Secondary growth leads to thicker roots and causes primary tissues like epidermis, hypodermis, cortex, and endodermis to be lost. Secondary roots develop a suberized cortical layer that prevents the entry of water, although it can enter through lenticels when they are present.
Only the largest main and lateral roots of dicot plants and gymnosperm show a typical secondary growth. Secondary growth begins when the procambium meristem, between the xylem and phloem, becomes the vascular cambium meristem. Depending on the number of phloem bundles, there is initially formed a variable number of segments of vascular cambium (Figure 1). At the same time, the portions of the pericycle close to the xylem spoke poles divide periclinaly and the cells that locate inner become vascular cambium meristem too. Later, the vascular cambium originated between xylem and phloem and that originated from the pericycle are connected to form a continuous structure: the root vascular cambium, which is a cylinder extending along the mayor axis of the root. Short after the continuous vascular cambium is formed, it produces secondary phloem toward the outer part and secondary xylem toward the inner part. The new layers of secondary xylem push the vascular cambium toward the surface of the root. In this way the perimeter of vascular cambium increases in length and xylem progressively accumulates so that the root increases in thickness.
In this way, the secondary root gets organized similarly to the secondary stem. Indeed, there is a continuity of the vascular bundles, as well as the vascular cambium, between the root and the stem. Unlike the primary growth, there is no transition zone between secondary stem and secondary root. The higher proportion of xylem and less delimited growth rings in the roots make possible to distinguish the root from the stem. In both, root and stem, the vascular cambium is composed of two types of cells: fusiform initial cells and radial initial cells. Fusiform initial cells differentiate in axially (vertical) oriented cells, whereas the radial initial cells give horizontally oriented cells.
The periderm is the outer structure of the secondary root. Periderm is derived from the phellogen meristem (cork cambium), after the formation of the secondary vascular tissue has begun.
From the surface to the inner part, the following structures can be distinguished in a secondary root:
Epidermis/cortex/periderm. The secondary growth of the root may be more or less advanced. At the beginning of the secondary growth, roots show epidermis and cortex with parenchyma cells (cortical parenchyma). In more advanced secondary growth, the epidermis and cortex is replaced by the periderm, which is a protective layer derived from the cock cambium (phellogen), a lateral meristem differentiated from the pericycle.
Secondary phloem. It is differentiated from the vascular cambium meristem, and it is laid toward the outer part. The formation of the secondary phloem lets to the separation between the primary phloem and the vascular cambium.
Vascular cambium. It is a lateral meristem responsible for the growing in thickness of the root. It produces secondary phloem toward the outer part and secondary xylem toward the inner part. As the root is getting thicker, the vascular cambium is increasing in size and moves away from the center of the root.
Secondary xylem. It is produced by the vascular cambium. It forms the wood of the root, and it is dead tissue in the thicker roots . The most recent xylem is the most superficial.
Primary xylem. It is generated during the primary growth and is found in the inner part of the root. It is death tissue in secondary roots.