Sexual Reproduction in Seedless Plants

Transcription

1 530 Section 1 Sexual Reproduction in Seedless Plants Objectives Summarize the life cycle of a moss. 13B Summarize the life cycle of a fern. 13B Compare and Contrast the life cycle of a moss with the life cycle of a fern. 13B Key Terms archegonium antheridium sorus Reproduction in Nonvascular Plants The carpet of green you often see near streams and in moist, shady places is usually made up of mosses or liverworts. As you learned in the previous chapter, these small, relatively simple plants are nonvascular plants. They do not have a vascular system for distributing water and nutrients. Mosses and liverworts do not usually thrive outside moist places because they must be covered by a film of water to reproduce sexually. Like all plants, nonvascular plants have a life cycle called alternation of generations. In this type of life cycle, a gamete-producing stage, or gametophyte, alternates with a spore-producing stage, or sporophyte. Gametophytes produce gametes (eggs and sperm) in separate multicellular structures. The structure that produces eggs is called an archegonium (ark uh GOHN ee uhm). The structure that produces sperm is called an antheridium (an thuhr IHD ee uhm). Sporophytes produce spores in a sporangium. The gametophytes of nonvascular plants are larger and more noticeable than are the sporophytes. This difference in size is very pronounced in the liverworts, as you can see in Figure 1. Figure 1 Reproductive structures of a liverwort Sporophytes, which grew from archegonia under The gametophytes of Marchantia, a common liverwort, the cap of a female stalk produce male and female gametes on separate stalks. Antheridia on top of a male stalk Sporophytes Male stalks Female stalks Archegonia under the cap of a female stalk

2 Life Cycle of a Moss The life cycle of a moss is summarized in Figure 2. Sexual reproduction results in a fertilized egg, or zygote. The diploid zygote grows into a new diploid sporophyte. As you can see, a moss sporophyte grows from a gametophyte and remains attached to it. The sporophyte consists of a bare stalk with a spore capsule (sporangium) at its tip. Spores form by meiosis inside the spore capsule. Therefore, as in all plants, the spores are haploid. The spore capsule opens when the spores are mature, and the spores are carried away by wind or water. When a moss spore settles to the ground, it germinates and grows into a leafy green gametophyte. Archegonia and antheridia form at the tips of the haploid gametophytes. Eggs and sperm form by mitosis inside the archegonia and antheridia. Remember, moss gametophytes grow in tightly packed clumps of many individuals. When water covers a clump of mosses, sperm can swim to nearby archegonia and fertilize the eggs inside them. The word archegonium comes from the Greek words archegonos, meaning first of a race. Knowing this makes it easier to remember that a new and genetically different individual grows from an archegonium when its egg is fertilized. 2 Diploid (2n) An adult sporophyte produces spores within its spore capsule. 3 Haploid (n) Spores grow into male and female gametophytes. Figure 2 Moss life cycle. In mosses, a sporophyte that consists of a spore capsule on a bare stalk alternates with a leafy, green gametophyte. Adult sporophyte Meiosis Spore capsule (sporangium) Spores Germinating spore 4 Gametophytes produce gametes inside antheridia and archegonia. Mitosis Young sporophyte Gametophytes Male Female 1 A zygote develops into a new sporophyte. Mitosis Antheridia Sperm Zygote Fertilization 5 Archegonia Sperm swim to and fertilize eggs inside the archegonia. Egg

3 532 Figure 3 Sori on a fern frond. Many sori are visible on this portion of a frond from a polypody fern. Each sorus consists of about sporangia. Reproduction in Seedless Vascular Plants You may recall that the seedless vascular plants include the whisk ferns, horsetails, club mosses, and ferns. The seedless vascular plants differ from the nonvascular plants because they have efficient water- and foodconducting systems of vascular tissue. Like the nonvascular plants, the seedless vascular plants thrive in moist, shady places. They can reproduce sexually only when a film of water covers the gametophyte. Eggs form in archegonia, and sperm form in antheridia. The archegonia and antheridia develop on the lower surfaces of the gametophytes. In most species of seedless vascular plants, both eggs and sperm are produced by the same individual. In some species, however, eggs and sperm are produced by separate gametophytes. Unlike nonvascular plants, seedless vascular plants have sporophytes that are much larger than their gametophytes. Some ferns, for example, have sporophytes that are as large as trees. On the other hand, the gametophytes of ferns are thin, green, heart-shaped plants that are less than 1 cm (0.5 in.) across. The sporophytes produce spores in sporangia. In horsetails and club mosses, sporangia develop in cones. In ferns, clusters of sporangia form on the lower surfaces of fronds, as shown in Figure 3. A cluster of sporangia on a fern frond is called a sorus. The word sorus comes from the Greek word soros, meaning a heap. Observing a Fern Gametophyte 13B You can observe the archegonia and antheridia of a fern gametophyte with a microscope. Materials prepared slide of a fern gametophyte with archegonia and antheridia, compound microscope Fern Gametophytes (56 ) Procedure 1. Examine a slide of a fern gametophyte under low power of a microscope. Move the slide until you can see a cluster of archegonia. Now, switch to high power, and focus on one archegonium. Draw and label what you see. 2. Switch back to low power, and move the slide until you can see several egg-shaped structures. These are antheridia. Now, switch to high power, and focus on one antheridium. Draw and label what you see. Analysis 1. Describe the appearance of an archegonium and an antheridium. 2. Critical Thinking Drawing Conclusions In which structure, an archegonium or antheridium, does the growth of a new sporophyte begin? Explain.

4 Life Cycle of a Fern The life cycle of a fern is summarized in Figure 4. A fertilized egg, or zygote, grows into a new sporophyte. The diploid sporophyte produces spores by meiosis. The haploid spores fall to the ground and grow into haploid gametophytes. Fern gametophytes produce gametes by mitosis eggs in archegonia and sperm in antheridia. Sperm swim to archegonia and fertilize the eggs inside them. Figure 4 Fern life cycle. In ferns, a large sporophyte with leaves called fronds alternates with a small, green, heart-shaped gametophyte. Diploid (2n) Haploid (n) 2 An adult sporophyte produces spores in clusters of sporangia. Meiosis Sporangium Mitosis 3 The spores grow into a gametophyte. Spores Lower surface 4 Gametophytes produce gametes inside antheridia and archegonia. Mature gametophyte Frond Adult sporophyte Antheridium Rhizome Roots Archegonium Mitosis 1 A zygote develops into a new sporophyte. 5 Sperm swim to the archegonia and fertilize eggs. Young sporophyte Mitosis Zygote Sperm Egg Fertilization Section 1 Review List five major steps in the life cycle of a moss. List five major steps in the life cycle of a fern. Critical Thinking Forming Reasoned Opinions Which reproductive structures, gametes or spores, are responsible for the dispersal (spread) of seedless plants? Justify your answer. 13A 13B 13B 13B Critical Thinking Analyzing Information What are the major differences between the moss life cycle and the fern life cycle? 13B TAKS Test Prep What is the function of an archegonium? 13B A to produce sperm C to produce spores B to produce eggs D to conduct water 533

5 Section 2 Sexual Reproduction in Seed Plants Objectives Distinguish the male and female gametophytes of seed plants. 13B Describe the function of each part of a seed. 13B Summarize the life cycle of a conifer. 13B Relate the parts of a flower to their functions. 13B Summarize the life cycle of an angiosperm. 13B Key Terms pollen grain ovule pollination pollen tube seed coat cotyledon sepal petal stamen anther pistil ovary double fertilization Reproductive Structures of Seed Plants Reproduction in seed plants (gymnosperms and angiosperms) is quite different from reproduction in seedless plants. For one thing, you need a microscope to see the gametophytes of seed plants, as Figure 5 shows. Also, spores are not released from seed plants. The spores remain within the tissue of a sporophyte and develop into two kinds of gametophytes male gametophytes, which produce sperm, and female gametophytes, which produce eggs. The tiny gametophytes of seed plants consist of only a few cells. A male gametophyte of a seed plant develops into a pollen grain, which has a thick protective wall. A female gametophyte of a seed plant develops inside an ovule (AHV yool), which is a multicellular structure that is part of the sporophyte. Following fertilization, the ovule and its contents develop into a seed. Because the gametophytes of seed plants are very small, seed plants are able to reproduce sexually without water. Wind and animals transport pollen grains to the structures that contain ovules. The transfer of pollen grains from the male reproductive structures of a plant to the female reproductive structures of a plant is called pollination. When a pollen grain reaches a compatible female reproductive structure, a tube emerges from the pollen grain. This tube, called a pollen tube, grows from a pollen grain to an ovule and enables a sperm to pass directly to an egg. Figure 5 Seed plant gametophytes The tiny gametophytes of seed plants develop within specialized structures that form in the reproductive parts of a flower. Pollen grains Pollen grains are transferred to a female structure during pollination. Male reproductive structure Ovules A pollen grain consists of only two or three cells. Female reproductive structure The female gametophyte within an ovule consists of only seven cells.

6 Seeds As you learned in the previous chapter, seeds contain the embryos of seed plants. A plant embryo is a new sporophyte. A seed forms from an ovule after the egg within it has been fertilized. The outer cell layers of an ovule harden to form the seed coat as a seed matures. The tough seed coat protects the embryo in a seed from mechanical injury and from a harsh environment. The seed coat also prevents the embryo from immediately growing into a young plant by keeping out water and oxygen. Deprived of water and oxygen, the embryo stops growing and cannot resume its growth until water and oxygen can pass through the seed coat. Often, a seed must be exposed to cold temperatures, or the seed coat must be damaged, before the seed can take in water and oxygen. Thus, seeds enable the embryos of seed plants to survive conditions that are unfavorable for plant growth for long periods of time. Seeds also contain tissue that provides nutrients to plant embryos. In gymnosperms, this nutritious tissue is part of the female gametophyte. The seeds of angiosperms, however, develop a nutritious tissue called endosperm. Endosperm originates at the same time an egg is fertilized. In some angiosperms, such as corn and wheat, endosperm is still present in mature seeds. In other angiosperms, such as beans and peas, the nutrients in the endosperm have already been transferred to the embryo by the time a seed is mature. Leaflike structures called cotyledons (kah tuh LEE duhnz), or seed leaves, are a part of a plant embryo. Cotyledons function in the transfer of nutrients to the embryo. The embryos of gymnosperms have two or more cotyledons. For example, pine embryos have eight cotyledons. In the flowering plants, the embryos of monocots have one cotyledon, and the embryos of dicots have two cotyledons. The structure of three types of seeds is shown in Figure 6. Interpreting Graphics After reading the chapter, trace or make a sketch of Figure 6 without the labels. On separate pieces of paper, write down the labels. Without referring to your book, match the labels with the correct parts of your sketch. Figure 6 Seed structure Seeds have many similarities and differences in structure. Pine seed Corn grain Bean seed Wing Endosperm (3n) Embryo Seed coat Seed coat Embryonic leaves Cotyledons Embryonic root Embryonic root Female gametophyte (n) Embryo Seed coat fused to ovary wall Cotyledons 535

7 536 Figure 7 Male and female pine cones. This branch of an Austrian pine has an immature seed cone and many pollen cones. Cones Seed plants are the most successful of all plants. The success of the seed plants is due in part to the specialized structures in which seeds develop. In angiosperms, the ovules (immature seeds) are completely enclosed by sporophyte tissue at the time of pollination. In gymnosperms, the ovules are not completely enclosed by sporophyte tissue until after pollination. The gametophytes of gymnosperms develop in cones, which consist of whorls (circles) of modified leaves called scales. Gymnosperms produce two types of cones. Male cones, or pollen cones, produce pollen grains within sacs that develop on the surface of their scales. Female cones, or seed cones, produce ovules on the surface of their scales. Many gymnosperms produce both male and female cones on the same plant. As shown in Figure 7, the numerous small pollen cones lie to the left of the large seed cone. In some gymnosperms, male and female cones form on separate plants. Pollen cones produce large quantities of pollen grains that are carried by wind to female cones. At the time of pollination, the scales of a female cone are open, exposing the ovules. When a pollen grain lands near an ovule, a slender pollen tube grows out of the pollen grain and into the ovule. The sperm moves through the pollen tube and enters the ovule. Thus, the pollen tube delivers a sperm to the egg inside the ovule. Seed cones close up after pollination and remain closed until the seeds within them are mature. This process can take up to two years. Observing the Gametophytes of Pines 13B You can observe the gametophytes of a pine with a microscope. Materials prepared slides of the following: male pine cone, female pine cone, pine ovule; hand lens; compound microscope Immature female pine cone Procedure 1. Examine prepared slides of male and female pine cones first with a hand lens and then under the low power of a microscope. 2. Make a sketch of each type of pine cone, and label the structures that you recognize. 3. Examine a prepared slide of a pine ovule under the low power of a compound microscope. Compare what you see with the photo above. 4. Draw a pine ovule, and label the following structures: scale, ovule, egg, pollen tube (if visible). Analysis 1. Compare and Contrast the structure and contents of male and female pine cones. 2. Critical Thinking Applying Information It takes 15 months for a pine pollen tube to grow through the wall of a pine ovule. How would you describe the rate of pollen-tube growth in pines?

8 537 Life Cycle of a Conifer Most gymnosperms are conifers, a group that includes pines. You can trace the stages in the life cycle of a pine in Figure 8. In pines, as in all plants, a diploid zygote results from sexual reproduction. The zygote develops into an embryo, which then becomes dormant (inactive). The embryo and the surrounding tissues form a seed. When their seeds are mature, seed cones open, and the seeds fall out. A pine seed has a wing that causes it to spin like the blade of a helicopter. Thus, pine seeds often travel some distance from their parent tree. When conditions are favorable for growth, the seeds grow into new sporophytes. An adult pine tree produces both male and female cones. Spores form by meiosis, which occurs inside immature cones. The spores grow into gametophytes, which produce eggs and sperm by mitosis. After pollination, a pollen tube begins to grow from each pollen grain toward the eggs inside an ovule. Fertilization occurs as a sperm fuses with an egg, forming a zygote that will grow into a new sporophyte. Figure 8 Conifer life cycle. In conifers, a very large sporophyte that produces cones alternates with tiny gametophytes that form on the scales of cones. Diploid (2n) Scale Immature seed cone Ovule Meiosis Meiosis Female spore 3 Haploid (n) Male and female spores form on the scales of Gametophytes the cones. Pollen (male gametophytes) 4 Spores develop into male and female gametophytes. Eggs (within female gametophyte) Sperm Adult sporophyte 2 Pollen cone An adult pine produces male and female cones. Male spore 5 Pollination Fertilization After pollination, sperm enter the ovule through a pollen tube, and fertilization occurs. Mitosis 1 The zygote and ovule develop into a seed, which grows into a new sporophyte. Mature seed cone Young sporophyte Mitosis Pine seed (with wing) Scale Pollen tube Zygote

9 538 Figure 9 Basic flower structure. The four basic parts of a flower sepals, petals, stamens, and pistils are arranged in concentric whorls. Stamen Anther Filament Sepal Flowers In angiosperms, gametophytes develop within flowers. The basic structure of a flower is shown in Figure 9. Flower parts are arranged in four concentric whorls. The outermost whorl consists of one or more sepals (SEE puhlz), which protect a flower from damage while it is a bud. The second whorl consists of one or more petals, which attract pollinators. The third whorl consists of one or more stamens (STAY muhnz), which produce pollen. Each stamen is made of a threadlike filament that is topped by a pollenproducing sac called an anther. The fourth and innermost whorl of a flower consists of one or more pistils, which produce ovules. Ovules Petal develop in a pistil s swollen lower portion, which is called the ovary. Usually, a stalk, called the style, rises from the ovary. Pollen lands on Stigma Style Pistil Ovary and sticks to the stigma the swollen, sticky tip of the style. Flowers may or may not have all four of the basic flower parts. A flower that has all four parts is called a complete flower. Flowers that lack any one of the four types of parts are called incomplete flowers. If a flower has both stamens and pistils, it is called a perfect flower. Flowers that lack either stamens or pistils are called imperfect flowers. Observing the Arrangement of Parts of a Flower 13A 13B TAKS 2, TAKS 3 You can see how the parts of flowers are arranged by dissecting flowers. Materials gloves, monocot flower, dicot flower, paper, tape Procedure 1. Put on gloves. Examine a monocot flower and a dicot flower. Locate the sepals, petals, stamens, and pistil of each flower. 2. Separate the parts of each flower, and tape them to a piece of paper. Label each set of parts. 3. Count the number of petals, sepals, and stamens in each flower. Record this information below each flower. Analysis 1. Compare and Contrast the appearance of the sepals and petals of each flower. 2. Critical Thinking Forming a Hypothesis For each flower, suggest a function for the petals based on their appearance. 3. Critical Thinking Justifying Conclusions Explain why each flower is from either a monocot or a dicot.

10 539 Flowers and Their Pollinators Many flowers have brightly colored petals, sugary nectar, strong odors, and shapes that attract animal pollinators. Flowers are a source of food for pollinators such as insects, birds, and bats. For example, bees eat nectar and collect pollen, which is a rich source of protein they feed to their larvae. A bee gets coated with pollen as it visits a flower and then carries that pollen to other flowers. Bees locate flowers by scent first and then by color and shape. Bee-pollinated flowers are usually blue or yellow and often have markings that show the location of nectar. Moths, which feed at night, tend to visit heavily scented white flowers, which are easy to find in dim light. Flies may pollinate flowers that smell like rotten meat. Many flowers are not pollinated primarily by insects. Red flowers, for instance, may be pollinated by hummingbirds. Some large white flowers that open at night are pollinated by nighttime visitors bats, as seen in Figure 10. Many flowers, such as those of grasses and oaks, are pollinated by wind. Wind-pollinated flowers are usually small and lack bright colors, strong odors, and nectar. Figure 10 Bat pollination. This lesser long-nosed bat pollinates an organ pipe cactus as it feeds on the pollen of the plant s flowers. Protecting Honeybees TAKS 3 Bees pollinate more species of plants than any other animal. The most familiar of the more than 20,000 species of bees is the European honeybee, Apis mellifera, which was imported to the United States in the 1600s. Beekeepers raise honeybees mainly for the honey they produce. However, the bees also benefit farmers by pollinating more than 90 kinds of crop plants, which are worth $10 billion a year. Threats to Honeybees Since 1990, the population of beekeeper-raised honeybees has decreased by 25 percent in the United States. Some of this decline is due to pesticide use and loss of food sources, but the major culprits are parasites, pests, and diseases. The most serious problem currently facing U.S. honeybees is the varroa mite. This tiny, blood-sucking parasite probably entered the United States in the 1980s. It now infests beehives throughout most of North America, causing approximately $160 million worth of damage each year. The small hive beetle is a pest that was introduced to the southern United States from Africa in The immature beetles tunnel through hives in search of honey and pollen, killing the developing bees and forcing entire colonies to abandon their hives. American foulbrood is a bacterial disease that attacks developing honeybees. Highly contagious, the disease has the potential to spread rapidly throughout the United States. Research to the Rescue Scientists at the Kika De La Garza Subtropical Agricultural Research Center in Weslaco, Texas are studying ways to defeat the biological threats to honeybees. The scientists test natural and synthetic chemicals for use in controlling varroa mites and small hive beetles. They are also working to develop new antibiotics that will be effective against the bacteria that cause American foulbrood. Topic: Crop Pollination Keyword: HX4053

11 Figure 11 Angiosperm life cycle. In angiosperms, a large sporophyte alternates with tiny gametophytes. Life Cycle of an Angiosperm Figure 11 summarizes the life cycle of an angiosperm. Following fertilization, the zygote and the tissues of the ovule develop into a seed, which grows into a new sporophyte. The adult sporophytes of angiosperms produce spores by meiosis. These spores grow into gametophytes. The female gametophytes grow inside the ovules, which develop within the ovary of a pistil. The male gametophytes, or pollen grains, are produced in the anther of a stamen. A pollen grain contains two sperm cells. One sperm fuses with the egg, forming the zygote. The other sperm fuses with the haploid nuclei of two other cells produced by meiosis. The fusing of three haploid (n) cells forms a triploid (3n) cell that develops into endosperm. This is a process called. double fertilization Diploid (2n) Haploid (n) 2 A flower produces male spores inside its anthers and female spores inside its pistil. Adult sporophyte Anther Stamen Meiosis Male spores Pollen grains (male gametophytes) 3 Spores develop into male and female gametophytes. Pollination Pistil Gametophytes Flower Ovule Meiosis Ovule 4 Pollination occurs when a pollen grain lands on the stigma of a pistil. Pollen tube Endosperm (3n) 1 Mitosis Seed Seed coat Sporophyte embryo Mitosis The zygote and ovule develop into a seed, which grows into a new sporophyte. 3n nucleus Zygote Mature female spore (female gametophyte) Pollen tube Double fertilization Egg Sperm 5 Sperm enter an ovule through a pollen tube, and fertilization occurs. Section 2 Review 540 Distinguish pollen grains from ovules. 5A 13B Describe the function of each part of a seed. 13A Summarize the life cycle of a conifer. 13B Critical Thinking Relating Concepts How is each part of a flower suited to its function? 13A Critical Thinking Summarizing Information What are the main events in the life cycle of an angiosperm? 13B TAKS Test Prep In angiosperms, pollen is produced in sacs called 13B TAKS 3 Bio 13A A sepals. C pistils. B anthers. D ovules.

12 Asexual Reproduction Section 3 Vegetative Reproduction Most plants are able to reproduce asexually. The new individuals that result from asexual reproduction are genetically the same as the parent plant. Plants reproduce asexually in a variety of ways that involve nonreproductive parts, such as stems, roots, and leaves. The reproduction of plants from these parts is called vegetative reproduction. Many of the structures by which plants reproduce vegetatively are modified stems, such as runners, bulbs, corms, rhizomes, and tubers. Table 1 describes these structures. Vegetative reproduction is faster than sexual reproduction in most plants. A single plant can spread rapidly in a habitat that is ideal for its growth by reproducing vegetatively. Therefore, a mass of hundreds or even thousands of individuals, such as a stand of grasses or ferns, may have come from one individual. To learn about one unique method of vegetative reproduction in one plant, look at Up Close: Kalanchoë, on the next two pages. Objectives Describe several types of vegetative reproduction in plants. 13B Distinguish sexual reproduction in kalanchoës from asexual reproduction in kalanchoës. 13B Recommend several ways to propagate plants. 13A 13B TAKS 3 Key Terms vegetative reproduction plant propagation tissue culture Table 1 Stems Modified for Vegetative Reproduction Name Description Examples Runner Horizontal, aboveground stem Airplane plant, Bermuda grass Bulb Very short, stem with thick, fleshy leaves; only in monocots Onion, daffodil, tulip Corm Very short, thickened, underground stem with thin, scaly leaves Gladiolus, crocus Rhizome Horizontal underground stem Iris, fern, sugar cane Tuber Swollen, fleshy, underground stem Potato, caladium

13 542 Up Close Kalanchoë TAKS 2, TAKS 3 Scientific name: Kalanchoë daigremontiana Size: Grows from 30 cm (1 ft) to 1 m (3 ft) tall Range: Native to southwestern Madagascar; cultivated worldwide Habitat: Semiarid tropical grassland with moist summers and well-drained, fertile soil Importance: Kalanchoës (kal an KOH eez) are grown as indoor potted plants and as outdoor perennials in warm climates. External Structures Leaves The fleshy leaves are bluish green, with purple markings and saw-toothed margins. Leaf blades range from 12 to 25 cm (4 to 10 in.) long. Leaves are arranged in pairs that are opposite one another. Flowers A cluster of flowers forms on a flowering stalk that grows from the end of a stem. The flowers are bell-shaped and about 2.5 cm (1 in.) long. Flower parts occur in fours. Each flower produces many tiny seeds. Flower Plantlet Plantlets Tiny new plants develop along leaf margins. These plantlets are a means of vegetative reproduction. When a plantlet falls to the ground, it grows into a new plant. Leaf cutting Stem and leaf cuttings Kalanchoës are often propagated vegetatively by planting stem and leaf cuttings. Air roots Air roots The roots that grow from the stems and from plantlets originate from stem tissue.

14 543 Internal Structures Leaf structure Kalanchoës are succulents, which means they have fleshy leaves and stems that store water. A kalanchoë leaf shows how some succulents are adapted for conserving water. A thick cuticle covers the leaf, and the epidermis (outer layer of cells) consists of several layers of cells. Relatively few, very small stomata dot the leaf surfaces. Cuticle Epidermis Mesophyll Epidermis Vascular bundle Stoma Central vacuole Large central vacuole The cells inside a leaf, called the mesophyll cells, have a large central vacuole that can hold a great deal of water. Organelles Mesophyll cell Night Day Mesophyll cells CO 2 CO2 CAM photosynthesis Kalanchoës belong to the Crassulaceae family, a group of succulent plants that are adapted to hot climates. Photosynthesis in kalanchoës involves a process called crassulacean acid metabolism (CAM). The stomata of CAM plants open only at night, unlike those of other plants. At night, the plants fix carbon dioxide by using it to make malic acid. The malic acid is stored in the large central vacuoles of the mesophyll cells. In daytime, the stomata remain closed, which prevents water loss. Carbon dioxide is released from malic acid during the day and used by the Calvin cycle to make sugar. Malic acid Central vacuole Cell wall Sugar Cell membrane Calvin cycle Cytoplasm

15 Figure 12 Budding pears. A bud from a desirable variety of pears is attached to a stem of another pear species. The bud will grow into a branch that produces the desirable variety of pears. Plant Propagation People grow plants for many purposes, such as for food, to beautify homes, or to sell. Most field crops, such as cereal grains, vegetables, and cotton, are grown from seed. Many other plants are grown from vegetative parts. Growing new plants from seed or from vegetative parts is called plant propagation. Plants are often propagated using the structures the plants produce for vegetative reproduction. Bulbs and corms divide as they grow, forming many pieces that can each grow into a new plant. Rhizomes, roots, and tubers can be cut or broken into pieces with one or more buds that can grow into new shoots. But people also grow plants from vegetative parts that are not specialized for vegetative reproduction. For example, pieces of plants, such as the stems of ivys and the leaves of African violets, are cut from the parent plant. The cuttings are then used to grow new plants. Figure 12 shows a method of propagating trees called budding. In another technique called tissue culture, pieces of plant tissue are placed on a sterile medium and used to grow new plants. Table 2 summarizes some of the methods of vegetative plant propagation that are widely used to grow plants. Table 2 Methods of Vegetative Plant Propagation Method Description Examples Budding Small stems from one plant are attached to Grape vines, hybrid roses, fruit and and grafting larger stems or roots of another plant. nut trees Taking Leaves or pieces of stems or roots are cut from African violets, ornamental trees and cuttings one plant and used to grow new individuals. shrubs, figs Tissue culture Pieces of tissue from one plant are placed on a sterile medium and used to grow new individuals. Orchids, potatoes, many houseplants Section 3 Review Describe four types of vegetative reproduction in plants, and give an example of each. 13B Classify methods of reproduction in kalanchoës as sexual or asexual. 13B Recommend five ways to propagate plants. 13B Critical Thinking Justifying Conclusions Why would someone choose to propagate a particular plant for commercial purposes by using vegetative structures instead of seed? TAKS Test Prep Bermuda grass reproduces asexually by means of horizontal, aboveground stems called 13B 13A TAKS 2 A corms. C tubers. B rhizomes. D runners. 13B

16 Key Concepts 1 Sexual Reproduction in Seedless Plants In mosses, the leafy green gametophytes are larger than the sporophytes, which consist of a bare stalk and a spore capsule. In the life cycle of a fern, the sporophytes are much larger than the gametophytes. The thin, green, heart-shaped gametophytes produce both sperm and eggs. Nonvascular plants and seedless vascular plants need water for fertilization because sperm must swim to eggs. 2 Sexual Reproduction in Seed Plants The tiny gametophytes of seed plants develop from spores that remain within sporophyte tissues. Male gametophytes develop into pollen grains. Female gametophytes develop inside ovules. A seed contains an embryo, which is a new sporophyte, and a supply of nutrients for the embryo. The cotyledons of an embryo help transfer nutrients to the embryo. A seed coat covers and protects a seed. In gymnosperms, male and female gametophytes develop in separate cones on the sporophytes. After fertilization, ovules develop into seeds, which grow into new sporophytes. Flowers have four types of parts petals, sepals, stamens, and pistils. Petals attract pollinators. Sepals protect buds and may also attract pollinators. Pollen forms in the anthers of stamens. Seeds develop in the ovary of a pistil. In angiosperms, male and female gametophytes develop in the flowers of the sporophytes. After fertilization, ovules develop into seeds, which grow into new sporophytes. 3 Asexual Reproduction Vegetative reproduction is the growth of new plants from nonreproductive plant parts, such as stems, roots, and leaves. Kalanchoës are succulents that are often grown as potted plants and readily reproduce either vegetatively or by seeds. People often grow plants from their vegetative structures. This is called vegetative propagation. Key Terms Section 1 archegonium (530) antheridium (530) sorus (532) Section 2 pollen grain (534) ovule (534) pollination (534) pollen tube (534) seed coat (535) cotyledon (535) sepal (538) petal (538) stamen (538) anther (538) pistil (538) ovary (538) double fertilization (540) Section 3 vegetative reproduction (541) plant propagation (544) tissue culture (544)

17 546 Using Key Terms 1. A structure that produces eggs in mosses and ferns is called a(n) 13B a. archegonium. c. ovule. b. sporangium. d. antheridium. 2. In seed plants, the transfers sperm from a pollen grain directly to an egg in an ovule. 13B a. pollinator c. endosperm b. seed coat d. pollen tube 3. In conifers, the sporophyte produces spores and gametophytes in 13B a. flowers. c. sori. b. cones. d. sporangia. 4. Which part of a flower produces eggs? 13B a. pistil c. stamen b. petal d. sepal 5. For each pair of terms, explain the difference in their meanings. a. pollen grain, ovule b. sepal, petal c. cotyledon, endosperm d. reproduction, propagation Understanding Key Ideas 6. Mosses and liverworts thrive in a moist environment because they need for reproduction. 13B a. bees c. water b. birds d. wind 7. The life cycle of a moss differs from the life cycle of a fern in that 13B a. the gametophyte is absent in ferns. b. the sporophyte is absent in mosses. c. moss spores do not form on leaves. d. the gametophytes of mosses are green. 8. In angiosperms, the zygote and the first cell of the endosperm form by 13B a. mitosis. b. meiosis. c. pollination. d. double fertilization. 9. Vegetative reproduction has not occurred when a new plant grows from a 13B a. leaf. c. stem. b. root. d. seed. 10. Which of the following structures do kalanchoës produce for vegetative reproduction? 13B a. seeds c. flowers b. plantlets d. bulbs 11. Which of the following structures is not used to propagate dicots vegetatively? 13B a. tubers c. bulbs b. rhizomes d. stem cuttings 12. Look at the flower in the photograph below. It is the flower of the unicorn plant. How is this flower probably pollinated? Justify your answer. 13B 13. What is being done to counter biological threats to honeybees in Texas? 14. What is the function of the fruits in which seeds mature? (Hint: See Chapter 23, Section 1.) 13B 15. Concept Mapping Make a concept map that explains how plants reproduce. Try to include the following terms in your map: archegonium, antheridium, egg, sperm, ovule, zygote, stamen, anther, pistil, ovary, fertilization, spore, and vegetative reproduction. 3E

18 Critical Thinking 16. Evaluating Conclusions All nonvascular plants require a film of water for sperm to swim through and fertilize eggs. Therefore, many people conclude that nonvascular plants are not able to survive in very dry climates, such as deserts. Is this a valid conclusion? Justify your answer. 13A 17. Justifying Conclusions A classmate has found a plant whose flowers lack petals and have many stamens. Your classmate tells you that the plant is wind-pollinated. Justify this conclusion. 13B 18. Applying Information Explain how pesticide use could reduce the number of plants in a geographic area. 19. Evaluating Methods You are asked to grow a large number of identical potted plants for a florist. The plants can be grown from either seeds or cuttings. Which method of plant propagation would you use? Justify your choice. 13B Alternative Assessment 20. Finding Information Use the media center or Internet sources to find out how the plants commonly sold in your local garden centers and plant nurseries are propagated. Write a report summarizing the most common method used to propagate each of the plants you researched. Explain why each plant is usually propagated by this method instead of another method. 13B 21. Working Cooperatively Go with two or three of your classmates to visit a wholesale plant grower. Orchid growers would be an excellent choice if one is in your area. Find out how tissue culture is used to propagate different kinds of plants. Try to find out information on two methods of vegetative propagation that are not described in this chapter. Prepare an illustrated report of your findings to share with the class. 13B 22. Career Connection Plant Breeder Use the media center or Internet to find out about the field of plant breeding. Write a report on your findings. Your report should include a job description, the training required, names of employers, growth prospects, and an average starting salary. 3D TAKS Test Prep Use the drawing of a plant seed below and your knowledge of science to answer questions 1 3. A B 1. Which structure is the embryonic root? 13B A A C C B B D D 2. Which structure is the source of nutrients for the embryo? F A 13B H C G B J D 3. What type of plant produced this seed? 13B A nonvascular C dicot plant D monocot B gymnosperm D C Test Choose the best possible answer for each question, even if you think there is another possible answer that is not given. 547