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© Cengage Learning 2016
Chapter 22
The Land Plants
© Cengage Learning 2016
22.2 Plant Ancestry and Diversity
• Plants
– Multicelled
– Photosynthetic eukaryotes
– Adapted to life on land
– Close relatives of red algae and green algae
– Contain cellulose cell walls and chloroplasts
• Chlorophylls a and b
– Embryophytes
• Embryos form within chamber of parental tissues
• Receive nourishment during development
© Cengage Learning 2016
Plant Ancestry and Diversity
• Adaptive radiation
• First plants evolved 500 million
years ago
– Photosynthetic cells had released
oxygen into the atmosphere and
ozone layer had formed
• Plant spore
© Cengage Learning 2016
Plant Ancestry and Diversity
• Plant spore
– Haploid cell with a cell wall
• Some plants have alternating generations
– A life cycle in which a diploid generation alternates with a haploid
one
– Diploid
– Haploid
© Cengage Learning 2016
Meiosis
Haploid (n)
Phase of Cycle sporesgametes
multicelled
gametophyte
zygote Diploid (2n)
Phase of Cycle
Fertilization
Stepped Art
multicelled
sporophytemitosis
© Cengage Learning 2016
Major groups of plants
• Criteria
– Vascular
– Seed production
– Types of seeds produced
– Development and morphology
© Cengage Learning 2016
© Cengage Learning 2016
Evolutionary Trends Among Plants
• Story of plant evolution
– Adapted to life on land
– Adapted to increasingly drier habitats
• Sporophyte dominates all vascular plant
life cycles
– Spores more likely to survive in dry
conditions
• Oak tree
– Large and complex sporophyte
– Gametophyte: only a few cells
© Cengage Learning 2016
Evolutionary Trends Among Plants
• Genetic factors
– Influenced sporophyte dominance
• Structural adaptations
– Cuticle
• Helps reduce evaporative water loss
– Stomata
• Open to allow gas exchange for
photosynthesis
• Close to conserve water
© Cengage Learning 2016
Evolutionary Trends Among Plants
• Vascular tissues
– Internal system of pipelines
– Xylem distributes water
– Phloem distributes sugars
– Lignin provides structural support
• Leaves
– Contain veins of vascular tissue
– Allow plant to capture sunlight
and exchange gases
© Cengage Learning 2016
Pollen and Seeds
• Pollen grains
– Walled, immature gametophyte
• Gives rise to male gametes
– Released by seed-bearing vascular
plants
– Travel to female gametophytes on the
wind or in insect bodies
– Allow plants to reproduce even in dry
environments
© Cengage Learning 2016
Pollen and Seeds
• Seeds
– Consist of an embryo
sporophyte and nutritive tissue
enclosed within a waterproof
seed coat
– Many have features that
facilitate their dispersal
• Angiosperms disperse seeds
inside a fruit
© Cengage Learning 2016
Bryophytes
• Mosses
– Most diverse and familiar group of bryophytes
– Example: Sphagnum (peat moss)
• 350 species
• One of the most economically important bryophytes
• Grow in peat bogs in high latitude regions of Europe,
Asia, and North America
• Blocks are cut, dried, and burned as fuel in Ireland
• Helps soil retain moisture
© Cengage Learning 2016
zygote2
sporophyte (2 n)
spor e (n) released
from sporangium
sperm released from
male gametangium
1gametophyte (n)
male
gametophyte
sperm
egg in female
gametangium
4
female
gametophyte
Fertilization Meiosis
Diploid (2n)
phase
Haploid ( n) phase 3
5
6
7
© 2016 Cengage LearningJane Burton/ Bruce Coleman Ltd.
① The leafy green part of a moss is the haploid gametophyte.
② The diploid sporophyte has a stalk and a capsule (sporangium). It is not photosynthetic.
③ Haploid spores form by meiosis in the capsule, are released, and drift with the winds.
④ Spores germinate and develop into male or female gametophytes with gametangia that produce eggs or sperm by mitosis.
⑤ Sperm swim to eggs.⑥ Fertilization produces a zygote.⑦ The zygote grows and develops into a
new sporophyte while remaining attached to and nourished by the female gametophyte.
© Cengage Learning 2016
Bryophytes
• Liverworts
– Among oldest known fossils of land plants
– Some gametophytes look leafy and others are
flat sheets
– Example: Marchantia
• Hornworts
– Has a pointy, hornlike sporophyte
– Spores form in upright capsule called
sporangium
– Contain chloroplasts
© Cengage Learning 2016
Seedless Vascular Plants
• Oldest vascular plant lineages
• Have flagellated sperm that swim to eggs
• Disperse by releasing spores directly to the
environment
• Two lineages survive today
– Lycophytes
– Monilophytes
© Cengage Learning 2016
Seedless Vascular Plants
• Club mosses
– Often grow on floor of temperate forests
• Whisk ferns and horsetails
– Native to the Southeastern United States
– Includes rushes
• Ferns
– Most diverse and familiar seedless
vascular plant
– Most live in the tropics
© Cengage Learning 2016
1 The familiar leafy form is the diploid sporophyte.
2 Meiosis in cells on the underside of fronds (leaves)
produces haploid spores.
3 After spores are released,
they germinate and grow into
tiny gametophytes that
produce eggs and sperm.
4 Sperm swim to eggs and
fertilize them, forming a
zygote.
5 The sporophyte begins its
development attached to the
gametophyte, but it continues
to grow and live independently
after the gametophyte dies.
3
sperm
egg
rhizome
zygote
mature
gametophyte
(underside)
spore
sori (clusters of sporangia) on underside of frond
Diploid (2n ) phase
Haploid (n ) phase
2
MeiosisFertilization
1mature sporophyte
5young sporophyte
female
gametangium
male
gametangium
4
photo, A. & E. Bomford/ Ardea, London; art, © 2016 Cengage Learning
© Cengage Learning 2016
Rise of the Seed Plants
• Advantages of seed-bearing plants
– Reproductive traits
– Structural traits
© Cengage Learning 2016
seed (embryo
sporophyte in ovule)
zygote (2n) in ovule
fertilization
develop into
Figure 22.21 p365
pollen sac (2n)
sperm-producing male
gametophytes (pollen grains)
microspores (n) in pollen sac
develop into
meiosis
release and pollination
delivers sperm to
Stepped Art
develop into
egg-bearing female
gametophytes in ovule
megaspores (n) in ovule
ovule (2n)
meiosis
© Cengage Learning 2016
Gymnosperms
• Vascular seed plants
– Produce seeds on the surface of ovules
– Some seeds enclosed in a fleshy or papery
covering
• Types
– Conifers
– Cycads
– Ginkgos
– Gnetophytes
© Cengage Learning 2016
Gymnosperms
• Conifers
– 600 species
– Trees and shrubs with woody cones
– Needle-like or scale-like leaves with a thick cuticle
– More resistant to drought and cold than flowering plants
– Evergreen
– Include the longest lived plants
• Some bristlecone pines are 4,000 years old
© Cengage Learning 2016
pollen tube
sperm (n)
1 Scales of pollen
cones hold pollen sacs.
2 Meiosis of cells in
pollen sacs yields microspores ( n).
8 Fertilization
produces a zygote
that develops into
a seed.
9 The seed
germinates and grows into a new sporophyte.
3 Microspores
develop into pollen grains (male game-tophytes) that are released and travel on the wind.
6 Megaspores
develop into egg-
bearing female
gametophyte s
inside the ovule.
7 A pollen grain alights on a scale of an ovulate cone. It germinates
and a pollen tube grows toward the ovule. Sperm form as the tube grows.
© 2016 Cengage Learning; photos, Robert & Linda Mitchell Photography; Robert Potts, California Academy of Sciences; R. J. Erwin/Science Source
5 Meiosis of cells
in ovules yields megaspores ( n).
4 Scales of
ovulate cones contain ovules .
eggs (n)
female
gametophyte
Inside
ovule
zygote (2 n)
seed coat
embryo seed
stored food
sporophyte (2n)
pollen cone with
many scales
One scale sectioned
through pollen sac
One scale sectioned
through ovule
Fer tilization
MeiosisMeiosis
ovulate cone with
many scales
© Cengage Learning 2016
Gymnosperms
• Cycads
– Native to dry tropics and subtropics
• Ginkgos
– One species remains today: Ginkgo biloba
• Native to China
– Deciduous
• Gnetophytes
– Tropical trees
– Desert shrubs
– Leathery vines
© Cengage Learning 2016
Angiosperms – The Flowering Plants
• Vascular seed plants that make
flowers and fruits
• Flower
– Specialized reproductive shoot
– Contain sepals, petals, and stamen
– Innermost part of the flower is the ovary
– After fertilization, ovule matures into seed
• Ovary becomes the fruit
© Cengage Learning 2016
4 An ovule forms on theovary wall. It contains a diploid cell that will undergo meiosis.
5 Meiosis
yields four
megaspores.
Three will
degenerate.
6 The remaining
megaspore devel-
ops into a female
gametophyte tha t
consists of an egg
and six other cells,
including a cell wit h
two nuclei.8 During double fertilization, one
sperm fertilizes the egg to form a zygote; the other fertilizes the cell with
two nuclei to form a triploid (3n) cell.
9 The ovule
develops into a seed.
7 Pollinatio noccurs, and the pollen grain germinates.A pollen tube grows to and through ovary tissue to the ovule, where it releases two sperm.
2 Meiosis
produces microspores.
3 Microspores
develop into pollen grains (male
gametophytes).
ovule
Double Fertilization
1 An anther has
two pollen sacs
with diploid cells
that give rise to
microspores.
pollen sac
of anther
(cutaway
view)
seed coat (2n)
embryo (2 n)
endosperm (3 n)
2n
pollen grain(n)
microspores (n)
megaspores (n)
mature female
gametophyte
in ovule
pollination
pollen
tube
sperm
mature male
gametophyte
seedMeiosis Meiosis
pollen tube
delivers
2 sperm to
ovule
egg
cell with
2 nuclei
© 2016 Cengage Learning
© Cengage Learning 2016
Angiosperms – The Flowering Plants
• 90 percent of all species today are angiosperms
• Angiosperm characteristics that give them a selective
advantage over gymnosperms
– Shorter life cycle
– Animal-pollinated flowers
– Enhanced seed dispersal
© Cengage Learning 2016
Angiosperm Diversity and Importance
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