What is secondary growth and explain the successive stages of a normal secondary growth seen in Dicot stems ?

The secondary growth may be defined as an increase in thickness due to the formation of secondary tissues cut off by the cambium and cork cambium in the stellar and extrastelar regions.

The secondary growth in dicot stem occurs due to activity of cambium at stellar region and activity of cork cambium at extrastelar region. Stelar secondary growth leads to formation of annual ring.

Origin and activity of cambium- Generally, dicot stem contains disjoint, collateral, open and endarch vascular bundles arranged in a ring. The fascicular cambium is present between xylem and phloem of each vascular bundle.

At the time of secondary growth, the parenchymatous cells of primary meduallary rays lying between the edges of fascicular cambium become meristamic. They divide, redivide, and form a strip interfascicular cambium. The interfascicular cambium joins with fascicular cambium of either side and forms a complete ring of cambium. Secondly, growth begins with the activity of this cambium ring. Cambium cuts down secondary phloem towards the periphery and secondary xylem towards the center. Normally, cambium produces more secondary xylem than secondary phloem. Due to formation of more secondary tissues, the primary tissues are pushed apart from each other and remain small patches or the completely crushed.

The secondary xylem consists of vessels, tracheids, wood fibres and wood paremchyma. Vessels are more abundant. The secondary phloem consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres or bast fibres. The cells of secondary phloem are arranged in radial rows. The cambial cells between vascular bundles cut down secondary phloem.

Annual ring formation- the activity of cambium is influenced by the variations of climate. It produces more vessels with inner cavity during spring called the spring wood and less vessels with narrow cavities,more tracheids and wood fibres in autumn called autumn wood or late wood. These two types together form a growth ring or annual ring. By counting the total number of annual ring, the approximate age of the plant can be determined. After a long period of secondary growth, two types of woods appear in the stem such as, sapwood, heartwood. Sapwood is recently formed wood and heartwood is earlier formed wood.