Trees With Pinnately Compound Leaves

Pinnately compounds leaves are those attached to either side of twig-connection petioles of varying lengths called rachises that form above the axil, or the leaf's true petiole attachment to the twig, and often are joined by smaller leaflets on the petioles. The term pinnate comes from the Latin word pinnātus which means feathered or winged (like a feather).

If you have a leaf like this, you will most likely have either a pinnately compound tree leaf or a leaf with multi- pinnate characteristics that form bipinnately compound tree leaves as illustrated and identified below.

There are many trees and shrubs with pinnately compound leaves in North America. The most common tree species with this leaf configuration are hickory, walnut, pecan, ash, box elder, black locust and honey locust (which is bipinnate). The most common shrubs and smaller trees are mountain-ash, Kentucky yellowwood, sumac along with invasive exotic mimosa, alanthus, and chinaberry trees.

Some pinnately compound leaves can branch again and will develop a second set of pinnately compound leaflets. The botanical term for leaves with these secondary leaf branches is called a bipinnately compound leaf.

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The Many Degrees of Compound Leaves

Pinnately Compound Leaves.

There are many degrees of "compoundness" in more complicated leaves (such as tri-pinnately compound). Leaf compoundness may cause some tree leaves to grow extra shoot systems on the leaf and can confuse the leaf identification beginner.

It is always possible to distinguish a compound leaf attachment to the stem from a leaflet attachment to the petiole and rachis. A leaf attachment to the stem is recognized because there are axillary buds found in the angle between a true branch stem and the leaf petiole. This angle between a stem and the leaf petiole is called an axil. However, there will not be axillary buds present in the axils of leaflet attachments to the leaf rachis.

It's important to note the axils of a tree's leaves because these define which level of compound the leaves are experiencing, from simple pinnately compound leaves to the multi-tiered tri-pinnately compound leaves.

Compound leaves also come in other varieties, including paripinnate, imparipinnate, palmate, biternate, and pedate, each of which is defined by how the leaves and leaflets attach to the petiole and rachis (and/or secondary rachis).

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Trees With Pinnate Leaves

Pinnate Leaf. Pinnate Leaf

Trees having a leaf that is pinnately compound will have leaflets growing from several places along the stalk or rachis — there can be as many as 21 leaflets and as few as three.

In most cases, you will see an oddly pinnate leaf. That simply means that there will be a single terminal leaflet followed by a series of opposing leaflets. This can also be referred to as imparipinnate as the number of pinnate leaflets on each petiole is uneven and therefore not paired. Leaflets at the top of these are typically larger than those closer to the base of the petiole

Hickory, ash, walnut, pecan and black locust are all pinnate-leafed trees that can be found in North America. Check out one next time you're on a walk and see if the leaflets on each petiole.

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Trees With Bipinnate Leaves

Illustration of Double Pinnate. Wikimedia Commons

Trees having a leaf where at least some of the leaves are doubly compound and the leaflets have mostly smooth margins are known as bipinnate. The leaflets on these petioles appear on the rachis then are further subdivided along secondary rachises.

Another botanical word for bipinnate is pinnule, which is the word used to describe leaflets that are further pinnately divided. This term is used to describe any leaflet that grows in such a way, but it is most commonly associated with ferns.

The most common North American tree species with bipinnate leaves is a honey locust, though Bailey Acacias, silk trees, flamegolds, chinaberries, and Jerusalem thorns are also all examples of trees with bipinnate leaves.

Bipinnate leaflets can be easily confused with tripinnate leaflets, so it's important for those attempting to identify the trees from their leaf configuration to note whether the leaflet attaches to the first rachis or a secondary rachis — if it's secondary, the leaf is tripinnate.

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Nix, Steve. "Trees With Pinnately Compound Leaves." ThoughtCo, Sep. 11, 2017, thoughtco.com/pinnately-compound-leaves-tree-leaf-key-1343484. Nix, Steve. (2017, September 11). Trees With Pinnately Compound Leaves. Retrieved from https://www.thoughtco.com/pinnately-compound-leaves-tree-leaf-key-1343484 Nix, Steve. "Trees With Pinnately Compound Leaves." ThoughtCo. https://www.thoughtco.com/pinnately-compound-leaves-tree-leaf-key-1343484 (accessed September 22, 2017).