The Parts of a Leaf
 
The                 Parts of a Leaf
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.
The Blade, or lamina,                 is the broad, flat part of the leaf.                 Photosynthesis occurs in the blade, which has                 many green food-making cells. Leaf blades differ                 from one another in several ways: (1) the types                 of edges, (2) the patterns of the veins, and (3)                 the number of blades per leaf.
The Types of Edges. Almost all narrow, grasslike leaves and needles                 leaves have a blade with a smooth edge, as do                 many broadleaf plants, particularly those that                 are native to warm climates. The rubber plant, a                 common house plant, is a good example of such a                 plant.
The leaves of many temperate                 broadleaf plants have small, jagged points called                 teeth along the blade edge. Birch and elm trees                 have such leaves. Some plants have hydathodes,                 tiny valvelike 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 glads. These glands produce                 liquids that protect the young leaf from                 plant-eating insects.
Some temperate broadleaf 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.
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.
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 -- pinnate (featherlike) and palmate (palmlike or                 handlike). 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 vein pattern. 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.
Narrow leaves and needle leaves                 are not net-veined. Narrow leaves have a                 parallel-vein pattern. Several large veins run                 alongside one another from the base of the blade                 to the tip. Small crossveins connect the large                 veins. Needle leaves are so small that they have                 only one or two veins running through the center                 of the blade.
The Number of Blades per Leaf. A leaf with only one blade is called a simple                 leaf. Apple and oak trees, grasses, and many                 other plants have simple leaves. 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.                 In 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. The leaflets in a palmately compound                 leaf all grow from the tip of the leafstalk.                 Clover, horse chestnut trees, and many other                 plants have palmately compound leaves.
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.
The Petiole is                 the stemlike part of the leaf that joins the                 blade to the stem. Within a petiole are tiny                 tubes that connect with the veins in the blade.                 Some of the tubes carry water into the leaf.                 Others carry away food that the leaf has made. 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. 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.
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.
World Book Encyclopedia Chicago: World Book-Childcraft International,                 Inc., 1979