O growth in thickness Root can be called secondary growth, to distinguish it from growth in extension. Generally speaking, during secondary growth meristematic cell cylinders develop which allow the emergence of new root tissues.
The two meristematic tissues involved in secondary root growth are the vascular exchange, which allows the central cylinder to grow, and the suberogen or felogen exchange, which allows the growth of the periderm (bark).
The vascular exchange (from Latin vasculum, vessel) is so-called because it gives rise to new conductive vessels during secondary root growth. The vascular cambium is formed from the procambium and the pericycle, which combine and delimit an internal area of the central cylinder, where there is only xylem.
By actively multiplying, the vascular exchange cells originate xylem vessels to the innermost region and phloem vessels to the outermost region. Gradually the area delimited by the exchange becomes increasingly cylindrical.
The vascular root cambio is a meristem of mixed origin, primary and secondary. This is because it originates from both the procambium, a primary meristem, and the pericycle, an already differentiated tissue that overdifferentiation.
Suberogen or felogen exchange
The suberogen exchange, also called felogen (from the Greek phellos, cork and genos, which it generates), is a meristematic cell cylinder located in the cortical region of the root under the epidermis. Felogen is a secondary meristem, as it originates from cell de-differentiation of the cortical parenchyma.
As we have seen, the activity of felogen produces feloderm and subber, the latter a dead tissue that externally protects roots and stems with secondary growth.