Plant 337

Oxalis species (Oxalidaceae)

Wood sorrel

European wood sorrel (Oxalis acetosella), the minor Andean food plant oca (Oxalis tuberosa) and the South African Bermuda-buttercup (Oxalis pes-caprae) are members of the large, cosmopolitan genus Oxalis. The genus is particularly species rich in South America and the Cape Region of South Africa.

The bright yellow, funnel-shaped flowers of Oxalis pes-caprae open when the sun is shining. In cloudy conditions, the flowers are closed, and trumpet shaped. In the mid-eighteenth century, Oxalis pes-caprae was introduced into European gardens as a horticultural novelty. Over the past 150 years, its distinctive flowers and clover-like leaves have become familiar sights in many parts of Europe, especially in the western Mediterranean. In this region, Oxalis pes-caprae is a major agricultural weed and can form dense mats that out-compete early spring growth of native plants in natural habitats. It is another plant categorised as an invasive species, after escaping from gardens.

In South Africa, populations of Oxalis pes-caprae may contain individuals with three different types of flowers. The differences are associated with the relative positions of the styles and anthers (tristyly); the flowers are described as long-styled, mid-styled and short-styled. Tristyly is a means of promoting crossing between different plants, therefore reducing the possibility that individual plants mate with themselves. In Bermuda-buttercup’s native distribution, most plants are fertile and tetraploid (they have four complete sets of chromosomes). In contrast, in the Mediterranean most plants are short-styled pentaploids (they have five complete sets of chromosomes). The brilliant floral displays, so attractive to insects, have no biological function as the plants do not produce seeds. Propagation and spread is through tiny, underground bulbs.

In 1891, the Lamarckian botanist George Henslow (1835-1925) explained the absence of variation in European Oxalis pes-caprae populations because they propagated and spread from a single plant introduced to Malta in the early nineteenth century. Detailed analysis of herbarium data shows European plants are derived from at least two independent introductions, whilst DNA suggests at least three main areas of introduction: Iberian Peninsula; Apennine Peninsula; and the Levant. Given the evidence of multiple introductions, an explanation now needs to be found for why the most common type of European Oxalis pes-caprae is a short-styled pentaploid.

Oxalis is derived from the Greek oxus (sour) because of the taste of the oxalic-acid-rich leaves. The acid was first extracted from the leaves of Oxalis acetosella, which is now in bloom in British woodlands, including the Arboretum.

Further reading

Castro S et al. 2007. Distribution of flower morphs, ploidy level and sexual reproduction of the invasive weed Oxalis pes-caprae in the western area of the Mediterranean region. Annals of Botany 99: 507-517.

Ferrero V et al. 2015. Invasion genetics of the Bermuda buttercup (Oxalis pes-caprae): complex intercontinental patterns of genetic diversity, polyploidy and heterostyly characterize both native and introduced populations. Molecular Ecology 2015; 24: 2143–2155.

Papini A et al. 2017. History vs. legend: Retracing invasion and spread of Oxalis pes-caprae L. in Europe and the Mediterranean area. PLoS ONE 12: e0190237.

Stephen Harris