plant evolution. the plant kingdom nearly all are multicelled vast majority are photoautotrophs...
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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
sporophyte’s
importancegametophyte’s importance
green algae bryophytes ferns gymnosperms angiosperms
zygote only, no sporophyte
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
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
Fertilization MeiosisDiploid Stage
Haploid Stage
malegametophytetip
femalegametophytetip
Zygote
Development of mature sporophyte (still attached to gametophyte)
Male gametophyte
Female gametophyte
Spores released
Egg
Sperm
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
• 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
zygote fertilization
Diploid Stage
Haploid Stage
egg
sperm
egg-producing structure
sperm-producing structure
mature gametophyte (underside)
Spores develop
meiosis
Sori
Spores are released
Spore germinates
rhizome
gametophyte
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
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
PineLife
Cycle Sporophyte
Female cone
Male cone
Ovule
Fertilization Meiosis
MicrosporesMegaspores
Pollen sac
Egg View insideovule
Pollen tube
Seed
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
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
stamen (microspores form here)
carpel (megaspores form here)
petal
sepal
ovule in an ovary
Flowering Plant Life
Cycle Double fertilization Meiosis Meiosis
Microspores
Female gametophyte
Pollination
Mitosis without cytoplasmic division
Two sperm enter ovule
Diploid
Haploid
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