However, the vascular cambium quickly unites the bundles to produce an ectophloic siphonostele which is the mature form. Cross Section of a Pine Twig - Note the Leaf Bases Secondary Xylem Secondary xylem is composed entirely of tracheids and ray parenchyma . Some tracheids have thicker walls and are more important for physical support than for water transport. The rays are uniseriate and there is no axial parenchyma in the xylem. Accumulated secondary xylem is called wood.
Conifers produce a light uniform wood which can be very strong.
Some species produce secondary phloem fibers and some do not. A Periderm develops and forms a substantial bark. These range in size from a few mm. There is only one centrally located vascular bundle in the leaf. However, it is often partitioned into separate strands.