video slide

PowerPoint Lectures forBiology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 29

Plant Diversity IHow PlantsColonized Land

Overview: The Greening of Earth

•Looking at a lush landscape, it is difficult toimagine the land without any plants or otherorganisms

•For more than the first 3 billion years of Earth’shistory, the terrestrial surface was lifeless

•Since colonizing land, plants have diversified intoroughly 290,000 living species

Concept 29.1: Land plants evolved from greenalgae

•Green algae called charophyceans are the closestrelatives of land plants

Morphological and Biochemical Evidence

•Many characteristics of land plants also appear ina variety of algal clades, mainly algae

•However, land plants share four key traits onlywith charophyceans:

–Rose-shaped complexes for cellulosesynthesis

–Peroxisome enzymes

–Structure of flagellated sperm

–Formation of a phragmoplast

Genetic Evidence

•Comparisons of both nuclear and chloroplastgenes point to charophyceans as the closest livingrelatives of land plants

LE 29-3

10 mm

40 µm

Chara,

a pond

organism

(LM).

Coleochaete orbicularis, a disk-shaped

charophycean (LM).

Adaptations Enabling the Move to Land

•In charophyceans a layer of a durable polymercalled sporopollenin prevents exposed zygotesfrom drying out

•The accumulation of traits that facilitated survivalon land may have opened the way to itscolonization by plants

Concept 29.2: Land plants possess a set of derivedterrestrial adaptations

•Many adaptations emerged after land plantsdiverged from their charophycean relatives

Defining the Plant Kingdom

•Systematists are currently debating theboundaries of the plant kingdom

•Some biologists think the plant kingdom should beexpanded to include some or all green algae

•Until this debate is resolved, we will retain theembryophyte definition of kingdom Plantae

LE 29-4

Viridiplantae

Streptophyta

Plantae

Red algae

Chlorophytes

Charophyceans

Embryophytes

Ancestral alga

Derived Traits of Plants

•Five key traits appear in nearly all land plants butare absent in the charophyceans:

–Apical meristems

–Alternation of generations

–Walled spores produced in sporangia

–Multicellular gametangia

–Multicellular dependent embryos

LE 29-5a

Apical

Meristem

of shoot

Developing

leaves

Shoot

Root

Apical meristem

100 µm

Apical Meristems

LE 29-5b

Mitosis

Alternation of Generations

Spores

Mitosis

Zygote

Gametes

Haploid multicellular

organism (gametophyte)

Diploid multicellular

organism (sporophyte)

MEIOSIS

FERTILIZATION

LE 29-5c

Walled Spores

Produced in Sporangia

Multicellular

Gametangia

Multicellular,

Dependent Embryos

Longitudinal section of

Sphagnum sporangium (LM)

Spores

Sporangium

Sporophyte

Gametophyte

Sporophyte and sporangium

of Sphagnum (a moss)

Archegonia and antheridia

of Marchantia (a liverwort)

Male gametophyte

Antheridium

with sperm

Female gametophyte

Archegonium

with egg

Maternal

tissue

Embryo

2 µm

10 µm

Wall

ingrowths

Placental

transfer

cell

•Additional derived traits such as a cuticle andsecondary compounds evolved in many plantspecies

The Origin and Diversification of Plants

•Fossil evidence indicates that plants were on landat least 475 million years ago

•Fossilized spores and tissues have been extractedfrom 475-million-year-old rocks

LE 29-6

Fossilized spores.Unlike the spores ofmost living plants,which are singlegrains, these sporesfound in Oman are ingroups of four (left;one hidden) and two(right).

Fossilizedsporophyte tissue.The spores wereembedded in tissuethat appears to befrom plants.

•Those ancestral species gave rise to a vastdiversity of modern plants

•Land plants can be informally grouped based onthe presence or absence of vascular tissue

LE 29-7

Ancestral

green alga

Origin of land plants

(about 475 mya)

Origin of vascular plants

(about 420 mya)

Origin of seed plants

(about 360 mya)

Land plants

Vascular plants

Seed plants

Seedless vascular plants

Bryophytes

Liverworts

Hornworts

Mosses

Lycophytes

Pterophytes

Gymno-

sperms

Angio-

sperms

Charophyceans

Concept 29.3: The life cycles of mosses and otherbryophytes are dominated by the gametophyte stage

•Bryophytes are represented today by three phylaof small herbaceous (nonwoody) plants:

–Liverworts, phylum Hepatophyta

–Hornworts, phylum Anthocerophyta

–Mosses, phylum Bryophyta

•Debate continues over the sequence of bryophyteevolution

•Mosses are most closely related to vascular plants

Bryophyte Gametophytes

•In all three bryophyte phyla, gametophytes arelarger and longer-living than sporophytes

•Sporophytes are typically present only part of thetime

Animation: Moss Life Cycle Animation: Moss Life Cycle

LE 29-8

Male

gametophyte

“Bud”

Spores develop into

threadlike protonemata.

Protonemata

“Bud”

The haploidprotonemataproduce “buds”that grow intogametophytes.

Raindrop

Sperm

Antheridia

Most mosses have separatemale and female gametophytes,with antheridia and archegonia,respectively.

Egg

Haploid (n)

Diploid (2n)

Key

A sperm swimsthrough a film ofmoisture to anarchegonium andfertilizes the egg.

Archegonia

Rhizoid

Female

gametophyte

Gametophore

Spores

Sporangium

Peristome

MEIOSIS

Meiosis occurs and haploidspores develop in the sporangiumof the sporophyte. When thesporangium lid pops off, theperistome “teeth” regulate gradualrelease of the spores.

The sporophyte grows along stalk, or seta, that emergesfrom the archegonium.

FERTILIZATION

(within archegonium)

Archegonium

Zygote

Embryo

Calyptra

Young

sporophyte

Attached by its foot, thesporophyte remainsnutritionally dependent on thegametophyte.

The diploid zygotedevelops into asporophyte embryo withinthe archegonium.

Capsule

(sporangium)

Seta

Foot

Mature

sporophytes

Capsule with

peristome (SEM)

Female

gametophytes

•Bryophyte gametophytes

–Produce flagellated sperm in antheridia

–Produce ova in archegonia

–Generally form ground-hugging carpets andare at most only a few cells thick

•Some mosses have conducting tissues in thecenter of their “stems” and may grow vertically

Bryophyte Sporophytes

•Bryophyte sporophytes

–Grow out of archegonia

–Are the smallest and simplest of all extantplant groups

–Consist of a foot, a seta, and a sporangium

•Hornwort and moss sporophytes have stomata

LE 29-9a

Gametophore of

female gametophyte

Marchantia polymorpha,

a “thalloid” liverwort

Foot

Seta

Sporangium

500 µm

Marchantia sporophyte (LM)

LE 29-9b

Plagiochila

deltoidea,

a “leafy”

liverwort

LE 29-9c

An Anthroceros

hornwort species

Sporophyte

Gametophyte

LE 29-9d

Polytrichum

commune,

hairy cap

moss

Sporophyte

Gametophyte

Ecological and Economic Importance of Mosses

•Sphagnum, or “peat moss,” forms extensivedeposits of partially decayed organic materialknown as peat

•Sphagnum plays an important role in the Earth’scarbon cycle

LE 29-10

A peat bog.

Gametophyte

Sporangium at

tip of sporophyte

Living

photo-

synthetic

cells

Dead water-

storing cells

100 µm

Closeup ofSphagnum.

Note the “leafy”

Gametophytes andtheir offspring, thesporophytes.

Sphagnum “leaf” (LM). Thecombination of living photosyntheticcells and dead water-storing cellsgives the moss its spongy quality.

“Tolland Man,” a bog mummy dating from 405–100 B.C. Theacidic, oxygen-poor conditions produced by Sphagnum canpreserve human or animal bodies for thousands of years.

Concept 29.4: Ferns and other seedless vascularplants formed the first forests

•Bryophytes and bryophyte-like plants were theprevalent vegetation during the first 100 millionyears of plant evolution

•Vascular plants began to diversify during theCarboniferous period

•Vascular plants dominate most landscapes today

Origins and Traits of Vascular Plants

•Fossils of the forerunners of vascular plants dateback about 420 million years

•These early tiny plants had independent,branching sporophytes

•They lacked other derived traits of vascular plants

Life Cycles with Dominant Sporophytes

•In contrast with bryophytes, sporophytes ofseedless vascular plants are the larger generation,as in the familiar leafy fern

•The gametophytes are tiny plants that grow on orbelow the soil surface

Animation: Fern Life Cycle Animation: Fern Life Cycle

LE 29-12

Spore

Sperm

Antheridium

Egg

Haploid (n)

Diploid (2n)

Key

Young

gametophyte

Sorus

Sporangium

MEIOSIS

FERTILIZATION

Archegonium

Zygote

New

sporophyte

Mature

sporophyte

Sporangium

Gametophyte

Fiddlehead

Transport in Xylem and Phloem

•Vascular plants have two types of vascular tissue:xylem and phloem

•Xylem conducts most of the water and mineralsand includes dead cells called tracheids

•Phloem consists of living cells and distributessugars, amino acids, and other organic products

Evolution of Roots

•Roots are organs that anchor vascular plants

•They enable vascular plants to absorb water andnutrients from the soil

•Roots may have evolved from subterranean stems

Evolution of Leaves

•Leaves are organs that increase the surface areaof vascular plants, thereby capturing more solarenergy that is used for photosynthesis

•Leaves are categorized by two types:

–Microphylls, leaves with a single vein

–Megaphylls, leaves with a highly branchedvascular system

•According to one model of evolution, microphyllsevolved first, as outgrowths of stems

LE 29-13

Vascular tissue

Microphylls

Megaphylls

Sporophylls and Spore Variations

•Sporophylls are modified leaves with sporangia

•Most seedless vascular plants are homosporous,producing one type of spore that develops into abisexual gametophyte

•All seed plants and some seedless vascular plantsare heterosporous, having two types of sporesthat give rise to male and female gametophytes

Classification of Seedless Vascular Plants

•There are two phyla of seedless vascular plants:

–Lycophyta includes club mosses, spikemosses, and quillworts

–Pterophyta includes ferns, horsetails, andwhisk ferns and their relatives

LE 29-14a

Selaginella apoda,

a spike moss

LE 29-14b

Isoetes

gunnii,

a quillwort

LE 29-14c

Diphasiastrum tristachyum, a club moss

Strobili

(clusters of

sporophyllis)

LE 29-14d

Psilotum

nudum,

a whisk

fern

LE 29-14e

Equisetum

arvense,

field

horsetail

Vegetative stem

Strobilus on

fertile stem

LE 29-14f

Athyrium filix-femina, lady fern

Phylum Lycophyta: Club Mosses, Spike Mosses, andQuillworts

•Giant lycophytes thrived for millions of years inmoist swamps

•Surviving species are small herbaceous plants

Phylum Pterophyta: Ferns, Horsetails, and WhiskFerns and Relatives

•Ferns are the most diverse seedless vascularplants, with more than 12,000 species

•They are most diverse in the tropics but alsothrive in temperate forests

•Some species are even adapted to aridclimates

The Significance of Seedless Vascular Plants

•The ancestors of modern lycophytes, horsetails,and ferns grew to great heights during theCarboniferous, forming the first forests

•These forests may have helped produce theglobal cooling at the end of the Carboniferousperiod

•The decaying plants of these Carboniferousforests eventually became coal