Fig. 8. SEM showing transverse section of callus of control wound after 4 weeks. Differentiation of callus into xylem and phloem has occurred. Gum ducts have ceased to produce gum. Xylem present at the time of wounding (OX) and xylem formed after wounding (NX) are visible. Necrotic tissue cutoff by NP is visible at upper right (arrow). X23.(From Wisniewski et al., 1984).