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Chapter 29
Plant Diversity I:
How Plants
Colonized Land
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: The Greening of Earth
•
Looking at a lush landscape, it is difficult to imagine the land without
any plants or other organisms
•
For more than the first 3 billion years of Earth’s history, the terrestrial
surface was lifeless
•
Since colonizing land, plants have diversified into roughly 290,000
living species
•
Plants supply oxygen and are the ultimate source of most food eaten
by land animals
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Concept 29.1: Land plants evolved from green
algae
•
Green algae called charophytes are the closest relatives of land plants
•
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 only with charophytes:
–
Rose-shaped complexes for cellulose synthesis
–
Peroxisome enzymes
–
Structure of flagellated sperm
–
Formation of a phragmoplast
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• 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
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Fig. 29-3
5 mm
Chara species, a pond organism
Coleochaete orbicularis, a
disk-shaped charophyte that
also lives in ponds (LM)
40 µm
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Adaptations Enabling the Move to Land
•
In charophytes a layer of a durable polymer called sporopollenin
prevents exposed zygotes from drying out
•
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
•
The accumulation of traits that facilitated survival on land may have
opened the way to its colonization by plants
•
Some biologists think the plant kingdom should be expanded to include
some or all green algae
•
Until this debate is resolved, we will retain the embryophyte definition
of kingdom Plantae
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Fig. 29-4
Red algae
Chlorophytes
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Plantae
Embryophytes
Streptophyta
Charophytes
Viridiplantae
ANCESTRAL
ALGA
Derived Traits of Plants
• Four key traits appear in nearly all land plants
but are absent in the charophytes:
– Alternation of generations (with multicellular,
dependent embryos)
– Walled spores produced in sporangia
– Multicellular gametangia
– Apical meristems
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Gametophyte (n)
Fig. 29-5a
produces haploid gametes
by mitosis
Mitosis
Mitosis
n
sporophyte produces
haploid spores
n
Spore
2 gametes from
different plants
form a zygote
Gamete
FERTILIZATION
MEIOSIS
2n
zygote develops into a
multicellular diploid
sporophyte
Mitosis
Sporophyte
(2n)
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Alternation of Generations
•
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
embryo
wall ingrowths
placental transfer cell
(blue outline)
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maternal
tissue
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
sporangium containing spores
sporophyte
gametophyte
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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, are the site of sperm production
and release
femalegemetophyte
Archegonium with egg
Antheridium with sperm
male gametophyte
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Apical Meristems
•
Plants sustain continual growth in their apical meristems
•
Cells from the apical meristems differentiate into various tissues
apical meristem
developing leaves
shoot
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root
Other Characteristics
• epidermis in many species has a covering named cuticle
which acts as waterproof preventing desiccation and
protects against microbal infections
• secondary compounds are produced in secondary
metabolic pathways; examples are terpenes, alkaloids,
tannins that have bitter taste, strong odors, toxic effect on
consumers and pathogens
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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
fossilized spores with more
than one grain
fossilized sporophyte tissue
with spores embebed in it
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• Those ancestral species gave rise to a vast
diversity of modern plants
• 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
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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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Table 29-1
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Fig. 29-7
origin of land plants (about 475 mya)
origin of vascular plants (about 420 mya)
origin of seeded plants
~305 mya
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Concept 29.2: 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
• Mosses are most closely related to vascular
plants
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Bryophyte
• In all three bryophyte phyla, gametophytes are
larger and longer-living than sporophytes
• Sporophytes are typically present only part of
the time
mature sporophyte
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Fig. 29-8-3
1. spores develop
into threadlike
protonema
protonema produces
"buds" that grow into
haploid gametes
sperm swim through
moisture to reach the
egg
meiosis occurs,
spores develop
in capsule. When
mature lid opens
sporophyte grows a long stalk
(seta) which emerges from the
archegonium
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Life Cycle of Moss
• 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 (capsule), which discharges spores through a
peristome which acts like teeth allowing it to open and
close according to the environmental conditions
• Hornwort and moss sporophytes have stomata for gas
exchange
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Fig. 29-9a
Ph. Hepatophyta (Liverworth)
Thallus
Hepatophyta derives from Hepaticus= liver
thalloid= flattened shape of gametophytes
Gametophore of
female gametophyte
Sporophyte
Foot absorbs nutrients
Seta sends nutrients
to the capsule
gemma cup
asexual reproduction
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produces spores by
meiosis
500 µm
Marchantia polymorpha,
a “thalloid” liverwort
Capsule
(sporangium)
Fig. 29-9c
Ph. Anthocerophyta
anthos= top; keras=horn
sporophyta lacks seta, consists only of a sporangium
horn splits open releasing spores
(Hornworths)
Sporophyte
Gametophyte
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Fig. 29-9d
Polytrichum commune,
hairy-cap moss
Capsule
Seta
Sporophyte
(a sturdy
plant that
takes months
to grow)
Gametophyte
Ph. Bryophyta
(Mosses)
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Bryo= mosses
sperm requires water to reach egg
male and female gametophytes
The Ecological and Economic Importance of
Mosses
• Moses 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
Annual Nitrogen loss Kg/Ha
6
3
0
with moss
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without moss
• Sphagnum, or “peat moss,” forms extensive deposits of
partially decayed organic material known as peat
• Sphagnum is an important global reservoir of organic
carbon because of its acidic and oxygen-poor conditions
(a) Peat being harvested
(b) “Tollund Man,” a bog mummy
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Concept 29.3: 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
• Because seedless vascular plants have
flagellated sperm they are usually restricted to
moist environments
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Origins and Traits of Vascular Plants
• Fossils of the forerunners of vascular plants
date back about 420 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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Fig. 29-12
Aglaophyton major
branching is typical of vascular plants
www.adonline.id.au/plantevol/images
sporophyte
fossilized sample
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Life Cycles with Dominant Sporophytes
• In contrast with bryophytes, sporophytes of
seedless vascular plants are the larger
generation, as in the familiar leafy fern
• The gametophytes are tiny plants that grow on
or below the soil surface
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Fig. 29-13-3
1. single spore develops
into bysexual gametophyte
prothallus
Key
Haploid (n)
Diploid (2n)
MEIOSIS
Spore
dispersal
Spore
(n)
Sporangium
Sporangium
Antheridium
Young
gametophyte
Mature
gametophyte
(n)
Archegonium
Egg
Mature
sporophyte
(2n)
New
sporophyte
Zygote
(2n)
FERTILIZATION
Sorus
sorus is a cluster of
sporangia
Gametophyte
Fiddlehead
zygotedevelops into a sporophyte
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Sperm
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
• Phloem consists of living cells and distributes
sugars, amino acids, and other organic products
• Water-conducting cells are strengthened by
lignin and provide structural support
• Increased height was 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
• According to one model of evolution, microphylls evolved
first, as outgrowths of stems
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Fig. 29-14
Overtopping
growth
Vascular tissue
Sporangia Microphyll
Other stems
become reduced and
flattened.
(a) Microphylls
Megaphyll
(b) Megaphylls
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Webbing
develops.
Sporophylls and Spore Variations
• Sporophylls are modified leaves with
sporangia
• Sori are clusters of sporangia located on the
underside of the sporophyll
• Strobili are cone-like structures formed from
groups of sporophylls
strobilus
sori
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Fig. 29-UN3
Seedless
Homosporous spore production
Sporangium
on sporophyll
Single
type of spore
Typically a
bisexual
gametophyte
Eggs
Sperm
Seeded
Heterosporous spore production
Megasporangium
on megasporophyll
Megaspore
Female
gametophyte
Eggs
Microsporangium
on microsporophyll
Microspore
Male
gametophyte
Sperm
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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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Fig. 29-15a
Lycophytes (Phylum Lycophyta)
2.5 cm
Lyco=wolf
small plants
Isoetes
Strobili
(clusters of
gunnii,
a quillwort sporophylls)
1 cm
Selaginella apoda,
a spike moss
Diphasiastrum tristachyum, a club moss
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Fig. 29-15e
Pterophytes (Phylum Pterophyta)
ptero= winged
more than 12,000 species
tropics, temperate and arid habitats
share traits with seed plants (overtopping growth, megaphyll leaves and branching roots
Psilotus, Equisetum
Athyrium
filix-femina,
lady fern
Equisetum
arvense,
field
horsetail
Psilotum
nudum,
a whisk
fern
Vegetative stem
2.5 cm
1.5 cm
25 cm
Strobilus on
fertile stem
observe similarity
with fossil
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Fig. 29-15f
Fern Athyrium filix-femina
Has megaphylls = fond
Homosporous most of them
fonds are made of leaflets
25 cm
fond
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Fig. 29-15g
Horsetail Equisetum arvense
brushy appearance
vegetative and cone bearing stems
homosporous
known as arthrophytes (joints in stems)
small leaves
stem is the main photosynthetic organ
large air canals
Vegetative stem
1.5 cm
Strobilus on
fertile stem
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Fig. 29-15h
Whisk fern Psilotum nudum
2.5 cm
dichotomus branching
scalelike outgrowths that lack vascular tissue
no roots, rhizoids instead
"living fossils"
3 fused sporangia (yellow knob)
homosporous
bysexual gametophytes
gametophytes grow underground
gametophytes measure aprox. 1cm length
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Fig. 29-16
Concept of carboniferous era
The End
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