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The Land Plants
Chapter 23 Part 1
Impacts, Issues
Beginnings and Endings
 Land plants provide oxygen, food, and shelter to
humans and animals, but human activities are
pushing some species toward extinction
23.1 Evolution on
a Changing World Stage
 Changes in atmospheric conditions and shifts in
positions of continents affected the evolutionary
history of land plants
Evolution of Land Plants
 Land plants (embryophytes) evolved from a
lineage of green algae (charophytes) after the
ozone layer made life on land possible
red algae
chlorophytes
charophytes
land plants
p. 370
Evolution of Land Plants
 Bryophytes include three early diverging land
plant lineages
 Nonvascular seedless plants evolved next
 The first seed plants were gymnosperms, from
which angiosperms (flowering plants) evolved
Evolutionary Tree for Land Plants
Fig. 23-2a, p. 370
bryophytes
seedless
vascular
plants
gymnosperms
angiosperms
(flowering
plants)
Seed
Plants
Vascular
Plants
Land
Plants
ancestral green alga
Fig. 23-2a, p. 370
bryophyte (moss)
Fig. 23-2b (1), p. 370
seedless vascular plant (fern)
Fig. 23-2b (2), p. 370
gymnosperm (conifer)
Fig. 23-2b (3), p. 370
angiosperm (monocot)
Fig. 23-2b (4), p. 370
Diversity of Modern Land Plants
Timeline for Plant Evolution
 Continental movements that caused global
climate to become drier favored groups that
were better adapted to drought (seed plants)
•
•
•
•
•
Silurian: Small seedless vascular plants
Carboniferous: Large seedless vascular plants
Devonian: First seeds (gymnosperms)
Permian-Jurassic: Drought-tolerant conifers
Jurassic-Cretaceous: Flowering plants
Timeline for Plant Evolution
Gondwana 425
Origin of
first land
plants
(bryophytes)
by 475 mya.
Origin of
seedless
vascular
plants.
Ordovician Silurian
488
443
425 mya
Bryophytes
and seedless
vascular
plants
diversify.
Seed plants
arise by 385
mya.
Devonian
416
342 mya
Tree-sized
lycophytes
and horsetails
live in swamp
forests. First
conifers
arise late in
Carboniferous.
Carboniferous
359
Pangea 255 mya
Ginkgos,
cycads
appear. Most
horsetails and
lycophytes
disappear by
the end of
the Permian.
Permian
299
Adaptive
radiations of
ferns, cycads,
conifers;
by start of
Cretaceous,
conifers are
dominant trees.
65 mya
Flowering plants
appear in the early
Cretaceous,
undergo adaptive
radiation and
become dominant.
Triassic Jurassic Cretaceous Tertiary
251
200
146
66
Millions of years ago (mya)
Fig. 23-3, p. 371
Early Seedless Vascular Plants
 Cooksonia and Psilophyton
23.2 Evolutionary Trends Among Plants
 Over time, the spore-producing bodies of plants
became larger, more complex, and better
adapted to dry habitats
From Haploid to Diploid Dominance
 Plants shifted from gametophyte-dominated life
cycle (in bryophytes) to sporophyte-dominated
life cycle (in other plants)
• Gametophyte: Haploid stage that forms gametes
by mitosis
• Sporophyte: Diploid stage that forms spores by
meiosis; a sporangium helps protect and
disperse spores
Life Cycle of Land Plants
Fig. 23-5a, p. 372
mitosis
multicelled
sporophyte
(2n)
zygote
(2n)
fertilization
DIPLOID
meiosis
HAPLOID
gametes
(n)
mitosis
multicelled
gametophyte
(n)
spores
(n)
mitosis
Fig. 23-5a, p. 372
Fig. 23-5b, p. 372
zygote is only
diploid phase
green algae
bryophytes
ferns
gymnosperms
angiosperms
Fig. 23-5b, p. 372
Animation: Haploid to diploid dominance
Roots, Stems, and Leaves
 Life on land favored water conserving features
• Cuticle: Waxy layer that restricts evaporation
• Stomata: Openings across the cuticle
cuticle (waxy layer
at leaf surface)
Fig. 23-6a, p. 373
stoma (opening
across cuticle)
Fig. 23-6b, p. 373
Roots, Stems, and Leaves
 In vascular plants, a system of vascular tissue
reinforced by lignin distributes materials through
leaves, stems, and roots of sporophytes
• Xylem: Distributes water and minerals
• Phloem: Distributes products of photosynthesis
Vascular Tissues
xylem
Fig. 23-7a, p. 373
xylem
phloem
Fig. 23-7b, p. 373
Pollen and Seeds
 Bryophytes and seedless plants release spores
 Only seed-bearing vascular plants release
pollen grains and seeds
• Pollen grain: A walled, immature gametophyte
that will give rise to the sperm
• Seed: An embryo sporophyte and some nutritive
tissue enclosed inside a waterproof coat
Two Lineages of
Seed-Bearing Vascular Plants
 Gymnosperms
• Cycads, conifers, ginkgos, and gnetophytes
 Angiosperms (flowering plants)
• Most modern plants
• Seeds form inside floral tissue that later develops
into a fruit
Dispersal Methods: Spores and Seeds
23.1-23.2 Key Concepts
Milestones in Plant Evolution
 The earliest known plants date from 475 million
years ago
 Ever since then, environmental changes have
triggered divergences, adaptive radiations, and
extinctions
 Structural and functional adaptations of lineages
are responses to some of the changes
23.3 The Bryophytes
 Bryophytes include three land plant lineages –
liverworts, hornworts, and mosses – with a
gametophyte-dominated life-cycle
Characteristics of Bryophytes
 Nonvascular (no xylem or phloem)
 Sperm swim through water to eggs
 The sporophyte forms on, and is nourished by,
the dominant gametophyte
 Spores are the dispersal form
Liverworts
 Gametophyte has a thallus that attaches to soil
or surfaces by rootlike rhizoids
 Reproduces sexually, or asexually by producing
gemmae in cups on the gametophyte
Liverwort: Marchantia
thallus (leaflike part) with gemmae cups
Fig. 23-9a, p. 374
asexually-produced gemmae in cup
Fig. 23-9b, p. 374
sperm-producing structure of a male plant
Fig. 23-9c, p. 374
egg-producing structure of a female plant
Fig. 23-9d, p. 374
Hornworts
 Have a hornlike sporophyte with its base
embedded in gametophyte tissue; spores form
in an upright capsule (sporangium)
• Sporophyte has chloroplasts
• Gametophyte has nitrogen-fixing bacteria
sporophyte
gametophyte
p. 374
Mosses
 Sporophyte consists of a capsule (sporangium)
embedded in gametophyte
 Life cycle
• Haploid spores form in the capsule by meiosis,
germinate, and develop into gametophytes
• Eggs and sperm form in gametangia
• After fertilization, zygote develops into sporophyte
Moss Life Cycle: Polytrichum
G Zygote grows, develops
into a sporophyte while still
attached to gametophyte.
A Mature moss sporophyte
consists of a capsule atop
a stalk. It is still attached
to the gametophyte.
sporophyte
zygote
Diploid Stage
Haploid Stage
gametophyte
fertilization
F Drops of rain
disperse sperm,
which swim to
eggs and fertilize
them.
spermproducing
antheridium
D Sperm
form at tips
of male
gametophyte.
eggproducing
archegonium
E Eggs form at tip of
female gametophyte.
rhizoids
meiosis
B Meiosis of cells
inside the capsule
forms spores, which
are released when the
capsule pops open.
male
female
immature
gametophytes
C Spores germinate, grow, and
develop into gametophytes.
Fig. 23-10, p. 375
G Zygote grows, develops
into a sporophyte while still attached
to gametophyte.
A Mature moss sporophyte
consists of a capsule atop a
stalk. It is still attached to the
gametophyte.
sporophyte
zygote
Diploid Stage
gametophyte Haploid Stage
fertilization
F Drops of rain
disperse sperm, which spermswim to
producing
eggs and fertilize them. antheridium
rhizoids
D Sperm
form at tips
of male
gametophyte.
meiosis
B Meiosis of cells
inside the capsule
forms spores, which
are released when the
capsule pops open.
male
eggproducing
archegonium
E Eggs form at tip of female
gametophyte.
female
immature
gametophytes
C Spores germinate, grow, and
develop into gametophytes.
Stepped Art
Fig. 23-10, p. 375
Animation: Moss life cycle
Importance of Mosses
 Mosses are the most diverse group of
bryophytes; peat mosses (Sphagnum) are
ecologically and commercially important
Fig. 23-11a, p. 375
sporophyte
gametophyte
Fig. 23-11b, p. 375
23.3 Key Concepts
Early-Diverging Plant Lineages
 Three plant lineages (mosses, hornworts, and
liverworts) are commonly referred to as
bryophytes, although they are not a natural
group
 The gamete-producing stage dominates their life
cycle, and sperm reach the eggs by swimming
through droplets or films of water
23.4 Seedless Vascular Plants
 A sporophyte with lignified vascular tissue
(xylem and phloem) dominates the life cycle
 Two lineages of seedless vascular plants
• Lycophytes (club mosses, spike mosses)
• Monilophytes (whisk ferns, horsetails, ferns)
Some Lycophytes
 Club moss (Lycopodium)
• Spores form inside a soft, cone-shaped strobilus
 Spike moss (Selaginella) “resurrection plant”
• The most drought-tolerant vascular plants
Whisk Ferns
 Whisk ferns (Psilotum)
• Have rhizomes (underground stems) but no
roots
• Photosynthetic stems appear leafless
• Spores form in fused sporangia at tips of
branches
Horsetails
 Horsetails (Equisetum)
• Have rhizomes, hollow stems with silica deposits,
and nonphotosynthetic leaves
• Photosynthesis occurs in stems and leaflike
branches
• Spores in strobili form tiny gametophytes
Seedless Vascular Plants
 Club moss (Lycopodium), whisk fern (Psilotum),
and horsetails (Equisetum)
strobilus
Fig. 23-12a, p. 376
sporangia
Fig. 23-12b, p. 376
leaflike branch
Fig. 23-12c, p. 376
strobilus
Fig. 23-12d, p. 376
Ferns – No Seeds, Much Diversity
 Ferns
• The most diverse seedless vascular plants
• Most sporophytes have leaves and roots that
grow out from rhizomes
• Spores are dispersed from clusters of sporangia
(sori) on lower surfaces of frond leaves
• Many live as epiphytes attached to another plant
Fern Life Cycle: Woodwardia
The sporophyte
(still attached to
the gametophyte)
grows, develops.
zygote
rhizome
Diploid Stage
fertilization
egg
sperm
meiosis
Haploid Stage
eggproducing
archegonium
spermproducing
antheridium
sorus
Spores develop.
mature
gametophyte
(underside)
Spores are
released.
C
A spore
germinates,
grows into a
gametophyte.
Stepped Art
Fig. 23-13, p. 377
The sporophyte
(still attached to
the gametophyte)
grows, develops.
zygote
rhizome
sorus
Diploid Stage
fertilization
egg
sperm
Haploid Stage
eggproducing
archegonium
spermproducing
antheridium
meiosis
Spores develop.
mature
gametophyte
(underside)
Spores are
released.
A spore
germinates,
grows into a
gametophyte.
Fig. 23-13, p. 377
Animation: Fern life cycle
Fern Diversity
Fig. 23-14a, p. 377
Fig. 23-14b, p. 377
Fig. 23-14c, p. 377
Animation: Alternations of generations
Animation: Evolutionary tree for plants
Animation: Marchantia, a liverwort
Animation: Milestones in plant evolution
Animation: Seedless vascular plants