Tree biology is the study of the life processes of trees, including their growth, structure, and evolution. Trees differ from other plants in their ability to undergo secondary growth, which allows them to grow radially and produce wood. Secondary growth results in distinct layers, including the bark on the outside and xylem on the inside. The bark is divided into periderm (outer bark), phloem (inner bark), and vascular cambium. The xylem (wood) contains growth rings, tracheids, vessel elements, fibers, and parenchyma tissues. Trees also have a living symplast and nonliving apoplast that allow for transport and provide structural support.