3.2 Seed Plants

  • Bryophytes began initial colonization of land

    • Liverworts→mosses→ ferns

    • Still, require water for reproduction

  • Gymnosperms and Angiosperms = Seedplants

    • Evolved from spore-bearing plants known as pro gymnosperms

    • Angiosperms (flowering plants) are most recently evolved

Progymnosperms

  • first naked seedplants

  • Transitional group

    • Conifer-life trait of wood production from secondary growth

    • Fern-like reproduction

  • Successful when climate changed from wet to dry

  • All are extinct

Evolution of Seed Plants

Two structures that allowed seed plants to dominate dry land:

Seed (came second)

  • Protects and provides food for embryo

  • Dormancy allows survival of harsh periods before germination

  • Later development of fruits enhanced seed dispersal

    • Coevolution with seed plants and animals

Pollen (came first)

  • Replaced flagellated sperm

  • Encased in a protective coat

  • Pollen tube delivers male nucleus to the egg cell

  • Can be delivered by wind, bees, birds

  • Primitive gymnosperms (ginko and cycads) have pollen with a flagellated sperm

The Seed

  • Fertilization of haploid female gametophyte by haploid male gametophyte creates the diploid embryo

  • Seed coat protects the embryo

  • Endosperm stores food for the developing embryo

Evolution of Seed Plants

  • All are heterosporous

  • Male (micro-) gametophyte

    • Within pollen grains super tiny

    • Dispersed by wind or a pollinator

    • No water needed

  • Female (mega-) gametophyte

    • Develop within an ovule

    • Enclosed within diploid sporophyte tissue in angiosperm

  • Monoecious: “one home” - male and female on same plant

  • Dioecious: “two home” - Male and female on separate plants

Gynmopsperm

  • Unclear if monophyletic or paraphyletic

  • Dominant sporophytes, gand ametophytes are microscopic nd not independent

  • Plants with “naked seeds”

    • Seed not enclosed in ovary (fruit) but sheltered by sporophylls

  • Lack of flowers and fruit

  • Four living groups

    • Coniferophytes

    • Cycadophytes

    • Gnetophytes

    • Ginkgophytes

Conifers

  • Dominant and most diverse phylum

  • Mostly monoecious

  • Pines, sources, firs, cedars, sequoias

    • Coastal redwood - tallest living vascular plant

    • Bristlecone pine - oldest living tree

  • Needle-like leaves with thick cuticles reduce water evaporation and allow snow to slide off preventing broken branches.

  • Most are evergreen (don’t lose leaves in the fall)

  • “Softwood”: contains tracheids but no vessel elements (the simple phylum that produces softwood)

Female and Male Strobili (cones)

  • Megastrobilus or seed cones produes megagametophytes

  • Microstrobilius or pollen cones produce microgametophytes

Megastrobilus or Seed Cones

  1. Sporogenesis: Diploid megasporocyte (megaspore mother cell) divides via meiosis. One large haploid cell survives = megaspore

  2. Gametogenesis: megaspore divides via mitosis and developed into megagmaetophyte with egg cell inside

    • Contains archegonium with egg cell

    • Small opening called micropyle

Microstrobilus or Pollen Cone

  1. Sporogensis: diploid micrsporocyte (microspore mother cell) divide via meiosis to produce four haploid mircopoes = pollen tetrad.

  2. Gametogensis: one microspore divides via mitosis to produce a generative cell and a tube cell. The generative cell then dvived via mitosis to produce two sperm cells. The tube cell growns into the pollen tube that the sperm travels down for fertilization to occur.

Conifer Life cycle

  • Micro and mega sporocyte undergo meiosis → hapolid gamtes

  • Archegonium has a sticky pollen trap near microplye → pollination

  • Pollen tuben growns slowly towards the gametophyte

  • One sperm nuclei fertilizes the egg→ embryo→ seed

  • Entire process can take 1-2 years after pollination

Cycads

  • Slow-growing gymnosperms of tropical and subtropical regions

  • Dominant and very large during Mesozoic and Jurassic period, but ~100 small species left

  • Sporophytes resemple palm tree

  • Female cones can weigh 45kg

  • Dioecious

  • Flagellated sperm

Ginkgos

  • Dioecious

  • Only one extant species, no longer found in the wild: Gingko biloba

  • Pollen with flagellated sperm

  • Fan-like leaves

  • Fruit-like structures with 3 layers of seed coat; not a true fruit because it does not come from the ovary

Gnetophytes

  • Dioecious

  • Angiosperm like features mostly due to convergent evolution:

    • Vessels in xylem of Gnetum and Welwitschia forms “hardwood”

    • Double fertilization in Ephedra and Gentum results in 2 competing embryos, only one survies

    • Fruit-lie ovule coverings

    • Leaf arrangement and venation

      • Welwitschia: Parallel venation

      • Gnetum: netted venation and opposite leaves

  • Conatin three (unusual) genera

    • Ephedra: Horsetail-lie leafless desert shrubs that contain high levels of ephedrine

    • Gnetum: Tropical tree

    • Welwitschia: One pieces of long lived desert plant wth two large continuely growing leaves

Angiosperms - Flowering Plants

  • >300,000 spp

  • Success is due to flowers and fruit

    • Flower: Pollination and embryo protection

    • Fruit: seed protection and dispersal

  • Monophyletic phylum: Anthophyta

  • Heterosporous: all seed plants

  • Sporophyte is dominant

  • Three groups: basal angiosperm, monocots, and dicots

Flowers

  • Modified stems bearing modified leaves

  • Houses the gametophytes

    • Most are hermaphroditic: flowers that have both male & female parts

    • Monoecious: separate male & female flowers on the same plant

    • Dioecious: Separate male & female plants

Female Gamete Production

  • Meiosis in megasporocyte =4 megaspores (3 small, 1 large)

  • Megaspore survives, via mitosis 8 nuclei and 7 cells (center cell has 2 nuclei)

    • Three cell at each end of embryo sac

      • Antipodal cells (Not really talked about) (one end)

      • Synergids and eggs (Opposite end)

      • Center cell contain polar nuclei→ endrosperm of seed (where fertilization happens)

Male Gamete Production

  • Meiosis of mother spore cell(original cell) in microsporangium produces four microspores

  • Each mircposre forms a pollen grain

  • Pollen contains 2 cells:

    • 1 generative cell that later divies into two sperm

    • 1 cell becomes pollen tube

Double Fertilization

  • Pollen lands on stigma and pollen tube geins to grow

    • Synergids guide pollen tube to egg

  • First fertilization event: 1 sperm + 1 egg = diploid zygote

  • Second fertilization event: other sperm fuses with polar nuclei = triploid cell → endrosperm (food reserve)

  • Zygote develops into embryo with radicle and one or two cotyledons

Basal Angiosperms

  • Branches off before monocots and dicots

  • Magnolidae

    • Magnolias: Dark shiny leaves and large fragrant flower

    • Laurels: fragrant leaves and small flowers

Monocots and Eudicots

  • Monocots do not produce true wood

    • Grasses and palm trees

    • Herbaceous (doesn’t produce true wood)

  • Eudicots do produce true wood

    • Secondary Growth (Oak trees)

  • Remember all vascular plants have primary growth at the apical meristems (roots & shoots)

  • Secondary growth deals with expanding girth and creating wood

  • Wood if proliferating xylems, (; which are responsible for transporting water and nutrients, along with phloem that transports sugars and other metabolic products. This growth results in the thickening of stems and roots, allowing trees like oak to reach impressive heights and support a larger canopy. )

Mono/di cot = # of seed leaves - Cotyledon