Periderm

• 1. Apical meristem
  2. Protoderm
  3. Epidermis
  4. Cork cambium
  5. Cork
  6. Phelloderm
  7. Procambium
  8. Primary phloem
  9. Primary xylem 
  10. Fascicular cambium
  11. Vascular cambium
  12. Secondary xylem
  13. Secondary phloem
  14. Cork cambium
  15. Ground meristem
  16. Pith
  17. Cortex
  18. Interfascicular cambium
  19. Cork cambium

  
• Periderm

  • protective tissue of secondary origin 
  • replaces the epidermis in the secondary plant body 
  • The epidermis is still beginning to be shed off, because of secondary growth, the epidermis will be pushed out  
• What tissues comprise the periderm?

  1. Phellogen - cork cambium, produces the periderm
  2. Phellem - cork, outer derivatives
  3. Phelloderm - cork parenchyma, resembles cortical or phloem parenchyma, inner derivatives
• 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 
  • periderm is a part of the bark

  
• Bark is divided into two layers 

  The tissue layers thus separated die, bringing about a distinction between the nonliving outer bark and living inner bark

  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  
• 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  
  • In roots
  • Protective Tissue in Monocotyledons
  • Palms
  • Asparagales
• 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 
• Surface wounds 

  • 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 
  • Sometimes cuts off parts of the phloem
• In roots

  • In 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
• Protective Tissue in Monocotyledons 

  • Bundles are scattered, no opportunity for secondary growth
  • Most monocots do not have a vascular cambium and cork cambium 
  • Cordyline fructicosa 
  • The monocots do not develop a type of periderm like that of eudicots and conifers (Weisse, 1897; Philipp, 1923) 
  • The rhytidome does not exist in woody monocots, but the successive layers of phellem are separated by suberized undivided cortical cells (Philipp, 1923)
• Palms 

  • Do not have a true periderm 
  • Extremely hard epidermis  
  • Lignin in tangential and radial walls of the epidermal cells
  • Parenchyma cells (lignified) are dividing continuous leading it to become lignified 
  • Palm trees are not trees at all and belong to the grass family. Botanists define trees as woody plants with secondary growth. Palms lack both of these. They create their tough, "wood" epidermis through primary thickening and lignification
• 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:

  1. Phellogen
  2. Phellem
  3. Phelloderm
  4. Lenticels

  
• 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 
  • Usually one layer
  • May come from epidermis, ground tissues, secondary phloem, subepidermal layers, parenchyma cells, cells of cortex or phloem
• 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 

  • 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 
  • Produced by lenticel phellogen

  
• How does the anatomical arrangement of the lenticel relate to its function? 

  • The presence of intercellular spaces in the complementary tissue (open arrangement) permits entry of air through the periderm
• Filling cells

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

  
• Secondary Thickening Meristem

  • Initiates secondary growth in monocots
• Closing tissue

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