Plant 172

Helleborus species (Ranunculaceae)



Helleborus is a small genus of Eurasian herbs whose members are most familiar as the hellebores that brighten our gardens in late-winter and early-spring. The genus is broadly divided into two groups, caulescent species with visible above ground stems and acaulescent species without above ground stems. Helleborus flowers have radial symmetry but the showy parts are not petals, they are sepals. The petals are a ring of tubular structures (nectaries) between the sepals and numerous pollen-producing anthers. At the centre of the flower is a cluster of unfused carpels.

As the flowers mature, the nectaries and anthers fall off; the sepals remain attached to the plant until late in the season. Manipulative experiments have shown the persistent sepals are important for the development and maturation of the seeds that are contained inside inflated fruits called follicles.

The nectar of hellebores contains populations of natural yeasts. Experimental studies which manipulate yeast densities in the nectar of stinking hellebore (Helleborus foetidus) have shown the yeasts ferment the nectar. Fermentation produces heat, increasing the temperature of nectar and altering the within-flower thermal microenvironment. These observations have led to the hypothesis that increased temperature of the hellebore blooms makes them attractive to the few pollinators that are on the wing during the winter months.

All hellebores are toxic and produce a burning sensation in the mouth when chewed. The Christmas rose (Helleborus niger), native to the Alps and Appenines, is cardiotoxic and has been used as a medicine since antiquity. Hellebores have also been used as an ancient form of chemical warfare. The second-century CE Greek geographer Pausanias reported that eight centuries earlier the Athenian statesman Solon ordered hellebore roots to be thrown into the water supplies of the cities his armies besieged. The cities were overthrown as their populations suffered the effects of violent diarrhoea.

In 1648, Jacob Bobart the Elder was growing three sorts of hellebore in the Oxford Physic Garden. Two native caulescent species from Oxfordshire, Helleborus foetidus and Helleborus viridis, and the introduced medicinal acaulescent species, Helleborus niger. Today, there is a vast range of ornamental hellebores available. Much of this colour and morphological variation occurs in hybrids produced by crossing the eastern Mediterranean Helleborus orientalis with its closely related species. In these garden hybrids sepal colours range from blackish-grey through purple and red to yellow, white and green, whilst in double and semi-double hellebores nectaries and anthers become petaloid.

Further reading

Herrera CM 2005. Post-floral perianth functionality: contribution of persistent sepals to seed development in Helleborus foetidus (Ranunculaceae). America Journal of Botany 92: 1486-1491.

Herrera CM and Pozo MI 2010. Nectar yeasts warm the flowers of a winter-blooming plant. Proceedings of the Royal Society, Series B 277: 1827-1834.

Zhao L et al. 2011. Floral organogenesis of Helleborus thibetanus and Nigella damascena (Ranunculaceae) and its systematic significance. Botanical Journal of the Linnean Society 166: 431-443.

Stephen Harris