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 resources27.2 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 happensafter 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 underground roots or stems In second year, they use most of stored carbs to produce flower and seeds rather than 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