Angiosperm life cycle
Transcription
Angiosperm life cycle
Today’s lecture 1. 2. 3. 4. 5. Finish pine life cycle as typical gymnosperm Compare seed plants to non-seed plants Angiosperm reproductive characters Angiosperm life cycle Angiosperm advancements Pinus life cycle –seed plant Pages 442-443 text Conifer sperm are non-flagellate Spermatogenous cell divides in pollen tube to produce 2 sperm pollen tube sperm nuclei egg nucleus egg cell cytoplasm Pinus life cycle –seed plant Pages 442-443 text Nucellar apex Cycads & Ginkgo have multiflagellate sperm Seed evolution and changing life cycles Homospory, heterospory and the seed habit Seed evolution steps homospory Megasporangium surrounded by sterile telomes = branches heterospory Free telome tips Evolution of the seed/ovule Homospory Produce 1 size of spore Land plant life cycles Exosporic gametophyte development Free living gametophytes with multicelluar sex organsantheridia and archegonia Free sporing plants (=shed their spores from the sporangia) Need water for spore germination, gametophyte development and fertilization to take place Heterospory Produce 2 sizes of spores (megaspores and microspores) Endosporic gametophyte development (inside of spore wall) Free sporing plants Only need water for fertilization as gametophytes develop inside the spore wall Seed habit • Plants produce 2 sizes of spores • Endosporic gametophytes • Megasporangium never sheds its megaspore – i.e., it is not free sporing • Has eliminated the need for water in the completion of the life cycle • New to seed plants-the integument, pollen tubes that deliver the sperm to the archegonia or close to it. Cycads-swimming sperm Conifers-non-flagellate sperm Angiosperm life cycle Definitions • Angiosperms = flowering plants • “Angio” = container “sperm” = seeds 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 stigma stamen anther style filament pistil ovary petal ovules sepal receptacle FLOWER pedicel Flower parts 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 Anther x.s. 4 pollen sacs Vascular Bundle of Connective Monocot flower x.s. Note: in this flower the sepals and petals often look alike (colored) therefore = tepals Lily flower 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 Angiosperm Ovule Central cell Nucellus 7 celled embryo sac = megagametophyte 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 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 Tube nucleus 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 Capsella young dicot embryo 2n 2n 3n Capsella mature embryo l.s. Angiosperm life cycle Pages 472-473
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Angiosperms and course summary
• Endosporic gametophytes
• Megasporangium never sheds its megaspore –
i.e., it is not free sporing
• Has eliminated the need for water in the
completion of the life cycle
• New to seed plants-the ...
Seed plants
- but not for gametophyte growth which takes place inside
of the spore wall and
- not for spore germination, although needed for release of
sperm