Cross section: Secondary Growth in Pinus root magnification: 100x

In young roots, the vascular cylinder is surrounded by two rings of cells, the pericycle and endodermis, and above these layers the cortex and epidermis. In older roots underling activity of the cork cambium replaces epidermal and a portion of cortical tissues with a protective zone of cork rich periderm.

A well developed cortex of 4 to 5 layers of thin-walled parenchymatous cells contains clusters of sclerenchymatous stone cells and small resin canals.

The inner-most layer of the cortex is marked by a single- layered endodermis of brown suberized cells containing dark staining tannin.

Just below the endodermis is a multi-layered pericycle containing tannin and starch grains.

The vascular cylinder or stele consists of an outer narrow ring of phloem,and deep to this, the vascular cambium. The vascular cambium produces annual growth of secondary phloem towards the outside of the root and secondary xylem towards center of the root. Crushed primary phloem is present outside the secondary phloem. Because of greater production of xylem, the bulk of the vascular cylinder is dominated by radially arranged rays of secondary xylem interspaced with medullary rays of parenchyma cells. Annual growth rings of spring and summer wood is difficult to distinguish in roots . The xylem has no true vessels, consisting entirely of tracheids. The phloem consists of sieve tubes and phloem parenchyma, lacking companion cells.