| Dwarf mistletoes (Arceuthobium, Viscaceae) are highly derived dioecious angiosperms parasitic on many gymnosperm hosts worldwide. Several mistletoe species are capable of inducing an unusual form of isophasic infection in which the internal (endophytic) system proliferates even into the apical buds of its hosts. Studies of the internal endophytic system have for the most part focused on the parasite within host secondary tissues. The present study characterizes the growth pattern of the isophasic endophytic system of Arceuthobium douglasii within the dormant apical buds of Pseudotsuga menziesii, documenting a consistent pattern of growth occurring even into the preformed leaves of the host. The apparently non-intrusive growth of the parasite appears to be developmentally synchronized with that of the host. I describe the ultrastructure of the parasite/host interface within apical buds of P. menziesii parasitized by A. douglasii and of Pinus contorta parasitized by A. americanum; no symplastic connections were observed. |
