Download Ch 21

Chapter 21
The Diversity of Plants
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
At a Glance





What Are the Key Features of Plants?
How Do Plants Affect Other Organisms?
What Is the Evolutionary Origin of Plants?
How Have Plants Adapted to Life on Land?
What Are the Major Groups of Plants?
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.1 What Are the Key Features of Plants?
 Most plants have the ability to photosynthesize
 The multicellular embryo of a plant is retained
within and receives nutrients from the tissues of
the parent plant
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.1 What Are the Key Features of Plants?
 Plants exhibit alternation of generations (a
multicellular diploid generation alternates with a
multicellular haploid generation)
– A diploid sporophyte plant produces haploid spores
through meiosis
– The spores divide by mitosis and develop into haploid
gametophyte plants
– Haploid gametophyte plants produce haploid male and
female gametes through mitosis
– Gametes of opposite sex fuse to form diploid zygotes,
which divide by mitosis and develop into diploid
sporophytes
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Alternation of Generations
embryo
2n
sporophyte
(2n)
DIPLOID
GENERATION
zygote 2n
MEIOTIC CELL
DIVISION
FERTILIZATION
n
n
egg
HAPLOID
GENERATION
n
n
n
n spores
sperm
gametophyte
(n)
haploid (n)
diploid (2n)
Fig. 21-1
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.2 How Do Plants Affect Other Organisms?
 Plants play a crucial ecological role
– Plants make vital contributions to the food, air,
soil, and water that sustain life on land
– Plants capture energy that other organisms use
–Through photosynthesis, plants provide food,
directly or indirectly, for all the animals, fungi,
and nonphotosynthetic microbes on land
– Plants help maintain the atmosphere
–Plants produce oxygen gas as a by-product of
photosynthesis, continually replenishing
oxygen in the atmosphere
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.2 How Do Plants Affect Other Organisms?
 Plants play a crucial ecological role (continued)
– Plants build soil
–Dead plant material is decomposed by fungi,
prokaryotes, and other decomposers into
organic matter
–Decomposed plant tissue helps the soil hold
water and nutrients, making it more fertile
–The roots of living plants help hold the soil
together, preventing erosion by wind and
water
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Plants Protect Soil
Fig. 21-2
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.2 How Do Plants Affect Other Organisms?
 Plants help keep ecosystems moist
– Plants take up and store water from the soil,
slowing the rate at which water escapes
terrestrial ecosystems
– Plants also slow the rate of water runoff and
flooding
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.2 How Do Plants Affect Other Organisms?
 Plants provide humans with necessities and
luxuries
– Plants provide shelter, fuel, and medicine
–Wood is used to construct housing
–Wood is an important fuel for warming and
cooking in many parts of the world
–Coal is another important fuel and is derived
from the remains of ancient plants that have
been transformed by geological processes
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.2 How Do Plants Affect Other Organisms?
 Plants provide shelter, fuel, and medicine (continued)
– Many medicines and drugs were originally found in
and extracted from plants; for example, aspirin, the
heart medication digoxin, Taxol, the malaria drug
quinine, and pain-killers such as morphine and
codeine
– Humans have domesticated a host of useful plant
species
– Plants provide pleasure through flowers, gardens, lawns,
and parks and through coffee, tea, and wine
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.3 What Is the Evolutionary Origin of Plants?
 The ancestors of plants were photosynthetic
protists such as the freshwater green algae
known as stoneworts
– Plants and green algae have similar DNA and
use the same type of chlorophyll and accessory
pigments in photosynthesis
– Both plants and green algae store food as starch
and have cell walls made of cellulose
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Chara, a Stonewort
Fig. 21-3
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.3 What Is the Evolutionary Origin of Plants?
 The ancestors of plants were aquatic
– Life in freshwater had many advantages for the
ancestors of plants
–The organism was bathed in a nutrient-rich
solution
–It was supported by buoyancy and not likely to
dry out
–Gametes and zygotes were carried by water
currents
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.4 How Have Plants Adapted to Life on Land?
 The invasion of the land brought many
advantages to plants
– Direct access to sunlight unimpeded by water
that might block its rays
– Access to nutrient-rich soil
 Life on land also imposed some challenges
– There was no structural support from water, nor
the presence of a wet medium to transport
gametes
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.4 How Have Plants Adapted to Life on Land?
 Plant bodies resist gravity and drying
– Near universal adaptations to land include:
–The development of roots that anchor the
plant and absorb nutrients from the soil
–The development of a waxy cuticle that covers
the surfaces of leaves and stems and limits
the evaporation of water
–The presence of pores called stomata in the
leaves and stems that open and allow gas
exchange, but close when water is scarce
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Author Animation: Adaptations to Life on Land
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.4 How Have Plants Adapted to Life on Land?
 Plant bodies resist gravity and drying
(continued)
– Other adaptations to life on land are found in
only some plants
–The presence of conducting cells that
transport water and minerals upward from the
roots and move photosynthetic products from
the leaves to the rest of the plant
–The synthesis of the stiffening substance
lignin, which is a rigid polymer and supports
the plant body
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.4 How Have Plants Adapted to Life on Land?
 Plant embryos are protected, and some plants have
sex cells that disperse without water
– Seed plants have well-protected and well-provisioned
embryos and illustrate waterless dispersal of sex cells
– The key adaptations of these plant groups are seeds,
pollen, flowers, and fruits
– Pollen, because of its light weight, permits the
dispersal of gametes on the wind and the bodies of
insects
– Flowers attract animal pollinators that deliver the
pollen more precisely than the wind does
– Fruits attract animal foragers, who eat the fruit and
disperse the seeds in their feces
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Two major groups of land plants arose from
ancient green algal ancestors
– The nonvascular plants (called bryophytes)
require a moist environment to reproduce, and
therefore they straddle the boundary between
aquatic and terrestrial life
– The vascular plants (called tracheophytes)
have been able to colonize dry habitats
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Author Animation: Bryophytes
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Author Animation: Tracheophytes
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
The Evolutionary Tree of Some Major Plant
Groups
Nonvascular plants
Vascular plants
seed plants
Liverworts
Mosses
Ferns
Gymnosperms
Angiosperms
Flowers
and fruits
Seeds and pollen
True vascular tissue
and lignin
Ancestral green alga
Biology: Life on Earth, 9e
Fig. 21-4
Copyright © 2011 Pearson Education Inc.
Table 21-1
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Nonvascular plants lack conducting structures
– They lack true roots, stems, or leaves
– Instead, they have rhizoids, rootlike anchoring
structures
– Because they must instead rely on slow diffusion
or poorly developed conducting tissues to
distribute water and other nutrients, nonvascular
plants are of limited body size (most less than
2.5 cm tall)
–Their absence of a stiffening agent in the cell
walls also limits their size
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Nonvascular plants include the hornworts,
liverworts, and mosses
– Hornwort sporophytes generally have a spiky
shape that appears hornlike
– The gametophytes of certain liverwort species
have a lobed form reminiscent of a liver
– All hornworts, liverworts, and most mosses are
found in moist areas near banks of streams and
ponds and in forests
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Nonvascular plants include the hornworts,
liverworts, and mosses (continued)
– Mosses are the most diverse, and some have a
waterproof covering that retains moisture,
permitting them to invade drier areas, such as
deserts, bare rock, and northern and southern
climes where humidity is low and water scarce
–Mosses of the genus Sphagnum are common
in northern regions of the world
–Accumulations of dead mosses form peat,
which is used as a fuel
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Nonvascular Plants
Fig. 21-5
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 The reproductive structures of nonvascular
plants are protected
– Gametes develop within protected structures on
the haploid gametophyte
–Archegonia (singular, archegonium) produce
eggs
–Antheridia (singular, antheridium) produce
sperm
–Some nonvascular plant species have both
archegonia and antheridia on the same plant
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Life Cycle of a Moss
sperm
male
gametophyte
female
gametophyte
1 Mitotic cell
division produces
sperm in an
antheridium and
an egg in an
archegonium
2 Sperm
swim through
water to reach
the egg
egg
FERTILIZATION
6 The spores
germinate and
develop into
gametophytes
3 Following fertilization,
a sporophyte develops
and begins to grow upward
from the gametophyte
4 At maturity, the
sporophyte will
produce haploid spores
within a capsule
5 Haploid spores
are liberated from the
capsule and disperse
capsules
MEIOTIC CELL
DIVISION
haploid (n)
diploid (2n)
Fig. 21-6
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 The reproductive structures of nonvascular
plants are protected (continued)
– Reproduction involves several steps
–Sperm swim to an egg
–Fertilization occurs, and a diploid sporophyte
develops within the archegonium of the
gametophyte
–The sporophyte produces encapsulated
haploid spores via meiosis
–Haploid spores disperse and germinate into
new haploid gametophytes
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Vascular plants have conducting cells that also
provide support
– These plants are distinguished by specialized
groups of tube-shaped conducting cells
– These cells are impregnated with the stiffening
agent lignin
– The diploid sporophyte generation is dominant
– Vascular plants can be divided into two groups:
the seedless vascular plants and the seed plants
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 The seedless vascular plants include the club
mosses, horsetails, and ferns
– These plants have swimming sperm and require
water for reproduction
– They do not produce seeds, but propagate by
spores
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Club mosses and horsetails are small and
inconspicuous
– Present-day club mosses are only a few inches
tall
– Their leaves are small and scalelike
– Lycopodium (ground pine) grows on temperate
forest floors
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Some Seedless Vascular Plants
Fig. 21-7
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Club mosses and horsetails are small and
inconspicuous (continued)
– Present-day horsetails (Equisetum) rarely
exceed 3 feet in height
– Their leaves are reduced to scales on branches
– Their outer layer of cells contain silica (glass)
–Their abrasive texture led early European
settlers in North America to use them to scour
pots and floors
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Ferns are broad-leaved and more diverse
– With 12,000 species, ferns are the largest and
most diverse group of seedless vascular plants
– Their well-developed, broad leaves emerge from
coiled fiddleheads
– Ferns are the only seedless vascular plants that
have broad leaves
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Ferns are broad-leaved and more diverse
(continued)
– Reproduction in ferns requires water
– Gametes are produced in archegonia and
antheridia on the tiny haploid fern gametophyte;
the sperm swim to reach the egg
– After fertilization, the zygote develops into a
diploid sporophyte, which grows upward from the
parent gametophyte
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Ferns are broad-leaved and more diverse
(continued)
– A mature sporophyte produces haploid spores in
structures called sporangia
– The spores are dispersed by the wind
– After landing in a suitable place, the spore
germinates and develops into a gametophyte
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Author Animation: Life Cycle of a Fern
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Life Cycle of a Fern
gametophyte
sperm
1 Mitotic cell division
produces sperm in an
antheridium and an egg
in an archegonium
2 Sperm
swim through
water to reach
the egg
egg
FERTILIZATION
6 The spores
germinate and
develop into
gametophytes
5 Haploid
spores are
liberated from
the sporangia
and disperse
MEIOTIC CELL
DIVISION
3 Following fertilization, a
sporophyte develops and
begins to grow upward from
the gametophyte
4 At maturity,
the sporophyte
will produce
haploid spores
within sporangia
sporophyte
sporangium
haploid (n)
diploid (2n)
masses of
sporangia
Fig. 21-8
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 The seed plants are aided by two important
adaptations: pollen and seeds
– Pollen grains are tiny male gametophytes that
carry sperm-producing cells
–They are dispersed by wind or pollinators
–They eliminate the need for sperm to swim to
the egg
– Seeds have three components: an embryonic
sporophyte plant, a food supply for the embryo,
and a protective outer coat
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Seeds
embryo
stored
food
seed
coat
Pine seed
(gymnosperm)
(a) Seeds
Biology: Life on Earth, 9e
Bean seed
(angiosperm)
Fig. 21-9
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 The seed plants are aided by two important
adaptations: pollen and seeds (continued)
– Gametophytes are greatly reduced in size and
depend on the sporophyte for nutrition
–The female gametophyte is a small group of
haploid cells that produce the egg
–The male gametophyte is the pollen grain
itself
– Seed plants include the gymnosperms (that lack
flowers) and the angiosperms (that have flowers)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms are nonflowering seed plants
– Gymnosperms evolved earlier than the flowering plants
– They were the first fully terrestrial plants to evolve
– Early gymnosperms coexisted with the forests of
seedless vascular plants that prevailed during the
Carboniferous period
– Despite their success, most of these early gymnosperms
are now extinct
– Today, four groups of gymnosperms survive: ginkgos,
cycads, gnetophytes, and conifers
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms are nonflowering seed plants
(continued)
– Present-day ginkgos are represented by a single
species, Ginkgo biloba (maidenhair tree)
– They have been cultivated extensively,
particularly in Asia, and might otherwise be
extinct
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms are nonflowering seed plants
(continued)
– Ginko trees are either male or female
–Female trees bear foul-smelling, fleshy seeds
– Ginkgos (primarily males) have been extensively
planted in U.S. cities because they are resistant
to pollution
– Ginkgo extract purportedly improves memory
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms are nonflowering seed plants
(continued)
– Cycads are restricted to warm climates
– Although abundant during the Jurassic period,
today only approximately 160 species of cycads
survive
– They are most abundant in tropical or subtropical
climates
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms are nonflowering seed plants
(continued)
– Cycads have large, finely divided leaves and bear a
superficial resemblance to palms or large ferns
– Cycads grow slowly and live for a long time
– Most cycads are around 3 feet in height, although
some species can reach 65 feet
– One Australian specimen is estimated to be 5,000
years old
– Cycad tissues contain a potent toxin that can harm
grazing livestock
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms are nonflowering seed plants
(continued)
– About half of all cycad species are classified as
threatened or endangered
– The main threats to cycads are habitat
destruction, competition from introduced species,
and harvesting for the horticultural trade
–A large specimen of a rare cycad can sell for
thousands of dollars
– Because cycads grow slowly, recovery of
endangered populations is uncertain
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms (continued)
– Gnetophytes include 70 species of shrubs, vines,
and small trees
–Leaves of the Ephedra species contain
alkaloid
compounds used
as stimulants and
appetite
suppressants
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms (continued)
 The gnetophyte Welwitschia
mirabilisis is found only in the
extremely dry deserts of
southwest Africa
– Welwitschia has a deep taproot
that can extend as far as 100
feet down into the soil
– Above ground, it has only a stem
and two leaves, which remain on
the plant for its entire life and
may last for 2,000 years
– The strap-like leaves continue to
grow for that entire period.
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms are nonflowering seed plants
(continued)
– The conifer group includes pines, firs, spruce,
hemlocks, and cypresses
– Conifers are most abundant in cold latitudes and
at high elevations
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Gymnosperms are nonflowering seed plants
(continued)
– Their adaptations to these climates include:
–Retention of green leaves throughout the year
(they are evergreens)
–Thin, needle-like leaves covered with
waterproofing material to reduce evaporation
–An “antifreeze” in their sap, enabling them to
continue transporting nutrients in belowfreezing temperatures
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Conifer seeds develop in cones
– A conifer tree is a diploid sporophyte and develops male
and female cones
– Male cones are relatively small
– They produce pollen (the male gametophytes) by
meiosis
– The pollen is dispersed by the wind
– The pollen grain germinates and forms a pollen tube if
it lands near a female gametophyte
– The pollen tube slowly burrows into the female
gametophyte (a process that may take 14 months)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Conifer seeds develop in cones (continued)
– Female cones consist of numerous woody
scales arranged spirally around a central axis
–Two ovules (immature seeds) are located at
the base of each scale
–Cells within each ovule undergo meiosis to
produce haploid female gametophytes
–The female gametophytes produce egg cells
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Conifer seeds develop in cones (continued)
–The pollen tube releases sperm when it
comes into contact with an egg
–Fertilization occurs and the seed develops
–Seeds are released when the cone is mature
–The seeds are dispersed by the wind
–The seeds germinate to form sporophyte trees
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Life Cycle of the Pine
scale of a
male cone
MEIOTIC CELL
DIVISION IN
MALE SCALE
male cone
mature
sporophyte
1 Male cone
scales give rise to
pollen; each female
cone scale contains
two ovules
2 Pollen
is liberated
and carried
by the wind
scale of a
female cone
ovule
spore-forming
cell
3 Pollen lands
on the scale of a
female cone and
a pollen tube
begins to grow
MEIOTIC CELL
DIVISION IN
FEMALE SCALE
female cone
female
gametophyte
seedlings
7 The seed
germinates and the
embryo develops into
a sporophyte tree
6 The fertilized egg
develops into an embryo,
which is encased in a seed
seed
4 As the pollen tube grows,
meiotic cell division in the ovule
leads to development of the
female gametophyte
eggs
pollen
tube
5 When the pollen tube
reaches an egg within
the female gametophyte,
a sperm nucleus moves
through the tube and
fertilization occurs
embryo
FERTILIZATION
haploid (n)
sperm
nucleus
diploid (2n)
Fig. 21-11
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Gymnosperms
Fig. 21-10
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Angiosperms are flowering seed plants
– Angiosperms produce flowers and fruits
– They are the most diverse and widespread of all
plants, with more than 230,000 species
– They have a broad range in size
–The smallest is duckweed (3 mm in diameter)
–The largest is the eucalyptus tree (100 meters
in height)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Angiosperms
Fig. 21-12
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Angiosperms are flowering seed plants
(continued)
– Three major adaptations have contributed to the
dominance of angiosperms
–Flowers
–Fruits
–Broad leaves
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Flowers attract pollinators
– Flowers are reproductive structures in which
both male and female gametophytes are formed
– Flowers are 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
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Flowers attract pollinators (continued)
– Flowers develop on the dominant sporophyte
plant
–Male gametophytes (pollen) develop inside
anthers
–The female gametophyte develops from an
ovule inside the ovary
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Flowers attract pollinators (continued)
– Pollination occurs when a pollen grain lands on
the stigma of a flower
– Fertilization occurs when the growing pollen tube
releases sperm into the ovule
– The fertilized ovule develops into a seed
– The seed is dispersed and germinates to form a
sporophyte plant
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Life Cycle of a Flowering Plant
pollen grain
stigma
pollen tube
MEIOTIC CELL
DIVISION
flower
stigma
3 After
pollination,
a pollen
tube begins
to grow
anther
ovule
ovary
1 Cells in anthers give
rise to pollen; in the
ovary, each ovule will
give rise to a female
gametophyte
2 Pollen is liberated
and carried by the
wind or by an animal
egg
spore-forming
cell
MEIOTIC CELL
DIVISION
ovule
mature
sporophyte
diploid (2n)
Biology: Life on Earth, 9e
sperm
nuclei
egg
4 When the pollen tube
reaches the egg within the
female gametophyte, sperm
nuclei move through the
tube and fertilization occurs
6 The seed germinates
and the enclosed
embryo develops into a
sporophyte plant
seedling
haploid (n)
female
gametophyte
female
gametophyte
FERTILIZATION
5 In each ovule, a fertilized
egg gives rise to an embryo,
which is enclosed in a seed
fruit
seed
embryo
Fig. 21-13
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 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 the digestive tract unharmed)
–Burrs cling to animal fur
–Winged fruits are carried through the air
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Author Animation: Flowers and Fruits
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Broad leaves capture more sunlight
– The broad leaves of angiosperms collect more
sunlight for photosynthesis than the narrow
leaves of gymnosperms
– Temperate angiosperms drop their leaves to
conserve water when it is in short supply (fall,
winter)
– Tropical and subtropical angiosperms are
evergreen
–They may shed leaves during the dry season
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Broad leaves capture more sunlight (continued)
– The photosynthetic advantage is offset by the
fact that broad, tender leaves are more
appealing to herbivores than the tough, waxy
needles of conifers
– Angiosperm defenses include:
–Physical defenses (thorns, spines, resins)
–Chemical defenses (plant tissue is poisonous
or distasteful)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 Broad leaves capture more sunlight (continued)
– Many defensive compounds have been exploited
by humans for medicinal and culinary uses
–Medicines (aspirin, codeine)
–Stimulants (nicotine, caffeine)
–Spicy flavors (mustard, peppermint)
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 More recently evolved plants have smaller
gametophytes
– Earliest plants are similar to today’s nonvascular
plants
–The gametophyte dominates
–The small, dependent sporophyte is attached
to the larger gametophyte
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
21.5 What Are the Major Groups of Plants?
 More recently evolved plants have smaller
gametophytes (continued)
– Later plants (seedless vascular plants)
–The sporophyte dominates
–The small gametophyte is independent of the
sporophyte
– Most recently evolved plants (seed plants)
–The sporophyte dominates
–The microscopic, dependent gametophyte is
attached to the larger sporophyte
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.