Lecture 17: Flowers and Reproduction PDF

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Summary

This lecture covers flowers and reproduction, including sexual and asexual methods in plants. It analyzes flower structure and types, and details plant reproduction processes from micro- and megagametophyte formation to fertilization and seed development.

Full Transcript

Lecture 17: Flowers and Reproduction Reproduction Sexual Asexual – 2 parents – Single parent – Progeny are genetically – Progeny are genetically unique identical to the parent – Because offspring are a...

Lecture 17: Flowers and Reproduction Reproduction Sexual Asexual – 2 parents – Single parent – Progeny are genetically – Progeny are genetically unique identical to the parent – Because offspring are a – Crops: get consistent, combination of parental predictable results traits, they may be better suited to the environment Asexual Reproduction Many plants can reproduce by fragmentation Sexual Reproduction Sporophyte (2n) à spores – Division: meiosis – Female: megaspore; Male: microspore Gametophyte (1n) à gametes – Division: mitosis – Female: egg; Male: sperm Flowers Whorls – Sepals, petals, stamens, carpels All whorls present: complete Any whorls missing: incomplete Sepals Petals Stamens Carpels Male Gametophyte Formation Microspore divides by mitosis – Tube cell and generative cell Generative cell divides again to form 2 sperm cells When pollen lands on a stigma, it germinates – Pollen tube grows through style toward ovary – Sperm cells carried down pollen tube Female Gametophyte Formation Megaspore divides 3 times by mitosis – Produces 8 nuclei Nuclei move around the large cell – 3 at one end, 3 at the other end, 2 in the middle Walls form, now a mature gametophyte – 2 synergids, 3 antipodals, 1 egg, 1 central cell (2 nuclei) Double Fertilization One sperm cell fertilizes the egg – Diploid zygote The other sperm fertilizes the central cell – Sperm nucleus fuses with polar nuclei – Forms triploid endosperm Endosperm grows quickly – Nuclei divide a bunch, walls come later Embryo Development Zygote divides, growing by mitosis – Embryo proper and suspensor form Cotyledons form early – 1: monocot; 2: dicot Eventually takes on a cylindrical form – Radicle, hypocotyl, epicotyl Embryo Development Monocots – Endosperm intact and feeding embryo – Albuminous seeds Dicots – Cotyledons store nutrients – Endosperm used up; energy, resources converted into cotyledons – Exalbuminous seeds Fruit Development Ovary develops 3 layers – Exocarp (epicarp), mesocarp, endocarp Protect seeds and help to disperse them Fruit types: – Single ovary: simple fruit – Separate carpels fuse: aggregate fruit – All carpels on an inflorescence fuse: multiple fruit Pollen Swapping Cross-pollination – Pollen from another plant fertilizes the carpels Self-pollination – Pollen from the same plant fertilizes the carpels – Not asexual reproduction Many plants avoid selfing – Stigmas, anthers mature at different times; compatibility barriers; imperfect flowers Stamen/Style Maturation Times If anthers and stigmas mature at different times, selfing is not possible Anthers mature first Stigma/Pollen Incompatibility Chemical reactions at stigma prevent self- produced pollen from germinating Flowering Flowers lacking either stamens or carpels are imperfect Flowers with both stamens and carpels are perfect Monoecious Plants Either flowers are perfect, or both staminate and carpellate flowers on the same plant Encourages cross- pollination, but selfing possible Examples: cattails, corn Dioecious Plants Staminate and carpellate flowers on different plants All flowers are imperfect Cross-pollination necessary Examples: dates, marijuana, and papaya Animal Pollinated Flowers Results in coevolution between flowers and pollinators Symmetry – Actinomorphic (radial) means pollinator can approach from any direction – Zygomorphic (bilateral) means pollinator can approach only from 1 direction comfortably Actinomorphic Zygomorphic Wind Pollinated Flowers Typically lack perianth Zygomorphy is not beneficial Flowers are small Huge amounts of pollen are necessary Stigmas are large and feathery Tend to grow in dense populations Ovary Location Epigynous – Inferior ovary; perianth fused to ovary wall Ovary Location Hypogynous – Superior ovary; ovary clearly above perianth Ovary Location Perigynous – Semi-inferior; perianth fused along part of ovary wall Flower Number Single flower – A single flower on a single pedicel Flower Number Inflorescence – Many flowers grouped or clustered together – Many pedicels in a short space, or many pedicels fused together Inflorescences: Determinate Determinate – Inflorescence apex is converted to a flower – Limited reproductive potential – Typically the terminal flower opens first Inflorescences: Indeterminate Lowest or outermost flowers open first New flowers being initiated at the same time Fruits Dispersed by animals, gravity, wind, water If animals are to disperse the seeds, part must be edible; seed/embryo must be protected Fruit Types True fruit: only ovarian tissue Accessory fruit: other tissues, too Fruit Types: Dry Dehiscent Indehiscent – Single carpel – Single carpel Legume Caryopsis Follicle Achene – Compound gynoecium Samara Capsule – Compound gynoecium Schizocarp Nut Fruit Types: Other Fleshy Compound Fruits – Berry – Aggregate fruits – Pome – Multiple fruits – Drupe – Pepo – Hesperidium Seed Dispersal Wind: must be light, catch the wind Water: can be heavy, large Animal – Dry fruits cling – Fleshy must be ripe, seeds protected

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