Anchor plant
Colletia is a genus of five large, shrubby species endemic to the temperate parts of southern South America. The genus was named Colletia by the eighteenth-century French naturalist Philibert Commerson to honour his seventeenth-century, botanical countryman Philibert Collet. Commerson was the naturalist on Louis-Antoine de Bougainville's circumnavigation of the world between 1766 and 1769; he collected Colletia in South America. There were many notable events in Commerson's travels, two of which were: (i) the discovery of the familiar, garishly-coloured, bougainvillea (Bougainvillea spectabilis) around Rio de Janeiro (Brazil); and (ii) Commerson's 'male' assistant turned out to be female; Jeanne Baret was the first woman to circumnavigate the globe. In 1790s Oxford, in his undergraduate lectures, John Sibthorp, the Sherardian Professor of Botany, dismissed Commerson as a 'Libertine' who named genera after 'his several Mistresses'.
Colletia paradoxa must be one of the most bizarre-looking shrubs found growing in British gardens; the species was appropriately christened. It is a native of the dry regions of southern Brazil and Uruguay, but was introduced into British horticulture in 1824. From a distance, the anchor plant, which can reach three metres in height, appears to be a mass of flattened, grey-green, waxy, thorny 'leaves' that are arranged, in tiers, strictly opposite each other. On closer examination, the 'leaves' are spiny, triangular stems modified for photosynthesis (cladodes). The leaves proper are small, flattened structures, found below the cladodes and are usually only visible on the youngest growth; the leaves are deciduous.
The anchor plant produces small, short-tubed, white flowers with a feint almond odour and red, three- or four-chambered capsules. In Britain, the shrub produces tight clusters of flowers below the cladodes from late autumn through the winter. Most Colletia species, like Colletia paradoxa, have small, pale-coloured, insect-pollinated flowers. However, Colletia ulicina, which is endemic to central Chile, has clusters of long, tubular, red flowers. Field observations have shown this species is pollinated by both hummingbirds and bumblebees. Bird pollination is very rare in the family Rhamnaceae.
Colletia paradoxa, together with the other members of the genus, is capable of fixing atmospheric nitrogen into a form that the plant can use. Nitrogen fixation in Colletia, which uses actinomycetes (a sort of bacteria) inside nodules on the plant's roots, means that members of the genus can grow on low-nutrient soils. Colletia paradoxa is well-adapted to growth in dry regions, whilst its spines are excellent protection against grazing animals.
Further reading
Bond G and Becking JH 1982. Root nodules in the genus Colletia. New Phytologist 90: 57-65.
Medan D and Montaldo NH 2005. Ornithophily in the Rhamnaceae: The pollination of the Chilean endemic Colletia ulicina. Flora 200: 339-344.
Stephen Harris