Leaves

Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll.

The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species maximize its chances of survival in a particular habitat. Usually, the leaves of plants growing in tropical rainforests have larger surface areas than those of plants growing in deserts or very cold conditions, which are likely to have a smaller surface area to minimize water loss.

Structure of a Typical Leaf

Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Some leaves are attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant stem are called sessile leaves. Small green appendages usually found at the base of the petiole are known as stipules. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin. The figure below shows the structure of a typical eudicot leaf.

Extended description

The leaf is drawn pointing from upper left (its tip) to lower right (its petiole), with six leader lines in this order: ‘Tip,’ at the upper-left point of the blade; ‘Midrib,’ pointing to the central vein running down the blade’s length; ‘Margin,’ pointing to the blade’s outer edge on the right; ‘Vein,’ pointing to one of the side veins branching from the midrib; ‘Lamina,’ pointing to the broad green blade itself on the left; and ‘Petiole,’ pointing to the narrow tan stalk at the lower right where the blade meets the stem.

Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation pattern. Monocots and dicots differ in their patterns of venation (see the figure below). Monocots have parallel venation; the veins run in straight lines across the length of the leaf without converging at a point. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. One extant plant, the Ginkgo biloba, has dichotomous venation where the veins fork.

Leaf Arrangement

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 are classified as either alternate, spiral, or opposite. Plants that have only one leaf per node have leaves that are said to be either alternate—meaning the leaves alternate on each side of the stem in a flat plane—or spiral, meaning the leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as whorled.

Leaf Form

Leaves may be simple or compound (see the figure below). In a simple leaf, the blade is either completely undivided—as in the banana leaf—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. 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.

Leaf Structure and Function

The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata (see the figure below): openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing.

The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce the rate of transpiration by blocking air flow across the leaf surface (see the figure below).

Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or “middle leaf.” The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma (see the figure below). The palisade parenchyma (also called the palisade mesophyll) has column-shaped, tightly packed cells, and may be present in one, two, or three layers. Below the palisade parenchyma are loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts.

Extended description

Panel (a) is a rectangular cross-section diagram, read top to bottom. A row of large green rectangular cells is labeled ‘Upper epidermis’ on the left; a thin line above them is labeled ‘Cuticle’ on the right. Below that, tall column-shaped green cells packed side by side are bracketed ‘Palisade parenchyma’ on the right; a bracket on the left spanning this layer and the one below it is labeled ‘Mesophyll.’ Below the palisade layer, loosely spaced oval teal cells are bracketed ‘Spongy parenchyma’ on the right. A second row of large green rectangular cells at the bottom is labeled ‘Lower epidermis’ on the left; a thin red line below them is labeled ‘Cuticle’ on the right. A gap between two of the lower-epidermis cells is labeled ‘Stoma,’ flanked by two cells labeled ‘Guard cells’; a red arrow curves down into the gap labeled ‘CO2’ and a second red arrow curves up out of it labeled ‘O2.’ Panel (b), below panel (a), is an unlabeled black-and-white scanning electron micrograph of the same cross-section, showing tall columnar cells at the top over a looser, more porous layer beneath, with a white scale bar reading ‘200 µm’ in the lower right corner.

Like the stem, the leaf contains vascular bundles composed of xylem and phloem (see the figure below). The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.

Leaf Adaptations

Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface area: two attributes that aid in reducing water loss. In hot climates, plants such as cacti have leaves that are reduced to spines, which in combination with their succulent stems, help to conserve water. Many aquatic plants have leaves with wide lamina that can float on the surface of the water, and a thick waxy cuticle on the leaf surface that repels water.

Evolution Connection. Plant Adaptations in Resource-Deficient Environments

Roots, stems, and leaves are structured to ensure that a plant can obtain the required sunlight, water, soil nutrients, and oxygen resources. Some remarkable adaptations have evolved to enable plant species to thrive in less than ideal habitats, where one or more of these resources is in short supply.

In tropical rainforests, light is often scarce, since many trees and plants grow close together and block much of the sunlight from reaching the forest floor. Many tropical plant species have exceptionally broad leaves to maximize the capture of sunlight. Other species are epiphytes: plants that grow on other plants that serve as a physical support. Such plants are able to grow high up in the canopy atop the branches of other trees, where sunlight is more plentiful. Epiphytes live on rain and minerals collected in the branches and leaves of the supporting plant. Bromeliads (members of the pineapple family), ferns, and orchids are examples of tropical epiphytes (see the figure below). Many epiphytes have specialized tissues that enable them to efficiently capture and store water.

Some plants have special adaptations that help them to survive in nutrient-poor environments. Carnivorous plants, such as the Venus flytrap and the pitcher plant (see the figure below), grow in bogs where the soil is low in nitrogen. In these plants, leaves are modified to capture insects. The insect-capturing leaves may have evolved to provide these plants with a supplementary source of much-needed nitrogen.

Many swamp plants have adaptations that enable them to thrive in wet areas, where their roots grow submerged underwater. In these aquatic areas, the soil is unstable and little oxygen is available to reach the roots. Trees such as mangroves (Rhizophora sp.) growing in coastal waters produce aboveground roots that help support the tree (see the figure below). Some species of mangroves, as well as cypress trees, have pneumatophores: upward-growing roots containing pores and pockets of tissue specialized for gas exchange. Wild rice is an aquatic plant with large air spaces in the root cortex. The air-filled tissue—called aerenchyma—provides a path for oxygen to diffuse down to the root tips, which are embedded in oxygen-poor bottom sediments.

Summary

Leaves are the main site of photosynthesis. A typical leaf consists of a lamina (the broad part of the leaf, also called the blade) and a petiole (the stalk that attaches the leaf to a stem). The arrangement of leaves on a stem, known as phyllotaxy, enables maximum exposure to sunlight. Each plant species has a characteristic leaf arrangement and form. The pattern of leaf arrangement may be alternate, opposite, or spiral, while leaf form may be simple or compound. Leaf tissue consists of the epidermis, which forms the outermost cell layer, and mesophyll and vascular tissue, which make up the inner portion of the leaf. In some plant species, leaf form is modified to form structures such as tendrils, spines, bud scales, and needles.

Key terms

• compound leaf — leaf in which the leaf blade is subdivided to form leaflets, all attached to the midrib.
• cuticle — waxy protective layer on the leaf surface.
• lamina — leaf blade.
• palmately compound leaf — leaf type with leaflets that emerge from a point, resembling the palm of a hand.
• petiole — stalk of the leaf.
• phyllotaxy — arrangement of leaves on a stem.
• pinnately compound leaf — leaf type with a divided leaf blade consisting of leaflets arranged on both sides of the midrib.
• sessile — leaf without a petiole that is attached directly to the plant stem.
• simple leaf — leaf type in which the lamina is completely undivided or merely lobed.
• stipule — small green structure found on either side of the leaf stalk or petiole.
• venation — pattern of veins in a leaf; may be parallel (as in monocots), reticulate (as in dicots), or dichotomous (as in Ginkgo biloba).
• whorled — pattern of leaf arrangement in which three or more leaves are connected at a node.

Practice

Identify the parts of a typical leaf

Describe the internal structure and function of a leaf

Compare and contrast simple leaves and compound leaves

List and describe examples of modified leaves

This section is adapted from Biology 2e, Section 30.4: Leaves by Mary Ann Clark, Jung Choi, Matthew Douglas, and OpenStax, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: the glossary’s lowercase “ginkgo biloba” capitalized to the genus form Ginkgo biloba (reported as a source defect); figures re-encoded as WebP with a custom alt written from each image rather than reusing the source’s own alt text; two figures re-kinded after inspection from the manifest’s file-extension “photo” guess to “diagram” — Figure_30_04_01 (a labeled line drawing of a leaf’s parts, not a photograph) and Figure_30_04_02abc (a composite of three leaf photos each paired with a hand-drawn venation-pattern outline below it, drawn for print rather than captured); a longdesc added to Figure_30_04_01 and Figure_B30_04_03, walking each diagram’s leader lines and cell labels in reading order since neither is carried by its one-line caption; the “Plant Adaptations in Resource-Deficient Environments” Evolution Connection and both Link to Learning notes rendered as callouts, the Evolution Connection keeping its three figures inside it; the end-of-section Review Questions and Critical Thinking Questions adapted into the closing interactive Practice block (multiple choice and self-check respectively); rubric checkpoints added to each self-check, decomposing its model answer (the source solution) into check-off clauses with no new claims; ten text-in items added to round out the Practice groups — eight glossary-recall items (lamina, stipule, sessile, venation, cuticle, simple leaf, palmately compound leaf, pinnately compound leaf) and two cloze items built from the Section Summary (epidermis, needles). No source defect was found in this section’s exercises or glossary. One review-question distractor is replaced: the source’s “epidermis” is true by this section’s own statement that guard cells, which are epidermal, contain chloroplasts, so the page offers “endodermis” instead; key unchanged and the change is disclosed in the source-key ledger (erratum 437).