Cork

Abstract

Cork occurs in stems, roots, petioles as well as in some fruits or bud scales. It is usually formed from phellogen. Cork cells are usually tightly packed, dead and filled with air. Suberin is deposited along with waxes on the primary wall of cork cells. In addition to natural cork in intact plants, wound cork can develop after wounding. There are also specific structures of cork in some groups of plants, such as aerenchymatous phellem, polyderm, interxylary cork and the storied cork which is formed without the involvement of phellogen in some monocotyledons. Cork creates a physico‐chemical barrier that protects against water loss and pathogen invasion, but in some cases, it facilitates an exchange of gases between a plant's roots and its stem. Cork is characterised by low density; high elasticity; high resistance to compression; a low Poisson's ratio; low thermal, water and gas conductivity and the slow propagation of sound waves.

Key Concepts

  • Typical cork with tightly packed cells forms a physico‐chemical barrier, protecting a plant against water loss and pathogen invasion.
  • Loosely arranged cork cells may facilitate gas exchange in flooded plants.
  • Cork is usually formed in plants during secondary growth through activity of the phellogen, a secondary meristem.
  • In monocotyledons with abnormal secondary growth, the storied cork is formed which develops from parenchyma cells.
  • The cell walls of cork are subject to various modifications, the most important of which is adcrustation with suberin.
  • The degree of cork cell wall suberisation may be modified depending on the environment.
  • Cork can be considered as a natural form of the so‐called cellular solid structures known in technology.

Keywords: phellem; cork; phellogen; cork; phelloid; periderm; interxylary cork; polyderm; storied cork; suberin; solid structure

Figure 1. Drawing of cork of cork oak (Quercus suber L.) by Robert Hook, published in 1665 in ‘Micrographia’. Reproduced from Robert Hook (1665).
Figure 2. (a, b) Cork on pear (Pyrus communis L. ‘Conference’) fruits. (a) Layer of cork visible on the surface of the fruit – brown spots marked with a white arrow. (b) SEM image of cork with a clearly tile‐like form of tissue. (c, d) Cork on the shoot of a small‐leaved lime (Tilia cordata L.). (c) Two‐year old shoot with visible cork layers on the surface of the shoot (yellow arrow). (d) SEM image showing a splitting epidermis and a visible layer of cork cells (yellow arrow). Courtesy of A. Konarska.
Figure 3. Cross section of the periderm of elder (Sambucus nigra L.) with the epidermis still visible – e, closely arranged layer of cork – c, phellogen – p, phelloderm – pd. Courtesy of B. Łotocka.
Figure 4. Cork of cork oak (Quercus suber L.). (a) The layer of cork harvested from the trunk of an oak with the marked sections photographed under an SEM. (b) Radial longitudinal section (red). (c) Cross section (blue). (d) Tangential longitudinal section (green). Specimen of P. Staniszewski.
Figure 5. Suberised cork cell wall. (a) Lamellar structure of the cell walls of cork from an over 10‐year‐old trunk of white willow (Salix alba L.), where the light bands are aliphatic and the dark bands are aromatics domains. ml – middle lamella, cw1 – cell wall of a single cell, cw2 – cell wall of an adjacent second cell. Courtesy of E. Kurczyńska. (b) Model for the structure of suberin. Two lamellae, the poly(aliphatic) and a fraction of the poly(phenolic) domains in a portion of suberised potato cell wall, are shown including two lamellae, the poly(aliphatic) and a fraction of the poly(phenolic) domains. The poly(phenolic) domain is shown restricted to the primary cell wall and covalently attached to carbohydrate units. The poly(aliphatic) domain is represented by a linear, glycerol‐based polyester. The phenolic‐rich zones would yield the darker (electron rich) bands observed in TEM images while the predominantly aliphatic zones would yield the light bands observed in TEM images. Reproduced with permission from Bernards MA (2002) © NRC Research Press.
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Pereira H (ed.) (2011) Cork: Biology, Production and Uses. Amsterdam: Elsevier.

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Zajączkowska, Urszula(May 2016) Cork. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002080.pub2]