Plant Diversity I

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Chapter 29 Plant Diversity I How Plants
Colonized Land
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Figure 29.2 Charophyceans Chara (top),
Coleochaete orbicularis (bottom)
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Figure 3.2 Water transport in plants
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Figure 29.14 Three clades that are candidates
for designation as the plant kingdom
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• I.  Overview of land plant evolution
•     
• A.  Four main groups of land plants
• Bryophytes
• Pteridophytes
• Gymnosperms and
• Angiosperms
•                        
• ? Groups are distinguished from algae by
  reproduction (life cycle) that involves the
  development of a multi-cellular embryo attached
  to the mother plant for its protection and
  nourishment.

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• 1.  Bryophytes liverworts, hornworts, mosses
•                                    
• - Bryophytes have no vascular tissues.
•  
• The rest three groups are all vascular plants. 
• Vascular plants have cells that are joined to
  produce tubes that transport water and nutrients
  throughout the plant.
• Bryophytes live in damp/moist environments and
  are small so they dont need vascular
  tissue. They are sometimes called non-vascular
  plants.
• Algae that we saw in last chapter live in water
  and dont need vascular tissue because nutrients
  come from surrounding water.

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The vascular plants are, in order of their
evolution 2.  Pteridophytes ferns,
horsetails, lycophytes     a.  seedless
plants 3.  Gymnosperms conifers, ginkgo,
cycads, gnetopsids      a.  early seed
plants      b.  produce naked seeds   4. 
Angiosperms flowering plants       a.  seeds
protected by growing in ovaries       b. 
majority of modern plants are in this group
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Some highlights of plant evolution
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• B.  Charophyceans - green algae most closely
  related to land plants.
•  
• 1.  Both charophyceans and land plants are
• multi-cellular,
• eukaryotic,
• photoautotrophs.  
• 2.  Both have rosette cellulose-synthesizing
  complexes rose-shaped arrays of proteins
  that synthesize the cellulose components that
  make up plant cell walls. 
• Because all these features are shared between
  the groups, both land plants and charophyceans
  must have a common ancestor.

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Charophyceans, closest algal relatives of the
plant kingdom
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In order to grow on land, the land plants needed
to evolve terrestrial adaptations to
survive. C.  Terrestrial adaptations can be used
to distinguish land plants from charophycean
algae. These adaptations are 1.  Apical
meristems 2.  Multi-cellular, dependent embryos
3.  Alternation of generations 4.  Spore walls
contain sporopollenin 5.  Multi-cellular
gametangia                         1.  Apical
meristems                                    
a.  localized areas of cell division at tips of
roots and shoots.                        
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Apical meristems of plant shoots and roots
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2.  Multi-cellular, dependent embryos             
                        a.  Embryo develops
within female tissue female plant provides
nutrition (sugars, proteins).
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                                    b. 
Placental transfer cells that enhance the
transfer of nutrients from the parent to the
embryo.   Figure 29.5 (p. 579) Placental
transfer cell in a liverwort (a bryophyte) ? See
Text book.
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3.  Alternation of generations                    
                 Two multi-cellular body
forms                                     a. 
Gametophyte (haploid) that produces gametes. 
Gametes fuse to form zygotes that develop
into   b.  Sporophytes (diploid) that produce
spores.  Spores are haploid cells that can
develop into a new organism without fusing with
another cell.  
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Alternation of generations  a generalized scheme
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•  4.  Spore walls contain sporopollenin
•                                    
• a.  Sporopollenin is a polymer that makes the
  walls of plant spores very tough and resistant to
  harsh conditions.
•  
• b.  Sporopollenin is the most durable organic
  material known.
•  
• c.  Spores are produced by sporangia (cells in
  the sporophyte) through the process of meiosis.
•  
• d.  Durable spores are an adaptation for
  surviving on land.
• Can withstand long periods of adverse conditions.
  
• Easily transported by wind and water.
•  

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A fern spore
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5.  Multi-cellular gametangia                     
                a.  Gametangia are the
gametophyte forms of bryophytes, pteridophytes,
and gymnosperms.  Gametes are produced within
these organs.                                    
b.  Female gametangia are called archegonia à
(produce and retain egg cells)   c.  Male
gametangia are called antheridia à (produce sperm)
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Gametangia
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6.  Other terrestrial adaptations common to many
land plants                                    
a.  Epidermis covered by a waxy cuticle to
prevent excess loss of water.  Pores (stomata) in
cell layer can be opened and closed to allow O2
out and CO2 in.  
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Cuticle of a stem from Psilotum (a pteridophyte).
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b.  Except for bryophytes, land plants have
vascular tissue in roots, stems, and
leaves.                                           
      - Xylem consists of dead cells that carry
water and nutrients from roots to the rest of the
plant.                                            
     - Phloem consists of living cells that
distribute sugars and amino acids throughout the
plant.
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Xylem and phloem in the stem of Polypodium, a
fern (a pteridophyte)
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II.  Origin of land plants             A. 
Theory is that land plants evolved from
charophycean algae over 500 million years ago. 
                        Evidence               
          1.  Homologous chloroplasts             
              2.  Homologous cell walls made of
cellulose                           3. 
Homologous peroxisomes                          
4.  Similar DNA sequences  
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B.  Alternation of generations in plants may have
originated by delayed meiosis                     
    Zygote à Sporophyte à Many, many
spores   1.  Occurs on land because its more
difficult to produce zygotes.  (No water for
swimming sperm)   2.  By producing sporophyte,
many gametophytes can be produced from one zygote
because many, many spores are produced.  This
maximizes output of sexual reproduction.
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C.  Adaptations to shallow water pre-adapted
plants for living on land                        
1.  Charophycean algae inhabit shallow waters
and need to survive when water levels drop.  Lead
to increasing ability to survive entirely on dry
land.
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III.  Bryophytes             A.  Gametophyte is
the dominant generation in the life cycles of
bryophytes            
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Bryophytes
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B.  Life cycle of bryophytes             1. 
Bryophyte sporophytes produce and disperse huge
numbers of spores.
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The life cycle of Polytrichum, a moss
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C.  Ecological and economic benefits of
bryophytes                         1. 
Bryophytes were the worlds only plants for 100
million years.   2.  Peat bogs are made mostly of
moss called sphagnum.  They contain 400 billion
tons of carbon and cut down the amount of
greenhouse gases. Peat is harvested, dried, and
used as a fuel.   3.  Sphagnum is harvested for
use as a soil conditioner and plant packing
material.  
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Sphagnum, or peat moss
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IV.  Origin of vascular plants             -
Pteridophytes ferns               -
Gymnosperms fir trees             - Angiosperms
flowering plants   A.  Vascular plants evolved
additional terrestrial adaptations                
         1.  Xylem and phloem 2.  Dominant
sporophyte generation independent of the
gametophyte ? Different from the bryophytes   B. 
Cooksonia evolved over 400 million years ago à
oldest known vascular plant                       
 
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Cooksonia, a vascular plant of the Silurian
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V.  Pteridophytes  seedless vascular
plants   Examples of pteridophytes (seedless
vascular plants) next page.
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A.  Pteridophytes provide clues to evolution of
roots and leaves                         1. 
There is evidence that roots evolved from
subterranean portions of stems.   2.  There are
two types of leaves                              
       a.  Leaves of lycophytes are
microphylls.  Microphylls are small leaves with a
single, unbranched vein.   b.  Leaves of other
modern vascular plants are megaphylls. 
Megaphylls are typically larger and have a
branched vascular system.
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Hypotheses for the evolution of leaves
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B.  Sporophyte-dominant life cycle evolved in
seedless vascular plants (Pteridophytes)          
               1.  Alternation of
generations   2.  Dominant sporophyte versus
dominant gametophyte in bryophytes.   3.  Plants
are dispersed to new environments as spores no
seeds present                        
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The life cycle of a fern
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Figure 29.24b Fern sporophyll, a leaf
specialized for spore production
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 C.  Importance of Pteridophytes                  
       1.  Dominant plants in Carboniferous
period   2.  Extensive beds of coal from these
plants
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Artists conception of a Carboniferous forest
based on fossil evidence