Transcript Document

CHAPTER 30
LECTURE
SLIDES
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Overview of Green Plants
Chapter 30
Defining Plants
• All green algae and the land plants shared
a common ancestor a little over 1 BYA
– Kingdom Viridiplantae
– Not all photoautotrophs are plants
• Red and brown algae excluded
• A single species of freshwater green algae
gave rise to the entire terrestrial plant
lineage
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• The green algae split into two major
clades
– Chlorophytes – Never made it to land
– Charophytes – Did – sister to all land plants
• Land plants…
– Have multicellular haploid and diploid stages
– Trend toward more diploid embryo protection
– Trend toward smaller haploid stage
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Green plants
Streptophyta
Land plants
Bryophytes
Tracheophytes
Euphyllophytes
Red Algae
Green algae
Green algae
Chlorophytes
Charophytes
Seed plants
Liverworts
Mosses
Hornworts
Lycophytes
Ferns + Allies
Gymnosperms
Angiosperms
Ancestral alga
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• Adaptations to terrestrial life
– Protection from desiccation
• Waxy cuticle and stomata
– Moving water using tracheids
• Tracheophytes have tracheids
– Xylem and phloem to conduct water and food
– Dealing with UV radiation caused mutations
• Shift to a dominant diploid generation
– Haplodiplontic life cycle
• Mulitcellular haploid and diploid life stages
• Humans are diplontic
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Haplodiplontic Life Cycle
• Multicellular diploid stage – sporophyte
– Produces haploid spores by meiosis
– Diploid spore mother cells (sporocytes) undergo
meiosis in sporangia
• Produce 4 haploid spores
• First cells of gametophyte generation
• Multicellular haploid stage – gametophyte
– Spores divide by mitosis
– Produces gametes by mitosis
– Gametes fuse to form diploid zygote
• First cell of next sporophyte generation
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• All land plants are haplodiplontic
• Relative sizes of generations vary
• Moss
– Large gametophyte
– Small, dependent sporophyte
• Angiosperm
– Small, dependent gametophyte
– Large sporophyte
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Green algae
Liverworts
Charophytes
– Chlorophytes – Gave rise to
aquatic algae
– Streptophytes – Gave rise
to land plants
• Modern chlorophytes closely
resemble land plants
Chlorophytes
• Green algae have two
distinct lineages
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– Chloroplasts are biochemically
similar to those of the plants
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Chlorophytes
• Early green algae probably resembled
Chlamydomonas reinhardtiii
– Individuals are microscopic
– 2 anterior flagella
– Most individuals are haploid
– Reproduces asexually and sexually
– Not haplodiplontic
• Always unicellular
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• Volvox
– Colonial chlorophyte
– Hollow sphere of a
single layer of 500–
60,000 cells
– Individual cells each
have 2 flagella
– Few cells are
specialized for
reproduction
• Asexual or sexual
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• Ulva
– Multicellular
chlorophyte
– Haplodiplontic life cycle
• Gametophyte and
sporophyte have
identical appearance
• No ancestral
chlorophytes gave rise
to land plants
© Dr. Diane S. Littler
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Charophytes
• Clade of
streptophytes
• Also green algae
• Distinguished
from chlorophytes
by close
phylogenetic
relationship to
land plants
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• Charophytes have haplontic life cycles
– Evolution of diplontic embryo and haplodiplontic life
cycle occurred after move to land
• 2 candidate Charophyta clades
– Charales
– Coleochaetales
• Both charophyte clades form green mats around
the edges of freshwater ponds and marshes
• One species must have successfully inched its
way onto land through adaptations to drying
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Bryophytes
• Closest living descendants of the first land
plants
• Called nontracheophytes because they
lack tracheids
– Do have other conducting cells
• Mycorrhizal associations important in
enhancing water uptake
– Symbiotic relationship between fungi and
plants
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• Simple, but highly adapted to diverse terrestrial
environments
• 24,700 species in 3 clades
– Liverworts
– Mosses
– Hornworts
• Gametophyte – conspicuous and photosynthetic
– Sporophytes – small and dependent
• Require water for sexual reproduction
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Liverworts (phylum Hepaticophyta)
• Have flattened
gametophytes with
liverlike lobes
– 80% look like mosses
• Form gametangia in
umbrella-shaped
structures
• Also undergo asexual
reproduction
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Mosses (phylum Bryophyta)
• Gametophytes consist of small, leaflike
structures around a stemlike axis
– Not true leaves – no vascular tissue
• Anchored to substrate by rhizoids
• Multicellular gametangia form at the tips of
gametophytes
– Archegonia – Female gametangia
– Antheridia – Male gametangia
• Flagellated sperm must swim in water
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Hornworts (phylum Anthocerotophyta)
•
•
•
•
Origin is puzzling – no fossils until Cretaceous
Sporophyte is photosynthetic
Sporophyte embedded in gametophyte tissue
Cells have a single large chloroplast
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Tracheophyte Plants
• Cooksonia, the first vascular
land plant
– Appeared about 420 MYA
– Phylum Rhyniophyta
• Only a few centimeters tall
– No roots or leaves
– Homosporous – only 1 type of
spore
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Vascular tissues
• Xylem
– Conducts water and dissolved minerals upward from
the roots
• Phloem
– Conducts sucrose and hormones throughout the plant
• Enable enhanced height and size in the
tracheophytes
• Develops in sporophyte but not gametophyte
• Cuticle and stomata also found in land plants
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Tracheophytes
• Vascular plants include seven extant phyla
grouped in three clades
1. Lycophytes (club mosses)
2. Pterophytes (ferns, whisk ferns, and horsetails)
3. Seed plants
• Gametophyte has been reduced in size relative
to the sporophyte during the evolution of
tracheophytes
• Similar reduction in multicellular gametangia has
occurred as well
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• Stems
– Early fossils reveal stems but no roots or leaves
– Lack of roots limited early tracheophytes
• Roots
– Provide transport and support
– Lycophytes diverged before true roots appeared
• Leaves
– Increase surface area for photosynthesis
– Evolved twice
• Euphylls (true leaves) found in ferns and seed plants
• Lycophylls found in seed plants
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Lycophyll Origins
Stem with
vascular tissue
Stem, leafy tissue
without vascular tissue
Stem, leafy tissue
with vascular tissue
Single
vascular strand
(vein)
Euphyll Origins
Branching stems
with vascular tissue
Unequal
branching
Branches in Photosynthetic tissue
single planes
“webs” branches
Branched
vascular strands
(veins)
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• 400 million years between appearance of
vascular tissue and true leaves
– Natural selection favored plants with higher stomatal
densities in low-CO2 atmosphere
– Higher stomatal densities favored larger leaves with a
photosynthetic advantage that did not overheat
• Seeds
– Highly resistant
– Contain food supply for young plant
– Lycophytes and pterophytes do not have seeds
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Chlorophytes
Charophytes
Liverworts
Mosses
Hornworts
Lycophytes
Ferns + Allies
Gymnosperms
Angiosperms
Flowers
Fruits
Seeds
Euphylls
Stems, roots, leaves
Dominant sporophyte
Vascular tissue
Stomata
Multicellular embryo
Antheridia and archegonia
Cuticle
Plasmodesmata
Chlorophyll a and b
Ancestral alga
• Fruits in the flowering plants (angiosperms) add
a layer of protection to seeds and attract animals
that assist in seed dispersal, expanding the
potential range of the species
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Seed Plants
Ferns and Allies
Lycophytes
Lycophytes
Hornworts
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• Worldwide distribution – abundant in
tropics
• Lack seeds
• Superficially resemble true mosses
• Sporophyte dominant
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Pterophytes
• Phylogenetic
relationships among
ferns and their relatives
is still being sorted out
• Common ancestor gave
rise to 2 clades
• All form antheridia and
archegonia
• All require free water for
flagellated sperm
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Whisk ferns
• Found in tropics
• Sporophyte consists of
evenly forking green stems
without true leaves or roots
• Some gametophytes
develop elements of
vascular tissue
– Only one known to do so
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Horsetails
• All 15 living species are
homosporous
• Constitute a single species,
Equisetum
• Sporophyte consists of ribbed,
jointed photosynthetic stems
that arise from branching
rhizomes with roots at nodes
• Silica deposits in cells –
scouring rush
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Ferns
• Most abundant group of
seedless vascular plants
– About 11,000 species
• Coal formed from forests
300 MYA
• Conspicuous sporophyte
and much smaller
gametophyte are both
photosynthetic
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• Fern life cycle
differs from
that of a moss
• Much greater
development,
independence
, and
dominance of
the fern’s
sporophyte
• Gametophyte
lacks vascular
tissue
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• Fern morphology
– Sporophytes have rhizomes
– Fronds (leaves) develop at the tip of the rhizome as
tightly rolled-up coils (“fiddleheads”)
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Fern reproduction
• Produce distinctive sporangia in clusters
called sori on the back of the fronds
• Diploid spore mother cells in sporangia
produce haploid spores by meiosis
• Spores germinate into gametophyte
– Rhizoids but not true roots – no vascular
tissue
• Flagellated sperm
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The Evolution of Seed Plants
• Seed plants first appeared 305–465 MYA
– Evolved from spore-bearing plants known as
progymnosperms
• Success attributed to evolution of seed
– Protects and provides food for embryo
– Allows the “clock to be stopped” to survive
harsh periods before germinating
– Later development of fruits enhanced
dispersal
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Stored food
Integument
(seed coat)
Embryo
b: © Biology Media/Photo Researchers, Inc.
312 m
• Seed
– Embryo protected by integument
• An extra layer or 2 of sporophyte tissue
• Hardens into seed coat
– Megasporangium divides meiotically inside ovule to
produce haploid megaspore
– Megaspore produces egg that combines with sperm
to form zygote
– Also contain food supply for embryo
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• Seed plants produce 2 kinds of
gametophytes
• Male gametophytes
– Pollen grains
– Dispersed by wind or a pollinator
– No need for water
• Female gametophytes
– Develop within an ovule
– Enclosed within diploid sporophyte tissue in
angiosperms
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Angiosperms
• Plants with “naked seeds”
• There are four living groups
Gymnosperms
Gymnosperms
Ferns and Allies
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– Coniferophytes
– Cycadophytes
– Gnetophytes
– Ginkgophytes
• All lack flowers and fruits of angiosperms
• All have ovule exposed on a scale
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Conifers (phylum Coniferophyta)
• Most familiar gymnosperm phylum
• Pines, spruces, firs, cedars, and others
– Coastal redwood – Tallest living vascular plant
– Bristlecone pine – Oldest living tree
• Found in colder and sometimes drier
regions of the world
• Conifers are sources of important products
– Timber, paper, resin, and taxol (anti-cancer)
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• Pines
– More than 100 species,
all in the Northern
hemisphere
– Produce tough
needlelike leaves in
clusters
– Leaves have thick
cuticle and recessed
stomata to retard water
loss
– Leaves have canals with
resin to deter insect and
fungal attacks
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• 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
– Each contains a megasporangium
• Each will become a female gametophyte
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• Female cones usually take 2 or more seasons to
mature
• During the first spring, pollen grains drift down
between open scales
– Pollen grains drawn down into micropyle
– Scales close
• A year later, female gametophyte matures
– Pollen tube is digesting its way through
– Mature male gametophyte has 2 sperm
• 15 months after pollination, pollen tube reaches
archegonium and discharges contents
– One sperm unites with egg = zygote
– Other sperm degenerates
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Cycads (phylum Cycadophyta)
• Slow-growing
gymnosperms of tropical
and subtropical regions
• Sporophytes resemble
palm trees
• Female cones can weigh
45 kg
• Have largest sperm cells
of all organisms!
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Gnetophytes (phylum Gnetophyta)
• Only gymnosperms
with vessels in their
xylem
• Contain three
(unusual) genera
– Welwitschia
– Ephedra
– Gnetum
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Ginkgophytes (phylum Ginkgophyta)
• Only one living species
remains
– Ginkgo biloba
• Flagellated sperm
• Dioecious
– Male and female
reproductive structures
form on different trees
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Angiosperms
• Flowering plants
• Ovules are enclosed in diploid tissue at
the time of pollination
• Carpel, a modified leaf that covers seeds,
develops into fruit
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Ovules
(seeds)
Carpel
(fruit)
Ovules
Cross section
Modified leaf
with ovules
Folding of leaf
protects ovules
Fusion of
leaf margins
(bottom right): © Goodshoot/Alamy RF
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• Angiosperm origins are a mystery
– Origins as early as 145–208 MYA
– Oldest known angiosperm in the fossil record
is Archaefructus
– Closest living relative to the original
angiosperm is Amborella
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• Flower morphology
– Modified stems bearing modified leaves
– 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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• Flower whorls
– Outermost whorl – sepals
– Second whorl – petals
– Third whorl – stamens (androecium)
• Pollen is the male gametophyte
• Each stamen has a pollen-bearing anther and a
filament (stalk)
– Innermost whorl – gynoecium
• Consists of one or more carpels
• House the female gametophyte
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• Carpel has 3 major regions
– Ovary – swollen base containing ovules
• Later develops into a fruit
– Stigma – tip where pollen lands
– Style – neck or stalk
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Nucellus
Megaspore
mother cell
Integuments
Micropyle
Stalk of ovule (funiculus)
b.
• Single megaspore mother cell in ovule
undergoes meiosis
– Produces 4 megaspores
• 3 disappear
• Nucleus of remaining megaspore divides mitotically
– Daughter nuclei divide to produce 8 haploid nuclei
• 2 groups of 4
– Integuments become seed coat
• Form micropyle
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• Embryo sac = female gametophyte
– 8 nuclei in 7 cells
– 8 haploid daughter nuclei (2 groups of 4)
• 1 from each group of 4 migrates toward center
– Functions as polar nuclei – may fuse
• Egg
– 1 cell in group closest to micropyle
– Other 2 are synergids
• Antipodals
– 3 cells at other end – no function
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• 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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• Pollination
– Mechanical transfer of pollen from anther to
stigma
– May or may not be followed by fertilization
– 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
• No flagella on sperm
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• Double fertilization
– One sperm unites with egg to form the diploid
zygote
• New sporophyte
– Other sperm unites with the two polar nuclei
to form the triploid endosperm
• Provides nutrients to embryo
• Seed may remain dormant for many years
– Germinate when conditions are favorable
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