Xylem plant cells. Coloured scanning electron micrograph (SEM) of a longitudinal section through the xylem in the stem of a woody plant. The xylem is mainly composed of tracheids (brown, centre). Tracheids are long tubular cells that have walls strengthened with lignin, a woody fibre that helps to support the structure of the plant. The xylem transports water and mineral nutrients from the roots throughout the plant. Magnification: x400 when printed 10 centimetres wide.