Plant Reproduction 24 and Response 694 S ON CU O F TEKS 10B THE GREEN LEMONS For years, a California warehouse had stored freshly picked green lemons before they were shipped to market. The warehouse managers knew that the lemons would always be ripe and yellow Pollen grains from the common and ready to ship to market about five ­ragweed (SEM 1000∙) days after they arrived. Or so they thought. One year, for safety reasons, they decided to replace the warehouse’s kerosene heaters with modern electric ones. Then, much to their surprise, when they began to pack their first shipment of five-day-old lemons, they had to stop. The fruit they expected to ship were still a bright, and very unripe, green. All the lemons had been grown under similar conditions. A variable had changed after the lemons were harvested, and the lemons had failed to ripen. What could it have been? Look for clues in this chapter to the mystery of the still- green lemons. Then, solve the mystery. Never Stop Exploring Your World. Finding the solution to The Green Lemons mystery is only the beginning. Take a video field trip with the ecogeeks of Untamed Science to see where this mystery leads. Texas Essential Knowledge and Skills READINESS TEKS: 10B Describe the interactions that occur among systems that perform the functions of transport, reproduction, and response in plants. SUPPORTING TEKS: 5B Examine specialized cells, including roots, stems, and leaves of plants; and animal cells such as blood, muscle, and epithelium. 12B Compare variations and adaptations of organisms in different ecosystems. TEKS: 2G Analyze, evaluate, make inferences, and predict trends from data. Also covered: TEKS 2H. Plant Reproduction and Response 695 Reproduction in 24.1 Flowering Plants In this lesson you will learn about flowers and the angiosperm life cycle (TEKS 10B). In the Quick Lab, you will examine specialized leaves (TEKS 5B). Key Questions THInk ABOUt It What makes a flower beautiful? The symmetry of What are flowers? its petals, its rich colors, and, sometimes, its fragrance. But, at the heart of it, what’s behind all this beauty? The answer is, simply, angiosperm How does fertilization in sexual reproduction. To a plant, the whole point of a flower is to bring angiosperms differ from ­fertilization in other plants? gametes together for reproduction and to protect the resulting zygote and embryo. What is vegetative reproduction? The Structure of Flowers TEKS 10B Vocabulary What are flowers? stamen • anther • carpel • stigma • pistil • embryo sac • You may think of flowers as decorative objects that brighten our double fertilization • endosperm • world, and so they are. However, the beauty of flowers also reflects the vegetative reproduction • stunning evolutionary success of the angiosperms, or flowering plants. grafting The structure of a typical angiosperm flower is shown in Figure 24–1. Flowers are reproductive organs that are composed of four Taking Notes different kinds of specialized leaves: sepals, petals, stamens, and Two-Column Table Construct carpels. Flower development is triggered by interactions with the a two-column table with the plant’s genetic system when a gene called LEAFY is activated, causing headings Male Gametophyte and Female Gametophyte. As an ordinary meristem to develop into a flower. you read, take notes on the Sepals and Petals The outermost circle of floral parts contains characteristics of each type of gametophyte. the sepals (see pulz). In many plants, the sepals are green and closely Stamen Carpel resemble ordinary leaves. Sepals enclose the bud before it opens, and Anther Style Stigma protect the flower while it is develop- Filament Ovary ing. Petals, often brightly colored, are found just inside the sepals. The colors and shapes of such petals attract insects and other pollinators to the flower. Figure 24–1 The Parts of a Flower This diagram shows the parts of a typical flower. The flowers of some angiosperm species, however, Petal do not have all the parts shown here. Ovary Sepal Ovule 696 Stamens Within the ring of petals are the structures that produce ELPS 4.G.2 male and female gametophytes. Thestamens are the male parts of Use the “Visual Summary” on the flower. Each stamen consists of a stalk called a filament with an page 701 to clarify what you are anther at its tip. Anthers are the structures in which pollen grains— reading and to help you recall main the male gametophytes—are produced. In most angiosperm species, ideas. Then, show the visual to a small group. Use it to help retell or the flowers have several stamens. If you rub your hand on the anthers summarize what happens during the of a flower, a yellow-orange dust may stick to your skin. This dust is life cycle of a flowering plant. made up of thousands of individual pollen grains. Carpels The innermost floral parts are the carpels. Carpels produce and shelter the female gametophytes and, later, seeds. 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 or feathery portion known as the stigma, which is specialized to capture pollen. Botanists sometimes call a single carpel or several fused carpels a pistil. Online Journal Make a two-column table labeled Male and Figure 24–2 Variety Among Flowers Female. Then list and define the structures that make up a flower Flowers vary greatly in structure. in the appropriate columns. Form a Hypothesis How might it be an advantage for Variety in Flowers Flowers vary greatly in shape, color, and size, as a plant to have many flowers shown in Figure 24–2. A typical flowering plant produces both male clustered in a single structure? and female gametophytes. In some species, however, male and female gametophytes are produced on different plants. In some species, many flowers grow close together to form a composite structure that looks like a single flower, as seen in the Queen Anne’s lace at right. ▼ Iris The drooping petal-like Queen Anne’s Lace Some structures are in fact modified flowerlike structures are sepals. The fuzzy yellow stripe actually clusters of many running down the center guides individual flowers. ▼ bees and other pollinators to the male and female parts at the interior of the flower. Wild Rose Passion Flower Some flowers This flower has have stamens and pistils many stamens you can easily count. In this surrounding a dramatic flower, five stamens tight cluster of lie beneath three pistils.▼ carpels at the center. ▼ Plant Reproduction and Response 697 TEKS 5B What Is the Structure of a Flower? 1 Examine2 3 a4 flower 5 carefully.6 7 8 Make9 a Analyze and Conclude detailed drawing of the flower and label as 1. Observe Are many parts as you can. Note whether the the anthers in anthers are above or below the stigma. this flower 1 2 Remove3 4 an5 anther6 and7 place8 9 it on located above a slide. While holding the anther with or below the forceps, use the scalpel to cut one or more stigma? How thin slices across the anther. CAUTION: Be could this affect what happens to the pollen careful with sharp tools. Place the slide on a produced by the anthers? Explain your answer. flat surface before you start cutting. 2. Apply Concepts What structures did you identify 1 2 3 Lay4 the5 slices6 flat7 on8 the9 microscope in the anther? What is the function of these structures? slide and add a drop of water and a 3. Apply Concepts What structures did you identify coverslip. Observe the slices with the in the ovary? What is the function of these structures? microscope at low power. Make a labeled 4. Draw Conclusions Which parts of the flower will drawing of your observations. become the seeds? Which parts will become the fruit? 2 3 4 Repeat5 6 steps7 2 and8 93 with the ovary. The Angiosperm Life Cycle TEKS 10B How does fertilization in angiosperms differ from fertilization in other plants? Like other plants, angiosperms have a life cycle that shows an alterna- tion of generations between a diploid sporophyte phase and a haploid gametophyte stage. Recall that in vascular plants, including ferns and gymnosperms, the sporophyte plant is much larger than the gameto- phyte. This trend continues in angiosperms, where male and female gametophytes live within the tissues of the sporophyte. Development of Male Gametophytes The male gametophytes— the pollen grains—develop inside anthers. This process is shown in the top half of Figure 24–3. First, meiosis produces four haploid spore cells. Each spore undergoes one mitotic division to produce the two haploid nuclei of a single pollen grain. The two nuclei are surrounded by a thick wall that protects the male gametophyte from dryness and damage when it is released. The pollen grains stop growing until they are released from the anther and land on a stigma. Online Journal Make a flowchart that records the stages of development of an angiosperm’s male gametophyte. 698 Chapter 24 • Lesson 1 Development of Female Gametophytes While the male gameto- phytes are forming, female gametophytes develop inside each carpel of a flower. The ovules—the future seeds—are enveloped in a protective ovary—the future fruit. How do the female gametophytes form? As shown in the bottom half of Figure 24–3, a single diploid cell goes through meiosis to produce four haploid cells, three of which disintegrate. The remaining cell undergoes mitosis, producing eight nuclei. These eight nuclei and the surrounding membrane are called the embryo sac.