Charcoal. Coloured scanning electron micrograph (SEM) of a section through a sample of charcoal, showing its internal structure. Charcoal is produced by the slow heating of wood in the absence of oxygen (pyrolysis). This process preserves the cell structure of the plant tissue. Here, the xylem tracheid cells and phloem sieve tube cells of the vascular tissue can still be seen. Magnification: x200 when printed 10 centimetres wide.