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SECONDARY GROWTH.pptx

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SECONDARY GROWTH.pptx

  1. 1. Secondary Growth
  2. 2.  Secondary Growth –  The growth which responsible for increase in height of plant is called primary growth & produces primary tissues.  It is brought about by the activity of the meristems. The meristem which involved in increase in height of plant either be apical or intercalary meristem. These meristematic tissue responsible for primary growth of plant.  Many monocots & some herbaceous dicots possess only primary growth.  In Gymnosperms, woody as well as herbaceous dicots, & some monocots the stem & root continue to grow, not only in length but also in diameter. This is called secondary growth.  Secondary growth can be defined as production of secondary tissues by the activity of the vascular cambium & cork cambium in the stelar & extra-stelar regions leading to an increase in the girth of the stem or root. The vascular cambium and cork cambium are also called as lateral meristem because they present at lateral side of plant.  Complete secondary growth occur in two region ,stelar and extrastelar region .
  3. 3. Secondary growth in Dicot stem Stelar secondary growth  Stelar secondary growth occur in plant due to vascular cambium . In plant there are two kind of vascular cambium present inside stele i.e. interfascicular and fascicular cambium  Cambium which is located in between two vascular bundle called as interfascicular and cambium present between xylem and phloem of single vascular bundle. Parenchyma cells which are a part of the medullary ray & are in line with the fascicular cambium start becoming meristematic. Thus, interfascicular cambium is formed.  Fascicular & interfascicular cambium now join hands to form a complete cambial ring.  The vascular cambium i.e., fascicular cambium is composed of two types of cells – the fusiform initials & the ray initials.  The fusiform initials are elongated & spindle shaped & produce vascular tissues. The fusiform initials are elongated tapering cells that give rise to all cells of the vertical system of the secondary phloem and xylem (secondary tracheary elements, fibres, and sieve cells and the associated companion cells).  The ray initials are smaller & isodiametric & give rise to the xylem & phloem rays. They produce the vascular rays, which constitute the horizontal system of secondary tissues; this horizontal system acts in the translocation and storage of food and water.
  4. 4.  The cambial cells divide tangentially & produce secondary xylem on the inner side & secondary phloem on the outer side.  The amount of secondary xylem produced is more than the amount of secondary phloem.  Secondary xylem is composed of xylem vessels & fibres. There is less of wood parenchyma.  Secondary phloem consists of sieve tubes, companion cells & phloem parenchyma.  Due to increase in the girth, the cells of the non-vascular tissue, i.e., medullary rays, pith, hypodermis & cortex become tangentially compressed.  The secondary tissues put pressure on the primary phloem lying immediately outside & therefore it gets crushed. T.S of Dicot Stem Showing normal growth
  5. 5. FASICULAR CAMBIUM X p Interfascicular cambium T.S of dicot stem showing interfascicular and fascicular cambium
  6. 6. 1 2 3 4 Normal growth in T.S of dicot stem Secondary growth in dicot stem
  7. 7. Extrastelar secondary growth  When a large amount of secondary tissues are produced in the stelar region, the epidermis gets stretched & ultimately ruptures.  To protect the inner tissues, the epidermis is replaced by a new secondary tissue called periderm.  The periderm consists of three layers which are meristematic in nature – i) Cork cambium /Phellogen ,  ii)First cork cell /The phellem or cork produced by the phellogen towards the outer side &  iii) The phelloderm or secondary cortex, a tissue that resembles cortical parenchyma & consists of the inner derivatives of the phellogen.  Phellogen originates in the primary cortex through dedifferentiation. It is reversion of permanent parenchymatous cells into meristematic cells.
  8. 8.  The phelloderm is composed of cells that remain arranged in definite radial rows. They help in storage of food.  Phellem or cork is composed of cells which are arranged in distinct radial rows, without intercellular spaces & are without protoplasts at maturity.  They are filled with either air or with highly coloured organic substances which may be resinous or tanniferous compounds.  In the big perennial trees, the phellogen originates successively towards the inner side.  The earlier formed phellogen does not function throughout the life but is replaced by many successive phellogen layers.  This causes an accumulation of dead tissue on the outer side of the stem & root.  Such composite periderm is called Rhytidome (shell bark). It consists of cork, cortical layers & dead phloem placed alternately.  Bark is a non-technical term used to refer to all the tissues lying outside the vascular cambium, but often used to designate the phellem.
  9. 9. Kinds of bark 1. Ring Bark Continuous bark of equal thickening is called ring bark.  It is formed around the stem in the form of a complete ring. In ring bark cork cambium is continuous.  A complete distinct ring bark is formed in this plant. Its bark was used as a writing material as a paper in ancient period.  Example: Bhojpatra (Betula utilis) and it is also formed in Eucalyptus. Ring Bark In bhojpatra Tree
  10. 10.  Ring Bark In Eucalyptus Tree
  11. 11. 2. Scaly Bark  Discontinuous bark of unequal thickening is called scaly bark.  This bark is formed around the stem in the form of pieces or fragments. In scaly bark the ring of cork cambium is not continuous.  This scaly bark formed in Neem (Azadirachta indica), Mango (Mangifera indica) and Imli ( indica) etc. Scaly Bark is also formed in Neem (Azadirachta indica)
  12. 12. Tamarindus indica (Imli)
  13. 13.  Lenticels –  These are structurally differentiated portions of the periderm that are characterized by relatively loose arrangement of the cells without any suberization.  During periderm formation, gaseous exchange between the internal living cells & the outer atmosphere becomes difficult; lenticels take over this function of gaseous exchange.  Lenticels are first formed beneath the stomata. They originate either before, simultaneously with or after the initiation of the periderm.  Parenchyma near the sub-stomatal chamber loses chlorophyll & divides in many planes to form a mass of colourless cells with inter cellular spaces.  When phellogen is formed, it also produces the same type of loosely arranged cells on the outer side instead of normal cork cells.  They increase in numbers & ultimately the epidermis ruptures & these cells called complementary cells are exposed, forming a lenticel.
  14. 14. Functions of Secondary Meristem 1. Healing of wounds  When any plant part gets injured wound is formed there.  Boundary of the wound is raised outside and composed of similar type of living cells (parenchyma) called callus.  Living cells of wound are responsible to form a cambium.  This is called wound cambium. It is also called inducible cambium.  This newly formed cambium forms cork towards the outside.  This cork covers the wound entirely. Wound cambium is lateral meristem.
  15. 15. 2. Abscission  Falling of any plant organ is called as abscission.  Abscission takes place due to formation of abscission layer at the base of plant organ and it is composed of parenchyma.  Middle lamella is dissolved in abscission layer during abscission and primary walls also dissolve partially or completely.  Sign of leaf fall on stem is called leaf scar and it is a type of wound. The living cells of leaf scar are responsible to form cork cambium, which produce cork.  Cork covers the wound. At the site of abscission protective layer is found which is suberized.
  16. 16.  3. Knots  Knot is formed when branches are embedded inside the main stem. In most cases knots are caused by the natural growth of the tree.  As a tree grows and increases the circumference of its trunk, the growing trunk begins to overtake the branches that grow out from it. Knots form around these branches, building up trunk material as the tree continues to expand.  The wood of the knot is typically tougher than the surrounding wood and may form a bulge around the branch emerging from its center and known as tight knot.  If a branch becomes injured or otherwise dies while still attached to the tree, a loose knot forms as the trunk grows larger. Loose knots are similar to tight knots, but instead of having living wood in the center of the knot there is only a dark plug of dead or decaying material.

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