Structure of the host-parasite interface of Boschniakia hookeri Walpers (Orobanchaceae)

The host-parasite interface of Boschniakia hookeri Walpers (Orobanchaceae) parasitic on Gaultheria shallon Pursh is radially grooved, and shows several significant contrasts to that known for Orobanche. Unlike the latter, Boschniakia has a large, perennial interface, and the host surface is made up almost entirely of differentiated xylem. The xylem-xylem contact involves both fiber-tracheids and vessel elements on the host side, and vessel elements on that of the parasite; open xylem connections across the interface are relatively common. The phloic continuity and transfer-like cells known from Orobanche have no counterpart in Boschniakia. No meristematic zone is present along the interface, the initiation of the latter being brought about by an early inversion of the host cambium. The term “tuber”, even though not completely appropriate, is used for the swollen Boschniakia organ directly above the interface, and the term “soma” rejected.