Fig.10. Penetration of the bark of yellow birch by the canker rot pathogen Inonotus obliquus. The pathogen produces a wedge of fungus material into the bark. The larger white arrows show the boundary of the wedge that penetrated the bark three years before the tree was dissected. The smaller white arrows show an earlier boundary. The double black arrows in the bark show the tip of the wedge. The white pointer indicates the position where the wedge meets the living cambium. The black arrows in the xylem indicate the barrier zone formed in response to infection. Photo courtesy A. L. Shigo.