The cork cambium produces an outer protective layer.

The cork cambium maintains a protective layer around a stem that is actively increasing in diameter (see Fig. 31.11). The cork cambium forms initially from cortex cells that de-differentiate—that is, they regress to their earlier state—to become meristem cells. However, with time, this layer of actively dividing cells becomes increasingly distant from its source of carbohydrates—the phloem. As the cork cambium becomes cut off from its carbohydrate supply, a new cork cambium forms within the secondary phloem. Thus, each cork cambium is relatively short- lived, and in many cases, new cork cambia form in patches rather than as continuous layers. As a result, the bark of many trees has a patchy and fissured appearance (Fig. 31.13).

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FIG. 31.13 Outer bark. (a) Outside and (b and c) inside, showing how successive cork cambia are produced. The growth of many, discontinuous cork cambia results in the patchy bark of pine and walnut trees.

The cells produced by the cork cambium are called cork. As cork cells mature, they become coated with suberin, a waxy compound that protects against mechanical damage and the entry of pathogens, while also forming a barrier to water loss. However, this layer of waxy, nonliving cells impedes the inward diffusion of oxygen needed to meet the respiratory demands of living cells. The outer bark cells are less tightly packed in regions called lenticels, allowing oxygen to diffuse into the stem (Fig. 31.14).

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FIG. 31.14 Lenticels in (a) water birch (Betula occidentalis) and (b) basswood (Tilia americana). Lenticels allow oxygen to diffuse through the outer bark to supply the respiratory needs of the cells within the stem.

Quick Check 2 Why do plants have two types of lateral meristem?

Quick Check 2 Answer

One of a plant’s lateral meristems produces new vascular tissues; the other maintains an outer protective layer.