SECONDARY GROWTH IN FLOWERING PLANTS:-

·         The increase in girth in dicotyledonous plant is called secondary growth.

·         There are two lateral meristem are present in this growth named- Vascular cambium  and cork cambium.

Vascular cambium:-            

·         The Meristematic layer which is responsible for cutting off or formation of vascular tissue i.e. xylem and phloem called vascular cambium. In young stem, it is present patches as a single layer between xylem and phloem. Later it forms a complete ring.

Ø  Formation of Cambial ring:-

·         The cells of cambium present between primary xylem and phloem is called intrafascicular cambium. While the cells of medullary rays, present between two vascular bundles which adjoining the intrafascicular cambium called interfascicular cambium. Both of these cambium form continuous ring of cambium.

Ø  Activity of cambium  ring:-

·         After formation of cambium ring, it become activated and cut off or forms new cell both toward outer and inner side. The cells cut off toward pith become mature and form secondary xylem.

·         Inner cambium ring is more active than outer cambium ring. So the amount of secondary xylem is more than secondary phloem.

·         As the formation of secondary xylem, primary and secondary phloem will be crushed.

·         The cambium forms many narrow bands of parenchyma cells called secondary medullary rays. It is present in the radial direction where it passes through secondary xylem and phloem.

Ø  Physiological factors:-

·         Spring wood or Early wood- In spring season the cambium ring is very active and produce a large number of xylary elements having vessels with wide cavity. It is lighter in colour having and has low density.

·         Autumn wood or Late wood- In winter season the cambium is less active and produces fever xylary elements having vessels with narrow cavity. It is dark in colour and has high density.

·         Both the woods show alternate concentric rings called annual ring which represent the age of trees.

Ø  Heartwood and Sapwood:-

·         In old tree stems, the central portion of secondary xylem is dark brown in colour called heartwood. Its colour is dark brown due to deposition of some organic compounds like oils, gums, aromatic substances, tannins, resins etc. It is hard, durable and resistant from microbes. It doesn’t control the conduction of water but gives mechanical support to the stem.

·         The peripheral region of secondary xylem is lighter in colour called sapwood. It functions to control the conduction of water and minerals from roots to stems and leaves.

            

Fig- Secondary Growth in a Dicot stem (a) Formation of vascular cambium, (b) Formation of secondary vascular tissue and Phellogen, (c) T.S. of Secondary Growth in Stem.

Cork Cambium:-

·         As the increase in growth of vascular cambium the peripheral cortical and epidermis gets broken so a new protective layer is form Meristematic tissue preset at the region of cortex called cork-cambium or phellogen.

·         Phellogen is as couple of few thick layer made up of narrow, thin walled, reticular cells. These cells are bipolar so cut off or divided into both inner and exterior side. In outer differentiation forms cork or phellem while forms secondary cortex or phelloderm. Here cork impervious to water due to suberin deposition in the cell-wall and secondary cortex is parenchymatous.

·         Phellogen + phellem + Phelloderm = Periderm.

·         All tissues exterior to the vascular cambium called Bark. Viz., Periderm and secondary phloem. Bark formed in early season called early bark or soft bark while at the end of season called late bark or hard bark.

·         In woody trees the phloem cut off into loosely arranged parenchymatous cells and these cells soon ruptures to form many lens shaped openings called lenticels. Lenticels take part in gaseous exchange between outer atmosphere and internal tissues.

SECONDARY GROWTH IN ROOTS:-

·         In dicot roots, the vascular cambium is completely secondary in nature and originated from the lateral meristem present at the region of pericycle tissues.

      

Fig- Stages of Secondary Growth in Dicot Root.(a) Formation of vascular cambium, (b) Formation of Phellogen, Primary vascular rays, secondary xylem and Phloem, (d) T.S. of at mature stages.

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