Tree Leaf Key: Compound Leaves

A Quick and Easy Way to Identify 50 Common North American Trees

A compound leaf is one whose blade has two or more sub-units called leaflets attached to the same stalk or petiole. Classification of trees with these types of leaves can further be defined by whether or not the leaves and leaflets all start from the same point, which can help in identifying the specific genus of a tree based off of its leaves, bark, and seeds.

Once you understand that you have a compound leaf, you can then determine which type of compound leaf it is: palmate, pinnate, or bipinnate. In palmately compound leaves, the leaflets form and radiate from a single point of attachment called the distal end of the petiole or rachis. Another way to describe the palmate form is that the whole leaf structure is "palm-like" and shaped like the palm and fingers of your hand.

Pinnately compound leaves will have twig-connecting petioles of varying lengths with rows of smaller sub-leaves above the axil. These leaflets form on either side of an extension of the petiole or rachis, and although they may look like a number of small leaves, each of these leaflet groups is actually considered one leaf. Bipinnately compound leaves, consequently, are pinnately compound leaves whose leaflets further pinnately divide.

All three of these descriptions of leaves belong to the arrangement classification within a system called morphology that is used to study plants and name them by genus and species. Common leaf morphology includes classification by leaf venation, shape, margins, and the arrangement of the stem. By identifying leaves through these six classifications, herbalists and nature lovers alike can more accurately assess what type of plant he or she is looking at.

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Palmately Compound Leaves

Chestnut leaf with clipping path Chestnut
Joakim Leroy/E+/Getty Images

Palmately compound leaves originate from one point at the end of the petiole and can come in sets of three or more, depending on the genus of the tree bearing the leaves.

In palmately compound leaves, each leaflet is part of the individual leaf, all branching off from the axil. This may lead to confusion between palmately compound and simple leaf arrangements, as some simple leaves form on branches in a similar shape to palmate clusters of leaflets.

Palmately compound leaves do not have rachises as each palmate branches out directly from the petiole, though each petiole may also branch off to other petioles.

Some common examples in North America are poison ivy, the horse chestnut tree, and the buckeye tree. When trying to identify a tree or plant as a palmately compound, make sure that the leaflets are indeed attached to one point on the petiole, otherwise, you may be working with a different classification of the leaf.

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Pinnately Compound Leaves

Pinnately compound leaves are another classification of the arrangement of leaves that can be used to determine which genus a tree belongs to. These leaflets (called pinnule) form in rows along or on either side of the middle vein that's known as the rachis, which all form one leaf attached to the petiole or stem.

Pinnately compound leaves are common in North America as exampled by the abundance of walnut, pecan, and ash trees in the United States, all of which have pinnately compound leaves.

These pinnately compound leaves can compound again, branching off secondary rachises and forming new leaflets called pinna. That subsect of pinnate leaf arrangement belongs to a separate category called bipinnately and tripinnately compound leaves.

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Bipinnately and Tripinnately Compound Leaves

Bipinnately compound leaves. Image by Starr Environmental under a Flickr Creative Commons Attribution License

Often confused with shoot system plants, those like the silk tree or some common ferns that have complex leaf systems belong to arrangement known as bipinnately or tripinnately compound leaves. Essentially, these plants have leaflets that grow off of secondary rachises.

The distinguishing factor of plants like these, what makes them truly bipinnate, is that the auxiliary buds are found in the angle between the petiole and the stem of pinnate leaves but not in the axils of leaflets.

These leaflets are twice or thrice divided, but all still account for one leaf branching off the stem. Because the leaflets form on primary and secondary veins in this type of compound leaf, the leaflets formed on the secondary are given the name pinna.

The royal poinciana, pictured on the left, is an excellent example of bipinnately compound foliage. Though it seems otherwise, this is only one leaf.