Development of vascular cambium & secondary growth in a root

Schematic diagrams explaining initiation of the vascular cambium, and secondary growth in the vascular cylinder of a tetrarch root.

A = vascular cylinder of a root at the completion of primary growth. The vascular cambium (blue) is initiated between primary phloem and primary xylem.

B = cells of the pericycle (green) located opposite protoxylem arms start dividing periclinally to form a continuous layer of cambium.

C = the vascular cambium produces secondary xylem to the inside, and secondary phloem to the outside (shaded); the primary vascular tissues are white. The primary phloem is being pushed outward.

D = a smooth ring of vascular cambium is formed, producing secondary xylem and phloem; the primary xylem remains in the center of the stem, the primary phloem has been crushed. Patches of pericycle remaining outside the crushed phloem may subsequently give rise to cork cambium. Note the increase in thickness of the vascular cylinder due to secondary growth.

1 = primary xylem, 2 = endodermis, 3 = primary phloem, 4 = pericycle, 5 = vascular cambium, 6 = secondary phloem, 7 = secondary xylem

Illustration adapted from: Weier, Stocking & Barbour, 1974, Botany: An Introduction to Plant Biology, 5th Ed. This material is used by permission of a subsidiary of John Wiley & Sons, Inc.