13.1: Leaf Parts and Arrangement

In angiosperm anatomy, a leaf can be identified by where it emerges from the node. In a node, a leaf emerges below the axillary bud.

Figure \(\PageIndex{1}\): A diagram of a node. Each node is comprised of a leaf (on the bottom) and an axillary bud (on the top, located in the axil where the petiole meets the stem). Diagram by Nikki Harris, CC-BY 4.0 with labels added.

Leaf Parts

Leaves are generally composed of a few main parts: the blade and the petiole.

Figure \(\PageIndex{2}\): A leaf is usually composed of a blade and a petiole. The blade is most frequently the flat, photosynthetic part. The petiole is a stem that attaches the leaf blade to the main stem of the plant. As plants have radiated, diversified, and adapted to different environments, you'll see that there are many variations on this theme. The photo on the left is a palmate leaf, the diagram on the right is a pinnate leaf. Photo by Maria Morrow, CC-BY 4.0. Diagram on the right from Chapter 3.4.2, Botany, by Algiers, Ha, and Morrow.

The Leaf Blade and Vascular Arrangement

The leaf blade is (usually) the flat, photosynthetic part of the blade. In eudicots, the leaf will have a central midvein (also called the midrib), with smaller veins branching off from there. This type of vein organization is called netted venation. The edge of the blade is the margin.

Figure \(\PageIndex{3}\): The adaxial (left) and abaxial (right) surfaces of a eudicot leaf blade. The margin is the edge of the leaf and its morphology can be useful for identification. This leaf margin could be described as serrate, much like a serrated knife. In the center of the leaf, a large central vein stands out. This is the midrib or midvein. Many smaller diameter veins branch out from the midvein. Photos by Maria Morrow, CC-BY 4.0.

In monocots, the veins may be all approximately the same size or there may be a larger midvein, but the veins run parallel to each other. This is called parallel venation.

Figure \(\PageIndex{4}\): A plantain (Plantago sp.) leaf. This monocot has several large veins that run parallel to each other, lacking any true midvein. Photo by Maria Morrow, CC-BY 4.0.

The Petiole

Most leaves have a stem that attaches the blade of the leaf to the rest of the plant. This is the petiole. However, in some plants, the leaves do not have a petiole and the blade is directly attached to the plant stem. These leaves are sessile (lacking a petiole).

Figure \(\PageIndex{5}\): The first image shows a branch of a tree with petiolate leaves. Each leaf blade is attached to the main branch by a short stem (petiole). In the second image, the leaf blades are attached directly to the main stem (sessile). Photos by Maria Morrow, CC-BY 4.0.

Stipules

Some plants will have paired appendages found at the base of the leaf. These are called stipules. Stipules can look leaf-like or take on other forms (e.g. spines or tendrils).

Figure \(\PageIndex{6}\): From left to right: sheath, stipules and ocrea. Diagram from Chapter 3.4.2, Botany, by Algiers, Ha, and Morrow.

Figure \(\PageIndex{7}\): Stipules of Hibiscus. Each leaf on this plant has two long, flat, green projections at the base. These are the stipules, adapted for photosynthesis (flat and green). Image by Nadiatalent, CC BY-SA 3.0, via Wikimedia Commons.

Figure \(\PageIndex{8}\): A variety of stipule morphologies. First image: Ton Rulkens from Mozambique, CC BY-SA, via Wikimedia Commons. Second image: Christian Fischer, CC BY-SA, via Wikimedia Commons. Third image: Frank Vincentz, CC BY-SA, via Wikimedia Commons. Fourth image: Filo gèn', CC BY-SA, via Wikimedia Commons.

Compound Leaves

A compound leaf looks like a branch with leaves emerging from it. However, these leaves are not accompanied by axillary buds (i.e. they do not emerge from a node). This branch is in fact a compound leaf composed of leaflets. Though there are no buds at the base of the leaflets, there will be a bud at the base of the compound leaf's petiole.

Figure \(\PageIndex{9}\): Compound leaves. The leaf on the left is pinnately compound, petiolules attach to a central rachis. The leaf on the right is palmately compound, petiolules all meet at the tip of the petiole and radiate outward. Diagrams by Michael G. Simpson. Redrawn and color: User:RoRo, Public domain, via Wikimedia Commons.

A compound leaf can be palmate or pinnate. In a pinnate compound leaf, the leaflets are attached to a central stem called a rachis. The leaflets can be sessile or may be attached to the rachis by petiolules. Compound leaves can be even more complex by being compounded multiple times.

Figure \(\PageIndex{10}\): Leaves with one, two and three levels of hierarchy. Please note that the last leaf is ovate on the first and second level but circular on the third level of hierarchy. Diagram from Chapter 3.4.2, Botany, by Algiers, Ha, and Morrow.

Figure \(\PageIndex{11}\): Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant (Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut (Aesculus hippocastanum), the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (Carya floridana). The (d) honey locust has double compound leaves, in which leaflets branch from the veins. (credit a: modification of work by "BazzaDaRambler"/Flickr; credit b: modification of work by Roberto Verzo; credit c: modification of work by Eric Dion; credit d: modification of work by Valerie Lykes).

Leaf Arrangement

In angiosperms, leaf arrangement is determined by how many leaves emerge per node.

Figure \(\PageIndex{12}\): Types of leaf arrangement. In shoot A (far left), the leaves emerge one at a time. Shoot A has alternate leaf arrangement. In shoot B (middle), two leaves emerge per node, directly across from each other. Shoot B has opposite leaf arrangement. In shoot C, five leaves emerge at each node. Three or more leaves from a node is a whorled leaf arrangement. Diagram by Nikki Harris, CC-BY 4.0 with labels added.

Alternate Leaf Arrangement

In alternate leaf arrangement, one leaf emerges per node, giving the appearance of alternating leaves.

Figure \(\PageIndex{13}\): This salal (Gaultheria shallon) branch has alternate leaf arrangement. The leaves emerge with one per node. In this plant, a small bend occurs in the stem at each node, giving it a zig-zag shape. Photo by Maria Morrow, CC-BY 4.0.

Opposite Leaf Arrangement

In opposite leaf arrangement, two leaves emerge per node. Think of this as how your arms are opposite each other on your torso.

Figure \(\PageIndex{14}\): Both of the plants shown in the images above have opposite leaves, with two leaves emerging at each node opposite from each other on the stem. Photos by Maria Morrow, CC-BY 4.0.

Whorled Leaf Arrangement

In whorled leaf arrangement, three or more leaves emerge per node. I try to remember this as "around the whorled (world)", because the leaves often encircle the stem.

Figure \(\PageIndex{15}\): This member of the lily family has whorled leaves. There are between 5 and 8 leaves emerging at each node, making the plant look as if it were a tiered cake. Photo by Maria Morrow, CC-BY 4.0.