Many folks remember the Poison Ivy/Oak warning as “Leaves of three, let them be.” However, the real saying is “Leaflets three, let it be” because a poison ivy/oak leaf is a compound leaf that is divided into 3 leaflets. I suppose it doesn’t really matter if you get into a patch of it, but you might like to learn the difference between simple leaves and compound leaves to help better identify other plants in the future.
By the way, the picture above is Trillium decumbens, one of the “nice” plants with three true leaves.
Leaf identification can seem to be such a daunting task when you are standing in a forest, surrounded by many different trees and shrubs. You can see the differences – big, little, shiny, hairy, smooth edges, jagged edges – but it’s too much to take in all at once. I find it easier to consider the differences on paper and then find examples to illustrate them - like a basic leaf identification primer.
First, separate the leaves into two types: Broadleaf Leaves and Needled Leaves. This has nothing to do with leaves being evergreen, just the type of leaf. You are probably quite familiar with Pine trees having “needles”; needles are a type of leaf. Other leaves are considered to be “broadleaf” – leaves from plants like oaks, hickories, magnolias, and maples. This post is only going to deal with broadleaf identification such as the Florida anise shown here.
|Illicium floridanum, Florida anise|
The next separation is to determine if the leaf is simple or compound. This brings us back to Poison Ivy (Toxicodendron radicans, eastern poison ivy). When you see the familiar look of 3 “leaves”, you are actually looking at one compound leaf that is divided into 3 leaflets. This becomes more evident in the fall when the plant sheds it’s leaves; at that time the leaf sheds itself from the twig at the end of the petiole (the stalk that holds the leaf) which is below the 3 leaflets. As an example, here is a picture of a buckeye (Aesculus sp.) with 5 leaflets.
|Buckeye, Aesculus sp.|
If it were 5 leaves, each one would fall off, leaving the stalk in place. The picture shows the leaflets separating, but more importantly you can also see the separation of the petiole from the twig itself and that is the key. The whole structure falls away, revealing that it was a leaf holding those 5 leaflets.
You could also look more closely at the leaflets and see that there is no bud for next year’s growth next to them; that is because you are not looking at a true twig. A twig has leaf buds: the bud can be found at the base of the petiole as shown in this picture of an oak (Quercus sp.). Notice that where the petiole meets the twig you can see the bud for next year’s leaf.
|Oak, Quercus sp.|
So, a compound leaf has many leaflets. Some of the common plants with compound leaves are: Hickory (Carya sp), Walnut (Juglans sp), Pecan (Carya illinoinensis), Sumac (Rhus sp), Ash (Fraxinus sp), Elderberry (Sambucus sp), Buckeye (Aesculus sp), Poison Ivy (Toxicodendron sp.), Locust (Robinia sp), and Kentucky Coffeetree (Gymnocladus dioicus).
Plants with simple leaves have a single leaf, but there is still a lot of variation. Once you’ve determined that you have a plant with a single leaf, you will want to determine if the leaf is lobed and if it is toothed, two important distinguishing characteristics.
What is a “lobed” leaf? The dictionary defines it as “a leaf having deeply indented margins”. Another source compares it to “having fingers”. Examples are often the best way to understand. Examples include Sugar Maple, Red Oak, some of the Sassafras leaves (you may already know that Sassafras has both lobed and unlobed leaves), and this delightful Parsley Hawthorn (Crataegus marshallii) which I photographed this fall:
The spaces between the lobes are called “sinuses” and they can be very shallow or very deep - even on the same plant. I find this especially true of Oaks. One fall I picked up a variety of White Oak leaves (Quercus alba). I was fascinated by the variation. Here is a picture of the ones I collected showing the variation in shape and depth of sinus.
|Quercus alba leaves|
Even without lobing, a leaf can be identified by looking at the margins, that is, the edges. Are the edges perfectly smooth? Then they are considered to be “entire”. Florida anise (Illicium floridanum) has such a leaf (see the earlier picture above).
If the edges are not perfectly smooth and they are not lobed, then consider if the leaf margin is toothed.
Terminology exists to describe toothed margins, some of the most common are: dentate, crenate, serrate, biserrate. Suffice it to say that just knowing that a leaf is toothed may not get you very far without some other details. American beech (Fagus grandifolia) is a very common tree in my area that has a toothed leaf.
|American beech, Fagus grandifolia|
Note that you can have a leaf that is both lobed and toothed – see the Mapleleaf viburnum (Viburnum acerifolium picture below). For identification purposes, consider the pattern of the lobes first when researching it.
Of course the basic shape of the leaf is also very important. Again terminology exists to help define the shapes: lanceolate, oblong, obovate, ovate, oval, cordate are some of the most familiar ones, and you might have to consider more than one term - one tree that I was researching had the leaves described as "oblanceloate or oblong or obovate or oval". But even with the shape determined, leaves can be similar, so you might also want to look at the base of the leaf (where it joins the petiole, or what many people consider the stem). In certain areas where species overlap, two types of native deciduous magnolia can be quickly distinguished by that spot alone: Magnolia tripetala has an attenuate base while Magnolia macrophylla has an auriculate base. Here are pictures of each from my own yard:
I consider leaf shape and leaf arrangement (alternate vs. opposite) to be the basics of identification for most people. From there you can take it further by examining the leaf for hairs, stipules and other characteristics. Such details may be necessary to distinguish one species from another, but hopefully you can use the information here to at least get you to the right genus!
A good reference book for plant terminology is "Plant Identification Terminology - An Illustrated Glossary" by James G. Harris and Melinda Woolf Harris.