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The Colonization of Land
The Evolution of Land Plants
Retracing biological thought…
From the time of Linnaeus (1707-1778) until fairly
recently (1969)*, the diversity of life was organized
into two main groups: plants and animals. The plant
kingdom was thought to include plants, algae, fungi,
and later, bacteria (i.e. organisms we could see, but that
were clearly not animals).
What are the problems associated with classifying Euglena
and fungi, according to this scheme?
More recent classifications have recognized the
differences among these groups, and treat the plant
kingdom as limited to the Embryophyta, photosynthetic
eukaryotes that have multicellular, dependent embryos.
* The 5-kingdom classification scheme was proposed by Robert H.
Whittaker, of Cornell University.
What challenges exist in the
terrestrial environment?
Consider the 4 main groups of land plants:
Bryophytes (mosses)
Pteridophytes (ferns)
Gymnosperms (conifers)
Angiosperms (flowers)
What are the benefits of
living in a watery
environment?
What challenges exist in the
terrestrial environment?
Consider the abiotic needs:
water, nutrients, space,
exposure to UV & wind, etc.
What challenges exist in the
terrestrial environment?
Consider the major themes in
the study of life:
metabolism, growth,
homeostasis, reproduction, etc.
In other words, how do we define
“land plants”?
• Kingdom Plantae
– Eukaryotic
– Multicellular
– Photosynthetic autotrophs
General Characteristics of
Land Plants
• Chloroplasts, containing
chlorophyll a and b
• Cell walls contain
cellulose
• Starch is the energy
storage molecule
What is the primary function
of the central vacuole?
Adaptations to life “on the border”
Related to characteristics of
growth and reproduction:
How compatible are dry land and
swimming sperm? How big can a
plant body get? How do sessile
organism disperse?
• Apical meristems
• Dependent embryos
• Alternation of generations
– Sporophyte produces
durable spores
– Gametophyte produces
gametes
Meristems
…persistent populations of small
unspecialized cells with embryonic
characteristics
Are there cells in
humans that serve a
similar function?
Fig. 35.12
Fig. 29.3
Reproduction
• parental tissues provide nutrition
• haploid dominant  diploid
dominant …genetic advantage?
• motile gametes  nonmotile
gametes
• seedless  seeds
• Water disbursement  wind and
animals
– gametes, spores and seeds
Bryophytes
How are male gametes
transported to female gametes in
these two groups of land plants?
Conifers
Alternation of Generations
When does the haploid stage occur in humans? Is it
multicellular (as in plants)? Do humans have alternation of
generations?
Plants
Humans
Haploid Dominant  Diploid Dominant
The gametophyte is haploid. What type of cell
division will produce gametes?
Fig. 30.1
What is the advantage of a larger sporophyte for seedless plants?
Additional terrestrial adaptations
cuticle
• Water conservation
– Cuticle
– Stomata
• Transport
– Xylem
– Phloem
• Secondary compounds
xylem
stomata
phloem
Where do you think there would be more stomata,
on the upper or lower leaf surface? Why?
Fixed
Carbon
secondary
metabolites
Modified Leaves
Speculate as to why natural selection favored the leaf morphology
of each plant shown.
Tendrils
Spines
Succulent
Bracts
Underground Stem Storage
Speculate as to the adaptive advantage of these structures for these
plants.
Iris
Rhizomes (lateral stem)
Ginger, many grasses, some ferns...
Tubers
Rhizomes:
potatoes, yams...
Bulbs
Onion, Daffodil, Tulip, Lilies...
What is the most likely ancestor to
the group of land plants?
• How are land plants
different than the
photosynthetic
protists (algae)?
• How are they similar?
Spirogyra
Euglena
Brown algae
Dinoflagellate
The search for structural and
molecular homology*
Fig. 10-9
• Plant cell walls contain
cellulose
– but also present in some
dinoflagellates and
phaeophyta (browns)
*see p. 438
•Plant cells contain
chlorophylls a and b in
chloroplasts
–but also present in
Euglena, some
dinoflagellates and
green algae
The closest relatives to the land
plants are the Charophyceans
• Rosette cellulosesynthesizing complexes
– as opposed to synthesizing
proteins arranged in a line
• Molecular comparisons
Which molecules are
compared?
• Similarities in cytokinesis
• Similarities in sperm cell
structure
Proposed
phylogeny
of land
plants
Wood --> advantage?
Leaves --> advantage?
2. What other
innovation occurred
along with seeds?
1. What major innovations allowed the
initial invasion of land?
I think we should
rename the plant
kingdom
“Terrestrial Algae”.
But algae don’t
flower or produce
seeds!
I think the most
important
difference is that
plants protect
their embryos.
Isn’t the big
difference based
on whether they
have swimming
sperm?