Bark, outer covering of the stems and branches of woody plants. The protective outer bark usually consists of cork, while the inner bark contains food-conducting phloem and the reproductive cells of the cork cambium. Bark is usually formed in annual rings, the outer layers bursting and splitting with each new growth. Bark helps to protect the tree from extremes of climate and from various pests and diseases. Some varieties of bark have medicinal uses, others are sources of textile fibers and dyes.