Seed plants are well adapted to the demands of life on land,

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1 24 1 Reproduction With Cones and Flowers Seed plants are well adapted to the demands of life on land, especially in how they reproduce. The gametes of seedless plants, such as ferns and mosses, need water for fertilization to be successful. Water allows gametes to move from plant to plant. The gametes of seed plants, however, can achieve fertilization even when the plants are not wet from rain or dew. So, they can reproduce nearly anywhere. The way in which seed plants reproduce has allowed them to survive the dry conditions on land. Alternation of Generations All plants have a life cycle in which a diploid sporophyte generation alternates with a haploid gametophyte generation. Gametophyte plants produce male and female gametes sperm and eggs. When the gametes join, they form a zygote that begins the next sporophyte generation. In some plants, the two stages of the life cycle are distinct, independent plants. In most ferns, for instance, the gametophyte is a small, heart-shaped plant that grows close to the ground. The sporophyte is the familiar fern plant itself made up of graceful fronds. Where are these two generations in seed plants? You may remember from Mendel s work on peas that such plants are diploid. Therefore, in seed plants, the familiar, recognizable form of the plant is the diploid sporophyte. If the sporophyte is what we recognize as the plant, then where is the gametophyte? The answer may surprise you. As shown in Figure 24 1, the gametophytes of seed plants are actually hidden deep within tissues of the sporophyte plant. In gymnosperms they are found inside cones, and in angiosperms they are found inside flowers. Cones and flowers represent two different methods of reproduction. Gametophyte (N) Sporophyte (2N) Print: Bryophytes Ferns Seed plants SECTION RESOURCES Teaching Resources, Section Review 24 1, Chapter 24 Design an Experiment Reading and Study Workbook A, Section 24 1 Adapted Reading and Study Workbook B, Section 24 1 Investigations in Forensics, Investigation 7 Lesson Plans, Section 24 1 Time Saver Key Concepts What are the reproductive structures of gymnosperms and angiosperms? How does pollination differ between angiosperms and gymnosperms? Vocabulary pollen cone seed cone ovule pollen tube sepal petal stamen filament anther carpel ovary style stigma embryo sac endosperm double fertilization Reading Strategy: Making Comparisons Before you read, preview Figure 24 4 and Figure As you read, compare the life cycles of gymnosperms and angiosperms. Figure 24 1 An important trend in plant evolution is the reduction of the gametophyte and the increasing size of the sporophyte. Bryophytes consist of a relatively large gametophyte and smaller sporophytes, which include a capsule and stalk. Seedless vascular plants, such as ferns, have a small gametophyte and a larger sporophyte. Seed plants have an even smaller gametophyte that is contained within sporophyte tissues. Interpreting Graphics How does the relative size of the haploid and diploid stages of plants differ between bryophytes and seed plants? Technology: itext, Section 24 1 Animated Biological Concepts Videotape Library, 34 Angiosperm Reproduction Transparencies Plus, Section 24 1 Section FOCUS Objectives Identify the reproductive structures of gymnosperms and angiosperms Explain how pollination and fertilization differ between angiosperms and gymnosperms. Vocabulary Preview Ask students to predict which Vocabulary terms refer to the reproductive parts of gymnosperms, or cone-bearing plants, and which terms refer to the reproductive parts of angiosperms, or flowering plants. (Pollen cone and seed cone are parts of gymnosperms. Sepal, petal, stamen, anther, carpel, ovary, style, and stigma are parts of angiosperms. Both types of plants have pollen tubes and ovules.) Reading Strategy Suggest that students find each highlighted, boldface term in the text; read its definition; and then locate it in Figure 24 4 or Figure INSTRUCT Alternation of Generations Use Visuals Figure 24 1 Have students compare and contrast the differences in the sizes of the diploid sporophyte and the haploid gametophyte in different plant groups. Review the life cycle for each plant group shown, if needed. Challenge students to make inferences about the evolutionary advantage of a reduced haploid gametophyte stage. Have them consider water requirements, complexity of structure, and size. Answer to... Figure 24 1 Seed plants have a highly reduced haploid or gametophyte stage; the visible plant is nearly all diploid sporophyte. Reproduction of Seed Plants 609

2 24 1 (continued) Life Cycle of Gymnosperms Observing Obtain a small pine branch that has both pollen cones and seed cones. Point out how the two types of cones are arranged on the branch so that pollen from a pollen cone is likely to fall on a seed cone. Call students attention to the scales on a seed cone and how they are arranged. Remove some of the scales, and let students examine the base of the scales. (Even if the seeds have been shed, an impression of the seeds still remains.) Also, have students examine the scales of a seed cone that has been soaked in water. (The scales are closed.) Ask: What is the function of the scales of a seed cone? (To produce and protect the seeds) Demonstration Dust some pollen grains from a cone on a microscope slide. Prepare a wetmount slide and focus on low power. Use a microprojector or have students take turns observing the pollen. Suggest that students sketch what they see. Ask: How is the structure of the pollen grain related to its function? (A pine pollen grain has two tiny wings on either side of its rounded center that aid in its dispersal by wind.) Figure 24 2 Reproduction in gymnosperms takes place in structures called cones. In this pine tree, pollen cones, shown on the top, produce male gametophytes, which are pollen grains. Seed cones, such as the one shown on the bottom, produce female gametophytes that develop into a new embryo following fertilization. Figure 24 3 Pollen grains are male gametophytes. Pollen is carried by the wind until it reaches a female cone. Inferring Male and female cones are distributed on a plant such that pollen usually lands on a different plant from where it started. Why might this strategy have evolved? Life Cycle of Gymnosperms Pine trees and other gymnosperms are diploid sporophytes. As you will see, this sporophyte develops from a zygote that is contained within a seed. How and where is this seed produced? Reproduction in gymnosperms takes place in cones, which are produced by a mature sporophyte plant. Gymnosperms produce two types of cones: pollen cones and seed cones. Pollen Cones and Seed Cones Pollen cones, shown in Figure 24 2, are also called male cones. Pollen cones produce the male gametophytes, which are called pollen grains. As tiny as it is, the pollen grain makes up the entire male gametophyte stage of the gymnosperm life cycle. One of the haploid nuclei in the pollen grain will divide later to produce two sperm nuclei. The more familiar seed cones, which produce female gametophytes, are generally much larger than pollen cones. Near the base of each scale are two ovules in which the female gametophytes develop. Within the ovules, meiosis produces haploid cells that grow and divide to produce female gametophytes. These gametophytes may contain hundreds or thousands of cells. When mature, each gametophyte contains a few large egg cells, each ready for fertilization by sperm nuclei. Pollination The gymnosperm life cycle typically takes two years to complete. The cycle begins in the spring as male cones release enormous numbers of pollen grains. This pollen is carried by the wind, as shown in Figure Some of these pollen grains reach female cones. There, some pollen grains are caught in a sticky secretion on one of the scales of the female cone. This sticky material, known as a pollination drop, ensures that pollen grains stay on the female cone. What are pollen cones and seed cones? Pollen Grains (magnification: 750 ) 610 Chapter 24 Less Proficient Readers Because the double fertilization process is complicated, encourage students to carefully reread that subsection, and have each student draw a flowchart to illustrate the process. In their flowcharts, they should indicate which cells are haploid, diploid, and triploid. English Language Learners Invite students to diagram several very different types of flowers, such as orchids, magnolias, and daisies. Students should label each with the following: sepals, petals, stamen, anthers, ovary, style, and stigma. They may also include labels in their native languages. Advanced Learners Encourage students to write a biography of any mature seed plant. Their biographies should incorporate all of the important events in the plant s life cycle, including formation as a seed, germination, transportation away from the parent plant, and growth to maturity.

3 Haploid (N) Diploid (2N) Mature sporophyte Seed cone Pollen cone Cone scale Ovules Pollen grain (N) (male gametophytes) Diploid cell (2N) Ovule MEIOSIS FERTILIZATION Four haploid cells (N) Female gametophyte (N) Egg cells Discharged sperm nucleus Pollen tube Use Visuals Figure 24 4 Make sure students understand the diagram. Point out all the steps where the next drawing in the sequence is an enlargement of the previous drawing. For example, explain how the single cone scale at the top of the diagram is just one of many cone scales on the seed cone in the previous drawing. Similarly, the drawing that shows the diploid cell in the cone scale is an enlargement of the previous drawing of the cone scale. Add that the same holds true for the male pollen cone and its pollen grains. Then, review the entire life cycle of a pine tree, and have students follow along in the diagram. As you identify the structures involved in each stage, students should locate them in the diagram. Check students comprehension of the life cycle by asking: Which parts of the plant are haploid? (The haploid cell in the ovule, the female gametophyte, and the male gametophytes in the pollen grains) Why is the zygote diploid? (Because fertilization has occurred) Seedling Germinated seed Seed Fertilization and Development If a pollen grain lands near an ovule, the grain splits open and begins to grow a structure called a pollen tube, which contains two haploid sperm nuclei. Once the pollen tube reaches the female gametophyte, one sperm nucleus disintegrates, and the other fertilizes the egg contained within the female gametophyte. If sperm from another pollen tube reaches the female gametophyte, more than one egg cell may be fertilized, but just one embryo develops. As shown in Figure 24 4, fertilization produces a diploid zygote the new sporophyte plant. This zygote grows into an embryo. During this time, it is encased within what will soon develop into a seed. The seed consists of three generations of the life cycle. The outer seed coat is part of the old sporophyte generation, the haploid cells surrounding the embryo are part of the female gametophyte, and the embryo is the new sporophyte plant. Zygote (2N) (new sporophyte) Gametophyte tissue (N) Embryo (2N) Seed coat (old sporophyte) Figure 24 4 This illustration shows the life cycle of a typical gymnosperm. A pine tree the mature sporophyte produces male and female cones. Male cones produce pollen, and female cones produce ovules located on cone scales. If an egg is fertilized by the sperm, it becomes a zygote that is nourished by the female cone. In time, the zygote develops into a new sporophyte plant. Classifying Classify each of the following terms as to whether they belong to the haploid or diploid stage of the pine tree s life cycle: pollen tube, seed cone, embryo, ovule, seedling. Applying Concepts As students watch, cut a pine seed in half. Point out the three layers of the seed: the outer seed coat, the gametophyte, and the embryo. You may want to provide students with a hand lens to examine the three layers. Explain that the seed consists of three generations of the pine tree. Ask: Which generation of the pine tree is represented by each layer of the seed? (The outer seed coat is part of the old sporophyte plant, the cells surrounding the embryo are part of the female gametophyte, and the embryo is the new sporophyte plant.) BACKGROUND In an attempt to simplify the terminology, pollen grains often are referred to as gametes. It is important, though, to be clear about exactly what is a gamete and what is a spore. Recall that a gamete is a cell that must fuse with another gamete to form a new organism. Pollen grains, therefore, are not gametes; they are spores because they grow by mitosis into a new organism the gametophyte. In angiosperms, the true male gametes are the two sperm nuclei that appear in the pollen tube. The true female gametes are the egg cell and the polar nuclei that fuse with the sperm nuclei to form the embryo and endosperm, respectively. Answers to... Pollen cones produce the male gametophytes. Seed cones produce the female gametophytes. Figure 24 3 Because it increases genetic variation Figure 24 4 Haploid stage: pollen tube; diploid stage: seed cone, embryo, ovule, seedling Reproduction of Seed Plants 611

4 24 1 (continued) Structure of Flowers Classifying Display pictures of a wide variety of angiosperms. Include trees, shrubs, perennial and annual herbaceous plants, ground covers, grasses, and water plants. Give students a chance to view the pictures, and then have them brainstorm a list of characteristics that they think all angiosperms share. (Students might identify green leaves, flowers, and fruits, among other possible shared characteristics.) Then, ask: How do angiosperms differ from gymnosperms? (Students might say that angiosperms have broad leaves instead of needles and that they produce flowers and fruits instead of cones.) Using Models Give students a chance to create a model of a flower using whatever materials they find around the classroom or at home. For example, they might use colored construction paper for sepals and petals; toothpicks for filaments; modeling clay for anthers, stigma, and ovary; cornmeal for pollen; a drinking straw for the style; and dry peas for ovules. Invite students to share their models with the class and identify the parts of each model. Stamen Anther Filament Ovary Carpel Stigma Style Ovule Sepal Ovary Petal Figure 24 5 This diagram shows the parts of a typical flower. The flowers of some species, however, may not have all the parts shown here. Flowers are reproductive organs that include sepals, petals, stamens, and carpels. For: The Structure of a Flower activity Visit: PHSchool.com Web Code: cbp-7241 Structure of Flowers You may think of flowers as decorative objects that brighten the world. However, the presence of so many flowers in the world is visible evidence of something else the stunning evolutionary success of the angiosperms, or flowering plants. Flowers are the key to understanding why angiosperms have been so successful. Flowers are reproductive organs that are composed of four kinds of specialized leaves: sepals, petals, stamens, and carpels. These structures are shown in the flower in Figure Sepals and Petals The outermost circle of floral parts contains the sepals, which in many plants are green and closely resemble ordinary leaves. Sepals enclose the bud before it opens, and they protect the flower while it is developing. Petals, which are often brightly colored, are found just inside the sepals. The petals attract insects and other pollinators to the flower. Because they do not produce reproductive cells, the sepals and petals of a flower are sometimes called sterile leaves. Stamens and Carpels Within the ring of petals are the structures that produce male and female gametophytes. The male parts consist of an anther and a filament, which together make up the stamen. The filament is a long, thin stalk that supports an anther. At the tip of each filament is an anther, an oval sac where meiosis takes place, producing haploid male gametophytes pollen grains. In most angiosperms, each flower has several stamens. If you rub your hand on the anthers of a flower, a yellow-orange dust may stick to your skin. This is pollen, which consists of thousands of individual pollen grains. The innermost floral parts are carpels, also called pistils, which produce the female gametophytes. Each carpel has a broad base forming an ovary, which contains one or more ovules where female gametophytes are produced. The diameter of the carpel narrows into a stalk called the style. At the top of the style is a sticky portion known as the stigma, where pollen grains frequently land. Some flowers have several carpels fused together to form a single reproductive structure called a compound carpel. Anthers Carpel For: The Structure of a Flower activity Visit: PHSchool.com Web Code: cbe-7241 Students can interact with the art online. Carpel Anthers Tulip Wild Rose FACTS AND FIGURES Flowery facts In many flowers, both sepals and petals are brightly colored and help attract pollinators. In other plants, sepals are smaller and thicker than petals and green in color. In these plants, the sepals function is to protect the more fragile flower bud from damage. Some angiosperm plants have evolved unique ways to attract pollinators. For example, an African plant, Anchomanes difformis, does the botanical equivalent of burning incense. The flower has a foot-tall structure called a spadix. The plant s tissues generate heat and warm the spadix to about 40 C, so that it produces a sweet aroma that attracts the beetles that pollinate the flower. Most angiosperm flowers contain both pistils and stamens. However, some species including date palms, willows, and poplars produce unisexual flowers that have only stamens (staminate flowers) or pistils (pistillate flowers). 612 Chapter 24

5 What is the structure of a flower? Materials flower, forceps, scalpel, microscope slide, dropper pipette, coverslips, microscope Procedure 1. Examine a flower carefully. Make a detailed drawing of the flower and label as many parts as you can. Note whether the anthers are above or below the stigma. 2. Remove an anther and place it on a slide. While holding the anther with forceps, use the scalpel to cut one or more thin slices across the anther. CAUTION: Be careful with sharp tools. 3. Lay the slices flat on the microscope slide and add a drop of water and a coverslip. Observe the slices with the microscope at low power. Make a labeled drawing of your observations. 4. Repeat steps 2 and 3 with the ovary. Flowers vary greatly in shape, color, and size, as shown in Figure A typical flower produces both male and female gametophytes. In some plants, however, male and female gametophytes are produced in separate flowers on the same individual. Corn, for example, has separate male and female flowers on the same plant. The tassel is a flower that produces male gametophytes, and the silk is the style of a flower that contains the female gametophyte. In other cases, many flowers grow together to form a composite structure that looks like a single flower, as shown in the sunflower. What are the male structures in a typical flower? The female structures? Ray flowers Disk flowers Analyze and Conclude 1. Observing Are the anthers in this flower located above or below the stigma? How could this affect what happens to the pollen produced by the anthers? Explain your answer. 2. Applying Concepts What structures did you identify in the anther? What is the function of these structures? 3. Applying Concepts What structures did you identify in the ovary? What is the function of these structures? 4. Drawing Conclusions Which parts of the flower will become the seeds? The fruit? Figure 24 6 Flowers vary enormously in structure. The tulip has only a single carpel, whereas the wild rose has many carpels. Some flowerlike structures are actually clusters of many individual flowers. In the sunflower, disk flowers toward the inside of the cluster are reproductive, whereas ray flowers toward the outside are nonreproductive and form what look like petals. Formulating Hypotheses How might it be an advantage for a plant to have many flowers together in a single structure? Objective Students will be able to observe the structures of a flower and conclude which structures become seeds and which structures become the fruit. Skills Focus Observing, Applying Concepts, Drawing Conclusions Materials flower, forceps, scalpel, microscope slide, dropper pipette, coverslips, microscope Time 20 minutes Strategy Before students cut their flower, make sure they have noted whether the anthers are above or below the stigma. Analyze and Conclude 1. Self-pollinated flowers typically have anthers higher than the stigma, and pollen falls directly from the anthers onto the stigma. Many crosspollinated plants have taller stigmas that receive windblown or animalborne pollen from other flowers. 2. Students may be able to observe mature or immature pollen in the anthers. The pollen grains form male gametophytes that can fertilize female gametophytes and form zygotes that will grow into new plants. 3. Students may be able to observe mature or immature ovules in the ovary. The ovules produce female gametophytes that can be fertilized by male gametophytes. 4. The ovules will become the seeds. Generally, the ovary becomes the fruit, although other parts of the flower may also contribute to fruit formation. Sunflower Answers to... The male structures are the stamen and anthers; the female structures are the carpels, ovary, style, and stigma. Figure 24 6 Many flowers together in a single structure might attract more insects, which might improve the chances of pollination. Reproduction of Seed Plants 613

6 24 1 (continued) Life Cycle of Angiosperms Use Visuals Figure 24 7 Some students might be confused by the figure. Check their comprehension by asking: Where does fertilization take place? (Inside the ovary) How do pollen grains reach the ovary? (By growing pollen tubes down through the style) After fertilization, how many sets of chromosomes are contained within the endosperm? (Three) How many sets of chromosomes does the embryo have? (Two) From which part of the plant does the seed coat develop? (The outer part of the ovule) Address Misconceptions Students may not fully recognize the alternation of generations in the angiosperm life cycle. Review the definitions of sporophyte and gametophyte. Then, superimpose the angiosperm life cycle on the general plant life cycle in Figure 22 2 on page 552. Make sure students can identify the gametophyte stage and the sporophyte stage in angiosperms. Emphasize the single mitotic divisions that the haploid cells of the male and female gametophytes undergo. Identify pollen as the spores of the sporophyte. Haploid (N) Diploid (2N) Stigma Style Mature sporophyte Seedling (2N) (new sporophyte) Figure 24 7 This illustration shows the life cycle of a typical angiosperm an iris. The developing seeds of a flowering plant are protected and nourished inside the ovary, which is located at the base of the flower. Reproduction in angiosperms takes place within the flower. After pollination, the seeds of angiosperms develop inside protective structures. Ovary Endosperm Seed coat Ovary (2N) Embryo (2N) Fruit Anther (2N) MEIOSIS Egg cell Sperm Pollen tube Endosperm (3N) Zygote (2N) Pollen grains (N) (male gametophyte) Haploid cell (N) Ovule Embryo sac (N) (female gametophyte) Polar nuclei Life Cycle of Angiosperms Pollen tubes FERTILIZATION Reproduction in angiosperms takes place within the flower. Following pollination and fertilization, the seeds develop inside protective structures. The life cycle of angiosperms is shown in Figure You can think of the angiosperm life cycle as beginning when the mature sporophyte produces flowers. Each flower contains anthers and an ovary. Inside the anthers the male part of the flower each cell undergoes meiosis and produces four haploid spore cells. Each of these cells becomes a single pollen grain. The wall of each pollen grain thickens, protecting the contents of the pollen grain from dryness and physical damage when it is released from the anther. The nucleus of each pollen grain undergoes one mitotic division to produce two haploid nuclei. The pollen grain, which is the entire male gametophyte, usually stops growing until it is released from the anther and deposited on a stigma. FACTS AND FIGURES All about angiosperms There are at least 250,000 known species of angiosperms. Given their numbers, it is not surprising that they show great variability. For example, the length of time for completion of the angiosperm life cycle ranges from less than a month to as long as 150 years. Pollen tubes also show great variation. In corn, the pollen tube may be as long as 50 cm, but in most plants the pollen tube is much shorter. In addition, a pollen tube may complete its growth in less than 24 hours, but in some plants it takes over a year. The size of flowers varies greatly as well. The smallest flowers are those of the tiny duckweed Wolffia columbiana. Its flowers are only about 0.1 mm long. The largest flowers are those of the Rafflesia plant, which is indigenous to Indonesia. Its huge blooms can grow to 1 m in diameter and attain a mass of 9 kg. 614 Chapter 24

7 The ovary of the flower contains the ovules, in which the female gametophyte develops. A single diploid cell goes through meiosis to produce four haploid cells, three of which disintegrate. The remaining cell undergoes mitosis to produce eight nuclei. These eight nuclei and the surrounding membrane are called the embryo sac. The embryo sac, contained within the ovule, is the female gametophyte of a flowering plant. One of the eight nuclei, near the base of the gametophyte, is the egg nucleus the female gamete. If fertilization takes place, this cell will become the zygote that grows into a new sporophyte plant. Inside the ovary, the cells of the growing embryo begin to differentiate. That is, they begin to specialize, developing from a ball of cells into an embryonic sporophyte. The new sporophyte, or seedling, is shown in Figure Pollination Where does the female gametophyte develop? Once the gametophytes have developed inside the flower, pollination takes place. Most gymnosperms and some angiosperms are wind pollinated, whereas most angiosperms are pollinated by animals. These animals, mainly insects, birds, and bats, carry pollen from one flower to another. Because wind pollination is less efficient than animal pollination, wind-pollinated plants, such as the oak tree in Figure 24 8, rely on favorable weather and sheer numbers to get pollen from one plant to another. Animal-pollinated plants have a variety of adaptations, such as bright colors and sweet nectar, to attract animals. Animals have evolved body shapes that enable them to reach nectar deep within certain flowers. Insect pollination is beneficial to insects and other animals because it provides a dependable source of food pollen and nectar. Plants also benefit because the insects take the pollen directly from flower to flower. Insect pollination is more efficient than wind pollination, giving insect-pollinated plants a greater chance of reproductive success. Botanists suggest that insect pollination is the factor largely responsible for the displacement of gymnosperms by angiosperms during the past 100 million years. B C Figure 24 8 Most angiosperms are pollinated by animals, although some are pollinated by wind. The shape of a flower often indicates how it is pollinated. The flowers of an oak tree (A) are typical of wind-pollinated flowers in that they are small, are not brightly colored, and produce vast amounts of pollen. To attract insects and other animals, many animal-pollinated flowers are large and brightly colored. The rose flower (B) is pollinated by a variety of insects, whereas the trumpet creeper flower (C) has a tube shape that is adapted specifically to the long beak of a hummingbird. A Pollination Applying Concepts Have students imagine that they are botanists who have just discovered a new plant that has not yet been identified. They note that the plant has tiny green flowers that are difficult to see against the background of green leaves. Ask: How do you think this plant is pollinated? (By the wind, because it does not have large colorful flowers to attract animal pollinators) Make Connections Health Science Point out that many people are allergic to the pollen of flowers. Explain that allergies to pollen are actually reactions to proteins in the coat of the pollen grain and that people may be allergic to certain pollens but not to others because each type has a different protein coat. One of the most common pollen allergies is the allergic reaction known as hay fever. You might want to take a poll of students to see how many have this type of allergy. Explain that hay fever is not an allergy to hay but to certain windpollinated plants. Ask: Why might wind-pollinated plants create more problems for allergy sufferers than animal-pollinated plants? (Windpollinated plants usually produce more pollen because these plants release their pollen into the air in large amounts, and it is easily carried all over by wind.) Answer to... The female gametophyte develops in the ovules, which are contained in the ovary of the flower. Reproduction of Seed Plants 615

8 24 1 (continued) Fertilization in Angiosperms Use Visuals Figure 24 9 Have students identify the parts of the corn seed that are haploid (none), diploid (embryo), and triploid (endosperm). Review the process of double fertilization. Discuss its adaptive advantage. 3 ASSESS Evaluate Understanding Trace Figure 24 5 and give students a copy without the labels. Then, have students label the parts of the flower shown in the diagram. Reteach Review the life cycle of gymnosperms and angiosperms as students follow the stages shown in Figure 24 4 and Figure Seed plants: Dominant stage Diploid (2N); Zygote formation Two sperm nuclei in a pollen tube reach a female gametopyhte. One sperm nucleus disintegrates and the other fertilizes the egg, forming a diploid zygote; Occurrence of meiosis In gymnosperms, meiosis occurs in the pollen grains and in the ovules. In angiosperms, meiosis occurs in anthers and in ovules. Chlamydomonas: Dominant stage Haploid (N); Zygote formation Gametes gather in large groups, and then and + gametes form pairs, which join flagella and shed their cell walls and fuse, forming a diploid zygote; Occurrence of meiosis The thick-walled zygote divides and produces four flagellated haploid cells. Embryo Seed coat Endosperm Embryonic leaves Cotyledon Primary root Figure 24 9 The endosperm of a corn seed develops through the process of double fertilization. After one sperm nucleus fertilizes the egg cell, the zygote forms. Then the other sperm nucleus fuses with the two polar nuclei to form a triploid cell, which develops into the endosperm. Predicting What will happen to the endosperm when the seed begins to grow? 24 1 Section Assessment 1. Key Concept What are the reproductive structures of gymnosperms? 2. Key Concept Describe the flower and how it is involved in reproduction. 3. Key Concept Are angiosperms typically wind pollinated or animal pollinated? How does this process occur? Fertilization in Angiosperms If a pollen grain lands on the stigma of an appropriate flower of the same species, it begins to grow a pollen tube. The generative nucleus within the pollen grain divides and forms two sperm nuclei. The pollen tube now contains a tube nucleus and two sperm nuclei. The pollen tube grows into the style. There, it eventually reaches the ovary and enters the ovule. Inside the embryo sac, two distinct fertilizations take place. First, one of the sperm nuclei fuses with the egg nucleus to produce a diploid zygote. The zygote will grow into the new plant embryo. Second, the other sperm nucleus does something truly remarkable it fuses with two polar nuclei in the embryo sac to form a triploid (3N) cell. This cell will grow into a food-rich tissue known as endosperm, which nourishes the seedling as it grows. As shown in Figure 24 9, a seed of corn, a monocot, contains a rich supply of endosperm. In many dicots, including garden beans, the cotyledons absorb the endosperm as the seed develops. The cotyledons then serve as the stored food supply for the embryo when it begins to grow. Because two fertilization events take place between the male and female gametophytes, this process is known as double fertilization. Double fertilization may be one of the reasons why the angiosperms have been so successful. Recall that in gymnosperms, the food reserve built up in seeds is produced before fertilization takes place. As a result, if an ovule is not fertilized, those resources are wasted. In angiosperms, if an ovule is not fertilized, the endosperm does not form, and food is not wasted by preparing for a nonexistent zygote. 4. What is endosperm? Where does it form in a flowering plant? 5. Critical Thinking Inferring Many flowers have bright patterns of coloration that directly surround the reproductive structures. How might this type of coloration be advantageous to the plant? Alternation of Generations Review the life cycle of the green alga Chlamydomonas in Section Make a compare-and-contrast table comparing alternation of generations in seed plants and Chlamydomonas. Include which stage (haploid or diploid) of each organism s life cycle is dominant, the process by which zygotes form, and when meiosis occurs. If your class subscribes to the itext, use it to review the Key Concepts in Section Answer to... Figure 24 9 As the seed develops, the food stored in the endosperm is absorbed by the cotyledon and then used by the growing embryo. 616 Chapter Section Assessment 1. The reproductive structures of gymnosperms are pollen cones, pollen grains, seed cones, ovules, and pollen tubes. 2. Flowers are reproductive organs that are composed of four kinds of specialized leaves: sepals, petals, stamens, and carpels. The stamens produce male gametophytes, and the carpels produce female gametophytes. 3. Angiosperms are typically pollinated by animals. Insects, birds, and bats carry pollen from one flower to another as they gather nectar. 4. A food-rich tissue that nourishes a seedling as it grows; inside the embryo sac 5. Bright patterns of coloration might attract insects and other animals to the reproductive structures of the flower and increase the chances of pollination.

9 What s your favorite flower? Perhaps your answer was the rose. Roses are the world s most popular ornamental flowers. They come in many colors. Chances are you ve seen red, white, pink, or even yellow roses. But have you ever seen a blue rose? Probably not! Roses do not have the enzymes to produce blue pigments, so even the best efforts of plant breeders have not produced a blue rose. What Makes a Flower? Botanists have discovered that flower development is controlled by a series of genes. By manipulating these genes, scientists have produced plants that will flower earlier and much faster than normal. Changing the color of a flower, however, has proved a little more difficult. Knowing that petunias often produce blue flowers, in 1991 Australian researchers isolated the gene for the enzyme that produces blue pigment. Then, they transferred this blue gene to a rose. To their disappointment, however, the new roses were just as red as ever. Apparently, flower color is a tricky and unpredictable business particularly in roses that involves complex interactions with other genes and pigments. Violet Carnations? When the Australian scientists turned from roses to carnations, they produced a carnation with unique violet flowers. Again, they inserted the gene from the blue petunias into a carnation plant. The result, shown in the photos, was a deep violet carnation unlike any ever seen in nature. In 1999, these genetically modified carnation plants were introduced for sale in Europe and the United Using Technology to Design Flowers States. A number of biotech companies are hoping to master the intricacies of color genetics in flowers. Someday, one of these companies may have what they ve all been seeking a blue rose. Research and Decide 1. Use library or Internet resources to learn more about the relationship between genetics and new flower varieties. Then, choose a common food crop and find out how breeders have modified the plants to give the crop specific traits. 2. Suppose you are a plant geneticist and you want to create a new color of lily. Decide which flower color you would like to produce. Then, write down the scientific steps that you would take to produce the new flower color. After students have read this feature, you might want to discuss one or more of the following: Genetic engineering methods, such as the use of restriction enzymes and DNA insertion, that are used to isolate a gene from one plant or insert it into another plant Other traits of flowers, besides color, that breeders might try to modify, such as season of bloom, length of bloom period, size of flowers, number of petals, and number of flowers Examples of plants produced by breeders that have unusual characteristics, such as plants that have black or nearly black flowers (for example, columbine, hollyhock, viola, and sweet pea) Reasons why varieties of plants bred for certain traits, such as color, may not be as hardy as the standard varieties Research and Decide 1. Students should choose any common food crop that has been genetically modified, such as tomatoes, potatoes, squash, or corn. They should describe how the plants have been modified to exhibit certain traits. 2. Students should select any color they would like to produce in a lily plant, such as yellow, pink, red, or orange. The steps they would take might include first isolating a gene from another plant that codes for the color of their choice and then inserting the gene into a lily plant. For: Links from the authors Visit: PHSchool.com Web Code: cbe-7241 Students can research genetically modified plants on the site developed by authors Ken Miller and Joe Levine. Reproduction of Seed Plants 617