Plant Diversity I

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Plant Diversity I

• Level 1 Biological Diversity
• Jim Provan

Campbell Chapter 29
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Main groups of land plants

• Four main groups
• Bryophytes (mosses, liverworts and hornworts)
• Differ from algae due to adaptation to
  terrestrial life
• Mainly reproductive differences e.g. developing
  embryo attached to mother plant
• Pteridophytes (ferns, lycophytes, horsetails
  etc.)
• Vascular system not found in bryophytes
• Some bryophytes have a rudimentary
  water-conducting system
• Gymnosperms (conifers etc.)
• Seed plants embryo, food supply and protective
  covering
• Angiosperms
• Flowering plants

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Highlights of plant evolution
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Ten phyla of extant plants
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Plants probably evolved from green algae called
charophytes

• Evidence suggests plants and charophytes evolved
  from a common ancestor
• Homologous chloroplasts accessory pigments,
  stacked thylakoids, DNA evidence
• Biochemical cellulose and peroxisomes
• Cell division dispersed nuclear envelope,
  persistence of spindle etc.
• Sperm ultrastructure
• Phylogenetics

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Differences between plants and charophycean algae

• Apical meristems
• Light and CO2 found above ground
• Water and nutrients mainly found below ground
• Multicellular, dependent embryos
• Alternation of generations
• Occurs in some algae but not charophyceans (no
  sporophyte generation) evolved independently in
  land plants
• Differs from standard haploid/diploid life cycles
  in that both stages are represented by
  multicellular bodies
• Walled spores produces in sporangia
• Multicellular gametangia

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Alternation of generations
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Alternation of generations (continued)

• Most plants reproduce sexually and most are also
  capable of asexual reproduction all plants have
  life cycles with an alteration of generations
• Haploid gametophyte generation produces and
  alternates with a diploid sporophyte generation ?
  gametophyte
• Life cycles are heteromorphic i.e. have different
  morphology
• Sporophyte is larger and more noticeable, except
  in mosses
• Comparison of life cycles among divisions is
  instructive
• Points to an important trend in plant evolution -
  reduction of gametophyte generation and dominance
  of diploid sporophyte
• Some features are adaptations to terrestrial
  environment - replacement of flagellated sperm by
  pollen

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Alternation of generations in plants may have
originated by delayed meiosis

• Evolved independently in various groups of algae
• Does not occur among modern charophytes -
  occurrence in plants had a separate origin from
  alternation of generations in algae
• Appearance in plants analogous to occurrence in
  algae
• Coleochate may hold clues to evolution
• Parental thallus retains eggs - zygote also
  attached to parent
• Thallus cells grow around zygote which enlarges,
  undergoes meiosis and releases haploid swimming
  spores
• Haploid spores develop into new individuals
• Only diploid stage is zygote - no multicellular
  diploid stage. This would arise if meiosis was
  delayed until after mitotic division of zygote

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Alternation of generations in plants may have
originated by delayed meiosis
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Adaptations to shallow water preadapted plants
for living on land

• Many modern charophytes live in shallow water, as
  did their ancestors
• 440mya (Ordovician ? Silurian), climatic changes
  caused fluctuations in water levels
• Natural selection favoured plants tolerant to
  periodic drying - preadaptation to terrestrial
  life
• Waxy cuticles
• Protection of gametes
• Protection of developing embryos
• Eventually, accumulated adaptations made it
  possible for ancestral plants to live above water
  line

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Bryophytes
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The embryophyte adaptation evolved in bryophytes

• The embryophyte condition was a pivotal
  adaptation -
• Antheridium (male gametangium) produces
  flagellated sperm cells
• Archegonium (female gametangium) produces egg -
  fertilisation/embryo development occur within
  female organ
• Bryophytes are not totally independent of aquatic
  habitat
• Need water for sperm to swim in
• Have no vascular tissue - diffusion

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The gametophyte is the dominant generation in the
life cycle of bryophytes
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Mosses (Division Bryophyta)

• A tight pack of many moss plants forms a spongy
  mat that can absorb water
• Each plant grips the substratum with rhizoids
• Photosynthesis occurs in the upper parts of
  plants
• Cover about 3 of land surface and contain vast
  amounts of organic carbon

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Liverworts (Division Hepatophyta)

• Less conspicuous than mosses
• Sometimes have bodies divided into lobes
• Live cycle similar to mosses sporangia have
  elaters to aid dispersal
• Can reproduce asexually from gemmae
• Most diverse in tropical forests

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Hornworts (Division Anthocerophyta)

• Resemble liverworts but sporophytes are
  horn-shaped and grow from mat-like gametophyte
• Photosynthetic cells have a single, large
  chloroplast
• Most closely related to vascular plants

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Additional terrestrial adaptations in vascular
plants

• Regional specialisation of the plant body
  evolution of roots to absorb water and
  stems/leaves to make food
• Structural support no buoyancy from water, so
  lignin became embedded into cell walls
• Vascular system
• Xylem conducts water and minerals from roots.
  Composed of dead, tube-shaped cells and provides
  structural support
• Phloem conducts food throughout the plant.
  Composed of living cells organised into tubules
  and distributes sugars etc.
• Pollen and seeds to transport gametes
• Increased dominance of the diploid sporophyte

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Pteridophytes
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The sporophyte-dominant life cycle

• Exemplified by ferns
• Familiar leafy plant is the sporophyte
• Gametophytes are small and grow on or below the
  soil surface
• Vascular plants display two distinct reproductive
  strategies
• Sporophyte of homosporous plants (e.g. ferns)
  produces a single type of spore which develops
  into a bisexual gametophyte with both male and
  female sex organs
• Sporophyte of heterosporous plants produces two
  types of spore
• Megaspores develop into female gametophytes
  (archegonia)
• Microspores develop into male gametophytes
  (antheridia)

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A sporophyte-dominant life cycle evolved in
seedless vascular plants
MITOSIS
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Lycophytes (Division Lycophyta)

• Includes club mosses and ground pines
• Dominated land through the Carboniferous Period
  (340-280mya)
• Some species of Lycopodium are epiphytes
• Sporangia borne on sporophylls
• Spores develop into inconspicuous gametophytes
• Mostly homosporous

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Horsetails (Division Sphenophyta)

• Survived through Devonian and at peak during
  Carboniferous
• Only genus is Equisetum
• Lives in damp locations and has flagellated sperm
• Homosporous
• Conspicuous sporophyte generation
• Has photosynthetic, free-living gametophytes

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Ferns (Division Pterophyta)

• Co-existed with lycopods and horsetails in
  Carboniferous forests
• Larger leaves with branched system of veins -
  fronds are compound leaves
• Homosporous
• Specialised sporophylls
• Gametophyte is small and fragile
• Embryo develops within archegonium