BI111 Lecture Notes - Lecture 15: Red Algae, Charophyta, Ordovician

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Lecture 15 - Land Plants
Focusing on how they changed over time
what were the ancestral traits, what were the derived traits?
What were the selective pressures that shaped the way they evolved
Importance of Land Plants
Important evolutionary event
drastically influenced terrestrial, aquatic and atmospheric conditions
created new environmental niches
300,000 + living species, 10 phyla
Important to today's ecological processes
carbon, water, nitrogen, etc.
primary producers in terrestrial food web
domestication and cultivation of plants
land plants make up a large part of our diet
Evolution of land plants falls at a really important time on earth
for long periods of time life was restricted to aquatic environments
450 million years ago first plants colonize above surface of water
all the other animals evolving due to land plants
Land Plants first evolved in the Ordovician period
early to middle ordovician period
mild climate with moist atmosphere
marine and freshwater organisms had dramatically changed the atmosphere
(cyanobacteria use water as a reproducing agent, producing atmospheric O2 as a
byproduct), CO2 levels were still 10X greater than today
Origin of Land Plants
between land and water
seaweed living half in water and half out
First species to make it onto land:
most likely a type of green algae
Charophyte relative (“Stonewort”)
multicellular with cellulose cell walls
2 forms of chlorophyll (like land plants today)
most cells are photoautotrophs (use light to feed itself)
storage of products of photosynthesis as amylose starch (sugars stored as
starches, phenotype that evolved in ocean from common ancestor)
no specialized transport or support structure; body supported by water, where
nutrients obtained. (no root system) (kind of like seaweed that floats in water,
does not need support its buoyant and floats in water).doesn't need system of
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Challenges and Opportunities
Why did the plants make their way to land anyway?
1. land fewer predators
2. less competition
3. driving force had to do with sunlight
a. limit on how deep you can go as an algae and still have photosynthetic
activity (very little makes it down to any real depth)
b. air is far less absorbent of light energy than water (huge amount of energy
you can use to fix carbon and make more offspring)
Challenge #1 - Dryness
early plants had no control over water content, were restricted to moist areas
when wet lots of water in bodies, when dry dried out completely
charophytes and algae rely on water for spreading gametes
Poikilohydric (variable relating to water)
Selection for Regulation of Water
Adaptation #1 - Cuticle
stops water to be lost across membrane, keeping it inside body
present in all bryophytes (mosses, liverworts, hornworts)
problem : prevents CO2 uptake (plants need to exchange gases across
solutions: Pores (need to control H2O loss) (pores occur across cuticle layer,
but a regular pore is still a problem because it’s not regulated)
Adaptation #2 - Stomata
present in mosses and hornworts (not in liverworts)
guard cells that can be selectively open/closed depending on if plant wants to
exchange gasses or keep water in.
Other Bryophyte Characteristics
Liverwort, Moss, Hornworts
No vascular tissues for movements of water and nutrients - in wet habitats
only in moist safe areas
typically small, low lying, lack of lignin
No roots, only filamentous rhizoids (anchors) (this is why it’s very easy to remove
moss from rock)
limited with getting nutrients out of soil
some weak symbiotic associations with fungi (key evolutionary event)
Gametophyte (haploid) phase dominant - multicellular haploid phase of life cycle is
depend on water, need it to reproduce, don't have major internal structures, don't
grow tall (low lying)
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Haplontic Life Cycle: Zygotic Meiosis
charophyte (green algae) start with production of gamates (single haploid unicellualr
organisims that fuse together to for zygote) (zygote is diploid (2 coppies of every
chromosones (like humans))
In charaphyte zygote doesnt last long, immedialy undergoes meosis creating haploid cells
once formed they undergo mitosis coppying themselves making a multicellular haploid
Haplodiplontic Life Cycle:
zygote undergoes mitosis (copies itself) to make multicellular individual (sporophyte)
Sporophyte produces spores (haploid unicellular structures) undergo mitosis (make
multicellular individual)
Multicellular individual is gametophyte
If you see moss on a rock you're seeing the gametophyte life cycle (haploid phase) haploid
phase dominant means it spends most of its life in that phase, very little time spent in the
sporophytic phase.
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