Pharmacognosy Lecture Notes - Bark

The Bark

  • The bark consists of all tissues of the stem or the root outside the vascular cambium.

Young Stem Structure

  • Pith
  • Vascular Cambium
  • Cork Cambium
  • Epidermis
  • Cortex
  • Phloem
  • Xylem

Old Stem Structure

  • Cork (phellem)
  • Cork Cambium (phellogen)
  • Secondary Cortex (phelloderm)
  • Primary Cortex
  • Primary Phloem
  • Secondary Phloem
  • Vascular Cambium
  • Secondary Xylem
  • Primary Xylem

Bark Definition

  • Bark includes all the tissues outside the vascular cambium.

Shapes of Barks

  • Flat: Quite flat and very thick (e.g., Quillaia).
  • Curved: Slightly concave on the inner side (e.g., Wild cherry).
  • Recurved: The concave side is the outer one (e.g., Pomegranate).
  • Channeled: Deeply concave on the inner side (e.g., Cassia).
  • Single Quill: One edge of bark covers the other (e.g., Cascara & Cinchona).
  • Double Quill: Both edges curved inward (e.g., Frangula).
  • Compound Quill: Smaller quills are packed into bigger ones (Cinnamon).

Bark Surfaces

  • Outer Surface
  • Inner Surface

Outer Surface Features

  • Epiphytes:
    • Lichens (e.g., Cinchona)
    • Liverwort (e.g., Cascara - stem bark only)
    • Mosses (e.g., Cascara - stem bark only)
  • Lenticles: Breathing pores for exchange gases. Their shape and abundance may aid in bark identification.
  • Cracks & Fissures: Formed due to increased growth and lack of elasticity (e.g., Cinchona calisaya).
  • Wrinkles & Furrows: Result from shrinking in the softer tissues.
  • Smooth

Inner Surface Features

  • The color and condition of the inner surface are of diagnostic value.
  • Smooth (e.g., Quillaia).
  • Finely striated (e.g., Cinnamon - fine parallel longitudinal ridge).
  • Coarse striated (e.g., Cascara - coarse parallel longitudinal ridge).
  • Corrugated (e.g., Cascara - transverse parallel wrinkles).

Fracture Types

  • Short: The fractured surface is smooth.
  • Granular: The fractured surface exhibits small rounded projections.
  • Splintery: Jagged projecting points are formed.
  • Fibrous: Fine fibrous threads extend from the broken surface.
  • Lamellated: The fractured region breaks into tangentially arranged layers.

Difference between Stem Bark and Root Bark

  • Stem Bark
    • The outer surface is usually lighter in color than the inner surface.
    • The outer surface shows epiphytes (Lichens, mosses, and liverworts).
  • Root Bark
    • Both the outer and the inner surface have the same dark color.
    • Epiphytes are absent.

Bark Structure

  • Commercial bark may consist of all or some of the following tissues, starting from outside inward:
    • Rhytidoma
    • Periderm:
      • Cork (Phellem)
      • Cork Cambium (Phellogen)
      • Secondary Cortex (Phelloderm)
    • Primary Cortex
    • Pericycle
    • Primary Phloem
    • Secondary Phloem

Detailed Bark Structure

  • Rhytidoma
    • Formed of outer dead or collapsed tissues of the epidermis, cortex, or phloem.
    • Usually removed during preparation of commercial bark.
  • Cork or Phellem
    • Protective tissue of secondary origin developed in stems and roots.
    • Formed of dead polygonal cells, which are either slightly radially elongated or radially flattened.
    • Cells are usually uniform in shape and arranged in radial rows, tightly fitted together, and lacking intercellular spaces.
    • Typically appears dark brown due to the presence of brown or reddish cell contents.
  • Phellogen
    • By division, produces rows of cork cells to the outside and unsuberised cells forming the phelloderm to the inside.
  • Phelloderm (secondary cortex)
    • Unsuberised cells, arranged in one, two, or several layers, usually devoid of coloring matter.
    • May be parenchymatous, collenchymatous, or sclerenchymatous.
  • Cortex (primary cortex)
    • Absent in many barks due to decortication (e.g., Cinnamon).
    • If present, may contain sclereid cells, fibers, oil-cells, mucilage cells.
    • Cells may contain small starch granules, and crystals of calcium oxalate may be present.
  • Pericycle
    • Varies in extent; may be formed of one to many cells thick.
    • May be parenchymatous (indistinguishable from the cortex e.g., Cinchona and Cascarilla) or sclerenchymatous (e.g., Cinnamon and Witch-hazel).
  • Phloem (Bast)
    • Primary phloem is usually collapsed or totally absorbed.
    • The phloem is mainly composed of secondary phloem constituting the inner bark, traversed by medullary rays.
    • Characterized by the presence of sieve tubes, which may be accompanied by companion cells and phloem parenchyma.
    • Commonly contains bast fibers, sometimes sclereid cells.
    • May also show oil cells (e.g., Canella, Cinnamon), mucilage cells (e.g., Cassia).
    • The parenchyma usually contains starch granules and sometimes crystals of calcium oxalate.

Tissue Layers

  • Outer or Shell Bark
    • Rhytidoma (dead collapsed tissues)
    • Periderm:
      • Cork (Phellem)
      • Cork cambium (phellogen).
      • 2ry cortex (phelloderm).
    • Primary cortex.
  • Middle Bark
    • Pericycle.
    • 1'y phloem.
  • Inner Bark
    • 2ry phloem (Bast).
  • Wood
    • Cambium.
    • 2ry xylem
    • 1' xylem
    • Pith