Gross xylem structure of the interface of Psittacanthus ramiflorus (Loranthaceae) with its hosts and with a hyperparasite

Abstract

An emended interpretation is offered of the gross xylem structure and development of the haustorium of Psittacanthus ramiflorus (DC.) G. Don (Loranthaceae) and the woodrose induced on its hosts, based on decorticated material. At the point of entry, the host cambium is split or perhaps locally destroyed but regenerates, eventually in an inverted position, with its initials apparently assuming a position perpendicular to the mother branch. The resultant cambial ‘front’ is apparently strongly stimulated in some areas while nearly inactive in others, resulting in a basket-like structure of branching and anastomosing arms of xylem which eventually fuse completely. The parasite develops successive, concentric layers of xylem strands which connect perpendicularly to the inner margin of the woodrose. These xylem strands fuse upwardly, merging into tongue-like layers of normal xylem applied to older xylem. Brief reference is made to variation of woodroses in other species of Psittacanthus and to the latter's xylem connection to a hyperparasitic Phoradendron (Viscaceae).