Secondary growth

• Secondary growth


  Increase in the diameter of stems and roots in plants due to the new cells produced by lateral meristems
• Plants with secondary growth

  • Woody perennial plants including all gymnosperms species and many dicot species
• Lateral meristems


  Vascular cambium and cork cambium
• Role of vascular cambium

  1. Adds secondary xylem (wood) towards primary xylem

  2. Adds secondary phloem towards primary phloem

  3. Increases vascular flow and support for the shoots
• Role of cork cambium


  Produces tough thick covering consisting mainly of wax impregnated cells that protect the stem from water loss and from invasion of insects, bacteria and fungi
• Primary growth and secondary growth in woody plants

  Occur simultaneously
• Primary growth


  Adds new cells and lengthens stems and roots in the younger regions of a plant
• Secondary growth


  Increases the diameter of stems and roots in older regions where primary growth has ceased
• Some epidermal cells have specialized functions such as guard cells (stomata) or trichomes (hairs).
• Secondary vascular tissue


  Produced by the action of vascular cambium
• Vascular cambium in a typical woody stem

  • Consists of a continuous cylinder of undifferentiated cells of often only a single cell layer in thickness
  • Located outside the pith and primary xylem and to the inside of the cortex and primary phloem
• Vascular cambium in a typical woody root

  • Forms laterally exterior to the primary xylem and interior to the primary phloem and pericycle
• Vascular cambium divides


  1. Increases circumference of the vascular cambium

  2. Adds secondary xylem to the inside of the cambium

  3. Adds secondary phloem to the outside
• Vascular cambium in cross section

  Appears as a ring of initials
• Vascular cambium initials


  • Some are elongated and oriented parallel to the axis of stem or root
  • Some are shorter and oriented perpendicular to the axis of stem or root
• Elongated initials


  Produce cells such as tracheids, vessel elements, parenchyma and fibers of the xylem, as well as sieve-tube elements companion cells, phloem fibers and phloem parenchyma
• Shorter initials


  Produce vascular rays-mostly parenchyma cells that connect secondary xylem and phloem, store carbohydrates and aid in wound repairing
• Secondary growth continues over many years

  Layers of secondary xylem (wood) accumulate
• Secondary xylem cell walls

  • Heavily lignified and account for the hardness and strength of wood
• Early stages of secondary growth

  Epidermis pushed outwards, causing it to split, dry and fall off the stem or root
• Cork cambium


  A cylinder of dividing cells that arises in the outer layer of cortex in stems and in the outer layer of pericycle in the roots
• Cork cambium produces


  Cork cells to the exterior
• Cork cambium and tissues it produces

  Collectively called periderm
• Mature cork cells


  • Deposit a waxy, hydrophobic material called suberin in their walls and become dead cells
• Cork tissues


  Function as a barrier that helps protect the stem or root from water loss, physical damages and pathogens
• Periderm


  Each cork cambium and the tissues it produces comprise a layer of periderm which is impermeable to water and gasses
• Lenticels


  Small pores present in the periderm for gaseous exchange
• Lenticels


  • Formed by loosely arranged cork cells
  • Appear as horizontal slits
• Further growth of stem or root

  1. Breaks the layer of cork cambium

  2. Cork cambium lacks meristematic activity

  3. Cork cells are produced
• New cork cambium initiation

  Produces a new layer of periderm
• New cells added


  Outer regions of cork crack and peel off in many tree trunks
• Due to the tissue layers produced by vascular cambium and cork cambium, girth of the stem or root increases in secondary growth
• Bark


  • All tissues out of the vascular cambium (cork is commonly and incorrectly referred to as bark)
  • Main components are secondary phloem and periderm
• As a woody plant ages, the older layers of secondary xylem no longer transport water and minerals. • These layers are called heartwood because they are close to the centre of the stem or root.
• The newest outer layers of secondary xylem, still transport xylem sap are known as sapwood
• Spring wood


  Wood that develops early in spring in temperate regions
• Spring wood


  • Xylem tissue with xylem vessels that have large lumens and thin walls
  • Maximizes delivery of water to new leaves
• Summer wood


  Wood produced during the rest of the growing season in temperate regions
• Summer wood


  • Xylem tissue with xylem vessels that have thick walls and small lumens
  • Do not transport much water but provide more support
• Annual ring


  The two woods (spring wood and summer wood) collectively known as an annual ring