Secondary Growth of Roots
Secondary tissues comprise the greatest volume of the root mass of woody perennial plants. Primary tissues continue to form in the feeder roots, but the supporting root structure consists of secondary tissues produced by the lateral meristems, the vascular cambium, and one or more cork cambia. The usually unobserved underground root systems of most trees are as massive as the huge aerial bodies and counterbalance the aboveground weight thus keeping the tree upright and stable.
Roots produce branch roots and secondary tissues at the expense of the primary tissues. Cells in the primary tissue are broken and discarded as secondary growth proceeds. New lateral roots form endogenously (from within the root) and push outward from the pericycle, destroying cortex and epidermal tissues on their way to the soil.
Initiation of secondary growth takes place in the zone of maturation soon after the cells stop elongating there. The vascular cambium differentiates between the primary xylem and phloem in this zone and pericycle cells divide simultaneously with the procambium initials. The result is a cylinder of cambium encircling the primary xylem.
The vascular cambium almost immediately begins producing xylem cells inward and phloem cells toward the outside of the root, in the process flattening the primary phloem against the more resistant endodermis. Concomitant differentiation of cork cambia in the pericycle adds other areas of cell division in the stele. The combination of periderm and vascular tissue production not only physically breaks the remaining cells of the cortex and epidermis, but the lignified and suberized new cell walls laid down by the cambia effectively isolate the outer tissues as well from their source of supplies in the interior of the root. Their death is inevitable.
By the end of the first year, secondary growth has obliterated all but the central core of primary xylem cells and a few fibers of primary xylem pushed against the periderm. The zones at this time, therefore, from outside to inside are: periderm, pericycle, primary phloem, secondary phloem, vascular cambium, secondary xylem, and primary xylem.