How Plants Colonized Land
Chapter 29
Overview:
• Origins of Mulitcellularity
• Quick look at over all diversity in Plant Kingdom
• Characteristics of Plants
– Special features, structures
• Origin from green algae
– Link to Charophyceans
– Evidence to support
– Dates
• Alternation of generations
– Origin of embryophytes
• Bryophyte (moss) lifecycle – in Lab
• Pteridophyte (fern) lifecycle – in Lab
Multicellularity
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Arose independently several times in Eukarya
Caused another new wave in evolution
Origins in simpler colonial forms – Volvox
Cellular specialization and Division of labor
Escape cell size limitations
– Membrane area to cytoplasm volume ratio
• Multicellularity solves ratio limits Fig 6.7
The Plant Kingdom
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Origins over 475 MYA
10 Divisions
4 Basic lifecycles
Green algae that
evolved onto land
• Evolved becoming
more terrestrial,
independent from
water
• Then coevolved with
pollinators, dispersal
• An overview of land plant evolution
Land plants
Vascular plants
Figure 29.7
Angiosperms
Origin of seed plants
(about 360 mya)
Origin of vascular
plants (about 420 mya)
Origin of land plants
(about 475 mya)
Ancestral
green alga
Seed plants
Gymnosperms
Pterophyte
(ferns, horsetails, whisk fern)
Seedless vascular plants
Lycophytes
(club mosses, spike mosses, quillworts)
Mosses
Hornworts
Liverworts
Charophyceans
Bryophytes
(nonvascular plants)
Major events:
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Living on land, spores
Vascular tissues
Pollen, Seeds
Flowers & Fruits
The Plant Kingdom? Who is in / out?
Viridiplantae
Streptophyta
Plantae
Red algae
Chlorophytes
Ancestral alga
Charophyceans
Embryophytes
• We’ll see all
but the
hornworts in
lab
Table 29.1
Evolutionary Sequence
• Green Algae (Charophyceans) gave rise to:
• Mosses (475 MYA) and liverworts, which gave rise to:
• Ferns (420 MYA) and related plant groups which
gave rise to:
• Gymnosperms (360 MYA) Conifers and related plant
groups which gave rise to:
• Angiosperms (140 MYA) (flowering plants) which
have form two groups:
– Monocots
– Eudicots
Charophyceans- plant’s green algae ancestor
Chara
Coleochaete
Modern examples of charophyceans
Original Traits
• Found in ancestral
species and new
species
Derived Traits
• Not found in ancestral
species, new to
daughter species
Older terms: primitive and advanced traits
What’s new in Plants:
Some Derived Characteristics of Plants
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Growth by divisions in Apical Meristems
Multicellular dependent embryos
Alternation of generations
Spores
Multicellular gametangia
Cuticle
Transport tissues
Secondary compounds
Growth by cell divisions in Apical Meristems
• Localized regions of cell division
• In shoot tips, roots
Multicellular
dependent
diploid embryos
• Land plants are called embryophytes
• Haploid charophyceans
Spores
• Walls of resistant sporopollenin
• Dry out and travel in the wind
• Zygotes of Charophyceans protected by
sporopollenin – precursor to spore walls?
• Disperse then grow into gametophyte plants
Multicellular gametangia
Archegonia
Make egg at base of
Vase like column
Antheridia
produce many sperm
cells that swim to egg
All haploid tissues, gametes form by mitosis
Cuticle
• Waxy covering layer prevents water loss, and
microbial attack
• Stomata allow for gas exchange
• Thicker layers in plants adapted to arid
conditions
Transport tissues
• Xylem carries water up from the roots to the
leaves
• Phloem carries a sugary solution through out
the plant
Secondary Compounds
• Metabolic side branches off common
pathways
• Provide benefit to plant ( defense, etc.)
• Can be used by People as Flavorings, drugs
insecticides, etc.
APICAL MERISTEMS
Apical
meristem
of shoot
Developing
leaves
Apical meristems of plant shoots
and roots. The light micrographs
are longitudinal sections at the tips
of a shoot and root.
Apical meristem
of root
Shoot
Root
100 µm
100 µm
Haploid multicellular
organism (gametophyte)
Mitosis
Mitosis
n
n
n
ALTERNATION OF GENERATIONS
Spores
n
n
Gametes
MEIOSIS
FERTILIZATION
2n
2n
Zygote
Mitosis
Diploid multicellular
organism (sporophyte)
Alternation of generations: a generalized scheme
WALLED SPORES PRODUCED IN SPORANGIA
Spores
Sporangium
Longitudinal section of
Sphagnum sporangium (LM)
Sporophyte
Gametophyte
Sporophyte and sporangium
of Sphagnum (a moss)
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
Placental transfer cell
Origin of plants
• How did the sporophyte generation come
about?
• Preadaptations may have lead the
charyophyceans onto land
Hypothesis for origin of alternation of generations
• Delayed meiosis maximizes output of sexual
reproduction.
– More spores can be produced per fertilization event
• Adapting to drying conditions with fewer surviving
spores
• Lineages separate before sporophytes evolved much
Preadaptation
• Evolutionary adaptation that was selected for
under one set of conditions
• This trait then gives an advantage for a new
situation with different conditions.
Preadaptation
• Evolutionary adaptation that was selected for
under one set of conditions
• This trait then gives an advantage for a new
situation with different conditions.
Feathers arose first for insulation, then
helped with flight
Adaptation to shallow waters preadapted
Charophyceans to life on land
• Adapted to periodic drying during low tides,
droughts
– Leads to cuticle?
• Adapted to higher light intensities
– Leads to common chloroplast structure?
• Zygote protected from drought within
archegonia with a layer of sporopollenin
– Leads to spore wall?
Alternation of Generations
• Separate multicellular haploid and diploid phases
– (2n) Sporophyte make spores by meiosis
– (n) Gametophyte makes gametes by mitosis
• Sperm and egg (moss & fern)
• Pollen and Ovule (gymnosperm & angiosperm)
• The sporophyte and gametophyte are very different
in morphology
– Vascular tissues only appear in sporophyte phase
• Sporophyte becomes more dominant in new plant
groups
• Charophyceans lack sporophyte phase
Charophycean life cycle
Characteristics that Plants share with the
green algae group Charophyceans
• Autotrophic Multicellular Eukaryote
• Have cell walls made of cellulose
– Made by rosette cellulose-synthesizing complexes
– 20-26% of wall material, closest match in algae
• Chloroplast similarities
– have chlorophyll a & b, use β-carotene as accessory
– Thylakoids stacked in grana
– Chloroplast DNA comparisons
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Peroxisome enzymes
Cell plate formation by phragmoplast
Nuclear membrane breaks down during mitosis
Sperm ultrastructure - biflagellate
Gene sequences – rRNA, Cytoskeleton proteins
Switch to sporophyte dominance
Figure 29.13 Hypotheses for the
evolution of leaves
Vascular tissue
(a) Microphylls, such as those of lycophytes, may have
originated as small stem outgrowths supported by
single, unbranched strands of vascular tissue.
(b) Megaphylls, which have branched vascular
systems, may have evolved by the fusion of
branched stems.
What’s new in Mosses?
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Spores / sporangia
Sporophyte phase
Upright growth on land
Cuticle
Multicellular gametangia
The Bryophytes
• Bryophytes are represented by three divisions:
– Division Hepatophyta - liverworts
– Division Anthocerophyta - hornworts
– Division Bryophyta – mosses
Liverworts and hornworts
are believed to be more
similar to what early
plants were like.
Bryophyte lifecycle: moss
• Haploid dominant
• No vascular
tissues
• Filamentous
protonema stage
• Swimming sperm
• Disperse by
spores
• Dependent
sporophyte
• Dioecious
gametophytes
• No true leaves
• Rhizoids, not
roots
What’s new in Ferns?
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Vascular tissue
True roots and stems and leaves
Stomata
Sporophyte (2n) dominate
Pteridophytes evolved over 400 MYA
Seedless, Vascular plants (having Xylem &
Phloem). Today represented by two divisions:
Pterophyta: Ferns, Horsetails (Equisetum)
Lycophyta: Club moss
Cooksonia, an extinct plant over 400 million
years old, is the earliest known vascular
plant.
The branched sporophytes
were up to 50cm tall with
small lignified cells, much
like the xylem cells of
modern pteridophytes.
Vascular tissue
• Allows plants to grow taller
• More support by lignified xylem tracheids
• Can pull water up from soil
– Can tolerate soil that is drier on the surface
• Form parts of true leaves and roots.
• Only found in diploid tissue
– Lead to sporophyte dominance?
Fern Lifecycle
• Diploid dominate
• Gametophyte still independent, short lived,
– monoecious in fern (Pteridophyta)
– dioecious in club moss (Lycophyta)
• Sporophyte
– in Lycophyta is Monecious
– in Pteridiophyta is Dioecious
• Spores disperse plant
• Sporophyte perennial, monoecious
Figure 29.23 The life cycle of a fern
LYCOPHYTES (PHYLUM LYCOPHYTA)
Strobili
(clusters of
sporophylls)
Isoetes
gunnii,
a quillwort
Selaginella apoda,
a spike moss
Diphasiastrum tristachyum, a club moss
Psilotum
nudum,
a whisk
fern
PTEROPHYTES (PHYLUM PTEROPHYTA)
Equisetum
arvense,
field
horsetail
Vegetative stem
Athyrium
filix-femina,
lady fern
Strobilus on
fertile stem
WHISK FERNS AND RELATIVES
HORSETAILS
FERNS
Carboniferous forest based on fossil
evidence