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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.