The Difference Between a Simple and Compound Tree Leaf

One of the most fascinating aspects of tree morphology, meaning the way in which individual specimens are shaped, is the study of the shape of individual leaves. All trees, whether grown ornamentally or in the wild, have a leaf structure that can be classified as either simple, pinnately compound, double or bi-pinnately compound, or palmately compound. Here's a guide to what those look like:

Simple

Leaf Anatomy
Steve Nix

A simple leaf is a single leaf that is 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

Silhouettes Green Ash Leaf
Stephen G. Saupe

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

If you have a doubt as to whether you are looking at a leaf or a leaflet, 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. On a compound leaf, you should expect a bud node at the base of each stem/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

Leaf Morphology
Compound Leaflet Arrangments.

David Perez/Wikimedia Commons

The term pinnation, when talking about a tree's leaf, refers to how multi-divided leaflets arise from both sides of a common axis, or 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. Also called "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. Also called "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. It is also called "aternipinnada."

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

Double Pinnately Compound

Illustration of Double Pinnate

Steve Nix

This compound leaf arrangement has several names, including bi-pinnate, double pinnate, and twice pinnate. In this case, leaflets are arranged on what are actually secondary stems, which grow off a main stem, or rachis. 

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

Palmately Compound

Silhouettes Buckeye Leaf
Stephen G. Saupe

The palmately compound leaf is easy to recognize because it looks like a palm frond, with its distinctive hand-and-finger shape. Here, 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.