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Cork originates from a layer of cambium (=phellogen) that itself is formed as a secondary meristem from a layer of collenchyma or parenchyma immediately beneath the epidermis. In contrast to sclerenchyma cells, collenchyma cells are alive and they have retained the potency to de-differentiate. Cork cambium cells only divide periclinally so that the typical rows of daughter cells arise: cork cells (=phellem) are mainly generated toward the outside. In a lesser extent also cork parenchyma (=phelloderm) is made toward the inside. Cork cambium, cork cells and cork parenchyma together are also named periderm. Mature cork cells are dead; their cell walls contain suberine, a fatty substance that repels water. The layer of cork provides protection against desiccation, but it also isolates tissues in the inner parts of the stem or trunk so thoroughly that exchange of gas with the outer world is impeded. 'Breathing' is yet achieved by so-called lenticels. The cork cambium starts to generate numerous parenchyma cells toward the surface, in most cases at the level of a stoma. These thin-walled parenchyma cells, which eventually degenerate, force an interruption of the sealing created by the cork layer and focus an opening for gas-exchange.
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