Fig. 11.5-8. Transverse section of pea stem (Pisum sativum). This micrograph shows the metaxylem and metaphloem of a vascular bundle. Notice that parenchyma makes up a large fraction of the xylem, maybe one-quarter of the of the xylem volume is parenchyma, and if you count the cells, there are only about 25 lignified, red-stained vessel elements in the xylem here but there is a much greater number of parenchyma cells: most of the cells of pea xylem differentiate as parenchyma rather than as tracheary elements. I emphasize this because you may read statements such as “Xylem cells are dead at maturity” or “Xylem is a dead tissue” – people often focus exclusively on the conducting cells and forget about the xylem parenchyma cells. There has been little research on the role of parenchyma in xylem, but notice the intimate contact between parenchyma cells and vessel elements: do the parenchyma cells actively pump material into or out of the vessels? Could they adjust the pH or concentration of salts in the xylem sap, perhaps such that the solution in one vessel has different characters from neighboring vessels?