Transcript Chapter 30

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
• 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
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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
• Also green algae
• Distinguished from
chlorophytes by close
phylogenetic
relationship to land
plants
• 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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• Charophytes have haplontic life cycles
– Evolution of diplontic embryo and haplodiplontic life
cycle occurred after move to land
• 2 candidate Charophyta clades
– Charales
– Coleochaetales
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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
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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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• 400 million years between appearance of
vascular tissue and true leaves
• 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
• 15 living species
• Constitute a single genus,
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
• 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)
• 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 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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• 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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