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Lecture 5

BIOC50H3 Lecture 5: Lecture Five

Biological Sciences
Course Code
Jason Weir

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Lecture Five
The Evolution of Plants
- Plants that come to the land
- 11 points you need to know about the evolution of plants?
POINT 1: The lineage leading up to green algae and plants obtained photosynthetic chloroplasts
following an endosymbiotic incorporation of a photosynthetic cyanobacteria
- The chloroplast was originally a free floating cyanobacteria and it gets incorporated
through endosymbiosis to the ancestor leading to plants and green algae
PONT 2: What are plants: they are a clade of green algae
- Land plants are known as embryophytes
- Land plant are a clade of green algae
- Algae:
o Going back up to 2.1 bya in the form of Grypania
- Red algae go back to 1200 mya
- Green algae: go back up to 750 mya (still disputing)
- These are prior to Ediacaran
- Looking at the phylogeny of green algae:
o All of land plants are one clade of green algae
o The closest relatives to green algae: It’s charales and coleochaetales
- We can assume that land plants may have evolved from a fresh water ancestor rather than
a marine ancestor
POINT 3: Either the green algae Charales (pondweed) or Coleochaetales is the closest living
relative of land plants
What difficulties might a green algae have in colonizing a terrestrial environment and becoming
land plants for the first time?
- Dehydration
- Difference in oxygen and C02 levels
- Direct radiation from the sun would cause desiccation or high mutations
- Soil might have different acidic soil levels
What advantages might there be in a colonizing a land? Why not just stay in the water forever
instead of moving to the land?
- Less competition and might be no predators
o Land is an empty resource that can be colonized if you get the right
- There is a higher concentration of C02
o If you adapt to it…it’ll be great
- Increased level of sunlight compared to the water environment

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The next 8 points deal with plant adaptations to a terrestrial land:
- POINT 4: Land plants are known as embryophytes, due to adaptations to support the
spores and embryo in a gaseous atmosphere
o All embryophytes have a derived reproductive organ that distinguishes them from
green algae
It consists of one or more multicellular layers that help protect in the
developing embryo
o The protective layers around the spores of embryophytes results in Y shaped
trilete markings on spores of embryophytes
Shown on the fossil records… it indicates of land embrophytes rather than
aquatic algae
Embryopytes probably evolved in early Ordovician
- POINT 5: To survive on land, you would need a waxy cuticle
o Almost all embryophytes have waxy cuticles
o Waxy structure covers the surface of cell and provides a barrier between it and the
gaseous environment
o Prevents moisture from escaping the cells
o Cuticle evolves around the origin of land plants
Perhaps in the ancestor of all bryophytes
- POINT 6: Adaptation 3: Stomata
o You need it for carbon dioxide to enter while water and oxygen to exit
o This allows respiration
o Stomata not in liverworts
o Hornworts have an open stomata
Probably first evolution of the stomata
o Mosses: stomata close when dehydrated
It prevents desiccation
Opens stomata to allow for CO2
- POINT 7: Adaptation 4: Vascular system with xylem and tracheid
o Early plants lacked specialized vascular systems and was restricted to moist
environments…such as mosses (they don’t grow tall); they are called bryophytes
(liverworts, hornworts and mosses)
o Vascular system revolutionizes plant evolution because it gives structural support
for the plant and it also allowed for water and minerals to be transported around
the plant…make it not dependent in the environment
Aglaophyton: fossils from early Devonian
- One of the first fossils that are evolving towards vascular tissues
o Aglaophtyon has features intermediate between bryophytes and vascular plants
o They had stomata and intercellular spaces for gas flow but was not fully
- Probably more than 15 cm
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