Lecture 17: Flowers and Reproduction PDF

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