidacea, which modern botanists have reduced to a tribe of the geranium family (geraniaceoe). The genus contains about 230 species, mostly herbs, or a few having somewhat woody stems; many have bulb-like rootstocks; some have no stem above ground, and all have leaves of three or more leaflets. The flowers are regular, with five sepals and as many petals; stamens ten, often united at the base, with the alternate ones shorter; ovary five-lobed, five-celled, with five distinct styles; the membranaceous, oblong capsule five-celled, each cell opening on the back and liberating two or more seeds; flowers solitary or in many-flowered clusters. Some species produce inconspicuous and particularly fruitful flowers, which are fertilized in the bud. The genus is widely distributed, but the greater number of species are natives of tropical America and southern Africa. Three species are found in the Atlantic states, one of which, the yellow wood sorrel (0. strictci), is very abundant, and makes its appearance in cultivated grounds as a weed; it has running subterranean shoots, leafy branching stems, which are at first erect, and then spreading upon the ground, and small yellow flowers in clusters of five or six on axillary peduncles.

Our other two species are stemless, their leaves and scapes arising from a rootstock or scaly bulb. The common wood sorrel, 0. acetosella, also a native of Europe and Asia, is common in woods from Pennsylvania northward to Canada, and it extends to the Pacific. The long petioles bear three obovate, delicate green leaflets, and the flower stalks, 2 to 5 in. high, bear each a solitary flower, with white petals beautifully veined with red. The foliage is pleasantly sour, owing to the presence of binoxalate of potash. Before the discovery of the method of preparing oxalic acid artificially, it was obtained from this plant, 500 lbs. of the herbage yielding 4 lbs. of the crystals of the binoxalate of potash, known as the salt of sorrel. This oxalis shares with white clover (trifolium re-pens) the credit of being the true shamrock; Bentham regards this as the real shamrock for the reason that it is a native of Ireland, while the clover is of comparatively recent introduction. The violet wood sorrel, 0. violacea, more abundant southward, has a similar habit to the preceding, but its flower stalks each bear several flowers in an umbel, and they are of a violet color.

A large number of the exotic species of oxalis are in cultivation, and are favorite plants in greenhouse and window culture, producing an abundance of bright cheerful flowers with the simplest treatment. There are both stem-less species and those with long trailing stems among the cultivated ones, and their flowers are white or nearly so, yellow, rose-colored, and crimson, often with two colors in the same flower, as in 0. versicolor; this is one of the finest, and has its white petals edged on the outside with crimson, so that the flowers when quite closed appear red, when fully open white, and when only partially open white striped with red lines on the under side. 0. Bowiei, 0. flava, 0. speciosa, 0. rosacea, and others are common in cultivation, and several are treated as border or bedding plants. Nearly all the species are sensitive to the action of light, drooping their leaflets and taking a position of sleep at nightfall, and many of them only open their flowers in the sunshine; one species, 0. sensitiva, from India, contracts its leaves when touched, and is nearly as irritable as the true sensitive plant. The leaves of several species, especially the common wood sorrel (0. acetosella), are mixed with salad to impart a pleasant acidity, and the tubers of others are used as food.

The oca of the Peruvians consists of the tubers of 0. crenata, which has spreading stems about 2 ft. high, yellow flowers, and tubers much like a small potato in appearance, tapering at the end toward the plant; there are a white and a red variety, the difference being solely in the color of the tubers. These are cultivated in Peru for their acid leaf stalks, and especially their tubers, which when boiled are farinaceous and nutritious, but have an acid taste that is disliked by most persons; it is said that this may be removed by exposing the tubers to the sun for several days. When the potato disease appeared in Europe this oxalis was one of the various substitutes tested, but on account of the small size of the tubers and the light yield they are not likely to come into general use. The Mexican 0. Deppei is a sternless species, with four leaflets to the leaves, and flowers which are red and sufficiently showy for it to be cultivated as a garden plant; the roots are parsnip-shaped, about 4 in. long, and have at the top numerous small bulblets by which it can be propagated.

The root is boiled and dressed with white sauce like salsify, and is regarded as very easy of digestion; it is very slow in forming its tubers, and likely to succeed better in southern than in northern gardens.

Common Wood Sorrel (Oxalis acetosella).