A tree is a vascular plant of a woody substance that exhibits both primary and secondary growth. The basic sections of a tree are the root system, the trunk, the branches and the foliage. The function and description of each part are detailed below.
Vascular plants are plants with tissues that specialize in moving resources through the organism. This allows them to grow much larger than other plants. Primary growth refers to the increase in length of a plant’s stem, root or branch. Secondary growth, on the other hand, refers to increase in width. In trees, secondary growth can be seen in the way they add new a ring for each year.
The trunk is the stem of a tree.
Phloem is the innermost layer of bark. This tissue conducts sugar from the leaves (where it is converted from sunlight via photosynthesis) to the rest of the tree. The xylem is the heartwood of the tree --- the hard, durable tissue that provides structure and keeps the tree upright. This inner wood is technically dead, but it still allows for the transfer of water.
The bark, of course, is the outer layer or skin of the tree.
The terms branch, limb and bough are interchangeable, although bough is used by some to describe the end of the branch where it splits off into twigs (think the part that is used in Christmas wreaths). These terms refer to the units that grow off of the trunk. The purpose of a branch is to support the foliage so that it can receive sunlight. This determines the way branches grow. In dense stands, species that are less shade tolerant will lose their lower branches, as they can’t harness enough sunlight. This also explains why lower branches are longer in many conifers, since they need to reach out past the branches above to get sunlight. You’ll notice that there’s usually more foliage at the end of the branch than there is close to the trunk, where its shaded.
Branches experience both primary and secondary growth, since they need the extra thickness for support as they increase in length. Growing off the branches are twigs, the small shoots from which the foliage sprouts. Spreading out like this allow the tree to cover more surface area with its foliage, and to yield more seeds to increase its reproductive chances.
Every part of the tree needs water and sugar. Leaves play an important role for both resources. Water comes from the ground, and is drawn in by the roots, moving upward to the leaves. The movement of water is done through transpiration, a process where the leaves release water into the atmosphere. This creates negative pressure in the cells of the xylem tissue, forcing water upward. Sugar comes from the leaves, and is conducted through the phloem in a sticky substance we know as sap.
Leaves: Note that the foliage of conifers (commonly referred to as needles) are leaves, technically speaking. They carry out the same exact function, despite their different appearance. Chlorophyll is the substance responsible for photosynthesis, and is also what makes the leaves green. Deciduous trees lose the color in their leaves, and then the leaves themselves, in the fall. This means that they are surviving the winter on reserves.
Seeds: In spring, the leaves of deciduous trees return. Along with them come blossoms, usually in the form of a flower. These flowers attract pollinators, like bees. This leads to the production of fruit, which contains the seed that is necessary for reproduction. With conifer trees, the seed is contained within a cone, which is often the most reliable way to ID a conifer tree.
The root system
The roots provide structural strength, and draw water and nutrients from the soil for the rest of the tree. The taproot shoots downward in relatively straight fashion, providing an anchor for the trunk and crown. Like the trunk and branches, roots grow both in length and thickness. Smaller roots branch off of the taproot, and even smaller roots branch off them.
Roots have small hair-like fibers growing off of them that “grab” around clumps of soil. This compacts the soil, so that it can give the support the tree needs. Most of the root system is in the top 6-12” of the soil, as that is where most of the water is. The roots continue to multiply and to grow outward to cover more surface area. Expect the roots of a tree to extend roughly as far as the canopy.