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The Transition to
Terrestrial Life
1.
First plants - algae - still thrive in a range of
aquatic habitats today (not primitive - simple
compared to more complex groups; highly
evolved and well adapted to the niche they
occupy.)
2.
Aquatic environment is predictable
(~stable). Why venture onto land?
Selection pressure may have been
competition!
3.
Selective pressures on pioneer land plants
–
Desiccation
–
Water for reproduction - need free
standing water for fusion of gametes
–
Support - buoyancy supports and
spreads the algal thallus. On land, plants
would be plastered on the mud
–
Water for spore dispersal - to colonize
new terrestrial habitats spores would
have to be released in air not water
Evolutionary Trends in
the Transition to Land
1.
~ 400 million years ago freshwater, green,
filamentous algae invaded the land.
– probably isomorphic alternation of generations
– probably heterotrichous.
2.
Selection favors individuals more able to withstand
periods without submergence (e.g. at pond margins,
on wet mud)
3.
Gametophytes need water for reproduction
– basal part of the gametophyte developed with
loss of the upper portion.
– sterile jacket of cells evolved to protect the
developing gametes during periods of exposure.
4.
Sporophytes - spore dispersal was originally in
water.
– Spores need to be dispersed in air.
– upper spore-bearing part of the plant would need
to be held above water
5.
Appearance of first conducting tissues
6.
The generations began to diverge.
Highlights of Plant
Evolution
1.
2.
3.
4.
5.
Silurian (~425 MYA) first land plants
(cuticle, gametangia, vascular
tissues)
Devonian (~400 MYA) seedless
vascular plants (ferns)
Late Devonian (~360 MYA) Seed Plant embryo with food and a coat
Evolution of gymnosperms (coexist
with ferns for 200 MY
Cretaceous (~130 MYA) emergence
of flowers
Changes Needed in the
Sporophyte Generation
• Cuticle - a new non-cellular, waxy, water-proof
layer
• Sporopollenin - a similar waxy waterproofing
appeared on the surface of spores (prevented
desiccation during travel through air).
• Multiaxial filament - (vs flimsy uniaxial filament)
for erect plant retains turgor at the core of plant
even when the outer cell layers lose water
• Transport system - evolution of the stele, (xylem
and phloem) for transporting water, minerals and
organic material when simple diffusion is no
longer sufficient for a large plant body
• Ventilation system - evolution of stomata -pores
in the plant surface - and intercellular air spaces
for transport of gasses
• Anchorage System - the lower portion of the
sporophyte developed an anchorage system that
was more than a few thread-like rhizoids.
The Plant Fossil
Missing Links
1.
Cooksonia - late Silurian with terminal
sporangia
2.
Zosterophyllum - Devonian ancestor of
club mosses and ground pines with lateral
sporangia near stem tips
3.
Rhynia - Devonian ~ 400 MYA (probably
the ancestor of all other extant plants)
Early 1900's, fossils were found
–
Called Rhynia (after the quarry in
Scotland).
–
erect dichotomously branched stems
–
sporangia borne at the tips.
–
spread by an underground rhizome
bearing rhizoids.
Summary of
Bryophytes
1.
Embryophytes (Like other land plants, produce an
embryo); distinct lineage from other land plants.
–
Did not give rise to the vascular plants
–
but they probably were the earliest land plants
probably evolved from green algal ancestors,
closely related to the Charophytes.
–
well-adapted to moist habitats.
2.
Similarities to land plants
–
multicellular sex organs, ( gametes enclosed
by a sterile jacket of cell)s
–
are parenchymatous, not filamentous
–
retain the zygote within the female sex organ
for development
–
have cutin (a cuticle) on the plant and spores
3.
Differences from land plants
–
no lignin (usually)
–
small, low-lying, (generally)
–
Depend on free standing water for
reproduction
–
no true roots, only filamentous rhizoids
–
dominant generation is the gametophyte;
sporophyte is parasitic on the gametophyte.
Reproduction in
Bryophytes: Vegetative
1. fragmentation - pieces of the
gametophyte breaking off (sole
means of dispersal in the Arctic)
2. gemmae - specialized propagules
produced mitotically.
Reproduction in
Bryophytes: Sexual
1.
Often dioecious - male and female sex organs are
borne on separate gametophytes.
2.
sex organs in clusters - examples
–
surrounded by sterile hairs
–
born on the head of a stalk
3.
At maturity, antheridium bursts releasing the sperm
cells or antherozoids.
–
can only swim a few cm so that if the archegonia
are not adjacent the sperm rely on raindrops to
"splash-launch" them
4.
chemical exuded by the egg cell attracts the sperm
–
which swims down the neck of the archegonium
–
fertilizes the egg.
5.
resulting zygote represents the beginning of the
sporophyte generation
Sporophyte
Generation
1.
Develops within the archegonium
remains parasitic on this for its entire life
(except a few moss sporophytes which
develop photosynthetic capacity.)
2.
Sporophyte generation is short-lived.
–
Capsule - produces spores by meiosis
–
Stalk - holds above the body of the
gametophyte.
3.
HAPLOID spores - dispersed by air currents
–
Germinate after landing somewhere moist
4.
Gametophyte generation begins when spore
germinates
–
First have filamentous stage (protonema)
–
Cells are full of chloroplasts.
–
Grows into dominant gametophyte
EVOLUTION OF
WATER TRANSPORT
1.
•
•
Evolution from aquatic to terrestrial
habitats
Supply of water
System of delivery
2.
Exchange rate of water for carbon is
poor
3.
Evolutionary trend is innovations for
greater efficiency