Secondary growth similar to stems

Greater xylem parenchyma than stems

Wider vessels than in stems

Cross-section of a basswood (Tilia americana) eight-year-old root. Note the general similarity in structure to the secondary xylem in stems (compare its structure with that of the same species in the unit on Woods).

Rings of large vessels characterize the early wood; the late wood appears as a more compressed region of growth layer. Rays are numerous and uniseriate. Outside of the vascular cambium zone is found a sizeable zone of secondary phloem. The densest outer region of the root is periderm. The center of the root (top left) is primary xylem.

Overall, secondary growth in roots shows a smaller proportion of lignification and greater proportion of parenchyma cells than in stems. Root vessels are often wider than in stems. The parenchyma cells are longer and thus believed to be more ancestral than those found in stems.