Plants without Seeds: From Water to Land

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Plants without Seeds From Water to Land
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28 Plants without Seeds From Water to Land

• 28.1 How Did the Land Plants Arise?
• 28.2 How Did Plants Colonize and Thrive on Land?
• 28.3 What Features Distinguish the Vascular
  Plants?
• 29.4 What Are the Major Clades of Seedless
  Plants?

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28.1 How Did the Land Plants Arise?

• Land plants are monophyletic, all descend from a
  single common ancestor.
• One synapomorphy Development from an embryo
  protected by tissues of the parent plant.
• Also called embryophytes.

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28.1 How Did the Land Plants Arise?

• Land plants retain derived features they share
  with green algae
• Chlorophyll a and b
• Starch as a storage product
• Cellulose in cell walls

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Figure 28.1 What Is a Plant?
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28.1 How Did the Land Plants Arise?

• Plants can be defined in several ways
• Streptophytes include land plants and closely
  related green algae
• Green plantsstreptophytes plus all other green
  algae. All have chlorophyll b

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28.1 How Did the Land Plants Arise?

• There are ten major clades of land plants.
• Vascular plants, or tracheophytes (seven
  clades)all have conducting cells called
  tracheids.
• The seven groups of vascular plants constitute a
  clade.

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28.1 How Did the Land Plants Arise?

• Nonvascular land plants The remaining three
  cladesliverworts, hornworts, and mosses.
• These groups do not form a clade.

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28.1 How Did the Land Plants Arise?

• The closest relatives of the land plants are the
  Coleochaetales and the Charales.
• Both retain eggs in the parental organism, as do
  land plants.

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28.1 How Did the Land Plants Arise?

• Charales is thought to be the sister group of
  land plants based on synapomorphies
• Plasmodesmata
• Branching, apical growth
• Similar peroxisome contents, mechanics of mitosis
  and cytokinesis, and chloroplast structure

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Figure 28.2 The Closest Relatives of Land Plants
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28.2 How Did Plants Colonize and Thrive on Land?

• Plants first appeared on land between 400 and 500
  million years ago.
• Adaptations were needed to survive in a dry
  environment.
• Large plants needed a way to transport water to
  all parts of the plant.
• They also needed structural support and methods
  to disperse gametes.

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28.2 How Did Plants Colonize and Thrive on Land?

• Characteristics of land plants
• The cuticlewaxy covering that retards water loss
• Stomataopenings in stems and leaves regulate
  gas exchange (except liverworts)
• Gametangia enclosing gametes

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28.2 How Did Plants Colonize and Thrive on Land?

• Embryos in a protective structure
• Pigments that protect against UV radiation
• Spore walls containing sporopolleninprotects
  from dessication and decay
• Mutualistic relationships with fungi that promote
  nutrient uptake from soil

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28.2 How Did Plants Colonize and Thrive on Land?

• Ancient plants contributed to soil formation.
• Acids secreted by plants help break down rock.
• Organic material from dead plants contributes to
  soil structure.

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28.2 How Did Plants Colonize and Thrive on Land?

• Nonvascular land plants (liverworts, mosses, and
  hornworts) are thought to be similar to the first
  land plants.
• They have only a thin cuticle and grow in moist
  environments in dense mats.
• They are small, there is no system to conduct
  water from soil to plant body parts.

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28.2 How Did Plants Colonize and Thrive on Land?

• The growth pattern of nonvascular plants allows
  water to move through mats by capillary action.
• Minerals can be distributed through the small
  plants by diffusion.

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28.2 How Did Plants Colonize and Thrive on Land?

• Many can grow on marginal surfaces, including
  tree trunks, rocks, even buildings.
• This ability results from a mutualistic
  relationship with fungi called glomeromycetes.
• The earliest plants were colonized with these
  fungithey promote absorption of water and
  minerals.

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28.2 How Did Plants Colonize and Thrive on Land?

• All land plants have a life cycle with
  alternation of generations.
• Includes multicellular diploid and multicellular
  haploid stages
• Gametes are produced by mitosis. Meiosis produces
  spores that develop into haploid organisms

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Figure 28.3 Alternation of Generations in Plants
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28.2 How Did Plants Colonize and Thrive on Land?

• The multicellular diploid plant is the sporophyte
  (spore plant).
• Cells in sporangia undergo meiosis to produce
  haploid, unicellular spores.
• Spores develop into a multicellular haploid
  plantthe gametophyte (gamete plant).

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28.2 How Did Plants Colonize and Thrive on Land?

• Gametophytes produce haploid gametes by mitosis.
• Fusion of gametes (syngamy, or fertilization)
  results in a diploid zygote.
• The zygote develops into the multicellular
  sporophyte.

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28.2 How Did Plants Colonize and Thrive on Land?

• There is a trend toward reduction of the
  gametophyte generation in plant evolution.
• In nonvascular plants the gametophyte is larger,
  longer-lived, and more self-sufficient than the
  sporophyte.
• In plants that appeared later, this is reversed.

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28.2 How Did Plants Colonize and Thrive on Land?

• In nonvascular plants, the gametophyte generation
  is the familiar, photosynthetic form.
• The sporophyte may or may not be photosynthetic,
  but is always nutritionally dependent on the
  gametophyte, and is permanently attached.

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Figure 28.4 A Moss Life Cycle (Part 1)
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28.2 How Did Plants Colonize and Thrive on Land?

• The haploid gametophyte produces gametes in
  specialized sex organs called gametangia.
• Female Archegoniumproduces one egg.
• Male Antheridiumproduces many sperm with two
  flagella each.

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Figure 28.4 A Moss Life Cycle (Part 2)
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28.2 How Did Plants Colonize and Thrive on Land?

• Sperm must swim to archegonium, or be carried
  there by raindrop splashes.
• Egg or archegonium releases chemical attractants
  for sperm.
• Cells in archegonium break down to form a
  water-filled canal through which sperm travel.
• Water is required for all these events.

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28.2 How Did Plants Colonize and Thrive on Land?

• Egg and sperm form a diploid zygote.
• Zygote develops into a multicellular, diploid
  sporophyte embryo.
• Base of archegonium grows to protect embryo
  during early development.
• Sporophyte remains attached to gametophyte by the
  foot.

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28.3 What Features Distinguish the Vascular
Plants?

• Vascular plants include the club mosses, ferns,
  conifers, and angiosperms (flowering plants).
• The vascular system consists of tissue
  specialized for the transport of materials.
• Xylem conducts water and minerals from soil up to
  the rest of the plant.

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28.3 What Features Distinguish the Vascular
Plants?

• Some xylem cell walls have lignin, which also
  provides support.
• Phloem conducts products of photosynthesis
  throughout the plant.

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28.3 What Features Distinguish the Vascular
Plants?

• A single event probably launched the vascular
  plants
• In the mid-Silurian (430 Mya), the sporophyte
  generation of a now extinct plant produced a new
  cell type, the tracheid.
• Tracheids are the principal water-conducting
  elements of xylem in all vascular plants.

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28.3 What Features Distinguish the Vascular
Plants?

• Evolution of tracheids had two important
  consequences
• Transport of water and minerals
• Rigid structural supportallows plants to grow
  tall and compete for light, and aid in spore
  dispersal
• Tracheids set the stage for the invasion of land

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28.3 What Features Distinguish the Vascular
Plants?

• Vascular plants also have a branching,
  independent sporophyte.
• A branching sporophyte can produce more spores
  and develop in complex ways.
• Mature sporophyte is nutritionally independent
  from the gametophyte.

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Figure 28.5 The Evolution of Plants
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28.3 What Features Distinguish the Vascular
Plants?

• First vascular plants were successful on land
  because of the cuticle and protective layers for
  the gametangia, and because of the absence of
  herbivores.
• First fossils are from the Silurian. They made
  land more hospitable for animalsamphibians and
  insects arrived soon after plants.

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Ch. 28 Opener 1 Making It on Land
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28.3 What Features Distinguish the Vascular
Plants?

• Trees appeared during the Devonian. In the
  Carboniferous period (359297 Mya), forests of
  lycophytes (club mosses), horsetails, and tree
  ferns flourished in tropical swamps.
• Plant parts were buried in the swamps, and over
  millions of years, high pressure and temperature
  transformed them into coal.

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Figure 28.6 Reconstruction of an Ancient Forest
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28.3 What Features Distinguish the Vascular
Plants?

• During the Permian, the continents came together
  to form Pangaea.
• Extensive glaciation occurred late in the
  Permian.
• Lycophytefern forests were replaced by
  gymnosperms.
• A different group of seed plants (angiosperms)
  overtook the landscape about 65 million years ago.

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28.3 What Features Distinguish the Vascular
Plants?

• Earliest vascular plants (now extinct)
• Rhyniophytes (Silurian) had a simple vascular
  system and dichotomous branching, but lacked
  leaves and roots.
• They were anchored by rhizomes (horizontal
  portions of stem) and rhizoids (water-absorbing
  filaments).

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Figure 28.7 An Ancient Vascular Plant Relative
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28.3 What Features Distinguish the Vascular
Plants?

• Lycophytes (club mosses) first appeared in the
  Silurian.
• Monilophytes (ferns and fern allies) appeared in
  the Devonian.
• These groups had true roots and leaves, and two
  types of spores.

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28.3 What Features Distinguish the Vascular
Plants?

• Monilophytes and seed plants form a clade called
  euphyllophytes.
• Synapomorphies include overtopping growthnew
  branches grow beyond the othersan advantage in
  the competition for light.

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28.3 What Features Distinguish the Vascular
Plants?

• Roots probably originated from a rhizome or stem.
  Fossil evidence supports this hypothesis.
• Underground and aboveground stems would be
  subjected to very different selection pressures,
  and the two have evolved distinctive structures.

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28.3 What Features Distinguish the Vascular
Plants?

• Leaf A flattened photosynthetic structure
  arising from a stem or branch has true vascular
  tissue
• Microphyllssmall, one vascular strand
  (lycophytes). May have originated as sterile
  sporangia
• Megaphyllslarger, more complex (monilophytes and
  seed plants)

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Figure 28.8 The Evolution of Leaves (A)
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28.3 What Features Distinguish the Vascular
Plants?

• Megaphylls may have arisen from reduced and
  flattened branching stems.
• Flat plates of photosynthetic tissue developed
  between branches, and the end branches became the
  leaf veins.
• Advantage Increased photosynthetic surface area.

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Figure 28.8 The Evolution of Leaves (B)
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28.3 What Features Distinguish the Vascular
Plants?

• Small megaphylls first appeared in the Devonian.
  Large megaphylls did not appear until the
  Carboniferous.
• One theory High CO2 concentrations in the
  Devonian reduced selection for stomata. Fewer
  stomata were needed to take up CO2.

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28.3 What Features Distinguish the Vascular
Plants?

• Stomata also allow heat to be lost by the
  evaporation of water.
• If megaphylls had grown large during this time,
  with few stomata, it would have resulted in
  overheating.

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Figure 28.9 CO2 Levels and the Evolution of
Megaphylls (Part 1)
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Figure 28.9 CO2 Levels and the Evolution of
Megaphylls (Part 2)
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28.3 What Features Distinguish the Vascular
Plants?

• The most ancient vascular plants were
  homosporousa single type of spore.
• The spores produce one type of gametophyte that
  has both archegonium and antheridium.

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Figure 28.10 Homospory and Heterospory (Part 1)
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28.3 What Features Distinguish the Vascular
Plants?

• Heterosporous plants produce two types of spores
• Megaspore develops into female gametophytethe
  megagametophyte, which produces only eggs
• Microspore develops into male gametophytethe
  microgametophyte, produces only sperm

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Figure 28.10 Homospory and Heterospory (Part 2)
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28.3 What Features Distinguish the Vascular
Plants?

• Megaspores are produced in small numbers in
  megasporangia.
• Microspores are produced in large numbers in
  microsporangia.
• Heterospory evolved several times.

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28.4 What Are the Major Clades of Seedless Plants?

• Liverworts Hepatophyta9,000 species.
• Some have leafy gametophytes some have thalloid
  gametophytes.
• Sporophytes are
• very short, only
• a few mm.

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Figure 28.11 Liverwort Structures
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28.4 What Are the Major Clades of Seedless Plants?

• A stalk raises the simple sporangium above ground
  level to allow spores to be dispersed.
• Liverworts also reproduce asexually
• By simple fragmentation of the gametophyte
• And by gemmaelens-shaped clumps of cells in
  gemmae cups. Gemmae are dispersed by raindrops.

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28.4 What Are the Major Clades of Seedless Plants?

• The mosses Bryophyta15,000 species.
• Mosses (plus hornworts and vascular plants) have
  stomata,
• important in water
• and gas exchange.

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Figure 28.12 Mosses Grow in Dense Mats
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28.4 What Are the Major Clades of Seedless Plants?

• Moss gametophytes begin as branched, filamentous
  structuresthe protonema.
• Some filaments are photosynthetic, others are
  rhizoids that anchor the protonema.
• Tips of photosynthetic filaments form buds which
  produce the leafy moss shoots.

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Figure 28.4 A Moss Life Cycle (Part 1)
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28.4 What Are the Major Clades of Seedless Plants?

• Some moss gametophytes are too large to depend on
  diffusion for water transport.
• Cells called hydroids die and provide channels
  through which water can travel.
• Hydroids are functionally similar to tracheids.

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28.4 What Are the Major Clades of Seedless Plants?

• Sphagnum moss grows in cool, swampy places.
• The upper layers of moss compress lower layers
  that are beginning to decompose, forming peat
  which can be used as a fuel.
• Long ago, continued compression of peat led to
  the formation of coal.

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Figure 28.13 Sphagnum Moss
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28.4 What Are the Major Clades of Seedless Plants?

• Hornworts Anthocerophyta100 species.
• Gametophytes are flat plates of cells.

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28.4 What Are the Major Clades of Seedless Plants?

• Hornwort cells have a single, large chloroplast.
• The sporophyte has no stalk but has a basal
  region capable of indefinite cell division.
  Sporophytes can grow up to 20 cm.
• Hornworts have internal cavities filled with
  nitrogen-fixing cyanobacteria.

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Figure 28.14 A Hornwort
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28.4 What Are the Major Clades of Seedless Plants?

• The exact evolutionary position of the hornworts
  is still unclear.
• In some morphological analyses they are placed as
  the sister group to the mosses plus the vascular
  plants (the two groups that express apical cell
  division).

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28.4 What Are the Major Clades of Seedless Plants?

• In the seedless vascular plants, the large
  sporophyte is independent of the small,
  short-lived gametophyte.
• The single-celled spore is a resting stage.
• Must have water for at least one part of the life
  cyclefor the flagellated, swimming sperm.

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28.4 What Are the Major Clades of Seedless Plants?

• The lycophytes Club mosses, spike mosses, and
  quillworts 1,200 species.
• Roots branch dichotomously leaves are
  microphylls.

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28.4 What Are the Major Clades of Seedless Plants?

• Some club mosses have sporangia arranged in
  clusters called strobili.
• Others have sporangia on upper surfaces of leaves
  called sporophylls.

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Figure 28.15 Club Mosses
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28.4 What Are the Major Clades of Seedless Plants?

• Lycophytes were dominant during the Carboniferous
  period.
• One type of coalcannel coalis formed almost
  entirely from the spores of a tree lycophyte
  Lepidodendron.

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28.4 What Are the Major Clades of Seedless Plants?

• The monilophytesHorsetails, whisk ferns, and
  ferns, form a clade.
• Horsetails and whisk ferns are both monophyletic,
  ferns are not.

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28.4 What Are the Major Clades of Seedless Plants?

• Horsetails 15 species in one genusEquisetum.
• Silica in cell wallsscouring rushes.
• Have true roots sporangia are on short stalks
  called sporangiophore.
• Leaves are reduced megaphylls in whorls. Each
  stem segment grows from the base.

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Figure 28.16 Horsetails
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28.4 What Are the Major Clades of Seedless Plants?

• Whisk ferns 15 species in two genera.
• No roots but well-developed vascular system.
• Psilotum has scales instead of leaves.
• Tmesipteris has flattened, reduced megaphylls.

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Figure 28.17 A Whisk Fern
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28.4 What Are the Major Clades of Seedless Plants?

• Whisk ferns were once thought to be descendents
  of rhyniophytes.
• DNA analysis determined a more modern origin.
  Evolved from more complex ancestors by reduction
  or loss of megaphylls and true roots.

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28.4 What Are the Major Clades of Seedless Plants?

• Leptosporangiate ferns 12,000 species. Most
  ferns belong to this clade. Sporangia walls are
  only one cell thick, borne on a stalk.
• Sporophytes have true roots, stems, and leaves.
• Fern leaf starts development as a coiled
  fiddlehead.

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Figure 28.18 Fern Leaves Take Many Forms
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28.4 What Are the Major Clades of Seedless Plants?

• Fern life cycle
• Spore mother cells in sporangia form haploid
  spores by meiosis.
• Spores can be blown by wind and develop into a
  gametophyte far from parent plant.

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28.4 What Are the Major Clades of Seedless Plants?

• Fern gametophytes produce antheridia and
  archegonia, not always at the same time or on the
  same gametophyte.
• Sperm swim through water to an archegonium to
  fertilize egg.
• Zygote develops into independent sporophyte.

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Figure 28.19 The Life Cycle of a Homosporous Fern
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28.4 What Are the Major Clades of Seedless Plants?

• Most ferns are in shaded, moist environments
  because water is required for swimming sperm.
• Tree ferns can reach heights of 20 m.
• Sporangia occur on undersides of leaves in
  clusters called sori.

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28.4 What Are the Major Clades of Seedless Plants?

• Most ferns are homosporous two groups of aquatic
  ferns are heterosporous.
• Some genera have a tuberous gametophyte that
  depends on a mutualistic fungus for nutrition.
• In some genera, even the sporophyte embryo must
  be associated with the fungus before development
  can proceed.

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28.4 What Are the Major Clades of Seedless Plants?

• DNA research suggests that diversification of
  modern ferns is fairly recent.
• Ferns may have taken advantage of shady
  environments created by the expansion of seed
  plant forests.