PLANTS

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PLANTS
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40 MILLION YEARS OLD
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LAND COLONIZATION

• Earths early atmosphere lacked O2
• Oxygenic photosynthesis evolved 2.5 billion years
  ago
• O2 accumulated in the atmosphere
• Ozone (O3) formed
• Increasing O2 levels precipitated evolutionary
  change
• Mechanisms to tolerate O2 arose
• Mechanisms to use O2 evolved
• Ozone protected live from ultraviolet light
• Life previously remained primarily beneath the
  surface of water and mud
• This protection enabled organisms to colonize land

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LAND COLONIZATION

• Cyanobacteria were likely the first living cells
  to colonize the land
• Green algae and fungi together colonized the land
  later
• Green algae were ancestral to all plants

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PLANT FEATURES

• Multicellular eukaryotic photoautotrophs
• Cellulose-containing cell walls
• Store surplus carbohydrates as starch
• Some algae also fit the above description
• Chloroplasts contain chlorophylls a b
• All photosynthetic eukaryotes possess chlorophyll
  a
• Green algae also possess chlorophyll b
• How do we distinguish plants from multicellular
  green algae?

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PLANT FEATURES

• Plants are mainly terrestrial organisms
• Some have returned to the water
• Required resources are found in different places
• Light and O2 are found in the air
• Water and mineral nutrients are found in the soil
• Plants display structural specialization
• Subterranean organs (e.g., roots)
• Aerial organs (e.g., leaf-bearing shoots)
• Gas exchange typically regulated by stomata
• Water loss typically reduced by cuticle

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PLANT FEATURES

• Life cycles of all plants feature an alternation
  of generations
• This feature is not unique to plants
• Some algae also display an alternation of
  generations
• After fertilization of a female gamete, the
  zygote develops into an embryo which is retained
  and nourished by the parent
• This feature is lacking in algae
• Plants are termed embryophytes

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LIFE CYCLES

• The human life cycle features both diploid and
  haploid cells
• Multicellular individuals are formed primarily of
  diploid cells
• Gametes are transient unicellular haploid cells
• Plant life cycles are more complex

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PLANT LIFE CYCLES

• All plant life cycles feature an alternation of
  generations
• Sporophyte
• Multicellular diploid individual
• Produces haploid spores
• Gametophyte
• Multicellular haploid individual
• Produces haploid gametes

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PLANT ANCESTRY

• Plants represent a monophyletic group
• Evolved from a common ancestor
• Who was this common ancestor?
• Multiple lines of evidence indicate that plants
  evolved from a group of green algae termed
  Charophytes
• What is this evidence?

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PLANT ANCESTRY

• Homologous chloroplasts
• Only green algae possess the accessory pigments
  chlorophyll b and beta-carotene present in plants
• Biochemical similarity
• Of all green algae, charophytes possess a cell
  wall cellulose composition most similar to that
  of plants
• Charophytes are the only green algae possessing
  peroxisomal enzymes matching those of plants

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PLANT ANCESTRY

• Similarities in cell division
• Timing and degree of nuclear membrane dispersal
  (late prophase, completely) is similar between
  charophytes and plants
• In plants and in some charophytes, cytokinesis
  involves the cooperation of microtubules, actin,
  and vesicles in the formation of a cell plate
• Similarities in sperm ultrastructure
• Charophytes are more similar to plants than they
  are to other green algae in this regard

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PLANT ANCESTRY

• Genetic relationships
• Molecular analysis of highly conserved genes
  show charophytes to be the green algae most
  closely related to plants

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MAJOR PLANT GROUPS

• Four major periods of plant evolution
• New structures evolved, adaptive radiations
  followed
• Origin of plants from aquatic ancestors
• Diversification of vascular plants
• Origin of seeds
• Emergence of flowering plants

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PLANT EVOLUTION

• Plant ancestry can be viewed in terms of a nested
  set of monophyletic groups

flowering plants
green algae
gnetophytes
zygophytes, related groups
charophytes
lycophytes
horsetails
ginkgos
bryophytes
cycads
conifers
ferns
seed plants
euphyllophytes
vascular plants
embryophytes (land plants)
(closely related groups)
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OVERVIEW

• Nonvascular plants
• Lack tissue systems to conduct water
• Vascular plants
• Possess tissue systems to conduct water
• Seedless vascular plants
• Gymnosperms
• Seed-bearing vascular plants
• Angiosperms
• Flower- and seed-bearing vascular plants

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BRYOPHYTES

• Nonvascular plants
• lt19,000 species
• Previously grouped together in a single division
• Bryophyta
• Now separated into three divisions
• Bryophyta (mosses)
• Hepatophyta (liverworts)
• Anthocerophyta (hornworts)
• Still, the term bryophyte encompasses all three
  of these divisions of nonvascular plants

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Liverwort
Liverwort
Hornwort
Moss
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BRYOPHYTES

• Possess the embryophyte condition
• Gametes develop within gametangia
• Antheridium (male) and archegonium (female)
• Egg is fertilized within the archegonium
• Flagellated sperm must swim through film of dew
  or rainwater from antheridium to archegonium
• Zygote develops into an embryo within this
  structure

Archegonium
Antheridium
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BRYOPHYTES

• Most bryophytes have no vascular tissue
• Water on surface of plant must be imbibed like a
  sponge
• Distribution through plant is through diffusion,
  capillary action, and cytoplasmic streaming
• Slow processes
• Damp shady places are most common habitats for
  bryophytes
• Bryophytes lack lignin-fortified support tissues
• Low profile, sprawl horizontally

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BRYOPHYTES

• The gametophyte is the dominant generation in the
  life cycles of bryophytes

BRYOPHYTE
SPOROPHYTE (2n)
GAMETOPHYTE (n)
GR ALGA
FERN
GYMNOSPERM
ANGIOSPERM
BRYOPHYTE
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BRYOPHYTES
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BRYOPHYTES
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BRYOPHYTES
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MOSSES

• Division Bryophyta
• Many plants growing tightly together form a mat
• Help to hold one another up
• Spongy quality helps to retain water
• Substrate gripped with rhizoids
• Root-like elongated cells or filaments
• Analogous to roots of vascular plants
• Most photosynthesis occurs in upper part
• Stem-like and leaf-like appendages
• Analogous to these structures in vascular plants

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MOSSES

• Peat mosses (Sphagnum) cover gt3 or Earths
  terrestrial surface
• Probably the most abundant plants on Earth
• Greater density in northern latitudes
• Accumulated peat (living and dead) is major
  reservoir of organic carbon
• Resistant to microbial degradation
• Stabilizes atmospheric CO2

Moss sporangium
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Gametophytes
Sporophytes
Archegonium
Sporangium
Protonemata
Spores
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PEAT MOSS BOG
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PEAT MOSS BOG
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LIVERWORTS

• Less conspicuous than mosses
• Divided into lobes
• Resembles lobed animal liver
• Life cycle similar to that of moss
• Some sporangia contain coiled cells
• Spring out of opened capsule
• Aid in dispersal of spores
• Also reproduce asexually from small bundles of
  cells called gemmae
• Dispersed by raindrops

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LIVERWORT GEMMAE
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HORNWORTS

• Resemble liverworts
• Sporophytes are elongated capsules resembling
  horns
• Hornworts are the bryophytes most closely related
  to vascular plants

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ADAPTATIONS

• Bryophyte adaptations
• Gametangia
• Embryos
• Sporopollenin-walled spores
• Stomata
• Present in some
• Cuticles
• Present in some
• May or may not be homologous to cuticles in
  higher plants

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VASCULAR PLANTS

• Adaptations of vascular plants
• Differentiated bodies
• Subterranean root systems (water minerals)
• Aerial stems and leaves (photosynthesis)
• Vascular tissue
• Xylem (water minerals)
• Phloem (organic nutrients)
• Lignin
• Cell wall component providing mechanical support

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SEEDLESS VASCULAR PLANTS

• Key changes in early vascular plants
• Sporophyte generation is dominant
• Sporophyte is branched
• More sporangia ? more spores
• Raw material for evolution of more complex body
  parts
• e.g., Branches webs ? leaves

Cooksonia
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SEEDLESS VASCULAR PLANTS

• Dominated forest landscapes of Carboniferous
  period
• 360 - 290 million years ago
• Three living divisions
• Lycophytes
• Horsetails
• Ferns

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SEEDLESS VASCULAR PLANTS
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LYCOPHYTES

• Division Lycophata
• Evolved in Devonian period
• Prevalent in Carboniferous period
• Woody tree lineage
• Became extinct near end of Carboniferous period
• Herbaceous lineage
• Represented today by 1,000 species
• e.g., Club mosses ground pines
• (Which, incidentally, are neither mosses nor
  pines)
• Many species are epiphytes
• Use another species as substrate (not a parasite)
• Many species grow on temperate forest floors

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LYCOPHYTES
Club Moss
Ground Pine
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LYCOPHYTES

• Dominant sporophyte generation
• True of all vascular plants
• Sporangia borne on sporophylls
• Specialized leaves
• Discharged spores develop into inconspicuous
  gametophytes
• Nonphotosynthetic, nurtured by symbiotic fungi
• May live underground for ten years or more

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LYCOPHYTES

• Some species are homosporous
• Single type of spore
• Gametophyte with archegonia and antheridia
• Gametophyte produces both sperm and eggs
• Some species are heterosporous
• Megaspore ? female gametophyte ? eggs
• Microspore ? male gametophyte ? sperm

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HORSETAILS

• Division Sphenophyta
• Sometimes considered part of Division Pterophyta
• Ancient lineage of seedless vascular plants
• Dates back to Devonian period
• Prevalent during Carboniferous
• Modern survivors include 15 species in the genus
  Equisetum
• Most common in Northern hemisphere
• Generally found in damp locations, streambanks

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HORSETAILS

• Dominant sporophyte generation
• True of all vascular plants
• Homosporous
• Sporangia within cone-like structures
• Discharged spores develop into inconspicuous
  gametophytes
• Only a few millimeters long
• Photosynthetic, free-living
• Outer cell layer silica-embedded
• Abrasive, used before scouring pads

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WHISK FERNS

• Division Pterophyta
• Formerly Division Psilophyta
• Molecular analysis has indicated its close
  relatedness with ferns
• Simple body structure evolved secondarily
• Ancestors were more complex
• Lacks roots present in ancestor
• Possess subterranean rhizomes
• Small outgrowths of stems are likely reduced
  leaves

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FERNS

• Division Pterophyta
• Ancient ancestry
• Origins in Devonian period
• Prevalent in Carboniferous period
• Currently most prevalent seedless vascular plant
• gt12,000 species exist today
• Most diverse in tropics

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FERNS

• Leaves generally larger than those of lycophytes
• Evolved differently
• Multiple (not single) strands of vascular tissue

Lycophytes
Ferns
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FERNS

• Leaves (called fronds) are compound
• Divided into multiple leaflets
• Frond grows as tip (fiddlehead) unfurls
• Some leaves are specialized sporophylls
• Sporangia on underside
• Sometimes arranged in clusters termed sori
• Spring-like devices launch spores several meters
• Wind can disseminate widely

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SEEDLESS VASCULAR PLANTS
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Sori
Sporophyte
Sporangium / Spores
Sorus / Sporangia
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Spore
Sporangium / Spores
Germinating Spore
Sporophyte
Sporophyte
Gametophytes
Gametophyte
Archegonia
Sporophytes
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COAL FORESTS

• Seedless vascular plants were widespread during
  the Carboniferous period
• 360 290 million years ago
• Formed the fossil fuel coal
• Less extensive coal beds were also formed during
  other geological periods
• Most continents were flooded by shallow swamps
• Dead plants did not completely decay
• Peat formed
• Heat and pressure converted peat to coal

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SEED PLANTS

• Swamps began to dry up at the end of the
  Carboniferous period
• 290 million years ago
• Pangea supercontinent formation ? hotter and
  dryer continental interiors
• Flora and fauna changed dramatically
• Punctuated equilibrium
• Seed plants had already existed, but rose to
  prominence after this environmental change
• Ditto for reptiles, etc.

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SEED PLANTS

• Key adaptations of seed plants
• Reduction of the gametophyte
• Minute gametophytes retained within and protected
  by the sporophyte
• Advent of the seed
• Seeds replaced spores as a means of dispersing
  offspring
• Evolution of pollen
• Eliminated the liquid H2O fertilization
  requirement

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SEED PLANTS
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SEED PLANTS

• Reduction of the gametophyte
• Gametophyte generation becomes even more reduced
  in seed plants
• Gametophytes of bryophytes are dominant
• Gametophytes of seedless vascular plants develop
  as an independent generation
• Minute gametophytes of seed plants are retained
  within sporophyte tissue
• Protected from desiccation
• Reversal of the gametophyte-sporophyte
  relationship found in bryophytes

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SEED PLANTS

• Reduction of the gametophyte
• Shift toward diploidy may be response to damaging
  effects of ultraviolet radiation
• Mutagenic
• Light-filtering properties of water afford some
  protection to aquatic organisms
• Diploid organisms can tolerate some mutations
  that would be lethal to haploid individuals
• Sporophyte-dominance is an adaptation to
  terrestrial conditions

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SEED PLANTS

• Reduction of the gametophyte
• Why is the gametophyte reduced, not removed?
• Perhaps this haploid generation provides a means
  of screening and eliminating deleterious alleles
• Gametophyte tissue remains important in
  nourishing the embryonic sporophyte

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SEED PLANTS

• Advent of the Seed
• Spores are the resistant stage in the life cycle
  of bryophytes and seedless vascular plants
• Able to withstand harsh environments
• Seeds are also able to resist harsh environments
• Spores are the means by which bryophytes and
  seedless vascular plants disperse offspring
• Able to be dispersed over great distances
• Seeds became important in dispersing offspring

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SEED PLANTS

• Advent of the Seed
• A seed consists of a protective coat housing an
  embryonic sporophyte and its food supply
• The reduced gametophytes of seed plants develop
  within tissues of the parental sporophyte

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SEED PLANTS

• Advent of the Seed
• All seed plants are heterosporous
• Megasporangia ? megaspores ? female gametophytes
  ? eggs
• Microsporangia ? Microspores ? male gametophytes
  ? sperm

Ginkgo sperm
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SEED PLANTS

• Advent of the Seed
• Megasporangium is not simply a chamber
• Solid, fleshy structure
• Nucellus
• Additional layers of sporophyte tissue envelop
  the megasporangium
• Integuments
• Provides protection to the megaspore
• Integuments nucellus megaspore ovule

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SEED PLANTS

• Advent of the Seed
• Female gametophyte contains an egg cell
• Egg sperm ? zygote ? sporophyte embryo
• Entire ovule develops into a seed
• Resistant, dispersible, can remain dormant for
  years
• Can germinate under favorable conditions

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SEED PLANTS

• Evolution of Pollen
• Microspores develop into pollen grains
• Mature to form male gametophytes
• Protected by tough sporopollenin-containing coats
• Released from microsporangium
• Dispersed by wind or animals
• Some pollen grains lands near an ovule
• Pollen tube is produced
• Sperm discharged into female gametophyte
• Sperm lack flagella in conifers, angiosperms, etc.

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SEED PLANT CLADES

• Gymnosperms
• Monophyletic group of flower-less seed plants
• Angiosperms
• Monophyletic group of flowering seed plants
• Gymnosperms and angiosperms appear to have
  evolved from separate ancestors in the extinct
  progymnosperm group

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GYMNOSPERMS

• Probably descended from progymnosperms
• Seeds had evolved by end of Devonian period
• Adaptive radiation in Carboniferous and early
  Permian produced the gymnosperm divisions
• Largely replaced seedless vascular plants
• Better adapted to drier (Pangean) climate

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AGE OF DINOSAURS

• Gymnosperms dominated the Mesozoic era
• Terrestrial were supported by gymnosperms
• Mainly conifers and great palm-like cycads
• The Mesozoic era ends and the Cenozoic era begins
  65 million years ago
• Powerful meteorite impact
• Climate cooled, mass extinctions ensued
• Extinctions of many gymnosperms, some remained
• Extinctions of many animal species
• Most dinosaurs became extinct
• Which ones survived?

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GYMNOSPERMS

• Four divisions currently exist
• Ginkgophyta (ginkgo)
• Cycadophyta (cycads)
• Gnetophyta (gnetophytes)
• Coniferophyta (conifers)

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DIVISION GINKGOPHYTA

• Ginkgo biloba is the only living species
• Fanlike leaves turn gold and are shed in autumn

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DIVISION GINKGOPHYTA

• Ability to tolerate pollution makes the ginkgo a
  popular ornamental tree in cities
• Female trees produce fleshy seeds
• Seed coat emits repulsive odor
• Generally only male trees are planted

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DIVISION GINKGOPHYTA
Ginkgo female
Ginkgo male
Ginkgo sperm
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DIVISION CYCADOPHYTA

• Superficially resemble palms
• True palms are angiosperms
• Seeds develop on the surface of sporophylls
• Specialized reproductive leaves
• Packed together to form cones

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DIVISION GNETOPHYTA

• Consists of three genera very different in
  appearance
• Welwitschia
• Giant strap-like leaves
• Gnetum
• Grow in tropics as trees or vines
• Ephedra
• Mormon tea
• Shrub of American deserts

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DIVISION GNETOPHYTA

• Consists of three genera very different in
  appearance
• Welwitschia
• Giant strap-like leaves
• Gnetum
• Grow in tropics as trees or vines
• Ephedra
• Mormon tea
• Shrub of American deserts

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DIVISION GNETOPHYTA

• Consists of three genera very different in
  appearance
• Welwitschia
• Giant strap-like leaves
• Gnetum
• Grow in tropics as trees or vines
• Ephedra
• Mormon tea
• Shrub of American deserts

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DIVISION CONIFEROPHYTA

• Conifers
• Largest of the four gymnosperm genera
• 550 species
• e.g., Pines, firs, spruces, larches, yews,
  junipers, cedars, cypresses, redwoods, etc.
• A few of these species dominate vast forested
  regions of the Northern Hemisphere
• Reproductive structures are cones
• Cluster of scale-like sporophylls

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DIVISION CONIFEROPHYTA
Pine male pollen cones
Pine female ovulate cones
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DIVISION CONIFEROPHYTA

• Most conifers are evergreens
• Retain leaves throughout the year
• Some conifers have deciduous leaves
• Shed in autumn
• e.g., Dawn redwood, tamarack

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DIVISION CONIFEROPHYTA

• Coniferous trees are some of the Earths largest
  organisms

Sequoias
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DIVISION CONIFEROPHYTA

• Coniferous trees are some of the Earths oldest
  organisms
• Methuselah, a bristlecone pine, is over 4,600
  years old

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PINE LIFE CYCLE
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PINE LIFE CYCLE
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PINE LIFE CYCLE
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PINE SEED
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WATER TRANSPORT

• Tracheids conduct water in conifers
• Elongated, tapered cell
• Functions in support and water movement
• Relatively early type of xylem cell
• Angiosperms possess more specialized xylem
  elements

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ANGIOSPERMS

• Flowering plants
• Most diverse and widespread plants
• 300,000 known species of plants currently exist
• 250,000 are angiosperms
• All belong to Division Anthophyta

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ANGIOSPERMS

• Division Anthophyta
• Contains five major monophyletic lineages
• Amborella
• Water lilies
• Other early lineages
• Monocots
• Eudicots

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Amborella

• Oldest angiosperm lineage
• Represented by a single species
• Amborella trichopoda
• Possesses xylem similar to that of gymnosperms
• Tracheids
• Lacks the vessel elements of other angiosperms

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WATER TRANSPORT

• Vessel elements conduct water in angiosperms
• Shorter, wider cells
• Evolved from tracheids
• Arranged end-to-end to form continuous tubes
• More specialized for transporting water
• Less specialized for support
• Xylem reinforced by xylem fibers
• Also present in conifers
• Evolved from tracheids
• Thick, lignified cell walls
• Specialized for support

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WATER LILIES

• e.g., Nymphaea carulea
• Evolved on land
• Secondarily returned to the water

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OTHER EARLY LINEAGES

• e.g., Illicium floridanum, the star anise

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MONOCOTS

• e.g., Paphiopedilum furheyo, an orchid

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EUDICOTS

• e.g., Eschscholzia californica, the California
  poppy

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FLOWERS

• Defining reproductive adaptation of angiosperms
• Most produce both male and female gametophytes
• Perfect flowers
• Pollen is transferred from one flower to the
  female sex organs of the same or another flower
• Wind may facilitate this transfer
• Insects and other animals often facilitate the
  transfer
• Is this always sexual reproduction?
• Why might cross-pollination be beneficial?

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FLOWERS

• Compressed shoot with four whorls of modified
  leaves
• Sepals
• Petals
• Stamens
• Carpels

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FLOWER STRUCTURE

• Sterile Floral Parts
• Sepals
• Generally green
• Enclose flower before it opens
• Petals
• Generally brightly colored
• Aid in attracting insects and other pollinators
• (Petals of wind-pollinated flowers are typically
  drab)

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FLOWER STRUCTURE

• Reproductive Organs
• Stamens
• Male
• Stalk (filament) and terminal sac (anther)
• Carpels
• Female
• Stigma

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FLOWER STRUCTURE

• Reproductive Organs Stamens
• Male flower parts
• Pollen is produced within anthers, terminal
  sacs attached to stalk-like filaments

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FLOWER STRUCTURE

• Reproductive Organs Carpels
• a.k.a., Pistil
• Female flower parts
• One or many per flower
• Sticky stigma receives pollen
• Style leads to ovary, which houses ovules

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FLOWER ORIGINS

• Carpels likely resulted from the rolling of
  sporophylls
• Megasporangia-containing reproductive leaves

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FRUITS

• Aid in the protection and dispersal of angiosperm
  seeds
• Mature ovary
• Ovary wall thickens to form pericarp
• Thickened wall of fruit
• Growth triggered by pollination
• Fruit surrounds seed(s)

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FRUITS

• Various modifications in fruit assist in seed
  dispersal
• Some fruits function as kites or propellers, and
  are dispersed by the wind
• e.g., maple, dandelion
• Many fruits promote dispersal by animals
• e.g., cockleburs
• e.g., edible fruits

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TYPES OF FRUITS

• Simple fruit
• Derived from a single ovary
• Aggregate fruit
• Results from a single flower with multiple
  carpels
• Multiple ovaries fuse
• Multiple fruit
• Develops from an inflorescence, a tightly packed
  group of flowers
• Multiple ovaries fuse

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ANGIOSPERM LIFE CYCLE

• Heterosporous
• Microspores ? male gametophyte
• Megaspores ? female gametophyte
• Immature male gametophytes are contained within
  pollen grains
• Develop within anthers of a stamen
• Each pollen grain has two haploid cells
• Ovules contain female gametophyte
• Embryo sac
• Consists of a few cells, including one egg

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POLLINATION

• Pollen is released from the anther
• Carried to the stigma at tip of carpel
• Self-pollination may occur
• Many plants have mechanisms making
  cross-pollination more likely
• e.g., Male and female parts mature at different
  times
• e.g., Male and female parts physically distant
• e.g., Self-incompatibility
• What is the benefit of cross-pollination?

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POLLINATION

• Pollen grain germinates
• Occurs after adhering to the carpel
• (Now contains mature gametophyte)
• Pollen tube extended down style to ovary
• Pollen tube penetrates ovule
• Two sperm discharged
• Double fertilization

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FERTILIZATION

• Double fertilization
• Sperm egg ? diploid zygote
• Mitotic division produces embryo
• Rudimentary root
• One (monocots) or two (dicots) seed leaves
• Sperm 2 nuclei ? 3n nucleus
• Mitotic division produces energy-rich endosperm
• Ovule matures into a seed

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FERTILIZATION

• Why double fertilization?
• Synchronizes development of food storage with
  seed development
• Without fertilization, neither will occur
• Resources are not wasted on infertile ovules

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SEED

• Consists of
• Embryo
• Endosperm
• Sporangium
• Seed coat
• Surrounded by fruit
• Developed from ovary

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ANGIOSPERM LIFE CYCLE
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ANGIOSPERM LIFE CYCLE
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ANGIOSPERM RADIATION

• Radiation of angiosperms marks the transition
  from the Mesozoic era to the Cenozoic era
• Earliest angiosperms found are 130 million years
  old
• Adaptive radiation made angiosperms the dominant
  plants on Earth by the end of the Cretaceous 65
  million years ago
• Adaptive radiation followed a period of
  environmental disturbance

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COEVOLUTION

• Plants have influenced the evolution of animals
• Animals have influenced the evolution of plants
• e.g., Plant-herbivore coevolution
• e.g., Plant-pollinator coevolution

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COEVOLUTION

• Plant-pollinator coevolution is responsible for
  the diversity of flowers
• What does the pollinator gain?
• Nectar, pollen, etc.
• What does the plant gain?
• Cross-pollination

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MAJOR PLANT GROUPS
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PLANT REPRODUCTION

• EVEN MORE ON

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FLOWERS

• Compressed shoot with four whorls of modified
  leaves
• Sepals
• Petals
• Stamens
• Carpels

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TYPES OF FLOWERS

• Many flowers possess all four basic floral organs
• Complete flowers
• In some flowers, one or more of these basic
  organs are absent
• Incomplete flowers
• e.g., Grasses possess flowers lacking petals

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TYPES OF FLOWERS

• Many flowers possess both stamen and carpels
• Perfect flowers
• Bisexual flowers
• Some flowers lack either pistils or stamen
• Imperfect flowers
• Unisexual flowers
• Staminate of carpellate

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TYPES OF FLOWERS

• Some species possess both staminate and
  carpellate flowers on the same plant
• Monoecious
• e.g., Maize
• Some species possess staminate and carpellate
  flowers on different plants
• Dioecious
• e.g., Sagittaria

Carpellate
Staminate
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TYPES OF FLOWERS

• Some species possess both staminate and
  carpellate flowers on the same plant
• Monoecious
• e.g., Maize
• Some species possess staminate and carpellate
  flowers on different plants
• Dioecious
• e.g., Sagittaria, date palms, etc.

Staminate
Carpellate
122
POLLEN GRAIN

• Numerous microsporocytes exist within the
  sporangia of anthers
• Microsporocytes undergo meiosis
• Produce four microspores
• Each microspore divides once by mitosis
• Produce generative cell and tube cell
• These cells are encased in a cell wall
• Thick, resistant
• Sculpted into an elaborate pattern
• Unique to each plant species

123
POLLEN GRAIN

• Generative cell
• Eventually produces sperm
• Tube cell
• Encloses the generative cell
• Will ultimately produce the pollen tube
• Pollen grain
• Cell wall tube cell generative cell
• Immature male gametophyte
• Becomes a mature male gametophyte when the
  generative cell divides to form sperm

124
POLLINATION

• The transmission of pollen from an anther to the
  stigma of a carpel
• This transmission may be mediated by wind or by
  animal pollinators
• This transmission may occur within a single
  flower or between flowers

125
FERTILIZATION

• Some flowers normally self-fertilize
• Selfing
• e.g., Mendels peas
• Self-fertilization does not generate as much
  genetic diversity as cross-fertilization

126
FERTILIZATION

• The majority of angiosperms possess mechanisms
  reducing the likelihood of self-fertilization
• Stamens and carpels of some bisexual flowers
  mature at different times
• Physical arrangement of carpels and stamens may
  reduce likelihood of transmission within a
  flower
• Self-incompatibility

127
SELF-INCOMPATIBILITY

• The ability of a plant to reject its own pollen
  and that of closely related individuals
• Biochemical block prevents pollen from completing
  its development
• e.g., Suppresses pollen tube formation
• Analogous to animal immune systems
• Discriminates between self and non-self

128
MAIZE

• Most current crop plants were domesticated
  approximately 10,000 years ago
• Artificial selection gave rise to rapid changes
• e.g., Teosinte ? maize

129
BIOTECHNOLOGY

• Biotechnology is rapidly transforming agriculture
• Genetic modification of food plants to
  incorporate desirable phenotypes
• e.g., Insect- or virus-resistant plants
• e.g., Increased nutritional value

Virus-resistant papaya