Flowering plants, or angiosperms, include about 300,000 species

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1 36 Reproduction in Flowering Plants Dr. Jeremy Burgess/Science Photo Library/Photo Researchers, Inc. KEY CONCEPTS The flower is the site of sexual reproduction in angiosperms. A typical flower consists of four whorls: sepals, petals, stamens, and carpels. Pollen grains are transported to stigmas by a variety of agents, such as animals and wind. Double fertilization results in a plant embryo and endosperm. The seed is a mature ovule, and the fruit is a mature ovary. Many vegetative organs (roots, stems, and leaves) have evolved modifications for reproducing asexually. Flower of the common chickweed. This plant (Stellaria media) is a weedy annual native to Europe but widespread in North America. Each flower has five green sepals, five deeply notched yellow petals, three to five pollen-bearing stamens (this flower has three), and a single pistil. Flowering plants, or angiosperms, include about 300,000 species and are the largest, most successful group of plants. You may have admired flowers for their fragrances as well as their appealing colors and varied shapes (see photograph). The biological function of flowers is sexual reproduction. Their colors, shapes, and fragrances increase the likelihood that pollen grains, which produce sperm cells, will be carried from one plant to another. Sexual reproduction in plants includes meiosis and the fusion of reproductive cells egg and sperm cells, collectively called gametes. The union of gametes, called fertilization, occurs within the flower s ovary. Sexual reproduction offers the advantage of new gene combinations, not found in either parent, that may make an individual plant better suited to its environment. These new combinations result from the crossing-over and independent assortment of chromosomes that occur during meiosis, before the production of egg and sperm cells (see Chapter 11). Many flowering plants also reproduce asexually. Asexual reproduction often does not involve the formation of flowers, seeds, and fruits. Instead, offspring generally form when a vegetative organ (such as a stem, root, or leaf) expands, grows, and then becomes separated from the rest of the plant, often by the death of tissues. Because asexual reproduction requires only one parent and no meiosis or fusion of gametes occurs, the offspring of asexual reproduction are virtually genetically identical to one another and to the parent plant from which they came. 1 1 Somatic mutations can result in some variability among asexually derived offspring. 767

2 This chapter examines both sexual and asexual reproduction in flowering plants, including floral adaptations that are important in pollination; seed and fruit structure and dispersal; germination and early growth; and several kinds of asexual reproduction. We conclude with a discussion of the evolutionary advantages and disadvantages of sexual and asexual reproduction. THE FLOWERING PLANT LIFE CYCLE Learning Objectives 1 Describe the functions of each part of a flower. 2 Identify where eggs and pollen grains are formed within the flower. In Chapters 27 and 28 you learned that angiosperms and other plants undergo a cyclic alternation of generations in which they spend a portion of their life cycle in a multicellular haploid stage and a portion in a multicellular diploid stage. The haploid portion, called the gametophyte generation, gives rise to gametes by mitosis. When two gametes fuse during fertilization, the diploid portion of the life cycle, called the sporophyte generation, begins. The sporophyte generation produces haploid spores by meiosis. Each spore has the potential to give rise to a gametophyte plant, and the cycle continues. In flowering plants the diploid sporophyte generation is larger and nutritionally independent. The haploid gametophyte generation, which is located in the flower, is microscopic and nutritionally dependent on the sporophyte. We say more about alternation of generations in flowering plants after our discussion of the role of flowers as reproductive structures. It may be helpful for you to review Figure 28-13, which shows the main stages in the flowering plant life cycle. Darwin Dale/Photo Researchers, Inc. (a) Figure 36-1 Animated Floral structure (a) An Arabidopsis thaliana flower. (b) Cutaway view of an Arabidopsis flower. Each flower has four sepals (two are shown), four petals (two are shown), six stamens, and one long pistil. Four of the stamens are long, and two are short (two long and two short are shown). Pollen grains develop within sacs in the anthers. In Arabidopsis, the compound pistil consists of two carpels that each contain numerous ovules. Female floral parts Male floral parts Pollen grain (each will produce two sperm cells) PISTIL (consisting of one or more carpels) Stigma Style Ovary Anther Filament STAMEN Ovules (each producing one egg cell) Petal Receptacle Sepal (b) Peduncle 768 Chapter 36

3 Flowers develop at apical meristems How does a plant know it is time to start forming flowers? Correct timing in the switch from vegetative to reproductive development is crucial to ensure reproductive success. What happens, for example, if a plant flowers so late in the season that it does not have enough time to set seed before winter? A variety of environmental cues, such as temperature and day length, ensure proper timing, and different species are adapted to respond to distinct environmental cues. These environmental signals interact with a variety of plant hormones and developmental pathways. In recent years some biologists have focused on the molecular aspects of developmental pathways that initiate flowering in the model organism Arabidopsis. When environmental conditions induce flowering, many genes are activated or inactivated. One gene, the Flowering Locus C (FLC) gene, codes for a transcription factor that represses flowering. Another gene, called Flowering Locus D (FLD) gene, codes for a protein that removes acetyl groups from histones in the chromatin where the FLC gene is located. When deacetylation occurs, the FLC gene is not transcribed (that is, the repressive transcription factor is not produced), and the shoot apical meristem undergoes a transition from vegetative growth to reproductive growth. It is intriguing that the plant FLD protein is homologous to a mammalian protein that also removes acetyl groups from chromatin. Other genes are also involved in the initiation of flowering, and this area remains a focus of research interest. In Chapter 37 we discuss further the initiation of flowering. Each part of a flower has a specific function Flowers are reproductive shoots, usually consisting of four kinds of organs sepals, petals, stamens, and carpels arranged in whorls (circles) on the end of a flower stalk ( Fig. 36-1; also see Fig ). In flowers with all four organs, the normal order of whorls from the flower s periphery to the center (or from the flower s base upward) is as follows: Sepals petals stamens carpels The tip of the stalk enlarges to form a receptacle on which some or all of the flower parts are borne. All four floral parts are important in the reproductive process, but only the stamens (the male organs) and carpels (the female organs) participate directly in sexual reproduction sepals and petals are sterile. Sepals, which constitute the outermost and lowest whorl on a floral shoot, cover and protect the flower parts when the flower is a bud. Sepals are leaflike in shape and form and are often green. Some sepals, such as those in lily flowers, are colored and resemble petals ( Fig. 36-2). The collective term for all the sepals of a flower is calyx. The whorl just inside and above the sepals consists of petals, which are broad, flat, and thin (like sepals and leaves) but tremendously varied in shape and frequently brightly colored, which attracts pollinators. Petals play an important role in ensuring that sexual reproduction will occur. Sometimes petals fuse to form a tube or other floral shape. The collective term for all the petals of a flower is corolla. M. P. Land/Science Photo Library/Photo Researchers, Inc. Figure 36-2 Lily flower In lily (Lilium) the three sepals and three petals are similar in size and color. Just inside and above the petals are the stamens, the male reproductive organs. Each stamen has a thin stalk, called a fila ment, at the top of which is an anther, a saclike structure in which pollen grains form. For sexual reproduction to occur, pollen grains must be transferred from the anther to the female reproductive structure (the carpel), usually of another flower of the same species. At first, each pollen grain consists of two cells surrounded by a tough outer wall. One cell, the generative cell, divides mitotically to form two nonflagellate male gametes, known as sperm cells. The other cell, the tube cell, produces a pollen tube, through which the sperm cells travel to reach the ovule. One or more carpels, the female reproductive organs, are located in the center or top of most flowers. Carpels bear ovules, which are structures with the potential to develop into seeds. The carpels of a flower may be separate or fused into a single structure. The female part of the flower, often called a pistil, may be a single carpel (a simple pistil) or a group of fused carpels (a compound pistil) (see Fig ). Each pistil has three sections: a stigma, on which the pollen grains land; a style, a necklike structure through which the pollen tube grows; and an ovary, a juglike structure that contains one or more ovules and can develop into a fruit. Female gametophytes are produced in the ovary, male gametophytes in the anther Before we proceed, it may be helpful to relate the stages in alternation of generations to floral structure. As discussed in Chapters 27 and 28, angiosperms and certain other plants are heterosporous and produce two kinds of spores: megaspores and microspores ( Fig. 36-3). Each young ovule within an ovary contains a diploid cell, the megasporocyte, which undergoes meiosis to produce four haploid megaspores. Three of these usually disintegrate, and the fourth, the functional megaspore, divides mitotically to produce a multicellular female gametophyte, also called an embryo sac. The fe- Reproduction in Flowering Plants 769

4 Key Point In alternation of generations in flowering plants, the female and male gametophytes are microscopic and nutritionally dependent on the sporophyte plant. Young ovule (megasporangium) Megasporocyte Young pollen sac with numerous microsporocytes Meiosis Microsporocyte Functional megaspore Disintegrating megaspores (3) Meiosis Microspores (4) Mitosis (3 divisions) Single microspore Antipodal cells Mitosis Polar nuclei in a central cell Synergids Integuments Egg Female gametophyte (embryo sac) Pollen grain (immature male gametophyte) Generative cell (will divide to form 2 sperm cells) Tube cell Figure 36-3 Animated Development of female and male gametophytes (Left side) The female gametophyte, or embryo sac, develops within phyte becomes mature when its generative cell divides mitotically to the ovule. (Right side) The immature male gametophytes, or pollen produce two sperm cells. grains, develop within pollen sacs in the anthers. Each male gameto- male gametophyte, which is embedded in the ovule, typically contains seven cells with eight haploid nuclei. Six of these cells, including the egg cell (the female gamete), contain a single nucleus each; a large central cell has two nuclei, called polar nuclei. The egg and both polar nuclei participate directly in fertilization. Pollen sacs within the anther contain numerous diploid cells called microsporocytes, each of which undergoes meiosis to produce four haploid cells called microspores. Each microspore divides mitotically to produce an immature male gametophyte, also called a pollen grain, that consists of two cells, the tube cell 770 Chapter 36

5 and the generative cell. The pollen grain becomes mature when its generative cell divides to form two nonmotile sperm cells. Review How do petals differ from sepals? How are they similar? How do stamens differ from carpels? How are they similar? What are female gametophytes, and where are they formed? What are male gametophytes, and where are they formed? POLLINATION Learning Objectives 3 Compare the evolutionary adaptations that characterize flowers pollinated in different ways (by insects, birds, bats, and wind). 4 Define coevolution, and give examples of ways that plants and their animal pollinators have affected one another s evolution. Before fertilization can occur, pollen grains must travel from the anther (where they form) to the stigma. The transfer of pollen grains from anther to stigma is known as pollination. Plants are self-pollinated if pollination occurs within the same flower or a different flower on the same individual plant. When pollen grains are transferred to a flower on another individual of the same species, the plant is cross-pollinated. Flowering plants accomplish pollination in a variety of ways. Beetles, bees, flies, butterflies, moths, wasps, and other insects pollinate many flowers. Animals such as birds, bats, snails, and small nonflying mammals (rodents, primates, and marsupials) also pollinate plants. Wind is an agent of pollination for certain flowers; and water, for a few aquatic flowers. Many plants have mechanisms to prevent self-pollination In plant sexual reproduction, the two gametes that unite to form a zygote may be from the same parent or from two different parents. The combination of gametes from two different parents increases the variation in offspring, and this variation may confer a selective advantage. Some offspring, for example, may be able to survive environmental changes better than either parent can. Plants have evolved a variety of mechanisms that prevent selfpollination and thus prevent inbreeding, which is the mating of genetically similar individuals. Inbreeding can increase the concentration of harmful genes in the offspring. Some plants, such as asparagus and willow, have separate male and female individuals; the male plants have staminate flowers that lack carpels, and the female plants have pistillate flowers that lack stamens. Other species have flowers with both stamens and pistils, but the pollen is shed from a given flower either before or after the time when the stigma of that flower is receptive to pollen. These characteristics promote outcrossing (also called outbreeding), which is the mating of dissimilar individuals. Many species have genes for self-incompatibility, a genetic condition in which the pollen is ineffective in fertilizing the same flower or other flowers on the same plant. In other words, an individual plant can identify and reject its own pollen. Genes for self-incompatibility usually inhibit the growth of the pollen tube in the stigma and style, thereby preventing delivery of sperm cells to the ovules. Self-incompatibility, which is more common in wild species than in cultivated plants, ensures that reproduction occurs only if the pollen comes from a genetically different individual. In plants such as oilseed rape, self-incompatibility is based on a high degree of variation at a particular locus called the S locus; the many alleles at this locus are designated S 1, S 2, S 3, S 4, and so on. Here is an example of how the S locus blocks self-fertilization: A plant with the genotype S 1 S 2 produces pollen grains that land on a stigma of another plant with the genotype S 1 S 3. In this case, the presence of the S 1 allele in both the pollen and stigma triggers a self-recognition signaling cascade in surface cells of the stigma. As a result, the stigma cells do not undergo changes that allow the pollen grain to grow a pollen tube. Therefore, fertilization does not occur. The molecular basis of self-incompatibility in Arabidopsis and related plants is an area of active scientific interest. Arabidopsis can self-pollinate. Biologists have determined that the genes involved in self-incompatibility and outcrossing exist in Arabidopsis but have undergone mutations so that they are no longer functional. Thus, it appears that self-incompatibility in these plants is the ancestral (normal) condition. Flowering plants and their animal pollinators have coevolved Animal pollinators and the plants they pollinate have had such close, interdependent relationships over time that they have affected the evolution of certain physical and behavioral features in one another. The term coevolution describes such reciprocal adaptation, in which two species interact so closely that they become increasingly adapted to one another as they each undergo evolutionary change by natural selection. We now examine some of the features of flowers and their pollinators that may be the products of coevolution. Flowers pollinated by animals have various features to attract their pollinators, including showy petals (a visual attractant) and scent (an olfactory attractant). One reward for the animal pollinator is food. Some flowers produce nectar, a sugary solution, in special floral glands called nectaries. Pollinators use nectar as an energy-rich food. Pollen grains are also a protein-rich food for many animals. As they move from flower to flower searching for food, pollinators inadvertently carry along pollen grains on their body parts, helping the plants reproduce sexually ( Fig. 36-4). Biologists estimate that insects pollinate about 70% of all flowering plant species. Bees are particularly important as pollinators of crop plants. Crops pollinated by bees provide about 30% of human food. Plants pollinated by insects often have blue or yellow petals (see Fig. 36-4a). The insect eye does not see color Reproduction in Flowering Plants 771

6 Sexual reproduction produces some individuals with genotypes that are well adapted to the environment, but it also produces some individuals that are less well adapted. Therefore, sexual reproduction is usually accompanied by high death rates among offspring, particularly when selective pressures are strong. As discussed in Chapter 18, however, this aspect of sexual reproduction is an important part of evolution by natural selection. Every biological process involves trade-offs, and sexual reproduction is no exception. Although sexual reproduction has its costs, the adaptive advantages of sexual reproduction clearly outweigh any disadvantages. Review How does sexual reproduction in plants differ from asexual reproduction? What is an adaptive advantage of sexual reproduction? Why is sexual reproduction considered costly? SUMMARY WITH KEY TERMS Learning Objectives 1 Describe the functions of each part of a flower (page 768). Sepals cover and protect the flower parts when the flower is a bud. Petals play an important role in attracting animal pollinators to the flower. Stamens produce pollen grains. Each stamen consists of a thin stalk (the filament) attached to a saclike structure (the anther). The carpel is the female reproductive unit. A pistil may consist of a single carpel or a group of fused carpels. Each pistil has three sections: a stigma, on which the pollen grains land; a style, through which the pollen tube grows; and an ovary that contains one or more ovules. Learn more about flower structure by clicking on the figure in ThomsonNOW. 2 Identify where eggs and pollen grains are formed within the flower (page 768). Pollen forms within pollen sacs in the anther. Each pollen grain contains two cells. One generates two sperm cells, and the other produces a pollen tube through which the sperm cells reach the ovule. An egg and two polar nuclei, along with several other nuclei, are formed in the ovule. Both egg and polar nuclei participate directly in fertilization. Watch plant reproduction unfold by clicking on the figure in ThomsonNOW. 3 Compare the evolutionary adaptations that characterize flowers pollinated in different ways (by insects, birds, bats, and wind) (page 771). Flowers pollinated by insects are often yellow or blue and have a scent. Bird-pollinated flowers are often yellow, orange, or red and do not have a strong scent. Bat-pollinated flowers often have dusky white petals and are scented. Plants pollinated by wind often have smaller petals or lack petals altogether and do not produce a scent or nectar; wind-pollinated flowers make large amounts of pollen. 4 Define coevolution, and give examples of ways that plants and their animal pollinators have affected one another s evolution (page 771). Coevolution is reciprocal adaptation caused by two different species (such as flowering plants and their animal pollinators) forming an interdependent relationship and affecting the course of one another s evolution. For example, flowers with large, showy petals and scent have evolved in some plants, whereas hairy bodies that catch and hold sticky pollen grains have evolved in bees. 5 Distinguish between pollination and fertilization (page 775). Pollination is the transfer of pollen grains from anther to stigma. After pollination, fertilization, the fusion of gametes, occurs. Flowering plants undergo double fertilization. In the ovule, the egg fuses with one sperm cell, forming a zygote (fertilized egg) that eventually develops into a multicellular embryo in the seed. The two polar nuclei fuse with the second sperm cell, forming a triploid nutritive tissue called endosperm. Learn more about double fertilization by clicking on the figures in ThomsonNOW. 6 Trace the stages of embryo development in flowering plants, and list and define the main parts of seeds (page 775). A eudicot embryo develops in the seed in an orderly fashion, from proembryo to globular embryo to the heart stage to the torpedo stage. A mature seed contains both a young plant embryo and nutritive tissue stored in the endosperm or in the cotyledons (seed leaves) for use during germination. A tough, protective seed coat surrounds the seed. Watch embryonic development in flowering plants by clicking on the figure in ThomsonNOW. 7 Explain the relationships among the following: ovules, ovaries, seeds, and fruits (page 775). Ovules are structures with the potential to develop into seeds, whereas ovaries are structures with the potential to develop into fruits. Seeds are enclosed within fruits, which are mature, ripened ovaries. 8 Distinguish among simple, aggregate, multiple, and accessory fruits (page 775). Simple fruits develop from a single ovary that consists of one carpel or several fused carpels. Aggregate fruits develop from a single flower with many separate ovaries. Multiple fruits develop from the ovaries of many flowers growing in close proximity on a common axis. In accessory fruits, the major part of the fruit consists of tissue other than ovary tissue. 9 Summarize the influence of internal and environmental factors on the germination of seeds (page 782). Germination is the process of seed sprouting. Internal factors affecting whether a seed germinates include the maturity of the embryo; the presence or absence of chemical inhibitors; and the presence or absence of hard, thick seed coats. 786 Chapter 36

7 External environmental factors that may affect germination include requirements for oxygen, water, temperature, and light. For example, before germinating, dry seeds absorb water by imbibition. Learn more about germination and seedling growth by clicking on the figure in ThomsonNOW. 10 Explain how the following structures may be used to propagate plants asexually: rhizomes, tubers, bulbs, corms, stolons, plantlets, and suckers (page 783). Rhizomes, tubers, bulbs, corms, and stolons are stems specialized for asexual reproduction. A rhizome is a horizontal underground stem. A tuber is a fleshy underground stem enlarged for food storage. A bulb is a modified underground bud with fleshy storage leaves attached to a short stem. A corm is a short, erect underground stem covered by papery scales. A stolon is a horizontal aboveground stem with long internodes. Some leaves have meristematic tissue along their margins and give rise to detachable plantlets. Roots may develop adventitious buds that develop into suckers. Suckers produce additional roots and may give rise to new plants. 11 Define apomixis (page 783). Apomixis is the production of seeds and fruits without sexual reproduction. 12 State the differences between sexual reproduction and asexual reproduction, and discuss the evolutionary advantages and disadvantages of each (page 785). Sexual reproduction involves the union of two gametes; the offspring produced by sexual reproduction are genetically variable. Asexual reproduction involves the formation of offspring without the fusion of gametes; the offspring are virtually genetically identical to the single parent. The parental genotypes are not preserved in the offspring of sexual reproduction. Genetic diversity among offspring produced by sexual reproduction may let individuals survive in a changing environment or exploit new environments. Sexual reproduction is costly, because both male and female gametes must be produced and must meet. The parental genotype is preserved in asexual reproduction. Genetic similarity may be advantageous if the environment is stable. Most plant species whose reproduction is primarily asexual occasionally reproduce sexually, thereby increasing their genetic variability. TEST YOUR UNDERSTANDING 1. In flowering plants, the is/are large (multicellular) and nutritionally independent. (a) gametes (b) microspores (c) megaspores (d) mature gametophyte (e) mature sporophyte 2. The normal order of whorls from the flower s periphery to the center is (a) sepals petals carpels stamens (b) stamens carpels sepals petals (c) sepals petals stamens carpels (d) petals carpels stamens sepals (e) carpels stamens petals sepals 3. A pistil consists of (a) stigma, style, and stamen (b) anther and filament (c) sepal and petal (d) stigma, style, and ovary (e) radicle, hypocotyl, and plumule 4. The petals of a flower are collectively called a (a) calyx (b) capsule (c) carpel (d) cotyledon (e) corolla 5. The transfer of pollen grains from anther to stigma is (a) fertilization (b) double fertilization (c) pollination (d) germination (e) apomixis 6. The observation that insects with long mouthparts pollinate long, tubular flowers and insects with short mouthparts pollinate flowers with short corollas is explained by (a) coevolution (b) germination (c) double fertilization (d) apomixis (e) explosive dehiscence 7. The process of in flowering plants involves one sperm cell fusing with an egg cell and one sperm cell fusing with two polar nuclei. (a) coevolution (b) germination (c) double fertilization (d) apomixis (e) pollination 8. The nutritive tissue in the seeds of flowering plants that is formed from the union of a sperm cell and two polar nuclei is called the (a) plumule (b) endosperm (c) cotyledon (d) hypocotyl (e) radicle 9. The is a multicellular structure that anchors the embryo and aids in nutrient uptake from the endosperm. (a) proembryo (b) ovule (c) suspensor (d) cotyledon (e) pollen tube 10. After fertilization, the ovule develops into a and the ovary develops into a. (a) fruit; seed (b) seed; fruit (c) calyx; corolla (d) corolla; calyx (e) follicle; legume 11. In plants that lack endosperm in their mature seeds, the cotyledons function to (a) enclose and protect the seed (b) aid in seed dispersal (c) serve as an absorptive embryonic root (d) store food reserves (e) attach the embryo within the ovule 12. fruits develop from many ovaries of a single flower, whereas fruits develop from the ovaries of many separate flowers. (a) multiple; accessory (b) simple; accessory (c) aggregate; multiple (d) accessory; aggregate (e) simple; multiple 13. Apples, strawberries, and pears are examples of what kind of fruit? (a) accessory (b) simple (c) multiple (d) aggregate (e) legume 14. A horizontal underground stem that may or may not be fleshy and that is often specialized for asexual reproduction is called a (a) stolon (b) bulb (c) corm (d) rhizome (e) tuber 15. Place the following events in the correct order. 1. pollen tube grows into ovule 2. insect lands on flower to drink nectar 3. embryo develops within the seed 4. fertilization occurs 5. pollen carried by insect contacts stigma (a) 2, 5, 1, 4, 3 (b) 1, 4, 2, 5, 3 (c) 3, 2, 5, 1, 4 (d) 5, 1, 3, 4, 2 (e) 2, 5, 4, 3, 1 Reproduction in Flowering Plants 787

8 CRITICAL THINKING 1. Sketch the kinds of flowers that form simple, aggregate, multiple, and accessory fruits. 2. Using what you have learned in this chapter, speculate whether it is more likely that offspring of asexual reproduction develop in close proximity to or widely dispersed from the parent plant. Explain your reasoning. How could you design an experiment to test your hypothesis? 3. Which type of reproduction, sexual or asexual, might be more beneficial in the following circumstances, and why: (a) a perennial (plant that lives more than two years) in a stable environment; (b) an annual (plant that lives one year) in a rapidly changing environment; and (c) a plant adapted to an extremely narrow climate range? 4. Using what you have learned in this chapter, offer an explanation of why telephone poles and wires strung across grassy fields or plains often have tree seedlings growing under them. 5. Evolution Link. Is seed dispersal by ants an example of coevolution? Why or why not? Additional questions are available in ThomsonNOW at login 788 Chapter 36