Plant diversity I: How Plants Colonized Land, Ch 29, U305PP

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Plant diversity I How Plants Colonized Land, Ch
29, U305PP
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• For more than the first 3 billion years of
  Earths history
• The terrestrial surface was lifeless
• Since colonizing land
• Plants have diversified into roughly 290,000
  living species

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What IS a plant?

• Store food as starch
• Display the alternation of generations life cycle
• Photosynthetic, contain chlorophylls a b

Watch for this trend the importance of water
for reproduction
This is a monophyletic kingdom look for the
connections and trends in this kingdom
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• Concept 29.1 Land plants evolved from green
  algae
• Researchers have identified green algae called
  charophyceans as the closest relatives of land
  plants

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Morphological and Biochemical Evidence of
relatedness between charophyceans and land plants

• There are four key traits that land plants share
  only with charophyceans
• Rose-shaped complexes for cellulose synthesis
• Peroxisome enzymes
• Structure of flagellated sperm
• Formation of a phragmoplast, a region formed
  during cell division

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Genetic Evidence

• Comparisons of both nuclear and chloroplast genes
• Point to charophyceans as the closest living
  relatives of land plants

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Adaptations Enabling the Move to Land

• In charophyceans
• A layer of a durable polymer called sporopollenin
  prevents exposed zygotes from drying out
• The accumulation of traits that facilitated
  survival on land
• May have opened the way to its colonization by
  plants

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• Concept 29.2 Land plants possess a set of
  derived terrestrial adaptations
• Many adaptations
• Emerged after land plants diverged from their
  charophycean relatives
• Structurally living on land (support)
• Reproduction (how do you get gametes from male to
  female)

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Defining the Plant Kingdom

• Systematists
• Are currently debating the boundaries of the
  plant kingdom

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• Some biologists think that the plant kingdom
• Should be expanded to include some or all green
  algae

Campbell doesnt reflect this change quite yet ?
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Derived Traits of Plants

• Five key traits appear in nearly all land plants
  but are absent in the charophyceans
• Apical meristems
• Alternation of generations
• Walled spores produced in sporangia
• Multicellular gametangia
• Multicellular dependent embryos

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• Apical meristems and alternation of generations

A diploid, multicellular sporophyte produces
spores by meiosis which grow into haploid
gametophytes that make gametes which fuse to make
a zygote that develops into a sporophyte. Whew.
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• Walled spores multicellular gametangia and
  multicellular, dependent embryos

Spores
WALLED SPORES PRODUCED IN SPORANGIA
Sporangium
Sporophyte and sporangium of Sphagnum (a moss)
Longitudinal section of Sphagnum sporangium (LM)
Sporophyte
Gametophyte
MULTICELLULAR GAMETANGIA
Female gametophyte
Archegonium with egg
Antheridium with sperm
Archegonia and antheridia of Marchantia (a
liverwort)
Male gametophyte
MULTICELLULAR, DEPENDENT EMBRYOS
Embryo
Maternal tissue
2 µm
Embryo and placental transfer cell of Marchantia
10 µm
Wall ingrowths
Figure 29.5
Placental transfer cell
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• Additional derived units
• Such as a cuticle and secondary compounds,
  evolved in many plant species

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The Origin and Diversification of Plants

• Fossil evidence
• Indicates that plants were on land at least 475
  million years ago

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• Fossilized spores and tissues
• Have been extracted from 475-million-year-old
  rocks

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• Whatever the age of the first land plants
• Those ancestral species gave rise to a vast
  diversity of modern plants

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• An overview of land plant evolution

Land plants
Vascular plants
Bryophytes (nonvascular plants)
Seedless vascular plants
Seed plants
Mosses
Hornworts
Liverworts
Angiosperms
Gymnosperms
Charophyceans
Pterophyte (ferns, horsetails, whisk fern)
Origin of seed plants (about 360 mya)
Lycophytes (club mosses, spike mosses, quillworts)
Origin of vascular plants (about 420 mya)
Origin of land plants (about 475 mya)
Ancestral green alga
Figure 29.7
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• Concept 29.3 The life cycles of mosses and other
  bryophytes are dominated by the gametophyte stage
• Bryophytes are represented today by three phyla
  of small herbaceous (nonwoody) plants
• Liverworts, phylum Hepatophyta
• Hornworts, phylum Anthocerophyta
• Mosses, phylum Bryophyta

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• Debate continues over the sequence of bryophyte
  evolution
• Mosses are most closely related to vascular plants

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Bryophyte Gametophytes

• In all three bryophyte phyla
• Gametophytes are larger and longer-living than
  sporophytes

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• The life cycle of a moss

Note dominant Gametophyte generation
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• Bryophyte gametophytes
• Produce flagellated sperm in antheridia
• Produce ova in archegonia
• Generally form ground-hugging carpets and are at
  most only a few cells thick
• Some mosses
• Have conducting tissues in the center of their
  stems and may grow vertically

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Bryophyte Sporophytes

• Bryophyte sporophytes
• Grow out of archegonia
• Are the smallest and simplest of all extant plant
  groups
• Consist of a foot, a seta, and a sporangium
• Hornwort and moss sporophytes
• Have stomata

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NOTE, though no true vascular tissue, must have
water to facilitate reproduction- tied to water

• Bryophyte diversity

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Ecological and Economic Importance of Mosses

• Sphagnum, or peat moss
• Forms extensive deposits of partially decayed
  organic material known as peat
• Plays an important role in the Earths carbon
  cycle

Peat being harvested from a peat bog
(a)
Sporangium at tip of sporophyte
Gametophyte
Living photo- synthetic cells
Closeup of Sphagnum. Note the leafy
gametophytes and their offspring, the
sporophytes.
(b)
Dead water- storing cells
100 µm
Sphagnum leaf (LM). The combination of living
photosynthetic cells and dead water-storing
cells gives the moss its spongy quality.
(c)
(d)
Tolland Man, a bog mummy dating from 405100
B.C. The acidic, oxygen-poor conditions produced
by Sphagnum canpreserve human or other animal
bodies for thousands of years.
Figure 29.10 ad
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• Concept 29.4 Ferns and other seedless vascular
  plants formed the first forests
• Bryophytes and bryophyte-like plants
• Were the prevalent vegetation during the first
  100 million years of plant evolution
• Vascular plants
• Began to evolve during the Carboniferous period

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Origins and Traits of Vascular Plants

• Fossils of the forerunners of vascular plants
• Date back about 420 million years

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• These early tiny plants
• Had independent, branching sporophytes
• Lacked other derived traits of vascular plants

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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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• The life cycle of a fern

NOTE, still need water for reproduction, but
vascular tissue allows plants to grow bigger
Sporangia release spores. Most fern
species produce a single type of spore that gives
rise to a bisexual gametophyte.
The fern spore develops into a
small, photosynthetic gametophyte.
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Although this illustration shows an egg and
sperm from the same gametophyte, a variety of
mechanisms promote cross-fertilization between
gametophytes.
Key
Haploid (n)
Diploid (2n)
Antheridium
Young gametophyte
Spore
MEIOSIS
Sporangium
Sperm
Archegonium
Mature sporophyte
Egg
New sporophyte
Zygote
Sporangium
FERTILIZATION
Sorus
On the underside of the sporophytes reprodu
ctive leaves are spots called sori. Each sorus is
a cluster of sporangia.
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Fern sperm use flagella to swim from the
antheridia to eggs in the archegonia.
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Gametophyte
A zygote develops into a new sporophyte,
and the young plant grows out from an
archegonium of its parent, the gametophyte.
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Fiddlehead
Figure 29.12
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Transport in Xylem and Phloem

• Vascular plants have two types of vascular tissue
• Xylem and phloem

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• Xylem
• Conducts most of the water and minerals
• Includes dead cells called tracheids
• Phloem
• Distributes sugars, amino acids, and other
  organic products
• Consists of living cells

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Evolution of Roots

• Roots
• Are organs that anchor vascular plants
• Enable vascular plants to absorb water and
  nutrients from the soil
• 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 for photosynthesis

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• 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 first, as outgrowths of stems

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Sporophylls and Spore Variations

• Sporophylls
• Are modified leaves with sporangia
• Most seedless vascular plants
• Are homosporous, producing one type of spore that
  develops into a bisexual gametophyte

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

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Classification of Seedless Vascular Plants

• Seedless vascular plants form two phyla
• Lycophyta, including club mosses, spike mosses,
  and quillworts
• Pterophyta, including ferns, horsetails, and
  whisk ferns and their relatives

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• The general groups of seedless vascular plants

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Phylum Lycophyta Club Mosses, Spike Mosses, and
Quillworts

• Modern species of lycophytes
• Are relics from a far more eminent past
• Are small herbaceous plants

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Phylum Pterophyta Ferns, Horsetails, and Whisk
Ferns and Relatives

• Ferns
• Are the most diverse seedless vascular plants

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The Significance of Seedless Vascular Plants

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

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• The growth of these early forests
• May have helped produce the major global cooling
  that characterized the end of the Carboniferous
  period
• Decayed and eventually became coal