Viscum album L. (Santalaceae)



Typical habit of Viscum album. Viscum album fruits.

Viscum album is a woody, poisonous Eurasian shrub with yellow-green, oblong, leathery leaves parasitic on trees such as limes, hawthorns, poplars and apples. The most widely distributed subspecies is a generalist parasite, whilst other subspecies are specialist parasites. Strictly, mistletoes are hemiparasitic, producing sugars via photosynthesis and taking inorganic nutrients and water from their hosts. Individual plants may form immense globes festooning the branches of large trees. Indeed, infection levels may be so great that during the winter months otherwise deciduous trees appear covered in leaves.

Individual mistletoe plants are dioecious (i.e., male or female), with obscure, green, insect-pollinated flowers. Until mistletoes start to flower genders are indistinguishable, although mistletoe populations tend to be biased towards females. Female mistletoes attract most attention when they produce their clusters of small, white or yellowish, fleshy, single-seeded fruits. The very sticky, carbohydrate-rich fruit flesh appears to comprise two layers (viscin). The layer close to the seed is indigestible and rich in pectins, whilst the outer layer is apparently digestible and rich in cellulose.

Mistletoe seeds germinate in early spring, forming short-lived, torpedo-shaped, non-parasitic seedlings that stick to host branches. Eventually, seedlings start to develop a sophisticated piece of plumbing, the haustorium, and become parasitic as they bind their lives to their hosts. After about five years, mistletoe starts to flower and fruit.

Birds, such as thrushes, fieldfares and blackcaps, are major mistletoes dispersers but they consume the fruit differently: thrushes eat fruits whole, whereas blackcaps eat only the flesh. Consequently, bird behaviour appears to determine the distances over which mistletoe seeds disperse and the positions that mistletoe seedlings have on their hosts since birds deposit seeds in their faeces or by beak wiping. The Medieval Latin proverb turdus ipse sibi malum cacat [the thrush defecates its own destruction] reveals the biological knowledge of the saw's originators and one use for mistletoe - birdlime, a sticky substance used to coat tree branches and catch birds, including thrushes. Birdlime is illegal but trapping songbirds remains a European conservation issue.

The appearance and chemistry of mistletoe has endowed it with all manner of mystical, mythical and murderous powers in Eurasian folklore. The Roman writer Pliny records Celts attaching special properties to mistletoe parasitising oaks, whilst Norse mythology relates how the god Balder could only be killed by mistletoe. However, the most pervasive belief in the English-speaking world is the association of mistletoe and kissing, especially at Christmas.

Further reading

Zuber D 2004. Biological Flora of Central Europe: Viscum album L. Flora 199: 181-203.

Stephen Harris