Overview of Green Plants

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Overview of Green Plants

• Chapter 30

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Defining Plants

• The kingdom Viridiplantae includes land plants
  and green algae
• Red and brown algae are excluded
• All green plants arose from a single species of
  freshwater algae
• The green algae split into two major clades
• Chlorophytes Never made it to land
• Charophytes Did!

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Defining Plants
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Defining Plants

• Land plants have two major features
• 1. Protected embryos
• 2. Multicellular haploid and diploid phases

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Defining Plants

• Adaptations to terrestrial life
• 1. Protection from desiccation by a waxy cuticle
  and stomata
• 2. Evolution of leaves which increase
  photosynthetic surface area
• 3. Shift to a dominant vertical diploid
  generation

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Plant Life Cycles

• Humans have a diplontic life cycle
• Only the diploid stage is multicellular
• Plants have a haplodiplontic life cycle
• Multicellular diploid stage Sporophyte
• Multicellular haploid stage Gametophyte

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Plant Life Cycles

• Sporophyte produces haploid spores by meiosis
• Spores divide mitotically, producing the
  gametophyte
• Gametophyte produces gametes by mitosis
• Gametes fuse to form the diploid sporophyte

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Plant Life Cycles

• As more complex plants evolved
• 1. Diploid stage became the dominant portion of
  the life cycle
• 2. Gametophyte became more limited in size
• 3. Sporophyte became nutritionally independent

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Chlorophytes

• Green algae have two distinct lineages
• Chlorophytes Gave rise to aquatic algae
• Streptophytes Gave rise to land plants
• Chlamydomonas
• Unicellular chlorophyte with two flagella
• Have eyespots to direct swimming
• Reproduces asexually as well as sexually

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Chlorophytes

• Volvox
• -Colonial chlorophyte

-Hollow sphere of a single layer of 500-60,000
cells -A few cells are specialized for
reproduction
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Chlorophytes

• Ulva
• -Multicellular chlorophyte
• -Haplodiplontic life cycle
• -Gametophyte and sporophyte have identical
  appearance

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Charophytes

• Charophytes are green algae related to land plants

• There are two candidate clades

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Charophytes

• Charales (300 species)
• -Macroscopic
• -Plant-like plasmodesmata
• -Sister clade to land plants

Choleocaetales (30 species) -Microscopic -Plant
-like mitosis -Next closest plant relatives
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Bryophytes

• Bryophytes are the closest living descendants of
  the first land plants
• Called nontracheophytes because they lack
  tracheids (specialized transport cells)
• Simple, but highly adapted to diverse terrestrial
  environments
• Non-photosynthetic sporophyte is nutritionally
  dependent on the gametophyte

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Bryophytes

• Liverworts (phylum Hepaticophyta)
• -Have flattened gametophytes with liver-like
  lobes

-Form gametangia in umbrella-shaped
structures -Also undergo asexual reproduction
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Bryophytes

• Hornworts (phylum Anthocerotophyta)
• -Sporophyte has stomata

-Sporophyte is photosynthetic
-Cells have a single large chloroplast
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Bryophytes

• Mosses (phylum Bryophyta)
• Gametophytes consist of small, leaflike
  structures around a stemlike axis
• Anchored to substrate by rhizoids
• Multicellular gametangia form at the tips of
  gametophytes
• Archegonia Female gametangia
• Antheridia Male gametangia
• Mosses withstand drought, but not air pollution

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Features of Tracheophyte Plants

• Cooksonia, the first vascular land plant,
  appeared about 420 MYA

-Only a few centimeters tall -No roots or
leaves -Homosporous
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Features of Tracheophyte Plants

• Vascular tissues are of two types
• Xylem Conducts water and dissolved minerals
  upward from the roots
• Phloem Conducts sucrose and hormones throughout
  the plant
• These enable enhanced height and size in the
  tracheophytes
• Tracheophytes are also characterized by the
  presence of a cuticle and stomata

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Features of Tracheophyte Plants

• Vascular plants have gametophytes reduced in size
  and complexity relative to sporophytes
• Seeds
• Highly-resistant structures that protect the
  plant embryo
• Occur only in heterosporous plants
• Fruits in flowering plants add a layer of
  protection to seeds
• Also attract animals that disperse seeds

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Features of Tracheophyte Plants

• Vascular plants include seven extant phyla
  grouped in three clades
• 1. Lycophytes (club mosses)
• 2. Pterophytes (ferns and their relatives)
• 3. Seed plants

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Lycophytes

• Club mosses are the earliest vascular plants
• -They lack seeds

-Superficially resemble true mosses but they are
not related -Homosporous or heterosporous
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Pterophytes

• The phylogenetic relationships among ferns and
  their relatives is still being sorted out

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Pterophytes

• Whisk ferns
• -Saprophyte consists of evenly forking green
  stems without leaves or roots

-Some gametophytes develop elements of vascular
tissue -Only one known to do so
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Pterophytes

• Horsetails
• -All 15 living species are homosporous
• -Constitute a single species, Equisetum

-Consist of ribbed, jointed photosynthetic stems
that arise from branching rhizomes
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Pterophytes

• Ferns
• -The most abundant group of seedless vascular
  plants with about 11,000 species

-The conspicuous sporophyte and much smaller
gametophyte are both photosynthetic
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Pterophytes

• The fern life cycle differs from that of a moss
• Much greater development, independence and
  dominance of the ferns sporophyte
• Fern morphology
• Sporophytes have rhizomes
• Fronds (leaves) develop at the tip of the rhizome
  as tightly rolled-up coils
• They unroll and expand

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Pterophytes

• Fern reproduction
• -Most fern are homosporous
• -Produce distinctive sporangia in clusters
  called sori on the back of the fronds
• -Diploid spore mother cells in sporangia produce
  haploid spores by meiosis
• -At maturity, the spores are catapulted by
  snapping action

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The Evolution of Seed Plants

• Seed plants first appeared 305-465 MYA
• Evolved from spore-bearing plants known as
  progymnosperms
• The seed represents an important advance

1. Protects the embryo 2. Easily dispersed 3.
Introduces a dormant phase in the life cycle
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The Evolution of Seed Plants

• Seed plants produce 2 kinds of gametophytes
• -Male gametophytes
• -Pollen grains
• -Dispersed by wind or a pollinator
• -Female gametophytes
• -Develop within an ovule
• -Enclosed within diploid sporophyte tissue

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Gymnosperms

• Gymnosperms are plants with naked seeds
• Ovule is exposed on a scale at pollination
• There are four living groups
• Coniferophytes
• Cycadophytes
• Gnetophytes
• Ginkgophytes
• All lack flowers and fruits of angiosperms

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Gymnosperms

• Conifers (phylum Coniferophyta) are the largest
  gymnosperm phylum
• Include
• Pines, spruces, firs, cedars and others
• Coastal redwood Tallest tree
• Bristlecone pine Oldest living tree
• Conifers are sources of important products
• Timber, paper, resin and taxol (anti-cancer)

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Gymnosperms

• Pines
• -More than 100 species, all in the Northern
  hemisphere
• -Produce tough needlelike leaves in clusters
• -Leaves have
• 1. Thick cuticle and recessed stomata
• 2. Canals into which cells secrete resin

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Gymnosperms

• Pine reproduction
• -Male gametophytes (pollen grains) develop from
  microspores in male cones by meiosis
• -Female pine cones form on the upper branches of
  the same tree
• -Female cones are larger, and have woody
  scales
• -Two ovules develop on each scale

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Gymnosperms

• Pine reproduction
• -Each ovule contains a megasporangium called the
  nucellus
• -Surrounded by the integument
• -Opening Micropyle
• -One layer becomes the seed coat
• -While scales of female cone are open, pollen
  grains drift down between them
• -Are drawn to top of nucellus

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Gymnosperms

• Pine reproduction
• -While female gametophyte is developing, a
  pollen tube emerges from the pollen grain
• -It digests its way to the archegonium
• -Fifteen months after pollination, pollen tube
  reaches archegonium and delivers its sperm

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Gymnosperms

• Cycads (phylum Cycadophyta)
• -Slow-growing gymnosperms of tropical and
  subtropical regions

-Sporophytes resemble palm trees -Have
largest sperm cells of all organisms!
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Gymnosperms

• Gnetophytes (phylum Gnetophyta)
• -Only gymnosperms with vessels in their xylem

-Contain three (unusual) genera -Welwitschia
-Ephedra -Gnetum
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Gymnosperms

• Ginkgophytes (phylum Ginkgophyta)
• -Only one living species remains
• -Ginkgo biloba

-Dioecious -Male and female reproductive
structures form on different trees
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Angiosperms

• Angiosperms are the flowering plants
• -Ovules are enclosed in diploid tissue at the
  time of pollination

-The carpel, a modified leaf that covers seeds,
develops into fruit
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Angiosperms

• Angiosperm origins are a mystery
• -The oldest known angiosperm in the fossil
  record is Archaefructus
• -The closest living relative to the original
  angiosperm is Amborella

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Angiosperms
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Angiosperms
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Angiosperms

• Flower morphology
• -Primordium develops into a bud at the end of a
  stalk called the pedicel
• -Pedicel expands at the tip to form a
  receptacle, to which other parts attach
• -Flower parts are organized in circles called
  whorls

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Angiosperms

• Flower morphology
• -Outermost whorl Sepals
• -Second whorl Petals
• -Third whorl Stamens (androecium)
• -Each stamen has a pollen-bearing anther and
  a filament (stalk)
• -Innermost whorl Gynoecium
• -Consists of one or more carpels that house
  the female gametophyte

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Angiosperms
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Angiosperms

• Carpel structure
• -Three major regions
• -Ovary Swollen base containing ovules
• -Later develops into a fruit
• -Stigma Tip
• -Style Neck or stalk

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Angiosperm Life Cycle

• The female gametophyte (embryo sac) has 8 haploid
  nuclei arranged in two groups of four
• A nucleus from each group migrate toward the
  ovules center and become polar nuclei
• Cell walls form round remaining three nuclei
• At the micropyle end, one cell functions as the
  egg, and the other two are synergids
• At the other end, three cells are antipodals
• They eventually break down

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Angiosperm Life Cycle

• Pollen production occurs in the anthers
• -It is similar but less complex than female
  gametophyte formation
• -Diploid microspore mother cells undergo meiosis
  to produce four haploid microspores
• -Binucleate microspores become pollen grains

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Angiosperm Life Cycle

• Pollination is the mechanical transfer of pollen
  from anther to stigma
• -Pollen grains develop a pollen tube that is
  guided to the embryo sac
• -One of the two pollen grain cells lags behind
• -This generative cell divides to produce two
  sperm cells

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Angiosperm Life Cycle

• As the pollen tube enters the embryo sac, a
  double fertilization occurs
• One sperm unites with egg to form the diploid
  zygote
• Other sperm unites with the two polar nuclei to
  form the triploid endosperm
• Provides nutrients to embryo
• When the seed germinates, a young sporophyte
  plant emerges

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Angiosperm Life Cycle

• Angiosperms include
• Eudicots (about 175,000 species)
• Trees, shrubs, snapdragons, peas, other
• Use flowers to attract insect pollinators
• Monocots (about 65,000 species)
• Grasses, lilies, palms, irises, others
• Some rely on wind for pollination
• Note Self-pollination may also occur