The tissues comprising the cortical haustoria and sinkers in the endophytic system of Phoradendron flavescens were studied in plants parasitizing the walnut, Juglans Hindsii. Cortical haustoria-longitudinally oriented strands in the host bark-have a central cylinder containing xylem and phloem inclosed by a parenchymatous cortex. The xylem consists of tracheary elements and parenchyma cells. The phloem consists of sieve-tube members, companion cells, and parenchyma cells. Cortical haustoria having secondary growth contain, in addition, fibers in their secondary xylem. Sinkers-radially oriented extensions into the host wood-occur at intervals along a cortical haustorium. Each normal sinker consists of three distinct parts: (1) a parenchymatous part imbedded in the host phloem; (2) a meristem corresponding in position to the host cambium; (3) a part imbedded in host wood and composed of two kinds of cells, parenchyma cells and tracheary elements, both more or less isodiametric. Additionally, many sinkers ha...