Secondary growth is the increase in thickness due to formation of Secondary tissues by the activity of cambium and Cork cambium.
(1) Activity of vascular cambium
The meristem producing secondary xylem and phloem is called vascular cambium. As it occupies lateral position in the axis, it is called lateral meristem. Cambial cells differ from nomal meristems in being highly vacuolated. These occur in two forms, the fusiform initial which is much longer than wide and the ray initial which is almost isodiametric. Fusiform initials form axial system and ray initials from the radial system of zone of cambial initials. Former gives rise to axial system of secondary xylem and the latter to radial system.
Formation of cambium ring: At the time of secondary growth the cells of medullary rays in a line with fascicular cambium become meristematic and form new strips of secondary meristems called interfascicular cambium. These strips joinstrips of fascicular cambium on both sides and form a complete ring called cambium ring. In some dicot stem e.g. limum and Tilia the primarty xylem and phloem are not visible as separate vascular bundle. These look like close cylinder. The intrafascicular cambium forms complete cambium ring.
Formation of Secondary Tissues: At the beginning of cambial development divisions initiating the cambium within the vascular bundles frequentlyprecede those appearing in the interfascicular regions. If these regions are wide their initial cambial divisions may start next to the fascicular cambium and spread tangentialy. Then cambium ring as a whole forms new cells on both sides by tangential divisions and these cells are midified into secondary xylem cells. Secondary xylem consists of wood vessels, wood fibres, tracheids and wood parenchyma. The cells formed on outer surface of cambium get modified into secondary phloem which consists sieve tube, sieve plate, companion cells, bast fibres and phloem parenchyma.
The activity of cambium is on inner side, more of secondary xylem is formed. At maturity this secondary xylem forms main bulk of stem. By its pressure the cambium and phloem are pushed outwards. As a result the primary phloem is usually crushed the primary xylem, especially Meta xylem does not lose its identity for considerable time period. It is conical protuberance in the pith.
The secondary phloem formed on the outside also forces the primary phloem to outside where the latter becomes non functional. The cortex cells divide amitotically to increase the circumference to keep pace with the increasing width of secondary tissue. The epidermis also increases in circumference and then it breaks up. Large amount of secondary Xylem also exerts a pressure on developing secondary phloem which is accommodated in increasing amount of parenchyma and is not crushed.
Near ray initials the cambium form narrow bands of radially elongated parenchymatous cells both on the inner and outer side instead of xylem and phloem. These form secondary medullary rays and are one to few cells in thickness and one to many cells in height. In Tulia the cells of secondary medullary ray divide and grow in the phloem region so as to form dialated masses of parenchyma between two phloem patches.
As secondary xylem cylinder expands there must be a compensating expansion of the cambium in order to maintain unbroken character of its cylinder. Such an increase in circumference of the cambial ring is accomplished by an increase in total number of cambial cells due to occasional radial divisions in addition to the usually tangential divisions.
The cambial cells may also divide by oblique radial wall and the daughter cells move past one another until two initials reach normal length and lie side by side tangentially. In herbaceous stems cambium ring remains active only for one season while in woody plants it remains active for several years producing xylem and phloem.
Formation of Annual rings: In spring cambium form xylem vessels with wide cavities. In winter small diametered vessels are formed by cambium this is called wood or late wood. Two types of wood together form an annual ring and represent one year’s growth. We get alternate years of xylem vessel, with wide cavities and those with smaller cavities. The contrast between two is so sharp that the successive rings are distinct even to the naked eye. In T.S of a stem we can find several concentric rings of vessels with wider and smaller cavities i.e. annual rings. As one rings formed in a year, the age of the plant can be calculated by counting the number of rings.
Heart wood and sap Wood: In older stems where sufficient amount of secondary growth has taken place secondary wood loses the power of conduction and its cells are filled with tannin and other substances. It becomes hard and durable and is blackish in colour. This region is known as heart wood or duramen and its function is to give mechanical support to the plant. The outer region of secondary wood which consists of younger xylem cells is yellow in colour and is known as sap wood or alburnum. It does the function of conduction.
(2) ACTIVITY OF CORK CAMBIUM
Formation of Periderm: Secondary tissues formed by activity of cambium exert a pressure on outer tissues and the epidermis may be ruptured. Hypodermis and cortex are also adversely affected. To replace this peripheral tissue new secondary tissues are developed known as periderm. It includes cork cambium or phellogen, phellem or cork and secondary cortex or phelloderm.
Cork Cambium: It originates in outer layers of hypodermis cells. It consists of a raw of narrow thin walled rectangular cells living and active. Cork cambium becomes meristematic and forms new cells on both the sides.
Secondary Cortex: Newly formed cells of the cork cambium on the inner side get converted into parenchyma and form secondary cortex or phelloderm. Cells bear chloroplast and add to the primary cortex.
Cork: New cells formed on outside of the cork cambium lose their contents become filled with air and are arranged in rows at right angles to the surface forming dead impervious layer. The cell wall becomes suberised and inter cellular spaces are lost. This dead tissue acts as protective coating and is known as cork or phellem.
Formation of Bark: Cork cells being suberized check the outward passage of water. The outer tissue becomes dead and acts as the bark. Bark includes all dead cells outside cork cambium.
Protection is the function of cork and bark.