Download Leaves

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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?
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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.
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acute
alternate
auricle
awl-like leaves
collar
compound leaf
cordate
cuneate
cuticle
dichotomous venation
elliptical
entire
epidermis
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guard cells
lanceolate
leaf blade
leaf sheath
leaflets
leaves
ligule
lobe
margin
mesophyll
midrib
needle-like leaves
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netted veins
oblique
obovate
obtuse
opposite
ovate
palisade mesophyll
palmately compound
palmately netted
parallel veins
petiole
pinnately compound
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pinnately netted
rounded
scale-like leaves
serrate
simple leaf
sinus
spongy mesophyll
stomata
subopposite
trichomes
vernation
whorled
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I. Leaves are the primary foodproducing 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 needleshaped leaves.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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A. A leaf that has only
one blade on its petiole
is called a simple leaf.
Most plants have simple
leaves.
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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.
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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.
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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.
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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
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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.
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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.
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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).
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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.
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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.
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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?