Leaves are an essential part of every tree species. They allow the tree to survive by being a significant factor in photosynthesis. Not only are they functional, but leaves are also essential to a tree’s aesthetic. The leaf of a tree gives it a lot of its shape and texture.
Using leaves, you can often identify the genus and even the tree species. Leaves themselves don’t technically have scientific classifications since they are only a part of specific species. However, there are categories and ways to identify types of leaves.
This article will look through the various types of leaves, their categories, and the typical tree groups they characterize. Then, we break down the parts of a leaf and how they can dramatically affect the shape of the leaf. By the end, you will be a leaf expert, ready to go out and identify bushes, trees, and everything in between.
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Three Primary Types of Leaves
There are billions of leaves globally and thousands of different expressions and shapes. However, these large numbers are deceiving since there are only three primary types of leaves. Every type, shape, and category falls into one of these two main types. The major typologies are needle-like single leaves and compound leaves.
1. Needle-Like Leaves
Coniferous trees have needle-like leaves. These are relatively self-explanatory. They are leaves that are somewhat pointed or scaled. They usually have a waxy layer over the top of them, which helps them stay attached to the tree and insulate it during the winter.
The size of the needles on trees can vary quite a bit. Some trees, such as the ponderosa pine, have needles up to 10 inches (25.4 cm) long. On the other hand, the foxtail pine has the shortest needles on average. Their needles measure about 1 inch (2.5 cm) long.
Some other examples of needle-like leaves include cedars, larch, spruces, and pine.
In simple terms, a simple leaf is one where the leaf blade is undivided. The margins of the leaf may have teeth. They can also have multiple lobes. However, they have to have a single prominent vein or midrib running up the leaf to be a simple leaf.
Some well-known species with simple leaves include oak, mango, cherry, rose, and maple. If you look at each one of these, the leaves are significantly different, yet they all fall under the category of a simple leaf.
Compound leaves are the opposite of simple and encompass the rest of the deciduous leaf types. A compound leaf has distinctive parts that separate it from the midrib. They are all joined along a single stem, but they are all separated. There are two subcategories within the category of compound leaves. This helps make species identification easier since there are many of these leaves.
Pinnately Compound Leaf Type
A pinnately compound leaf is one whose leaflets are arranged on either side of the leaf’s axis or central stalk. They can even be bipinnately compound if they split off the central stalk twice.
Species with pinnately compound leaves common to North America include hickory, box elder, and walnut. An example of a tree with bipinnate leaves is the honey locust.
Palmately Compound Leaf Type
A palmately compound leaf has leaflets as well. However, these leaflets radiate or whorl around the petiole, which comes from the branch. The part of the name ‘palmate’ gives you a visual idea about the shape of these leaves. They look similar to the way our fingers do when spread out around our palms.
Common species with palmately compound leaves include the Ohio buckeye, Schefflera, poison ivy. However, palmately compound leaves are not as common as pinnately compound or simple leaves. That means if you see a tree with palmately compound leaves, your species list is quite a bit smaller.
The first step to identify what kind of tree you have is to figure out if it has needle-like leaves or not. Conifers will always have needle-like leaves. Sometimes these behave differently, but they still signify a conifer instead of a deciduous tree. For example, some needles will fall off seasonally instead of staying attached to the tree.
Typically, the two classifications are conifer or hardwood. However, this name can be deceiving since not all conifers produce softwood. Therefore, it is better to divide the two classifications into a coniferous group and a deciduous group. Even this classification can still have issues, but it is still the most widely understood.
Other than these three main leaf types, there are many subcategories. Identifying any plant species becomes much easier if you can figure out precisely what kind of leaf shape it has. We list each of these and describe them to understand them and some tree species they represent.
1. Linear Leaves
A linear leaf is the most basic version of a simple leaf. It doesn’t have any divisions. Instead, it is long and slender. Typically, both sides of the leaf stay close to the leaf’s midrib and end at a point.
The most prevalent example of a plant with linear leaves is grass. However, if you are looking for a flowering example, cornflowers often have linear leaves for their upper leaves.
An ovate leaf’s shape is much wider at the base on either side of the midrib. As it gets farther away from the branch, it tapers towards the apex of the leaf. A genuine ovate leaf will have a length-to-width ratio that ranges from 1.5:1 to less than 2:1.
Species with ovate leaves include blue beech, elm trees, and dogwood. Keep in mind the margins might make it difficult to determine the exact width of the leaf, such as in the case of holly.
An elliptical leaf is shaped like an ellipse or a flattened oval shape. Because of its formation, the sides won’t be parallel. As a result, it typically looks relatively flat on the top and the bottom of the leaf. Some will have rounded tops, and others will come to a slight point.
Elliptical leaves have shapes that are easily confused with other shapes, such as oblong leaves. That makes them more difficult to associate with a specific kind of tree. One example includes the Blackgum tree.
An oblong leaf is a leaf that has two rounded ends with sides that are parallel. Another name that some call this is ‘oval.’ Others believe that the shape is almost too rectangular to be an oval. Another identifier is that the sides are considerably longer than the ends.
There are quite a few trees that have oval or oblong leaves. Some of these are debatable but remember that only taking an example of one leaf isn’t enough to base their categorical shape on. These trees include boxwood, citrus trees, and some apple species.
A cordate-shaped leaf takes on a heart shape. The name of this type of leaf doesn’t describe its overall shape as much as it describes its basal shape. A cordate leaf will have deep indents where the leaf meets the midrib, making it look like the top of a heart.
Species with cordate leaves include the Eastern Redbud, one of the classic examples of a cordate leaf. The Littleleaf Linden and Katsuratree are also examples of trees that have cordate leaves.
Lanceolate leaves are typically quite long leaves. There most defining factor is that they have a lance-like head. The leaf starts very thin at its pedicel and then widens until it tapers to a top point. These leaves fall under simple leaves for the most part.
Lanceolate leaves are most often associated with willows. Most willow species have long, thin leaves that end in a lanced point.
Acicular leaf is the technical term for a needle-like leaf. Next time you want to impress your friends with your expansive vocabulary, you can pull that word out. Tell them to watch out for the shape ‘acicular leaves’ instead of just the boring needles.
Many conifers have acicular leaves. The subcategories that fall under that include pine trees, cedars, and firs.
A reniform leaf isn’t often seen on full-grown tree species. These leaves are similar in shape to the cordate leaves. Reniform leaves are deeper in their overall shape than cordate leaves, though. They look more like a kidney shape than a heart, with a more rounded edge.
A tree with reniform leaves is the Carolina Basswood, although the Eastern Redbud can have reniform leaves instead of cordate leaves.
You can glean the general shape of this leaf from its name, ORB-icular. These leaves are circular or very close to being circular with no divisions. These leaves should have a rounded shape, just like an orb is round.
Orbicular leaves are quite distinctive in their plants since they are not a traditional leaf shape. Examples of plants with an orbicular leaf shape include the Cranberry Cotoneaster, Quaking Aspen, and the Purple Common Tree. In addition, certain species of lilacs also have orbicular leaf shapes.
A sagittate leaf shape is another unique example of leaves. The sagittate leaves are those that look like arrowheads. They have a sharp triangular pattern based around their pedicel. These are not a leaf you will see on trees, with the possible exception of some exotic species. Instead, certain vining and aquatic plants are the only ones with genuinely sagittate leaves.
One example of a plant with this kind of leaf is the Sagittaria longiloba. As its Latin name would suggest, this aquatic plant has leaves shaped like an arrowhead.
This name is also suggestive of the shape of the leaf. If you are familiar with musical instruments, the lyre is from Ancient Greece. It is a stringed instrument similar to a harp. However, it was small enough to sit in the player’s lap and took a curved U-shape.
Leaves that have a lyrate shape have a shape similar to the lyre, with an immediately broad base that extends upwards and gets slightly wider towards the top. Some plants with a lyrate leaf include the Chocolate Flower, Ficus, and Overcup Oak.
Spatulate leaves are similar to lanceolate leaves. They are often quite long, start thin at the end that attaches to the branch and widen until the tip of the leaf. The difference is that instead of an end shaped like a lance, these have a broader spatula-esque shape.
They are easy to confuse since it all has to do with exactly how broad the top is compared to the bottom length of the leaf. A tree that has a spatulate leaf is a water oak.
A leaf with an oblique shape can also be combined with various other leaf types. The name “oblique” only has to do with the base of the leaf instead of its overall shape.
We are most familiar with the term ‘oblique’ in geometry, associated with a triangle without any equal sides. In a leaf, oblique means that the base of the leaf is unequal. The base will feature one side that is larger, wider, or rounder than the other.
Although there will rarely be a leaf with perfectly matching basal sides, oblique refers to those on a plant that is consistently mismatched.
Cuneate again references the base of the leaf more than the overall shape of the entire leaf. These leaves have a wedge shape at the bottom with an acute angle for their base.
Examples of leaves with a cuneate leaf include a Star Magnolia, Gray Dogwood, and White Oak.
A leaf might seem like a simple structure. However, each leaf has specific parts that function together to give it its full identity. Each of these sets leaf patterns apart from other categories. The main four parts in leaf anatomy include the apex, margin, surface, and base. We will break each one of these apart and their identifying categories.
The apex of a leaf is the top of the leaf that typically protrudes slightly. This area is typically where water droplets accumulate and then drain off from the leaf. It is necessary for water direction since water left to sit on a leaf can cause rot.
There are nine primary types of leaf apexes. Some botanists separate these even further, but we will stick to the primary categories. These include:
The margin of a leaf is its edge. The type of margin describes the shape and texture of the leaf’s edge.
- Entire: A smooth edge.
- Dentate: A toothed edge.
- Double dentate: Smaller teeth stacked on themselves.
- Lobate: Lobed edges or more prominent rounded edges.
- Serrate: Similar to a toothed edge, but with rounded indentations into the leaf’s surface.
- Crenate: A rounded, toothed edge to the leaf.
- Denticulate: A toothlike projection.
- Ciliate: A fringe on the leaf margin
- Serrulate: A leaf with margins notched like a saw blade
- Sinuate: Shallow, wavy margins.
- Spiny: Modified leafs that protect the plant from herbivors.
- Undulate: Waves that add a three-dimensional margin
The leaf’s surface is the most visually impactful but generally the least helpful in the leaf’s overall identification. The appearance of the leaf surface can give you indications of the plant’s health and life stage.
The primary identifying factors on a leaf’s surface are its venation and texture. Typically, a leaf’s texture will be smooth, fuzzy, or rough. The pattern of its veins can give you some indication of the type of leaf. For example, parallel venation typically belongs to bulbous plants such as tulips.
We described some of the bases of the leaves in the list of leaf shapes above, such as truncate, acute, cuneate, and sagittate. How the leaf attaches around the midrib indicates its overall shape and the type of plant it represents.
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How Leaves Function
Leaves primarily function to produce energy for the plant by means of photosynthesis. The greenness of a leaf is determined by the amount of chlorophyll it has. Chlorophyll is a substance that can allow the leaves to absorb energy from light. It changes UV rays into usable energy, much like our bodies change calories from food into usable energy.
Other than photosynthesis, there are four other essential functions of a leaf. One of the other highly important ones is transpiration. Transpiration is the release of water and oxygen from the tree. It is effectively how the plant can control its temperature and breathe out unusable gases.
Transpiration is crucial not only for the continuation of the plant’s life cycle but also for ours. Plants release oxygen, which is what we need to breathe. We then aid in their life cycle by releasing carbon dioxide. This exchange is the basis for why both animals and plants need each other for a functioning ecosystem.
You know that guttation happens when you see small drops of liquid appear on the leaves of a vascular plant. This is typically xylem sap and occurs typically during the night. This is an evolutionary development to help plants deal with the influx of too much water.
Plants also function as storage organs for the tree, containing essential water, energy, and nutrients until the plant needs to use it.
Finally, leaves are useful as a defense mechanism for a plant. They act as a sort of first wall against an oncoming attack. Their other purpose as a storehouse makes them somewhat expendable. A tree can afford to lose about 40% of its leaves before it starts to take on more permanent damage.
Leaves of all kinds, whether deciduous or evergreen, start as a bud. These are often the sign of spring since you can so clearly see them when the tree is otherwise bare. However, most trees continue to grow and lose leaves throughout the growing season.
Leaf buds for the new year initially grow and develop during the summer months. Many of them will be protected by bud scales, which are generally a richer brown than the plant’s main trunk.
Once spring arrives, the buds are triggered by extended hours of light and increased temperatures. The bud scales will off, and then the leaves will begin to open up and grow. When they unfurl and reach their full size, they will begin photosynthesizing.
They will continue to be helpful for the tree until the fall. Once the sunlight hours decrease, the tree will provide them with less and less chlorophyll until they take on their fall colors. The plant is shutting down in this state since leaves lacking chlorophyll cannot photosynthesize.
Eventually, the tree gets triggered to cut the pedicel of the leaf off from the main tree, literally. A thin film forms over the spot where the leaf was attached. Once it is complete, the leaf falls, and the tree has safely protected itself from pests or diseases that would try to infiltrate an open wound.
There are certain plants with enormous leaf structures. Since leaves are very important for photosynthesis, it is sometimes necessary for large plants to produce large leaves. Large leaves can result from a ground cover plant growing larger to maximize its chance to interact with sunlight.
The Raphia regalis is simply a very large plant. It is a palm use leaves that start at the tree’s base and gradually grow upward with it. That means the leaves grow to be as tall as the tree itself. The largest leaf ever measured on one of these palms was 82.3 feet (25.11 meters) long and 9.8 feet (3 meters) wide.
These plants have the smallest leaves that are technically still leaves (note that mosses and liverworts are not technically plants with real leaves.) The Wolffia genus has around ten plants that include the smallest flowering plants in the world. They are often called duckweed and are aquatic plants that look like flecks of cornmeal on the water.