Primary Growth: The growth in length of stem and roots is called primary growth. Primary growth happens because of activities in the apical meristem.
Secondary Growth: The increase in girth is called secondary growth. The tissues which are involved in secondary growth are; the two lateral meristems (vascular cambium and cork cambium).
The vascular cambium is responsible for cutting off vascular tissues. In case of young stem; it is present in patches as a single layer between the xylem and phloem. It forms a complete ring at a later stage.
In dicot stems, the cambium which is present between primary xylem and primary phloem is called intrafascicular cambium. The cells of medullary rays near these intrafascicular cambium become meristematic and form interfascicular cambium. This leads to the formation of a continuous ring of cambium.
Activity of Cambial Ring: The cambial ring becomes active and starts cutting off new cells; both on the inner and the outer sides. The cells which are cut off towards the pith mature into secondary xylem. The cells which are cut off towards the periphery mature into secondary phloem. The cambium is usually more active on the inner side than on the outer side. Hence, a large number of secondary xylem is produced compared to the secondary phloem. The secondary xylem; thus produced soon forms a compact mass.
Due to continued formation and accumulation of secondary xylem, the primary and secondary phloems gradually get crushed. But the primary xylem more or less remains intact, in or around the centre. At some places, a narrow band of parenchyma is formed by the cambium. This band of parenchyma passes through the secondary xylem and secondary phloem in radial directions. These are called secondary medullary rays.
Various physiological and environmental factors control the activity of cambium. Climatic conditions are not uniform through the year; in temperate regions. Cambium is very active during the spring season, while it is less active during the winters. Hence, during spring; a large number of xylem elements are formed which have wider vessels. During winter, fewer xylem elements are formed which have narrow vessels. The wood formed during summer is called spring wood or early wood, while the wood formed during winter is called autumn wood or late wood.
Spring wood is lighter in colour and has a lower density. The autumn wood is darker in colour and has a higher density.
The two kinds of wood appear as alternate concentric rings in transverse section of a trunk or branch of a tree. These are called annual rings and can provide rough estimate of the age of the tree.
In old trees, the greater part of secondary xylem is dark brown. This happens because of deposition of organic compounds; like tannins, resins, oils, gums, aromatic substances and essential oils in the innermost layers of the stem. Due to the presence of these substances, the wood becomes hard, durable and resistant to attacks by microorganisms and insect. This region is composed of dead elements with highly lignified walls. The wood in this region is called heartwood. The heartwood does not conduct water, but gives mechanical support to the stem. The peripheral region of the secondary xylem is lighter in colour. This is known as sapwood. Conduction of water and minerals takes place through sapwood.
Cork Cambium: When the girth of the stem increases due to secondary growth, the outer cortical and epidermal layers get broken. These layers need to be replaced by new protective layers. To fulfill this need, a meristematic tissue develops in the cortex region. This meristematic tissue is called cork cambium or phellogen.
Phellogen is composed of a couple of layers. This is made up of narrow, thin-walled and nearly rectangular cells.
The phellogen cuts off cells on both sides. The outer cells differentiate into cork or phellem. The inner cells differentiate into secondary cortex or phelloderm. Cork is impervious to water due to suberin deposition in the cell wall. The cells of secondary cortex are parenchymatous. Phellogen, phellem and phelloderm are collectively called periderm.
Due to activity of the cork cambium, pressure builds up on the remaining layers which are peripheral to phellogen. These layers finally die and slough off.
Bark: All tissues which lie exterior to the vascular cambium are called bark; in common language. Bark formed early in the season is called soft bark, while one formed late in the season is called hard bark.
Lenticels: The phellogen cuts off closely arranged parenchymatous cells on the outer side instead of cork cells. This happens at certain regions. These parenchymatous cells soon rupture the epidermis. This leads to the formation of lens-shaped openings called lenticels. Lenticels permit exchange of gases. Lenticels are usually found in woody trees.
The vascular cambium of the dicot root is completely secondary in origin. It originates from the tissue which is located just below the phloem bundles. A portion of pericycle tissue; above the protoxylem; forms a complete and continuous wavy ring. It subsequently becomes circular. Rest of the steps are similar as in dicot stem.
Secondary growth takes place in stems and roots of gymnosperms. But secondary growth does not happen in monocotyledonous.
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