Periderm

• Periderm


  Protective tissue of secondary origin that replaces the epidermis in the secondary plant body
• Components of periderm


  • Phellogen
  • Phellem
  • Phelloderm
• Phellogen


  Cork cambium, produces the periderm
• Phellogen


  • 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

  
• Phellem


  Cork, outer derivatives of the phellogen
• 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, with 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


  Cork parenchyma, resembles cortical or phloem parenchyma, inner derivatives of the phellogen
• Phelloderm


  • 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
• Lenticels


  • Arise beneath the stomata, where the phellogen cells are more active
  • Presence of intercellular spaces in the complementary tissue (open arrangement) permits entry of air through the periderm
  • Complementary/filling tissues: Loosely arranged cells in the lenticel, allows greater amount of intercellular spaces
  • Closing tissue: Compact, suberized cells identical to phellem cells, responsible for closing in or holding the loose filling cells
• Periderm formation is an important stage in the development of protective layers near injured or dead tissues
• The periderm arises in the pericycle through the process of dedifferentiation and redifferentiation of parenchymatous meristematic cells
• Most monocots do not have a vascular cambium and cork cambium
• The monocots do not develop a type of periderm like that of eudicots and conifers
• The rhytidome does not exist in woody monocots, but the successive layers of phellem are separated by suberized undivided cortical cells

  
• Monocots with secondary growth


  • Asparagales
  • 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)
• Palms do not have a true periderm, but have an extremely hard epidermis with lignin in tangential and radial walls of the epidermal cells
• Stem thickening in palms may also be caused by the development of adventitious roots deep within the stem
• Periderm


  Protective tissue of secondary origin that replaces the epidermis in the secondary plant body
• Components of periderm


  • Phellogen
  • Phellem
  • Phelloderm
• Phellogen


  Cork cambium, produces the periderm
• Phellogen


  • 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

  
• Phellem

  Cork, outer derivatives of the phellogen
• 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, with 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


  Cork parenchyma, resembles cortical or phloem parenchyma, inner derivatives of the phellogen
• Phelloderm


  • 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
• Lenticels


  • Arise beneath the stomata, where the phellogen cells are more active
  • Presence of intercellular spaces in the complementary tissue (open arrangement) permits entry of air through the periderm
  • Complementary/filling tissues: Loosely arranged cells in the lenticel, allows greater amount of intercellular spaces
  • Closing tissue: Compact, suberized cells identical to phellem cells, responsible for closing in or holding the loose filling cells
• Periderm formation is an important stage in the development of protective layers near injured or dead tissues
• The periderm arises in the pericycle through the process of dedifferentiation and redifferentiation of parenchymatous meristematic cells
• Most monocots do not have a vascular cambium and cork cambium
• The monocots do not develop a type of periderm like that of eudicots and conifers
• The rhytidome does not exist in woody monocots, but the successive layers of phellem are separated by suberized undivided cortical cells

  
• Monocots with secondary growth

  • Asparagales
  • 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)

  
• Palms do not have a true periderm, but have an extremely hard epidermis with lignin in tangential and radial walls of the epidermal cells
• Stem thickening in palms may also be caused by the development of adventitious roots deep within the stem