Parasitic plants are found in different plant families that are not related to each other. However, within the subclass of Rosidae and the order of Santalales, there is a group of plant families that contains a large number of species with a parasitic or hemiparasitic life style. The genus Viscum has been grouped earlier in the family Loranthaceae but it is now widely accepted to establish two different families, the Loranthaceae and the Viscaceae (Calder 1983). The major features distinguishing the two families are listed in a table by Calder (1983). According to this, the main macroscopic difference is that Loranthaceae have larger flowers that are bright coloured and represent both sexes, whereas the latter is monoecious and highly reduced. The family Viscaceae sens. str. comprises seven genera with approximately 400 species (Barlow 1983): Arceuthobium, Ginalloa, Notothixus, Korthalsella, Phoradendron, Dendrophthora, Viscum.
The genus Arceuthobium has its widest distribution with twenty-four species in North America, and the greatest concentration in the western United States and central Mexico. Arceuthobium oxycedri has a distribution between the 30th and 40th northern parallel from the Azores through northern Algeria and Southern Europe to the Himalaya. The other four species are found in restricted areas in East Asia and temperate Africa. Ginalloa is the smallest genus of the Viscaceae with five species in South East Asia. Notothixos is found mostly in tropical forests from Sri Lanka to eastern Australia and the Salomon Islands. Korthalsella has a distribution from eastern and southern Asia through the Malesian region to Australia and New Zealand and to many islands in the Indian and Pacific Ocean. Phoradendron is found in South America and in the southern parts of North America. Dendrophthora has a similar distribution as Phoradendron but goes less south and less north than the latter. The genus Viscum comprises about ninety species, among which two thirds are native to Africa (see Kirkup et al., this book), the rest is native to Eurasia and Australia. In Europe we have two Viscum-species: Viscum cruciatum ex Boiss. and Viscum album L.
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