The 3 Main Parts
Most leaves have two main parts:
(1) the blade and
(2) the petiole, or leafstalk.
The leaves of some kinds of plants also have a third part, called the stipules.
(1) the blade and
(2) the petiole, or leafstalk.
The leaves of some kinds of plants also have a third part, called the stipules.
1.) The Blade
The Blade is the broad, flat part of the leaf.
Photosynthesis occurs in the blade, which has many green food-making cells.
The main organelle in which photosynthesis takes place is called the chloroplast.
The main organelle in which photosynthesis takes place is called the chloroplast.
These small photosynthesis factories buried within the leaves contain chlorophyll, a green pigment inside the chloroplast membranes.
Chlorophyll absorbs a wide range of the spectrum of sunlight, giving the plant as much energy as it can for its reactions. The primary section of the light spectrum that chlorophyll doesn't absorb is green, which explains why leaves usually appear to be some shade of green. |
These green chloroplasts reside on the leaf's interior.
The surface of the leaf that can be touched is actually the epidermis, which protects the processes occurring beneath.
The blade is protected by a waxy layer on the top part of the epidermis called the cuticle.
The cuticle's function is to lessen water loss by not letting it diffuse out so easily during evaporation.
The surface of the leaf that can be touched is actually the epidermis, which protects the processes occurring beneath.
The blade is protected by a waxy layer on the top part of the epidermis called the cuticle.
The cuticle's function is to lessen water loss by not letting it diffuse out so easily during evaporation.
Leaf blades differ from one another in several ways:
(a) the number of blades per leaf,
(b) the types of leaf edges, and
(c) the patterns of the veins.
(a) the number of blades per leaf,
(b) the types of leaf edges, and
(c) the patterns of the veins.
(a.) The Number of Blades per Leaf
The number of blades per leaf divides leaves into 2 main categories:
(1.) Simple Leaf (2.) Compound Leaf |
(1.) Simple Leaf:
A leaf with only one blade is called a simple leaf. Apple and oak trees, grasses, and many other plants have simple leaves. (2.) Compound Leaf:
A leaf with more than one blade is known as a compound leaf. The blades of a compound leaf are called leaflets. The leaflets in a compound leaf may be arranged in a pinnate or palmate pattern. (a.) Pinnately Compound Leaves: The leaflets grow in two rows, one on each side of a central stalk, called the rachis. Plants with pinnately compound leaves include ash and walnut trees and garden peas. (b.) Palmately Compound Leaves: The leaflets all grow from the tip of the leafstalk. Clover, horse chestnut trees, and many other plants have palmately compound leaves. **Double Compound Leaf:
A few plants, including carrots, honey locust trees, and Kentucky coffeetrees, have double compound leaves, with each leaflet being divided into a number of still smaller leaflets. One double compound leaf looks more like a group of twigs and leaves than like a single leaf. |
(b.) The Types of Edges
The edges of leaves are divided into 3 main categories:
(1.) Smooth-edged (2.) Toothed-edged (3.) Lobed-edged |
(1.) Smooth-Edged:
Almost all narrow, grass-like leaves and needles leaves have a blade with a smooth edge, as do many broadleaved plants, particularly those that are native to warm climates. The rubber plant, a common house plant, is a good example of a plant that is native to a warm climate. (2.) Toothed-Edged:
The leaves of many temperate broadleaved plants have small, jagged points called teeth along the blade edge. Birch and elm trees have such leaves. Some plants have hydathodes, tiny valve-like structures that can release excess water from the leaf. The teeth of young leaves on many plants, including cottonwood and pin cherry trees, bear tiny glands. These glands produce liquids that protect the young leaf from plant-eating insects. (3.) Lobed-Edged:
Some temperate broadleaved plants, including sassafras trees and certain mulberry and oak trees, have lobed leaves. The edge of such a leaf looks as if large bites have been taken out of it. This lobing helps heat escape from the leaf. |
(c.) The Patterns of the Veins
Veins carry food and water in a leaf.
They also support the blade, much as the metal ribs support the fabric of an open umbrella. There are 3 patterns of veins. |
(1.) Net-like Patterned Veins:
In most broad leaves, the veins form a netlike pattern, with several large veins connected by smaller ones. The smallest veins supply every part of the blade with water. They also collect the food made by the green cells. There are two main types of net-vein patterns: (a.) pinnate (featherlike): Pinnately-veined leaves have one large central vein, called the midrib, which extends from the base of the blade to its tip. Other large veins branch off on each side of the midrib. The leaves of beech, birch, and elm trees have such a pinnate vein pattern. (b.)palmate (palmlike or handlike): A palmately-veined leaf has several main veins of about equal size, all of which extend from a common point at the base of the blade. The vein patterns of maple, sweet gum, and sycamore leaves are palmate. (2.) Parallel Veins:
Narrow leaves have a parallel-vein pattern. Narrow leaves and needle leaves are not net-veined. Several large veins run alongside one another from the base of the blade to the tip. Small cross veins connect the large veins. (3.) Centered Veins:
Needle leaves are so small that they have only one or two veins running through the center of the blade. |
This leaf is joined to the plant stem by the Petiole.
This leaf has no joining Petiole and is known as a sessile leaf.
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2.) The Petiole
The Petiole is the stem-like part of the leaf that joins the blade to the stem.
Inside a petiole are tiny tubes that connect with the veins in the blade.
In many trees and shrubs, the petioles bend in such a way that the blades receive the most sunlight, thus assuring that few leaves are shaded by other leaves. This is called "phototropism". The petiole also provides a flexible "handle" that enables the blade to twist in the wind and so avoid damage. In some plants, the petioles are much larger than the stems to which they are attached. For example, the parts we eat of celery and rhubarb plants are petioles. In contrast, the leaves of some soft-stemmed plants, particularly grasses, have no petioles. These leaves are called Sessile leaves. |
3.) The Stipules
The Stipules are two small flaps that grow at the base of the petiole of some plants. In some plants, the stipules grow quickly, enclosing and protecting the young blade as it develops Some stipules, such as those of willows and certain cherry trees, produce substances that prevent insects from attacking the developing leaf. In many plants, the stipules drop off after the blade has developed, but garden peas and a few other kinds of plants, have large stipules that serve as an extra food-producing part of the leaf. |
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Sources:
World Book Encyclopedia. Chicago:World Book-Childcraft International, Inc., 1979.
World Book Encyclopedia. Chicago:World Book-Childcraft International, Inc., 1979.