Bark comprises all the tissues outside the vascular cambium of a vascular plant. The majority of the bark of woody plants develops from three meristems: the vascular cambium that gives rise to the secondary phloem, the phellogen that gives rise to the cork and the dilatation meristem that produces parenchyma cells to prevent cracking when the axis increases in diameter. Bark tissues have a critical role in defending plants from pathogens and herbivores through their physical and chemical properties. They also defend from environmental hazards such as sun irradiation, desiccation, wind, flooding, hail, snow and even fire by forming a thick cork layer. The bark has a critical role in storage and transport of organic molecules and in many plants the bark also contributes to photosynthesis. Many of the various defensive and toxic substances found in barks are used by humans as medicines, spices and for various industries. Gene exploring in barks is expected to result in many beneficial molecules for agriculture, medicine, food and industry.

Key Concepts:

• Bark comprises all the tissues outside the vascular cambium of a vascular plant.
  
• The majority of the bark of woody plants develops from three meristems: the vascular cambium that gives rise to the secondary phloem, the phellogen that gives rise to the cork and dilatation meristem that produces parenchyma cells to avoid cracking when the axis increases in diameter.
  
• The formation of the bark is regulated by several plant hormones, mainly auxin, ethylene, jasmonates and gibberellin.
  
• After wounding the induced rise in the hormones ethylene and jasmonic acid induces the formation of defensive cork and traumatic resin or gum ducts in many species.
  
• The bark functions in storage, transport and defence from herbivores, pathogens and environmental stresses.
  
• Many bark products (e.g., fibres, food, medicine, resins, rubber, pigments and cork) have been used by humans since antiquity.