Leaf Anatomy and Morphology

Lesson Plan: NRES B5-5

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Anticipated Problems

1. What are the main external parts of a leaf, and what are the functions of a leaf?

2. What are the functions of the internal components of a leaf?

3. What are the functions of leaf veins, and what are some vein patterns found in leaves?

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Anticipated Problems

4. What are some of the major types of broadleaf plant leaves?

5. What are the major leaf arrangements?6. What are the major botanical terms used to

describe various leaf shapes and the various shapes found at the leaf margins, apexes, and basees?

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Terms acute alternate auricle awl-like leaves collar compound leaf cordate cuneate cuticle

dichotomous venation

elliptical entire epidermis guard cells lanceolate leaf blade leaf sheath leaflets

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Terms leaves ligule lobe margin mesophyll midrib needle-like leaves netted veins oblique

obovate obtuse opposite ovate palisade mesophyll palmately compound palmately netted parallel veins petiole

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Terms pinnately

compound pinnately netted rounded scale-like leaves serrate simple leaf sinus spongy mesophyll stomata

subopposite trichomes vernation whorled

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Leaves

Leaves are the primary food-producing organs of the plant. Designed to efficiently collect lightUses that light energy to produce food

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Broadleaf Plant

The large broad part of a leaf is the leaf blade, which provides a large surface area well suited for absorbing solar energy for photosynthesis. The blade has many layers that help the plant

move and store photosynthesis materials and by-products.

Many leaves are organized with one main vein running down the middle of the blade. This vein is called the midrib.

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Broadleaf Plant

The leaf blade is connected to the stem by the petiole, which is the lifeline between the leaf and the rest of the plant. The petiole is similar to a stem in that it has

xylem and phloem. Water and minerals flow into the leaf blade

and food flows out of the leaf blade through the petiole.

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Broadleaf Plant

The edge of the leaf blade is referred to as the margin. The margins can be one of many forms: wavy,

toothed, lobed, and entire or smooth.

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Distinct Structures of Grasses

An auricle is a pair of tiny appendages between the leaf blade and the sheath.

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Distinct Structures of Grasses

The collar is a light-colored band between the leaf blade and the sheath on the lower side of the leaf. Some are continuous, and some are divided.

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Distinct Structures of Grasses

The upper portion of a grass leaf is called the leaf blade.

The leaf sheath is the lower portion of a grass leaf.

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Distinct Structures of Grasses

A ligule is a membranous or hairy structure on the inside of a leaf at the junction of the leaf blade and the sheath.

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Distinct Structures of Grasses

Vernation is the arrangement of the youngest leaf in the bud shoot, either folded or rolled.

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Conifers: Types of Leaves

Awl-like leaves are shaped like an awl and are usually very sharp to the touch. (junipers)

Scale-like leaves overlap one another like fish scales or shingles on a roof. These leaves tend to be soft to the touch.

Needle-like leaves are characteristic of pines, firs, and spruce. (leaves are linear)

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Functions of a Leaf

A leaf is organized to collect sunlight and turn it, through photosynthesis, into food. The leaf has many layers of tissue to allow this to happen.

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Functions of a Leaf

On top of the leaf is a waxy non-cellular layer called the cuticle, which prevents water from escaping. Generally speaking, plants that live in bright

arid conditions have very thick cuticle layers.

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Functions of a Leaf

The next layer on the leaf also exists for protection. The epidermis is the skin-like layer of cells found on the top and the bottom surface of the leaf. The epidermis may be one or many layers

thick.

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Functions of a Leaf

Between the epidermal layers is the mesophyll. The bulk of photosynthetic activity for a plant takes place in the mesophyll cells. Throughout the mesophyll is a network of

veins containing xylem and phloem tissues. The veins are numerous enough so every cell

is reached for exchange of materials.

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Functions of a Leaf

Directly beneath the upper epidermis is a layer of cells that are standing on end and are packed very tightly.

These standing cells are responsible for most of the photosynthesis in the leaf and are called the palisade mesophyll.

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Functions of a Leaf

Located under the palisade mesophyll are loosely packed cells called spongy mesophyll, which forms air spaces that hold raw materials used and products of photosynthesis.

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Functions of a Leaf

Leaves have openings in the epidermis called stomata (singular: stoma). Stomata allow the diffusion of carbon dioxide, oxygen, and water.

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Functions of a Leaf

Specialized cells called guard cells control the opening and closing of the stomata.

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Functions of a Leaf

Another feature seen on some plant leaves are trichomes, which are specialized cells that appear as hairs on the leaves of some plants. They reduce water loss by slowing air

movement close to the leaf and discourage some pests from devouring the leaves.

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Functions of Leaf Veins

Veins in leaves are essential for the translocation of water, minerals, sugars, and other materials. Different types of plants have different vein

patterns. Most of these patterns can be categorized into

two groups.

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Functions of Leaf Veins

Monocots have leaves with parallel veins. While the veins may not be parallel in a strict mathematical sense, none of the veins on the leaf cross. It may look like they fused together at the top

or bottom of the blade. Examples: corn and grass plants

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Functions of Leaf Veins

Dicots have veins that connect and branch from each other. Veins in a branching pattern are called netted veins.

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Functions of Leaf Veins

Some leaves with netted veins have several smaller veins branching out of a dominant midrib, which is a condition known as pinnately netted.

Others have several dominant veins branching out from the petiole, known as palmately netted.

A few plants have a spreading vein pattern called dichotomous venation. ( a gingko leaf )

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Types of Broadleaf Plant Leaves

Some are adapted to hot, dry climates by storing water or being smaller.

Some have very large blades to collect the maximum light in a shady location.

In some leaves, the blade is broken into several sections.

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Simple Leaf

A leaf that has only one blade on its petiole is called a simple leaf. Most plants have simple leaves.

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Compound Leaf

A leaf with multiple blades (leaflets) is said to be a compound leaf. Two common types are:A palmately compound leaf

has all its leaflets attached to a common point.

A pinnately compound leaf has multiple leaflets attached along a rachis or axis.

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Major Leaf Arrangements

The arrangement of leaves on a stem varies from one genus to another. Leaves are arranged along stem in one of four major ways.

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Major Leaf Arrangements

1. When leaves and buds are arranged directly across from each other on a stem, they are said to be opposite.

2. Leaves and buds that are spaced along a stem in an alternating fashion are termed alternate.

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Major Leaf Arrangements

3. A third arrangement is subopposite. In this arrangement, leaves and buds are not spaced far enough apart to be called alternate or perfectly opposite.

4. When three or more leaves and buds are attached at a node, the arrangement is called whorled.

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Common Leaf Terms

Leaf shapes, bases, margins, and apices vary from species to species.

Knowing terminology regarding leaf shapes is useful in identifying and describing plants.

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Common Leaf Terms

Ovate leaves have an egg-like shape with the broadest part below the middle.

Obovate leaves are shaped like an egg with the broadest part above the middle.

Lanceolate refers to leaves that are much longer than wide with the broadest part below the middle and tapering to the apex.

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Common Leaf Terms

Cordate means shaped like a heart. Elliptical leaves have an ellipse shape with

the broadest part in the middle and tapering to the base and apex.

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Three Terms used for Margins

Entire means the margin lacks teeth. Serrate means saw-tooth with the teeth

pointing forward. A lobe is a projecting part or segment of

the leaf. The space between two lobes is a sinus. Maples and oaks have lobes and sinuses.

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Two Apices

Acute means having an apex whose sides are straight and taper to a point.

Obtuse means rounded and approaching semicircular.

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Review

What is the name of the one main vein running down the middle of the leaf blade?

What is the function of the cuticle? What is the difference between parellel veins

and netted veins? What are the four major ways that leaves are

arranged along stem? What leaf name means it is shaped like a

heart?