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Chapter 26
• The Colonization of Land by Plants and Fugi
Overview: The Greening of Earth
• For more than the first 3 billion years of Earth’s history, the terrestrial surface was lifeless
• Cyanobacteria likely existed on land 1.2 billion years ago
• Around 500 million years ago, small plants, fungi, and animals emerged on land
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• Since colonizing land, plants have diversified into roughly 290,000 living species
• Land plants are defined as having terrestrial ancestors, even though some are now aquatic
• Land plants do not include photosynthetic protists (algae)
• Plants supply oxygen and are the ultimate source of most food eaten by land animals
© 2011 Pearson Education, Inc.
1 m
Figure 29.1
Land plants evolved from green algae
• Green algae called charophytes are the closest relatives of land plants
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Morphological and Molecular Evidence
• Many characteristics of land plants also appear in a variety of algal clades, mainly algae
• However, land plants share four key traits with only charophytes
– Rings of cellulose-synthesizing complexes– Peroxisome enzymes– Structure of flagellated sperm– Formation of a phragmoplast
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1 m
Figure 29.2
30 nm
• Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives of land plants
• Note that land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes
© 2011 Pearson Education, Inc.
1 m
Figure 29.3
Chara species, a pond organism
Coleochaete orbicularis, adisk-shaped charophytethat also lives in ponds (LM)
40 m
5 mm
Adaptations Enabling the Move to Land
• In charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying out
• Sporopollenin is also found in plant spore walls• The movement onto land by charophyte ancestors
provided unfiltered sun, more plentiful CO2, nutrient-rich soil, and few herbivores or pathogens
• Land presented challenges: a scarcity of water and lack of structural support
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• The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plants
• Systematists are currently debating the boundaries of the plant kingdom
• Some biologists think the plant kingdom should be expanded to include some or all green algae
• Until this debate is resolved, we define plants as embryophytes, plants with embryos
© 2011 Pearson Education, Inc.
1 m
Figure 29.4
Red algae
Chlorophytes
Charophytes
Embryophytes
ANCESTRALALGA
Virid
iplan
taeStrep
top
hyta
Plan
tae
Derived Traits of Plants
• Four key traits appear in nearly all land plants but are absent in the charophytes
– Alternation of generations and multicellular, dependent embryos
– Walled spores produced in sporangia– Multicellular gametangia– Apical meristems
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Alternation of Generations and Multicellular, Dependent Embryos
• Plants alternate between two multicellular stages, a reproductive cycle called alternation of generations
• The gametophyte is haploid and produces haploid gametes by mitosis
• Fusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis
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• The diploid embryo is retained within the tissue of the female gametophyte
• Nutrients are transferred from parent to embryo through placental transfer cells
• Land plants are called embryophytes because of the dependency of the embryo on the parent
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1 m
Figure 29.5a
Gamete fromanother plant
Key
Haploid (n)
Diploid (2n)Gametophyte(n)
Mitosis Mitosis
Spore Gamete
MEIOSIS FERTILIZATION
Zygote
MitosisSporophyte(2n)
Alternation of generations
2n
n
n n
n
1 m
Figure 29.5b
Embryo
Maternal tissue
Embryo (LM) andplacental transfer cell (TEM)of Marchantia (a liverwort)
Wall ingrowths
Placental transfercell (outlined inblue)10 m
2 m
Walled Spores Produced in Sporangia• The sporophyte produces spores in organs called
sporangia• Diploid cells called sporocytes undergo meiosis
to generate haploid spores• Spore walls contain sporopollenin, which makes
them resistant to harsh environments
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1 m
Figure 29.5c
SporesSporangium
Longitudinal section ofSphagnum sporangium (LM)
Sporophyte
Gametophyte
Sporophytes and sporangia of Sphagnum (a moss)
Multicellular Gametangia• Gametes are produced within organs called
gametangia• Female gametangia, called archegonia, produce
eggs and are the site of fertilization• Male gametangia, called antheridia, produce and
release sperm
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1 m
Figure 29.5d
Femalegametophyte
Malegametophyte
Archegonia,each with anegg (yellow)
Antheridia(brown),containing sperm
Archegonia and antheridia of Marchantia (a liverwort)
1 m
Figure 29.5e
Apical meristemof shoot
Developingleaves
Shoot100 m100 mRoot
Apicalmeristemof root
Apical meristems of plantroots and shoots
• Additional derived traits include Cuticle, a waxy covering of the epidermis Mycorrhizae, symbiotic associations between
fungi and land plants that may have helped plants without true roots to obtain nutrients
Secondary compounds that deter herbivores and parasites
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The Origin and Diversification of Plants
• Fossil evidence indicates that plants were on land at least 475 million years ago
• Fossilized spores and tissues have been extracted from 475-million-year-old rocks
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1 m
Figure 29.6
(a) Fossilizedspores
Fossilizedsporophytetissue
(b)
• Those ancestral species gave rise to a vast diversity of modern plants
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1 m
Figure 29.7
Origin of land plants (about 475 mya)
Origin of vascular plants (about 425 mya)
Origin of extant seed plants (about 305 mya)
2
1
3
2
1ANCESTRALGREENALGA
500 450 400 350 300 50 0
Millions of years ago (mya)
Liverworts
Mosses
Hornworts
Lycophytes (clubmosses, spikemosses, quillworts)
Pterophytes (ferns,horsetails, whisk ferns)
Gymnosperms
Angiosperms
La
nd
pla
nts
Va
sc
ula
r pla
nts
No
nv
as
cu
lar
pla
nts
(bry
op
hy
tes
)
Se
ed
les
sv
as
cu
lar
pla
nts
Se
ed
pla
nts
3
1 m
Figure 29.7b
Liverworts
Mosses
Hornworts
Lycophytes (clubmosses, spikemosses, quillworts)
Pterophytes (ferns,horsetails, whisk ferns)
Gymnosperms
Angiosperms
Lan
d p
lants
Vascu
lar plan
ts
No
nvascu
larp
lants
(bryo
ph
ytes)
Seed
lessvascu
larp
lants
Seed
plan
ts
• Land plants can be informally grouped based on the presence or absence of vascular tissue
• Most plants have vascular tissue; these constitute the vascular plants
• Nonvascular plants are commonly called bryophytes
• Bryophytes are not a monophyletic group; their relationships to each other and to vascular plants is unresolved
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• Seedless vascular plants can be divided into clades
– Lycophytes (club mosses and their relatives)– Pterophytes (ferns and their relatives)
• Seedless vascular plants are paraphyletic, and are of the same level of biological organization, or grade
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• A seed is an embryo and nutrients surrounded by a protective coat
• Seed plants form a clade and can be divided into further clades
– Gymnosperms, the “naked seed” plants, including the conifers
– Angiosperms, the flowering plants
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1 m
Table 29. 1
Mosses and other nonvascular plants have life cycles dominated by gametophytes
• Bryophytes are represented today by three phyla of small herbaceous (nonwoody) plants
– Liverworts, phylum Hepatophyta– Hornworts, phylum Anthocerophyta– Mosses, phylum Bryophyta
• Bryophyte refers to all nonvascular plants, whereas Bryophyta refers only to the phylum of mosses
© 2011 Pearson Education, Inc.
1 m
Figure 29.UN01
Nonvascular plants (bryophytes)
Seedless vascular plantsGymnosperms
Angiosperms
Bryophyte Gametophytes
• In all three bryophyte phyla, gametophytes are larger and longer-living than sporophytes
• Sporophytes are typically present only part of the time
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Protonemata(n)
Key
Haploid (n)Diploid (2n)
“Bud”
“Bud”
Malegametophyte(n)
AntheridiaSperm
Egg
ArchegoniaGametophoreSpores
Sporedispersal
Peristome
Sporangium
Femalegametophyte(n) Rhizoid
FERTILIZATION(within archegonium)Zygote
(2n)
Archegonium
Embryo
Seta
Capsule(sporangium)
Foot
Youngsporophyte(2n)
MEIOSIS
Mature sporophytes
2 m
m
Capsule withperistome (LM) Female
gametophytes
1 m
Figure 29.8-3
• A spore germinates into a gametophyte composed of a protonema and gamete-producing gametophore
• The height of gametophytes is constrained by lack of vascular tissues
• Rhizoids anchor gametophytes to substrate• Mature gametophytes produce flagellated sperm
in antheridia and an egg in each archegonium• Sperm swim through a film of water to reach and
fertilize the egg
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Bryophyte Sporophytes
• Bryophyte sporophytes grow out of archegonia, and are the smallest and simplest sporophytes of all extant plant groups
• A sporophyte consists of a foot, a seta (stalk), and a sporangium, also called a capsule, which discharges spores through a peristome
• Hornwort and moss sporophytes have stomata for gas exchange; liverworts do not
© 2011 Pearson Education, Inc.
1 m
Figure 29.9a
Sporophyte
ThallusGametophore offemale gametophyte
Marchantia polymorpha,a “thalloid” liverwort
Marchantia sporophyte (LM)
FootSeta
Capsule(sporangium)
500
mPlagiochila deltoidea, a “leafy” liverwort
1 m
Figure 29.9c
Polytrichum commune,hairy-cap moss
Capsule
Seta
Sporophyte(a sturdyplant thattakes monthsto grow)
Gametophyte
The Ecological and Economic Importance of Mosses
• Mosses are capable of inhabiting diverse and sometimes extreme environments, but are especially common in moist forests and wetlands
• Some mosses might help retain nitrogen in the soil
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• Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material known as peat
• Peat can be used as a source of fuel• Sphagnum is an important global reservoir of
organic carbon• Overharvesting of Sphagnum and/or a drop in
water level in peatlands could release stored CO2 to the atmosphere
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1 m
Figure 29.11
Peat being harvested from apeatland
(a) “Tollund Man,” a bog mummydating from 405–100 B.C.E.
(b)
Ferns and other seedless vascular plants were the first plants to grow tall
• Bryophytes and bryophyte-like plants were the prevalent vegetation during the first 100 million years of plant evolution
• Vascular plants began to diversify during the Devonian and Carboniferous periods
• Vascular tissue allowed these plants to grow tall• Seedless vascular plants have flagellated sperm
and are usually restricted to moist environments
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1 m
Figure 29.UN03
Nonvascular plants (bryophytes)
Seedless vascular plants
Gymnosperms
Angiosperms
Origins and Traits of Vascular Plants
• Fossils of the forerunners of vascular plants date back about 425 million years
• These early tiny plants had independent, branching sporophytes
• Living vascular plants are characterized by Life cycles with dominant sporophytes Vascular tissues called xylem and phloem Well-developed roots and leaves
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1 m
Figure 29.12
Sporangia
Life Cycles with Dominant Sporophytes
• In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation, as in familiar ferns
• The gametophytes are tiny plants that grow on or below the soil surface
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1 m
Figure 29.13-3
Key
Haploid (n)Diploid (2n)
MEIOSISSporedispersal
Spore(n)
Younggametophyte
Rhizoid
Undersideof maturegametophyte(n)
Antheridium
Sperm
Archegonium
Egg
FERTILIZATIONZygote(2n)
Gametophyte
Newsporophyte
Maturesporophyte(2n)
Fiddlehead (young leaf)
Sporangium
Sorus
Sporangium
Transport in Xylem and Phloem
• Vascular plants have two types of vascular tissue: xylem and phloem
• Xylem conducts most of the water and minerals and includes dead cells called tracheids
• Water-conducting cells are strengthened by lignin and provide structural support
• Phloem consists of living cells and distributes sugars, amino acids, and other organic products
• Vascular tissue allowed for increased height, which provided an evolutionary advantage
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Evolution of Roots
• Roots are organs that anchor vascular plants• They enable vascular plants to absorb water and
nutrients from the soil• Roots may have evolved from subterranean stems
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Evolution of Leaves
• Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis
• Leaves are categorized by two types Microphylls, leaves with a single vein Megaphylls, leaves with a highly branched
vascular system
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• According to one model of evolution, microphylls evolved as outgrowths of stems
• Megaphylls may have evolved as webbing between flattened branches
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1 m
Figure 29.14
Vascular tissue Sporangia Microphyll
(a) Microphylls (b) Megaphylls
Overtoppinggrowth
Megaphyll
Otherstemsbecomereducedandflattened.
Webbingdevelops.
Sporophylls and Spore Variations
• Sporophylls are modified leaves with sporangia• Sori are clusters of sporangia on the undersides
of sporophylls• Strobili are cone-like structures formed from
groups of sporophylls
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• Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyte
• All seed plants and some seedless vascular plants are heterosporous
• Heterosporous species produce megaspores, which give rise to female gametophytes, and microspores, which give rise to male gametophytes
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Classification of Seedless Vascular Plants
• There are two phyla of seedless vascular plants– Phylum Lycophyta includes club mosses, spike
mosses, and quillworts– Phylum Pterophyta includes ferns, horsetails, and
whisk ferns and their relatives
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1 m
Figure 29.15a
Selaginellamoellendorffii,a spike moss
Isoetesgunnii,a quillwort
Diphasiastrum tristachyum,a club moss
Strobili(clusters ofsporophylls)
2.5 cm
1 cm
1 m
Figure 29.15b
Athyriumfilix-femina,lady fern
Equisetum arvense,field horsetail
Vegetative stem
Strobilus onfertile stem
Psilotumnudum,a whiskfern
4 cm
25 c
m
1.5
cm
Phylum Lycophyta: Club Mosses, Spike Mosses, and Quillworts
• Giant lycophytes trees thrived for millions of years in moist swamps
• Surviving species are small herbaceous plants• Club mosses and spike mosses have vascular
tissues and are not true mosses
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Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and Relatives
• Ferns are the most diverse seedless vascular plants, with more than 12,000 species
• They are most diverse in the tropics but also thrive in temperate forests
• Horsetails were diverse during the Carboniferous period, but are now restricted to the genus Equisetum
• Whisk ferns resemble ancestral vascular plants but are closely related to modern ferns
© 2011 Pearson Education, Inc.
The Significance of Seedless Vascular Plants
• The ancestors of modern lycophytes, horsetails, and ferns grew to great heights during the Devonian and Carboniferous, forming the first forests
• Increased growth and photosynthesis removed CO2 from the atmosphere and may have contributed to global cooling at the end of the Carboniferous period
• The decaying plants of these Carboniferous forests eventually became coal
© 2011 Pearson Education, Inc.
Fern Lycophyte trees Horsetail
Tree trunkcovered withsmall leaves
Lycophyte treereproductivestructures
1 m
Figure 29.16
1 m
Figure 29.UN02
1 m
Figure 29.UN04
Homosporous spore production
Heterosporous spore production
Sporangiumon sporophyll
Singletype of spore
Typically abisexualgametophyte
Eggs
Sperm
Megasporangiumon megasporophyll
Megaspore Femalegametophyte
Malegametophyte
Eggs
SpermMicrosporeMicrosporangiumon microsporophyll
Gametophyte
Mitosis Mitosis
Spore Gamete
MEIOSIS FERTILIZATION
Zygote
Mitosis
Sporophyte
Haploid
Diploid
Alternation of generations
4
21
3
Apical meristemof shoot
Developingleaves
Apical meristems
Archegoniumwith egg
Antheridiumwith sperm
Sporangium Spores
Multicellular gametangia Walled spores in sporangia
n
n n
n
2n
1 m
Figure 29.UN05
1 m
Figure 29.UN06
Overview: Transforming the World
• Seeds changed the course of plant evolution, enabling their bearers to become the dominant producers in most terrestrial ecosystems
• Seed plants originated about 360 million years ago
• A seed consists of an embryo and nutrients surrounded by a protective coat
• Domestication of seed plants had begun by 8,000 years ago and allowed for permanent settlements
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Figure 30.1
Concept 30.1: Seeds and pollen grains are key adaptations for life on land
• In addition to seeds, the following are common to all seed plants
– Reduced gametophytes
– Heterospory
– Ovules
– Pollen
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Advantages of Reduced Gametophytes
• The gametophytes of seed plants develop within the walls of spores that are retained within tissues of the parent sporophyte
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Figure 30.2
PLANT GROUP
Mosses and othernonvascular plants
Gametophyte
Sporophyte
Sporophyte(2n)
Gametophyte(n)
Dominant
Reduced, dependent ongametophyte for nutrition
Dominant
Reduced, independent(photosynthetic andfree-living)
Ferns and other seedlessvascular plants
Dominant
Reduced (usually microscopic), dependent on surroundingsporophyte tissue for nutrition
Seed plants (gymnosperms and angiosperms)
AngiospermGymnosperm
Microscopicfemalegametophytes(n) insidethese partsof flowers
Microscopic femalegametophytes (n) insideovulate cone
Microscopicmalegametophytes(n) insidethese partsof flowers
Microscopic malegametophytes (n)inside pollencone
Sporophyte (2n)Sporophyte (2n)
Sporophyte(2n)
Gametophyte(n)
Example
Heterospory: The Rule Among Seed Plants
• The ancestors of seed plants were likely homosporous, while seed plants are heterosporous
• Megasporangia produce megaspores that give rise to female gametophytes
• Microsporangia produce microspores that give rise to male gametophytes
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Ovules and Production of Eggs
• An ovule consists of a megasporangium, megaspore, and one or more protective integuments
• Gymnosperm megaspores have one integument• Angiosperm megaspores usually have two
integuments
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Figure 30.3-1
Immatureovulate cone
Integument (2n)
Spore wall
Megaspore (n)
(a) Unfertilized ovule
Megasporangium(2n)
Pollen grain (n)Micropyle
Pollen and Production of Sperm
• Microspores develop into pollen grains, which contain the male gametophytes
• Pollination is the transfer of pollen to the part of a seed plant containing the ovules
• Pollen eliminates the need for a film of water and can be dispersed great distances by air or animals
• If a pollen grain germinates, it gives rise to a pollen tube that discharges sperm into the female gametophyte within the ovule
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The Evolutionary Advantage of Seeds
• A seed develops from the whole ovule• A seed is a sporophyte embryo, along with its
food supply, packaged in a protective coat
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Figure 30.3-3
Immatureovulate cone
Integument (2n)
Spore wall
Megaspore (n)
Femalegametophyte (n)
Egg nucleus(n)
Dischargedsperm nucleus(n)
Pollen tubeMale gametophyte (n)
(a) Unfertilized ovule
Megasporangium(2n)
Pollen grain (n)Micropyle
(b) Fertilized ovule (c) Gymnosperm seed
Seedcoat
Sporewall
Foodsupply (n)
Embryo (2n)
• Seeds provide some evolutionary advantages over spores
– They may remain dormant for days to years, until conditions are favorable for germination
– Seeds have a supply of stored food
– They may be transported long distances by wind or animals
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Gymnosperms bear “naked” seeds, typically on cones
• Gymnosperms means “naked seeds”• The seeds are exposed on sporophylls that form
cones• Angiosperm seeds are found in fruits, which are
mature ovaries
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Figure 30.UN01
Nonvascular plants (bryophytes)
Seedless vascular plantsGymnosperms
Angiosperms
Gymnosperm Evolution
• Fossil evidence reveals that by the late Devonian period some plants, called progymnosperms, had begun to acquire some adaptations that characterize seed plants
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Figure 30.4
• Living seed plants can be divided into two clades: gymnosperms and angiosperms
• Gymnosperms appear early in the fossil record about 305 million years ago and dominated Mesozoic (251–65 million years ago) terrestrial ecosystems
• Gymnosperms were better suited than nonvascular plants to drier conditions
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• Angiosperms began to replace gymnosperms near the end of the Mesozoic
• Angiosperms now dominate more terrestrial ecosystems
• Today, cone-bearing gymnosperms called conifers dominate in the northern latitudes
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• The gymnosperm consist of four phyla– Cycadophyta (cycads)
– Gingkophyta (one living species: Ginkgo biloba)
– Gnetophyta (three genera: Gnetum, Ephedra, Welwitschia)
– Coniferophyta (conifers, such as pine, fir, and redwood)
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Phylum Cycadophyta• Individuals have large cones and palmlike leaves• These thrived during the Mesozoic, but relatively
few species exist today
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Figure 30.5a
Cycas revoluta
Phylum Ginkgophyta• This phylum consists of a single living species,
Ginkgo biloba• It has a high tolerance to air pollution and is a
popular ornamental tree
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Ginkgo bilobaleaves andfleshy seeds
Ginkgo biloba pollen-producing tree
Figure 30.5b
Phylum Gnetophyta• This phylum comprises three genera• Species vary in appearance, and some are
tropical whereas others live in deserts
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Figure 30.5d
Ovulate conesGnetum
Ephedra
Welwitschia
Phylum Coniferophyta• This phylum is by far the largest of the
gymnosperm phyla• Most conifers are evergreens and can carry out
photosynthesis year round
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Figure 30.5e
Douglas fir
Common juniper
European larch
Sequoia
Wollemi pine Bristlecone pine
The Life Cycle of a Pine: A Closer Look
• Three key features of the gymnosperm life cycle are
– Dominance of the sporophyte generation
– Development of seeds from fertilized ovules
– The transfer of sperm to ovules by pollen
• The life cycle of a pine provides an example
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• The pine tree is the sporophyte and produces sporangia in male and female cones
• Small cones produce microspores called pollen grains, each of which contains a male gametophyte
• The familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes
• It takes nearly three years from cone production to mature seed
© 2011 Pearson Education, Inc.
Key
Haploid (n)Diploid (2n)
Maturesporophyte(2n)
Pollencone
Microsporocytes(2n)
Microsporangia
Microsporangium (2n)
Pollengrains (n)
MEIOSIS
Figure 30.6-1
Key
Haploid (n)Diploid (2n)
Maturesporophyte(2n)
Ovulatecone
Pollencone
Microsporocytes(2n)
Microsporangia
Microsporangium (2n)Survivingmegaspore (n)
MEIOSIS
Megasporangium (2n)Pollengrain
Pollengrains (n)
MEIOSIS
Megasporocyte (2n)
Integument
Ovule
Figure 30.6-2
Key
Haploid (n)Diploid (2n)
Maturesporophyte(2n)
Ovulatecone
Pollencone
Microsporocytes(2n)
Microsporangia
Microsporangium (2n)
Archegonium
Survivingmegaspore (n)
MEIOSIS
Megasporangium (2n)Pollengrain
Pollengrains (n)
MEIOSIS
Femalegametophyte
Megasporocyte (2n)
Integument
Spermnucleus (n) Egg nucleus (n)
Pollentube
FERTILIZATION
Ovule
Figure 30.6-3
Key
Haploid (n)Diploid (2n)
Maturesporophyte(2n)
Ovulatecone
Pollencone
Microsporocytes(2n)
Microsporangia
Microsporangium (2n)
Seedling
Archegonium
Survivingmegaspore (n)
MEIOSIS
Megasporangium (2n)Pollengrain
Pollengrains (n)
MEIOSIS
Femalegametophyte
Megasporocyte (2n)
Integument
Spermnucleus (n) Egg nucleus (n)
Pollentube
Seed coat (2n)
FERTILIZATION
Foodreserves (n)
Seeds
Embryo(new sporophyte)(2n)
Ovule
Figure 30.6-4
The reproductive adaptations of angiosperms include flowers and fruits
• Angiosperms are seed plants with reproductive structures called flowers and fruits
• They are the most widespread and diverse of all plants
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Figure 30.UN02
Nonvascular plants (bryophytes)
Seedless vascular plantsGymnosperms
Angiosperms
Characteristics of Angiosperms
• All angiosperms are classified in a single phylum, Anthophyta, from the Greek anthos for flower
• Angiosperms have two key adaptations– Flowers
– Fruits
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Flowers
• The flower is an angiosperm structure specialized for sexual reproduction
• Many species are pollinated by insects or animals, while some species are wind-pollinated
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• A flower is a specialized shoot with up to four types of modified leaves
– Sepals, which enclose the flower
– Petals, which are brightly colored and attract pollinators
– Stamens, which produce pollen
– Carpels, which produce ovules
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• A stamen consists of a stalk called a filament, with a sac called an anther where the pollen is produced
• A carpel consists of an ovary at the base and a style leading up to a stigma, where pollen is received
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Stamen Anther
Filament
StigmaCarpel
Style
Ovary
Petal
Sepal
Ovule
Figure 30.7
Fruits
• A fruit typically consists of a mature ovary but can also include other flower parts
• Fruits protect seeds and aid in their dispersal• Mature fruits can be either fleshy or dry
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Animation: Fruit Development
Angiosperm Diversity
• Angiosperms comprise more than 250,000 living species
• Previously, angiosperms were divided into two main groups
– Monocots (one cotyledon)– Dicots (two dicots)
• DNA studies suggest that monocots form a clade, but dicots are polyphyletic
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• The clade eudicot (“true” dicots) includes most dicots
• The rest of the former dicots form several small lineages
• Basal angiosperms are less derived and include the flowering plants belonging to the oldest lineages
• Magnoliids share some traits with basal angiosperms but evolved later
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Basal Angiosperms• Three small lineages constitute the basal
angiosperms• These include Amborella trichopoda, water lilies,
and star anise
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Water lily
Star anise
Amborella trichopoda
Basal AngiospermsFigure 30.13a
Monocots• More than one-quarter of angiosperm species
are monocots
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Orchid
Monocots
Lily
Pygmy date palm
Anther
Filament
Stigma
Ovary
Barley, a grass
Figure 30.13c
Eudicots• More than two-thirds of angiosperm species are
eudicots
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California poppy Dog rose
Pyrenean oak
Snow pea Zucchini
EudicotsFigure 30.13d
Figure 30.13eMonocot
CharacteristicsEudicot
Characteristics
Embryos
One cotyledon Two cotyledons
Leafvenation
Veins usuallyparallel
Veins usuallynetlike
Stems
Vascular tissuescattered
Vascular tissueusually arranged
in ring
Roots
Root systemusually fibrous(no main root)
Taproot (main root)usually present
Pollen
Pollen grain withone opening
Pollen grain withthree openings
Flowers
Floral organsusually in
multiples of three
Floral organsusually in multiples
of four or five
Evolutionary Links Between Angiosperms and Animals
• Animals influence the evolution of plants and vice versa
– For example, animal herbivory selects for plant defenses
– For example, interactions between pollinators and flowering plants select for mutually beneficial adaptations
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Figure 30.14
• Clades with bilaterally symmetrical flowers have more species than those with radially symmetrical flowers
• This is likely because bilateral symmetry affects the movement of pollinators and reduces gene flow in diverging populations
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Figure 30.15
Human welfare depends greatly on seed plants
• No group of plants is more important to human survival than seed plants
• Plants are key sources of food, fuel, wood products, and medicine
• Our reliance on seed plants makes preservation of plant diversity critical
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Products from Seed Plants
• Most of our food comes from angiosperms• Six crops (wheat, rice, maize, potatoes, cassava,
and sweet potatoes) yield 80% of the calories consumed by humans
• Modern crops are products of relatively recent genetic change resulting from artificial selection
• Many seed plants provide wood• Secondary compounds of seed plants are used
in medicines
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Table 30.1
Threats to Plant Diversity
• Destruction of habitat is causing extinction of many plant species
• In the tropics 55,000 km2 are cleared each year• At this rate, the remaining tropical forests will be
eliminated in 200 years • Loss of plant habitat is often accompanied by
loss of the animal species that plants support
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• At the current rate of habitat loss, 50% of Earth’s species will become extinct within the next 100–200 years
• The tropical rain forests may contain undiscovered medicinal compounds
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Figure 30.16
A satellite imagefrom 2000 showsclear-cut areas inBrazil surroundedby dense tropicalforest.
By 2009, muchmore of this sametropical forest hadbeen cut down.
4 km
Figure 30.16a
A satellite imagefrom 2000 showsclear-cut areas inBrazil surrounded bydense tropical forest.
4 km