The Ultimate Guide to Simple and Compound Tree Leaves

Defining the characteristics of individual leaves is part of tree morphology, or the study of the physiological structure of trees. All trees have a leaf structure that can be classified as either simple or compound. Compound leaves can be classified further as pinnately compound, double or bi-pinnately compound, or palmately compound.

Identifying which category a tree leaf falls into can give you a better idea of what kind of tree you are observing. This guide provides key information on the differences between simple and compound leaves.

Simple Leaves

A simple leaf is singular and never divided into smaller leaflet units. It is always attached to a twig by its stem or the petiole.

The margins, or edges, of the simple leaf can be smooth, jagged, lobed, or parted. Lobed leaves will have gaps between lobes but will never reach the midrib. Maple, sycamore, and sweet gum are all examples of common North American trees with simple leaf structure.

Compound Leaves

In contrast to a single leaf, the compound leaf has leaflets, which are attached to the middle vein and have their own stalks. Envision a bunch of single leaves, all attached to the main stem, called a rachis, which in turn is attached to a twig.

It is sometimes confusing to know whether you're looking at a leaf or a leaflet. In these situations, locate lateral buds along the twig or branch. All leaves, whether simple or compound, will have a bud node at the place of petiole attachment to the twig. Compound leaves have a bud node at the base of each stem or petiole but no bud node at the base of each leaflet on midribs and the rachis of the compound leaf.

There are three types of compound leaves: pinnately, double pinnately, and palmately.

Pinnately Compound

The term pinnation refers to the process in which multi-divided leaflets arise from both sides of the rachis. There are three types of pinnate leaflet arrangement. Each of these categories defines leaflet morphology and are used by biologists to identify tree species:

• Even-pinnate leaflet arrangement: rachis divisions on pinnately compound leaves in which leaflets sprout in pairs along the rachis without a single terminal leaflet. This arrangement is also known as paripinnate.
• Odd-pinnate leaflet arrangement: rachis divisions on pinnately compound leaves in which there is a single terminal leaflet at the top of the structure rather than a terminal pair of leaflets. This arrangement is also known as imparipinnate.
• Alternate-pinnatel leaflet arrangement: rachis divisions on pinnately compound leaves in which leaflets sprout alternately along the rachis, usually with a single terminal leaflet. This arrangement is also known as aternipinnada.

Common pinnately compound leaf-shaped trees in North America include hickory, walnut, pecan, ash, box elder, and black locust.

Double Pinnately Compound

This compound leaf arrangement has several names, including bi-pinnate, double pinnate, and twice pinnate. Leaflets are arranged on what are actually secondary stems, which grow off the rachis. 

This is a rare arrangement for common North American trees, but some examples include the honey locust, the invasive mimosa, Kentucky coffeetree, and Hercules club. 

Palmately Compound

The palmately compound leaf is easy to recognize because it looks like a palm frond and has a distinctive hand-and-finger shape. Leaflets radiate out from the center of their attachment to the petiole or leaf stem, which is again attached to the twig.

The two trees native to North America that feature palmately compound leaves are buckeye and horse chestnut.