Tree Leaf Key: Compound Leaves

A Quick and Easy Way to Identify Common North American Trees

Palm Leaves
Palm Leaves.

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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 the leaves and leaflets all start from the same point, which can help in identifying the specific genus of a tree based on 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.

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 they are looking at.

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Palmately Compound Leaves

Chestnut leaf with clipping path Chestnut
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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.

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

POISONWOOD, Metopium toxiferum. Pinnately compound leaf
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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 several small leaves, each of these leaflet groups is actually considered one leaf.

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 subsection of pinnate leaf arrangement belongs to a separate category called bipinnately and tripinnately compound leaves.

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Bipinnately, Tripinnately Compound Leaves

Bipinnately compound leaves

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Bipinnately compound leaves are pinnately compound leaves whose leaflets further pinnately divide.

Often confused with shoot system plants, those like the silk tree or some common ferns that have complex leaf systems belong to the arrangement known as bipinnately or tripinnately compound leaves. Essentially, these plants have leaflets that grow off 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 here, is an excellent example of bipinnately compound foliage. Though it seems otherwise, this is only one leaf.