Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
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Companies, 2008
C HAPTER OUTLIN E
The Flower 73
Floral Organs 73
A CLOSER LOOK 5.1 Mad About
Tulips 74
Modified Flowers 75
Meiosis 77
Stages of Meiosis 78
A CLOSER LOOK 5.2 Pollen is
More Than Something to Sneeze
At 80
Meiosis in Flowering Plants 80
Male Gametophyte Development 80
Female Gametophyte Development 81
Pollination and Fertilization 83
Animal Pollination 83
A CLOSER LOOK 5.3 Alluring
Scents 84
Wind Pollination 84
Double Fertilization 86
Chapter Summary 86
Review Questions 86
Further Reading 87
KEY C ON C EPTS
1.
2.
3.
C H A P T E R
4.
5
Plant Life Cycle: Flowers
72
The inflorescence of Zantedeschia aethiopica (arum lily) is a spadix
(fleshy spike) surrounded by a large bract, called a spathe.
Angiosperms are unique among
plants in that they have their sexual
reproductive structures contained in a
flower.
Meiosis is a type of cell division that
reduces the number of chromosomes
from the diploid to the haploid number
and is an integral part of sexual
reproduction.
Pollination is the transfer of pollen from
the anther to the stigma and largely
occurs through the action of wind or
animals.
In angiosperms, reproduction is
accomplished through the process of
double fertilization.
Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
© The McGraw−Hill
Companies, 2008
CHAPTER 5
T
he natural beauty of flowers has always been a
source of inspiration, and the appearance of the first
flower of spring lightens the heart of anyone weary
of winter. (See A Closer Look 5.1—Mad about Tulips.) But
what role do flowers play in the lives of plants? Their beauty
notwithstanding, flowers play a pivotal role in the life cycle
of angiosperms since they are the sites of sexual reproduction. The events leading to flowering are very complex and
may include internal factors such as plant hormones (see
A Closer Look 6.1—The Influence of Hormones on Plant
Reproductive Cycles) and biological clocks (internal rhythms
that regulate the timing of biological functions) as well as
external factors such as temperature and day length. The
interconnection between these internal and external features
allows plants to coordinate their reproduction with the environment. This chapter will emphasize the reproductive role
of flowers once they have developed.
THE FLOWER
Flowers, unique to angiosperms, are essentially modified
branches bearing four sets of specialized appendages or
floral organs. These appendages are grouped in whorls and
consist of sepals, petals, stamens, and carpels. They are
inserted into the receptacle, the expanded top of the pedicel
(peduncle), or flower stalk (fig. 5.1).
73
Plant Life Cycle: Flowers
Floral Organs
The outermost whorl consists of the sepals, leafy structures
that cover the unopened flower bud; they are usually green
and photosynthetic. The whole whorl of sepals of a single
flower is called the calyx. The petals that make up the next
whorl of flower parts are collectively called the corolla.
Often brightly colored and conspicuous, the petals function
by attracting animal pollinators. Together, the calyx and
corolla constitute the perianth.
In the center of the flower, the male and female structures can be found. The androecium, the whorl of male
structures, is composed of stamens, each of which consists of
a pollenproducing anther supported on a stalk, the filament.
Each anther houses four chambers where pollen develops.
The pollen chambers can be seen in the cross section of the
anther, Figure 5.1. The gynoecium is the collective term for
the female structures, or carpels, which are located in the
middle of the flower. Flowers can have one to many carpels.
(The old term “pistil,” which referred to one or more carpels,
will not be used in this book.) A gynoecium with just one
carpel is illustrated in Figure 5.2a. If many carpels are present, they may either be fused together (fig. 5.2b) or remain
separate. Carpels, whether individual or fused, consist of
a stigma, style, and ovary (fig. 5.1). Contained within the
basal ovary are one to many ovules (structures that will
eventually become seeds); rising from the top of the ovary
Stigma
Petal
Pollen chamber
Pollen
Anther
Style
Stamen
Carpel
Sepal
Ovary
Pedicel
Ovule
Filament
Stamen
Carpel
Figure 5.1 Flower structure.
Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
© The McGraw−Hill
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A CLOSER LOOK 5.1
Mad About Tulips
The flower, the crowning characteristic of the angiosperms,
have a narrow elongated perianth with the flower resembling
technically is a modified branch bearing specialized leaves
a star. The familiar tulips of horticulture have rounded broad
that are integral to sexual reproduction. But flowers have
sepals and petals with a bowl-shaped flower. The tulip can
meaning beyond this technical definition. Few of us could
produce seeds, but it takes 7 years before a tulip grown
imagine or want a world without flowers. We are attracted
from seed will flower. A quicker way is to grow the tulips
to them because of their beauty of color, form, and frafrom bulbs. A bulb, such as the familiar onion, is actually an
grance. Perhaps we appreciate them
underground stem with fleshy storage
all the more because their beauty is
leaves. As the bulb grows, two to four
To see a World in a Grain of Sand,
so delicate and ephemeral. They have
new bulbs develop within the layers, or
And a Heaven in a Wild Flower,
been praised in song and poem and
skirts, of the original bulb. The bulbs can
Hold Infinity in the palm of your hand,
have decorated our homes and persons.
be divided and planted and will flower
And Eternity in an hour.
Certain flowers are so revered that they
within the few months of a single grow—William Blake (1757–1827),
become representations of human emoing season. In addition, since propagation
“Auguries of Innocence”
tions or nations. Yet, in a bizarre epiby bulbs is a method of asexual reprosode in history, the desire for a beautiful
duction, the integrity of the flower, its
flower created a frenzy that brought a nation and its people
form and color, remains true to the parent plant whereas
to economic ruin.
tulip flowers grown from seeds, as the products of sexual
The story begins in 1554 in the Ottoman Court of
reproduction, may be quite variable.
Suleiman the Magnificent in Constantinople. Some years
In 1593, Carolus Clusius, who had been the director of
earlier, the Turks had been the first to bring into cultivation
the Imperial Medicinal Garden in Vienna, was persuaded to
a wild flower whose beauty had captivated the court and
come to the new University of Leiden, in the Netherlands,
all who saw it. Ogier Ghislain de Busbecq, the ambassador
and establish a physick, or medicinal, garden. Naturally, he
for the Austrian-Hapsburg Empire, was so entranced by the
brought many exotic plants with him, including his collection
floral beauties that he had some of the flowers sent back to
of tulips. Although the tulip was known in Holland, it was still
Vienna. The Turks called the flower lale, but Busbecq called
a rarity and as such a status symbol for the wealthy. Clusius
them tulipam, a corruption of dulban, meaning turban. The
was loath to part with any of his bulbs. Under the darkness
Ottoman court favored tulips with elongated pointed petof night, thieves broke into the garden and stole most of his
als and, through selected crosses, had achieved this effect.
tulip collection. The thieves lost no time in propagating and
Perhaps Busbecq thought the form of the flower resembled a
selling the tulips.
turban. In any case, the first tulips arrived in Europe in 1554.
As tulips became more available, their popularity spread
Wild tulips (Tulipa spp.) originated in Central Asia and
across Europe but especially in Holland. Part of the fascinathe Caucasus Mountains. Tulips are monocots belonging to
tion was due to a phenomenon called breaking. A small numthe lily family, the Liliaceae. There are about 120 species of
ber of tulips, perhaps just one or two in 100, were prone to
tulip, and typically a single flower is borne on a stalk. Most
spontaneous, unpredictable, eruptions of color, with stripes
tulips have three sepals and three petals apiece, all similarly
or feathers of bold color against a contrasting background.
vibrantly colored, six stamens, and a central gynoecium of
Today, the variety of tulips with contrasting perianth colors
three carpels. The shape of flower and petals varies; some
are called Rembrandts because the Dutch painter painted
is a slender column called the style. The expanded tip of
the style is the stigma, which functions in receiving pollen.
Flowers containing all four floral appendages are known as
complete flowers.
Although flowers have been described in terms of only
four floral appendages, some flowers may have additional
floral structures called bracts, which are found outside the
calyx. Bracts may appear leaflike or petal-like and be of various sizes. The showy red “petals” of poinsettia are actually
bracts.
74
In Chapter 3, the vegetative differences between monocots and dicots were described; there are also easily recognizable differences in the floral structures (fig. 5.3). Monocots
generally have their floral parts in threes or multiples of
three; for example, lilies have three sepals, three petals, six
stamens, and a three-part ovary (formed from the fusion of
three carpels). On the other hand, dicots generally have a
numerical plan of four or five or multiples; a wild geranium
flower contains five sepals, five petals, 10 stamens, and five
fused carpels with separate stigmas.
Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
some of the most famous tulips of the time. We now know
that the breaks are caused by the tulip breaking virus spread
by the peach-potato aphid (Myzus persicae), an insect that
sucks the sap of plants. Since peach trees were practically a
staple in every Dutch garden, there was a ready source of
aphids to spread the disease.
The color of tulips is due to the presence of two types of
pigment. The base color of all tulips is either white or yellow. On top of this there are anthocyanin pigments of reds
and blues. The base and anthocyanin pigments combine to
produce the final color. The tulip breaking virus causes the
suppression of the anthocyanins in infected cells of the perianth; in these cells, the base color breaks through in striking
contrast with the more vibrant color of the uninfected cells.
Although the flower palette may be spectacular, the broken
tulips are in fact diseased. The virus eventually weakens the
bulb, and fewer and fewer offshoots are produced with each
generation. Thus, the very nature of breaking ensures rarity.
Since the cause of breaking at the time was unknown and
appeared to be unpredictable, many superstitious practices
arose in a vain attempt to encourage breaking. One suggested sprinkling pigments on a tulip field. When it rained,
the pigments would dissolve and be absorbed by the roots.
When the pigments were transported to the flower, its color
would be transformed!
Tulips that had a particularly pleasing color pattern with
symmetrical breaks were the most desired. One of the most
famous broken tulips was Semper Augustus (box fig. 5.1),
with its carmine red feathers against a white background.
A single bulb commanded a price of what would be today
$4,600!
By 1634, tulips were no longer being bought to grow in
a personal garden but for resale at a profit. Tulipomania had
begun. People sold their homes and possessions to invest in
the tulip trade for a sure profit. Tulip clubs were formed to
buy and sell the hottest properties. The Dutch government
established a Tulip Notary that dealt exclusively with the
tulip trade. At first, sales took place between the end of the
growing season in June and September when the bulbs were
ready for replanting. Later, the sales took place throughout
the year because people were buying and selling ownership
to future bulbs. They bought and sold papers for bulbs that
never left the ground.
On February 2, 1637, there were the first signs that
the tulip business was souring. Tulips were up for sale but
Modified Flowers
The basic pattern of flower structure is often modified. In
flowers like tulips and lilies, the sepals are brightly colored
and identical to the petals. In such flowers, the petals and
sepals are often referred to as tepals. In contrast, flowers of
the grasses possess neither sepals nor petals; these flowers are
incomplete (fig. 5.4a). In fact, flowers lacking any of the four
floral structures are known as incomplete flowers. Flowers
with both stamens and carpels are called perfect flowers
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Box Figure 5.1 Semper Augustus tulip, the most expensive
tulip sold during tulipomania. Anonymous Dutch Artist,
seventeenth century, Norton Simon Art Foundation.
could no longer command the expected price. Sellers got
worried and tried to sell their bulbs at lower and lower
prices. Eventually, confidence in the market dropped when
there were more sellers than buyers, and the prices of bulbs
plummeted. The madness for tulips that had inspired wild
speculation ended in an economic crisis, but the tulip and the
Netherlands are forever linked.
even if sepals or petals are lacking. Some flowers, such as
squash and holly, are unisexual; they are either staminate or
carpellate. Incomplete flowers lacking either stamens or carpels are imperfect. A single plant may have both staminate
and carpellate flowers; this plant is said to be monoecious.
Alternatively, dioecious plants have only unisexual flowers
on a single individual. Corn, squash, and pecans are familiar
monoecious species (see fig. 5.10), while spinach, date palms,
and some hollies are dioecious.
75
Levetin−McMahon: Plants
and Society, Fifth Edition
76
UNIT II
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
© The McGraw−Hill
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Introduction to Plant Life: Botanical Principles
Ovule
Carpel
(b)
(a)
Figure 5.2 Examples of gynoecia. (a) Gynoecium composed of a single carpel. (b) Gynoecium composed of three carpels.
Bracts
Grass
(a) Incomplete flower
(a)
Stamen
Petal
Ovary
Sepal
Hypogynous
Perigynous
Epigynous
(b) Position of ovary and floral parts
Regular flower
Irregular flower
(c) Symmetry
(b)
Figure 5.3 Monocot and dicot flowers. (a) Coast rose gentian,
Sabatia arenicola, illustrates a dicot flower with flower parts in
multiples of five. (b) Michigan lily, Lilium michiganense, a monocot,
shows flower parts in multiples of three.
Figure 5.4 Modifications of the basic floral design result in
diverse flower types. (a) Incomplete flowers lack one or more of
the four floral organs. Grass flowers lack both sepals and petals.
(b) Various positions of the floral whorls in relation to the ovary
are possible. (c) Regular flowers can be bisected along many
planes, but irregular flowers can be bisected along only one.
Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
© The McGraw−Hill
Companies, 2008
CHAPTER 5
One feature that is important in the classification of
flowers is the position of the ovary in relation to the other
floral parts. If the sepals, petals, and stamens are inserted
beneath the ovary, this arrangement is referred to as a superior ovary. The ovary is inferior if the sepals, petals, and
stamens are inserted above it. Corresponding terms that refer
to these arrangements are hypogynous, epigynous, and
perigynous. Hypogynous (below the gynoecium) flowers
have flower parts inserted beneath a superior ovary; epigynous (on the gynoecium) flowers have flower parts inserted
above an inferior ovary; and perigynous (around the gynoecium) flowers have the bases of the flower parts fused into a
cuplike structure surrounding a superior ovary (fig. 5.4b).
Flowers can also be described by their pattern of symmetry. Regular flowers (actinomorphic) such as the tulip, lily,
rose, and daffodil display radial symmetry; they can be dissected into mirror-image halves along many lines. Irregular
flowers (zygomorphic) such as the orchid, iris, snapdragon,
and pea display bilateral symmetry. They can be dissected
into mirror-image halves along only one line (fig. 5.4c).
Some flowers are borne singly on a stalk, but in many
cases, flowers are grouped in clusters called inflorescences.
Sometimes what is commonly called a single flower is
actually an inflorescence, as in the case of sunflowers, daisies, and chrysanthemums. The dogwood flower is also an
Plant Life Cycle: Flowers
inflorescence, but here the pink or white “petals” are bracts
surrounding a cluster of small flowers. The arrangement of
flowers in the cluster determines the type of inflorescence,
with many patterns possible: spike, raceme, panicle, umbel,
head, and catkin (fig. 5.5). Often the type of infloresence is
an important characteristic in classification.
To understand the flower’s role in sexual reproduction,
it is necessary to learn about a special form of cell division,
meiosis, that occurs within stamens and carpels.
MEIOSIS
Sexual reproduction, whether in a plant or an animal, is
basically the fusion of male and female gametes, sperm
and egg, to produce a zygote, which will develop into a
new individual. When the egg is fertilized by the sperm,
the zygote receives an equal number of chromosomes from
each gamete. Gametes are different from most other cells in
angiosperms because they are haploid (containing only one
set of chromosomes) whereas other body cells are diploid
(containing two complete sets of chromosomes). During
the process of fertilization, the diploid number is restored
in the zygote. When the chromosomes in a diploid cell are
examined microscopically, it can be seen that there are two
Pedicels
(a)
(b)
(c)
Peduncle
(d)
(e)
77
(f )
(g)
Figure 5.5 Inflorescence types: (a) spike, (b) raceme, (c) panicle, (d) umbel, (e) compound umbel, (f) head, (g) catkin, which is a
unisexual inflorescence.
Levetin−McMahon: Plants
and Society, Fifth Edition
78
UNIT II
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
Introduction to Plant Life: Botanical Principles
of each kind of chromosome. These pairs of chromosomes
are known as homologous chromosomes; the members of a
pair are derived from the contributing haploid gametes. The
homologous chromosomes not only look alike, but they also
carry genes for the same traits.
Meiosis has a major role in all sexually reproducing
organisms because it is the process that reduces the number
of chromosomes from the diploid to the haploid number. This
reduction compensates for the doubling that occurs during
fertilization. Without meiosis, the number of chromosomes
would double with each generation.
In animals, gametes are produced directly by meiosis;
however, in plants the products of meiosis are haploid spores.
Spores are reproductive units formed in a sporangium. The
diploid plant that undergoes meiosis to form these spores is
known as a sporophyte. Spores develop into haploid gametophytes that produce gametes, egg or sperm (fig. 5.6). The
sporophyte and gametophyte are the two stages in the life
cycle of each plant. (Later in this chapter the gametophytes of
flowering plants will be studied.) The process of fertilization
brings together egg and sperm to produce a genetically unique
zygote. Sexual reproduction in this way introduces variation
into a population whereas offspring produced by asexual
reproduction are genetic clones.
Ovary
Sporophyte
Diploid (2N)
Meiosis
Haploid (N)
Microspore
Sperm
Male gametophyte
Female
gametophyte
Meiosis is a specialized form of cell division that consists of
two consecutive divisions and results in the formation of four
haploid cells (fig. 5.7). Both the first and second divisions of
meiosis are divided into four stages: prophase, metaphase,
anaphase, and telophase. Recall that these are the same names
used for the stages of mitosis.
During the first meiotic division, the chromosome number is halved; in fact, it is often called the reduction division.
The most significant events occur during prophase I.
Prophase I
At the beginning of prophase I, the chromosomes appear
threadlike. As in mitosis, the DNA is duplicated during the S
phase of the preceding interphase, so that each chromosome
actually consists of two chromatids. As the chromosomes
continue to condense and coil, the homologous chromosomes
pair up gene for gene in a process called synapsis. Since each
chromosome is doubled, the synapsed homologous chromosomes actually consist of four chromatids. As the synapsed
chromosomes continue to condense, breaks and exchanges
of genetic material can occur between the chromatids in an
event called crossing over. This results in chromatids that are
complete but have new genetic combinations. Soon synapsis
starts to break down, and the homologous chromosomes repel
each other; however, they are held together at points where
crossing over occurred. These places are referred to as chiasmata (sing., chiasma). While these chromosome events are
occurring, the nucleolus and nuclear membrane break down,
leaving the chromosomes free in the cytoplasm. Prophase I is
the longest and most complex stage of meiosis (fig. 5.7 and
fig. 7.6).
During the next stage, metaphase I (fig. 5.7), the homologous
chromosome pairs line up at the equatorial plane (across the
center of the cell). Spindle fibers that actually begin to appear
in late prophase attach to the centromeres of each homologous
pair. Two types of spindle fibers occur those that run from
pole to pole and those that run from one pole to a centromere.
Recall that spindle fibers are composed of microtubules.
Zygote
Egg
Stages of Meiosis
Metaphase I
Anther
Fertilization
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Megaspore
Figure 5.6 Alternation of diploid (sporophyte) and haploid
(gametophyte) generations in a flowering plant.
Anaphase I
Homologous chromosomes separate during anaphase I
(fig. 5.7); they are pulled by the spindle fibers to opposite
poles of the cell. During metaphase I, the orientation of the
homologous chromosome pairs occurred by chance. As a
result, chromosomes from each parent are mixed randomly
into a number of possible combinations during this separation process. In contrast to the events in mitotic anaphase,
in meiotic anaphase the chromatids of each chromosome are
still united; it is only the homologous pairs that are separating. By the end of this stage, the chromosome number has
been halved.
Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
© The McGraw−Hill
Companies, 2008
CHAPTER 5
Plant Life Cycle: Flowers
79
Interphase
Meiosis I
Early Prophase I
Late Prophase I
Early Metaphase I
Anaphase I
Telophase II
Anaphase II
Metaphase II
Telophase I
Prophase II
Meiosis II
Figure 5.7 Stages of meiosis.
Telophase I
Telophase I (fig. 5.7) is similar to telophase of mitosis in
that the spindle disappears, the chromosomes become less
distinct, and the nuclear membrane may reform. Cytokinesis
generally follows, dividing the cell into two daughter cells,
each with half the number of chromosomes of the original
parent cell.
Second Meiotic Division
In some organisms, an interphase occurs between the two
meiotic divisions; in other cases the cells proceed directly
from telophase I to prophase II. The second meiotic division is essentially similar to mitosis; the chromatids, which
are still joined together, finally separate. Prophase II is
identical to mitotic prophase; in each cell the chromosomes
become evident and the nuclear membrane breaks down
(fig. 5.7). During metaphase II, the chromosomes line up at
the equatorial plane of each cell and spindles appear, with
the spindle fibers stretching from pole to pole and pole to
centromere. During anaphase II, the chromatids separate,
pulled to the poles by the spindle fibers. Cytokinesis occurs
in telophase II, and the nuclear membranes and nucleoli
reappear as the single-stranded chromosomes become
threadlike chromatin. By the end of telophase II, four haploid cells are produced. Because of crossing over and the
random associations of parental chromosomes, the four
cells contain unique genetic combinations that differ from
Levetin−McMahon: Plants
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II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
© The McGraw−Hill
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A CLOSER LOOK 5.2
Pollen Is More Than Something to Sneeze At
The essential role that pollen plays in the life cycle of seed
plants is well documented. Less well known is the significance
that palynology (the study of pollen) has had in many diverse
fields: petroleum geology, archeology, criminology, anthropology, aerobiology, and the study of allergy.
When pollen is released by wind-pollinated plants, only
a tiny percentage reaches the stigma. At the proper season,
pollen is so abundant that clouds of it can be seen emanating
from vegetation disturbed by wind or shaking (box fig. 5.2a).
Most of it is carried by the wind and eventually settles back
(a)
to the ground. It is this excess pollen that is the focus of
study.
The distinctive ornamentation on the outer wall of a
pollen grain allows for the identification of most types
of pollen, sometimes even to the species level (box
fig. 5.2b). Under certain conditions, pollen can be preserved, leaving a record of area vegetation. Some fossil
pollen dates back over 200 million years and has revealed
information about the changing vegetation patterns over
evolutionary time.
(b)
Box Figure 5.2a A cloud of pollen can be seen wafting from
Box Figure 5.2b The pollen grain shown here has netlike
the pollen cones when a cedar branch is disturbed.
ridges in the exine.
each other and the parent cell from which they originated.
This result contrasts with the process of mitosis, in which
the two daughter cells are genetically identical to the parent
cell (see Chapter 2, Cell Division).
microspore mother cells; in fact, the pollen chambers
are technically referred to as microsporangia (sing.,
microsporangium). Each microspore mother cell undergoes meiosis to produce four microspores (male spores).
Initially, the four spores stay together as a tetrad, but
eventually they separate and each will develop into a pollen
grain, an immature male gametophyte. In the development
of the pollen grain, the microspore undergoes a mitotic
division to produce two cells, a small generative cell and
a large vegetative cell (or tube cell). Also, the wall of the
microspore becomes chemically and structurally modified
into the pollen wall. The pollen wall consists of an inner
layer, the intine, and an outer layer, the exine, which may
be ornamented with spines, ridges, or pores. When the
Meiosis in Flowering Plants
Within the flower, meiosis occurs during the formation of
pollen in the stamen and the formation of ovules within the
carpel (fig. 5.8).
Male Gametophyte Development
During the development of the stamen, certain cells in
the pollen chambers of the anther become distinct as
80
Levetin−McMahon: Plants
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II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
Palynology is essential to the petroleum industry; an
examination of fossil pollen from core samples can determine if an area is likely to be a rich source of oil. Certain fossil species in a particular region are known to be associated
with oil deposits; palynologists look for the pollen of these
indicator species in core samples.
Archeologists have sought the help of palynologists in
determining when agriculture originated in certain areas and
what plants were consumed by ancient peoples. Examination
of the fossil pollen can pinpoint the shift from gathering native
vegetation to cultivating cereal grains. Pollen residues found
in storage vessels or coprolites (fossilized feces) give direct
evidence of the diet of prehistoric groups; for example, the
Viking recipe for mead was determined by examining pollen
scrapings in drinking horns.
Pollen has proved instrumental in solving many criminal
cases. The scene of a crime or the whereabouts of a suspect
at the time of the crime can often be determined by analyzing pollen clinging to the victim’s body or to the shoes and
clothing of a suspect.
Anthropologists have learned that pollen has symbolic
meaning to several Native American tribes in the Southwest.
Among the Navajos, pollen is revered as a symbol of life
and fulfillment; it is used in sacred ceremonies and chants
throughout the stages of life from birth to death (box
fig. 5.2c). The mystique of pollen has even been adopted by
current health food fadists who claim that bee pollen (pollen
collected by bees) is a power food that cures ailments, prevents disease, and promotes fitness. A few athletes take daily
bee pollen supplements to maintain a winning edge, but most
nutritionists discount these claims and even express concern
about allergic reactions.
Airborne pollen is well known to trigger hay fever,
asthma, and other allergic reactions in sensitized individuals.
Despite its name, hay fever is not caused by hay but is due to
pollen from inconspicuous flowers of wind-pollinated trees,
grasses, and weeds. This pollen is responsible for the misery
of at least the 15% of the U.S. population identified as allergy
sufferers. Aerobiologists study airborne pollen to document
the species responsible and the factors influencing pollen
pollen grains are fully developed, they are released as the
anthers open, or dehisce. (See A Closer Look 5.2— Pollen
Is More Than Something to Sneeze At for further discussion on pollen.)
Female Gametophyte Development
Within the ovary one or more ovules develop; an ovule consists of a megasporangium enveloped by one or two layers
of tissue called integuments. The integuments completely
surround the megasporangium except for an opening called
the micropyle. The ovule first appears as a bulge in the ovary
wall. During the development of the ovule, one cell becomes
distinct as a megaspore mother cell; it is surrounded by
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(c)
Box Figure 5.2c Pollen has symbolic meaning to several
Native American tribes. This painting by Harrison Begay illustrates
a Navajo women gathering corn pollen. (“Navajo woman and
child gathering corn pollen,” Harrison Begay, 0237.48 from the
Collection of the Gilcrease Museum, Tulsa.)
abundance and distribution. Their findings suggest that there
should be greater care in the selection of landscaping plants
so that hay fever–causing plants are avoided. Allergies and
their causes will be discussed further in Chapter 21.
tissue called the nucellus. The megaspore mother cell
undergoes meiosis to produce four megaspores; generally
three of these degenerate, leaving one surviving megaspore.
This megaspore then undergoes a series of mitotic divisions, eventually producing a mature female gametophyte,
which is often called the embryo sac. In the typical pattern
of development, a series of three mitotic divisions produces
eight nuclei within the greatly enlarged megaspore. These
eight nuclei are distributed with three (the egg apparatus)
near the micropyle end of the ovule, three antipodals at the
opposite end, and two polar nuclei in the center. The egg
apparatus consists of two synergids and one egg. Cell walls
soon develop around the egg, synergids, and antipodals; at
this stage the female gametophyte is mature (fig. 5.8).
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82
UNIT II
II. Introduction to Plant
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5. Plant Life Cycle: Flowers
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Introduction to Plant Life: Botanical Principles
Flower
Stigma
Sporophyte
Style
Ovary
Ovule
Meiosis
Megaspore
mother cell
Young
sporophyte
Four megaspores
Nucleus
Germination
Microspore
mother cells
Anther
Micropyle
Fruit
Seed coat
Seed
Integuments
Pollen chamber
with microspores
Ovary develops
into fruit: ovule
develops into seed
Three megaspores
degenerate
2N
Zygote develops
into embryo
N
Tetrad
Each microspore matures
into a pollen grain
Embryo
Endosperm forms when
the two polar nuclei
and one sperm unite
On the stigma, the
pollen germinates and
produces two sperm
Pollen grain
Vegetative cell
nucleus
Endosperm (3n)
Pollen tube
Zygote
Sperm
Fertilization
Generative cell
nucleus
Antipodals
Polar nuclei
Polar nuclei
Egg
Egg
Synergids
Pollination
Micropyle
Figure 5.8 Reproductive cycle of an angiosperm.
The eight nuclei are
produced by three
successive mitotic
divisions of the
megaspore nucleus.
They become rearranged
in what is now called the
embryo sac or female
gametophyte
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5. Plant Life Cycle: Flowers
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CHAPTER 5
POLLINATION AND FERTILIZATION
Pollination involves the transfer of pollen from the anther
to the stigma. Pollen transfer within the same individual
plant is known as self-pollination. Cross-pollination
involves the transfer of pollen from one plant to another.
It is obvious that cross-pollination prevents the potentially detrimental effects of inbreeding, and most perfect
flowers have physiological mechanisms to prevent selffertilization. Pollination can be accomplished by various
methods. Large showy flowers usually attract animal pollinators whereas small inconspicuous flowers are often
wind pollinated.
Animal Pollination
Although bees are the most familiar animal pollinators, a
host of species are involved in the transfer of pollen. Other
insects such as wasps, flies, ants, butterflies (fig. 5.9a),
and moths are equally important pollinators for many
flowers. Even larger animals, such as birds (fig. 5.9b) and
83
bats, are efficient pollinators for some species. Pollination
is accomplished inadvertently when the animal visitor,
dusted with pollen from one flower, visits a second flower
of the same species.
Color and scent are what attract animals to flowers.
Certain colors are associated with specific pollinators; for
example, bee-pollinated flowers are often yellow, blue,
purple, or some combination of those colors. Many birdpollinated flowers, such as columbine and trumpet creeper,
are red. In addition, many white or light-colored flowers are pollinated in the evening by night-flying visitors.
Various contrasting color patterns (nectar guides) seen
on petals serve to direct insects toward the nectar. Often,
nectar guides cannot be seen by the human eye but are
visible in ultraviolet light, which can be perceived by
certain insects. To the insect eye, the nectar guides seem
like airport lights lining a runway. Essential oils, volatile
oils that impart a fragrance, attract pollinators by scent.
The essential oils of flowers such as the rose, orange, and
jasmine have been used in perfumes for hundreds of years
(see A Closer Look 5.3—Alluring Scents). Not all scents
(a)
Figure 5.9 Flowers and their animal pollinators. (a) Butterflypollinated flowers have a broad expanse for the butterfly to land.
(b) Hummingbird-pollinated flowers are often tubular, allowing the
bird to insert its beak to reach the nectar.
Plant Life Cycle: Flowers
(b)
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5. Plant Life Cycle: Flowers
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A CLOSER LOOK 5.3
Alluring Scents
Since earliest times the fragrances of certain plants, owing to
their essential oils, have been valued as a source of perfumes.
It is difficult to pinpoint when people first began using plant
fragrances to scent their bodies, but by 5,000 years ago the
Egyptians were skilled perfumers, producing fragrant oils that
were used by both men and women to anoint their hair and
bodies. Fragrances were also used as incense to fumigate
homes and temples in the belief that these aromas could
ward off evil and disease. In fact, our very word perfume
comes from the Latin per meaning through and fumus meaning smoke, possibly referring to an early use of perfumes as
incense.
Today most perfumes are a mixture of several hundred
scents that are carefully blended, using formulas that are
highly guarded secrets (table 5.A). Many of these scents are
now synthetics that resemble the natural essences from
plants, but many costly perfumes still rely on the natural
essential oils extracted directly from plants.
Various methods are used to extract the essential oils
from plant organs including distillation, solvent extraction,
expression, and enfleurage. The method used depends to
a large extent on the location and chemical properties
are appealing to humans; for example, the carrion flower
(Stapelia sp.), which is fly-pollinated, gives off an aroma
of rotting meat.
In most cases, the flower provides a reward of nectar,
pollen, or both to the animal. Nectar is a sugary liquid produced in glands called nectaries found in the epidermis of a
floral organ. Many children have tasted its sweetness when
they sipped the nectar of honeysuckle (Lonicera japonica)
blossoms. The amount of nectar produced by flowers varies
greatly; flowers pollinated by birds generally produce copious
amounts of nectar.
Flowering plants and their animal pollinators are a
classic example of coevolution. Coevolution is a case of
reciprocal adaptations as two interacting species modify and
adjust to each other over time. Adaptations occur that make
a flower more attractive to a specific type of pollinator, thus
ensuring a greater chance of a successful pollination. The
pollinator, in turn, changes in ways that enhance its efficiency in exploiting the nutritional rewards offered by the
flower. As the evolutionist Charles Darwin (see Chapter 8)
observed,
84
Table 5.A Commonly Used Plant
Materials for Essential Oil Extraction in
the Perfume Industry
Plant Organ
Source
Flowers
Roses, carnations, orange blossoms,
ylang-ylang, violets, lavender
Leaves and stems
Mints, rosemary, geranium, citronella,
lemon grass
Seeds and fruits
Oranges, lemons, nutmeg
Roots
Sassafras
Rhizomes
Ginger
Bark
Cinnamon, cassia
Wood
Cedar, sandalwood, pine
Gums
Balsam, myrrh
of the essential oil. Distillation, one of the most common
methods used, employs exposing the tissue to boiling water
or steam, thereby volatilizing the essential oil, which can
Thus, I can understand how a flower and a bee
might slowly become either simultaneously or one
after the other modified and adapted in the most
perfect manner to each other.
Concept Quiz
The coevolution of flowers and animal pollinators is one of
the marvels of nature and can greatly enhance the rate of
successful pollinations. Few flowers, however, are so specialized that they can be pollinated by only one type of animal.
Why would extreme specialization between pollinator and flower
be at once both an advantage and a drawback?
Wind Pollination
As described, animal-pollinated flowers have a variety of
mechanisms used to attract the pollinator; wind-pollinated
Levetin−McMahon: Plants
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II. Introduction to Plant
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5. Plant Life Cycle: Flowers
then be separated from the condensate (box fig. 5.3). Since
this method employs heat, only the most stable essential
oils can be extracted by distillation. For solvent extraction,
plant material is immersed in an organic solvent at room
temperature; later, the essential oil is recovered from
the solution. At no time is heat used, thus avoiding damage to temperature-sensitive essential oils. Expression is
the simplest method and is mainly used to express the oil
from citrus rinds with mechanical pressure. In enfleurage,
flower petals are layered on trays containing cold fat that
absorbs the essential oils from the blossoms. The petals are
continually replaced until the fat is saturated with the floral
essence. The essential oil is then extracted from the fat
with alcohol. Enfleurage is slow and labor intensive and is
used to extract only those delicate essential oils that would
be destroyed by other methods. Regardless of the method
used, tremendous quantities of plant material are needed to
produce even small quantities of the pure essential oil; for
example 60,000 roses are needed for 1 ounce (28 grams)
of rose oil.
Once the essential oils have been extracted and blended
for the characteristic fragrance of a particular perfume, fixatives are added to retard the evaporation of highly volatile
essential oils. The fixatives may be plant or animal oils, such
as musk oil from the musk deer. Today, however, most animal oils have been replaced by synthetics.
The final perfume concentrate is then diluted with alcohol
and a small amount of distilled water: perfumes generally contain 18%–25% concentrate; eau de parfum contains 10%–15%;
eau de cologne, 5%–8%; and eau de toilette, 2%–4%.
flowers, on the other hand, have a much simpler structure.
Color, nectar, and fragrance, which play an integral role in
animal pollination, are usually not prominent in the windpollinated flower.
Wind-pollinated flowers are often small and inconspicuous, usually lacking petals and sometimes even sepals; these
drab flowers are frequently arranged in inflorescences such
as the catkins of oak, pecan (fig. 5.10), and willow and the
panicles, racemes, and spikes of the grasses. Although most
grasses have perfect flowers, many other wind-pollinated
species are imperfect. Stamens and stigmas are also modified
for this method of pollination. The filaments are usually long,
allowing the anthers to hang free away from the rest of the
flower, thereby enabling the pollen to be caught by the wind.
Stigmas are often feathery, increasing the surface area for
trapping pollen.
Although individual flowers are small, their small size
is offset by the large number of flowers formed and by the
production of copious amounts of dry, lightweight pollen. A
single stamen of corn contains between 2,000 and 2,500 pollen grains, with the whole plant producing about 14 million
pollen grains. One wind-pollinated plant whose pollen causes
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Box Figure 5.3 Rose petals undergo distillation to extract
rose oil, one of the perfume industry’s most valued scents.
misery to hay fever sufferers is ragweed (Ambrosia spp.); one
healthy ragweed plant can release 1 billion pollen grains. It is
estimated that 1 million tons of ragweed pollen are produced
in the United States each year!
Figure 5.10 Staminate catkins of pecan, Carya illinoensis, a
wind-pollinated species.
85
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86
UNIT II
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
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Introduction to Plant Life: Botanical Principles
Concept Quiz
In many perfect flowers, the stamens and carpels mature at
different times. For instance, the anthers of a flower may
release pollen while the stigma is still immature and unreceptive, and by the time the stigma is receptive, the anthers have
released all of their pollen and are empty.
What is the advantage of this adaptation?
Double Fertilization
Once pollination has been accomplished, the stage is set for
fertilization. Recall that the pollen grain at the time of pollination contains a tube cell and a generative cell. On a compatible stigma, the pollen grain germinates; a pollen tube begins
growing down into the style toward the ovary. The vegetative
nucleus is generally found at the growing end of the pollen
tube while behind it the generative nucleus divides mitotically, producing two nonmotile sperm. The pollen tube continues to grow until it reaches and grows into the micropyle
of an ovule, penetrating the ovule at one synergid. The vegetative nucleus, synergids, and antipodals usually degenerate
during the fertilization process, leaving the two sperm, egg,
and polar nuclei as the remaining participants (fig. 5.8).
Both sperm are involved in fertilization. One sperm fertilizes the egg to produce a zygote that will develop into an
embryo. The zygote produced from the fusion of haploid egg
and sperm is diploid; this restores the chromosome number
for the sporophyte generation. The second sperm fertilizes
or fuses with the two polar nuclei, producing the primary
endosperm nucleus, which develops into endosperm, a
nutritive tissue for the developing embryo. The fusion of the
haploid sperm and polar nuclei normally produces triploid
endosperm. Endosperm development generally begins immediately, followed by division of the zygote to produce the
embryo. This double fertilization is a distinctive feature of
angiosperm reproduction.
The value of endosperm as a food source for the human
population cannot be overemphasized. The nutritive value of
wheat, rice, and corn, the world’s major crops, is due to the
large endosperm reserves in these grains. The development
of early civilizations in various parts of the world is linked
to the cultivation of these grains, which provided stable food
sources.
After fertilization, changes begin to occur within the
whole flower. Sepals, petals, and stamens often wither and
drop off as the ovary greatly expands, becoming a fruit.
Within the ovary, each fertilized ovule becomes a seed containing an embryo and nutritive tissue; the integuments of
the ovule develop into the seed coat, the outer covering of
the seed. Occasionally, fruit forms without fertilization. This
process is referred to as parthenocarpy and, understandably,
results in seedless fruit. Parthenocarpy occurs naturally in
certain fruits, such as bananas (see Chapter 14) and navel
oranges (see Chapter 6). Hormone applications can induce
artificial parthenocarpy in other fruits. (See A Closer Look
6.1—The Influence of Hormones on Plant Reproductive
Cycles.) The discussion of fruits and seeds continues in
Chapter 6.
CHAPTER SUMMARY
1. Flowers, the characteristic reproductive structures of
angiosperms, are composed of sepals, petals, stamens,
and carpels. Modifications of the basic floral organs are
common, often resulting in incomplete and imperfect
flowers.
2. Meiosis is a form of cell division that reduces the number
of chromosomes from diploid to haploid. The process
consists of two consecutive divisions, with the reduction
in chromosome number occurring in the first division.
The most significant events of meiosis occur in prophase
I, when synapsis occurs, and anaphase I, when the homologous chromosome pairs separate.
3. In angiosperms, meiosis occurs before the formation
of male and female gametophytes, which are small and
relatively short-lived. Ovules, which include the female
gametophytes, develop within the carpels; the pollen
grains, or male gametophytes, develop in the stamen.
4. Pollen is transferred passively by animals or wind from
stamen to stigma. Insect-pollinated flowers typically have
bright, showy petals and fragrant aromas and are rich in
nectar. Pollen in these flowers is often sticky, adhering
to the insect body. Wind-pollinated flowers are usually
small and inconspicuous but produce copious amounts
of dry, lightweight pollen. Only a small amount of pollen
from wind-pollinated plants reaches the female organ.
Most pollen grains settle to the ground where they can
leave a lasting record in the sediment.
5. Before fertilization, the pollen tube grows down the style
into the ovary and ovule. The generative nucleus gives
rise to two sperm. Within the ovule, double fertilization occurs as one sperm fertilizes the egg, producing
the zygote, while the second sperm fuses with the polar
nuclei, giving rise to the primary endosperm nucleus.
After fertilization, the ovary becomes a fruit, and each
ovule becomes a seed.
REVIEW QUESTIONS
1. Describe the parts of a flower, and indicate some common
modifications.
2. Detail the events of meiosis. Why is prophase I of meiosis
an important stage?
3. Describe the male and female gametophytes.
4. What is the general appearance of a wind-pollinated
flower? of an animal-pollinated flower? Take a walk in a
Levetin−McMahon: Plants
and Society, Fifth Edition
II. Introduction to Plant
Life: Botanical Principles
5. Plant Life Cycle: Flowers
CHAPTER 5
5.
6.
7.
8.
garden or tour a greenhouse and try to determine whether
the flower is wind- or animal-pollinated by examining the
flower’s structure.
What is meant by double fertilization?
What fields use the study of pollen as a tool? What types
of information have been gained from this approach?
Self-pollination occurs when the stigma is pollinated by
pollen from the same flower or another flower of the
same plant. Cross-pollination results when the stigma of
one flower is pollinated by the pollen from a different
individual. What is the advantage of self-pollination? the
disadvantage? What is the advantage of cross-pollination?
the disadvantage?
A number of flowering plants are adapted to aquatic environments. Investigate how pollen is tranferred in eel grass
(Valisneria), a dioecious angiosperm.
FURTHER READING
Berger, Terry. 1977. “Tulipomania” Was No Dutch Treat to
Gambling Burghers. Smithsonian 8(1): 70–77.
Bryant, Vaughan, Jr. 2000. Does Pollen Prove the Shroud
Authentic? Biblical Archaeology Review 26(6): 36–44.
Buchmann, Stephen L., and Gary Paul Nabhan. 1996. The
Forgotten Pollinators. Islands Press/Shearwater Books,
Washington, DC.
Coen, Enrique. 2002. The Making of a Blossom. Natural
History 111(4): 48–55.
Finnell, Rebecca B., ed. 1999. The Flower Issue. Natural
History 108(4): 1–100.
Freinkel, Susan 2004. Roses are blue, violets are red.
Discover. Vol 25(4): 28–29.
Green, Timothy. 1991. Making Scents Is More Complicated
Than You’d Think. Smithsonian 22(5): 52–61.
© The McGraw−Hill
Companies, 2008
Plant Life Cycle: Flowers
87
Hansen, Eric. 2000. Orchid Fever: A Horticultural Tale of
Love, Lust, and Lunacy. Pantheon Books, New York,
NY.
Klesius, Michael. 2002. The Big Bloom. National Geographic
202(1): 102–121.
Meeuse, Bastian, and Sean Morris. 1984. The Sex Life of
Flowers. Rainbird Publishing, London.
Milius, Susan 2006. Nectar: The First Soft Drink. Science
News 169(19): 298–300.
Moize, Elizabeth A. May, 1978. Tulips: Holland’s Beautiful
Business. National Geographic 153(5): 712–728.
Newman, Cathy. 1984. Pollen: Breath of Life and Sneezes.
National Geographic 166(4): 496–521.
Newman, Cathy, and Robb Kendrick. 1998. Perfume:
The Essence of Illusion. National Geographic 194(4):
94–119.
Pavord, Anna. 1999. The Tulip. Bloomsbury Publishing,
London.
Pollan, Michael. 2001. The Botany of Desire. Random House,
New York, NY.
Schwartz, David M. 2000. Birds, Bees, and Even NectarFeeding Bats Do It. Smithsonian 31(1): 58–71.
Sessions, Laura A. and Steven D. Johnson. 2005. The Flower
and the Fly. Natural History (3): 58–63.
Tanner, Ogden. 1985. The Flowers That Afflict Us with “A
Sort of Madness.” Smithsonian 16(8): 168–178.
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