Trees have a few basic parts: Roots, Shoots, Flowers and Fruits.
The primary part is the root system. Roots are primary in two ways: the first part of a new plant emerging from a seed is the root, which shows how critical the root-soil connection is for plants. In the landscape, root function is critical. Roots make up roughly 25% of the mass of a tree, and have the following functions:
They anchor the tree to the ground. Trees have to stand up against gravity and wind, so the roots spread widely (NOT deeply) to hold up the crown (see figure below).
They take in and distribute water and nutrient elements from the soil ecosystem, both of which are used throughout the tree for many purposes. In order to do this, roots need to be able to get fresh oxygen from the soil.
Everything above ground is part of the shoot system. Roughly 75% of the tree mass is in the shoot system. Much of that is in the form of wood. Sapwood has active water vessels (xylem), heartwood is old sapwood that no longer has active xylem. Older wood can be thought of as being dead, but healthy dead. The cells don’t have living contents, but they are not being decayed. This kind of wood is very strong yet flexible.
Landscape trees should have strong trunks to support their branch systems. Poor structure and previous damage are indicators of greater chance of failure in the trunk system.
The branches grow out of the trunk. They start out as new twigs, and over the years certain ones keep growing, becoming the woody scaffold holding up the leaves. Branches can have strong or weak attachment to their parent stem (larger branch or trunk); weakly attached branches fail more often.
The leaves are where photosynthesis occurs and they are very important for the tree. Leaves are also expendable; most trees lose their leaves every year. The loss of a small percentage of a tree’s leaves usually doesn’t hurt a tree in any real way.
Most trees will also produce flowers and fruits. These are the reproductive parts, leading to new individual trees formed in the seeds found in the fruit. Trees with showy and/or sweet smelling flowers can add a nice dimension to a landscape. However, many fruits might be considered a nuisance in a typical urban landscape setting. Fruits can be fleshy (like an apple or peach) or dry (like a locust pod or an acorn).