Plant 377

Mirabilis jalapa L. (Nyctaginaceae)

Four-o’clock plant

Mirabilis jalapa is a bushy, tuberous perennial, native to dry regions of tropical America, which is widely naturalised across tropical and temperate regions and grown as a popular ornamental. The attraction of the plant is its fragrant, vividly coloured flowers. Individual flowers may be plain coloured (yellow, red, magenta, pink, white) or patterned with stripes and spots or broken into coloured sectors. Moreover, flowers on the same plant may have different colours and patterns at different stages of their development. Flower colour is associated with two pigments: yellow betaxanthins and violet betacyanins. A flower-derived crimson dye is sometimes used for colouring cakes and jellies.

As Mendel’s laws of inheritance were being rediscovered in the early twentieth century, Mirabilis jalapa became a model plant for investigating the genetic basis of flower colour. When dark-pink-flowered plants were crossed with white-flowered ones, light-pink-flowered offspring are produced rather than dark-pink-flowered ones as expected. The answer is that dark-pink genes are not dominant to white genes, but that both show incomplete dominance.

Mirabilis flowers have no petals – the coloured parts are sepals. Flowers open in the late afternoon and close in the morning, giving rise to the name ‘four-o’clock plant’. The flowers are pollinated by long-tongued hawkmoths and other nocturnal insects attracted by their sweet-smelling fragrance. Curiously, visible fluorescence emitted by betaxanthin is absorbed by betacyanin to create contrasting fluorescent patterns, which may also have a role in pollinator attraction.

The ‘maruellous varietie [of flowers] doth not without cause bring admiration’ led to the plant’s generic name, which means ‘wonderful’. Carolus Linnaeus’ adoption of the specific epithet jalapa refers to the mistaken belief that Mirabilis jalapa was the source of the medicinal purgative jalap (it really comes from Ipomoea jalapa – one of the morning glories).

An Aztec medicinal plant (tlaquilin) represented in a coloured herbal called the Badianus Manuscript, which was made in 1552, is evidence that Mirabilis jalapa was cultivated in pre-Columbian Mexico. By the mid-sixteenth century, Mirabilis had arrived in Europe, making it one of the oldest ornamentals in European gardens to have originated in the Americas. Within a century it was grown in European gardens, including the Oxford Physic Garden, because of it ‘rarenesse, beautie, and sweetnesse’. In mild conditions, the plant’s tuberous roots mean it can overwinter. More commonly, it readily regenerates from pea-sized, spherical fruits. The distinctive, single-seeded fruits are covered by the blackened, wrinkled, leathery remains of the flower.

Further reading

Bogle AL 1974. The genera of Nyctaginaceae in the southeastern United States. Journal of the Arnold Arboretum 55: 1-37.

Gandía-Herrero F et al. 2005. Floral fluorescence effect. Nature 437: 334.

Le Duc A 1995. A revision of Mirabilis section Mirabilis (Nyctaginaceae).

Stephen Harris