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The Flower and Sexual Reproduction

The Flower and Sexual Reproduction. Chapter 13. Significance of the Flower. Flowers and fruit least affected by environment Appearance of flowers and fruits important to understanding evolutionary relationships among angiosperms. Function of Flowers.

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The Flower and Sexual Reproduction

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  1. The Flower and Sexual Reproduction Chapter 13

  2. Significance of the Flower • Flowers and fruit least affected by environment • Appearance of flowers and fruits important to understanding evolutionary relationships among angiosperms

  3. Function of Flowers • To facilitate the important events of gamete formation and fusion

  4. Steps in Sexual Cycle • Production of special reproductive cells after meiosis • Pollination • Fertilization • Seed and fruit development • Seed and fruit dissemination • Seed germination

  5. Flower Parts • Four whorls of modified leaves • Sepals • Petals • Stamens • Carpels

  6. Flower Parts

  7. Flower Parts • Perianth • Collective term for calyx and corolla • Protects stamens and pistil(s) • Attracts and guides movements of some pollinators

  8. Androecium • Whorl of stamens • Consists of • Filament • Anther • Made up of four elongated lobes called pollen sacs

  9. Androecium • Pollen sac • Contains microsporocytes • Each microsporocyte • Divides by meiosis to produce four haploid microspores • Each microspore nucleus divides mitotically to form two-celled pollen grain (male gametophyte)

  10. Pollen • Contains tube cell and generative cell • Elaborate cell wall • wall pattern genetically determined • Varies among plants • Contains sporopollenin • Resists decay • Reason pollen grains make good fossils

  11. Mature Pollen • Anther wall splits • Releases pollen • Pollen transported to stigma (pollination) • Pollen absorbs water • Secretes proteins • Some involved in pollen recognition and compatibility reactions • Pollen grain germinates

  12. Gynoecium • Female organs • Simple pistil • Single folded carpel • Compound pistil • Several separate carpels or a group of fused carpels • Ovary • Chambers called locules

  13. Gynoecium • Placenta • Tissue within ovary to which ovule is attached • Types of placentation • Parietal • On ovary wall • Axile • On axis of ovary • Central placentation • Ovules form on central column

  14. Gynoecium • Style • Often withers after pollination • Stigma • May have hairs that help hold pollen grains • Sometimes secretes sticky fluid that stimulates pollen growth

  15. Gynoecium • Ovule • Structure that eventually becomes the seed • As it matures, forms 1 or 2 outer protective layers called integuments • Micropyle – small opening in integuments where pollen tube enters • Consists of 1 or 2 outer protective integuments, micropyle, megasporocyte, and nucellus • Megasporocyte • Enlarges in preparation for meiosis • Embedded in tissue called nucellus

  16. Gynoecium • Embryo sac • Female gametophyte plant (haploid) • Megasporocyte • Undergoes meiosis • Produces 4 megaspores (1n) • 3 megaspores nearest micropyle disintegrate • 1 remaining megaspore develops into mature embryo sac

  17. Gynoecium • Stages in embryo sac development • Series of 3 mitotic divisions form 8 nucleate embryo sac • Nuclei migrate • Cell wall forms around nuclei

  18. Gynoecium • Within embryo sac • At micropylar end of embryo sac • Egg cell and 2 synergic cells • All 3 of the above cells sometimes called egg apparatus • Center • Polar nuclei lie in center of central cell • Opposite end • 3 antipodal cells

  19. Double Fertilization • Generative cell within pollen grain divides by mitosis to form 2 sperm cells • 1 sperm cell fuses with egg to form diploid (2n) zygote • 1 sperm fuses with the 2 polar nuclei • Forms triploid (3n) primary endosperm nucleus • Divides to become food reserve tissue called endosperm

  20. Double Fertilization • Double fertilization actually refers to • Fusion of egg and sperm • Fusion of sperm with polar nuclei

  21. Flower Development • Shoot apex transformed into floral apex • Broadening of apical dome • General increase in RNA and protein synthesis • Increase in rate of cell division in apical dome • Bracts • 1st organs to form from floral apex • Flower itself is really a shortened and modified stem.

  22. Flower Types • Complete flower • Has all four sets of floral whorls (sepals, petals, stamens, carpels) • Incomplete flower • Lacks one or more of the sets of floral whorls

  23. Flower Types • Perfect flower • Bisexual flowers • Have both male and female flower parts • Imperfect flower • Unisexual flowers • Flowers will be either • Staminate (stamen bearing)  male • Pistillate (pistil bearing)  female

  24. Flower Types • Monoecious • Plant with staminate and pistillate flowers on one individual plant • Dioecious • Staminate and pistillate flowers on separate individual plants

  25. Flower Symmetry • Regular symmetry • Any line drawn through center of flower divides flower into two similar halves • Irregular symmetry • Only one line can divide flower into two similar halves

  26. Fusion of Flower Parts • Connation • Union of parts of same whorl • Adnation • Union of flower parts from different whorls

  27. Ovary Position • Superior ovary • Ovary located above the points of origin of the perianth and androecium • Inferior ovary • Ovary located below the points of attachment of the perianth and stamens

  28. Inflorescences • Clusters or groups of flowers • Types • Raceme • Spike • Umbel • Head • Cyme

  29. Types of Inflorescences

  30. Self-Pollination and Cross-Pollination • Joseph Koelreuter • 1760s • 1st to demonstrate importance of pollen to plant reproduction • Christian Sprengel • Correctly distinguished between self-pollinating and cross-pollinating species • Described role of wind and insects as pollen vectors

  31. Self-Pollination and Cross-Pollination • Koelreuter and Sprengel • Founders of study called pollination ecology

  32. Self-Pollination and Cross-Pollination • Two types of pollination • Self-pollination (selfing) • Cross-pollination (outcrossing)

  33. Self-Pollination and Cross-Pollination • Outcrossing or cross-pollination • Insured by separation of sexes into different individual plants • Self-pollination prevented by • Different maturation times for stigma and anther of same plant • Inhibition of pollen tube growth through style • Inhibition of zygote formation

  34. Self-Pollination and Cross-Pollination • Advantages of self-pollination • Means of reproduction for scattered populations in extreme habitats • Common among plants in disturbed habitats • Saves pollen and the metabolic energy to produce it • Increases probability that pollen will reach stigma because distance traveled and travel time are short

  35. Apomixis • Sexual reproduction in which no fusion of sperm and egg occurs • Parthenogenesis • Embryo develops from unfertilized egg • Adventitious • Embryo arises from diploid tissue surrounding the embryo sac

  36. Pollination Syndrome • Unique set of pollen traits that adapt a plant for pollination

  37. Pollinators • Animals • Visit flowers for some reward • Incidentally transfer pollen • Rewards include • Pollen • Nectar

  38. Pollinators • Pollen • Excellent food for animals • Contains • 15-30% protein • 15% sugar • 3-13% fat • 1-7% starch • Trace amounts of vitamins, essential elements, secondary substances • Highly noticeable • Distinctive odor

  39. Pollinators • Nectar • Sugary water transported by phloem into secretory structures called nectaries • Contains • 15-75% sugar • Minor amounts of amino acids • All 13 essential amino acids needed for insects are present

  40. Biotic Pollen Vectors • Beetles • Among oldest insect groups • Flowers pollinated by beetles typically have primitive traits • Regular symmetry • Large, simple flowers • Bowl shaped architecture • Floral parts not fused • Many beetle-pollinated species are tropical

  41. Biotic Pollen Vectors • Flies • No single syndrome of floral traits for fly pollination • Bees and butterflies • Active by day • Need landing platform • Harvest nectar as reward

  42. Biotic Pollen Vectors • Moths • Active by night or at dawn and dusk • Harvest nectar as reward • Moth pollinated flowers • White or faintly colored • Emit heavy odors • Fringed blossom rim • Are pendant or horizontal • Have no nectar guides • Often closed during day • Have long, narrow tubes with pools of nectar at their base

  43. Biotic Pollen Vectors • Butterflies • Flowers pollinated by butterflies • Vividly colored • Emit faint odors • Have broad blossom rim • Are erect • Exhibit prominent nectar guides

  44. Biotic Pollen Vectors • Birds • Not recognized by botanists as pollinators until relatively recently • Bird pollinated flowers • Scarlet to red to orange in color • Generally lack nectar guides • Deep tubes usually without a landing platform • Are pendant or horizontal • Have abundant nectar • Emit no odor

  45. Biotic Pollen Vectors • Bats • Bat pollinated flowers • Open at night • Positioned below foliage of parent tree hanging pendant or attached to trunk or low limbs • Drab white, green, or purple • Strong musty odor at night • Large, tough flowers • Lots of pollen and nectar

  46. Abiotic Pollen Vectors • Wind-pollinated flowers • Small • Colorless • Odorless • Nectarless • Petals often lacking or reduced to small scales • Positioned to dangle or wave in open • Stigmas enlarged and elaborate and often extend outside of flower

  47. Abiotic Pollen Vectors • Pollen from wind-pollinated flowers • Generally smoother, smaller, drier than animal-pollinated species • Often changes shape from spherical to Frisbee shape on release to dry air • More pollen grains/ovule than animal-pollinated flowers

  48. Aquatic Plants • Many aquatic plants produce flowers that project above water surface • Vectors are usually wind and insects • Some produce flowers at water surface • Pollen floats from anther to stigma

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