Among terrestrial plants the vascular plants are the largest and the most con- spicuous. Because of the presence of lignified supporting and conducting tissues such as xylem, terrestrial vascular plants can develop large upright forms. Composed mainly of xylem tissue, the stems of trees are characterized by elongated cells with thickened walls, impregnated by lignin. Diagrams of xylem cells (vessels, tracheids and fibers) can be seen in textbooks...