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Transcript
Chapter 20 Lecture Outline Seed Plants: Angiosperms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Outline Introduction Phylum Magnoliophyta – The Flowering Plants • Development of Gametophytes • Pollination • Fertilization and Development of the Seed • Apomixis and Parthenocarpy • Trends of Specialization and Classification in Flowering Plants Pollination Ecology Herbaria and Plant Preservation Introduction Angiosperms = flowering plants Seeds enclosed in carpel that resembles a leaf that has folded over and fused at the margins. • Pistil composed of a single carpel, or two or more united carpels. Seed develops from ovule within carpel. Ovary becomes a fruit. Bleeding hearts Introduction Phylum Magnoliophyta – The Flowering Plants Has been divided into two large classes: • Magnoliopsida - Dicots – DNA and cladistic evidence suggest that two groups of dicots should be recognized. • Liliopsida - Monocots Flower is modified stem bearing modified leaves. • Most primitive flower: – – – Long receptacle Many spirally arranged flower parts that are separate and not differentiated into sepals and petals Flattened and numerous stamens and carpels Phylum Magnoliophyta – The Flowering Plants Heterosporous Sporophytes dominant. Female gametophytes wholly enclosed within sporophyte tissue and reduced to only a few cells. At maturity, male gametophytes consist of a germinated pollen grain with three nuclei. Phylum Magnoliophyta Development of gametophytes - Female: • Diploid megasporocyte differentiates in ovule. – Undergoes meiosis and produces four haploid megaspores. o Three degenerate. • Remaining cell enlarges and nucleus divides to produce 8 nuclei (without walls). • Outer two layers of ovule differentiate into integuments that later become seed coat. – Micropyle at one end of ovule Phylum Magnoliophyta Development of gametophytes - Female: • 8 nuclei form two groups, 4 near each end of cell. • One nucleus from each group migrates to cell middle and form central cell. • Cell walls form around remaining six nuclei. – – Egg and two synergids closest to micropyle Three antipodals at opposite end - No apparent function • Female gametophyte (megagametophyte, embryo sac) = large sac containing 8 nuclei and 7 cells Phylum Magnoliophyta Development of gametophytes - Male: • Formation of male gametophytes takes place in anthers. • Four patches, corresponding to pollen sacs, of microsporocyte cells differentiate in anther. • Each microsporocyte undergoes meiosis to produce quartet of haploid microspores. Anther with microspores Phylum Magnoliophyta Development of gametophytes - Male: • Microspores undergo three important changes: – Divide once by mitosis to form a small generative cell inside the larger tube cell o Nucleus of tube cell = vegetative nucleus – Members of each quartet of microspores separate. – Wall becomes two-layered. o Outer layer = exine « Finely sculptured « Contains chemicals that may react with chemicals in stigma • Generative nucleus will later divide to produce two sperm. Pollen grain Phylum Magnoliophyta Pollination: • Pollination - Transfer of pollen grains from anther to stigma – Self-pollination - Pollen grains germinate on stigma of same flower. • Fertilization - Union of sperm and egg • Pollination by insects, wind, water, animals or gravity. Phylum Magnoliophyta Fertilization and development of the seed: • After pollination, further development of male gametophyte may not take place unless pollen grain is: – – From a different plant of the same species. From a variety different from that of the receiving flower. • Pollen tube grows between cells of stigma and style until it reaches ovule micropyle. • Vegetative nucleus stays at tips of pollen tube, while generative cell lags behind and divides into two sperm. • Pollen tube enters female gametophyte, destroying synergid in the process, and discharges sperms. Phylum Magnoliophyta Fertilization and development of the seed: • Mature male gametophyte = germinated pollen grain with its vegetative nucleus and two sperms within tube cell Phylum Magnoliophyta Fertilization and development of the seed: • Double fertilization: – One sperm unites with egg, forming zygote, then embryo. – Other sperm unites with central cell nuclei, producing triploid endosperm nucleus that develops into endosperm tissue. – o Endosperm tissue = nutritive tissue for embryo o Endosperm becomes extensive part of seed in some monocots, such as corn and other grasses. « Wheat, rice and corn - Major source of nutrition for humans due to nutritional quality of endosperm o Endosperm absorbed into cotyledons in most dicots. Ovule becomes seed, ovary matures into fruit, integuments harden into seed coat. Phylum Magnoliophyta Lifecycle of typical flowering plant: Phylum Magnoliophyta Fertilization and development of the seed: • Other types of (female) gametophyte development: – Female gametophyte can have from 4 to 16 nuclei or cells at maturity. – Endosperm may be 5x, 9x or 15x. – Example in lilies: o All four haploid megaspore nuclei produced from megasporocyte remain functional. o Three of nuclei unite forming 3x nucleus and forth nucleus remain haploid. « Results in female gametophyte with four 3x nuclei and four 1x nuclei « One central cell nucleus is 3x and other is 1x. « Fertilization results in 5x endosperm nucleus. Phylum Magnoliophyta Fertilization and development of the seed: • Lily (female) gametophyte development: Phylum Magnoliophyta Apomixis and parthenocarpy: • Apomixis - Without fusion of gametes but with the normal structures otherwise being involved – Embryo from diploid nutritive cell or other diploid cell of ovule, instead of from zygote. o Results in a vegetatively propagated plant • Parthenocarpy - Fruits develop from ovaries with unfertilized eggs. – Results in seedless fruits o Navel oranges and bananas Phylum Magnoliophyta Trends of specialization and classification in flowering plants: • First historical classifications for convenience. • Modern botanists group plants according to natural relationships based on evolution. • Fossil record suggests flowering plants first appeared about 160 million years ago during late Jurassic. • Flowering plants then developed during Cretaceous and Cenozoic. • Dominant plants today Phylum Magnoliophyta Trends of specialization and classification in flowering plants: • First pistil from leaflike structure with ovules along margins = carpel. – Edges of blade rolled inward and fused together. • Separate carpels of primitive flowers fused together to form compound pistil consisting of several carpels. Phylum Magnoliophyta Trends of specialization and classification in flowering plants: • Inferior ovary (epigynous flower) - Receptacle or other flower parts fused to ovary and grown up around it. – Calyx and corolla appear to be attached to top of ovary. • Superior ovary (hypogynous flower) - Ovary produced on top of receptacle. – Other flower parts attached around ovary base. • Perigynous flowers - Flower parts attached to corolla tube of fused petals, creating floral tube that is not attached to ovary. Phylum Magnoliophyta Trends of specialization and classification in flowering plants: • Complete flower - Has calyx, corolla, stamens and pistil • Incomplete flower - Corolla or other flower parts missing. • Perfect flower - Both stamens and pistil present. • Imperfect flower - Either stamens or pistil missing. – – Monoecious species - Male and female imperfect flowers on same plant. Dioecious species - Plant bears only male flowers and other plants bear only female flowers. Male flower Female flower with inferior ovary Phylum Magnoliophyta Trends of specialization and classification in flowering plants: • Primitive flowering plants: – Simple leaves. – Flower with numerous, spirally arranged parts, not fused to each other – Flowers radially symmetrical = regular. – Flowers with both stamens and pistils o Complete and perfect flowers – Superior ovary (hypogynous flower) – Still many plants today whose flowers have Magnolia primitive features. Phylum Magnoliophyta Trends of specialization and classification in flowering plants: • Specialized flowering plants: – Flower parts fewer and definite in number. – Spiral arrangements compressed to whorls – Bilaterally symmetrical flowers = irregular – Reduction and fusion of parts o Incomplete or imperfect flowers – Inferior ovary Orchid Pollination Ecology Pollinators have coevolved with plants. • Twenty thousand bee species among current-day pollinators. • Bee-pollinated flowers: – – Generally brightly colored, mostly blue or yellow Often have lines or other distinctive markings, which may function as honey guides to lead bees to nectar. o Bees see UV light (humans do not). « Some flower markings visible only in UV light. In ordinary light In UV light Pollination Ecology Beetle-pollinated flowers: • Strong, yeasty, spicy or fruity odor • White or dull in color - Beetles do have keen visual senses. • Some do not secrete nectar, but furnish pollen or food on petals in special storage cells. Fly-pollinated flowers: • Smell like rotten meat • Dull red or brown Pollination Ecology Butterfly- and moth-pollinated flowers: • Often have sweet fragrances • White or yellow for night-flying moths • Sometimes red, often blue, yellow or orange for butterflies • Nectaries at bases of corolla tubes or spurs for long tongues. Bird-pollinated flowers (hummingbirds and sunbirds): • • • • • Often bright red or yellow Little if any odor - Birds do not have a keen sense of smell. Large and part of sturdy inflorescence Copious amounts of nectar - Birds highly active. Long floral tubes Pollination Ecology Bat-pollinated flowers: • Primarily in tropics • Open at night when bats are foraging • Dull in color • Large enough for bat to insert head or consist of ball-like inflorescence containing large numbers of small flowers Pollination Ecology Orchid flowers: • Have pollinators among all types mentioned • Some of adaptations between orchid flowers and pollinators are extraordinary. • Pollen grains produced in little sacs called pollinia (singular: pollinium) with sticky pads at base. • Members of Ophrys have modified petal that resembles female bumble bee or wasp. – Male bees or wasps try to copulate with flower. o Pollinia deposited on their head. Ophrys Herbaria and Plant Preservation Herbaria (singular: herbarium) - Libraries of dried, pressed plants, algae, and fungi, arranged and labeled. Methods: • Fungi and bryophytes dried and stored in small packets. • Plant press used for vascular plants. Herbaria and Plant Preservation Methods: • Vascular plant specimens mounted on 100% rag herbarium paper. • Specimens stored so retrieval of specimens is easily accomplished. Review Introduction Phylum Magnoliophyta – The Flowering Plants • Development of Gametophytes • Pollination • Fertilization and Development of the Seed • Apomixis and Parthenocarpy • Trends of Specialization and Classification in Flowering Plants Pollination Ecology Herbaria and Plant Preservation