Early evolution of the haustorial system in Loranthaceae mistletoes, and its relationship to the organization of the haustorium in root hemiparasitic Santalales

It is generally believed that aerial mistletoes evolved from some root hemiparasitic ancestor of the Santalales. Olacaceae seem the likely ancestral group for the Loranthaceae. However, little data have been presented in support of this view based on comparative anatomy of the haustorium in the various groups of Santalales. The present contribution reviews this information derived from studies made by the author on Olacaceae, Santalaceae, Opiliaceae and Loranthaceae spanning 40 years. The hypothesis is that if aerial mistletoes originated from some root parasitic ancestor, then structural features present in the haustorium of root parasitic Santalales might be expected to have persisted in the haustorium of loranth mistletoes. While such features might be retained in the most primitive, or least derived Loranthaceae, these may not all have been carried over into more advanced members of the family.