Sclerenchyma

• The term sclerenchyma refers to a tissue composed of cells with secondary walls, often lignified, whose principal function is mechanical or support
• The word is derived from the Greek skleros, meaning “hard” and enchyma, an infusion
• Sclerenchyma have lignified secondary walls
• They function mainly in mechanical support and are found in mature and non-elongating parts of the plant body. Also function for protection and water conduction or transport.
• Sclerenchyma cells may or may not retain their protoplasts at maturity.
• Sclerenchyma cells are usually divided into two categories, fibers and sclereids.
• Fibers are described as long cells, and sclereids as relatively short cells.
• Fibers serve as supporting elements in plant parts that are no longer elongating
• Fibers serve as supporting elements in plant parts that are no longer elongating.

  • Many fibers retain their protoplasts at maturity
  • Fibers are widely distributed in the plant body
  • Usually found in small groups in ground tissues but more extensively found in vascular tissue

  
• Fibers are derived from the procambium and vascular cambium, ground meristem and protoderm
• Type of fibers

  Xylary Fibers: fibers that are found in xylem tissues
• Type of fibers

  type of xylary fibers: Libriform fibers - longer (than fiber-tracheids) and have thicker walls, have simple pits
• Type of fibers

  type of xylary fibers: fiber-tracheids - transitional stage between tracheid and libriform in thickness and size, have bordered pits
• Type of fibers;

  Extraxylary Fibers: fibers located outside the xylem
• Type of fibers;

  type of extraxylary fibers: phloem fibers - initially started as sieve-element, arise from primary phloem (parenchymatous sieve elements) or as part secondary phloem

  
• Type of fibers;

  Type of extraxylary fibers: cortical fibers - found and originate in the cortex

  
• Type of fibers;

  type of extraxylary fibers: perivascular fibers - located in the periphery of vascular cylinders
• Bundle sheath or bundle caps: associated with vascular bundles

  • Bundle sheath extensions: some extend to the dermal tissues

  • Bundle cap: the sheath does not surround the bundle, only found on one side (particularly the area of the phloem)
• Sclereids are said to arise through secondary sclerosis of parenchyma cells
• Sclereids typically are short cells with thick secondary walls, strongly lignified, and provided with numerous simple pits.

  • Many sclereids retain living protoplasts at maturity.
  • Secondary wall typically appears multilayered
• Type of sclereid:

  1. Astrosclereids
  2. Brachysclereids
  3. Macrosclereids
  4. Osteosclereids
  5. Trichoslcereids
  6. Filiform
• Astrosclereids

  • star-shaped, star-cells, with lobes or arms diverging from a central body
  • often found in the leaves of eudicots
  • function is assumed to be one of support
  • often covered with calcium oxalate crystals
  • resistant to decay

  
• brachysclereids:

  • stone cells, roughly isodiametric, smallest form
  • widely distributed in cortex, phloem and pith of stems and in flesh of fruits
  • secondary wall consists of many thin concentric layers, which are laid down to the interior of the cell
• Macrosclereids

  • Elongated, rod-like
  • Found in epidermis of legume seed coats
  • Cortex, phloem and pith of stems and in flesh of fruits
  • Prevent desiccation, keep dormancy--promoting hormones from leaching out, and deter herbivory
• Osteosclereids

  • bone-shaped
  • found in subepidermal layer of seed coats
  • prevent desiccation, keep dormancy-promoting hormones from leaching out, and deter herbivory
• Trichoslcereids

  • hair-like with branches projecting into intercellular spaces
  • intergrade with filiform sclereids
• Filiform

  • long slender cells resembling fibers
  • intergrade with trichosclereids
• Sclereids also arise from different meristems

  1. Procambium
  2. Cork cambium
  3. Protoderm
  4. Ground meristem
• Stem - not seen in most stem only seen in particular species
• Hoya

  • continuous cylinder of sclereids occurs on the periphery of the vascular region in the stem
  • groups of sclereids in the pith of stems
  • These sclereids have moderately thick walls and numerous pits
• LEAVES - common in leaves, it is distributed as either terminal positions or diffuse

  1. Terminal: confined to the ends of the small veins
  2. Diffuse: solitary sclereids or groups of sclereids dispersed throughout the tissue without any spatial relationship to the vein endings

  • In some protective foliar structures, the sclereids form part of the entire epidermis
• Fruits - occur in various locations in fruits. found in fleshy region of fruits – either singly or in cluster; in hard shells of fruits and stony endocarp.

  • In pear (Pyrus) and quince (Cydonia), single or clustered stone cells, or brachysclereids, are scattered in the fleshy parts of the fruit
• SEEDS - In seeds of bean (Phaseolus), pea (Pisum), and soybean (Glycine), columnar macrosclereids comprise the epidermis and prismatic sclereids or bone-shaped osteosclereids occur beneath the epidermis.