Parts of a Leaf
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 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
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
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.
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,
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