XVI International Botanical Congess

Between eight and ten percent of all vascular species spend part to all of their life cycle anchored in the crowns of woody hosts without direct access to earth soil, but less than ten percent of this group do so as aerial parasites. Moreover, some 80 families contain at least one autotropic epiphyte, while the mistletoes belong exclusively to Santalales. I consider this evolutionary asymmetry relative to opportunities for plants to use woody flora simply as perches versus sources of water and mineral nutrients and sometimes fixed carbon as well. Factors that account for the many fewer heterotrophic than autotrophic lineages in woodland canopies appear to be related to aspects of water and ion relations and seedling establishment that distinguish these two life strategies. Nevertheless, aerial parasitism offers certain advantages that permit the mistletoes to exploit a greater variety of kinds of forest habitats than their autotrophic counterparts.