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

p. 370

red algae chlorophytes charophytes land plants

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

Fig. 23-2a, p. 370

seedless

vascular

plants

angiosperms

(flowering

plants)bryophytes gymnosperms

Seed

Plants

Vascular

Plants

Land

Plants

ancestral green alga

Fig. 23-2b (1), p. 370bryophyte (moss)

Fig. 23-2b (2), p. 370seedless vascular plant (fern)

Fig. 23-2b (3), p. 370gymnosperm (conifer)

Fig. 23-2b (4), p. 370angiosperm (monocot)

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

Fig. 23-3, p. 371

Gondwana 425 425 mya 342 mya Pangea 255 mya 65 mya

Origin of

first land

plants

(bryophytes)

by 475 mya.

Origin of

seedless

vascular

plants.

Bryophytes

and seedless

vascular

plants

diversify.

Seed plants

arise by 385

mya.

Tree-sized

lycophytes

and horsetails

live in swamp

forests. First

conifers

arise late in

Carboniferous.

Ginkgos,

cycads

appear. Most

horsetails and

lycophytes

disappear by

the end of

the Permian.

Adaptive

radiations of

ferns, cycads,

conifers;

by start of

Cretaceous,

conifers are

dominant trees.

Flowering plants

appear in the early

Cretaceous,

undergo adaptive

radiation and

become dominant.

Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Tertiary

Millions of years ago (mya)

488 443 416 359 299 251 146 66200

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

Fig. 23-5a, p. 372

mitosismulticelled

sporophyte

(2n)

zygote

(2n)

fertilizationDIPLOID

meiosis

HAPLOID

gametes

(n)

spores

(n)multicelled

gametophyte

(n)mitosis mitosis

Fig. 23-5b, p. 372

Fig. 23-5b, p. 372

zygote is only

diploid phase

green algae ferns gymnosperms angiospermsbryophytes

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

Fig. 23-6a, p. 373

cuticle (waxy layer

at leaf surface)

Fig. 23-6b, p. 373

stoma (opening

across cuticle)

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

Fig. 23-7a, p. 373

xylem

Fig. 23-7b, p. 373

xylem phloem

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

Fig. 23-9a, p. 374

thallus (leaflike part) with gemmae cups

Fig. 23-9b, p. 374

asexually-produced gemmae in cup

Fig. 23-9c, p. 374

sperm-producing structure of a male plant

Fig. 23-9d, p. 374

egg-producing structure of a female plant

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

p. 374

sporophyte

gametophyte

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

Fig. 23-10, p. 375

A Mature moss sporophyte

consists of a capsule atop

a stalk. It is still attached

to the gametophyte.

G Zygote grows, develops

into a sporophyte while still

attached to gametophyte.

sporophytezygote

fertilization meiosisDiploid Stage

Haploid Stagegametophyte

B Meiosis of cells inside the capsule forms spores, which are released when the capsule pops open.

F Drops of rain

disperse sperm,

which swim to

eggs and fertilize

them.rhizoids

sperm-producing antheridium

maleD Sperm form at tips of male gametophyte. immature

gametophytes egg-

producing

archegonium

female

E Eggs form at tip of

female gametophyte.

C Spores germinate, grow, and

develop into gametophytes.

Fig. 23-10, p. 375

meiosis

B Meiosis of cells inside the capsule forms spores, which are released when the capsule pops open.

male

sperm-producing antheridium

D Sperm form at tips of male gametophyte.

femaleegg-

producing

archegonium

E Eggs form at tip of female

gametophyte.

zygote

fertilization

F Drops of rain

disperse sperm, which

swim to

eggs and fertilize them.

G Zygote grows, develops

into a sporophyte while still attached

to gametophyte.

Stepped Art

C Spores germinate, grow, and

develop into gametophytes.

immature gametophytes

A Mature moss sporophyte

consists of a capsule atop a

stalk. It is still attached to the

gametophyte.

sporophyte

gametophyte

rhizoids

Diploid Stage

Haploid Stage

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

Fig. 23-11b, p. 375

sporophyte

gametophyte

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)

Fig. 23-12a, p. 376

strobilus

Fig. 23-12b, p. 376

sporangia

Fig. 23-12c, p. 376

leaflike branch

Fig. 23-12d, p. 376

strobilus

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

fertilization

zygote

Diploid Stagerhizome

Fig. 23-13, p. 377

egg-

producing

archegonium

egg

sperm-

producing

antheridium

A spore

germinates,

grows into a

gametophyte.

sperm

mature

gametophyte

(underside)

C

Haploid Stagemeiosis

Spores are

released.Spores develop.

The sporophyte

(still attached to

the gametophyte)

grows, develops.

Stepped Art

sorus

Fig. 23-13, p. 377

The sporophyte

(still attached to

the gametophyte)

grows, develops.

zygote rhizome sorusDiploid Stage

fertilizationHaploid Stage

meiosis Spores are

released.egg-

producing

archegoniumegg

Spores develop.

mature

gametophyte

(underside)sperm-

producing

antheridium

A spore

germinates,

grows into a

gametophyte.

sperm

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