Epidermis

• Epidermis


  Outermost layer of cells on the primary plant body

  
• Dermal tissues


  Tissues that make up the epidermis

  
• Epidermis


  • Usually one layer
  • Develop from the protoderm
• Epidermis in different types of plants

  • Shoot epidermis - outermost layer of apical meristem
  • Root epidermis - rhizodermis/epiblem, dermatocalyptrogen

  
• Cuticle is absent or very thin in root epidermis
• Root hairs form from trichoblast which function in increasing absorption of the roots

  
• Epidermis in different types of plants

  • Angiosperms - outermost layer of SAM, dermatogen
  • Gymnosperms - lateral derivatives of apical initials
  • Pteridophytes - common origin with deeper lying tissues

  
• In woody plants, epidermis is replaced by periderm
• In monocots, epidermis is replaced by a special type of periderm
• Epidermis is retained in plants with little or no secondary growth

  
• Epidermis may still be present in plants that have undergone secondary growth

  
• Acer pensylvanicum (striped maple)


  • Striped maple is named for the conspicuous vertical white stripes that traverse its greenish-brown bark
• Epidermis


  • Outermost layer
  • Complex
  • Living protoplasts
  • Compact
  • Typically uniseriate
  • Presence of cuticle

  
• The epidermis is a complex tissue composed of a variety of cell types which work together as a unit
• Epidermal cells have living protoplasts at maturity
• The epidermal cells are tightly joined together with no intercellular spaces
• Velamen


  Multiple epidermis in aerial roots and some leaves, provides support, prevents water loss and assists in absorption

  
• Hypodermis


  Resembles a multiseriate epidermis, found right next (below) the true epidermis, ultimately derived from the ground meristem

  
• Cuticle


  A coating of waxes and oils that covers the epidermis of the aerial parts of the plant

  
• Functions of the cuticle

  • Reduction of water loss by transpiration
  • Regulation of gas exchange
  • Mechanical protection
  • Protection against pathogens
  • Mechanical support
• Other functions of the epidermis

  • Protection against UV radiation
  • Dynamic compartments of metabolic products
  • Absorb water and solutes
  • Act as lenses
  • Site of perception involved in circadian leaf movements and photoperiodic induction
• Structural components of the epidermis


  • Pavement cells
  • Stomata
  • Trichomes
  • Others
• Pavement cells


  • Typically tabular
  • Varied shapes in surface view
  • Wavy anticlinal walls
  • Living protoplasts at maturity
  • Walls vary in thickness
• Pavement cells have functional chloroplasts and protein crystals, and vacuoles with high amount of water and anthocyanins
• Cuticle


  Serves as first protective barrier between aerial surface and environment, barrier to movement of water, lipid components include cutin, cuticular proper, additional layer of cutin, and epicuticular wax
• Other wall features


  • Pectin layer is continuous with the middle lamella and bonds the cuticle to the outer walls
  • Teichodes are channels along the outer wall that function in foliar absorption and excretion
  • Plasmodesmata in outer walls of epidermal cells
• Stomata


  Openings in the epidermis each bounded by a pair of guard cells, regulate exchange of water vapor and CO2
• Occurrence of stomata in leaves


  • Amphistomatic - found on both surfaces
  • Epistomatic - found in the upper epidermis
  • Hypostomatic - found in the lower epidermis

  
• Level of position of stomata in the epidermis

  • On the same level as adjacent epidermal cells
  • Raised above the surface of the epidermis
  • Sunken below the surface of the epidermis (stomatal crypt)

  
• Guard cells


  • Pair of specialized epidermal cells, distinguished by green color and unique shape, eudicots have crescent-shaped, monocots have dumbbell-shaped
• Guard cells


  • Lined by cuticle
  • Prominent nucleus, numerous mitochondria
  • Poorly developed chloroplasts
  • Chemical ions involved in closing and opening of the stomatal pore
• Stomatal development


  1. Starts from asymmetric division of protodermal cell

  2. Cell with shorter, densely staining cytoplasm, and large nucleus serves as the precursor (stomatal meristemoid)

  3. Equal division gives rise to the guard cells

  
• Subsidiary cells


  Border the guard cells, differ in size, shape, arrangement and sometimes in content from ordinary epidermal cells, may support guard cell function, arise from guard mother cell or other protodermal cell
• Origin of subsidiary cells

  • Mesogenous - all subsidiary and neighboring cells arise from where the guard cells originated
  • Perogenous - none of the subsidiary or neighboring cells have a common origin with the guard cells
  • Mesoperigenous - if one subsidiary or neighboring cell is directly related ontogenetically with the guard cells; the others are not
• Stomata


  Pores in the epidermis of leaves and stems that allow gas exchange
• Guard cell development


  1. Asymmetric division of protodermal cell

  2. Cell with shorter, densely staining cytoplasm and large nucleus serves as precursor (stomatal meristemoid)

  3. Equal division gives rise to guard cells
• Subsidiary cells


  • Border the guard cells
  • Differ in size, shape, arrangement and sometimes in content from ordinary epidermal cells
  • May support guard cell function
  • Arise from guard mother cell or other protodermal cell
• Mesogenous origin


  All subsidiary and neighboring cells arise from where the guard cells originated
• Perogenous origin


  None of the subsidiary or neighboring cells have a common origin with the guard cells
• Mesoperigenous


  If one subsidiary or neighboring cell is directly related ontogenetically with the guard cells; the others are not