In a mature tree the layers of xylem usually constitute more than 95 percent of the diameter of the stem. The xylem layers are called wood and the layers outside the cambium are called bark. The growth of wood for each year appears as a distinct ring called an annual, or growth, ring. The width of each ring is affected by climate and other variables, and archaeologists have studied tree rings to determine the climatic conditions and variations in environment of former times. Xylem carries water and dissolved mineral nutrients upward from the soil to the leaves.
Water is used in the leaves in a process of food manufacture called photosynthesis.
Food is transported downward to the roots by the phloem. Trees add to our enjoyment of the outdoors and contribute to the value of our property. With age or disease weakening a tree, it can cause injury to property and individuals, interfere with power and telephone lines, crack sidewalks or even the foundation of a home.