The growth in thickness by the activity of secondary tissues is called secondary thickening. It involves stelar growth by the activity of vascular cambial ring and extra stelar growth by the activity  of cork cambium.
Dicot stem
secondary Growth in Dicotyledonous Stem
1. The cambial ring formed is circular in cross section from the beginning onwards.
2. The cambial ring is partially primary (fascicular cambium) and partially secondary (interfascicular cambium).
3. Periderm orginates from the cortical cells (extra stelar in origin).
4. In Dicot stem, for mechanical support xylem is with comparatively smaller vessels, greater fibers and less parenchyma.
5. More amount of cork is produced for protection.
6. Lenticels on periderm are very prominent.

Dicot root
DICOT ROOT SECODARY THICKENING
1. The cambial ring formed is wavy in the beginning and later becomes circular.
2. The cambial ring is completely secondary in origin.
3. Periderm originates from the pericycle (intra stelar in origin).
4. In Dicot root, xylem is with big thin walled vessels with few fibers and more parenchyma.
5. Less amount of cork is produced as root is underground.
6. Lenticels on periderm are not very prominent.

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