The Difference Between a Simple and Compound Tree Leaf

1
The Simple or Single Leaf

Leaf Anatomy
Leaf Anatomy. Steve Nix

Common tree species with a single or simple leaf are primarily exclusive to maples, elms, oaks, birch, beech and cherries in North America. You will never see these trees with any other leaf arrangement. The actual leaf blade is attached singly and will always be attached to twigs by the petiole.

In a simple leaf, the blade is a single leaf that is never divided into smaller leaflet units. The true leaf is only attached at a tree bud. In contrast, compound tree leaves always have leaflets attached at a rachis where there is never a bud node. So a simple or single leaf attachment is always attached to a twig with the petiole (see the leaf structure illustration with parts labeled).

Simple leaves can have an entire or toothed edge (or the edge margin). These margins can be either unlobed or have protuberances that form lobes. Lobed leaves will have gaps between lobes but will never reach the midrib.

2
The Compound Leaf

Silhouettes Green Ash Leaf
Silhouettes Green Ash Leaf. Stephen G. Saupe

Common tree species with compound leaves are primarily exclusive to hickories, ash and some locusts in North America. You will always see these trees with leaflet arrangements attached to a leaf rachis which is in turn attached to twigs at a bud node. This combination of leaflets becomes the true leaf and can be confusing when identifying the actual leaf.

Trying to clear some of the confusion, a rachis is a biological term for main axis or "shaft" and often used to describe bird feather structure where the barbs are attached to that shaft. In botany and specifically in a compound tree leaf, the rachis is the main axis where only leaflets (not leaves) are attached. The rachis end then becomes the leaf "petiole" and is where the leaf is attached to the 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. There are no buds at the base of each leaflet. You should expect a bud node at the base of each petiole but no bud node at the base of each leaflet on midribs and the rachis of the compound leaf

  • A pinnately compound leaf:  Leaflets are arranged on both sides of  a common rachis or leaf stalk and have a feathery look.
  • A bipinnately or double pinnately compound leaf
  • A palmately compound leaf: Leaflets that radiate from a central point.

3
The Pinnately Compound Leaf

Leaf Morphology
Compound Leaflet Arrangments. Wikimedia Commons; David Perez

 First, the term pinnation, when talking about a tree's leaf, is where multi-divided leaflets arise  from both sides of a common axis called a rachis. This is to say that once-divided leaf blades having leaflets arranged on both sides of a rachis constitute a pinnately compound leaf.

There are three types of pinnate leaflet arrangement. Each of these categories defines leaflet morphology which is a major way to Identify a tree. The following types relate in order to the image I provide, from left to right.

Even-pinnate leaflet arrangement - rachis divisions on pinnately compound leaves in which leaflets sprout in pairs along the rachis without a single terminal leaflet. It is also called "paripinnate".

Odd-pinnate leaflet arrangement - rachis divisions on pinnately compound leaves in which there is a lone terminal leaflet rather than a terminal pair of leaflets. It is also called "imparipinnate".

 

alternipinnada 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 "alternate-pinnate".

4
The Double Pinnately Compound Leaf

Illustration of Double Pinnate. Wikimedia Commons

This compound leaf arrangement has several names including bi-pinnate, double pinnate and twice pinnate. The leaflets are actually arranged on side branches off a main axis or rachis. That is to say, they are on a secondary axis or rachis and are in fact "twice pinnate leaflets off of leaflets".

This is an unusual arrangement to appear in common North American trees and very important as a tree leaf marker for positive tree identification. The most common tree(s) that express bipinnate leaf structure is our native honey locust and the invasive mimosa. Other small but less common trees are Kentucky coffeetree and Hercules club.

5
The Palmately Compound Leaf

Silhouettes Buckeye Leaf
Silhouettes Buckeye Leaf. Stephen G. Saupe

The palmately compound leaf is easy to recognize and looks like a "palm frond" or a hand and fingers. There are very few common trees with this leaflet arrangement. The leaflets of this true leaf radiate from the center of their attachment to the petiole or leaf stalk which is again attached to the twig.

In North America, there are only several trees that have a palmately compound leaf, These trees are buckeye and horse chestnut.