One of the first steps in plant identification is determining the leaf shape of the plant species you are working with. Depending on the plant identification key you are using, there may be as many as a dozen different leaf shape types listed. Basic leaf shapes include round, ovate, oblong, deltoid and linear. Before you try to determine the shape of a plant's leaves, you'll need to first understand basic leaf anatomy and a few simple botanical terms.
Basic Leaf Anatomy
Leaves grow in a huge variety of shapes, sizes, textures and colors. Both deciduous and evergreen trees have leaves; in evergreen trees, needles and scales are considered leaves.
In the most simple terms, a leaf blade is the broad, flattened part of a plant. Leaves are attached to stems by petioles, which are like short stalks. The edges of leaves are called the margins.
Plant leaves can be simple or compound. If a plant has only one blade per petiole, it is called a simple leaf. When the blade of a leaf is made up of several smaller leaflets, this is referred to as a compound leaf.
There are three types of leaf arrangements – called phyllotaxy – in plants: alternate, opposite and whorled. Plants that display alternate leaf arrangement have one leaf growing from each node, which is the place where a leaf attaches to the stem. In opposite leaf arrangement, two leaves grow across from each other at a single node. According to The University of Texas at Austin Biodiversity Center, a plant has whorled leaves when three or more leaves grow from a single node.
Different Shapes of Leaves
It can be challenging to determine leaf shape from leaf arrangement, so it's important to know the difference between leaves and leaflets. Leaves attach to petioles, whereas leaflets – which are smaller parts of a leaf blade – attach to midribs. A midrib is the central vein of a leaf.
As you look at a single leaf, take in the overall shape. Is it longer than it is wide? If so, it may be classified as linear or oblong. According to The Seed Site, sword-shaped leaves are long and pointed, while lance-shaped leaves are long and wider at the middle than at the ends.
Check out the illustration provided by the University of Illinois to visualize 17 leaf shape types. These include basic shapes like elliptical and orbicular, along with more unusual shapes such as perfoliate and flabellate. Heart-shaped leaves are referred to as cordate, and many of the leaves of evergreens are called scale-like. Rhomboid leaves are shaped like diamonds, and spear-shaped leaves look exactly like they sound.
Other Leaf Characteristics
After you decide whether a leaf is simple or compound, the leaf arrangement and the shape of a plant's leaves, there are several other characteristics that can help you determine what species you are working with. Leaf edges, or margins, often provide help in identifying plants. From smooth to serrated, and finely serrate to lobed, there are many descriptive terms for leaf margins, as illustrated by the University of Georgia Extension.
Another characteristic of leaves is the way that the veins are arranged. Venation is the botanical term for the patterning of veins in leaves. There are three basic ways that veins are arranged in leaves: parallel, pinnate and palmate. Palmate veins diverge from a central point, while pinnate veins look like a ladder, with smaller veins spaced along a main vein. Parallel veins run alongside each other.
The texture of a plant's leaves can also help you determine its identity. Checking whether a leaf is smooth or rough and thick or thin can often provide clues about the genus and species of a plant.
About the Author
Meg Schader is a freelance writer and copyeditor. She holds a Bachelor of Science in agriculture from Cornell University and a Master of Professional Studies in environmental studies from SUNY College of Environmental Science and Forestry. Along with freelancing, she also runs a small farm with her family in Central New York.