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What are mistletoes?

The familiar white-berried Christmas mistletoe is just one of many hundreds of mistletoe species worldwide. All are plant parasites, and in their native lands many have similar folklore and superstitions to our own species. Most grow in the tropics and subtropics, with only a few species in northern temperate zones

All mistletoe species grow as parasites on the branches of trees and shrubs. They rely on their hosts for water and mineral nutrient supply. Technically speaking they are only hemi-parasites as most have green leaves and can photosynthesise - but many do use some metabolites from the host

The host-parasite interface is a woody ‘gall’ known as a haustorium. Some cross-sections of a mistletoe haustorium are shown on the left. The picture shows mistletoe on apple - the mistletoe is the paler wood at the top, the host is the lower half.

The diagram is from a 19th century mistletoe paper, showing how several 'sinkers' of mistletoe tissue can be seen in some cross-sections, often belonging to different mistletoe plants.

Some species, such as the Dwarf Mistletoes of North America, can be serious forestry pests, with infestations causing significant damage to timber crops. Other species are more benign, often occurring in small numbers on isolated trees. Some, particularly those in the Loranthaceae family, can be quite attractive, with colourful flowers. A few species have medicinal properties, and have been used as cures since ancient times.

Mistletoes are all related to each other, and were once grouped into one plant family. Today they are classified into a number of families, though most are in the Loranthaceae (over 900 species) and the Viscacaeae (over 400 species). Smaller families include the Eremolepidaceae and Misodendraceae. Some examples of Viscaceae and Loranthaceae species are pictured below:

Viscum cruciatum, a red-berried mistletoe from Spain - An African mistletoe, from the mangrove swamps of Kenya

Viscum cruciatum - from South-West Europe, and Agelanthus sansibarensis - from Africa

There are about 1400 mistletoe species in total, with estimates varying according to the classification systems used, and whether subspecies are upgraded to species. The European mistletoe Viscum album is divided into four subspecies, three in Europe and one in Asia.

All mistletoes are parasitic on trees and shrubs. Strictly speaking they are only hemi-parasitic as most have green leaves to photosynthesise but rely on the host for water and mineral nutrients. Many pose intriguing biological and evolutionary questions for the biologist, but to most people they are only important for their local folklore and the problems some can cause in forestry plantations.