3.4.1: External Structure of Leaves

Learning Objectives

• Identify the main parts of a leaf.
• Compare petiolate and sessile leaves.
• Distinguish among alternate, opposite, and whorled phyllotaxes.
• Compare simple, pinnately compound, and palmately compound leaves.
• Compare parallel, pinnate, and palmate venation in leaves.
• Recognize common leaf margins and shapes.

Each leaf typically has a flat, wide portion called the blade (lamina), which is also the widest part of the leaf (Figure \(\PageIndex{1}\)). Some leaves are attached to the plant stem by a stalklike petiole and are called petiolate leaves (Figure \(\PageIndex{2}\)). Although petioles are narrow and often resemble stems, they are considered part of the leaf. A petiolate leaf thus consists of the blade and the petiole. Petioles usually attach at to the margin (edge) of the blade along the base, but in peltate leaves, the petiole is attached underneath the blade (Figure \(\PageIndex{3}\)). Leaves that do not have a petiole and are directly attached to the plant stem are called sessile (apetiolate) leaves (Figure \(\PageIndex{4}\)). In a special type of sessile leaves called perfoliate leaves, the stem passes through the center of the blade (Figure \(\PageIndex{4}\)). Many leaves have a midrib, which travels the length along the center of the leaf. The midrib contains the main vein (primary vein) of the leaf as well as supportive ground tissue (collenchyma or sclerenchyma).

Figure \(\PageIndex{1}\): A typical eudicot leaf. Many leaves consist of a stalk-like petiole and a wide, flat blade (lamina). The midrib extends from the petiole to the leaf tip and contains the main vein. Additional veins branch from the midvein. The margin is the edge of the leaf.

Figure \(\PageIndex{2}\): The petiolate leaves of the geranium consist of a petiole and blade (lamina). The wide lamina is attached to the stalk-like petiole. In the sessile leaves of Aster amellus, the blade is attached directly to the stem. Left image by Melissa Ha (CC-BY), and right image by Enrico Blasutto (CC-BY-SA).

Figure \(\PageIndex{3}\): In the peltate leaves of nasturtium, petioles attach underneath the leaf blade, similar to an umbrella. Image by Melissa Ha (CC-BY).

Figure \(\PageIndex{4}\): The stem runs through the center of the blade in the perfoliate leaves of miner's lettuce. Image by Wikimediaimages/Pixabay (Pixabay license).

Small, green appendages usually found at the base of the petiole are known as stipules (Figure \(\PageIndex{5}\)). Other structures located near leaf base sheath (typical for grasses, lilies, and related species) and ocrea (typical for buckwheat family, Polygonaceae).

Figure \(\PageIndex{5}\): Left: In Hibiscus, two small, narrow, green stipules emerge at the base of each petiole. Middle: This grass blade (lamina) is attached to a sheath, which wraps around the stem. Right: The ocrea of Persicaria maculosa from the buckwheat family is constructed of fused stipules. Left image by Nadiatalent (CC-BY-SA), middle image by Harry Rose (CC-BY), and right image by Martin Olsson (CC-BY-SA).

Phyllotaxy

The arrangement of leaves on a stem is known as phyllotaxy. The number and placement of a plant’s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves arrangements are classified as either alternate, opposite, or whorled (Figure \(\PageIndex{6}\)). Plants that have only one leaf per node have leaves that are said to be alternate, and alternate leaves often spiral up the stem. In an opposite leaf arrangement, two leaves arise at the node (with the leaves usually connecting opposite each other along the branch). Pairs of opposite leaves may face all in the same direction, or each pair can rotate at 90\(^\circ\) (decussate). If there are three or more leaves connected at a node, the leaf arrangement is classified as whorled, and each whorl can also rotate. Each type of spiral phyllotaxis has its own angle of divergence.

Figure \(\PageIndex{6}\): Types of phyllotaxis (leaf arrangement): (a) alternate— with only one leaf attaching to each node, (b) and (c) opposite— with two leaves attaching at each node, and (d) whorled— with more than two leaves attached at each node. While both (b) and (c) illustrate opposite arrangements, leaf pairs are perpendicular to each other in (b) and aligned in (c).

Simple and Compound Leaves

Leaves may be simple or compound (Figure \(\PageIndex{7}\)). In a simple leaf, the blade is either completely undivided (Figures \(\PageIndex{8, 12, 13}\))—or it has lobes, but the separation does not reach the midrib, as in the maple leaf. In a compound leaf, the leaf blade is completely divided, forming leaflets, as in the locust tree. Each leaflet may have its own stalk but is attached to the rachis. A palmately compound leaf resembles the palm of a hand, with leaflets radiating outwards from one point (Figure \(\PageIndex{8}\)). Examples include the leaves of poison ivy, the buckeye tree, or the familiar houseplant Schefflera sp. (common name “umbrella plant”). Pinnately compound leaves take their name from their feather-like appearance; the leaflets are arranged along the midrib, as in rose leaves (Rosa sp.), or the leaves of hickory, pecan, ash, or walnut trees (Figure \(\PageIndex{8}\)). Trifoliate leaves, such as in clover or strawberry, have only three leaflets (Figure \(\PageIndex{9}\))

Figure \(\PageIndex{7}\): Compound leaves (left) and simple leaves (right). While these plants appear similar, each true leaf has an axillary bud above. On the left, the axillary bud is above each compound leaf (but not each leaflet). On the right, each leaf has a bud above it, identifying them as leaves rather than leaflets. Leaflets are always on the same plane with in a leaf (such that the leaf is flat), but this is not the case for multiple simple leaves on the same branch.

Figure \(\PageIndex{8}\): Leaves may be simple or compound. In simple leaves, the blade is continuous. The (a) banana plant (Musa sp.) has simple leaves. In compound leaves, the blade 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 (rachis), 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)

Figure \(\PageIndex{9}\): The trifoliate leaves of strawberry each consist of three leaflets. Each leaflet has a dentate margin, consisting of symmetrical teeth. Image by Melissa Ha (CC-BY).

Simple leaves have just one level of hierarchy whereas compound leaves have two or more levels of hierarchy (Figure \(\PageIndex{10}\)). Pinnately compound leaves with two levels of hierarchy are unipinnate. Those with three levels of hierarchy are called bipinnate (twice pinnate; doubly compound), and those with four levels of hierarchy are tripinnate. For compound leaves, a botanist could describe characteristics of the leaf overall as well as the shape, margin, and venation (see below) of the leaflets.

Figure \(\PageIndex{10}\): Leaves with one (simple), two (unipinnate), and three (bipinnate) levels of hierarchy. The blade of the simple leaf is continuous. In the unipinnate leaf, the blade is divided into leaflets, which attach on either side of the midrib (rachis). In the bipinnate leaf, each leaflet is further subdivided. The dotted lines indicate the shape of each level, which is not necessarily the same for each level.

Venation

Leaf veins are vascular bundles coming to the leaf from stem. The arrangement of veins in a leaf is called the venation pattern. Frequently, there is one or more main vein (primary vein) and secondary veins that branch from it. Tertiary veins branch from secondary veins (Figure \(\PageIndex{11}\)).

Figure \(\PageIndex{11}\): The main (primary) vein of this leaf is the largest, central vein. Secondary veins emerge from the primary vein. Tertiary veins (at arrows) emerge from the secondary veins. Image modified from freesvg (public domain).

There are three main arrangements of the most prominent (major) of the leaf. They may all run longitudinally (parallel venation, Figure \(\PageIndex{12}\)), they may branch from a single midvein (pinnate venation, Figure \(\PageIndex{8a, 12}\)), or they may arise from a single point at the base of the leaf (palmate venation; Figure \(\PageIndex{13}\)). Parallel venation is found in monocots, and pinnate and palmate venation is common in eudicots. The maidenhair tree, ginkgo (Ginkgo biloba), has a unique venation pattern in which each vein divides into two similar parts. This is known as dichotomous venation (Figure \(\PageIndex{12}\)).

Figure \(\PageIndex{12}\): (a) Tulip (Tulipa), a monocot, has leaves with parallel venation, with veins running up the leaves in the same direction. They do not intersect. (b) Linden (Tilia cordata) is a eudicot with pinnate and reticulate venation. Veins radiate out from the midrib, and smaller veins radiate out from these.The (c) Ginkgo biloba tree has dichotomous venation with veins radiating out from the petiole. Each of these veins branches into two veins. (credit a photo: modification of work by “Drewboy64”/Wikimedia Commons; credit b photo: modification of work by Roger Griffith; credit c photo: modification of work by "geishaboy500"/Flickr; credit abc illustrations: modification of work by Agnieszka Kwiecień)

Figure \(\PageIndex{13}\): In leaves with palmate venation, multiple main veins branch from a central point, which is near the base of the leaf, similarly to how the fingers radiate from the palm of a hand. Left image cropped from Maksim (CC-BY-SA), and right image by Maksim (CC-BY-SA).

The less prominent (minor) veins of a leaf may form a branching, netlike pattern, which is called reticulate venation (Figure \(\PageIndex{14}\)). In contrast, the minor veins are arranged neatly, forming a ladderlike structure in percurrent venation.

Figure \(\PageIndex{14}\): Minor veins may demonstrate reticulate venation (left) or percurrent (right). Left image cropped from Katifori E, Magnasco MO (2012) Quantifying Loopy Network Architectures. PLoS ONE 7(6): e37994 (CC-BY), and right image by RoRo (public domain).

Margins

The margin describes the outline of a simple leaf or leaflet (Figure \(\PageIndex{15}\)). Leaves with smooth margins are called entire. Those with irregularly wavy margins are undulate. Lobate (lobed) leaves may be palmately lobate (with lobes outlining the palmate venation pattern of veins arising from a single point at the base of the leaf) or pinnately lobate (with lobes outlining the pinnate venation pattern of secondary veins branching from a midvein; Figure \(\PageIndex{16}\)). Toothed leaves have small projections (teeth) and indentations that do not align with the venation. For serrate leaves, the sharp teeth point forward, like a serrated knife. For dentate leaves, sharp teeth are symmetrical, like equilateral triangles. For crennate leaves, the teeth are rounded rather than sharp.

Figure \(\PageIndex{15}\): Examples of leaf margins. Ciliate margins have fine hairs. Crenate margins have rounded teeth, dentate margins have symmetrical teeth, and denticulate margins have fine dentition (small teeth). Serrate margins have forward-facing teeth, like a serrated knife. Doubly serrate margins are serrate with sub-teeth. Entire margins are smooth. Lobate margins are indented, but not to the midline. Serrulate margins have fine serration. Sinuate margins have wave-like indentations. Spiny margins have sharp, stiff points, and undulate leaf margins are widely, irregularly wavy. Image cropped from McSush/Debivort (CC-BY-SA).

Figure \(\PageIndex{16}\): Lobed leaves have indentations that match the venation pattern. Oak leaves (left) are pinnately lobate. The leaves on the right are palmately lobate. Images by Melissa Ha (CC-BY).

Shape

There are several general leaf shapes (Figure \(\PageIndex{17}\)). The widest part of ovate leaves is near the based whereas the widest portion of obovate leaves is near the apex (tip). If the widest portion of the leaves is equidistant form the apex and base, the leaf may be elliptic or oblong. In elliptic leaves, a single wide middle portion tapers as it approaches the apex or base. Oblong leaves are similarly, but there is a longer section in the middle of these leaves that is uniformly wide. Linear leaves are long and thin. Lanceolate leaves are lance-shaped and somewhat intermediate between ovate and linear.

Figure \(\PageIndex{17}\): Six main leaf shapes. Ovate leaves are egg-shaped, but the base is the widest portion. Obovate are egg-shaped, but the base is the narrowest portion. Elliptic leaves have a wide center and taper at the base and tip. Oblong leaves are elongated. Linear leaves are long, and lanceolate leaves are lance-shaped. Image modified from McSush/Debivort (CC-BY-SA).

The leaf shape can be described in further detail using specific terminology that applies to the leaf apex (Figure \(\PageIndex{18}\)) and leaf base (Figure \(\PageIndex{19}\)). The leaf apex could be rounded, acute (forming an angle less than 90º), obtuse (forming an angle greater than 90º), attenuate (tapering to a point), or acuminate (dramatically curving inward to a narrow point). The base of the leaf blade could be rounded, acute, obtuse, truncate (straight), cuneate (wedge-shaped or triangular), or cordate (indented in the center, like an upside-down heart).

Figure \(\PageIndex{18}\): Five leaf apex shapes. The leaf apex could be rounded, acute (forming an angle less than 90º), obtuse (forming an angle greater than 90º), attenuate (tapering to a point), or acuminate (dramatically curving inward to a narrow point). Image modified by Melissa Ha (CC-BY) from Shipunov (public domain).

Figure \(\PageIndex{19}\): Six leaf base shapes. The base of the leaf blade could be rounded, acute (forming an angle less than 90º), obtuse (forming an angle greater than 90º), truncate (straight), cuneate (wedge-shaped or triangular), or cordate (indented in the center, like an upside-down heart). Image modified by Melissa Ha (CC-BY) from Shipunov (public domain).

Hetrophylly

Heterophylly refers to a plant having more than one kind of leaf. A plant can have both juvenile leaves and adult leaves, water leaves and air leaves, or sun leaves and shade leaves. In California live oak, leaves growing closely to the ground have sharp teeth, presumably to deter herbivores such as deer. Leaves at the top of the plant are too high for herbivores to reach and have entire margins. California sycamore has both palmately lobed leaves and small, perfoliate leaves (Figure \(\PageIndex{20}\)).

Figure \(\PageIndex{20}\): The California sycamore exhibits heterophylly because it has two different leaf types. The stem pierces a small, circular perfoliate leaf in the center of the image. The other larger leaves are palmately lobate. Image by Melissa Ha (CC-BY).

Attributions

Curated and authored by Melissa Ha using the following sources:

• 30.4 Leaves from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 5.3 The Leaf from Introduction to Botany by Alexey Shipunov (public domain).
• 9.2 Introduction to Leaf Anatomy from Botany Lab Manual by Maria Morrow (licensed CC-BY)