By definition, a tree must have a central stem and reach a height of at least 20 feet at maturity. Other woody plants are called shrubs. Through these identifying characteristics trees can be divided into groups. Leaf shapes sometimes vary on the same tree but do not lose their identity. The first division can be made with leaves which occur opposite on the stem and those which occur alternately.
Leaves are simple or compound.
Leaves are lobed or oval, and, they are broad oval or slim oval. To be a leaf the stem of the leaf must leave the main branch at a point where a bud occurs. In compound leaves there is no bud at the base of the leaflet. Compound leaves are further divided into palmately and pinnately compound. Examples are ash, walnut, butternut, locust and hickory.