Plant Diversity II

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Plant Diversity II

• Level 1 Biological Diversity
• Jim Provan

Campbell Chapter 30
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Reproductive adaptations of seed plants

• Three life cycle modifications led to the success
  of terrestrial plants
• Reduction of the gametophyte retained in the
  moist reproductive tissue of the sporphyte
• Origin of the seed
• Zygotes developed into embryos packaged with a
  food supply within a protected seed coat
• Seeds replaced spores as the main means of
  dispersal
• Evolution of pollen plants were no longer tied
  to water for fertilisation

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Reduction of gametophytes in seed plants
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In seed plants, the seed replaced the spore as
the main means of dispersing offspring

• Relatively harsh terrestrial environment
• Bryophytes and seedless vascular plants release
  spores
• Seeds are more hardy because of their
  multicellular nature
• Seed is a sporophyte embryo and a food supply
  surrounded by a protective coat
• All seed plants are heterosporous
• Development of seed associated with
  megasporangia
• Seed plant megasporangia are fleshy structure
  called nucelli
• Additional tissues (integuments) surround
  megasporangium
• Resulting structure is called an ovule
• Female gametophyte develops in wall of megaspore,
  is fertilised (embryo) and resulting ovule
  develops into a seed

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From ovule to seed
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Pollen became the vehicle for sperm cells in seed
plants

• Microspores develop into pollen grains which
  mature to form the male gametophytes of seed
  plants
• Pollen grains coated with a resistant polymer,
  sporopollenin
• Can be carried away by wind or animals (e.g.
  bees) following release from microsporangia
• A pollen grain near an ovule will extend a tube
  and discharge sperm cells into the female
  gametophyte within the ovule
• In some gymnosperms, sperm are flagellated
  (ancestral)
• Other gymnosperms (including conifers) and
  angiosperms do not have flagellated sperm cells

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Gymnosperms

• Descended from Devonian progymnosperms
• Seedless
• Seeds evolved late Devonian
• Climatic changes during Permian led to lycopods,
  horsetails and ferns being replaced by conifers
  and cycads
• Lack enclosed chambers (ovaries) in which seeds
  develop

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Four divisions of extant gymnosperms
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Conifers are the largest division of gymnosperms

• Mostly evergreens e.g. pines, firs, spruces,
  larches, yews, cypresses etc.
• Include the tallest, largest and oldest living
  organisms
• Needle-shaped leaves adapted to dry conditions
• Thick cuticle covers leaf
• Stomata in pits, reducing water loss
• Megaphylls cf. other leaves

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The life cycle of a pine
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Angiosperms
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Angiosperms (flowering plants)

• Flowering plants are the most widespread and
  diverse (250,000 species)
• Only one division (Anthophyta), with two classes
• Monocotyledons
• Dicotyledons
• Less dependent on wind pollination - use insects
  and animals

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Evolution of vascular tissue in angiosperms

• Conifers have water-conducting cells called
  tracheids

• Angiosperms have vessel elements
• More specialised for transport
• Less specialised for support

• Xylem reinforced by fibres
• Specialised for support - thick lignified wall
• Evolved in conifers (conifers lack vessel
  elements)

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The flower is the defining reproductive
adaptation of angiosperms

• Sepals sterile, enclose bud
• Petals sterile, attract pollinators
• Stamen produces pollen
• Carpel evolved from seed-bearing leaf that
  became rolled into a tube
• Stigma sticky structure that receives pollen
• Ovary protects ovules, which develop into seeds
  after fertilisation

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Fruits help disperse the seeds of angiosperms

• Fruits are ripened ovaries that protect dormant
  seed and aids in its dispersal
• Modifications of fruits that aid dispersal
  include
• Seeds within fruits that are shaped like kites or
  propellors (e.g. maple)
• Burr-like fruit that cling to animal fur
• Edible fruit - tough seeds pass through digestive
  tract

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Life cycle of an angiosperm
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Angiosperms and animals shaped one anothers
evolution

• Coevolution reciprocal evolutionary responses
  among two or more interacting species
• Coevolution led to diversity of flowers
• Flower-specific pollinators
• Usually adapted for types of pollinators
• Attraction of ripening fruits
• Soft, fragrant and sugary
• Attractive change of colour

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Plants transformed the atmosphere and the climate

• Plants decreased atmospheric carbon dioxide,
  resulting in global cooling

• Cooler environment made terrestrial life more
  habitable for other organisms