Transcript Seed Plants

Teresa Audesirk • Gerald Audesirk • Bruce E. Byers
Biology: Life on Earth
Eighth Edition
Lecture for Chapter 21
The Diversity of Plants
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 21 Outline
• 21.1 What Are the Key Features of Plants? p.
404
• 21.2 What Is the Evolutionary Origin of Plants?
p. 405
• 21.3 How Have Plants Adapted to Life on Land?
p. 406
• 21.4 What Are the Major Groups of Plants? p.
407
Section 21.1 Outline
• 21.1 What Are the Key Features of
Plants?
– Plants Have Alternating Multicellular Haploid
and Diploid Generations
– Plants Have Multicellular, Dependent
Embryos
– Plants Play a Crucial Ecological Role
– Plants Provide Humans with Necessities and
Luxuries
– Plants Are Adapted to Life on Land
Key Features of Plants
• Multicellularity
• Ability to photosynthesize (most)
• Exhibit alternation of generations (a
multicellular diploid generation alternates
with a multicellular haploid generation)
Alternation of Generations
• Diploid sporophyte plant produces
haploid spores through meiosis
• Spores divide by mitosis and develop into
haploid gametophyte plants
• Haploid gametophyte plant produces
haploid gametes through mitosis
• Gametes fuse to form diploid zygotes,
which divide by mitosis and develop into
diploid sporophytes
FIGURE 21-1
Alternation of
generations in
plants
As shown in this
generalized
depiction of a
plant life cycle,
a diploid
sporophyte
generation
produces haploid
spores through
meiosis.
The spores develop into a haploid gametophyte generation that
produces haploid gametes by mitosis. The fusion of these
gametes results in a diploid zygote that develops into the
sporophyte plant.
Multi-cellular Dependent Embryos
• Have multi-cellular, dependent embryos
– Zygotes develop into multi-cellular embryos
– Embryos are retained within, and receive
nutrients from the gametophyte parent
Crucial Ecological Role
• Through photosynthesis, plants provide
food, directly or indirectly, for ALL of the
animals, fungi, and non-photosynthetic
microbes on land
• Plants produce oxygen gas as a byproduct
of photosynthesis, continually replenishing
oxygen in the atmosphere
Crucial Ecological Role
• Plants help create and maintain soil
– Dead plant material is decomposed by fungi,
prokaryotes, and other decomposers
– Decomposed plant tissue becomes part of the
soil, making it more fertile
– Roots of living plants help hold soil together,
preventing erosion by wind and water
Human Necessities and Luxuries
• Plants provide shelter
– Wood is used to construct housing
• Plants provide fuel
– Wood: important fuel for warming and cooking
in many parts of the world
– Coal: derived from the remains of ancient
plants that have been transformed by
geological processes
Human Necessities and Luxuries
• Plants provide medicine
– Many medicines and drugs were originally
found in and extracted from plants, e.g.
aspirin, Taxol, morphine
• Plants provide pleasure
– Flowers, gardens, and lawns
– Coffee, tea, and wine
Section 21.2 Outline
• 21.2 What Is the Evolutionary Origin of
Plants?
– Green Algae Gave Rise to Plants
– The Ancestors of Plants Lived in Fresh Water
The Plant Evolutionary Tree
• Certain anatomical features represent
milestones in the evolution of plants
– Appearance of vascular tissue and lignin
– Appearance of pollen and seeds
– Appearance of flowers and fruits
FIGURE 21-2 Evolutionary tree of some major plant groups
Green Algae
• Several lines of evidence support the
hypothesis that green algae gave rise to
plants:
DNA comparisons show that green algae
are plants’ closest living relatives
Both use the same type of chlorophyll and
accessory pigments in photosynthesis
Both store food as starch
Both have cell walls made of cellulose
Section 21.3 Outline
• 21.3 How Have Plants Adapted to Life
on Land?
– Plant Bodies Resist Gravity and Drying
– Plant Embryos Are Protected and Plant Sex
Cells May Disperse Without Water
Terrestrial Adaptations
• Roots or root-like structures
– Anchor plant
– Absorb water and nutrients from soil
• Waxy cuticle covers leaves and stems
– Reduces evaporative water loss
Terrestrial Adaptations
• Stomata (singular, stoma)
– Allow gas exchange when open
– Reduce evaporative water loss when closed
• Conducting vessels
– Transport water and nutrients throughout
plant
• Lignin
– Stiffening agent found in cell walls; supports
plant body
Reproduction Without Water
• Pollen
– A reduced male gametophyte that allows wind
(instead of water) to carry sperm to eggs
• Seeds
– Nourish, protect, and help disperse
developing embryos
• Flowers
– Attract pollinators
• Fruits
– Attract animals to disperse seeds
Section 21.4 Outline
• 21.4 What Are the Major Groups of
Plants?
– Bryophytes Lack Conducting Structures
– Vascular Plants Have Conducting Vessels
That Also Provide Support
– The Seedless Vascular Plants Include the
Club Mosses, Horsetails, and Ferns
– The Seed Plants Dominate the Land, Aided by
Two Important Adaptations: Pollen and Seeds
Section 21.4 Outline
• 21.4 What Are the Major Groups of
Plants? (continued)
– Gymnosperms Are Nonflowering Seed Plants
– Angiosperms Are Flowering Seed Plants
– More Recently Evolved Plants Have Smaller
Gametophytes
Major Groups of Plants
• Bryophytes (non-vascular plants)
– Lack well-developed structures for conducting
water and nutrients
• Vascular plants (tracheophytes)
– Have a complex vascular system
Table 21-1 Features of the Major Plant Groups
The Bryophytes
• Lack true roots, stems, or leaves
• Have rhizoids, root-like anchoring
structures
• Limited body size (most less than 2.5 cm tall)
– Non-vascular
– Cell walls lack a stiffening agent
• Most are restricted to moist habitats
– Motile sperm must swim to egg
The Bryophytes
• Gametophyte generation is dominant
• Sporophyte remains attached to and is
nutritionally dependent on parental
gametophyte
• Include liverworts, hornworts, and mosses
FIGURE 21-3a Bryophytes
The plants shown here are less than
inch (about 1 centimeter) in height.
(a) The horn-like sporophytes of
hornworts grow upward from
archegonia that are embedded in
the gametophyte body.
(b) Liverworts grow in moist,
shaded areas. This female
plant bears umbrella-like
archegonia, which hold the
eggs. Sperm must swim up
the stalks through a film of
water to fertilize the eggs.
FIGURE 21-3c
Bryophytes
(c) Moss plants,
showing the stalks
that carry sporebearing capsules.
FIGURE 21-3d
Bryophytes
(d) Mats of Sphagnum
moss cover moist bogs
in northern regions.
The Bryophytes: Reproduction
• Gametes develop within protected
structures on gametophyte
– Archegonia (singular, archegonium) produce
eggs
– Antheridia (singular, antheridium) produce
sperm
• Archegonia and antheridia may be located
on the same plant or on different plants
FIGURE 21-4 Moss life cycle
The leafy green gametophyte
(lower right) is the haploid
generation that produces
sperm and eggs. The sperm
must swim through a film of
water to the egg. The zygote
develops into a stalked, diploid
sporophyte that emerges from
the gametophyte plant. The
sporophyte is topped by a
brown capsule in which
haploid spores are produced
by meiosis. These are
dispersed and germinate,
producing another green
gametophyte generation.
(Inset) Moss plants. The short,
leafy green plants are haploid
gametophytes; the reddish
brown stalks are diploid
sporophyte
The Bryophytes: Reproduction
• Sperm swim to egg
• Fertilization occurs and diploid sporophyte
develops within archegonium of
gametophyte
• Sporophyte produces encapsulated
spores via meiosis
• Haploid spores disperse and germinate
into new gametophytes
The Vascular Plants
• Have roots, stems, and leaves
• Have vessels impregnated with the
stiffening agent lignin
• Sporophyte generation is dominant
• Include the seedless vascular plants and
the seed plants
The Seedless Vascular Plants
• Gametes develop within archegonia and
antheridia
• Motile sperm swim to egg
• Formed the first forests
– Gave rise to present-day coal deposits
• Include club mosses, horsetails, and ferns
Club Mosses
• Present-day club mosses are only a few
inches tall
• Leaves are small and scalelike
• Lycopodium (ground pine) grows on
temperate forest floors
FIGURE 21-5a Some seedless vascular plants
Seedless vascular plants are found in moist woodland habitats.
(a) The club mosses (sometimes called ground pines) grow in
temperate forests. This specimen is releasing spores.
Horsetails
• Present-day horsetails (Equisetum) rarely
exceed a meter in height
• Leaves reduced to scales on branches
• Outer layer of cells contain silica (glass)
– Abrasive texture led early European settlers to
call them “scouring rushes”
FIGURE 21-5c Some seedless
vascular plants
Seedless vascular plants are
found in moist woodland habitats.
(b) The giant horsetail extends
long, narrow branches in a series
of rosettes. Its leaves are
insignificant scales. At right is a
cone-shaped spore-forming
structure.
Ferns
• Largest and most diverse group of
seedless vascular plants
• Have well-developed, broad leaves
• Leaves emerge from coiled fiddleheads
• Reach significant heights in tropics
FIGURE 21-5b Some seedless
vascular plants
Seedless vascular plants are found
in moist woodland habitats. (c) The
leaves of this deer fern are
emerging from coiled fiddleheads.
(d) Although most fern
species are small,
some, such as this tree
fern, retain the large
size that was common
among ferns of the
Carboniferous period.
Ferns: Reproduction
• Sporophyte produces haploid spores
within sporangia (singular, sporangium)
• Spores disperse and germinate into tiny,
independent gametophytes
– Archegonium produces egg
– Antheridium produces sperm
• Sperm swim to egg
• Fertilization occurs and zygote develops
into a diploid sporophyte
FIGURE 21-6 Fern life cycle
The dominant plant body
(upper left) is the diploid
sporophyte. Haploid spores,
formed in sporangia located on
the underside of certain leaves,
are dispersed by the wind to
germinate on the moist forest
floor into inconspicuous haploid
gametophyte plants. On the
lower surface of these small,
sheet-like gametophytes, male
antheridia and female
archegonia produce sperm and
eggs. The sperm must swim to
the egg, which remains in the
archegonium. The zygote
develops into the large
sporophyte plant. (Inset)
Underside of a fern leaf,
showing clusters of sporangia.
The Seed Plants
• Produce pollen and seeds
– Pollen grains contain sperm-producing cells
• Dispersed by wind or pollinators
• Eliminate need for sperm to swim to egg
– Seeds consist of:
• Embryonic plant
• Seed coat (protects embryo)
• Food supply (nourishes emerging plant)
FIGURE 21-7a,b Seeds
Seeds from (a) a gymnosperm and (b) an angiosperm. Both
consist of an embryonic plant and stored food confined within a
seed coat.
Seeds
• May remain dormant for days, months, or
years
• May possess adaptations for dispersal by
wind, water, and animals
FIGURE 21-7d Seeds
Seeds exhibit diverse adaptations for dispersal, including (c) the
dandelion’s tiny, tufted seeds that float in the air and (d) the
massive, armored seeds (protected inside the fruit) of the coconut
palm, which can survive prolonged immersion in seawater as they
traverse ocean.
The Seed Plants
• Gametophytes greatly reduced in size and
dependent on sporophyte for nutrition
– Female gametophyte is a small group of
haploid cells
– Male gametophyte is pollen grain
• Include gymnosperms and angiosperms
Gymnosperms
• Non-flowering seed plants
• First fully terrestrial plants to evolve
• Includes ginkgos, cycads, gnetophytes,
and conifers
Gymnosperms: Ginkgos
• Present-day ginkgos represented by a
single species, Ginkgo biloba (maidenhair
tree)
• Trees are either male or female
– Female trees bear foul-smelling, fleshy seeds
• Male trees extensively planted in U.S.
cities (resistant to pollution)
• Ginkgo extract purportedly improves
memory
FIGURE 21-8a Gymnosperms
(a) This ginkgo, or maidenhair tree, is female and bears fleshy
seeds the size of large cherries.
Gymnosperms: Cycads
• Probably evolved from ferns
• Most abundant in tropical or subtropical
climates
• Are either male or female
• Grow slowly and live for a long time
– One Australian specimen estimated to be
5000 years old
FIGURE 21-8b Gymnosperms
(b) A cycad. Common in the age of dinosaurs, these are now
limited to about 160 species. Like ginkgos, cycads have separate
sexes.
Gymnosperms: Gnetophytes
• 70 species of shrubs, vines, small trees
• Leaves of Ephedra species contain
compounds used as stimulants and
appetite suppressants
• The leaves of gnetophyte Welwitschia
mirabilis may be hundreds of years old…
FIGURE 21-8c Gymnosperms
(c) The leaves of the gnetophyte Welwitschia may be
hundreds of years old.
Gymnosperms: Conifers
• Include pines, firs, spruce, hemlocks, and
cypresses
• Most abundant in cold latitudes and at
high elevations
• Adapted to dry, cold conditions:
– Retain green leaves throughout the year
(evergreen)
– Thin, needle-like leaves covered with
waterproofing material to reduce evaporation
– Produce an “antifreeze” in sap
FIGURE 21-8d Gymnosperms
(d) The needle-shaped leaves of conifers are protected
by a waxy surface layer.
Conifer Seeds Develop in Cones
• Male cones are relatively small
– Produce pollen (male gametophytes) by
meiosis
– Pollen dispersed by wind
– Pollen grain germinates and forms a pollen
tube if it lands near female gametophyte
– Pollen tube slowly burrows into female
gametophyte (may take 14 months)
Conifer Seeds Develop in Cones
• Female cones consist of numerous woody
scales arranged spirally around a central
axis
– Two ovules (immature seeds) located at base
of each scale
– Cells within each ovule undergo meiosis to
produce haploid female gametophytes
– Female gametophytes produce egg cells
Conifer Seeds Develop in Cones
• Pollen tube releases sperm when it comes
into contact with egg
• Fertilization occurs and seed develops
• Seeds released when cone is mature
– Seeds dispersed by wind
– Seeds germinate to form sporophyte trees
FIG 21-9 Pine life cycle
The pine tree is the sporophyte
generation (upper left) and
bears both male and female
cones. Haploid female
gametophytes develop within
the scales of female cones and
produce egg cells. Male cones
produce pollen, the male
gametophytes. A pollen grain,
dispersed by the wind, may land
on the scale of a female cone. It
then grows a pollen tube that
penetrates the female
gametophyte and conducts
sperm to the egg. The fertilized
egg develops into an embryonic
plant enclosed in a seed. The
seed is eventually released
from the cone, germinates, and
grows into a sporophyte tree.
Seed Plants: Angiosperms
• Seed plants that produce flowers and fruits
• Most diverse and widespread of all plants
• Have broad range in size
– Smallest is duckweed (3 mm in diameter)
– Largest is eucalyptus tree (100 meters in
height)
FIGURE 21-10a Angiosperms
(a) The smallest angiosperm is the duckweed, found floating on ponds.
These specimens are about 1/8 inch (3 millimeters) in diameter. (b) The
largest angiosperms are eucalyptus trees, which can reach 325 feet
(100 meters) in height. Conspicuous flowers, such as those on a
eucalyptus tree (b, inset), entice insects and other animals that carry
pollen between individual plant
Seed Plants: Angiosperms
• Three major adaptations have contributed
to dominance of angiosperms
– Flowers
– Fruits
– Broad leaves
Flowers
• Flowers are reproductive structures in
which both male and female
gametophytes are formed
• Believed to have evolved when
gymnosperm ancestors formed an
association with animals
– Animals benefited by eating some of the
protein-rich pollen
– Plants benefited by using animals as
pollinators
Flowers
• Most flowers are showy and attract animal
pollinators (e.g. insects)
Flowers
• Some flowers are inconspicuous and rely
on wind for pollination
FIGURE 21-10b Angiosperms
Both (c) grasses and many trees, such as (d) this birch, in which flowers are
shown as buds (green) and blossoms (brown), have inconspicuous flowers and
rely on wind for pollination.
Life Cycle of an Angiosperm
• Flowers develop on dominant sporophyte
plant
– Male gametophytes (pollen) develop inside
anthers
– Female gametophyte develops from an ovule
inside the ovary
• Egg develops within female gametophyte
Life Cycle of an Angiosperm
• Pollination occurs when pollen grain lands
on the stigma of a flower
• Fertilization occurs when growing pollen
tube releases sperm into the ovule
• Fertilized ovule develops into a seed
• Seed is dispersed and germinates to form
a sporophyte plant
FIGURE 21-11 Life cycle of a
flowering plant
The dominant plant body (upper
right) is the diploid sporophyte,
whose flowers normally produce
both male and female
gametophytes. Male
gametophytes (pollen grains) are
produced within anthers. The
female gametophyte develops
from a spore within the ovule, and
contains one egg cell. A pollen
grain that lands on a stigma
grows a pollen tube that burrows
down to the ovule and into the
female gametophyte. There it
releases its sperm, one of which
fuses with the egg to form a
zygote. The ovule gives rise to the
seed, which contains the
developing embryo and its food
source. The seed is dispersed,
germinates, and develops into a
mature sporophyte.
Fruits Encourage Seed Dispersal
• Fruits are mature ovaries that contain
developing seeds
• Various fruit adaptations help disperse
seeds
– Edible fruits entice animals to eat them (seeds
pass through digestive tract unharmed)
– Burrs cling to animal fur
– Winged fruits are carried through the air
Broad Leaves
• Broad leaves of angiosperms collect more
sunlight for photosynthesis than narrow
leaves of gymnosperms
• Temperate angiosperms drop leaves to
conserve water when it is in short supply
(fall, winter)
• Tropical and subtropical angiosperms are
evergreen
– May shed leaves during dry season
Broad Leaves
• Photosynthetic advantage is offset by fact
that broad, tender leaves are more
appealing to herbivores than tough, waxy
needles of conifers
• Angiosperm defenses include
– Physical defenses (thorns, spines, resins)
– Chemical defenses (make plant tissue
poisonous or distasteful)
Chemical Defenses
• Many defensive compounds have been
exploited by humans for medicinal and
culinary uses
– Medicines (aspirin, codeine)
– Stimulants (nicotine, caffeine)
– Spicy flavors (mustard, peppermint)
Smaller Gametophytes ‘Evolve’
• Earliest plants (nonvascular plants)
– Gametophyte dominates
– Small, dependent sporophyte attached to larger
gametophyte
• Later plants (seedless vascular plants)
– Sporophyte dominates
– Small gametophyte is independent of sporophyte
• Recently evolved plants (seed plants)
– Sporophyte dominates
– Microscopic, dependent gametophyte attached to
larger sporophyte