Types of Leaves

Leaves give us many clues to the identification of trees. Follow the steps carefully for observing leaves and try and identify the trees in your neighbourhood. But first here are a few basic observations to help you identify the different parts of a leaf.

Every leaf grows from a bud in the spring. It is formed from a very thin, often green, skin called a blade. The blade may have one or more leaflets depending on the type of leaf. The leaf may be supported by a small stem called a leafstalk. When there is no stalk, the leaf is said to be sessile. A network of veins running through the blade is easily seen.

You will notice that many trees shed their leaves in the autumn. These leaves are called deciduous leaves. Almost all broadleaf trees have deciduous leaves. However, some trees retain the dead leaves attached to their branches during winter. These are called marcescent leaves. Oak leaves are often marcescent. In winter, try to find an oak by looking for dead leaves attached to the branches.

When observing leaves, it is preferable to gather twigs, because many characteristics are influenced by the arrangement of the leaves on the twigs.

The first step is to determine the type of leaf you are observing.

If the blade consists of one leaflet, it is a simple leaf. However, if the blade is made up of several leaflets attached to a leafstalk, the specimen is a compound leaf. Take a close look at the illustrations to distinguish between a simple leaf and a compound leaf.

The Arrangement of Leaves

The arrangement of leaves on twigs is an essential clue to the identification of trees. A number of tree species can be identified by this using this observation criterion.

Leaves may occur in pairs at one position on opposite sides of the stem. These are called opposite leaves. The red maple has leaves in opposite pairs. Try to find other species with opposite leaves.

When leaves occur at different positions on the stem, they are called alternate leaves. Look at a twig from an American elm to see whether the leaves are alternate.

If the leaves are arranged in star-like fashion all around the stem at the same position, they are called whorled leaves. However, whorled leaves are found far more frequently on plants than on trees.

The Contour of Leaves

The shape, or more precisely, the contour of a leaf is extremely useful for identifying a tree. So make sure you pay special attention to the contours of leaves. The description of the four different types of edges will help you classify leaves according to their contours.

Smooth-edge leaves as the name suggests have a smooth contour with no particular projections on the edges. Willow leaves are a perfect example of a smooth-edge leaf.

Lobed leaves have a contour that is divided by empty spaces called a sinuses that separate two lobes. The sinuses and lobes may be either rounded or pointed. The oak leaf is an example of a lobed leaf. To see the difference between a lobed leaf with rounded sinuses and lobes, and another with pointed sinuses and lobes, compare a bur oak leaf with a red oak leaf. Which of the two has pointed lobes?

Leaves with pointed teeth of the same size are called toothed-edge leaves. Take a look at a Balsam poplar; it has leaves with toothed edges.

Some leaves have two sizes of teeth on their contour, large teeth which themselves contain smaller pointed teeth. These are called double-toothed leaves. You’ll see the small teeth more clearly with a magnifying glass. Why not practise by observing a white birch leaf.

The Arrangement of Veins

Veins in leaves are the equivalent of the blood system in the human body. This network of vessels runs through the leaf and supports the blade. Veins can change the direction of the leaf to enhance photosynthesis. The arrangement of veins is a useful clue to tree identification.

When the leaf has a network of main veins on either side of the midvein, the veins are said to be arranged pinnately. The midvein is the vein that runs down the centre of the leaf and the main veins are either attached to the midvein or start at the base of the leaf. Find a beech leaf or a willow leaf to observe veins that are pinnately arranged.

Veins that are arranged palmately start at the base of the blade and are laid out in the shape of a hand. The best example of palmately arranged veins is the maple leaf.

Leaves with parallel veins have veins running side by side the whole length of the blade. Such an arrangement is found more often in plants than in trees.