HOST-FORMED TYLOSES IN VESSELS OF THE MISTLETOE PHORADENDRON (VISCACEAE)

The xylem-tapping mistletoe, Phoradendron, forms vessel-to-vessel conn ections with its dicotyledonous host, Juglans. Contact vessels of the host may become embolized as evidenced by the presence of tyloses. Tyl oses 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 embolized host vessels, but also those of the parasite with w hich they were continuous. Some parasite vessels may terminate in vess el members lacking perforations at the juncture with host vessels; the se vessels lacked tyloses. In parasitic angiosperms in general tyloses within haustoria may be formed by either host or parasite. Several fu nctional roles are reported for tyloses including; absorption of nutri ents, defence against invasion of the parasite, and the occlusion of e mbolized vessels, as exemplified in the present study.