One thing most trees have in common is their structure. If you tunnel into a tree’s trunk or branch, you encounter different layers. The first is bark: the outer layer of dead, protective cells, and then the live, spongy layer underneath called the phloem. Next, you find the cambium, a single layer of cells where the tree’s new wood and bark are formed. Finally, you come to the xylem—commonly called wood. Wood is made up of tightly packed, strong fibers and hollow tubes, like straws, that conduct water and nutrients through the tree.
As trees grow, the cambium produces a new layer of wood each year, called an annual ring or growth ring. The rings in the center of a trunk are the oldest. Wide rings usually indicate good growth conditions for that year, and narrow rings indicate poor conditions. Over time, some of the wood in the center is not needed for water and nutrient movement. This wood dies one ring at a time and becomes heartwood, which is often dark in color. The active, living wood is referred to as sapwood and is usually lighter in color.
A good way to get to know a tree is to look at its leaves. Leaves grow from a stem at regular points called nodes. The leaf is composed of a stalk (known as the petiole) and the expanded area called the blade. The form of a tree’s leaves can give you information about its species. Tree leaves tend to follow one of four forms: simple—with undivided blades; palmate—with the blade divided into leaflets that spread from the tip of the stalk; pinnate—with the blade divided into leaflets along the sides of the stalk (like a feather); or bipinnate—with the blade divided into sets of leaflets along sides of the stalk.
Whether or not a tree loses its leaves provides another clue to its identity. Deciduous trees drop all their leaves once a year, when the leaves die and fall off. Partly deciduous trees typically lose only a portion of their leaves at once, although they sometimes drop at the same time, depending on conditions. Evergreen trees keep their leaves year round; each leaf lasts one to several years and is replaced when it dies. With some exceptions, deciduous trees tend to have broad leaves and evergreen trees tend to have needlelike or scaly leaves.