Bring several different leaves or plants with different leaves into class. Ask students to compare and contrast the leaves. Use student responses to form a list of how the leaves are the same and how they are different. Begin to build on the student list. Do the similarities on the list have something to do with photosynthesis? (Similarities may be that leaves are green or have a large flat surface.) What words are used to describe the differences? Can we choose better terms?

1 Identify the main external parts of a leaf (from a broadleaf plant and from a narrowleaf plant), and describe the functions of leaves.

2 Identify and describe the functions of the internal components of a leaf.

3 Describe the function of leaf veins, and discuss common vein patterns in leaves.

4 Distinguish between the types of broadleaf plant leaves.

5 Identify and describe the patterns of leaf arrangement.

6 Identify the major botanical terms used to describe various leaf shapes and the various shapes found at the leaf margin, apex, and base.

_ 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 _ 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 _ pinnately compound

_ pinnately netted _ rounded _ scale-like leaves _ serrate _ simple leaf _ sinus _ spongy mesophyll _ stomata _ subopposite _ trichomes _ vernation _ whorled

I. Leaves are the primary food-producing organs of the plant. Leaves are designed to efficiently collect light and use that light energy to produce food. They come in a great variety of shapes, sizes, and colors. Many plants (e.g., soybeans, apples, and maples) have broad leaves. Grasses have long, narrow leaves. Coniferous plants (e.g., pines and spruces) have needle-shaped leaves.

A. There are different parts to the leaf of a broadleaf plant.

1. 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.◦ a. The blade is supported by a system of veins that contain

xylem and phloem. These veins prevent the blade from collapsing under its own weight.

◦ b. Many leaves are organized with one main vein running down the middle of the blade. This vein is called the midrib. All of the veins, the petiole, and the midrib help position the blade in a way that it is facing the light source.

2. 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.

3. 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.

B. Grasses have distinct structures.

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

2. 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.

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

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

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

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

C. Conifers have three different types of leaves.

1. Awl-like leaves are shaped like an awl and are usually very sharp to the touch. Junipers have awl-like leaves.

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

3. Needle-like leaves are characteristic of pines, firs, and spruce. These leaves are linear.

II. 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.

A. 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.

B. 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.

C. 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.

1. 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.

2. 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.

D. Leaves have openings in the epidermis called stomata (singular: stoma). Stomata allow the diffusion of carbon dioxide, oxygen, and water. Specialized cells called guard cells control the opening and closing of the stomata.

E. 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. Soybeans have a hairy leaf. In the case of desert plants, trichomes serve to reduce the intensity of light that reaches the plant by reflecting some light.

III. 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.

A. 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. Corn and grass plants are good examples of monocot leaves.

B. Dicots have veins that connect and branch from each other. Veins in a branching pattern are called netted veins. Some leaves with netted veins have several smaller veins branching out of a dominant midrib, which is a condition known as pinnately netted. Other leaves have several dominant veins branching out from the petiole, which is known as palmately netted. A few plants have a spreading vein pattern called dichotomous venation. A gingko leaf has this type of veins.

IV. There are many different types of leaves. Some leaves are adapted to hot, dry climates by storing water or being smaller. Some leaves have very large blades to collect the maximum light in a shady location. In some leaves, the blade is broken into several sections.

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

B. In some leaves, the blade is divided into three or more sections. A leaf with multiple blades (leaflets) is said to be a compound leaf. There are many different kinds of compound leaves, but two common types are:

1. A palmately compound leaf has all its leaflets attached to a common point.

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

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

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

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

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

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

VI. Leaf shapes, bases, margins, and apices vary from species to species. Knowing terminology regarding leaf shapes is useful in identifying and describing plants.

A. Common leaf shape terms

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

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

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

4. Cordate means shaped like a heart. 5. Elliptical leaves have an ellipse shape

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

B. Common leaf bases

1. Cuneate means wedge-shaped with essentially straight sides.

2. Rounded means the base is rounded. 3. Oblique means lopsided (one side of the

leaf base is wider or larger than the other).

C. Three terms used for margins

1. Entire means the margin lacks teeth. 2. Serrate means saw-tooth with the teeth

pointing forward. 3. 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.

D. Two apices

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

2. Obtuse means rounded and approaching semicircular.

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

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

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

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