Mistletoes are unique enough among canopy flora to merit separate treatment. Certain relic terrestrial forms parasitize roots of other plants, but they will be mentioned only in passing; the principal focus will be on aerial mistletoes which are here defined as shrubby hemiparasites growing attached to branches. These unusual plants deviate from “true” epiphytes in form, diversity, physiology, and impact on hosts. Most mistletoes belong to Santalales, a sizable, predominantly tropical, order. Xylem rather than phloem supply is reputed to be the usual consequence of santalalean parasitism, but, as noted later, advanced forms as well (e.g., Arceuthobium) take host substrates. Mistletoes have long occupied a place in European folklore and continue to figure prominently in certain holiday rituals of the Western world. Their destructive qualities are widely recognized. Fortunately, enough scientific curiosity has been aroused by these remarkable organisms to encourage a hard look at their biology. In fact, vegetative and reproductive activity is better known for these plants than for any other like-size assemblage of forest-canopy residents. In this chapter, that information is summarized and aerial mistletoes are contrasted with the true, fully autotrophic, epiphytes.

Systematics and biogeography

The mistletoe habit is polyphyletic, having arisen at least three or four times in Santalales and again in Laurales. The largest mistletoe family is santalalean Loranthaceae with some 900 species distributed unevenly among about 65 genera. Second in size and much more uniform in floral structure is Viscaceae, a group of perhaps 400 species in just seven genera.