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Plant Evolution
The Plant Kingdom
• Nearly all are multicelled
• Vast majority are photoautotrophs
– Energy from sun
– Carbon dioxide from air
– Minerals dissolved in water
Setting the Stage for Plants
• Earth’s atmosphere was originally oxygen
free
• Ultraviolet radiation bombarded the
surface
• Photosynthetic cells produced oxygen and
allowed formation of a protective ozone
layer
Invading the Land
• Cyanobacteria were probably the first to
spread into and up freshwater streams
• Later, green algae and fungi made the
journey together
• Every plant is descended from species of
green algae
Adaptations to Land
•
•
•
•
Root systems
Shoot systems
Vascular tissues
Waxy cuticle for water
conservation
Evolutionary Trend in
Plant Life Cycles
• Algae and bryophytes put most energy
into making gametophytes
• Land plants put energy into structures
that produce spores and retain, nourish,
and protect gametes
Evolutionary Trend
zygote only, no
sporophyte
spo
r
oph
gam
yte’
s
etop
hyte impor
tan
’s im
ce
por
tanc
e
green algae bryophytes
ferns
gymnosperms
angiosperms
Milestones in Plant Evolution
charophytes
bryophytes lycophytes
horsetails ferns
cycads ginkgos conifers gnetophytes flowering plants
seed plants
plants with true leaves
vascular plants
land plants
(closely related groups)
Pollen
• Pollen grains are sperm-bearing male
gametophytes that develop from
microspores
• Allows transfer of sperm to egg without
water
• Can drift on air currents, or be carried
by pollinators
Seeds
• Ovules are female reproductive
structures that become seeds
• Consist of:
– Female gametophyte with egg cell
– Nutrient-rich tissue
– Jacket of cell layers that will form
seed coat
Nonvascular Plants
• Bryophytes
• Include 24,000 species of:
Liverworts
Hornworts
Mosses
Bryophytes
• Small, nonvascular, nonwooody
• Gametophyte dominates life cycle;
has leaflike, stemlike, and rootlike
parts
• Usually live in wet habitats
• Flagellated sperm require water to
reach eggs
Moss Life Cycle
Development of
mature
sporophyte (still
attached to
gametophyte)
Zygote
Fertilization
Diploid Stage
Meiosis
Haploid Stage
Spores
released
male
gametophyte
tip
Sperm
Egg
Male gametophyte
female
gametophyte
tip
Female gametophyte
Marchantia: A Liverwort
• Reproduces asexually by way of
gemmae cups
• Sexual reproduction
• Gametophytes are male or female
• Gametes are produced on elevated
structures
Vascular Plants
• Majority of plants
• Have internal tissues that carry water
and solutes
• Two groups
– Seedless vascular plants
– Seed-bearing vascular plants
Seedless Vascular Plants
• Produce spores but no seeds
• Main groups:
Lycophytes
Horsetails
Ferns
Seedless Vascular Plants
• Like bryophytes:
– Live in wet, humid places
– Require water for fertilization
• Unlike bryophytes:
– Sporophyte is free-living and
has vascular tissues
Ferns
• 12,000 species, mostly tropical
• Most common sporophyte structure
– Perennial underground stem (rhizome)
– Roots and fronds arise from rhizome
– Young fronds are coiled “fiddleheads”
– Spores form on lower surface of some
fronds
Fern Life Cycle
The sporophyte
(still attached to
the gametophyte)
grows, develops
Sori
rhizome
zygote
Diploid Stage
Haploid Stage
fertilization
meiosis
Spores develop
egg
sperm
eggproducing
structure
Spores are
released
mature
gametophyte
(underside)
spermproducing
structure
gametophyte
Spore
germinates
Seed-Bearing Vascular Plants
• Gymnosperms arose first
– Cycads
– Ginkgos
– Conifers
• Angiosperms arose later
– Monocots
– Dicots
Seed-Bearing Plants
• Microspores that give rise
to pollen grains
• Megaspores inside ovules
• More water-conserving
than seedless vascular
plants
Special Traits of
Seed-Bearing Plants
• Pollen grains
– Arise from megaspores
– Develop into male gametophytes
– Can be transported without water
• Seeds
– Embryo sporophyte inside nutritive tissues
and a protective coat
– Can withstand hostile conditions
Gymnosperms
• Plants with “naked seeds”
• Seeds don’t form inside an ovary
Conifers
Ginkgos
Cycads
Conifer Characteristics
• Woody trees or shrubs
• Most are evergreen
• Bear seeds on exposed cone scales
• Most produce woody cones
Cycads
•
•
•
•
Only 130 living species
Two species of Zamia are native to U.S.
Palmlike appearance
Pollen-bearing and seed-bearing cones
on different plants
Ginkgos
• Only surviving species, Ginkgo biloba,
is native to China
• Deciduous tree with fan-shaped leaves
• Trees are male or female
• Female trees produce seeds covered
with a fleshy, foul-smelling coat
Pine Cones
• Woody scales of a “pine cone” are the
parts of where megaspores formed
and developed into female
gametophytes
• Male cones, where microspores and
pollen are produced, are not woody
Pine
Life
Cycle
Female cone
Sporophyte
Ovule
Male cone
Pollen sac
Seed
Fertilization
Egg
Pollen tube
View inside
ovule
Meiosis
Microspores
Megaspores
Angiosperms
• Flowering plants
• Dominant land plants (260,000 species)
• Defining feature: Ovules and (after
fertilization) seeds are enclosed in an
ovary
• Two classes: Monocots and dicots
Plants and Pollinators
• Pollen:
– Sperm packed inside a nutritious package
– Transferred first by wind currents
– Later transferred by insects
• Plants that attracted insect pollinators
with flowers had a reproductive
advantage
Pollen Formation
• Each anther has four pollen sacs
• Inside the pollen sacs, cells undergo
meiosis and cytoplasmic division to form
microspores
• Microspores undergo mitosis to form
pollen grains
Double Fertilization
• A pollen tube grows down through the ovary tissue
• It carries two sperm nuclei
• When pollen tube reaches an ovule, it penetrates
embryo sac and deposits two sperm
• One fertilizes the egg, other fuses with both nuclei of
endosperm mother cell
Endosperm Formation
• Occurs only in angiosperms
• Fusion of a sperm nucleus with the two nuclei
of the endosperm mother cell produces a
triploid (3n) cell
• This cell will give rise to the endosperm, the
nutritive tissue of the seed
Seeds and Fruits
ovule
wall of
ovary
• The seed is the mature ovule
• The fruit is the mature ovary
Structure of a Seed
• Protective seed coat is derived from
integuments that enclosed the ovule
• Nutritious endosperm is food reserve
• Embryo has one or two cotyledons
– Monocot has one
– Eudicot has two
Seed Formation
• Fertilization of the egg produces a diploid
sporophyte zygote
• The zygote undergoes mitotic divisions to
become an embryo sporophyte
• Seed: A mature ovule, which encases an
embryo sporophyte and food reserves
inside a protective coat
Seed Dispersal
• Fruit structure is adapted to mode of dispersal
• Some modes of seed dispersal:
– Wind currents
– Water currents
– Animals
Double Fertilization
• Distinctive feature of angiosperms
• Male gametocyte delivers two sperm to
an ovule
• One fertilizes egg; other fertilizes a cell
that gives rise to endosperm that
supports embryo
Flower Structure
petal
stamen
(microspores
form here)
sepal
carpel
(megaspores
form here)
ovule in an
ovary
Flowering
Plant Life
Cycle
Diploid
Double fertilization
Haploid
Pollination
Two
sperm
enter
ovule
Meiosis
Microspores
Female gametophyte
Meiosis
Mitosis
without
cytoplasmic
division
Deforestation
• Result of demand for wood as fuel and
lumber; cultivation of land for agriculture
• Greatest occurrence in Brazil,
Indonesia, Columbia, and Mexico