Leaves

Leaf arrangement, shape, size, color, venation, texture, and margin all provide clues to a tree's identity.

Refer to Appendix 2, Leaf Key to Common Trees in Maryland, for further explanations or pictures of different leaves.

Leaves are produced in set intervals along the twig. In most tree species, a single leaf occurs at the point of attachment or node, and leaves appear to alternate with each other on the twig, in an alternate arrangement. Less frequently, a pair of leaves emerges from the node, in an opposite arrangement. If more than two leaves appear at a common node, they are whorled.

Each leaf consists of a blade extending from a leafstalk, which is attached to a woody stem. If the blade is all in one piece, it is known as a simple leaf. A compound leaf has two or more separate blades, or leaflets, attached to the central leafstalk. A leaflet may look like a leaf, but leaflets are distinct: they have no bud at the base and leave no definite leaf scar when picked. Hickories, walnut, and the ashes are examples of trees with compound leaves.

The leaves, or needles, of pines grow in fascicles, or bundles. The number of needles making up the fascicle varies from species to species. For example, loblolly pine needles grow in fascicles of 2 or 3, while white pine needles grow in fascicles of 5.

For illustrations of leaf arrangements, see Appendix 2, the publication, Leaf Keys to Common Trees in Maryland (page 3, first two rows).

The size and shape of a leaf are characteristic for a species and of tremendous value in tree identification. The common leaf shapes are illustrated in Appendix 2, pages 7-25. Occasionally, leaf variations appear in the same tree; for example, the sassafras has three distinctly different leaf shapes.

The margin, the edge of the leaf blade, can be smooth, toothed, or lobed. Appendix 2 illustrates common leaf margins (page 3, third row).

Leaves exhibit various textures-thin or thick, and smooth, waxy, or pubescent (hairy). Some leaves are smooth on top and hairy underneath.

In broadleaf trees, leaf veins usually follow one of four patterns: parallel, or closed venation, in which the veins run nearly parallel to each other, from the base to the apex of the leaf, resulting in a "closed" system; palmate venation, in which three or more secondary veins branch from the base of the leaf; pinnate venation, in which the midrib extends the length of the leaf, with secondary veins branching off; or arcuate venation, which is a modification of pinnate venation. Figure 7-1 illustrates leaf venation.

Particularly in the fall, color offers a clue to leaf identity. However, even in the spring and summer, leaves vary in intensity of color.