Explain the process of secondary growth in the stems of woody angiosperms with the help of schematic diagrams. What is its significance?

The process and structures associated with secondary growth in dicot stems are, (i) Formation of the cambial ring:In dicot stems, an intrafascicular cambium is present between the xylem and phloem, which is primary in nature. The parenchymatous cells of each medullary ray, lying between the intrafascicular cambium of the adjacent bundles divide and become meristematic and form a strip of cambium called the interfascicular cambium. Both types of cambia join together and to form a complete ring of vascular cambium, which is partly primary and partly secondary meristematic in origin.The cambial ring becomes active and begins to cut off new cells, both towards the inner and outer sides, by vertical or oblique divisions of the elongated fusiform initials. The derivatives, which cut-off towards the outer side get differentiated into the secondary phloem, whereas those produced on the inner side are differentiated into secondary xylem. The cambium is generally more active on the inner side than on the outer side. As a result, the xylem increases more rapidly in bulk than the phloem and soon forms a compact mass. This forms the main bulk of the plant body. Due to the continued formation of secondary xylem, both the primary and secondary phloem of the earlier years gets gradually crushed.(ii) Annual rings:The activity of the cambium is under the control of a series of physiological and environmental factors. The cell of the cambium divide rapidly and several layers of the undifferentiated cells become visible. Consequently, a larger volume of xylem tissue is produced, having comparatively large, thin-walled and light-staining components. One light and one dark coloured zone comprise one year’s growth and this is known as the annual ring or growth ring. Since each annual ring corresponds to the growth of wood in one year, one can estimate the age of the tree to some degree of accuracy by counting these rings.Due to the addition of secondary phloem and secondary xylem elements, the outermost layer of the cortex becomes highly stretched and may crack open. During this process, a few layers of meristematic tissue arise in the cortex. This is called the cork cambium. The cells of the cork cambium are rectangular and these cut-off cells on both the sides. Those formed on the outer side become sub-raised and constitute the cork, which is impervious to water and air. The inner cells become parenchymatous and may contain chloroplasts. These constitute the secondary cortex. The cork cambium and the secondary cortex are collectively known as the periderm. Periderm cells are living and some of them take part in the conduction of the metabolites.(iii) Lenticels:These are openings or breaks in the cork tissue, which look like raised spots on the surface of the stem and permit the exchange of gases between the outer atmosphere and the internal tissues of the stem cells. These occur in most trees. Lenticels consist of a pore formed due to the rupture in the epidermal layer. Below the pore, a loose mass of thin-walled, rounded parenchymatous cells, which is known as the complementary tissue is visible. The lenticels may remain scattered or get arranged in longitudinal or vertical rows. Rows of lenticels may occur opposite the medullary rays, thus facilitating free exchange of gases.(iv) Heartwood:After certain years of growth, the xylem elements of the stem of a number of trees develop dark brown coloration, especially in the central or innermost layers. This region comprises of dead elements with highly lignified walls and is called heartwood.