Levetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles5. Plant Life Cycle: Flowers © The McGraw−Hill Companies, 200872CHAPTER OUTLINE The Flower 73Floral Organs 73 A CLOSER LOOK 5.1 Mad About Tulips 74Modified Flowers 75 Meiosis 77Stages of Meiosis 78 A CLOSER LOOK 5.2 Pollen is More Than Something to Sneeze At 80Meiosis in Flowering Plants 80 Male Gametophyte Development 80 Female Gametophyte Development 81 Pollination and Fertilization 83Animal Pollination 83 A CLOSER LOOK 5.3 AlluringScents 84Wind Pollination 84 Double Fertilization 86 Chapter Summary 86 Review Questions 86 Further Reading 87 KEY CONCEPTS 1. Angiosperms are unique among plants in that they have their sexual reproductive structures contained in a flower.2. Meiosis is a type of cell division that reduces the number of chromosomes from the diploid to the haploid number and is an integral part of sexual reproduction.3. Pollination is the transfer of pollen from the anther to the stigma and largely occurs through the action of wind or animals.4. In angiosperms, reproduction is accomplished through the process of double fertilization. 5Plant Life Cycle: Flowers The inflorescence of Zantedeschia aethiopica (arum lily) is a spadix (fleshy spike) surrounded by a large bract, called a spathe. CHAPTERLevetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles5. Plant Life Cycle: Flowers © The McGraw−Hill Companies, 2008CHAPTER 5Plant Life Cycle: Flowers 73 The natural beauty of flowers has always been a source of inspiration, and the appearance of the first flower of spring lightens the heart of anyone weary of winter. (See A Closer Look 5.1—Mad about Tulips.) But what role do flowers play in the lives of plants? Their beauty notwithstanding, flowers play a pivotal role in the life cycle of angiosperms since they are the sites of sexual reproduc-tion. The events leading to flowering are very complex and may include internal factors such as plant hormones (see A Closer Look 6.1—The Influence of Hormones on Plant Reproductive Cycles) and biological clocks (internal rhythms that regulate the timing of biological functions) as well as external factors such as temperature and day length. The interconnection between these internal and external features allows plants to coordinate their reproduction with the envi-ronment. This chapter will emphasize the reproductive role of flowers once they have developed. THE FLOWER Flowers, unique to angiosperms, are essentially modified branches bearing four sets of specialized appendages or floral organs. These appendages are grouped in whorls and consist of sepals, petals, stamens, and carpels. They are inserted into the receptacle, the expanded top of the pedicel(peduncle), or flower stalk ( fig. 5.1 ).Floral Organs The outermost whorl consists of the sepals, leafy structures that cover the unopened flower bud; they are usually green and photosynthetic. The whole whorl of sepals of a single flower is called the calyx. The petals that make up the next whorl of flower parts are collectively called the corolla.Often brightly colored and conspicuous, the petals function by attracting animal pollinators. Together, the calyx and corolla constitute the perianth.In the center of the flower, the male and female struc-tures can be found. The androecium, the whorl of male structures, is composed of stamens, each of which consists of a pollenproducing anther supported on a stalk, the filament.Each anther houses four chambers where pollen develops.The pollen chambers can be seen in the cross section of the anther, Figure 5.1 . The gynoecium is the collective term for the female structures, or carpels, which are located in the middle of the flower. Flowers can have one to many carpels. (The old term “pistil,” which referred to one or more carpels, will not be used in this book.) A gynoecium with just one carpel is illustrated in Figure 5.2a . If many carpels are pres-ent, they may either be fused together ( fig. 5.2b ) or remain separate. Carpels, whether individual or fused, consist of a stigma, style, and ovary ( fig. 5.1 ). Contained within the basal ovary are one to many ovules (structures that will eventually become seeds); rising from the top of the ovary Figure 5.1Flower structure.StamenStamenAntherFilamentPetalSepalCarpelPedicelPollen chamberCarpelStigmaStyleOvaryOvulePollenLevetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles5. Plant Life Cycle: Flowers © The McGraw−Hill Companies, 200874A CLOSER LOOK 5.1The flower, the crowning characteristic of the angiosperms, technically is a modified branch bearing specialized leaves that are integral to sexual reproduction. But flowers have meaning beyond this technical definition. Few of us could imagine or want a world without flowers. We are attracted to them because of their beauty of color, form, and fra-grance. Perhaps we appreciate them all the more because their beauty is so delicate and ephemeral. They have been praised in song and poem and have decorated our homes and persons. Certain flowers are so revered that they become representations of human emo-tions or nations. Yet, in a bizarre epi-sode in history, the desire for a beautiful flower created a frenzy that brought a nation and its people to economic ruin.The story begins in 1554 in the Ottoman Court of Suleiman the Magnificent in Constantinople. Some years earlier, the Turks had been the first to bring into cultivation a wild flower whose beauty had captivated the court and all who saw it. Ogier Ghislain de Busbecq, the ambassador for the Austrian-Hapsburg Empire, was so entranced by the floral beauties that he had some of the flowers sent back to Vienna. The Turks called the flower lale, but Busbecq called them tulipam, a corruption of dulban, meaning turban. The Ottoman court favored tulips with elongated pointed pet-als and, through selected crosses, had achieved this effect. Perhaps Busbecq thought the form of the flower resembled a turban. In any case, the first tulips arrived in Europe in 1554.Wild tulips (Tulipa spp.) originated in Central Asia and the Caucasus Mountains. Tulips are monocots belonging to the lily family, the Liliaceae. There are about 120 species of tulip, and typically a single flower is borne on a stalk. Most tulips have three