Host-Formed Tyloses in Vessels of the Mistletoe Phoradendron (Viscaceae)

Abstract

The xylem-tapping mistletoe, Phoradendron, forms vessel-to-vessel connections with its dicotyledonous host, Juglans. Contact vessels of the host may become embolized as evidenced by the presence of tyloses. Tyloses may also occur in parasite vessels contiguous with embolized host vessels. Differences between parasite and host in nuclear size, shape and intensity of staining make it possible to determine the origin of the tyloses present in contact vessels. Host-formed tyloses occluded not only emboli zed host vessels, but also those of the parasite with which they were continuous. Some parasite vessels may terminate in vessel members lacking perforations at the juncture with host vessels; these vessels lacked tyloses. In parasitic angiosperms in general tyloses within haustoria may be formed by either host or parasite. Several functional roles are reported for tyloses including; absorption of nutrients, defence against invasion of the parasite, and the occlusion of embolized vessels, as exemplified in the present study.