Plant Evolution and Diversity

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Plant Evolution and Diversity

• Unit 10 Plants
• AP Biology

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Plants

• Multicellular, photosynthetic eukaryotes.
• Evolution marked by adaptations to land existence.

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Water vs. Terrestrial

• WATER
• Water filters light (disadvantage)
• Provides for plentiful water
• Offers support for the body of a plant
• Carbon dioxide is not as plentiful in water and
  diffuses slower in water than in air.
  (disadvantage)

• LAND
• Plentiful Light
• Carbon dioxide more plentiful and diffuses faster
  in air than in water
• Constant threat of desiccation (disadvantage)
• Need to protect all phases of reproduction (from
  drying out) such as sperm, egg and embryo
• Need to have an efficient means of dispersing
  embryo within seed.
• Vascular system needs to evolve

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Evolution of Plants
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Evolution of Plants

• Plants evolved from a freshwater green algal
  species some 425 450 million years ago.
• Evidence in support of this
• Green plants and algae contain chlorophyll a as
  the main pigment in photosynthesis and
  chlorophyll b as an accessory pigment.
• Green plants and algae store excess glucose as
  starch
• Green plants and algae have cellulose in their
  cell walls.

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More Evidence

• Comparison of RNA sequences between land plants
  and green algae suggest that land plants are MOST
  closely related to a freshwater green algae known
  as charophytes.
• Freshwater exists in bodies of water on LAND, and
  natural selection would have favored those
  specimens BEST ABLE TO make the transition to the
  land itself.
• Land was barren without any competition.

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Charophytes most like land plants

• CHARALES- macroscopic species
• Stoneworts some species are encrusted with
  calcium carbonate deposits
• Filamentous algae with whorls of branches that
  occur at multicellular nodes.

• COLEOCHAETE microscopic species
• Looks like a flat pancake.
• Body composed of elongated branched filaments of
  cells that spread flat across the substrate and
  form a 3D cushion.

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Charophytes

• Charales

• Coleochaete

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Charophytes

• Live in water
• Haploid life cycle
• Cellulose cell walls
• Cytokinesis nearly identical to land plants
• Apical cells produce cells that allow filaments
  to increase in length. At the nodes, other cells
  can divide asymmetrically to produce reproductive
  structures
• Plasmodesmota provides a means of communication
  between neighboring cells.
• Placenta (designated cells) transfer nutrients
  from haploid cells of the previous generation to
  the diploid zygote. Care for their zygote.

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Land Plants - Embryophytes

• Alternation of generation life cycle
• Protect a multicellular sporophyte embryo
• Gametangia produce gametes
• Apical tissue produces complex tissues
• Waxy cuticle prevents water loss.

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Alternation of Generations Life Cycle

• All land plants have an alternation of
  generations life cycle.
• The sporophyte generation (2n) is named because
  it produces haploid spores that grow into the
  gametophyte generation (1n)
• The gametophyte generation (1n) is named because
  it produces haploid gametes that unite in
  fertilization to produce sporophyte.

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Alternation of Generations
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Alternation of Generations
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Alternation of Generations

• In the plant life cycle, the zygote becomes a
  multicellular sporophyte with one or more
  sporangia that produces many windblown spores.
• The production of so many spores would most
  likely have assisted land plants in colonizing
  the land environment.

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Dominant Generation

• Land plants differ as to which generation is
  dominant that is more conspicuous.
• In moss (BRYOPHYTE), the gametophyte is dominant,
  but in ferns, pine trees, and peach trees, the
  sporophyte is dominant.
• In the history of land plants, only the
  sporophyte evolves vascular tissue therefore,
  the shift to sporophyte dominance is an
  adaptation to the life on land.
• As the sporophyte becomes dominant, the
  gametophyte becomes microscopic and dependent on
  the sporophyte.

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Traits of Land Plants

• The zygote and the multicellular 2n embryo are
  retained and protected from drying out. Because
  they protect the embryo, an alternate name for
  the land plant clade is embryophyta.
• The sporophyte generation produces at least one
  or more multicellular sporangia.
• Sporangia produces spores by meiosis. Spores
  (and pollen grains) have a wall that contains
  sporopollenin a special molecule that prevents
  drying out.

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Traits of Land Plants continued

• Spores become the gametophyte generation (1n)
  that bears multicellular gametangia, which have
  an outer layer of sterile cells and an inner mass
  of cells that become the gametes.
• Male gametangium antheridium
• Female gametangium archegonium

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Traits of Land Plants continued

• Exposed parts of land plants are covered with
  waxy cuticle.
• Stomata little openings that allow gas exchange
• Presence of apical tissue. Apical tissue has the
  ability to produce complex tissues and organs.

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Bryophytes Nonvascular Plants

• First to colonize land
• Liverworts, hornworts, mosses
• Low-lying
• Nonvascular plants that prefer moist locations
• Dominant gametophyte produces flagellated sperm
  NEEDS water
• Unbranched, dependent sporophyte produces
  windblown spores.

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Vascular Plants -

• LYCOPHYTES (CLUB MOSSES), FERNS, SEED PLANTS
• Dominant, branched sporophyte has vascular
  tissue.
• Xylem tissue contains lignin
• Xylem transports water
• Phloem transports organic nutrients.
• Typically has roots, stems, and leaves
• Gametophyte is eventually dependent upon
  sporophyte.

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Lycophytes (Club Mosses)

• Leaves are microphylis with single, unbranced
  vein.
• Sporangia borne on sides of leaves produce
  windblown spores
• Independent and separate gametophyte produces
  flagellated sperm

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Ferns (Pteridophytes)

• Leaves are megaphylis with branced veins.
• Dominant sporophyte produces windblown spores in
  sporangia borne on leaves
• Independent and separate gametophyte produces
  flagellated sperm

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Seed Plants

• Gymnosperms and angiosperms
• Leaves are megaphylls
• Dominant sporophyte produces heterospores that
  become dependent male and female gametophytes
• Male gametophyte is pollen grain and female
  gametophyte occurs within ovule which becomes a
  seed.

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Seed Plants - Gymnosperms

• Cycads, ginkgoes, conifers, gnetophytes
• Usually large, cone-bearing
• Existing as trees in forests
• Sporophyte bears pollen cones
• Sporophyte pollen cones produce windblown pollen
• Seed cones produce seeds.

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Seed Plants - Angiosperms

• Diverse
• Live in all habitats
• Sporophyte bears flowers which produce pollen
  grains and bear ovaries with ovules within ovary.
• Following double fertilization, ovules become
  seeds that enclose a sporophyte embryo and
  endosperm (nutrient tissue)
• Fruit develops from ovary.

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Plant Evolution and Diversity

• Embryo protection (bryophytes)
• Apical growth (Bryophytes)
• Vascular Tissue (Ferns)
• Microphylls (club mosses)
• Megaphylls (Ferns)
• Seeds (Gymnosperms and angiosperms)
• Fruit (angiosperms)