Anatomical and developmental tissue relationships between Arceuthobium spp. endophytic tissues and host vascular tissues were examined by light and transmission electron microscopes. The host-parasite pairs studied were Psudotsuga menziesii/A. douglasii, Tsuga heterophylla/A. tsugense, Pinus sabiniana/A. occidentale, and P. lambertiana/A. californicum. The morphological form and growth characteristics of A. douglasii in different aged host tissues was found to be coordinated with growth and maturation of the host. It is proposed that morphological forms of Arceuthobium endophytic tissue be categorized as (1) primary, (2) diffuse-secondary, or (3) localized-secondary in order to semantically clarify the relationship of endophytic morphology with primary or secondary growth stages of host tissue. In localized-secondary endophytic forms, the parasite integrates with host rays to form multiseriate infected rays. At both the light and electron microscope level, parasite cells can usually be identified by their distinctive chromocentric nuclei and abundant lipid bodies or lipid ghosts. Sinker cells of Arceuthobium have unusual plastids which resemble etioplasts and which do not store starch. Sinker cells have distinctive mitochondria with unusually large nucleoid areas. They have abundant endoplasmic reticulum. Wall/plasmalemma specializations increase the membrane surface area in relation to cell volume in sinker cells. The walls of host and parasite are fused at the middle lamella common to both organisms and the organisms share a common apoplast. Pit-like regions in the fused walls of the host/parasite interface were commonly seen in light microscope studies, although such interspecific pitting is seen less than intraspecific pitting. It was determined that, whereas intraspecific pits are traversed by complete plasmodesmata, interspecific pits had no plasmatic channels, or, only half plasmodesmata on the host side of the pit. In one case a half plasmodesmata was seen on the Arceuthobium side of the host/parasite interface. On the basis of the electron microscope studies of the host/parasite interface it appears that plasmatic connections between host and parasite do not normally occur. Because of this plasmatic isolation it can be concluded that nutrient acquisition does not involve direct flow of nutrients via interspecific symplastic bridges. Therefore, photosynthate, normally housed within phloem cells, must be leaked into the common apoplast of both host and parasite before becoming available for absorption into the parasite symplast. Since host and parasite lack symplastic continuity but share a common apoplast, apoplastically-mobile herbicides should be tested for their ability to accumulate in parasite tissues. Cytopathological effects on the host cells were relatively mild although a significant increase in the ratio of radial to axillary vasculature was noted in infected tissue. Other modifications included a tendency for increased numbers of specialized phloem parenchyma (Strasburger cells) in infected rays. No apparent anomalies were observed in conjunction with host sieve cell structure and development. The structure and ontogeny of healthy P. sabiniana phloem was also studied. It was found that the sieve element reticulum (SER) of mature sieve cells is derived from plastids during the maturation of the sieve cell. After maturation some of the SER membranes seem to disassociate into individual 60 (ANGSTROM) fibrils. These fibrils then reassociate and appear to be condensed into paracrystalline bodies which, in turn, participate in the generation of new membranes in the mature sieve cell. Because of their plastid origin, the SER membranes may have energy transducing and ionic pump capabilities that commonly are associated with plastid membranes. Such specialized functions of SER may contribute to the translocating capacities of sieve cells.