Fig.
7.5-2.
Longitudinal section of stem of spurge (Euphorbia). This vascular bundle
has very small tracheary elements along its inner edge (at the bottom of the
micrograph, arrows); these are called protoxylem and they are small because the
cells stopped expanding while they – and all the surrounding internode tissues
– were still young and close to the shoot apical meristem. Just above these
are tracheary elements that are slightly wider (metaxylem); they delayed their
maturation and spent more time growing before beginning to deposit their
secondary wall. The widest tracheary elements just at the edge of the mass of
fibers underwent the most prolonged period of enlargement and delayed
maturation, so they were very large by the time they began depositing their
secondary wall. By the time they started to deposit their secondary wall, the
shoot apical meristem had grown upward and was relatively far away.