Plant Life Cycle - Flowers

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Levetin−McMahon: Plants and Society, Fifth Edition

II. Introduction to Plant Life: Botanical Principles

© The McGraw−Hill Companies, 2008

5. Plant Life Cycle: Flowers

CHAPTER OUTLINE The Flower 73 Floral Organs 73

A CLOSER LOOK 5.1 Mad About Tuli Tu lips ps 74 Modified Flowers 75

Meiosis 77 Stages of Meiosis Meiosis 78

A CLOSER LOOK 5.2 Pollen is More Than Something to Sneeze At 80 Meiosis in Flowering Flowering Plants 80 Male Gametophyte Development 80 Female Gametophyte Gametophyte Development Development 81

Pollination and Fertilization 83 Animal Pollinatio Pollination n 83

A CLOSER LOOK 5.3 Alluring Scen Sc ents ts 84 Wind Pollination Pollination 84 Double Fertilization Fertilization 86

Chapter Summary 86  Review Questions 86 Further Reading 87

KEY CONCEPTS 1.

Angiosperms are unique among plants in that they have their sexual reproductive structures contained in a flower.

2.

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

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.

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

© The McGraw−Hill Companies, 2008

5. Plant Life Cycle: Flowers

CHAPTER 5

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.

 T  

 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, sepals,   petals, stamens, and  carpels. They are inserted into the receptacle, the expanded top of the pedicel ( fig. 5.1). 5.1). (peduncle),, or flower stalk (fig. (peduncle)

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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. 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. 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. 5.2a. If many carpels are present, they may either be fused together (fig. ( fig. 5.2b) 5.2b) or remain separate. Carpels, whether individual or fused, consist of a stigma, style, and ovary (fig. 5.1). 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 Companies, 2008

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 Heave n 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), (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 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.

In Chapter 3, the vegetative differences between monocots and dicots were described; there are also easily recognizable differences in the floral structures (fig. ( fig. 5.3). 5.3). Monocots generally have their floral parts in threes or multiples of

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.

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.

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Levetin−McMahon: Plants and Society, Fifth Edition

II. Introduction to Plant Life: Botanical Principles

5. Plant Life Cycle: Flowers

© The McGraw−Hill Companies, 2008

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  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

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.

The basic pattern of flower structure is often modified. In flowers like tulips and lilies, the sepals are brightly colored

even if sepals or petals are lacking. Some flowers, such as squash and holly, are unisexual; they are either staminate staminate or  or carpellate. Incomplete flowers lacking either stamens or car-

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. (fig. 5.4a). 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

pels 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), 5.10 ), while spinach, date palms, and some hollies are dioecious.

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Levetin−McMahon: Plants and Society, Fifth Edition

76

UNIT II

II. Introduction to Plant Life: Botanical Principles

© The McGraw−Hill Companies, 2008

5. Plant Life Cycle: Flowers

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

Stamen

(a)

Petal Sepal

Ovary 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

© The McGraw−Hill Companies, 2008

5. Plant Life Cycle: Flowers

II. Introduction to Plant Life: Botanical Principles

CHAPTER 5

Plant Life Cycle: Flowers

77

One feature that is important in the classification of flowers is the position of the ovary in relation to the other

inflorescence, but here the pink or white “petals” are bracts surrounding a cluster of small flowers. The arrangement of

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, 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. ( fig. 5.4b). 5.4b). Flowers can also be described by their pattern of symmetry. Regular flowers (actinomorph (actinomorphic) ic) 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,

flowers in the cluster determines the type of inflorescence, with many patterns possible: spike, raceme, panicle, umbel, 5.5). Often the type of infloresence is head, and catkin (fig. 5.5). 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.

and pea display bilateral symmetry. They can be dissected into mirror-image halves along only one line (fig. (fig. 5.4c). 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

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

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

Pedicels (a)

(b)

(c)

Peduncle

(d)

(e)

(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

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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. ( fig. 5.6). 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.

Sporophyte Diploid (2N) Meiosis

Haploid (N) Microspore Sperm

Male gametophyte Female gametophyte

Megaspore

Figure 5.6 Alternation of diploid (sporophy (sporophyte) te) and haploid (gametophyte) generations in a flowering plant.

Meiosis is a specialized form of cell division that consists of two consecutive divisions and results in the formation of four haploid cells (fig. (fig. 5.7). 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 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 chiasas  chias). While these chromosome events are mata (sing., chiasma chiasma). 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. ( fig. 5.7 and fig. 7.6).

During the next stage, metaphase I (fig. ( fig. 5.7), 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.

Ovary

Egg

Stages of Meiosis

 Metaphase I

Anther Zygote

ion Fertilizat Fe rtilization

© The McGraw−Hill Companies, 2008

 Anaphase I Homologous chromosomes separate during anaphase I (fig. 5.7); 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

© The McGraw−Hill Companies, 2008

5. Plant Life Cycle: Flowers

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. (fig. 5.7) 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). 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 and Society, Fifth Edition

II. Introduction to Plant Life: Botanical Principles

5. Plant Life Cycle: Flowers

© The McGraw−Hill Companies, 2008

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). 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

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. (fig. 5.8). 5.8).

Male Gametophyte Development During the development of the stamen, certain cells in the pollen chambers of the anther become distinct as

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division to produce two cells, a small generative cell  cell  and a large vegetative 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 exine,, which may be ornamented with spines, ridges, or pores. When the

 

Levetin−McMahon: Plants and Society, Fifth Edition

II. Introduction to Plant Life: Botanical Principles

5. Plant Life Cycle: Flowers

© The McGraw−Hill Companies, 2008

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

(c)

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

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.

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.)

Within the ovary one or more ovules develop; an ovule con-

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

sists 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

eight nuclei are distributed with three (the egg apparatus  apparatus ) near the micropyle end of the ovule, three antipodals antipodals at  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. ( fig. 5.8). 5.8).

Female Gametophyte Development

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

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Levetin−McMahon: Plants and Society, Fifth Edition

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UNIT II

II. Introduction to Plant Life: Botanical Principles

© The McGraw−Hill Companies, 2008

5. Plant Life Cycle: Flowers

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



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 Pollination

Synergids 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

 

Levetin−McMahon: Plants and Society, Fifth Edition

II. Introduction to Plant Life: Botanical Principles

© The McGraw−Hill Companies, 2008

5. Plant Life Cycle: Flowers

CHAPTER 5

POLLINATION AND FERTILIZATION

Plant Life Cycle: Flowers

83

bats, are efficient pollinators for some species. Pollination is accomplished inadvertently when the animal visitor,

Although bees are the most familiar animal pollinators, a host of species are involved in the transf er of pollen. Other

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.

insects such as wasps, flies, ants, butterflies (fig. (fig. 5.9a), 5.9a), and moths are equally important pollinators for many flowers. Even larger animals, such as birds (fig. ( fig. 5.9b) 5.9b) and

The essential oils of flowers such as the rose, orange, and  jasmin e h ave been used in perfum perfumes es for hundreds of years (see A Closer Look 5.3—Alluring Scents). Not all scents

Pollination involves the transfer of pollen from the anther to the stigma. Pollen transfer within the same individual plant is known as self-pollination. 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

(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.

(b)

 

Levetin−McMahon: Plants and Society, Fifth Edition

II. Introduction to Plant Life: Botanical Principles

© The McGraw−Hill Companies, 2008

5. Plant Life Cycle: Flowers

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

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

are appealing to humans; for example, the carrion flower (Stapelia sp.), which is fly-pollinated, gives off an aroma

Thus, I can understand how a flower and a bee might slowly become either simultaneously or one

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  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 effi-

after the other modified and adapted in the most perfect manner to each other.

ciency 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

Leav Le aves es an and d ste stems ms

Mints, Mint s, ro rose sema mary ry,, ger geran aniu ium, m, ci citr tron onel ella la,, lemon grass

Seeds and fruits

Oranges, lemons, nutmeg

Roots

Sassafras

Rhizomes

Ginger

Bark

Cinnamon, cassia

Wood

Cedar, sandalwood, pine

Gums

Balsam, myrrh

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 and Society, Fifth Edition

II. Introduction to Plant Life: Botanical Principles

5. Plant Life Cycle: Flowers

© The McGraw−Hill Companies, 2008

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%.

Box Figure 5.3 Rose petals undergo distillation to extract rose oil, one of the perfume industry’s most valued scents.

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 wind-

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

pollinated 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. ( fig. 5.10), 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

in the United States each year!

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

Figure 5.10 Staminate catkins of pecan, Carya illinoensis, a wind-pollinated species.

85

 

Levetin−McMahon: Plants and Society, Fifth Edition

86

UNIT II

II. Introduction to Plant Life: Botanical Principles

5. Plant Life Cycle: Flowers

© The McGraw−Hill Companies, 2008

Introduction to Plant Life: Botanical Principles

certain fruits, such as bananas (see Chapter 14) and navel oranges (see Chapter 6). Hormone applications can induce

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. (fig. 5.8). 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 fertilizatio fertilization n 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, parthenocarpy and, results in seedless fruit. Parthenocarpy occurs naturally in

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 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 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 down the style 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. meiosis. Why is is prophase I of meiosis an important stage? 3. Describe the the male and female gametophytes. 4. What is the general appearance 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

garden or tour a greenhouse and try to determine whether the flower is wind- or animal-pollinated by examining the flower’s structure. 5. What is meant by double fertilization? 6. What fields use the study of of pollen as a tool? What types of information have been gained from this approach? 7. Self-pollination occurs occurs when the stigma is pollinated 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? 8. A number of of flowering plants plants are adapted adapted to aquatic envienvironments. Investigate how pollen is tranferred in eel grass (Valisneria), a dioecious angiosperm.

© 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.

FURTHER READING

Pavord, Anna. 1999. The Tulip. Bloomsbury Publishing, London.

Berger, Terry. 1977. “Tulipomania” Was No Dutch Treat to Gambling Burghers. Smithsonian 8(1): 70–77.

Pollan, Michael. 2001. The Botany of Desire. Random House, New York, NY.

Bryant, Vaughan, Jr. 2000. Does Pollen Prove the Shroud Authentic? Biblical Archaeology Review 26(6): 36–44.

Schwartz, David M. 2000. Birds, Bees, and Even NectarFeeding Bats Do It. Smithsonian 31(1): 58–71.

Buchmann, Stephen L., and Gary Paul Nabhan. 1996. The Forgotten Pollinators. Islands Press/Shearwater Books, Washington, DC.

Sessions, Laura A. and Steven D. Johnson. 2005. The Flower and the Fly. Natural History (3): 58–63.

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.

Tanner, Ogden. 1985. The Flowers That Afflict Us with “A Sort of Madness.” Smithsonian 16(8): 168–178.

ONLINE LEARNING CENTER Visit www.mhhe.com/le  www.mhhe.com/levetin5e vetin5e for online quizzing, web links to chapter-related material, and more!

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