In woody plants, the epidermis is sloughed off and replaced by a periderm after the cork cambium begins producing new tissues that increase the girth of the stem or root. The periderm constitutes the outer bark and is primarily composed of somewhat rectangular and boxlike cork cells, which are dead at maturity.
While the cytoplasm of cork cells is still functioning, it secretes a fatty substance, suberin, into the walls. This makes cork cells waterproof and helps them protect the phloem and other tissues beneath the bark from drying out, mechanical injury, and freezing temperatures. Some cork tissues, such as those produced by the cork oak, are harvested commercially and are used for bottle corks and in the manufacture of linoleum and gaskets.
Some parts of a cork cambium form pockets of loosely arranged parenchyma cells that are not impregnated with suberin. These pockets of tissue protrude through the surface of the periderm; they are called lenticels and function in gas exchange between the air and the interior of the stem. The fissures in the bark of trees have lenticels at their bases.
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