Periderm

• PERIDERM

  • protective tissue of secondary origin

  • replaces the epidermis in the secondary plant body
• In some slides (lab), in some instances, both the epidermis and periderm are present • The epidermis is still beginning to be shed off, because of secondary growth, the epidermis will be pushed out
• Bark - may be used most appropriately to designate all tissues outside the vascular cambium. Includes the secondary phloem, the primary tissues that may still be present outside the secondary phloem, the periderm, and the dead tissues outside the periderm.
• Inner bark

  • Vascular cambium to cork cambium
  • Living part of the bark
• Outer bark (rhytidome)

  • phellem and old non-functional phloem
  • Essentially dead in nature
• Identify

  A) Secondary Xylem

  B) inner bark

  C) outerbark

  D) cork cambium

  E) vascular cambium

  F) spring wood

  G) xylem rays

  
• Function

  • Reduces the loss of water and solutes from interior tissues

  • Protects a plant from unfavorable environmental conditions

  • Inhibits water loss

  • Allow gaseous exchange (thru lenticels)

  • It supplements the secondary vascular tissues (xylem) in stiffening stems
• Occurrence

  • Surface of roots and stems with secondary growth
  • Herbaceous dicots
  • In abscission zones and surface of wounds
  • Abscission zones
  • Surface wounds
  • In Roots
  • Protective Tissue in Monocotyledons
• Abscission zones

  • Periderm develops along surfaces that are exposed after abscission of plant parts, such as leaves and branches.
  • Abscission: occurs when plant organs (leaves, fruits) naturally shed off from the plant body
  • Scar: forms at abscission zones
• Periderm formation is also an important stage in the development of protective layers near injured or dead (necrosed) tissues (wound periderm or wound cork), whether resulting from mechanical wounding or invasion of parasites.
• Roots - the deep-seated periderm always arises in the pericycle through the process of dedifferentiation and redifferentiation of parenchymatous meristematic cells. Typically, the pericycle will develop a new phellogen every growing season, and the periderm will accumulate to seal off the root from the soil.
• Cordyline fructicosa - The rhytidome does not exist in woody monocots, but the successive layers of phellem are separated by suberized undivided cortical cells. They undergo diffused Secondary growth (secondary thickening meristem). They also can develop secondary vascular bundles.
• Palms

  • Do not have a true periderm
  • Extremely hard epidermis
  • lignin in tangential and radial walls of the epidermal cells
• Asparagales - Monocots that have true secondary growth, Derivatives are products of meristematic activities but not similar to the derivatives produced by eudicots.

  • Only have one type of initial within their cambium
  • Ray-less; do not have ray initials
  • Secondary thickening meristems (STM)
• Components

  • Phellogen
  • Phellem
  • Phelloderm cells
• Phellogen

  • Commonly appears as a continuous tangential layer (lateral meristem) of rectangular cells
  • It is a temporary meristem and a new phellogen must differentiate each growing season
  • Cells are living and retain protoplasts, vacuolated and may contain tannins and chloroplasts
  • Renewal of the phellogen takes place by repeated periclinal division in the parenchyma cells positioned outside the phloem
• Components

  A) phellem

  B) phellogen

  C) phelloderm

  
• Phellem - Cells are nonliving at maturity, may store tannins and resins

  • Prismatic in shape or irregular, elongated parallel with the long axis of the stem
  • Compactly arranged in radial rows
  • May have thick or thin walls
  • Thick: deposition of suberin , wax, cellulose
  • Phelloids: phellem-like cells free of suberin
  • Compressible, resilient, highly impervious to water, resistant to oil, resistant to enzymes
• Phelloderm cells

  • Cell shape is similar to phellem cells, resemble cortical or phloem parenchyma cells
  • Distinguishable by their position in the same radial files as the phellem cells
  • Have thinner walls and have numerous intercellular spaces among them
  • Cells are living at maturity, may contain crystals and may eventually become sclerified
• Lenticels

  • Specialized region of the periderm that allows for gaseous exchange between the atmosphere and the interior living tissues of the plant.
  • Arise beneath the stomata, where the phellogen cells are more active
• filling cells

  • Complementary/filling tissues
  • Loosely arranged cells in the lenticel
  • Allows to have greater amount of intercellular spaces
• closing tissue

  • Compact, suberized cells
  • Identical to phellem cells
  • Responsible for closing in or holding the loose filling cells