Leaves

Leaf Structure and Arrangment

Most leaves have similar essential structures, but differ in venation patterns and leaf arrangement (or phyllotaxy).

Learning Objectives

Sketch the basic structure of a typical leaf

Key Takeaways

Key Points

  • Each leaf typically has a leaf blade ( lamina ), stipules, a midrib, and a margin.
  • Some leaves have a petiole, which attaches the leaf to the stem; leaves that do not have petioles are directly attached to the plant stem and are called sessile leaves.
  • The arrangement of veins in a leaf is called the venation pattern; monocots have parallel venation, while dicots have reticulate venation.
  • The arrangement of leaves on a stem is known as phyllotaxy; leaves can be classified as either alternate, spiral, opposite, or whorled.
  • Plants with alternate and spiral leaf arrangements have only one leaf per node.
  • In an opposite leaf arrangement, two leaves connect at a node. In a whorled arrangement, three or more leaves connect at a node.

Key Terms

  • petiole: stalk that extends from the stem to the base of the leaf
  • lamina: the flat part of a leaf; the blade, which is the widest part of the leaf
  • stipule: small green appendage usually found at the base of the petiole

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. Leaves also have stipules, small green appendages usually found at the base of the petiole. 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.

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Parts of a leaf: A leaf may seem simple in appearance, but it is a highly-efficient structure. Petioles, stipules, veins, and a midrib are all essential structures of a leaf.

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. Monocots have parallel venation in which the veins run in straight lines across the length of the leaf without converging. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. Ginkgo biloba is an example of a plant with dichotomous venation.

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Venation patterns: (a) Tulip (Tulipa), a monocot, has leaves with parallel venation. (b) The netlike venation in this linden (Tilia cordata) leaf distinguishes it as a dicot. (c) The Ginkgo biloba tree has dichotomous venation.

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, opposite, or whorled. Plants that have only one leaf per node have leaves that are said to be either alternate or spiral. Alternate leaves alternate on each side of the stem in a flat plane, and spiral leaves are arranged 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.

Types of Leaf Forms

Leaves may be categorized as simple or compound, depending on how their blade (or lamina) is divided.

Learning Objectives

Differentiate among the types of leaf forms

Key Takeaways

Key Points

  • In a simple leaf, the blade is completely undivided; leaves may also be formed of lobes where the gaps between lobes do not reach to the main vein.
  • In a compound leaf, the leaf blade is divided, forming leaflets that are attached to the middle vein, but have their own stalks.
  • The leaflets of palmately-compound leaves radiate outwards from the end of the petiole.
  • Pinnately-compound leaves have their leaflets arranged along the middle vein.
  • Bipinnately-compound (double-compound) leaves have their leaflets arranged along a secondary vein, which is one of several veins branching off the middle vein.

Key Terms

  • simple leaf: a leaf with an undivided blade
  • compound leaf: a leaf where the blade is divided, forming leaflets
  • palmately compound leaf: leaf that has its leaflets radiating outwards from the end of the petiole
  • pinnately compound leaf: a leaf where the leaflets are arranged along the middle vein

Leaf Form

There are two basic forms of leaves that can be described considering the way the blade (or lamina) is divided. Leaves may be simple or compound.

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Simple and compound leaves: Leaves may be simple or compound. In simple leaves, the lamina is continuous. (a) The banana plant (Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. (b) In palmately compound leaves, such as those of the horse chestnut (Aesculus hippocastanum), the leaflets branch from the petiole. (c) In pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (Carya floridana). (d) The honey locust has double compound leaves, in which leaflets branch from the veins.

In a simple leaf, such as the banana leaf, the blade is completely undivided. The leaf shape may also be formed of lobes where the gaps between lobes do not reach to the main vein. An example of this type is the maple leaf.

In a compound leaf, the leaf blade is completely divided, forming leaflets, as in the locust tree. Compound leaves are a characteristic of some families of higher plants. Each leaflet is attached to the rachis (middle vein), but may have its own stalk. A palmately compound leaf has its leaflets radiating outwards from the end of the petiole, like fingers off the palm of a hand. Examples of plants with palmately compound leaves include poison ivy, the buckeye tree, or the familiar house plant Schefflera sp. (commonly called “umbrella plant”). Pinnately compound leaves take their name from their feather-like appearance; the leaflets are arranged along the middle vein, as in rose leaves or the leaves of hickory, pecan, ash, or walnut trees. In a pinnately compound leaf, the middle vein is called the midrib. Bipinnately compound (or double compound) leaves are twice divided; the leaflets are arranged along a secondary vein, which is one of several veins branching off the middle vein. Each leaflet is called a “pinnule”. The pinnules on one secondary vein are called “pinna”. The silk tree (Albizia) is an example of a plant with bipinnate leaves.

Leaf Structure, Function, and Adaptation

Leaves have many structures that prevent water loss, transport compounds, aid in gas exchange, and protect the plant as a whole.

Learning Objectives

Describe the internal structure and function of a leaf

Key Takeaways

Key Points

  • The epidermis consists of the upper and lower epidermis; it aids in the regulation of gas exchange via stomata.
  • The epidermis is one layer thick, but may have more layers to prevent transpiration.
  • The cuticle is located outside the epidermis and protects against water loss; trichomes discourage predation.
  • The mesophyll is found between the upper and lower epidermis; it aids in gas exchange and photosynthesis via chloroplasts.
  • The xylem transports water and minerals to the leaves; the phloem transports the photosynthetic products to the other parts of the plant.
  • Plants in cold climates have needle-like leaves that are reduced in size; plants in hot climates have succulent leaves that help to conserve water.

Key Terms

  • trichome: a hair- or scale-like extension of the epidermis of a plant
  • cuticle: a noncellular protective covering outside the epidermis of many invertebrates and plants
  • mesophyll: the inner tissue (parenchyma) of a leaf, containing many chloroplasts.

Leaf Structure and Function

The outermost layer of the leaf is the epidermis. It consists of the upper and lower epidermis, which are present on either side of the leaf. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis aids in the regulation of gas exchange. It contains stomata, which are openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing. Guard cells are the only epidermal cells to contain chloroplasts.

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 avert 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.

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Trichomes: Trichomes give leaves a fuzzy appearance as in this (a) sundew (Drosera sp.). Leaf trichomes include (b) branched trichomes on the leaf of Arabidopsis lyrata and (c) multibranched trichomes on a mature Quercus marilandica leaf.

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. The palisade parenchyma (also called the palisade mesophyll) aids in photosynthesis and has column-shaped, tightly-packed cells. It 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.

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Mesophyll: (a) (top) The central mesophyll is sandwiched between an upper and lower epidermis. The mesophyll has two layers: an upper palisade layer and a lower spongy layer. Stomata on the leaf underside allow gas exchange. A waxy cuticle covers all aerial surfaces of land plants to minimize water loss. (b) (bottom) These leaf layers are clearly visible in the scanning electron micrograph. The numerous small bumps in the palisade parenchyma cells are chloroplasts. The bumps protruding from the lower surface of the leaf are glandular trichomes.

Similar to the stem, the leaf contains vascular bundles composed of xylem and phloem. 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.

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Xylem and phloem: This scanning electron micrograph shows xylem and phloem in the leaf vascular bundle.

Leaf Adaptations

Coniferous plant species that thrive in cold environments, such as 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 succulent leaves that help to conserve water. Many aquatic plants have leaves with wide lamina that can float on the surface of the water; a thick waxy cuticle on the leaf surface that repels water.