Secondary growth may be defined as an increase in thickness due to the formation of secondary tissues. The secondary growth in dicotstem occurs by the activity of cambium at the stellar region and cork cambium at the extra stellar region.
Stelar Secondary growth: The vascular bundles in dicot stem are conjoint, collateral, open and endarch and are arranged in a ring. During secondary growth, the partnership enchymatous cells of primary medullary rays lying between the edges at the fascicular cambium (cambium strips between phloem and xylem in the vascular bundles) become meristamic. They divide and redivide to form strips of interfascicular cambium, which join the fascicular cambium strips and either side and form a complete ring of cambium.
Secondary growth begins with the activity of the cambium rings, which cuts off secondary phloem towards the periphery and the secondary xylem towards center. Normally it produces more secondary xylem than secondary phloem. Due to the formation of heavy secondary tissues, the primary tissues are pushed apart from each other and remain in small patches or are completely crushed. The secondary phloem consists of sieve tubes, companion cells, and phloem parenchyma and phloem fibres. On the other hand, the secondary xylem consists of vesssels, trachieds, xylem fibres and xylem parenchyma. Vessels are present abundantly.
The activity of cambium is influenced by the variations of climate. It produces more vessels with larger cavities during spring season called springwood or early wood and less vessels with narrow cavities in autumn season called autumn wood or late wood. These two types of ring together form a growth ring or annual ring. A number of annual rings are seen after heavy secondary growth.
Extra stellar secondary growth: When more secondary tissues are produced at the stellar region pressure is exerted towards epidermis. Epidermis ruptures and another ring of meristamic tissues originates in the region of hypodermis called cork cambium or phellogen. It cuts off cells on both sides, the cells produced towards outside are called cork or phellem and those towards innerside as phelloderm. While cork cells are dead and heavily suberised and lignified, cells of phelloderm are thin walled arranged in radial rows. Cock phellogen and phelloderm together constitute the periderm.
In old stems the dead tissues are produced by cork cambium at outer side form the bark and contain lense-shaped pores at the region of old stomata called lenticels.