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Angiosperms flowering plants Lecture #18 Definitions • Angiosperms = flowering plants • “Angio” = container “sperm” = seed Contained seeds – seeds of angiosperms are contained in a structure termed a carpel Reproductive Characters of angiosperms • Carpel encloses the seeds • Double integument for seed • Flowers – determinate stem tip with leaves and modified leaves that bear the pollen and seeds • 7-celled, 8 nucleate megagametophyte • 3-celled microgametophyte • Double fertilization • Endosperm Angiosperm phylogeny stigma stamen anther style filament pistil ovary petal ovules sepal receptacle FLOWER pedicel Flower – a determinate stem tip with modified leaves Sepal = leaf Petal = leaf Stamen = leaf bearing pollen sacs Carpel = leaf bearing ovules (makes up pistil) pedicel Carpels • The units that contain the seeds • Part of the ovary • If the ovary has more than one carpel you usually see more than one locule (chamber containing seeds) • You can sometimes tell how many carpels are in a flower by looking at the tip of the style. Number of style tips or lobes = number of carpels • Carpels are leaves that have rolled up to enclose the ovules. Later a number of these may have fused together to form syncarpous ovaries (i.e., they evolved from leaves) • Note: stamens are also modified leaves with attached pollen sacs Evolution of the angiosperm stamen Michelia x.s. Adaxial pollen sacs → marginal pollen sacs →reduced connective Magnolia flower mega micro All petals = corolla All sepals = calyx Calyx + corolla = perianth Pistil = stigma, style & ovary Gynoecium = all the carpels Androecium = all stamens Calyx = all the sepals Corolla = all the petals Stamen = filament & anther Anther = 4 pollen sacs Tepals = collective term used when sepals & petals look alike Flower parts Monocot flower Flower parts in 3’s or multiples of 3 Tepals Lilies – monocot flowers 2 whorls of perianth When the petals and sepals have the same color and morphology Tepals Lilium flower bud Petal Typical monocot flower with parts in 3’s Tepals Sepals Monocot flower x.s. Note: in this flower the sepals and petals often look alike (colored) therefore = tepals Anther x.s. 4 pollen sacs Vascular Bundle of Connective Eudicot flower x.s. – flower parts in 4’s or 5’s Ovary with 5 carpels – typical eudicot Angiosperm pollen Aperture = a specialized region of the wall that is thinner than the rest Monocolpate Colpus = elongated aperture with a length/ width ratio greater than 2. Monocolpate or monoporate Single aperture– shapes differ here Pore – circular or elliptical Aperture Tectate/collumellate angiosperm pollen wall structure Exine – sporopollenin 1. Tectum =roof 2. Columellaepillars or columns 3. Foot layer Intine – cellulose Sculptural elements Tricolpate Tricolporate Triporate e = equatorial view; p = polar view Angiosperm pollen grains Monocolpate Tricolpate Triporate Atactostele - monocots Eustele - eudicots No vascular cambium Have vascular cambium monocot Dicot stem Protoxylem cortex Early metaxylem vessel Endodermis Pericycle Late metaxylem vessel Dicot rootactinostele pith Monocot root – pith exarch medullated protostele Eudicot stem & leaves eudicot leaf monocot leaf Eudicot leaf with reticulate venation Eudicot leaf x.s. epidermis Sclerenchyma fibers collenchyma Eudicot leaf x.s. midvein stomata Eudicot leaf Monocot leaf with parallel venation Major and minor veins of various sizes Monocot leaf x.s. with parallel vascular bundles stoma substomatal chamber Monocot leaf x.s. Bulliform cells= enlarged cells in the epidermis of some monocot leaves that can lose water and collapse allowing the leaf to roll up to prevent water loss Vascular bundles in corn (monocot) leaf lacuna Eudicot embryos in seeds Capsella Monocot embryo Zea - corn Corn embryo - monocot (fruit wall) apex Hypocotyl apex Monocot embryo - corn Angiosperm life cycle Pages 472-473 Meiosis takes place in the pollen sac of the anther Microspore mother cells/ microsporocytes (2n) undergo meiosis Four microspores result from each division- these are Haploid (1n). Tapetum – nutritive tissue (also lays down the sporopollenin walls) Pollen tetrads – after meiosis Pollen shed in a 2-celled stage Microgametophyte is only 3 cells in total Tube cell or Tube cell (vegetative cell) controls The growth of the pollen tube Generative cell divides to form the 2 sperm Pollen Megagametophyte development Young developing ovule with megasporocyte or megaspore mother cell – undergoes meiosis mmc Inner integument micropyle nucellus Embryo sac = megagametophyte • Monosporic embryo sac development = after meiosis four haploid megaspores result. 3 abort and 1 goes on to divide and form the megagametophyte. Mono=one – Most angiosperms First division of the megaspore nucleus Most angiosperms have monosporic embryo sac development Two nucleate megagametophyte stage Second mitotic division Four-nucleate phase Angiosperm Ovule Central cell Nucellus 7 celled embryo sac = megagametophyte Mature embryo sac • 1. 2. 3. 4. Contains 8 nuclei, 7 cells Egg 2 synergids (“nurse cells”) 3 antipodal cells 2 polar nuclei in the central cell Note: There are no archegonia Gametophytes in angiosperms are very reduced Microgametophyte is 3 cells– mega. is 7cells A trend in reduction in the size of vascular plant gametophytes over time. Embryo sac = megagametophyte micropyle Egg & synergids at this end Central cell Polar nuclei 3 antipodal cells Central cell Pollen on stigmatic surface (stigma) Pollination in Angiosperms . The transfer of pollen from the anther to the stigma . Pollination droplet Integument Megaspore Ovule enclosed in carpel Pollination in gymnosperms Nucellus The transfer of pollen from the pollen sac to the micropyle of the seed/ovule Mature microgametophyte & pollen tube growth nucleus TubeTube nucleus Sperm Pollen grain Angiosperm Ovule Double fertilization Central cell 2 sperm Nucellus Pollen tube 7 celled embryo sac = megagametophyte Primary endosperm nucleus 3n (degenerate) xx x (degenerate) x x Zygote 2n Double fertilization • 1 egg + 1 sperm = 2n zygote • 2 polar nuclei + 1 sperm = 3n endosperm (primary endosperm nucleus) • Primary endosperm nucleus ÷ to form endosperm tissue cellular (or free nuclear) • “Endosperm” only found in angiosperms Becomes the food supply for the developing embryo (3n tissue) After double fertilization M I C R O P Y L A R E N D Endosperm divides 1st then the zygote M I C R O P Y L A R E N D Endosperm can be cellular or free nuclear Embryo development Capsella Summary of Capsella embryogeny 2n 2n 3n Capsella mature embryo l.s. Angiosperm life cycle Pages 472-473 Self quiz – label the parts Angiosperm Ovule Central cell Nucellus 7 celled embryo sac = megagametophyte
