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The Flower and Sexual Reproduction

The Flower and Sexual Reproduction

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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