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24–1 Reproduction With Cones and Flowers
Section 24–1
1 FOCUS
S
eed plants are well adapted to the demands of life on land,
especially in how they reproduce. The gametes of seedless
plants, such as ferns and mosses, need water for fertilization to be
successful. Water allows gametes to move from plant to plant. The
gametes of seed plants, however, can achieve fertilization even
when the plants are not wet from rain or dew. So, they can reproduce nearly anywhere. The way in which seed plants reproduce
has allowed them to survive the dry conditions on land.
Key Concepts
• What are the reproductive
structures of gymnosperms
and angiosperms?
• How does pollination differ
between angiosperms and
gymnosperms?
All plants have a life cycle in which a diploid sporophyte generation alternates with a haploid gametophyte generation.
Gametophyte plants produce male and female gametes—sperm
and eggs. When the gametes join, they form a zygote that begins
the next sporophyte generation. In some plants, the two stages
of the life cycle are distinct, independent plants. In most ferns,
for instance, the gametophyte is a small, heart-shaped plant
that grows close to the ground. The sporophyte is the familiar
fern plant itself made up of graceful fronds.
Where are these two generations in seed plants? You may
remember from Mendel’s work on peas that such plants are
diploid. Therefore, in seed plants, the familiar, recognizable
form of the plant is the diploid sporophyte.
If the sporophyte is what we recognize as the plant, then
where is the gametophyte? The answer may surprise you. As
shown in Figure 24–1, the gametophytes of seed plants are
actually hidden deep within tissues of the sporophyte plant. In
gymnosperms they are found inside cones, and in angiosperms
they are found inside flowers. Cones and flowers represent two
different methods of reproduction.
pollen cone • seed cone
ovule • pollen tube
sepal • petal • stamen
filament • anther • carpel
ovary • style • stigma
embryo sac • endosperm
double fertilization
Reading Strategy:
Making Comparisons
Before you read, preview Figure
24–4 and Figure 24–7. As you
read, compare the life cycles of
gymnosperms and angiosperms.
왗
Gametophyte (N)
Sporophyte (2N)
Ferns
Seed plants
Figure 24–1 An important trend in
plant evolution is the reduction of the
gametophyte and the increasing size of the
sporophyte. Bryophytes consist of a
relatively large gametophyte and smaller
sporophytes, which include a capsule and
stalk. Seedless vascular plants, such as ferns,
have a small gametophyte and a larger
sporophyte. Seed plants have an even
smaller gametophyte that is contained
within sporophyte tissues. Interpreting
Graphics How does the relative size of the
haploid and diploid stages of plants differ
between bryophytes and seed plants?
SECTION RESOURCES
Technology:
• Teaching Resources, Section Review 24–1,
Chapter 24 Design an Experiment
• Reading and Study Workbook A,Save
e
Section 24–1
• Adapted Reading and Study Workbook B,
Section 24–1
• Investigations in Forensics, Investigation 7
• Lesson Plans, Section 24–1
• iText, Section 24–1
• Animated Biological Concepts Videotape
Library, 34 Angiosperm Reproduction
• Transparencies Plus, Section 24–1
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24.1.1 Identify the reproductive
structures of gymnosperms
and angiosperms.
24.1.2 Explain how pollination and
fertilization differ between
angiosperms and
gymnosperms.
Vocabulary
Alternation of Generations
Bryophytes
Objectives
Vocabulary Preview
Ask students to predict which
Vocabulary terms refer to the reproductive parts of gymnosperms, or
cone-bearing plants, and which
terms refer to the reproductive parts
of angiosperms, or flowering plants.
(Pollen cone and seed cone are parts of
gymnosperms. Sepal, petal, stamen,
anther, carpel, ovary, style, and stigma
are parts of angiosperms. Both types of
plants have pollen tubes and ovules.)
Reading Strategy
Suggest that students find each highlighted, boldface term in the text;
read its definition; and then locate it
in Figure 24–4 or Figure 24–7.
2 INSTRUCT
Alternation of
Generations
Use Visuals
Figure 24 –1 Have students compare
and contrast the differences in the
sizes of the diploid sporophyte and the
haploid gametophyte in different plant
groups. Review the life cycle for each
plant group shown, if needed.
Challenge students to make inferences
about the evolutionary advantage of a
reduced haploid gametophyte stage.
Have them consider water requirements, complexity of structure, and
size.
Answer to . . .
Figure 24 –1 Seed plants have a
highly reduced haploid or gametophyte
stage; the visible plant is nearly all
diploid sporophyte.
Reproduction of Seed Plants
609
Life Cycle of Gymnosperms
24–1 (continued)
Pine trees and other gymnosperms are diploid sporophytes. As
you will see, this sporophyte develops from a zygote that is
contained within a seed. How and where is this seed produced?
Reproduction in gymnosperms takes place in cones,
which are produced by a mature sporophyte plant. Gymnosperms produce two types of cones: pollen cones and seed cones.
Life Cycle of
Gymnosperms
Build Science Skills
Observing Obtain a small pine
branch that has both pollen cones
and seed cones. Point out how the
two types of cones are arranged on
the branch so that pollen from a
pollen cone is likely to fall on a seed
cone. Call students’ attention to the
scales on a seed cone and how they
are arranged. Remove some of the
scales, and let students examine the
base of the scales. (Even if the seeds
have been shed, an impression of the
seeds still remains.) Also, have students examine the scales of a seed
cone that has been soaked in water.
(The scales are closed.) Ask: What is
the function of the scales of a seed
cone? (To produce and protect the
seeds)
Demonstration
Dust some pollen grains from a cone
on a microscope slide. Prepare a wetmount slide and focus on low power.
Use a microprojector or have students take turns observing the
pollen. Suggest that students sketch
what they see. Ask: How is the structure of the pollen grain related to
its function? (A pine pollen grain has
two tiny wings on either side of its
rounded center that aid in its dispersal
by wind.)
Pollen Cones and Seed Cones Pollen cones, shown in
Figure 24–2, are also called male cones. Pollen cones produce the
왖 Figure 24–2
Reproduction
in gymnosperms takes place in
structures called cones. In this pine
tree, pollen cones, shown on the
top, produce male gametophytes,
which are pollen grains. Seed cones,
such as the one shown on the
bottom, produce female gametophytes that develop into a new
embryo following fertilization.
male gametophytes, which are called pollen grains. As tiny as it
is, the pollen grain makes up the entire male gametophyte stage
of the gymnosperm life cycle. One of the haploid nuclei in the
pollen grain will divide later to produce two sperm nuclei.
The more familiar seed cones, which produce female
gametophytes, are generally much larger than pollen cones.
Near the base of each scale are two ovules in which the female
gametophytes develop. Within the ovules, meiosis produces
haploid cells that grow and divide to produce female gametophytes. These gametophytes may contain hundreds or thousands of cells. When mature, each gametophyte contains a few
large egg cells, each ready for fertilization by sperm nuclei.
Pollination The gymnosperm life cycle typically takes two
years to complete. The cycle begins in the spring as male cones
release enormous numbers of pollen grains. This pollen is
carried by the wind, as shown in Figure 24–3. Some of these
pollen grains reach female cones. There, some pollen grains are
caught in a sticky secretion on one of the scales of the female
cone. This sticky material, known as a pollination drop, ensures
that pollen grains stay on the female cone.
What are pollen cones and seed cones?
Figure 24–3 Pollen grains are male gametophytes.
Pollen is carried by the wind until it reaches a female
cone. Inferring Male and female cones are distributed
on a plant such that pollen usually lands on a different
plant from where it started. Why might this strategy have
evolved?
Pollen Grains
(magnification: 750ⴛ)
Less Proficient Readers
Because the double fertilization
process is complicated,
encourage students to carefully
reread that subsection, and
have each student draw a flowchart to illustrate the process.
In their flowcharts, they should
indicate which cells are haploid, diploid, and triploid.
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Chapter 24
English Language Learners
Invite students to diagram several very different types of
flowers, such as orchids, magnolias, and daisies. Students
should label each with the following: sepals, petals, stamen,
anthers, ovary, style, and stigma. They may also include
labels in their native languages.
Advanced Learners
Encourage students to write a
“biography” of any mature
seed plant. Their biographies
should incorporate all of the
important events in the plant’s
life cycle, including formation
as a seed, germination, transportation away from the
parent plant, and growth to
maturity.
Use Visuals
Cone
scale
Diploid cell (2N)
Haploid (N)
Diploid (2N)
Ovule
MEIOSIS
Seed
cone
Pollen
cone
Ovules
Four
haploid
cells (N)
Female
gametophyte
(N)
Pollen grain (N)
(male gametophytes)
Egg cells
Discharged
sperm nucleus
Mature
sporophyte
Pollen tube
FERTILIZATION
Seedling
Germinated
seed
Zygote (2N)
(new sporophyte)
Gametophyte tissue (N)
Embryo (2N)
Seed Seed coat
(old sporophyte)
Fertilization and Development If a pollen grain lands
near an ovule, the grain splits open and begins to grow a structure called a pollen tube, which contains two haploid sperm
nuclei. Once the pollen tube reaches the female gametophyte,
one sperm nucleus disintegrates, and the other fertilizes the egg
contained within the female gametophyte. If sperm from another
pollen tube reaches the female gametophyte, more than one egg
cell may be fertilized, but just one embryo develops. As shown in
Figure 24–4, fertilization produces a diploid zygote—the new
sporophyte plant. This zygote grows into an embryo. During this
time, it is encased within what will soon develop into a seed. The
seed consists of three generations of the life cycle. The outer seed
coat is part of the old sporophyte generation, the haploid cells
surrounding the embryo are part of the female gametophyte, and
the embryo is the new sporophyte plant.
왖 Figure 24–4 This illustration
shows the life cycle of a typical
gymnosperm. A pine tree—the
mature sporophyte—produces male
and female cones. Male cones
produce pollen, and female cones
produce ovules located on cone
scales. If an egg is fertilized by the
sperm, it becomes a zygote that is
nourished by the female cone. In
time, the zygote develops into a
new sporophyte plant. Classifying
Classify each of the following terms as
to whether they belong to the haploid
or diploid stage of the pine tree’s life
cycle: pollen tube, seed cone, embryo,
ovule, seedling.
Figure 24 – 4 Make sure students
understand the diagram. Point out all
the steps where the next drawing in
the sequence is an enlargement of
the previous drawing. For example,
explain how the single cone scale at
the top of the diagram is just one of
many cone scales on the seed cone
in the previous drawing. Similarly,
the drawing that shows the diploid
cell in the cone scale is an enlargement of the previous drawing of the
cone scale. Add that the same holds
true for the male pollen cone and its
pollen grains. Then, review the entire
life cycle of a pine tree, and have students follow along in the diagram. As
you identify the structures involved in
each stage, students should locate
them in the diagram. Check students’ comprehension of the life
cycle by asking: Which parts of the
plant are haploid? (The haploid cell
in the ovule, the female gametophyte,
and the male gametophytes in the
pollen grains) Why is the zygote
diploid? (Because fertilization has
occurred)
Build Science Skills
Applying Concepts As students
watch, cut a pine seed in half. Point
out the three layers of the seed: the
outer seed coat, the gametophyte,
and the embryo. You may want to
provide students with a hand lens to
examine the three layers. Explain that
the seed consists of three generations
of the pine tree. Ask: Which generation of the pine tree is represented
by each layer of the seed? (The
outer seed coat is part of the old sporophyte plant, the cells surrounding the
embryo are part of the female gametophyte, and the embryo is the new
sporophyte plant.)
BACKGROUND
In an attempt to simplify the terminology, pollen
grains often are referred to as gametes. It is
important, though, to be clear about exactly
what is a gamete and what is a spore. Recall that
a gamete is a cell that must fuse with another
gamete to form a new organism. Pollen grains,
therefore, are not gametes; they are spores
because they grow by mitosis into a new organism—the gametophyte. In angiosperms, the true
male gametes are the two sperm nuclei that
appear in the pollen tube. The true female
gametes are the egg cell and the polar nuclei that
fuse with the sperm nuclei to form the embryo
and endosperm, respectively.
Answers to . . .
Pollen cones produce the
male gametophytes. Seed cones produce the female gametophytes.
Figure 24 –3 Because it increases
genetic variation
Figure 24 –4 Haploid stage: pollen
tube; diploid stage: seed cone, embryo,
ovule, seedling
Reproduction of Seed Plants
611
24–1 (continued)
Structure of Flowers
Stamen
Stigma
Style
Anther
Filament
Carpel
Ovary
Build Science Skills
Classifying Display pictures of a
wide variety of angiosperms. Include
trees, shrubs, perennial and annual
herbaceous plants, ground covers,
grasses, and water plants. Give students a chance to view the pictures,
and then have them brainstorm a list
of characteristics that they think all
angiosperms share. (Students might
identify green leaves, flowers, and
fruits, among other possible shared
characteristics.) Then, ask: How do
angiosperms differ from gymnosperms? (Students might say that
angiosperms have broad leaves instead
of needles and that they produce flowers and fruits instead of cones.)
Build Science Skills
Using Models Give students a
chance to create a model of a flower
using whatever materials they find
around the classroom or at home.
For example, they might use colored
construction paper for sepals and
petals; toothpicks for filaments; modeling clay for anthers, stigma, and
ovary; cornmeal for pollen; a drinking
straw for the style; and dry peas for
ovules. Invite students to share their
models with the class and identify
the parts of each model.
Structure of Flowers
You may think of flowers as decorative objects that brighten the
world. However, the presence of so many flowers in the world is
visible evidence of something else—the stunning evolutionary
success of the angiosperms, or flowering plants. Flowers are the
key to understanding why angiosperms have been so successful.
Flowers are reproductive organs that are composed of four kinds of specialized leaves: sepals, petals,
stamens, and carpels. These structures are shown in the
flower in Figure 24–5.
Sepals and Petals The outermost circle of floral parts
Ovary
Petal
Sepal
Ovule
왖
Figure 24–5 This diagram
shows the parts of a typical flower.
The flowers of some species, however,
may not have all the parts shown
here.
Flowers are reproductive
organs that include sepals, petals,
stamens, and carpels.
For: The Structure of
a Flower activity
Visit: PHSchool.com
Web Code: cbp-7241
contains the sepals, which in many plants are green and closely
resemble ordinary leaves. Sepals enclose the bud before it opens,
and they protect the flower while it is developing. Petals, which
are often brightly colored, are found just inside the sepals.
The petals attract insects and other pollinators to the flower.
Because they do not produce reproductive cells, the sepals and
petals of a flower are sometimes called sterile leaves.
Stamens and Carpels Within the ring of petals are the
structures that produce male and female gametophytes. The
male parts consist of an anther and a filament, which together
make up the stamen. The filament is a long, thin stalk that
supports an anther. At the tip of each filament is an anther, an
oval sac where meiosis takes place, producing haploid male
gametophytes—pollen grains. In most angiosperms, each flower
has several stamens. If you rub your hand on the anthers of a
flower, a yellow-orange dust may stick to your skin. This is
pollen, which consists of thousands of individual pollen grains.
The innermost floral parts are carpels, also called pistils,
which produce the female gametophytes. Each carpel has a
broad base forming an ovary, which contains one or more
ovules where female gametophytes are produced. The diameter
of the carpel narrows into a stalk called the style. At the top of
the style is a sticky portion known as the stigma, where pollen
grains frequently land. Some flowers have several carpels fused
together to form a single reproductive structure called a compound carpel.
Anthers
For: The Structure of a Flower
activity
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Web Code: cbe-7241
Students can interact with the
art online.
Carpel
Anthers
Carpel
Tulip
Wild Rose
FACTS AND FIGURES
Flowery facts
In many flowers, both sepals and petals are
brightly colored and help attract pollinators. In
other plants, sepals are smaller and thicker than
petals and green in color. In these plants, the
sepals’ function is to protect the more fragile
flower bud from damage.
Some angiosperm plants have evolved unique
ways to attract pollinators. For example, an
African plant, Anchomanes difformis, does the
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Chapter 24
botanical equivalent of burning incense. The
flower has a foot-tall structure called a spadix. The
plant’s tissues generate heat and warm the spadix
to about 40°C, so that it produces a sweet aroma
that attracts the beetles that pollinate the flower.
Most angiosperm flowers contain both pistils and
stamens. However, some species—including date
palms, willows, and poplars—produce unisexual
flowers that have only stamens (staminate flowers) or pistils (pistillate flowers).
What is the structure of
a flower?
Materials flower, forceps, scalpel, microscope
slide, dropper pipette, coverslips, microscope
Procedure
1. Examine a flower carefully. Make a detailed
drawing of the flower and label as many parts as
you can. Note whether the anthers are above or
below the stigma.
2. Remove an anther and place it on a slide. While
holding the anther with forceps, use the scalpel to
cut one or more thin slices across the anther.
CAUTION: Be careful with sharp tools.
3. Lay the slices flat on the microscope slide and add
a drop of water and a coverslip. Observe the slices
with the microscope at low power. Make a labeled
drawing of your observations.
4. Repeat steps 2 and 3 with the ovary.
Analyze and Conclude
1. Observing Are the anthers in this flower located
above or below the stigma? How could this affect
what happens to the pollen produced by the
anthers? Explain your answer.
2. Applying Concepts What structures did you
identify in the anther? What is the function of these
structures?
3. Applying Concepts What structures did you
identify in the ovary? What is the function of these
structures?
4. Drawing Conclusions Which parts of the flower
will become the seeds? The fruit?
Flowers vary greatly in shape, color, and size, as shown in
Figure 24 –6. A typical flower produces both male and female
gametophytes. In some plants, however, male and female gametophytes are produced in separate flowers on the same individual. Corn, for example, has separate male and female flowers on
the same plant. The tassel is a flower that produces male gametophytes, and the silk is the style of a flower that contains the
female gametophyte. In other cases, many flowers grow together
to form a composite structure that looks like a single flower, as
shown in the sunflower.
What are the male structures in a typical flower? The
female structures?
Ray flowers
Disk flowers
Figure 24–6 Flowers vary enormously in structure. The tulip has
only a single carpel, whereas the
wild rose has many carpels. Some
flowerlike structures are actually
clusters of many individual flowers.
In the sunflower, disk flowers toward
the inside of the cluster are reproductive, whereas ray flowers toward
the outside are nonreproductive and
form what look like petals.
Formulating Hypotheses How
might it be an advantage for a plant
to have many flowers together in a
single structure?
Objective Students will be able to
observe the structures of a flower
and conclude which structures
become seeds and which structures
become the fruit.
Skills Focus Observing, Applying
Concepts, Drawing Conclusions
Materials flower, forceps, scalpel,
microscope slide, dropper pipette,
coverslips, microscope
Time 20 minutes
Strategy Before students cut their
flower, make sure they have noted
whether the anthers are above or
below the stigma.
Analyze and Conclude
1. Self-pollinated flowers typically
have anthers higher than the stigma,
and pollen falls directly from the
anthers onto the stigma. Many crosspollinated plants have taller stigmas
that receive windblown or animalborne pollen from other flowers.
2. Students may be able to observe
mature or immature pollen in the
anthers. The pollen grains form male
gametophytes that can fertilize
female gametophytes and form
zygotes that will grow into new
plants.
3. Students may be able to observe
mature or immature ovules in the
ovary. The ovules produce female
gametophytes that can be fertilized
by male gametophytes.
4. The ovules will become the seeds.
Generally, the ovary becomes the
fruit, although other parts of the
flower may also contribute to fruit
formation.
Sunflower
Answers to . . .
The male structures are
the stamen and anthers; the female
structures are the carpels, ovary, style,
and stigma.
Figure 24 – 6 Many flowers together
in a single structure might attract more
insects, which might improve the
chances of pollination.
Reproduction of Seed Plants
613
24 –1 (continued)
Anther (2N)
Haploid (N)
Pollen grains (N)
(male gametophyte)
Diploid (2N)
Life Cycle of
Angiosperms
Stigma
Use Visuals
Pollen
tubes
Figure 24 –7 Some students might
be confused by the figure. Check
their comprehension by asking:
Where does fertilization take
place? (Inside the ovary) How do
pollen grains reach the ovary? (By
growing pollen tubes down through the
style) After fertilization, how many
sets of chromosomes are contained
within the endosperm? (Three)
How many sets of chromosomes
does the embryo have? (Two) From
which part of the plant does the
seed coat develop? (The outer part
of the ovule)
MEIOSIS
Style
Haploid cell
(N)
Ovary
Ovule
Embryo sac (N)
(female gametophyte)
Ovary (2N)
Sperm
Pollen tube
Mature
sporophyte
Polar nuclei
Address Misconceptions
Students may not fully recognize the
alternation of generations in the
angiosperm life cycle. Review the
definitions of sporophyte and
gametophyte. Then, superimpose the
angiosperm life cycle on the general
plant life cycle in Figure 22–2 on
page 552. Make sure students can
identify the gametophyte stage and
the sporophyte stage in angiosperms.
Emphasize the single mitotic divisions
that the haploid cells of the male and
female gametophytes undergo.
Identify pollen as the spores of the
sporophyte.
Egg cell
Endosperm
(3N)
Embryo
(2N)
FERTILIZATION
Seedling (2N)
(new sporophyte)
Endosperm
Seed coat
왖
Figure 24–7 This illustration
shows the life cycle of a typical
angiosperm—an iris. The developing seeds of a flowering plant are
protected and nourished inside the
ovary, which is located at the base
of the flower.
Reproduction in
angiosperms takes place within
the flower. After pollination, the
seeds of angiosperms develop
inside protective structures.
Fruit
Zygote
(2N)
Life Cycle of Angiosperms
Reproduction in angiosperms takes place within the
flower. Following pollination and fertilization, the seeds
develop inside protective structures. The life cycle of
angiosperms is shown in Figure 24–7. You can think of the
angiosperm life cycle as beginning when the mature sporophyte
produces flowers. Each flower contains anthers and an ovary.
Inside the anthers—the male part of the flower—each cell undergoes meiosis and produces four haploid spore cells. Each of these
cells becomes a single pollen grain. The wall of each pollen grain
thickens, protecting the contents of the pollen grain from dryness
and physical damage when it is released from the anther.
The nucleus of each pollen grain undergoes one mitotic
division to produce two haploid nuclei. The pollen grain, which
is the entire male gametophyte, usually stops growing until it
is released from the anther and deposited on a stigma.
FACTS AND FIGURES
All about angiosperms
There are at least 250,000 known species of
angiosperms. Given their numbers, it is not surprising that they show great variability. For
example, the length of time for completion of the
angiosperm life cycle ranges from less than a
month to as long as 150 years. Pollen tubes also
show great variation. In corn, the pollen tube may
be as long as 50 cm, but in most plants the pollen
tube is much shorter. In addition, a pollen tube
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Chapter 24
may complete its growth in less than 24 hours,
but in some plants it takes over a year. The size of
flowers varies greatly as well. The smallest flowers
are those of the tiny duckweed Wolffia
columbiana. Its flowers are only about 0.1 mm
long. The largest flowers are those of the Rafflesia
plant, which is indigenous to Indonesia. Its huge
blooms can grow to 1 m in diameter and attain a
mass of 9 kg.
The ovary of the flower contains the ovules, in which the
female gametophyte develops. A single diploid cell goes through
meiosis to produce four haploid cells, three of which disintegrate.
The remaining cell undergoes mitosis to produce eight nuclei.
These eight nuclei and the surrounding membrane are called the
embryo sac. The embryo sac, contained within the ovule, is the
female gametophyte of a flowering plant. One of the eight nuclei,
near the base of the gametophyte, is the egg nucleus—the female
gamete. If fertilization takes place, this cell will become the zygote
that grows into a new sporophyte plant. Inside the ovary, the cells
of the growing embryo begin to differentiate. That is, they begin to
specialize, developing from a ball of cells into an embryonic sporophyte. The new sporophyte, or seedling, is shown in Figure 24 –7.
Where does the female gametophyte develop?
Pollination
Once the gametophytes have developed inside the flower, pollination takes place.
Most gymnosperms and some
angiosperms are wind pollinated, whereas most
angiosperms are pollinated by animals. These animals,
mainly insects, birds, and bats, carry pollen from one flower to
another. Because wind pollination is less efficient than
animal pollination, wind-pollinated plants, such as the
oak tree in Figure 24– 8, rely on favorable weather and
sheer numbers to get pollen from one plant to another.
Animal-pollinated plants have a variety of adaptations,
such as bright colors and sweet nectar, to attract animals. Animals have evolved body shapes that enable
them to reach nectar deep within certain flowers.
Insect pollination is beneficial to insects and other
animals because it provides a dependable source of
food—pollen and nectar. Plants also benefit because the
insects take the pollen directly from flower to flower.
Insect pollination is more efficient than wind pollination, giving insect-pollinated plants a greater chance of
reproductive success. Botanists suggest that insect
pollination is the factor largely responsible for the
displacement of gymnosperms by angiosperms during
the past 100 million years.
B
Pollination
Build Science Skills
Figure 24–8
Most angiosperms are pollinated by animals,
although some are pollinated by
wind. The shape of a flower often
indicates how it is pollinated. The
flowers of an oak tree (A) are
typical of wind-pollinated flowers
in that they are small, are not
brightly colored, and produce vast
amounts of pollen. To attract
insects and other animals, many
animal-pollinated flowers are large
and brightly colored. The rose
flower (B) is pollinated by a variety
of insects, whereas the trumpet
creeper flower (C) has a tube shape
that is adapted specifically to the
long beak of a hummingbird.
A
Applying Concepts Have students
imagine that they are botanists who
have just discovered a new plant that
has not yet been identified. They note
that the plant has tiny green flowers
that are difficult to see against the
background of green leaves. Ask:
How do you think this plant is pollinated? (By the wind, because it does
not have large colorful flowers to attract
animal pollinators)
Make Connections
Health Science Point out that
many people are allergic to the
pollen of flowers. Explain that allergies to pollen are actually reactions
to proteins in the coat of the pollen
grain and that people may be allergic
to certain pollens but not to others
because each type has a different
protein coat. One of the most common pollen allergies is the allergic
reaction known as “hay fever.” You
might want to take a poll of students
to see how many have this type of
allergy. Explain that hay fever is not
an allergy to hay but to certain windpollinated plants. Ask: Why might
wind-pollinated plants create more
problems for allergy sufferers than
animal-pollinated plants? (Windpollinated plants usually produce more
pollen because these plants release
their pollen into the air in large
amounts, and it is easily carried all over
by wind.)
C
Answer to . . .
The female gametophyte
develops in the ovules, which are contained in the ovary of the flower.
Reproduction of Seed Plants
615
Fertilization in Angiosperms
24 –1 (continued)
Fertilization in
Angiosperms
Use Visuals
Figure 24 –9 Have students identify
the parts of the corn seed that are
haploid (none), diploid (embryo), and
triploid (endosperm). Review the
process of double fertilization.
Discuss its adaptive advantage.
3 ASSESS
Seed coat
Endosperm
Embryo
Reteach
Review the life cycle of gymnosperms
and angiosperms as students follow
the stages shown in Figure 24–4 and
Figure 24–7.
Seed plants: Dominant stage—
Diploid (2N); Zygote formation—
Two sperm nuclei in a pollen tube
reach a female gametopyhte. One
sperm nucleus disintegrates and the
other fertilizes the egg, forming a
diploid zygote; Occurrence of
meiosis—In gymnosperms, meiosis
occurs in the pollen grains and in
the ovules. In angiosperms, meiosis
occurs in anthers and in ovules.
Chlamydomonas: Dominant stage—
Haploid (N); Zygote formation—
Gametes gather in large groups, and
then – and + gametes form pairs,
which join flagella and shed their cell
walls and fuse, forming a diploid
zygote; Occurrence of meiosis—The
thick-walled zygote divides and produces four flagellated haploid cells.
Cotyledon
Primary root
Evaluate Understanding
Trace Figure 24–5 and give students
a copy without the labels. Then, have
students label the parts of the flower
shown in the diagram.
Embryonic
leaves
왖
Figure 24–9 The endosperm of
a corn seed develops through the
process of double fertilization. After
one sperm nucleus fertilizes the egg
cell, the zygote forms. Then the
other sperm nucleus fuses with the
two polar nuclei to form a triploid
cell, which develops into the
endosperm. Predicting What will
happen to the endosperm when the
seed begins to grow?
If a pollen grain lands on the stigma of an appropriate
flower of the same species, it begins to grow a pollen tube.
The generative nucleus within the pollen grain divides and
forms two sperm nuclei. The pollen tube now contains a
tube nucleus and two sperm nuclei. The pollen tube grows
into the style. There, it eventually reaches the ovary and
enters the ovule.
Inside the embryo sac, two distinct fertilizations take
place. First, one of the sperm nuclei fuses with the egg
nucleus to produce a diploid zygote. The zygote will grow
into the new plant embryo. Second, the other sperm
nucleus does something truly remarkable—it fuses with
two polar nuclei in the embryo sac to form a triploid (3N)
cell. This cell will grow into a food-rich tissue known as
endosperm, which nourishes the seedling as it grows.
As shown in Figure 24–9, a seed of corn, a monocot,
contains a rich supply of endosperm. In many dicots,
including garden beans, the cotyledons absorb the endosperm as the seed develops. The cotyledons then serve as
the stored food supply for the embryo when it begins to
grow.
Because two fertilization events take place between
the male and female gametophytes, this process is known
as double fertilization. Double fertilization may be one
of the reasons why the angiosperms have been so successful. Recall that in gymnosperms, the food reserve built up
in seeds is produced before fertilization takes place. As a
result, if an ovule is not fertilized, those resources are
wasted. In angiosperms, if an ovule is not fertilized, the
endosperm does not form, and food is not wasted by
preparing for a nonexistent zygote.
24 –1 Section Assessment
1.
Key Concept What are
the reproductive structures of
gymnosperms?
2.
Key Concept Describe the
flower and how it is involved in
reproduction.
3.
Key Concept Are
angiosperms typically wind
pollinated or animal pollinated?
How does this process occur?
4. What is endosperm? Where does
it form in a flowering plant?
5. Critical Thinking Inferring
Many flowers have bright patterns of coloration that directly
surround the reproductive
structures. How might this type
of coloration be advantageous
to the plant?
Alternation of Generations
Review the life cycle of the
green alga Chlamydomonas in
Section 20–4. Make a
compare-and-contrast table
comparing alternation of
generations in seed plants and
Chlamydomonas. Include
which stage (haploid or
diploid) of each organism’s life
cycle is dominant, the process
by which zygotes form, and
when meiosis occurs.
24 –1 Section Assessment
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 24–1.
Answer to . . .
Figure 24–9 As the seed develops, the
food stored in the endosperm is absorbed
by the cotyledon and then used by the
growing embryo.
616
Chapter 24
1. The reproductive structures of gymnosperms
are pollen cones, pollen grains, seed cones,
ovules, and pollen tubes.
2. Flowers are reproductive organs that are
composed of four kinds of specialized leaves:
sepals, petals, stamens, and carpels. The stamens produce male gametophytes, and the
carpels produce female gametophytes.
3. Angiosperms are typically pollinated by
animals. Insects, birds, and bats carry pollen
from one flower to another as they gather
nectar.
4. A food-rich tissue that nourishes a seedling as
it grows; inside the embryo sac
5. Bright patterns of coloration might attract
insects and other animals to the reproductive
structures of the flower and increase the
chances of pollination.
Using Technology
to Design Flowers
W
hat’s your favorite flower? Perhaps your
answer was “the rose.” Roses are the world’s
most popular ornamental flowers. They come in
many colors. Chances are you’ve seen red, white,
pink, or even yellow roses. But have you ever seen
a blue rose? Probably not! Roses do not have the
enzymes to produce blue pigments, so even the
best efforts of plant breeders have not produced a
blue rose.
What Makes a Flower?
Botanists have discovered that flower development
is controlled by a series of genes. By manipulating
these genes, scientists have produced plants that
will flower earlier and much faster than normal.
Changing the color of a flower, however, has
proved a little more difficult. Knowing that petunias
often produce blue flowers, in 1991 Australian
researchers isolated the gene for the enzyme that
produces blue pigment. Then, they transferred this
“blue gene” to a rose. To their disappointment, however, the new roses were just as red as ever.
Apparently, flower color is a tricky and unpredictable
business—particularly in roses—that involves complex interactions with other genes and pigments.
Violet Carnations?
When the Australian scientists turned from roses
to carnations, they produced a carnation with
unique violet flowers. Again, they inserted the gene
from the blue petunias into a carnation plant. The
result, shown in the photos, was a deep violet carnation unlike any ever seen in nature. In 1999,
these genetically modified carnation plants were
introduced for sale in Europe and the United
States. A number of biotech companies are hoping
to master the intricacies of color genetics in flowers.
Someday, one of these companies may have what
they’ve all been seeking—a blue rose.
Research and Decide
1. Use library or Internet resources to learn more
about the relationship between genetics and
new flower varieties. Then, choose a common
food crop and find out how breeders have modified the plants to give the crop specific traits.
2. Suppose you are a plant geneticist and you
want to create a new color of lily. Decide which
flower color you would like to produce. Then,
write down the scientific steps that you would
take to produce the new flower color.
For: Links from the authors
Visit: PHSchool.com
Web Code: cbe-7241
After students have read this feature,
you might want to discuss one or
more of the following:
• Genetic engineering methods, such
as the use of restriction enzymes
and DNA insertion, that are used to
isolate a gene from one plant or
insert it into another plant
• Other traits of flowers, besides
color, that breeders might try to
modify, such as season of bloom,
length of bloom period, size of
flowers, number of petals, and
number of flowers
• Examples of plants produced by
breeders that have unusual characteristics, such as plants that have
black or nearly black flowers (for
example, columbine, hollyhock,
viola, and sweet pea)
• Reasons why varieties of plants
bred for certain traits, such as color,
may not be as hardy as the standard
varieties
Research and Decide
1. Students should choose any common food crop that has been
genetically modified, such as tomatoes, potatoes, squash, or corn. They
should describe how the plants have
been modified to exhibit certain
traits.
2. Students should select any color
they would like to produce in a lily
plant, such as yellow, pink, red, or
orange. The steps they would take
might include first isolating a gene
from another plant that codes for the
color of their choice and then inserting the gene into a lily plant.
Students can research genetically
modified plants on the site developed by authors Ken Miller and
Joe Levine.
Reproduction of Seed Plants
617