Figs 3.5-3.9. Figs. 3.5 and 3.6. Light micrographs of transverse and longitudinal section, respectively, through cambial zone and bark of healthy one-year-old hybrid poplar cutting showing cambial zone (CZ), secondary phloem (SP), secondary phloem fibers (SPF), primary phloem (PP), primary phloem fibers (PPF), sclerids (S), ray cells (R), dilated ray (DR), crystal-containing cells (CC), cortex (CTX), and periderm (PER). Figs. 3.7-3.9. Longitudinal sections through the outer bark of poplar cutting inoculated with Cytospora chrysosperma. Fig. 3.7. Section through a portion of bark one week after inoculation, showing intercellular stromatic wedge (SW) composed of large-diameter hyphae extending through the inner periderm region. Fig. 3.8. Section through a portion of bark two weeks after inoculation, showing intercellular stromatic wedge (SW) composed of multiple advancing tips extending through the inner periderm and cortical regions. Fig. 3.9. Section through a portion of bark three weeks after inoculation, showing smaller-diameter intracellular hyphae and loosely woven pycnidial stroma (PS) and immature pycnidium (PYC) breaking through the outer bark. Bar = 40 m in all micrographs.