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Cecie Starr Christine Evers Lisa Starr www.cengage.com/biology/starr Chapter 27 Plant Reproduction and Development (Sections 27.1 - 27.5) Albia Dugger • Miami Dade College 27.1 Plight of the Honeybee • Most food crops are flowering plants, which make pollen • The recent decline of honeybees used to pollinate crops is a huge threat to our agricultural economy • pollinator • An organism that moves pollen from one plant to another Importance of Insect Pollinators • Many plant species will not develop fruits (or only develop inferior fruits) unless they receive pollen from other flowers 27.2 Reproductive Structures of Flowering Plants • Petals and other parts of a typical flower are modified leaves that form in four spirals (whorls) at the end of a floral shoot • flower • Specialized reproductive shoot of an angiosperm sporophyte Flower Formation • The outermost whorl develops into a ring of sepals (a calyx ) • Petals form in the second whorl (the corolla) • A whorl of stamens (which produce male gametophytes) forms inside the ring of petals • The innermost whorl of modified leaves are folded and fused into carpels (which produce female gametophytes) Key Terms • stamen • Reproductive structure that produces male gametophytes • In most plants it consists of a pollen-producing anther on the tip of a filament • carpel • Reproductive structure that produces female gametophytes • Sticky or hairlike stigma together with an ovary and a style Carpels • Flowers may have one carpel, several carpels, or several groups of carpels that may be fused • A swollen region (the ovary) contains one or more ovules • A cell in the ovule undergoes meiosis and develops into the haploid female gametophyte Key Terms • ovary • In flowering plants, the enlarged base of a carpel, inside which one or more ovules form and eggs are fertilized • ovule • In a seed-bearing plant, a structure in which a haploid, egg-producing female gametophyte forms • After fertilization, matures into a seed Anatomy of a Typical Flower Anatomy of a Typical Flower stamen filament anther petal (all petals combined are the flower’s corolla) sepal (all sepals combined are flower’s calyx) carpel stigma style ovary ovule (forms within ovary) receptacle Fig. 27.2a.2, p. 430 Flower Structure • Flower structure varies among different plant species Flower Structure carpel structure varies ovary position varies ovule position varies within ovaries B Flower structure varies among different plant species. Fig. 27.2b, p. 430 Life Cycle of a Flowering Plant • The life cycle of flowering plants is dominated by a diploid, spore-producing sporophyte • Spores that form by meiosis inside flowers develop into haploid gametophytes, which produce gametes • At fertilization, a diploid zygote forms when male and female gametes meet inside an ovary Life Cycle of a Flowering Plant Life Cycle of a Flowering Plant zygote mature sporophyte (2n) in seed (2n) microspores (n) sperm (n) eggs (n) male gametophyte megaspores (n) female gametophyte Fig. 27.3, p. 430 Life Cycle of a Flowering Plant zygote mature sporophyte (2n) in seed (2n) microspores (n) sperm (n) eggs (n) male gametophyte female gametophyte megaspores (n) Stepped Art Fig. 27.3, p. 430 Pollinators • Sexual reproduction in flowering plants involves transfer of pollen, typically from one plant to another • Animal pollinators pick up pollen and transfer it to the flower of a different plant • A flower’s shape, pattern, color, and fragrance are adaptations that attract specific animal pollinators Coevolution • ~90% of flowering plants have coevolved animal pollinators • An animal’s reward for a visit to a flower may be nectar, oils, nutritious pollen, or even sex • Butterflies, hummingbirds, and honeybees feed on nectar • Bats, moths, and flies are attracted to specific odors • Some flowers have specializations that prevent pollination by everything other than a specific pollinator species Coevolution • Zebra orchid mimics the scent of a female wasp • Female burnet moths ready to mate Key Concepts • Structure and Function of Flowers • Flowers are shoots that are specialized for reproduction • Modified leaves form their parts • Gamete-producing cells develop in their reproductive structures • Other parts such as petals are adapted to attract and reward pollinators 27.3 A New Generation Begins • In flowering plants, fertilization has two outcomes: • It results in a diploid zygote, which becomes the mature sporophyte • It is the start of nutritive endosperm, which sustains rapid growth of the sporophyte seedling until true leaves form and photosynthesis begins 10 Steps in the Flowering Plant Life Cycle 1. An ovule forms inside a flower’s ovary; one cell enlarges 2. Four haploid (n) megaspores form by meiosis and cytoplasmic division of the enlarged cell 3. In one megaspore, three rounds of mitosis with no cytoplasmic division result in a single cell with eight nuclei 4. Uneven cytoplasmic divisions result in a seven-celled embryo sac (female gametophyte) with eight haploid nuclei 10 Steps in the Flowering Plant Life Cycle 5. Pollen sacs form in an anther 6. Four haploid (n) microspores form by meiosis and cytoplasmic division of a cell in the pollen sac 7. Mitosis and differentiation of a microspore produces a twocelled pollen grain, which enters dormancy, before being released from the anther Key Terms • megaspore • Haploid spore that forms in ovule of seed plants • Gives rise to an egg-producing gametophyte • microspore • Walled haploid spore of seed plants • Gives rise to a sperm-producing gametophyte • dormancy • Period of temporarily suspended metabolism 10 Steps in the Flowering Plant Life Cycle 8. Pollen grains are released from the anther; one lands on a stigma and germinates (pollination): • One cell in the pollen grain develops into a pollen tube • The other cell develops into two haploid sperm cells 9. The pollen tube grows down through tissues of the carpel, carrying the two sperm nuclei with it 10 Steps in the Flowering Plant Life Cycle 10. The pollen tube reaches the ovule, penetrates it, and releases the two sperm nuclei (double fertilization) • One sperm fertilizes the egg and forms a diploid zygote • The other fuses with the endosperm mother cell, forming a triploid (3n) cell which gives rise to triploid endosperm Key Terms • pollination • Arrival of pollen on a receptive stigma • double fertilization • Mode of fertilization in flowering plants in which one sperm cell fuses with the egg, and a second sperm cell fuses with the endosperm mother cell • endosperm • Nutritive tissue in the seeds of flowering plants Life Cycle of a Eudicot Life Cycle of a Eudicot 1 An ovule forms inside a flower’s ovary. A cell in the ovule enlarges. an ovule ovary wall seedling ovary seed Sporophyte (2n) 2 Four haploid (n) megaspores form by meiosis and cytoplasmic division of the enlarged cell. Three megaspores disintegrate. pollen tube sperm (n) Meiosis in ovary endosperm mother cell (n + n) egg (n) 10 The pollen tube reaches the ovule, penetrates it, and releases the two sperm nuclei. One nucleus fertilizes the egg. The other fuses with the endosperm mother cell. megaspores (n) 3 In the remaining megaspore, three rounds of mitosis with no cytoplasmic division result in a single cell with eight nuclei. Double fertilization 4 Uneven cytoplasmic divisions result in a seven-celled embryo sac female with eight haploid nuclei. This sac gametophyte is the female gametophyte. Fig. 27.5.1-4,10, p. 433 Life Cycle of a Eudicot pollen sac anther (cutaway view) filament 5 Pollen sacs form in an anther. a cell in the pollen sac Meiosis in anther 6 Four haploid (n) 6 microspores (n) 7 In this plant, 7 mitosis of a microspore followed by differentiation results in a twocelled pollen grain. 9 The pollen tube grows down through tissues of the carpel, carrying the two sperm nuclei with it. 8 Pollen grains are 8 pollen tube sperm cells stigma style 9 microspores form by meiosis and cytoplasmic division of a cell in the pollen sac. released from the anther. One lands on a stigma and germinates. A cell in the grain develops into a pollen tube; the other, into two haploid sperm cells. mature male gametophyte Fig. 27.5.5-9, p. 432 Life Cycle of a Eudicot pollen sac anther (cutaway view) filament 5 Pollen sacs form in an anther. a cell in the pollen sac Meiosis in anther 6 Four haploid (n) 6 microspor es (n) 7 In this plant, 7 mitosis of a microspore followed by differentiation results in a twocelled pollen grain. 9 The pollen tube grows down through tissues of the carpel, carrying the two sperm nuclei with it. 8 Pollen grains are 8 pollen tube sperm cells stigma style 9 microspores form by meiosis and cytoplasmic division of a cell in the pollen sac. mature male gametophyte released from the anther. One lands on a stigma and germinates. A cell in the grain develops into a pollen tube; the other, into two haploid sperm cells. Stepped Art Fig. 27.5.5-9, p. 432 ANIMATION: Pollination ANIMATION: Plant Life cycle To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 27.4 From Zygotes to Seeds and Fruits • After fertilization, mitotic cell divisions transform a zygote into an embryo sporophyte encased in a seed • As embryos develop inside the ovules of flowering plants, tissues around them form fruits • Water, wind, and animals disperse seeds in fruits The Embryo Sporophyte Forms • Double fertilization produces a zygote, which develops into an embryo sporophyte, and a triploid (3n) cell, which develops into nutrient-containing endosperm • When the embryo matures, layers of integuments separate from the ovary wall and become a protective seed coat • The embryo sporophyte, its reserves of food, and the seed coat become a mature ovule (seed) A Seed • seed • Embryo sporophyte of a seed plant packaged with nutritive tissue inside a protective coat A Seed root apical meristem seed coat embryo shoot tip cotyledons Fig. 27.6, p. 434 Fruits • Fruits include seed-containing “vegetables” such as beans, tomatoes, grains, and squash • Simple fruits(e.g. pea pod) form from a single ovary • Aggregate fruits (e.g. strawberry) form from separate ovaries of one flower • Multiple fruits (e.g. pineapple) form from fused ovaries of separate flowers • fruit • A seed-containing mature ovary of a flowering plant • Often with accessory tissues Parts of a Fruit Parts of a Fruit tissue derived from ovary wall carpel wall seed enlarged receptacle Fig. 27.7, p. 434 Aggregate Fruits Aggregate Fruits Fig. 27.8a, p. 435 Aggregate Fruits Fig. 27.8b, p. 435 Aggregate Fruits Fig. 27.8c, p. 435 Categories of Fruits • Based on tissues: • True fruits develop from the ovary wall • Accessory fruits include the ovary and other parts developed from petals, sepals, stamens, or receptacles • Based on appearance: • Dry fruits (e.g. acorns, grains, strawberries) • Fleshy fruits (e.g. cherries, berries, apples) Function of Fruits • The function of a fruit is to protect and disperse seeds • Specific dispersal vectors are reflected in a fruit’s form: • Water: Water-repellent outer layers • Wind: Lightweight with breeze-catching specializations • Animals: Hooks or spines that stick to feathers, feet, or fur • Animal feces: Colorful, fleshy, fragrant fruits Key Concepts • Plant Sexual Reproduction • In flowering plants, pollination is followed by double fertilization • After fertilization, ovules mature into seeds • As seeds develop, tissues of the ovary and other parts of the flower mature into fruits, which function to disperse seeds ABC Video: Doomsday Vault Seed Collection Animation: Double Fertilization 27.5 Asexual Reproduction in Plants • Most flowering plants can reproduce by vegetative reproduction, a form of asexual reproduction that produces genetically identical offspring • Triploid species are sterile and can only reproduce asexually • vegetative reproduction • Growth of new roots and shoots from extensions or fragments of a parent plant Tissue Culture Propagation • An entire plant may be cloned from a single cell with tissue culture propagation • This technique is used to improve food crops and to propagate rare ornamental plants such as orchids • tissue culture propagation • Laboratory method in which body cells are induced to divide and form an embryo Key Concepts • Asexual Reproduction of Plants • Many species of plants reproduce asexually by vegetative reproduction • Humans take advantage of this natural tendency by propagating plants asexually for agriculture and research