Oxalidaceae

Wood Sorrel Family

Oxalidaceae, the wood sorrel family, is a diverse family of flowering plants in the order Oxalidales, comprising about 5-7 genera and around 750-800 species. The family is dominated by the large genus Oxalis. Members are mostly herbs, but also include shrubs and small trees, characterized by their often compound leaves (typically trifoliate) which frequently exhibit 'sleep movements' (nyctinasty) and a distinctive sour taste due to the presence of oxalic acid. The family has a cosmopolitan distribution.

Oxalidaceae example - Oxalis acetosella leaves and flower

Overview

The Oxalidaceae family is well-known for the genus Oxalis, commonly called wood sorrels or shamrocks (though true shamrocks are clovers in the Fabaceae family). These plants are widespread globally, found in diverse habitats from forests to grasslands and disturbed areas. Many species are recognized by their clover-like, palmately trifoliate leaves that fold down at night or in response to touch, and their characteristic sour flavor.

While predominantly herbaceous, often growing from bulbs, tubers, or rhizomes, the family also includes shrubs and notable small trees like Averrhoa carambola (starfruit). Flowers are typically 5-merous with 10 stamens, and many species of Oxalis exhibit heterostyly, a genetic mechanism promoting outcrossing. Fruits are often explosive capsules that forcefully eject seeds, though berries occur in Averrhoa.

Economically, the family includes edible species like oca (Oxalis tuberosa), an Andean tuber crop, and starfruit (Averrhoa carambola). Some Oxalis species are grown as ornamentals, while others are considered weeds in gardens and agriculture due to their persistence and vegetative propagation. The high oxalate content requires caution, as excessive consumption can be harmful.

Quick Facts

Scientific Name: Oxalidaceae R.Br.
Common Name: Wood Sorrel Family
Number of Genera: Approximately 5-7 (e.g., Oxalis, Averrhoa, Biophytum, Dapania, Sarcotheca)
Number of Species: Approximately 750-800 (most in Oxalis)
Distribution: Cosmopolitan, especially diverse in tropical and subtropical regions (esp. Neotropics, Southern Africa).
Evolutionary Group: Eudicots - Rosids - Fabids - Oxalidales

Key Characteristics

Growth Form and Habit

Mostly herbs (annual or perennial), often possessing underground storage organs like bulbs, tubers, or rhizomes. Also includes some shrubs and small trees (e.g., Averrhoa). Some species can be climbing or acaulescent (stemless with basal leaves).

Leaves

Leaves are usually alternate (sometimes basal or appearing whorled), and typically compound. Most commonly palmately trifoliate (like clover), but can also be pinnately compound (Biophytum, Averrhoa) or even unifoliate through reduction. Leaflets often possess basal pulvini, enabling nyctinastic ('sleep') movements where leaflets fold downwards at night or under stress. Stipules are present or absent. A sour taste due to oxalic acid is characteristic of many species.

Inflorescence

Flowers can be solitary or arranged in various types of inflorescences, commonly cymes or umbel-like structures, often borne on long peduncles arising from the leaf axils or directly from the ground in acaulescent species.

Flowers

Flowers are typically bisexual, actinomorphic (radially symmetrical), and 5-merous.

Calyx: Consists of 5 distinct or slightly basally connate sepals, persistent.
Corolla: Consists of 5 distinct or slightly basally connate petals, often clawed, typically white, yellow, pink, rose, or violet. Petals are convolute (twisted) in bud.
Androecium: Consists of 10 stamens arranged in two whorls of 5. The outer whorl typically has shorter filaments than the inner whorl. Filaments are often connate at the base, forming a tube around the ovary.
Gynoecium: The ovary is superior, composed of 5 fused carpels forming 5 locules. Placentation is axile, with usually 1 to many ovules per locule. There are 5 distinct styles, each with a capitate or slightly bilobed stigma. Heterostyly (specifically tristyly or distyly) is common in many Oxalis species, involving different relative lengths of styles and stamens in different individuals to promote cross-pollination.

Fruits and Seeds

The fruit is most commonly a loculicidal capsule, which often dehisces explosively, ejecting the seeds. In the genus Averrhoa, the fruit is a fleshy, indehiscent berry. Seeds are small and often possess an aril (a fleshy appendage) that aids in dispersal, particularly in the explosive capsules where the aril turns inside out to eject the seed.

Chemical Characteristics

The family is characterized by the presence of high concentrations of soluble and crystalline oxalates (salts of oxalic acid), particularly potassium oxalate. These compounds are responsible for the characteristic sour taste and can be toxic if consumed in large quantities, potentially leading to kidney problems.

Field Identification

Identifying members of Oxalidaceae often relies on their distinctive leaves and flower structure:

Primary Identification Features

Leaves: Usually compound (most often palmately trifoliate 'clover-like'), leaflets often notched at the apex, exhibiting sleep movements (folding down).
Taste: Distinctly sour taste when a small part of a leaf is chewed (due to oxalates - use caution).
Flowers: 5-merous (5 sepals, 5 petals), with 10 stamens (often fused at base) and 5 distinct styles.
Habit: Mostly herbs, often low-growing, sometimes with bulbs or rhizomes.
Fruit: Often an elongated capsule that may split explosively; sometimes a fleshy berry (Averrhoa).

Secondary Identification Features

Heterostyly: In Oxalis, observing different flower forms within a population with varying style and stamen lengths (requires close examination of multiple plants).
Petals: Often twisted (convolute) in bud.
Stamens: Filaments often fused into a tube at the base; arranged in two whorls of different lengths.
Seeds: Small, often ejected forcefully from capsules, sometimes with an aril.

Seasonal Identification Tips

Spring/Summer/Fall: Most species flower during warmer months. Leaves are present throughout the growing season. Look for flowers and developing fruits.
Winter: Many herbaceous species die back to underground bulbs, tubers, or rhizomes. Some evergreen species persist. Dried capsules might remain on some plants.

Common Confusion Points

Fabaceae (Legume family - Clovers): True clovers (Trifolium) also have trifoliate leaves but belong to Fabaceae. Clovers have distinctly different flowers (pea-like, often in dense heads), 10 stamens (usually 9 fused + 1 free), a single style, and fruit is a legume (pod). Clovers lack the sour taste.
Geraniaceae (Geranium family): Some geraniums have palmately lobed or divided leaves. Flowers are also 5-merous, but typically have 5 or 10 stamens (not always in 2 distinct whorls) and a characteristic "crane's bill" fruit (schizocarp) that splits into 5 segments, often with coiled awns.
Other plants with trifoliate leaves: Always check flower structure (10 stamens, 5 styles) and taste for confirmation.

Field Guide Quick Reference

Look For:

Leaves: Usually compound (often trifoliate), show 'sleep movements'.
Taste: Sour (oxalates).
Flowers: 5 sepals, 5 petals.
Stamens: 10 (often fused at base).
Styles: 5, distinct.
Fruit: Capsule (often explosive) or berry.
Habit: Mostly herbs, often low-growing.

Key Distinctions:

Sour taste (vs. clovers).
10 stamens & 5 styles (vs. clovers, geraniums).
Explosive capsule common (vs. legume pod of clovers).
Leaflets often notched, fold down at night.
Heterostyly common in Oxalis.

Notable Examples

The Oxalidaceae family includes common wildflowers, weeds, cultivated ornamentals, and edible plants:

Oxalis acetosella

Common Wood Sorrel

A delicate perennial herb found in woodlands across temperate Europe and Asia. It has characteristic trifoliate leaves arising from a creeping rhizome and solitary white flowers with pink veins. It prefers shady, moist conditions.

Oxalis corniculata

Creeping Wood Sorrel

A cosmopolitan weedy species, often found in lawns, gardens, and disturbed areas. It is a low-growing perennial herb with creeping stems that root at the nodes. Leaves are trifoliate (often purplish), and flowers are small and yellow. It reproduces rapidly via seeds and stolons.

Averrhoa carambola

Starfruit / Carambola

A small tree native to Southeast Asia, cultivated in tropical regions worldwide for its distinctive fruit. Unlike most Oxalidaceae, it has pinnately compound leaves. The fruit is a fleshy berry with prominent ridges; when cut in cross-section, it resembles a star. Flowers are small and pinkish.

Biophytum sensitivum

Little Tree Plant / Sensitive Plant

An unusual small herb found in tropical Asia and Africa. It has a short, unbranched stem topped by a rosette of pinnately compound leaves that rapidly fold downwards when touched or disturbed (thigmonasty), similar to the true sensitive plant (Mimosa pudica, Fabaceae). Flowers are small, yellow or pinkish, borne in umbels.

Oxalis tuberosa

Oca

A perennial herbaceous plant cultivated in the Andes for its starchy, edible tubers. Oca tubers come in various colors (yellow, orange, red, purple) and have a slightly tangy flavor due to oxalates, which can be reduced by sun exposure after harvest. Leaves are trifoliate, and flowers are yellow.

Phylogeny and Classification

Oxalidaceae is the namesake family of the order Oxalidales, which belongs to the fabid (or nitrogen-fixing) clade within the rosids group of eudicots. This placement is strongly supported by molecular data.

Within the Oxalidales, Oxalidaceae is sister to the family Connaraceae. The order Oxalidales also includes several other families, primarily woody plants like Cunoniaceae, Elaeocarpaceae, Cephalotaceae (containing the unique Albany Pitcher Plant), and Brunelliaceae. The relationships among these families form a well-supported clade.

Position in Plant Phylogeny (APG IV)

Kingdom: Plantae
Clade: Angiosperms (Flowering plants)
Clade: Eudicots
Clade: Rosids
Clade: Fabids (Nitrogen-fixing clade)
Order: Oxalidales
Family: Oxalidaceae

Evolutionary Significance

Oxalidaceae showcases several interesting evolutionary features:

Chemical Defense/Adaptation: High oxalate concentration serves as a defense against herbivores but also requires physiological adaptations by the plant.
Leaf Movements: Nyctinastic (sleep) movements of leaflets are well-developed and represent an adaptation possibly related to water conservation or temperature regulation. Thigmonasty (touch response) in Biophytum is also notable.
Reproductive Strategies: Widespread occurrence of heterostyly in Oxalis is a classic example of a genetic mechanism promoting outcrossing. Explosive seed dispersal is an effective short-distance dispersal mechanism.
Life Form Diversity: Despite being dominated by herbs, the family shows diversity including bulbs, tubers, rhizomes, shrubs, and trees.
Vegetative Propagation: Many herbaceous species rely heavily on bulbs, bulbils, or rhizomes for persistence and spread, contributing to their success as weeds or colonizers.