Secondary Growth

Secondary growth develops  in the stem and root systems of dicotyledonous and Gymnospermous plants.  It is characterized by the development of secondary tissues which include new xylem and phloem conduits (within the axial system)  as well as the development of other secondary tissues, including fibers as well as a new protective layer, called the periderm, which replaces the primary epidermis as that tissue becomes damaged during secondary stem and root growth. Secondary growth commences in the vascular bundles and will spear to the interfascicular region between the bundles. In some plants, such as the Helianthus stem at left, secondary growth is limited but will result in the formation of a ring of secondary xylem and phloem, which will fill in the interfascicular regions between the vascular bundles.