Plant 340

Viola species (Violaceae)

Violets and pansies

Viola is a primarily northern hemisphere genus of some 500 species, which has attracted the interest of gardeners for centuries. The genus occupies many habitats from grassland, woodland and marsh to montane environments, for example, Viola cheiranthifolia is endemic to elevations above 2,700 metres on the volcano El Teide in Tenerife.

A typical five-petalled Viola flower has one plane of symmetry. The lower petal, which is broader than the others, usually has a pouch extending behind the flower. Two of the five stamens have nectar spurs that project into this pouch, where nectar oozing from the spurs collects. In addition to nectar rewards, insects are attracted to Viola flowers by petal features such as lines of pigmentation (nectar guides), iridescence due to epidermal features and extreme contrasts between the centre and margin of the flower when seen under ultraviolet light. Viola species often produce two flower types. Chasmogamous flowers open and attract pollinators, whilst cleistogamous flowers remain closed, producing selfed seed. This reproductive assurance mechanism ensures each plant produces at least some seed each year.

Ionone is a compound that contributes to Viola odorata’s distinctive odour. However, genetic differences in a specific olfactory receptor mean people experience ionone in different manners, which may contribute to the plant’s illusive scent.

When mature, Viola fruits split into three parts which, as they dry, contract; seeds become minute, explosively liberated projectiles. In some Viola species, additional dispersal occurs by ants carrying seeds to their nests; they are attracted to fatty bodies (elaiosomes) on the seeds.

Two common names are applied to Viola species; violets are small-flowered annual and perennial species, whilst pansies are large-flowered horticultural cultivars. Today’s lucrative garden pansy trade, which sees tens of millions of plants used for bedding each year, is the product of the efforts of Lady Mary Bennet in Surrey, Lord Gambier in Buckinghamshire, ambitious nineteenth-century nurserymen and generations of Victorian horticultural show breeders. The pansy originated through hybridisation, artificial selection and hit-and-miss breeding of the common, lowland, multicoloured European Viola tricolor, and two yellow, montane species: central and north-west European Viola lutea; and Central Asian Viola altaica. Generations of selection saw enthusiasts breed towards a standard as they vied for coveted prizes. Compared to the parental species, flower diameters more than doubled to over eight centimetres, floral outlines became circular and the spectrum of petal colours expanded to range from white through to deep purple, almost black.

Further reading

Beattie AJ 1969. The floral biology of three species of Viola. New Phytologist 68: 1187-1201.

Fuller R 1993. Pansies, violas and violettas. The complete guide. The Crowood Press.

Jaeger SR et al. 2013. A Mendelian trait for olfactory sensitivity affects odor experience and food selection. Current Biology 23: 1601-1605.

Stephen Harris