XVI International Botanical Congess

Evolution in aerial mistletoes has involved various structural modifications of the haustorial system. Here the secondary haustorium of Aprimitive@ Loranthaceae is compared with that in root hemi-parasitic Santalales. Cellular specialization within the endophytic system is also examined. In Alepis flavida (Loranthaceae) secondary haustoria exhibit collapsed layers, a flask-shaped vasculature, and an interrupted zone with vessels often containing stainable material. These features are absent in the primary haustorium, but occur in root hemi-parasitic Santalalean haustoria. The endophytic tissue usually shows little cellular specialization, with ordinary parenchyma and tracheary elements predominating. However, in Korthalsella and Phoradendron (Viscaceae) flange cells are differentiated. Phorodendron shows greatest diversity with both flange cells and flange cells combined with transfer cells. These are located at the interface against host vessels (or tracheids in conifer hosts) and abutting tracheary elements of the sinker.