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.

Keywords: bark; cork; defence; dilatation; periderm; rhytidome; storage

Figure 1.

Cross‐section of the stem of a small tree of Calotropis procera showing a microscopic view of the secondary xylem with the pores of the water conducting vessels (red bottom part); live part of the bark with the band of latex‐forming ducts in the middle and the outer layers of cork with a typical lenticel in the centre (the green stained).

Figure 2.

Longitudinal tangential section of the bark of a large tree of Ficus sycomorus showing a microscopic view of the bark in a region of old phloem where dilatation started. The axial fibres and parenchyma form a net of strands while the radial component, the rays (which are spindle‐shaped), start to dilate.



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Further Reading

Dusotoit‐Coucaud A, Kongsawadworakul P, Maurousset L et al. (2010) Ethylene stimulation of latex yield depends on the expression of a sucrose transporter (HbSUT1B) in rubber tree (Hevea brasiliensis). Tree Physiology 30: 1586–1598.

da Ponte‐e‐Sousa JCA, de A and Neto‐Vaz AM (2011) Cork and metals: a review. Wood Science and Technology 45: 183–202.

Saveyn A, Steppe K, Ubierna N and Dawson TE (2010) Woody tissue photosynthesis and its contribution to trunk growth and bud development in young plants. Plant, Cell and Environment 33: 1949–1958.

Schreiber L (2010) Transport barriers made of cutin, suberin and associated waxes. Trends in Plant Science 15: 546–553.

Serra O, Figueras M, Franke R, Prat S and Molinas M (2010) Unraveling ferulate role in suberin and periderm biology by reverse genetics. Plant Signaling & Behavior 5: 953–958.

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Lev‐Yadun, Simcha(May 2011) Bark. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0002078.pub2]