Instructions for printer-friendly copy.

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Credit: Brett Zimmerman on DeviantArt
Go buy his photos.

    Sexual Encounters
    of the Floral Kind

    Going from 2n

      to n

        to 2n again.

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    xxxxxxxxx When plants do it, it's pretty.

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    Animal Sex is Simple

    1. Inside adult animal ovaries or testes, diploid (2n) germline cells
    xxxproduce haploid (n) gametes (ova or sperm) via meiosis.

    2. Male and female gametes join to form a diploid zygote.

    3. Zygote grows into a new diploid individual via mitosis.

    4. As an adult, the new diploid individual produces haploid gametes via meiosis.

    ...and the cycle continues.

    Simple!

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    Plant Sex is...Complicated.

    Plants insert extra steps between Animal #1 and #2.

    1. In an adult diploid (2n) sporophyte plant,
    xxxdiploid germline cells produce haploid (n) cells (spores) via meiosis.

      1a. Haploid cell grows into a new haploid (n) individual via mitosis.
      xxxThis haploid individual is called a gametophyte.

      1b. As an adult, the gametophyte produces haploid (n) gametes via mitosis.

    2. Male gametes (sperm) and female gametes (ova) join to form a diploid zygote.

    3. Zygote grows into a new diploid individual (via mitosis).
    xxxThis diploid individual is called a sporophyte.

    4. As an adult, the sporophyte produces haploid spores via meiosis.

    ...and the cycle continues.

    Not quite so simple.

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The change in ploidy from parent to offspring is known as the Alternation of Generations.

sporophyte (2n) ---> gametophyte (n) ---> sporophyte (2n) ---> gametophyte (n) ---> sporophyte (2n) ---> etc.

Land plant alternation of generations is said to be heteromorphic.

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By IvoShandor via Wikimedia Commons

    Evolution of the Alternation of Generations

    A land plant is either a

    • non-vascular bryophyte (more primitive)

      • Gametophyte (n) is the dominant life cycle stage.
      • Sporophyte (2n) is small and ephemeral/seasonal.

    • vascular tracheophyte (more derived)

      • Sporophyte (2n) is the dominant life cycle stage.
      • Gametophyte (n) is small and ephemeral/seasonal.

      (The "dominant" life cycle stage is larger and more conspicuous.)

Across plant taxa, homologous life cycle stages have undergone extensive evolution, and may look different from taxon to taxon.

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Terminology: A typical fern (tracheophyte) will illustrate.

    Sporophyte (2n)

    (spore means "spore" and phyt means "plant")

    • The sporophyte is a diploid (2n) spore-producing plant.

    • When reproducing, the sporophyte produces specialized leaves called sporophylls.

    Sporophyll (2n)

    (spore means "spore" and phyll means "leaf")

    • The sporophyll is a spore-bearing leaf.

    • Because it is a sporophyte organ, the sporophyll is diploid (2n).

    • The sporophyll grows clusters of specialized 2n structures called sporangia.

    Sporangium (2n)

    (spore means "spore" and angion means "box")

    • The sporangium is a spore-producing structure.

    • Inside the"box", many diploid (2n) germline cells undergo meiosis.

    • The resulting haploid (n) daughter cells are called spores.

    Spore (n)

    (spore means "spore")

    • Spores (n) are the propagules produced by the sporophyte.

    • The sporangium breaks open to release mature spores.

    • A lucky spore will land on a moist, hospitable spot.

    • The spore will germinate and grow via mitosis into a gametophyte.

    Gametophyte (n)

    (gamete means "gamete" and phyt means "plant")

    • The gametophyte is a haploid (n), gamete-producing plant.

    • Gametogenesis takes place in a multicellular sex organ (n) called
      • a gametangium (generic term), which is either
        • an archegonium (female)
        • an antheridium (male)

By I, EncycloPetey

    Multicellular Sex Organs

    Plants sex organs are somewhat analogous to ovaries and testes.
    They are present only in the gametophyte.

    • The female archegonium produces an ovum.
    • The male antheridium produces sperm.

  • Sperm are released when the environment is wet.
  • They swim to the ovum, still nestled in the archegonium.
  • Fertilization takes place inside the archegonium.
  • This produces a diploid zygote

By I, EncycloPetey

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    Which Sex?

    (And you thought it couldn't get any worse.)

    Depending on its species, a plant--whether gametophyte or sporophyte--can be

    dioecious

    and reproductive structures are on separate individuals

    monoecious

    and reproductive structures are on a single individual, but on different branches.

    bisexual/
    hermaphroditic

    and components are on a single individual and are combined in a single branch/structure.

    • A megasporophyll develops megasporangia that produce female megaspores.
    • A megaspore will grow into a female gametophyte.
    • A female gametophyte has only archegonia, and produces only ova.

    • A microporophyll develops microsporangia that produce male microspores.
    • A microspore will grow into a male gametophyte.
    • A male gametophyte has only antheridia, and produces only sperm.

    • A generic sporophyll develops plain old sporangia that produce plain old spores.
    • A spore will grow into a bisexual gametophyte.
    • A bisexual gametophyte has both antheridia and archegonia.

    The fern gametophyte is a mature, bisexual plant.
    And it's about to suffer a cruel fate because of that diploid zygote inside its archegonium.

    Zygote (2n)

    (zygo means "yoke")

    • The zygote is diploid.

    • Inside the gametophyte's archegonium, the zygote will grow via mitosis.

    • Eventually, it will emerge from the archegonium as the new sporophyte (2n)

    New Sporophyte (2n)

    (spore means "spore" and phyt means "plant")

    • Like its grandparent, the new sporophyte is diploid.

      • The bryophyte sporophyte is annual.
      • An annual plant lives, reproduces and dies in one year/season.

      • The tracheophyte sporophyte is perennial.
      • A perennial plant lives (and usually reproduces) for more than one year/season.
        • The growing tracheophyte sporophyte crushes its gametophyte mom out of existence.
        • Life (almost always just one season) is short and cruel for the tracheophyte gametophyte.

    • The new sporophyte will mature, produce sporophylls and spores, and the cycle will continue.

Now that we've gotten all this straight...

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If Animals Underwent Alternation of Generations: A Disturbing Analogy

It is sometimes mentally difficult to equate animal and plant life cycles.
To help clear up this confusion (or maybe make it worse),
here is an Imaginary Animal Species that Undergoes a Plant-like Alternation of Generations.

I. The Sporophyte Generation

    An Imaginary, Dioecious Animal Sporophyte

    In our imaginary species, the sporophytes are dioecious.
    There are separate male and female sporophytes.

    • Sporophyte Brad is diploid and male.
    • He will produce microspores

    • Sporophyte Angelina is diploid and female.
    • She will produce megaspores

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    The Sporophyte Produces Spores via Meiosis

    In the springtime, small lesions begin to appear
    on Sporophyte Brad and Sporophyte Angelina.

    Each lesion/bump matures into a sporangium.

    Inside each sporangium, diploid cells undergo meiosis to produce haploid spores.

    • Sporophyte Brad produces (male) microspores
      in his microsporangia.
    • Sporophyte Angelina produces (female) megaspores
      in her megasporangia.

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    Spores are Released

    Once the spores are mature, they are ready to be dispersed.
    The sporangia rupture, releasing spores to the wind.

    Remember:

    • Sporophyte Brad releases (male) microspores.
    • A microspore (n) will grow into a male gametophyte (n).

    • Sporophyte Angelina releases (female) megaspores.
    • A megaspore (n) will grow into a female gametophyte (n).

    Sporophytes Also Can Be Monecious or Bisexual

    Our imaginary species above is dioecious.

    But, depending on species, sporophytes also can be monoecious or bisexual.

    A monoecious sporophyte produces both

    • megasporangia that yield megaspores
    • microsporangia that yield microspores

    As long as two different mating types of spores are produced,
    the next step is the same, whether the sporophyte is
    dioecious, monoecious, or bisexual.

    It's time for the gametophyte generation.

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    II. The Gametophyte Generation

    Carried by the wind, the spores tumble out into the world.

    Some land on rooftops and wither away in the heat.
    Some end up in Spike's food bowl and are ingested with the kibble.

    But some land on a patch of moist soil.

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    Male and Female Gametophytes

    Spores that land in a hospitable environment germinate

    They grow via mitosis into
    haploid gametophytes.

    The gametophytes look completely different from their diploid sporophyte parents.

    (For one thing, they are relatively small
    and inconspicuous.)

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    Male Gametophyte (n): Antheridia Produce Sperm

  • A male gametophyte (n) is also known as a microgametophyte.

  • Microgametophytes have male sex organs called antheridia.

  • Each antheridium produces many haploid sperm.

  • Although they are haploid, antheridia can be considered analogous to testes.

    Female Gametophyte (n): Archegonia Produce Ova

  • A female gametophyte (n) is also known as a megagametophyte.

  • Megagametophytes have female sex organs called archegonia.

  • Each archegonium produces one haploid ovum.

  • Although they are haploid, archegonia can be considered analogous to ovaries.

    Sperm and Ova Ready...

    In their verdant little glade,
    the mature male and female gametophytes
    wait for that fateful event they need for reproduction:

    A GOOD RAIN STORM.

    Go Sperm, Go!

    Rain provides the moisture sperm need to swim.

    Male gametophytes release their sperm, which follow
    the chemical trails exuded by the female gametophytes.

    Sperm swim up the female's legs and into her archegonium.
    One sperm fertilizes the waiting ovum.

    A diploid zygote is formed inside the archegonium.

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III. Return of the Sporophyte

The next sporophyte generation is growing inside the archegonium of the gametophyte.

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    A Pregnant Pause

    Our imaginary animal is now incubating a growing zygote
    inside her archegonium.

    The diploid zygote will begin to grow into a
    new, genetically unique, diploid sporophyte.

    But what does this mean for the gametophyte?

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    This Will Not End Well for Smurfette

    Lacking a muscular, innervated uterus, the female gametophyte
    cannot give birth to the new sporophyte.

    As her offspring grows, it gradually crushes her
    out of existence.

    Tell your parents about this next time they complain
    that you're always asking them for money.

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    Evolution: Gametophyte vs. Sporophyte

    In the earliest plants, the gametophyte (n) generation was long-lived and dominant.
    As plants evolved, the gametophyte became more and more reduced.

    In most extant plants, the sporophyte is the long-lived, dominant generation.

    What does this progression look like in

    • the gametophyte?
    • the sporophyte?

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I. Evolutionary Progression: Long-lived Gametophyte, Ephemeral Sporophyte

The earliest land plants were most likely gametophytes.
They probably gave rise to small, ephemeral sporophytes that died soon after producing spores.

Extant bryophytes show this ancestral pattern.

REQUIRED VIDEO:

    The Bryophyte Gametophyte is the Perennial Generation

    Bryophytes include
    • hornworts (Anthocerophyta)
    • liverworts (Hepatophyta)
    • mosses (Bryophyta)

    When you see any of these growing in a damp place,
    you are seeing haploid gametophytes, the dominant, perennial generation.

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    <--- Watch the video for a clear illustration of a typical bryophyte life cycle.

    Bryophyte Gametophytes

    Bryophyte gametophytes are small and close to the ground.
    They usually reproduce once a year, in the spring.

    <---- On the left, a liverwort gametophyte (Marchantia sp.)

    xxxx On the right, a moss gametophyte (Dicranum sp.) ---->

    A relatively undifferentiated plant body such as this
    is called a thallus (Greek thallus means "green shoot").


  • Liverwort archegoniophores resemble tiny palm trees.
  • Archegonia are on the underside of the "fronds".
    • Bryophyte Gametophytes can be
      Monoecious or Dioecious

      Depending on species, mosses and liverworts can produce antheridia and archegonia on the same plant (monoecy ) or on different plants (dioecy ).

      Moss sex is hard to distinguish without a microscope.

      But liverworts do it so cute that we have to show you.

      <--- Female liverworts bearing archegoniophores.

      xxxxMale liverworts bearing antheridiophores. --->

      When antheridia release sperm (wet environment needed)
      they swim to the archegoniophores and up the stalks.
      Sperm fertilize each ovum in the many archegonia.

      The new sporophyte generation begins.

  • Liverwort antheridiophores resemble tiny umbrellas.
  • Antheridia are on the upper surface.
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    II. Evolutionary Progression: Long-lived Sporophyte, Ephemeral Gametophyte

    Over the course of evolution, tracheophytes have reversed the ancestral pattern of dominant gametophyte and ephemeral sporophyte.

    As tracheophytes became more derived, the gametophyte generation became more and more reduced.

    In the most derived tracheophytes (flowering plants), the gametophyte
    is little more than a collection of cells devoted solely to reproduction.

      The Tracheophyte Gametophyte is the Annual Generation

      Tracheophytes can be seedless:
      • club mosses (Lycophyta)
      • ferns (Polypodiophyta)

      Or tracheophytes can be seed-bearing:

      • cycads (Cycadophyta)
      • ginkgoes (Ginkgophyta)
      • conifers (Coniferophyta)
      • gnetophytes (Gnetophyta)
      • flowering plants/anthophytes (Anthophyta)

      The vast majority of plants you see on a daily basis
      are diploid tracheophyte sporophytes, the dominant, perennial generation.

      The tracheophyte gametophyte is the small, ephemeral generation.


      The next few slides will illustrate the gametophytes of
      three different kinds of tracheophytes:
      • ferns
      • conifers
      • flowering plants

      Notice how

      • the (male and female) gametophytes become more and more reduced
      • the sporophyte has become more and more dominant

      ...over the course of evolution.

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      Seedless Tracheophyte Gametophytes

      Like bryophyte gametophytes, seedless tracheophyte gametophytes are small and bisexual.
      Unlike bryophyte gametophytes, they live only one season.

      <---- On the left, a fern gametophyte

      xxxx On the right, a horsetail gametophyte ---->

      The resemblance of seedless tracheophyte gametophytes
      to liverwort gametophytes is no mere coincidence.

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      Seed Plant Gametophytes are Small and Dioecious

      Seed plant sporophytes may be dioecious, monoecious or bisexual, depending on species.

      However, all seed plant gametophytes are dioecious.

      Seed plant gametophytes are so reduced and highly derived
      that they no longer resemble their liverwort ancestors.

      But they are completely homologous to them.

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      Seed Plant Gametophytes: Male

      The male gametophyte of a pine or flowering plant
      is a small, haploid plant you may know as a pollen grain.


      <---- pine male gametophyte (pollen)

      xxxx anthophyte male gametophyte (pollen) ---->

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      Seed Plant Gametophytes: Female

      The gametophytes of female seed plants
      are even more reduced.

      <---- pine female gametophyte (inside ovule)

      xxxxanthophyte female gametophyte (inside ovule) ---->


      The seed plant female gametophyte remains encased
      in sporophyte tissue throughout its life.

      Sporophyte integuments + gametophyte comprise the ovule .
      After fertilization, the ovule becomes a seed .
      The sporophyte integuments form the seed coat.

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    REQUIRED VIDEO:

      Life Cycle of a Conifer

      Like all plants, conifers undergo an alternation of generations.

      As you will see, however, the gametophyte generation has become
      even more reduced than in the seedless tracheophytes.

      The sporophyte has become the dominant, long-lived generation.

      <--- Watch the video for a clear illustration of a typical conifer life cycle.

      A Pine Tree Produces Male Gametophytes (pollen) in Male Cones

      <--- In the spring, whorls of microsporophylls form male (staminate) cones. (left)

      Each sporophyll bears a microsporangium. (right) --->

        Inside the microsporangium, meiosis is producing microspores.
      • Each microspore develops into a male gametophyte that
        • lacks antheridia
        • produces two sperm via mitosis
        • is a pollen grain

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      A Pine Tree Produces Female Gametophytes (inside ovules) in Female Cones

      <--- In the spring, whorls of megasporophylls form fleshy,
      xxxxx female (pistilate) strobili (cones).

      Each megasporophyll bears a megasporangium. --->

    • A megaspore develops into a female gametophyte that
      • is enclosed in a jacket of sporophyte tissue forming an ovule.
      • has archegonia, each of which...
      • ...produces an ovum via mitosis

      Like the male, these highly derived gametophytes no longer resemble
      their liverwort ancestors. But they are homologous to them.

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    REQUIRED VIDEO:
    (tiny error: antipodals are identified as synergids!)

      Life Cycle of a Flowering Plant

      Flowering plants (anthophytes) also undergo alternation of generations.

      In these most derived plants, the gametophyte generation has become
      even more reduced than in the conifers.

      The sporophyte is still the dominant, long-lived generation.

      The gametophytes are small and ephemeral.

      • Pollen is the microgametophyte.
      • It produces sperm although it lacks antheridia.

      • An 8-nucleate embryo sac is the megagametophyte.
      • It produces an ovum although it lacks archegonia.

      A Flower is a Cluster of Sporophylls

      The flower is unique to anthophytes.
      It is composed of concentric rings of modified leaves.
      From outermost to innermost, these are:
      • sterile sepals (collectively called the calyx)
      • sterile petals (collectively called the corolla)
      • microsporophylls called stamens (collectively, the androecium)
        • filament (stalk)
        • anther (microsporangium)
      • megasporophylls called carpels (collectively, the gynoecium)
        • stigma (pollen "landing pad")
        • style (stalk)
        • ovary (housing ovules)

        One or more fused carpels comprise a pistil.

      A Flowering Plant Produces Male Gametophytes (pollen) in Anthers

      <--- When a flower blooms, it reveals microsporophylls
      xxxxx called stamens.

      Each stamen bears a microsporangium called an anther. --->

    • Inside the anther, meiosis is producing microspores.
    • Each microspore develops into a microgametophyte that
      • lacks antheridia
      • produces two sperm via mitosis
      • is a pollen grain

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      A Flowering Plant Produces Female Gametophytes (inside ovules) in Ovules

      <--- When a flower blooms, it reveals megasporophylls
      xxxxx called carpels.

      Each carpel contains ovules holding a megagametophyte. --->

      The megagametophyte which

      • is very reduced
      • lacks archegonia
      • undergoes mitosis x3 to produce 8 (n) nuclei
        • three antipodals
        • two polar nuclei
        • two synergids
        • one ovum
      • is sometimes called the 8-nucleate embryo sac

      (Each of the eight nuclei has a specific function.)

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    III. Evolutionary Progression: Ephemeral Gametophyte, Long-lived Sporophyte

    Over the course of evolution, tracheophytes have reversed the ancestral pattern of dominant gametophyte and ephemeral sporophyte.

    As tracheophytes became more derived, the sporophyte generation became larger and longer-lived.

    Most of the plants you see from day to day are sporophytes of vascular plants.

      Ephemeral Bryophyte Sporophyte: Liverworts

      The liverwort sporophyte grows inside the archegonium of
      and remains attached to the gametophyte for the duration of its life.

      It produces spores via meiosis, releases them, and then
      withers away along with the seasonal archegoniophore.

      Ephemeral Bryophyte Sporophyte: Mosses

      The moss sporophyte grows from the archegonium of
      the gametophyte and remains attached for the duration of its life.

      It is little more than a stalk bearing a terminal sporangium.

      It produces spores via meiosis, releases them,
      then withers away.


      Long-lived Seedless Tracheophyte Sporophyte: Club Mosses

      The earliest tracheophytes swapped the dominant gametophyte
      for a dominant sporophyte, and never looked back.

      Club Mosses are not true mosses, but primitive tracheophytes.

      When reproducing, club mosses produce bisexual strobili
      consisting of both megasporophylls and microsporophylls.

      They are never lonely.


      Long-lived Seedless Tracheophyte Sporophyte: Ferns

      The familiar fern is a sporophyte.

      When reproducing, it develops clusters of sporangia
      on the underside of its fronds, making them sporophylls.

      Fern spores grow into seasonal, bisexual gametophytes.


      Long-lived Seedless Tracheophyte Sporophyte: Cycads

      Sometimes called "Sago Palms" for their superficial resemblance to palms,
      cycads are actually the most ancient seed plants.

      True palms are highly derived flowering plants.

      Cycads are dioecious, with separate individuals producing
      either male or female strobili.

      Dinosaurs once munched on cycad sporophytes.
      But don't you try it. Most are wicked toxic.


      Long-lived Seedless Tracheophyte Sporophyte: Ginkgos

      The ginkgo sporophyte is a beautiful
      tree that turns brillant gold in autumn.

      Ginkgos are dioecious, with separate male and female trees..

      They are smog-resistant, and popular as landscape trees in
      cities with air pollution problems.

      Ginkgo extracts are sometimes touted as a tonic
      to improve brain function. But there's no empirical evidence
      to suggest it actually works.


      Long-lived Seedless Tracheophyte Sporophyte: Conifers

      Conifer sporophytes are diverse and
      ecologically important..

      Conifers are monoecious, with separate branches producing
      either male or female strobili.


      Long-lived Seedless Tracheophyte Sporophyte: Anthophytes

      Anthophyte sporophytes are the most diverse
      and successful of all plant groups.

      Depending on species, they can be dioecious, monoecious, or bisexual.

      They can be herbaceous or woody.

      Your life depends on their health and diversity every day.

    And That's How Plants Do.

    I. TERMS RELATED TO THE (HAPLOID) GAMETOPHYTE GENERATION:

    II. TERMS RELATED TO THE (DIPLOID) SPOROPHYTE GENERATION: