Plant features: Multicellular eukaryotes Photosynthetic autotrophs (a few are parasitic...
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Transcript of Plant features: Multicellular eukaryotes Photosynthetic autotrophs (a few are parasitic...
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Plant features:Multicellular eukaryotes
Photosynthetic autotrophs (a few are parasitic heterotrophs)
Mostly terrestrial (some aquatic) - requires new modes of nutrition, support, protection & reproduction
Multicellular gametangia, stomates
Characteristic cellular features:Chloroplasts with thylakoid membranes and chlorophyll (other eukaryotic organelles, including mitochondria, also present)
Starch as a storage compound (in plastids)
Cellulose walls (structural strength)
Cuticle (waxy coating) on the outside
Meet the Plants
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Plants share several key features with their likely sister group, the charophyceans (green algae)
A common ancestor of modern charophyceans and plants moved from an aquatic habitat onto land
- this involves changes in size, gas exchange, protection for fragile embryos, protection from desication, ways for sperm to reach egg, mechanisms of dispersal (getting around)... etc
Bryophytes & Seedless Vascular Plants
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Kingdom Plantae 1st appeared about 475 million years ago, along with terrestrial fungi
most plants retain a symbiotic association with fungi
Stages in Land Plant Evolution:
1) Early land plants (~400 MYA) bryophytes and seedless vascular plants
2) Diversification (~390 MYA) abundance of seedless vascular plants
1) Seeds (~ 300 MYA) Gymnosperms (naked-seeded plants)
2) Flowers and fruits (~ 130 MYA) Angiosperms (flowering plants)
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3 contrasting opinions of what constitutes the “plant kingdom”
Standard definition: true plants have multicellular embryos
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3 contrasting opinions of what constitutes the “plant kingdom”
should we include their closest algal relatives, the charophyceans, who share so many features with land plants?
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should we include green algae (chlorophytes),who share the same photosynthetic machinery?
3 contrasting opinions of what constitutes the “plant kingdom”
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Polypodium sp., a terrestrial plant with greater cell & tissue complexity
Unlike green algae, terrestrial plants have different cell and tissue types
Coleochaete orbicularis, a disc-shaped charophycean showing simple structure
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Features shared by plants and charophyceans:Primary endosymbiosis (double membrane around chloroplast)Photosynthetic pigments (chlorophyll a, b and carotenoids)Starch storage in plastidsProduction of cell walls from cellulose Cell plate formation during cell divisionSporic life cycle
New features in terrestrial plants:Specialized tissues + organs (roots, stems, leaves) derived from the apical meristem, the growing tip of a shootAlternation of multicellular generationsMulticellular gametangia Walled spores produced in sporangiaMulticellular, dependent embryos
Evidence for common ancestor
of green algae & plants
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Plants organs derived from apical meristem
Above ground Below ground
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Alternation of Generations
All plants have multicellular haploid stages, and multicellular diploid stages, which produce each other
gametophyte (N)
sporophyte (2N)
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Walled spores
Sporophyte (2N) stage has multicellular structures called sporangia that produce the haploid spores through meiosis
spores = haploid cells that cangrow into the gametophyte bymitosis
protective outer layerallows spores to resist dry conditions
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Multicellular Gametangia
Gametophyte (N) stage has multicellular structures called gametangia that produce the haploid gametes
Archegonium: vase-shaped female organ holds single egg cell
Antheridium: male organ releases sperm
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Multicellular, Dependent Embryos
Multicellular embryos develop from zygote inside tissue of female parent, which provide embryo with nutrients during development
Placental transfer cells, analogous to mammalian placenta
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Non-vascular plants (7%)mosses, liverworts, hornworts - no roots or true leaves
Vascular plants (93%) have a system of tubes that transport water + nutrients throughout the plant
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Seedless vascular plants ferns + horsetails
Seed plantsgymnosperms + angiosperms
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Trends in Plant Evolution:
Increasing adaptation to a terrestrial environment
Progressive reduction ofgametophyte, increasingdominance of sporophyte
Increased protection(esp. of reproductive parts)
Increased height, structuralstrength, and more complex transport tissues
Campbell & Reece 2002
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Key innovations are special features that allow one group to speciate
6500 12,000
250,000
1000100
12,0
00 550
# of living species in different plant groups
100 1
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1) Early land plants (~438-408 MYA, Paleozoic Era)
Bryophytes and 1st vascular plants appeared at same time
Bryophytes features (e.g. moss)
Short
Lack elaborate vascular tissue
No true leaves, roots, or stems.
Dominant gametophyte
Flagellated (=swimming) sperm
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Bryophytes
(plants lacking vascular tissues)
They are all short -- why?
Typically associated with moist habitats
Most parts only 1 cell thick
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Bryophyte life cycle
gametophytes are anchored into soil by threadlike protonema and single-celled rhizoids
sporophytes = brown stalks growing out of the female gametophyte
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Bryophyte life cycle
1) spores can disperse by wind – why is dispersal important?
- allows colonization of new (maybe better) habitats
- allows “escape” for offspring if local conditions turn bad
- prevents inbreeding
2) sperm have to swim to reach egg – this requires environmental water
- limits where bryophytes can grow
- think about how this limited the ability of bryophytes to take over the world of dry land, compared to plant groups that evolved
later
- on the plus side: no need for pollinators, just rain or splashes
3) what features of the bryophyte body also limit:
- the size of these plants?
- the environment in which they can live?
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Sphagnum sp.
Forms extensive peat bogs in northern latitudes (arctic regions) around world
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Seedless vascular plants appeared ~ 420 MYA
Seedless vascular plant features (e.g., ferns) - Dominant sporophyte - Most lack true roots, stems, and leaves - Primitive vascular tissues
Diversification culminated in Carboniferous Period (~300-350 MYA), when our modern-day coal deposits were first formed
damp conditions favored the growth of huge forests of seedless vascular plants, limiting opportunities for seed plants until a global drying out
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Fossil Cooksonia, a seedless plant with water-conducting tissue in stems, but no true roots or leaves
Primitive vascular plants had branching sporophytes that were independent of the gametophyte for growth
Enable multiple sporangia per individual
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Lycopodium sp.
Psilotum sp.
Equisetum sp.(horsetail)
Polypodium sp. (fern)
Seedless Vascular Plants – have primitive vascular tissues
Phylum Pterophyta
Phylum Lycophyta
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Fern Life cycle
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Complex tissues and organs 1st appeared in seedless vascular plants
xylem - tubes of dead cells w/ lignin-reinforced cell walls, used for transporting water + dissolved minerals
phloem - living cells that transport sugars + other organic nutrients
roots - organ for uptake of nutrients from soil...-
leaves - increase surface area for photosynthesis
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Microphylls, single vein Megaphylls, w/ complex branching - found in Lycophytes, the vascular system club mosses
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Major evolutionary development: modified leaves called sporophylls on which sporangia grow
Fern sporophylls bear many sori, small clusters of sporangia on the underside of the sporophyll (look for these in lab)
In groups we will see later, sporophylls diversified in shape and structure; stopped looking like leaves
in gymnosperms
in angiosperms