Secondary Growth in Plants

Primary growth — the vertical lengthening of roots and stems — is driven entirely by apical meristems. Secondary growth, an increase in the girth (diameter) of the root or stem, is a separate process seen in dicotyledonous plants and gymnosperms, and it is driven by lateral meristems rather than apical ones.

In a dicot stem, secondary growth begins with the formation of a single, continuous ring of vascular cambium. Each vascular bundle already contains a strip of cambium lying between its primary xylem and primary phloem — this is the intrafascicular cambium ("intra-" = within the bundle). Between adjacent bundles, cells of the medullary rays that had already matured as permanent parenchyma regain the ability to divide — a process called dedifferentiation — and this reactivated tissue becomes the interfascicular cambium ("inter-" = between the bundles). Because the intrafascicular strips and the newly formed interfascicular patches all lie in a single plane around the stem, they join up into one uninterrupted cambial ring. This whole process of ring formation is triggered seasonally: with the return of favourable growing conditions, the intrafascicular cambium activates first, closely followed by dedifferentiation of the fusiform and ray initials of the medullary rays, which together completes the ring. …