Penetration of western hemlock, Tsuga heterophylla, by the dwarf mistletoe Arceuthobium tsugense, and development of the parasite cortical system

Germinating dwarf mistletoe (Arceuthobium tsugense) seeds and young infections on western hemlock (Tsuga heterophylla) were collected, sectioned, and viewed using standard microtechnique methods. The radicle had a biseriate epidermal layer. A penetrating structure formed in the apex of the radicle when it encountered a needle base. As the penetrating structure enlarged, it used the needle base to lever and force itself into the host branch. Once in contact with phellem, the penetrating structure produced uniseriate filaments that penetrated living host tissue. Established infections had wedge-like sinkers embedded in the xylem that were connected to both small-diameter cortical strands in the host phloem and large-diameter strands beneath the phelloderm. Xylem was in the center of the cortical strand, surrounded by a sheath of parenchymous cells. The larger strands had an additional outer sheath which was contiguous with the tissue that was connected to aerial shoots. This sheath had larger and more numerous cells than the inner sheath, which possessed large vacuoles filled with phenolic compounds. Connections between the sinkers and cortical strands were of the cell type found in the inner sheath. Chloroplasts were found throughout the sinkers and cortical strands, except in xylem elements. Unique morphological features of the endophytic system, such as double-sheathed cortical strands, may aid in identifying dwarf mistletoe species. Morphological features, such as the presence of a biseriate epidermal layer in the radicle and sheath structure of cortical strands, and the mode of pentration itself, may help to clarify phyletic relationships and be aids in the classification of dwarf mistletoes.