Cork is formed by a cambium layer that itself almost always arises as secondary meristem from the collenchyma or parenchyma layer directly under the epidermis. Unlike sclerenchymal cells, collenchymal cells are still alive and have retained the ability to differentiate. Cork cambium cells only divide periclinally, creating the typical rows of daughter cells. In this way mainly cork cells are produced to the outside and to a much lesser extent (sometimes not at all) cork parenchyma is produced to the inside. Cork cambium, cork cells and cork parenchyma together are also called periderm. Developed cork cells are dead and their cell walls contain suberine, a greasy substance that repels water.
The cork layer protects against dehydration, but in this way it closes tissue in the inner part of the stem so thoroughly that gas exchange with the outside world is prevented. 'Breathing' is made possible because the cambium - usually at the height of a stoma - locally deposits so many parenchyma cells to the surface that interruptions occur in the cork layer, the so-called lenticels (of lens or lens-shaped). Eventually these thin-walled parenchymal cells degenerate, but the gas exchange is guaranteed.
Source: Radboud University, The Netherlands
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