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Secondary Growth

Dicot Stem
The increase in the thickness of the dicot stem occurs in two ways:
  • Stelar secondary growth
  • Extra-stelar secondary growth
  1. Stelar secondary growth
Formation of Cambium Ring
The vascular bundle consists of the cambium in between the xylem and phloem. This is called "fasicular vascular bundle". In between the vascular bundles there are medullary rays from which arises the 'interfasicular vascular bundle'. These two vascular bundles fuse to form a continuous cambium ring.

Activity of the Cambium Ring
The cells of the cambium divide repeatedly and produce new cells on both the sides which gradually undergoes differentiation and forms secondary vascular bundle. Those cells, which are produced outside develop into 'secondary phloem or bast' and those cells, which are produced inside develop into 'secondary xylem or wood'. In most of the dicots, as the stem increases in thickness the primary xylem and phloem are crushed and removed. They are soon replaced by secondary xylem and phloem, which remain active for a long period of time.

Annual Rings
In temperate regions and cold regions the activity of the cambium is influenced by seasonal variations.
During spring, when leaves and flowers are formed the plant requires more of water and mineral salts. Hence the wood formed in this period shows more number of xylem vessels with wider lumens. This is known as springwood or early wood. The colour of this wood is light.

During autumn, the plants are less active and the wood produced in this period show less number of xylem vessels with narrow lumens. This is known as autumn wood or late wood. This wood is dark in colour. In this way two types of secondary xylem are produced in one year. They appear in the form of dark and light coloured circles alternately in the mature trunk. These are called "annual rings or growth rings or seasonal rings. By counting the number of rings the approximate age of trees can be estimated. This branch of science is called as 'dendrochronology'.

Heart Wood and Sap Wood
With the increase in the age of the tree, the wood undergoes a number of physical and chemical changes. The older wood loses water and stores food substances such as oils, gum, resins and tannins. Hence the older xylem present in the centre appears dark in colour. This is called 'heart wood' or 'duramen'. The heartwood gives mechanical strength to the tree. The newly formed secondary xylem is called sap wood or alburnum and is light in colour and actively conducts water and mineral salts.
  1. Extra-stelar Secondary Growth
    As the secondary xylem and secondary phloem are formed inside the stele, a pressure is exerted on the epidermis causing its rupture. Meanwhile a secondary protective layer called 'periderm' is formed in the cortex. The secondary growth of cortex takes place and this is called 'phellogen' or 'cork cambium'. The tissue produced outside is called cork or phellem and the tissue produced inside is called secondary cortex or phelloderm.
To facilitate gaseous exchange, in the cork tissue certain bulged, lens shaped cells are formed. These are called lenticels. They are made up of loose mass of thin walled complimentary cells. Lenticels also promote transpiration.

Since the cork is an impervious tissue, the supply of water is cut off to the cells situated outside it. Hence the outer layers become dry and get peeled off as strips. This is called bark. In some cases the bark is completely peeled off in thin sheets and is known as ring barks.

 Fig: Different Stages of Secondary Growth in Dicot Stem

Dicotyledons Root

Formation of Vascular Cambium
A portion of the conjunctive tissue below the phloem becomes meristematic and gives rise to the cambium. This strip of cambium extends laterally between the xylem and the phloem. At the same time a portion of the pericycle also becomes meristematic and forms a strip of cambium. Both the cambial strips join and form a continuous ring, one above the xylem and one below the phloem.

Activity of the Vascular Bundle
Due to the over production of secondary tissue, the cambium as well as the primary phloem are pushed outwards. The cambium now becomes circular. The cambial ring now becomes active and is cut off from the secondary tissue on either side. The cells, which are formed on the inner side of the cambium ring, are differentiated into elements of secondary xylem, which consists of xylem parenchyma and few fibres. The cells, which are formed on the outer side of the cambium ring are differentiated into elements of secondary phloem having sieve tubes, companion cells, phloem parenchyma and phloem fibres.

Formation and the Activity of Cork Cambium
The cells of the pericycle gives rise to thin walled rectangular cells called cork cambium or phellogen. It gives rise to the cork or the phellem. On the inner side the secondary cortex or phelloderm is formed. During this process the endodermis and general cortex gets disorganised. Lenticels are also present which may be scattered.

 Fig: Activity of Cork Cambium

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