27 1 Most Angiosperms Reproduce Asexually Sexual reproduction in angiosperms involves mitosis meiosis and the alternation of haploid and diploid generations Flower is angiosperm s structure for sexual reproduction Differences between angiosperm and vertebrate animal sexual reproduction Meiosis in plants produces spores after which mitosis produces gametes in animals meiosis usually produces gametes directly In most plants there are multicellular diploid sporophyte and haploid gametophyte life stages alternation of generations in animals there is no multicellular haploid stage In plants the cells that will form gametes are determined in the adult organism usually in response to environmental conditions in animals the germline cells are determined before birth Flower consists of four concentric groups of organs arising from modified leaves carpels stamens petals and sepals Parts of flower all derived from modified shoot apical meristem Carpels are female sex organs that contain developing female gametophytes Stames are male sex organs that contain developing male gametophyets Imperfect flowers have only male or only female sex organs while perfect flowers have both Imperfect flowers termed dioecious Perfect flowers termed monoecious Female gametophytes megagametophytes which are also called embryo sacs develop in megasporangia meiotic products that give rise to megagametophytes are called megaspores 3 out of 4 haploid megaspores undergo apoptosis programmed cell death Remaining haploid megaspore undergoes three mitotic divisions without cytokinesis producing 8 haploid nuclei all contained in a single cell Cell wall formation leads to elliptical 7 celled megagametophyte embryo sac with 8 nuclei At one end of megagametophyte are three small cells Egg cell female gamete and two cells called synergids participate in fertilization by attracting pollen tube and receiving the sperm nuclei prior to their movement to egg cell and central cell At opposite end of megagametophyte are 3 antipodal cells which ventually degenerate In large central cell are two polar nuclei Male gametophytes microgametophytes whch are also called pollen grains develop in microsporangia Meiotic products that give rise to microgametophytes are called microspores 4 haploid products of meiosis in microsporangia each develop ell wall and undergo a single mitotic division produces 4 two celled pollen grains that are released into environmental 2 cells in a pollen grain have different roles Generative cell divides by mitosis to form two sperm cells that participate in fertilization Tube cell forms elongating pollen tube that deliver the sperm to embryo sac Self pollination fertilized by pollen that lands on the stigma of a flower on same plant or even the same flower Leads to homozygosity which can reduce the reproductive fitness of offspring Seperation of male and female gametophytes Self fertilization not possible in dioecious species Physical separation of male and female flowers sufficient in monoecious species Genetic Self Incompatibility Pollen grain will fertilize female gamete only if plant is self compatible capable of self fertilization To prevent self fertilization many plants self incompatible Controlled by cluster of tightly linked genes called S locus for self incompatibility Pollen tube delivers sperm cells to embryo sac Pollen grans germinates when they land on stigma of compatible pistil Germination involves development of pollen tube Two fertilization events occur when sperm reaches embryo sac on sperm cell fuses with egg cell producing diploid zygote which forms the new sporophyte embryo Other sperm cell fuses with two polar nuclei in central cell of embryo sac forms triploid nucleus that undergoes rapid mitosis to form a specialized nutritive tissue called endosperm endosperm provides chemical building blocks for developing embryo while it is underground and cannot perform photosynthesis Double fertilization is a characteristic feature of angiosperm reproduction Fertilization initiates growth of embryo endosperm ineguments and carpel Interguments tissue layers immediately surrounding the megasporangium develop into seed coat Carpel ultimately becomes wall of fruit that encloses seed Fruit in angiosperms is ovary together with the seed it contains 2 main functions Protect the seed from damage by animals and infection by microbial pathogens Aid in seed dispersal Seed dispersal vital to species survival Plants who are perennial germinate near their parent will compete with parent for resources 27 2 Hormones and Signaling Determine the Transition from the Vegetative to the Reproductive State Flowering represents reallocation of energy and material away from making more roots stems and leaves vegetative growth to making flowers and gametes reproductive growth Plants fall into 3 categories based on when they mature and initiate flowering and what happens after they flower Annuals Complete their lives within a year Include many crops important to humans corn wheat rice and soybeans When environment is suitable annuals grow rapidly After flowering they channel most of their energy into development of seeds and fruits Then rest of plant withers and dies Biennials Take two years to complete their lives Include carrots cabbage and onions Less common then annuals Typically only produce vegetative growth during first year and store carbs in In second year they use most of stored carbs to produce flower and seeds rather than underground roots or stems vegetative growth Plant dies after seeds form Perennials Live three or more years Typically these plants flower each year but stay alive and keep growing next season Shoot apical meristem becomes an inflorescence meristem inflorescence is cluster of flowers Produces small leafy structures called bracts as well as new meristems These new meristems can be inflorescence or floral meristems Each give rise to single flower Each floral meristem produces four consecutive whorls spirals of organs sepals petals stamens and carpels seperated by very short internodes Responsible for determinate growth Meristem Identity Genes Expression of two meristem identity genes initiates a cascade of further gene expression that leads to flower formation Genes LEAFY and APETALA1 Evidence supports that APETALA1 plays role in switching shoot apical meristem cells from vegetative to reproductive Floral Identity Genes Specify successive whorls of flower Homeotic genes whose products are